Nanomaterials-driven in situ vaccination: a novel frontier in tumor immunotherapy

Abstract

In situ vaccination (ISV) has emerged as a promising strategy in cancer immunotherapy, offering a targeted approach that uses the tumor microenvironment (TME) to stimulate an immune response directly at the tumor site. This method minimizes systemic exposure while maintaining therapeutic efficacy and enhancing safety. Recent advances in nanotechnology have enabled new approaches to ISV by utilizing nanomaterials with unique properties, including enhanced permeability, retention, and controlled drug release. ISV employing nanomaterials can induce immunogenic cell death and reverse the immunosuppressive and hypoxic TME, thereby converting a “cold” tumor into a “hot” tumor and facilitating a more robust immune response. This review examines the mechanisms through which nanomaterials-based ISV enhances anti-tumor immunity, summarizes clinical applications of these strategies, and evaluates its capacity to serve as a neoadjuvant therapy for eliminating micrometastases in early-stage cancer patients. Challenges associated with the clinical translation of nanomaterials-based ISV, including nanomaterial metabolism, optimization of treatment protocols, and integration with other therapies such as radiotherapy, chemotherapy, and photothermal therapy, are also discussed. Advances in nanotechnology and immunotherapy continue to expand the possible applications of ISV, potentially leading to improved outcomes across a broad range of cancer types.

Background

Cancer remains a leading cause of mortality worldwide; conventional treatments such as surgery, radiotherapy, and chemotherapy provide robust but often limited efficacy, particularly in advanced-stage cancers [1]. These therapies are frequently hindered by treatment resistance, high recurrence rates, and substantial side effects, highlighting the urgent need for novel therapeutic approaches [2, 3].

In recent years, immunotherapy has revolutionized cancer treatment by harnessing the body’s immune system to recognize and eliminate cancer cells [4]. Despite its potential, immunotherapy has demonstrated variable success across different cancer types and is often associated with severe immune-related adverse effects [5]. A major challenge limiting its effectiveness is the immunosuppressive and hypoxic tumor microenvironment (TME), which impairs the immune system’s ability to mount a robust anti-tumor response [6, 7].

The development of in situ vaccination (ISV) represents a novel strategy aimed at overcoming the limitations of conventional immunotherapy [8]. ISV functions by stimulating the immune system directly within the tumor site, transforming a “cold” TME into a “hot” TME and thus enhancing the anti-tumor immune response [9]. Nanomaterials have emerged as a powerful tool in ISV due to their unique properties, including improved drug delivery, enhanced permeability and retention (EPR) effects, and ability to modulate the TME [10, 11]. Nanomaterials-based ISV holds promise for advanced-stage cancer patients who have exhausted other treatment options; it also constitutes a potential neoadjuvant therapy in early-stage patients to eliminate micrometastases and reduce recurrence rates [12].

Despite the promising preclinical results of nanomaterials-based ISV, multiple challenges remain in translating these findings into clinical practice (e.g., addressing the metabolism and potential toxicity of inorganic nanomaterials, optimizing treatment regimens, and validating efficacy in clinical trials). Furthermore, the integration of ISV with other therapies, such as chemotherapy, radiotherapy, photothermal therapy, and photodynamic therapy (PDT), requires careful optimization to maximize treatment outcomes.

This review summarizes recent advances in the mechanisms and applications of nanomaterial-based ISV in cancer therapy. It examines how nanomaterials enhance immunogenic cell death (ICD) and reverse the TME, evaluates ISV as both a neoadjuvant and adjunct therapy, and identifies future research directions to overcome current challenges in this field. With continued advancements in nanotechnology and immunotherapy, nanomaterials-based ISV represents a promising strategy for improving cancer treatment and patient outcomes.

In situ vaccination for tumors

Overview of ISV from an immunotherapy perspective

Immunotherapy has been widely implemented in clinical practice and has extensively transformed cancer treatment in recent years [4]. Among the primary forms of immunotherapy, immune checkpoint inhibitors (ICIs) and chimeric antigen receptor-modified T (CAR-T) cells have provided substantial clinical benefits for many patients [13, 14]. ICIs function by inhibiting immune checkpoint receptors, such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed death-1 (PD-1), which are predominantly expressed by T cells, as well as programmed death-ligand 1 (PD-L1), the ligand for PD-1 [15, 16]. CAR-T cell therapy involves harvesting T cells from the patient, genetically engineering them in vitro to express a chimeric antigen receptor (CAR), and re-infusing them into the same patient after lymphodepleting chemotherapy [17]. The CAR structure includes specific fragments that recognize tumor-associated antigens, enabling CAR-T cells to target tumor cells independently of the human leukocyte antigen-dependent mechanism [18]. Although these two immunotherapeutic approaches have extended survival in certain terminal patients, they exhibit important limitations. ICIs, for instance, can induce adverse events, including immune-related toxicities such as severe myocarditis, rash, and pneumonitis [19,20,21,22]. Similarly, CAR-T cell therapy is associated with cytokine release syndrome and neurotoxicity [23]. Furthermore, the efficacy of ICIs depends on PD-L1 expression, whereas the effectiveness of CAR-T cell therapy is compromised by antigen escape, on-target off-tumor effects, and the immunosuppressive microenvironment [24].

In addition to ICIs and CAR-T cell therapy, tumor vaccines have recently gained attention and made substantial progress. These vaccines consist of two key components: tumor antigens and immune adjuvants [12]. The tumor antigens include tumor-associated antigens (TAAs), tumor virus antigens, and neoantigens [25, 26]. TAAs are antigens normally present in cells but aberrantly expressed in tumor cells with respect to quantity, timing, or cellular context. Historically, tumor vaccine research has primarily focused on TAAs. However, because TAAs are also found in normal cells, the immune system often fails to recognize them as foreign and mount an effective response. Consequently, research has shifted toward neoantigens—mutated forms of normal antigens expressed on tumor cells [27]. Advances in sequencing technologies have facilitated the identification and characterization of neoantigens, but tumor heterogeneity continues to limit the broad applicability of neoantigen-targeted vaccines.

ISV represents a unique category of tumor vaccines, distinguished by the inclusion of adjuvants alone, unlike conventional tumor vaccines (Fig. 1) [11]. ISV can be administered to a broad range of cancer patients without requiring tumor antigen recognition, thereby eliminating the need for tumor genome sequencing; this modification greatly reduces both time and preparation costs. Moreover, ISV utilizes tumor antigens already present within the TME, enhancing tumor cell recognition and targeting. Importantly, ISV circumvents tumor escape mechanisms by avoiding antigen elimination, which occurs when only a limited number of antigens are targeted. Safety, a key concern in drug development, is enhanced with ISV through local administration, thus minimizing immune-related adverse effects [8]. Unlike systemic therapies, ISV requires lower drug concentrations, further contributing to its favorable safety profile. Considering these advantages, ISV holds substantial clinical potential and research value, offering a promising approach that may transform cancer treatment in the future.

Fig. 1

Comparison of conventional vaccination and ISV. (a) Conventional vaccination. (b) ISV. Copyright 2018, Wiley Interdiscip Rev Nanomed Nanobiotechnol

Mechanisms of ISV

In the human body, the immune system maintains a delicate balance, preventing attacks on normal tissues that could otherwise lead to autoimmune diseases while simultaneously identifying and eliminating abnormal cells, including tumor cells. However, the TME presents a unique challenge. Numerous studies have demonstrated that the TME plays a critical role in tumor initiation, progression, and metastasis [28, 29]. Within the TME, cancer-associated fibroblasts are prevalent and substantially contribute to tumor progression by recruiting immune cells and secreting various growth factors and cytokines, including vascular endothelial growth factor (VEGF), matrix metalloproteinases, and transforming growth factor β (TGFβ) (Fig. 2) [28, 30]. Tumor-associated macrophages (TAMs) also play a crucial role in the immune response within the TME [31]. Depending on the stimuli encountered, TAMs can polarize into either M1 (anti-tumor) or M2 (pro-tumor) phenotypes [32]. Unfortunately, M2 pro-tumor macrophages predominate in the TME and secrete a variety of immunosuppressive cytokines, further promoting tumor growth [28, 33].

Fig. 2

The TME is immunosuppressive. Abbreviations: DC, dendritic cell; tol, tolerogenic; IDO, indoleamine 2,3-dioxygenase; Arg, arginase; MDSC, myeloid-derived suppressor cell; NOX-2, nitric oxide synthase-2; PD(L)1, programmed death (ligand)-1; NO, nitric oxide; Treg, regulatory T cell; IL, interleukin. Copyright 2020, Int J Hyperthermia

In parallel, CD8 + T cells, which are critical for tumor cell eradication, often become dysfunctional and exhausted due to prolonged T-cell receptor (TCR) signaling [34,35,36]. Additionally, regulatory T cells (Tregs), a specialized subset of CD4 + T cells, are recruited into the TME by chemokines secreted by TAMs. These Tregs release immunosuppressive cytokines such as interleukin (IL)-10 and TGFβ, thereby suppressing anti-tumor immunity and facilitating immune escape [37, 38]. Tregs can also induce CTLA-4-independent immunosuppression, rendering CTLA-4-targeting treatments ineffective [39, 40]. Furthermore, myeloid-derived suppressor cells (MDSCs) are recruited and expand within the TME, where they inhibit CD8 + T-cell and natural killer (NK) cell function while simultaneously promoting angiogenesis and tumor invasion [41,42,43].

Beyond immunological alterations, hypoxic conditions and elevated reactive oxygen species (ROS) levels within the TME further drive immune escape, tumor progression, and metastasis [44,45,46]. Collectively, these factors establish the TME as an immunosuppressive milieu [47]. Although the benefits of ISV in modulating the TME are well established for localized solid tumors, its application to systemic malignancies, such as hematological cancers, remains challenging. These malignancies often exhibit widespread infiltration, complicating the direct administration of ISV approaches. Advanced nanotechnological strategies, such as functionalized nanoparticles capable of targeting specific tissues (e.g., bone marrow), may expand the applicability of ISV to systemic malignancies by enabling immune modulation at sites of tumor dissemination. ISV exerts its therapeutic effects by utilizing adjuvants to reverse the immunosuppressive TME, a key mechanism underlying its action (Fig. 3a) [48, 49]. The adjuvants used in ISV reduce the presence of immunosuppressive cells, including MDSCs, M2 pro-tumor macrophages, Tregs, and immature dendritic cells (DCs). Simultaneously, ISV promotes the expansion of immunostimulatory cells within the TME, such as M1 anti-tumor macrophages, CD8 + T cells, and activated DCs. Through conversion of the TME from an immunosuppressive to an immunostimulatory state, ISV restores the immune system’s anti-tumor functions, thereby enhancing its capacity to eliminate tumors.

Fig. 3

Mechanisms by which ISV initiates and stimulates an effective anti-tumor immune response. (a) Reversal of the immunosuppressive TME through the induction of proinflammatory cytokines, polarization of M2 pro-tumor macrophages into M1 anti-tumor macrophages, activation of DCs, and inhibition of immunosuppressive Tregs and MDSCs. (b–f) ICD-related mechanisms: (b) ICD leads to the release of tumor antigens (TAs); (c) recruitment, maturation, and activation of antigen-presenting cells (APCs) at the tumor site, enhancing TA uptake and antigen processing by APCs; (d) increased trafficking of APCs to lymph nodes, presentation or cross-presentation of TAs to T cells, activation of antigen-specific T cells, and differentiation into cytotoxic T lymphocytes (CTLs), T-helper (Th) cells, effector memory T cells (TEMs), central memory T cells (TCMs), and antibody-secreting B cells in the tumor-draining lymph node; (e) expansion and recruitment of immune cells, including CTLs, Th cells, NK cells, and granulocytes; (f) tumor targeting at distant sites by circulating effector T cells and antibodies, induction and activation of memory T cells (TEMs and TCMs), and establishment of long-term anti-tumor immune memory to prevent tumor relapse. Copyright 2018, Wiley Interdiscip Rev Nanomed Nanobiotechnol

Another key mechanism of ISV involves the induction of ICD. In the 1890s, William Coley observed that postoperative infections could lead to the regression of unresected tumors. He documented cases in which tumors shrank after the injection of bacteria, a phenomenon he reported in several publications [12]. However, this treatment approach did not gain widespread acceptance at the time. In the 1950s, researchers discovered that radiotherapy could induce an “abscopal effect,” whereby unirradiated tumors regressed after radiation treatment. The mechanism underlying this effect remained unclear until the 21st century, when scientists linked it to the immune system, leading to the recognition of ICD as a key contributor to systemic anti-tumor immunity [50,51,52].

Cancer cells are likely to die of hypoxia, injury, or other stressors. However, cancer cell death is non-immunogenic in some instances—it fails to trigger a robust anti-tumor immune response. In contrast, ICD leads to the release of damage-associated molecular patterns (DAMPs), which are recognized by innate pattern recognition receptors (PRRs) (Fig. 4) [53, 54]. DAMPs include molecules such as calreticulin (CRT), adenosine triphosphate (ATP), heat shock proteins 70 (HSP70) and 90 (HSP90), and high mobility group box 1 (HMGB1), whereas PRRs encompass Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), both of which constitute indicators of immunogenicity [12, 55]. Mutated DCs are subsequently recruited to the tumor site, where they phagocytose dying tumor cells and release tumor antigens (Fig. 3b–f) [56]. These antigen-presenting cells (APCs) then undergo activation and present tumor antigens to T cells, promoting the differentiation of T cells into cytotoxic T lymphocytes (CTLs), T-helper (Th) cells, and effector memory T cells (TEMs). This process results in the recruitment and activation of various immune cells, including NK cells, granulocytes, and B cells. Additionally, antibodies produced by B cells circulate throughout the body, targeting distant tumors and contributing to the “abscopal effect.” The activation of memory T cells also prevents tumor recurrence, providing long-term therapeutic benefits. For example, one study demonstrated that in a triple-negative breast cancer (TNBC) mouse model, the administration of ICD inducers, such as human neutrophil elastase (ELANE) and Hiltonol^® (a TLR3 agonist), activated the immune response and inhibited breast cancer growth, thus validating the ICD mechanism [57].

Fig. 4

ICD induces the release of DAMPs, including CRT, ATP, HSP70, HSP90, and HMGB1. Copyright 2020, Mol Oncol

Overall, reversal of the immunosuppressive TME to an immunostimulatory state and the induction of ICD are the two principal mechanisms by which ISV exerts its anti-tumor effects. Importantly, these mechanisms are not independent—they synergistically interact through immune cells and cytokines.

Clinical applications and clinical trials of ISV

Despite extensive research over the past few decades, only a limited number of ISV agents have received approval from the U.S. Food and Drug Administration (FDA) for clinical use (Table 1). The first of these is Bacillus Calmette-Guérin (BCG), which can inhibit bladder cancer growth, eradicate bladder tumors, and prevent recurrence when administered as an intravesical therapy (Fig. 5a) [58, 59]. Consequently, BCG is widely used in the postoperative treatment of bladder cancer and provides a statistically significant extension of patient survival [60]. Although BCG has been in use for decades, the precise mechanisms underlying its therapeutic effects have not been fully elucidated.

Table 1 Important clinical applications of in situ vaccination

Fig. 5

Mechanisms underlying key clinical applications of ISV. (a) BCG attaches to (step 1) and invades (step 2) the urothelium. BCG therapy induces both an innate immune response (step 3a) and an adaptive immune response (step 4a). Copyright 2018, Nature Reviews Urology. (b) IM disrupts the balance between mitochondrial pro-apoptotic (blue: Bax, Bak, Bid) and anti-apoptotic (yellow: Bcl-2, Bcl-xL, Mcl-1) proteins, favoring cytochrome c translocation into the cytosol. This translocation leads to caspase activation and cell death. Copyright 2023, International Journal of Molecular Sciences. (c) The extrinsic apoptosis pathway induced by IM. IM upregulates the expression of CD95 and CD95 ligand (FasR, Fas-APO1 receptor system) in basal carcinoma cells, activating the extrinsic apoptosis pathway through direct contact between basal carcinoma cells and CD4 + T cells. Copyright 2023, International Journal of Molecular Sciences. (d) T-VEC preferentially replicates in cancer cells due to lower protein kinase R (PKR) activity caused by overactive Ras signaling (1). T-VEC replication is also facilitated by disrupted type I interferon (IFN) signaling (2). After viral replication and propagation in the nucleus, mature virion assembly is completed in the cytoplasm and cell lysis is induced (3). Copyright 2016, Clinical Cancer Research. (e) Infiltration of CD4+, CD8+, and CD68 + cells increases, and the diversity of the TCR repertoire is significantly modified in the radiotherapy-activated NBTXR3 group compared with radiotherapy alone. Copyright 2022, Cancer Cell International

Another category of ISV agents approved by the FDA includes TLR7/8 agonists, such as imiquimod (IM) and resiquimod, which are used in the treatment of skin cancers (Fig. 5b–c). The TLR7/8 agonist is mechanism of action is relatively straightforward: these agents activate PRRs, thereby triggering an immune response [61, 62]. Several other TLR7/8 agonists are currently under development, offering the potential for new therapeutic options in the future [63, 64].

Talimogene laherparepvec (T-VEC), an oncolytic virus, has also been approved for clinical use in the treatment of melanoma [65]. T-VEC represents the first oncolytic virus to receive FDA approval for clinical application. Through genetic modification, T-VEC has been engineered to enhance its immunogenicity [66, 67]. However, considering its relatively recent introduction, the long-term clinical outcomes and efficacy of T-VEC require further evaluation.

Additionally, certain cytokines, such as IL-12 and granulocyte-macrophage colony-stimulating factor (GM-CSF), have been directly utilized as ISV agents. These cytokines stimulate Th cells, recruit DCs, and facilitate tumor antigen presentation [68, 69]. Furthermore, to improve drug targeting and minimize systemic side effects, researchers are investigating strategies such as ligand-functionalized nanoparticles and external stimuli, including ultrasound and magnetic fields [70]. These approaches guide nanoparticles to specific tumor sites, enhancing therapeutic precision and reducing off-target effects. Such advancements hold promise for extending ISV applications beyond localized tumors. However, this type of ISV therapy is often combined with other treatment modalities to enhance effectiveness.

Currently, multiple clinical trials investigating ISV are ongoing (Table 2). For instance, Poly-ICLC, a multifunctional immune modulator, has entered a phase II clinical study (NCT02423863) [71]. Poly-ICLC stimulates two PRRs, TLR3 and melanoma differentiation-associated protein 5 (MDA5), leading to the production of cytokines and chemokines. As a type of ISV against solid tumors, Poly-ICLC activates NK cells, CD4 + T cells, and CD8 + T cells to reverse the immunosuppressive TME. It can be administered as monotherapy or in combination with immunotherapy, chemotherapy, and radiotherapy [72]. A related phase I trial demonstrated that Poly-ICLC was well tolerated; one patient achieved stable disease and a progression-free survival (PFS) of 6 months after two treatment cycles, whereas another patient experienced disease progression [73]. Another phase II clinical study is investigating the efficacy of Poly-ICLC in solid tumors (NCT01984892). Among the eight patients enrolled, only one completed all study visits, achieving a disease-free survival of 41 weeks; the other patients discontinued due to disease progression. Notably, all eight patients remained alive at the 30-month review, and no serious adverse events were reported. Furthermore, a combined phase I and phase II clinical trial (NCT02643303) evaluated the number of patients experiencing treatment-emergent adverse events (TEAEs) and the median PFS. Although all participants developed TEAEs within 15 months, the median PFS ranged from 43 to 157 days across cohorts of patients with different tumor types after treatment with durvalumab, tremelimumab, and Poly-ICLC. Among the 14 patients with locally recurrent or metastatic breast cancer, 21.4% achieved a partial response after receiving durvalumab for 12 cycles, intratumoral tremelimumab for four cycles, and intratumoral Poly-ICLC for three cycles. In contrast, in trial NCT03490760, seven patients received durvalumab plus radiation therapy, achieving a median PFS of only 53 days. Because three of the seven patients experienced serious adverse events, this clinical trial has been terminated. In the future, these investigational drugs may receive FDA approval and become available for clinical use, potentially advancing the application of ISV in anti-tumor therapy. However, the limited number of ISV drugs approved for clinical use highlights several important challenges in their development. A key barrier lies in the biological complexity of ISV. Its efficacy depends on modulating the TME, which greatly varies across cancer types and individual patients, hindering the achievement of broad therapeutic effectiveness [74, 75]. Additionally, ISV therapies often require intratumoral injections, which may limit patient recruitment and complicate clinical trial designs. From a manufacturing perspective, scalability and reproducibility present major challenges, particularly for therapies that involve nanomaterials. Consistent nanoparticle size and stability across production batches remain difficult to ensure. Furthermore, safety concerns, including off-target toxicity and immune-related adverse events, have created obstacles to meeting stringent regulatory requirements. High development costs and prolonged clinical trial timelines have also contributed to the limited number of approved ISV therapies. Several clinical trials listed in Table 2 were terminated due to a combination of scientific, logistical, and financial factors. In some cases, early-phase studies demonstrated insufficient therapeutic efficacy, which did not justify further development. For example, the complex interactions between ISV agents and the TME can sometimes lead to suboptimal immune activation. Logistical constraints, such as challenges in recruiting patients eligible for intratumoral injection therapies, have also resulted in early trial termination. Resource limitations and unexpected safety concerns, including severe immune-related adverse events, have further impeded the progression of some trials. Despite these challenges, the future of ISV remains promising, with ongoing innovations aimed at overcoming these barriers. Advances in nanotechnology are expected to improve the stability and scalability of ISV platforms, whereas optimized trial designs and patient stratification strategies may enhance alignment between therapies and specific TME characteristics. Furthermore, the integration of ISV with complementary modalities (e.g., radiotherapy, chemotherapy, and ICIs) may provide synergistic effects, thereby enhancing therapeutic efficacy. As regulatory frameworks evolve to accommodate novel therapeutic approaches, ISV therapies have the potential to become a cornerstone of cancer immunotherapy.

Table 2 Important clinical trials regarding in situ vaccination

Enhancing ISV: the power of nanomaterials

The success of ISV largely depends on its ability to reverse the immunosuppressive TME and induce a robust immune response. Although conventional adjuvants and immunotherapy agents have demonstrated potential in achieving these effects, their clinical utility is often constrained by systemic toxicity, insufficient targeting, and suboptimal immune cell activation [76, 77]. Nanomaterials offer a considerable advantage in this context. Their unique properties, including enhanced permeability, retention, and controlled drug release, enable precise localization at the tumor site; this minimizes off-target effects while maximizing immunostimulatory potential [78, 79]. Moreover, certain nanomaterials actively modulate the TME by alleviating hypoxia, promoting immune cell infiltration, and inducing ICD, thus enhancing the overall efficacy of ISV [9, 80, 81]. These multifunctional capabilities position nanomaterials as essential in addressing the current limitations of ISV. The following section examines specific types of nanomaterials and their applications in tumor therapy, highlighting their roles in complementing and enhancing the mechanisms that underlie ISV.

Nanomaterials in ISV: enhancing Tumor-Targeted immunotherapy

Nanomaterials: A critical tool for enhancing ISV

Nanoparticles, typically ranging from 1 to 100 nm in size, play a pivotal role in enhancing ISV for cancer immunotherapy. These nanoparticles can be broadly classified into inorganic and organic categories [94, 95]. Organic nanoparticles, including ferritin, micelles, dendrimers, and liposomes, offer biocompatibility and flexibility for drug delivery [96]. In contrast, inorganic nanoparticles, such as metal and metal oxide nanoparticles, provide greater stability and unique physicochemical properties that make them highly suitable for tumor targeting and immune modulation [97]. Nanomaterials are characterized by their nanoscale dimensions, which confer distinct properties that enhance interactions with biological systems [98].

The application of nanomaterials in ISV has received substantial attention due to their ability to improve drug delivery, promote immune cell activation, and modulate the TME. Their high surface area allows for increased catalytic activity, which can be utilized to generate ROS and nitric oxide (NO), both of which play essential roles in disrupting the immunosuppressive TME [99,100,101,102,103]. Silver nanoparticles, for example, exhibit antibacterial effects and stimulate immune responses through ROS generation, thereby enhancing ISV efficacy [102].

Nanomaterials are engineered with diverse properties to meet specific therapeutic needs. In the context of ISV, nanoparticles have been designed to optimize the delivery of immunostimulatory agents and tumor antigens directly to the tumor site, maximizing immune activation while minimizing systemic toxicity. For instance, nanoparticles improve immune system responses by increasing tumor vasculature permeability and enhancing antigen presentation, both of which are critical for efforts to achieve a robust ISV-mediated immune response [104, 105]. These properties establish nanomaterials as an integral component in the ongoing development of ISV strategies, underscoring their transformative potential in cancer therapy.

Leveraging nanomaterial properties for effective ISV

Nanomaterials have become central to the development of ISV strategies due to their unique properties, which enhance both tumor targeting and immune system activation in anti-tumor treatments [106, 107]. Initially, nanomaterials were primarily valued for their role in drug delivery, utilizing the EPR effect. This effect enables nanomaterials to passively accumulate in tumors without active targeting because tumor vasculature typically exhibits larger pores that increase permeability relative to normal tissues [108,109,110]. The EPR effect is particularly important in ISV, where it facilitates the localized concentration of immunostimulatory agents or antigens within the TME, thereby enhancing local immune activation while minimizing systemic toxicity. Researchers have explored strategies to further optimize this effect by modifying the size, shape, and flexibility of nanoparticles, as well as incorporating NO-releasing agents to increase vascular permeability and improve nanomaterial delivery to the tumor site [111, 112].

In addition to passive targeting, nanomedicines are engineered to provide sustained drug release, a key requirement in ISV to maintain prolonged immune stimulation at the tumor site. The encapsulation of therapeutic agents within nanomaterials enables sustained and controlled release, ensuring a continuous presence of immune-activating molecules [113, 114]. For instance, liposomal formulations of doxorubicin significantly extend the drug’s half-life compared with free doxorubicin, resulting in prolonged exposure to tumor cells and enhancing ISV-induced immune responses [115]. Furthermore, the distinct size and structural properties of nanomaterials improve the solubility of poorly soluble drugs, which is essential for effective delivery and bioavailability in the TME [116,117,118]. The protective encapsulation of drugs within nanomaterials also stabilizes them against degradation caused by pH changes or temperature fluctuations, thus increasing their efficacy in ISV applications [119].

Importantly, nanomaterials are not limited to drug delivery roles; they also function as adjuvants that directly modulate immune responses within ISV. Certain nanomaterials, particularly those engineered to induce ICD, release DAMPs that recruit and activate immune cells, further stimulating the immune system [120,121,122]. By reversing the immunosuppressive characteristics of the TME, these nanomaterials facilitate a robust anti-tumor immune response. Moreover, nanomaterials can synergize with other therapies—such as chemotherapy, radiotherapy, and photothermal therapy—in the ISV framework, thereby enhancing overall therapeutic efficacy. For example, nanoparticles combined with radiotherapy can increase tumor immunogenicity; formulations incorporating chemotherapy agents such as paclitaxel, which also induces ICD, can generate a more potent immune response [123, 124]. These properties highlight the essential role of nanomaterials in ISV development, establishing them as indispensable tools for modern cancer immunotherapy.

Clinical applications of nanomaterials in ISV

Due to their unique properties, nanomaterials have achieved considerable success in clinical applications, and several nanomedicines have received FDA approval for cancer therapy (Table 3) [125]. Doxil, the first FDA-approved nanomedicine, encapsulates doxorubicin within liposomes, improving drug delivery while minimizing systemic toxicity, and is indicated for the treatment of ovarian carcinoma [126]. Another example is Genexol-PM, a polymeric micelle formulation of paclitaxel, which enhances anti-tumor efficacy and is approved for the treatment of breast cancer and non-small-cell lung cancer [127, 128]. These nanomedicines illustrate how nanoparticles improve conventional cancer therapies by increasing drug accumulation at tumor sites and reducing adverse effects.

Table 3 Important Anti-Tumor nanomedicines

Nanomaterials also play an increasingly important role in ISV strategies. One notable example is NBTXR3, a functionalized hafnium oxide nanoparticle that greatly amplifies the effects of radiotherapy by enhancing cancer cell immunogenicity, thus activating adaptive anti-tumor immune responses. Clinical trials have demonstrated that, when combined with radiotherapy, NBTXR3 provides superior immune activation relative to radiotherapy alone (Table 1) [140]. These nanoparticles serve dual functions by improving local tumor control and stimulating systemic immune responses, closely aligning with ISV’s objective of transforming tumors into “in situ” vaccines.

In addition to nanoparticles, nanoscale oncolytic viruses such as T-VEC and Delytact are integral to ISV. These viruses selectively infect tumor cells, inducing their destruction while simultaneously triggering systemic immune responses [141]. When delivered through nanoparticles, oncolytic viruses enable a synergistic approach that combines direct tumor lysis with immune activation.

Beyond FDA-approved nanomedicines, numerous other nanomaterials are currently undergoing clinical evaluation to further expand ISV applications [142, 143]. As additional nanomedicines gain regulatory approval, their integration into ISV strategies is expected to introduce new therapeutic options that enhance the precision and efficacy of cancer immunotherapy, ultimately improving patient outcomes.

Applications of ISV prepared with nanomaterials in tumor therapy

Reversal of the immunosuppressive and hypoxic TME

As described above, ISV transforms the immunosuppressive TME into an immunostimulatory state. Additionally, the TME is known to exist in a hypoxic state, which serves as a key driver of cancer malignancy. In addition to immune modulation, nanomaterials contribute to the reversal of tumor hypoxia [144]. This section outlines the applications of ISV formulated with nanomaterials in tumor therapy, focusing on roles in the reversal of immunosuppressive and hypoxic TMEs [145].

However, not all tumor microenvironments are conducive to injection-based drug delivery, particularly when tumors are encapsulated by dense fibrotic tissues, located in poorly accessible anatomical regions (e.g., central nervous system or bone), or surrounded by pathophysiological barriers such as high interstitial fluid pressure. These conditions limit the effectiveness of direct injection and require alternative delivery methods to achieve the benefits of ISV. One promising approach is catheter-based interventional delivery, which utilizes radiological imaging techniques (e.g., fluoroscopy, computed tomography, or magnetic resonance imaging guidance) to precisely position catheters within or near the tumor site [146, 147]. This method is particularly advantageous for delivering immune stimulants to deep-seated tumors or those within sensitive structures, such as liver metastases or brain tumors. For example, transarterial chemoembolization, a technique commonly utilized for hepatocellular carcinoma, could be adapted to administer ISV agents directly into the tumor vasculature, ensuring high local concentrations while minimizing systemic exposure [148]. Another potential technique is ultrasound-guided microbubble-assisted drug delivery, which enhances the permeability of dense or fibrotic tumor tissues [149, 150]. Microbubbles, in combination with ultrasound, temporarily disrupt the tumor’s extracellular matrix and vascular barriers, facilitating deeper penetration of immune stimulants or nanoparticles loaded with ISV components. There is evidence that this method improves drug distribution in desmoplastic tumors, such as pancreatic cancer [151]. Additionally, magnetic resonance-guided focused ultrasound (MRgFUS) presents another avenue for precise and non-invasive ISV agent delivery [152]. By generating localized heat or mechanical forces, MRgFUS increases vascular permeability, reduces interstitial fluid pressure, and enhances drug penetration in encapsulated tumors [153]. This method can be further optimized via heat-sensitive nanocarriers, which release immune stimulants exclusively at the tumor site, thereby integrating ISV with thermal targeting for synergistic therapeutic effects [154]. These interventional and image-guided techniques represent promising opportunities to expand the scope of ISV, adapting its principles to challenging tumor microenvironments. By integrating these advanced delivery methods with ISV, physical and physiological barriers may be overcome, thus enabling effective localized immune activation even in tumors historically considered inaccessible.

Naturally derived nanomaterials

Virus-like particles (VLPs), composed of multiple proteins, possess a rigid and repetitive surface structure that can form pathogen-associated structural patterns (PASPs) [155]. PASPs facilitate the cross-linking of B-cell receptors, a critical step in the early stages of B-cell activation [156]. Among VLPs, plant virus nanoparticles (PVNPs) have emerged as a promising anti-tumor therapy [157]. Some researchers have demonstrated that PVNPs stimulate the phagocytosis of APCs, although the precise mechanism remains unclear [158]. Upon activation, APCs release cytokines and chemokines that help reverse the immunosuppressive TME [159]. Additionally, PVNPs can activate TLRs (Fig. 6) [160]. Although PVNPs hold considerable potential in ISV development, their applications require further investigation. For example, cowpea mosaic virus (CPMV), a type of PVNP, can stimulate anti-tumor immune responses in mouse models of breast cancer, ovarian cancer, and other malignancies [161]. The strong anti-tumor effects of CPMV may be attributed to its ability to activate TLR2, TLR4, and TLR7. Another study demonstrated that potato virus X (PVX) can slow melanoma progression through ISV mechanisms [162]. Furthermore, dual-functional nanoparticles combining PVX and doxorubicin exhibit greater anti-tumor efficacy than either PVX or doxorubicin alone, demonstrating a synergistic effect. Similarly, papaya mosaic virus nanoparticles (PapMVs) significantly delay melanoma progression and prolong survival by increasing CD8 + T-cell infiltration and decreasing MDSCs [163]. PapMVs have also shown a synergistic effect when combined with PD-1 inhibitors. Notably, alfalfa mosaic virus greatly slowed tumor progression in a breast cancer model by increasing the levels of IFN-α, IFN-γ, IL-6, and IL-12 [164]. These findings underscore the potential importance of VLPs in ISV. Another class of naturally derived nanomaterials includes bacteria-derived nanoparticles. Minicells—nanosized vesicles secreted by bacteria—are primarily used to encapsulate chemotherapy drugs, rather than ISV applications [165]. However, many bacterial components function as pathogen-associated molecular patterns, which are recognized by PRRs [166, 167]. This finding suggests that bacteria-derived nanoparticles have potential applications in ISV.

Fig. 6

Mechanisms by which PVNPs activate immune responses. Copyright 2022, J Cancer Immunol (Wilmington)

Inorganic nanomaterials

Gold (Au) nanoparticles have been shown to promote DC maturation and DC-mediated lymphocyte proliferation [168]. Moreover, Au nanoparticles can conjugate with and inactivate TGF-β1, an immunosuppressive factor [169]. These nanoparticles also enhance tumor infiltration by T cells. In addition to their use as standalone agents, Au nanoparticles conjugated with CpG deoxynucleotides—functioning as photothermal agents—reverse the TME by increasing the CD8+/CD4 + T-cell ratio, thus exerting a therapeutic effect in lymphoma [170]. Furthermore, Nam et al. developed photothermally polydopamine-coated Au nanoparticles, which significantly increased CD8 + T-cell infiltration in a mouse model of colon carcinoma [171]. Their findings also indicated that these nanoparticles enhance the therapeutic efficacy of doxorubicin, suggesting a novel anti-tumor strategy. Luo et al. investigated the use of poly(lactic-co-glycolic acid) (PLGA) microspheres co-encapsulating hollow Au nanoparticles and metformin to inhibit cancer progression [172]. This formulation was combined with 2-deoxyglucose to reverse photothermal resistance and enhance anti-tumor efficacy.

Silica nanoparticles (SiNPs) can also stimulate immune responses [173]. When used for ISV in a melanoma mouse model, cationic SiNPs induced plasma membrane rupture and the release of TAAs, leading to tumor cell death. Yang et al. demonstrated that mesoporous SiNPs enhanced CTL infiltration while reducing Treg infiltration in a mouse model of hepatocellular carcinoma [174]. Moreover, mesoporous SiNPs improved the anti-tumor effects of radiotherapy. When complexed with cationic SiNPs, bis-(3’-5’)-cyclic dimeric guanosine monophosphate (cdG), an agonist of the stimulator of interferon genes (STING) pathway, induced higher levels of IFN-γ and tumor necrosis factor-alpha (TNF-α), thereby recruiting additional CD8 + T cells [175]. Similarly, Chen et al. demonstrated that mesoporous SiNPs modified with poly(ethylene glycol) (PEG) and an ammonium-based cationic molecule (TA), and loaded with negatively charged cdG, significantly increased the secretion of IL-6, IL-1β, and IFN-β, as well as immune cell infiltration; these changes reversed the immunosuppressive TME in a mouse model of breast cancer [176].

Copper (Cu) is able to catalyze intracellular hydrogen peroxide, leading to the production of ROS that induce tumor cell death [177, 178]. Jang et al. developed a nanoplatform in which lipopolysaccharide was used to coat copper sulfide (CuS) nanoparticles. After laser irradiation, this nanoplatform activated DCs via TLR4 and increased the expression levels of IL-12, IL-6, TNF-α, and IFN-γ in a mouse model of colon cancer [179]. Additionally, this nanoplatform prevented secondary tumor growth in the spleen and inhibited liver metastasis.

These examples illustrate the broad potential of inorganic nanomaterials in enhancing the immune response by reversing the immunosuppressive TME. Furthermore, these nanomaterials can be combined with other adjuvants and treatments to achieve greater anti-tumor efficacy. Although such strategies demonstrate considerable promise for solid tumors, their adaptations to systemic malignancies require further investigation. In systemic cancers, such as hematological malignancies, nanoparticles could be engineered to deliver immune modulators to specific tissues, including the bone marrow or lymph nodes. This targeted approach would enable ISV-like immune responses even in cancers with widespread dissemination, expanding the clinical applicability of ISV strategies. Additionally, chemotherapy drugs such as doxorubicin and epirubicin, typically utilized for their direct cytotoxic effects, can enhance ISV by inducing ICD. This process leads to the release of DAMPs and TAAs, which activate DCs and prime CTLs. For example, the combination of doxorubicin-loaded nanoparticles with TLR agonists enhances immune responses, converting immunologically “cold” tumors into “hot” tumors more effectively than either agent alone [180]. Moreover, nanomedicines offer unique opportunities for dual-drug delivery by co-encapsulating chemotherapy agents and immune-stimulating molecules, such as cytokines or TLR agonists, to simultaneously induce tumor cytotoxicity and immune activation. For instance, liposomal nanoparticles carrying both doxorubicin and CpG oligodeoxynucleotides have been shown to increase CD8 + T-cell infiltration while reducing Tregs in tumor models [181]. In addition to their roles in therapeutic delivery, certain nanomaterials exhibit intrinsic immunomodulatory properties. Au nanoparticles, for example, promote DC maturation, whereas polymer-based nanoparticles can induce macrophage polarization from the pro-tumor M2 phenotype to the anti-tumor M1 phenotype [182]. These properties further reinforce the potential of nanomedicine as both a drug delivery platform and an immune activator, supporting ISV strategies that reshape the TME to enhance anti-tumor immunity.

Liposome nanomaterials

Liposomes, a type of membrane vesicle, are widely utilized in drug development due to their excellent drug-carrying capacity. However, they can also be effectively used for ISV. Meraz et al. demonstrated that cationic liposomes increased the expression levels of IL-1β and TNF-α in mouse models of breast cancer [183]. Furthermore, the addition of monophosphoryl lipid A (MPL-A) and IL-12 to cationic liposomes activated PRRs, further enhancing IFN-γ expression and increasing CD8 + T-cell infiltration. Notably, MPL liposomes and IL-12 exhibited a synergistic anti-tumor effect. Zheng et al. developed a nanoplatform composed of folic acid-modified thermosensitive liposomes as the shell and simvastatin-loaded Au nanoparticles as the core [184]. After photothermal therapy, this nanoplatform induced tumor antigen release and increased CTL infiltration in a mouse model of melanoma, indicating reversal of the immunosuppressive TME. Lu et al. designed a formulation that combined CpG liposomes with lipid bilayer-coated cisplatin nanoparticles [185]. This combination increased IFN-γ and TNF-α expression in mouse models of melanoma, enhanced CTL infiltration, and reduced the presence of MDSCs and Tregs, thereby facilitating the transition from an immunosuppressive TME to an immunostimulatory TME.

Other researchers have developed liposome-based nanoparticles known as CRT NPs [186]. CRT is a calcium-binding protein expressed in the endoplasmic reticulum (ER). CRT NPs were able to increase the expression of DAMPs and enhance the anti-tumor effect of CTLA-4 inhibitors in mouse models of colon tumors. The combination of CRT NPs and CTLA-4 inhibitors increased CD8 + T-cell infiltration while reducing MDSC presence, effectively reversing the immunosuppressive TME. This combination also activated ICD, demonstrating the interplay between these two mechanisms. Similarly, Xia et al. developed a TNF-α-loaded liposome platform designed to release TNF-α within tumors, leading to tumor cell necrosis and subsequent tumor antigen release [187]. This platform promoted DC activation and T-cell infiltration, thus facilitating TME transition; it also activated ICD through the release of high mobility group box 1 (HMGB1) and lactate dehydrogenase. Additionally, this platform enhanced the anti-tumor efficacy of PD-1/PD-L1 inhibitors. Many liposome-based nanomedicines are not used in isolation but are instead combined with cytokines or other immunotherapeutic agents to enhance anti-tumor efficacy. This approach aligns with the principle that such nanomedicines, when utilized in ISV, act as adjuvants to improve the TME and potentiate the effects of other therapeutic agents.

Polymer nanomaterials

Similar to liposomal nanomaterials, polymer-based nanomaterials are widely used in the development of ISV. Polyethylenimine (PEI)-based nanoparticles encapsulating small interfering RNA (siRNA) have been shown to reverse the tolerogenic phenotype of human and mouse ovarian tumor-associated DCs by activating TLR5 [188]. Another study confirmed that cationic dextran and PEI can convert M2 pro-tumor macrophages to M1 anti-tumor macrophages while activating T cells [189]. Similarly, PEI has been demonstrated to enhance the function of CpG oligodeoxynucleotides by forming a PEI-CpG nanocomplex [190]. This PEI-CpG nanocomplex increased NK cell and T-cell infiltration in a mouse model of melanoma.

Although some polymer nanomaterials can independently reverse the immunosuppressive TME, they are often used in combination with other immunotherapeutic agents. For example, Zhang et al. developed a novel ISV strategy that integrated hyaluronic acid-functionalized polydopamine nanoparticles with IM and doxorubicin into a thermosensitive hydrogel [191]. This formulation promoted DC maturation and increased the proportion of CD8 + T cells in a mouse model of breast cancer. However, these effects could not be achieved using polymer nanomaterials alone. Another study demonstrated that PLGA-PEG nanoparticles loaded with doxorubicin, the TLR3 agonist Poly, the dual TLR7/8 activator resiquimod, and macrophage inflammatory protein-3α increased leukocyte infiltration in mouse models of lung carcinoma and colon adenocarcinoma [180]. Similarly, monomethoxy-poly(ethylene glycol)-poly(d, l-lactide-co-glycolide) (mPEG-PLGA) nanoparticles enhanced the function of oxaliplatin by activating CTLs. These polymer nanomaterials also induced ICD, illustrating the interplay between the two mechanisms [192].

Polymer nanomaterials can also be utilized in conjunction with other therapies. For instance, a study demonstrated that a polymer-shelled oxygenated dextran acetate succinate microparticle significantly increased oxygen levels and CD8 + T-cell infiltration in the TME within a mouse model of prostate cancer after radiation therapy [193]. This finding suggests that by reversing tumor hypoxia through polymer nanomaterials, radiation therapy may exert a stronger “abscopal effect.” Similarly, Chen et al. developed a PLGA-based nanomedicine encapsulating catalase and loading hydrophobic IM, a TLR7 agonist [124]. After radiation therapy, this nanomedicine released oxygen and converted macrophages from the pro-tumor M2 phenotype to the anti-tumor M1 phenotype, thereby reversing both hypoxic and immunosuppressive TMEs. Furthermore, this nanomedicine demonstrated synergistic effects when combined with a CTLA-4 inhibitor.

Chitosan nanomaterials

Chitosan nanomaterials, composed of glucosamine and N-acetylglucosamine, are unique because they are the only naturally occurring polycations [194]. This characteristic provides chitosan nanomaterials with strong bioadhesive properties, enabling them to effectively bind to negatively charged surfaces (e.g., mucosal tissues). Chitosan nanomaterials can also form a gel matrix through chemical cross-linking, a process that can be triggered by changes in pH or temperature, making them pH-sensitive. In addition to these advantages, chitosan nanomaterials exhibit antioxidant activity and effectively neutralize various radical species [195]. Moreover, chemical modifications can produce numerous chitosan derivatives, including trimethyl, glycol, acylated, and thiolated chitosan nanomaterials, which further enhance their functional properties. Consequently, chitosan nanomaterials have been widely utilized as drug delivery vehicles. However, several decades ago, researchers discovered that chitosan nanomaterials could also enhance immune responses when used as adjuvants [196, 197]. For example, chitosan nanomaterials augment the immunostimulatory effects of GM-CSF by promoting greater T-cell proliferation [198]. Thus, chitosan nanomaterials are increasingly employed as immune adjuvants.

Zaharoff et al. demonstrated that IL-12 co-formulated with chitosan exhibited greater anti-tumor efficacy than IL-12 alone, primarily through the activation of CD8 + T cells and NK cells in mouse models of colorectal and pancreatic cancer [199]. Although those authors did not characterize particle size, their data clearly indicated the ability of chitosan to reverse the immunosuppressive TME and exert anti-tumor effects. Another study revealed that IL-12 co-formulated with chitosan could eradicate orthotopic bladder tumors in mice after four intravesical treatments, an outcome not achieved with IL-12 alone or BCG therapy [200]. The mechanism underlying this enhanced anti-tumor effect is that IL-12 co-formulated with chitosan increases levels of IL-12, IL-6, TNF-α, and IFN-γ, while also promoting CD8 + T-cell and macrophage infiltration. In a study by Kim et al., rather than using chitosan nanoparticles for direct delivery of IL-12, mannosylated chitosan (MC) was utilized to introduce the IL-12 gene into DCs [201]. The results showed that MC induced higher expression levels of IL-12 and IFN-γ, thereby inhibiting tumor growth. Additionally, MC suppressed angiogenesis by reducing VEGF expression.

Chitosan nanomaterials can also be combined with other therapies. Guo et al. developed a platform that integrates photothermal ablation with immunotherapy [202]. This platform consists of chitosan-coated hollow CuS nanoparticles combined with CpG oligodeoxynucleotides. After photothermal ablation, this nanomedicine increased the infiltration of activated NK cells and CD8 + T cells while significantly enhancing DC maturation in breast cancer mouse models; it effectively reversed the immunosuppressive TME. Furthermore, this platform increased IFN-γ and IL-2 expression levels. In recent years, a novel N-dihydrogalactochitosan (GC) formulation, combined with near-infrared (NIR) laser therapy, has been developed [203]. Researchers have demonstrated that after thermal laser delivery, GC alters the TME by increasing IL-12 expression, promoting DC maturation, and activating T cells [204]. Another study confirmed that, after photothermal therapy, GC reduced the population of Tregs in a mouse model of squamous cell carcinoma [205].

Mixed formulations of organic/inorganic nanomaterials

Both organic and inorganic nanomaterials offer distinct advantages; combinations of these materials may enhance anti-tumor efficacy while reducing toxicity. Chen et al. demonstrated that fibrin gel-encapsulated calcium carbonate nanoparticles, when combined with an anti-CD47 antibody, increased M1 macrophage and CD8 + T-cell populations while reducing M2 macrophages, Tregs, and MDSCs in surgical wounds within mouse models of melanoma after tumor resection [206]. Additionally, this nanomedicine enhanced levels of IFN-γ and TNF-α expression. By converting an immunosuppressive TME into an immunostimulatory TME, this approach effectively controlled the growth of both primary and metastatic tumors in an incomplete tumor resection model. Similarly, Zhang et al. developed a platform comprising 3-aminopropyltriethoxysilane (APTES)-modified iron(II, III) oxide (Fe₃O₄) nanoparticles conjugated with CpG oligodeoxynucleotides [207]. This platform significantly increased NK cell and CD8 + T-cell infiltration relative to Fe₃O₄ or CpG alone, thereby inhibiting the growth of both primary and metastatic tumors in mouse models of breast and colon cancer by reversing the TME.

Mixed organic/inorganic nanomaterials can also be combined with other therapeutic strategies. For instance, Zhang et al. developed another organic/inorganic nanomedicine composed of PLGA modified with PEG and Gly-Arg-Gly-Asp-Ser (GRGDS) peptides, encapsulating a PD-1 inhibitor, iron oxide, and perfluoropentane [208]. After photothermal therapy, this nanomedicine increased CD8 + T-cell infiltration and prolonged survival in a mouse model of melanoma. Another study demonstrated that polydopamine-coated aluminum oxide (Al₂O₃) nanoparticles conjugated with CpG oligodeoxynucleotides induced DC maturation and increased the levels of IFN-γ and TNF-α expression after photothermal therapy [209]. This mixed organic/inorganic nanomedicine also increased CD8 + T-cell infiltration, thereby activating the anti-tumor immune response.

However, combinations of organic and inorganic nanomaterials do not consistently result in superior anti-tumor effects. For example, Hoopes et al. compared the efficacy of iron oxide nanoparticles (IONPs), VLPs derived from the cowpea mosaic virus, and a combination of IONPs and VLPs after radiation therapy in canine oral melanoma patients [210]. In their study, VLPs represented the organic nanomaterial component. Intriguingly, the IONP-VLP combination did not demonstrate superior anti-tumor effects compared with IONPs or VLPs alone. All three treatments increased immune cell infiltration, but their effects did not significantly differ. Therefore, efforts to identify optimal combinations of organic and inorganic nanomaterials that produce enhanced synergistic effects require further investigation.

Comparative analysis and challenges of nanomaterials for reversing the TME

The six categories of nanomaterials discussed—naturally derived, inorganic, liposomal, polymer-based, chitosan, and mixed organic/inorganic formulations—offer distinct advantages in modulating the TME and supporting ISV. Naturally derived nanomaterials, such as VLPs, exhibit strong immunogenicity and biocompatibility; however, source material variability and batch-to-batch reproducibility present challenges. Inorganic nanomaterials, including Au and Si nanoparticles, effectively deliver therapeutic payloads and enhance immune responses, but concerns persist regarding their long-term toxicity and clearance. Liposomal nanomaterials, which have achieved substantial clinical success with products such as Doxil, offer excellent biocompatibility and controlled drug release; nevertheless, stability during storage can be problematic. Polymer-based nanomaterials and their derivatives provide highly customizable platforms capable of incorporating diverse payloads, but scalability and synthesis complexity remain key obstacles. Chitosan-based systems uniquely combine bioadhesive properties with immunostimulatory capabilities, although their chemical stability and limited clinical applications restrict broader use. Mixed organic/inorganic materials harness the synergistic benefits of different material types and demonstrate considerable promise; however, their manufacturing complexity and potential for increased toxicity pose challenges. These findings are summarized in Table 4, which provides a comparative analysis of efficacy, safety, and translational potential across nanomaterial types.

Despite their potential, the development of nanomaterials for TME modulation faces critical challenges related to reproducibility, scalability, and safety. For instance, the reproducibility of naturally derived materials, such as plant-based VLPs, heavily depends on source-specific factors; synthetic inorganic nanoparticles require stringent control over size, shape, and surface chemistry to ensure consistent biological performance. Scaling production to meet clinical demands presents another major challenge, particularly for mixed organic/inorganic formulations and polymer-based nanomaterials, which often involve complex manufacturing workflows. Stability issues further complicate development, especially for lipid-based systems that are susceptible to degradation under suboptimal storage conditions. Furthermore, long-term safety remains a critical concern for inorganic nanoparticles, which may persist in tissues and exhibit potential toxicity. Efforts to address these challenges require a focus on standardization of synthesis methods, optimization of storage formulations, and validation of safety profiles through long-term preclinical and clinical studies. The integration of synergistic combinations of nanomaterials represents a promising strategy for enhancing efficacy and overcoming individual material limitations. Advancements in this area could facilitate next-generation ISV strategies that more effectively modulate the TME and improve cancer treatment outcomes.

Table 4 Comprehensive comparative analysis of nanomaterials for reversing the TME

By inducing ICD

The discovery of ICD has advanced the application of nanomaterials in cancer therapy. ICD induction only eliminates tumors at the treatment site; it also reduces distant tumors by transforming a “cold” TME into a “hot” TME [9]. Consequently, nanomaterials used in ISV are often combined with other anti-tumor therapies, such as photothermal therapy, chemotherapy, and radiotherapy. These therapies function as ICD inducers, enhancing overall anti-tumor efficacy. The occurrence of ICD can be confirmed by evaluating biomarkers such as CRT, ATP, and HMGB1. This section highlights several nano-platforms capable of inducing or enhancing ICD based on the therapeutic modalities with which they are combined.

Enhancing ISV via photothermal Therapy-Induced ICD

Tumor cells die upon exposure to elevated temperatures [211]. This heat-induced cell death releases DAMPs. Photothermal therapy exploits this mechanism by converting light energy into heat to induce tumor cell death [212, 213]. NIR light enhances the local surface plasmon resonance of certain nanoparticles [214], leading to either radiative or non-radiative decay, which subsequently converts photon energy into thermal energy [215]. By modifying the size and shape of nanoparticles, the heat generated can be precisely controlled. This property makes nanomaterials, particularly inorganic nanoparticles, excellent candidates for photothermal therapy due to their high molar extinction coefficients and superior resistance to photodegradation [216].

Cu catalyzes H₂O₂ to generate ROS, triggering tumor cell death. Additionally, Cu efficiently converts light energy into heat. Zhang et al. developed a copper-based metal-organic framework platform capable of producing hydroxyl radicals, thereby inducing ICD [217]. Under NIR light irradiation, this Cu-based nanoplatform significantly increased CRT expression relative to the control group, confirming ICD induction. Furthermore, this platform promoted DC maturation, increased activated CD8 + T-cell infiltration, and elevated the levels of IL-6, IFN-γ, and TNF-α in a mouse model of breast cancer. Wang et al. developed maleimide PEG-modified CuS nanoparticles [218]. After four photothermal treatments, these CuS nanoparticles increased CD8 + T-cell infiltration and upregulated the expression levels of IL-2, IL-6, IFN-γ, and TNF-α in a mouse model of breast cancer. Moreover, these CuS nanoparticles enhanced the anti-tumor effect of a PD-L1 inhibitor, effectively inhibiting tumor growth in both primary and distant tumors.

Indocyanine green (ICG) is another effective photosensitizer (PS) in the NIR spectrum, capable of converting light energy into heat [219]. Li et al. developed a nanoplatform incorporating ICG, hollow Au nanospheres, and hemoglobin (HB) liposomes [220]. After NIR light irradiation, this nanoplatform induced CRT expression, promoted DC maturation and activation, recruited CD8 + T cells, reduced Tregs, and increased IFN-γ and TNF-α expression levels in mouse models of colon carcinoma and melanoma. In addition to the ICD mechanism, this nanoplatform delivered oxygen via HB, effectively reversing tumor hypoxia within the TME.

Photothermal therapy can also be combined with other treatments to enhance anti-tumor efficacy. Liu et al. investigated a nanoplatform incorporating oxaliplatin, ICG, and an iron-based organic framework, MIL-100 (Fe) [221]. MIL-100 releases drugs and degrades under acidic TME conditions [222]. This nanoplatform, utilized in the ISV setting, releases ICG and oxaliplatin within the tumor, effectively integrating photothermal therapy with chemotherapy. Researchers confirmed that after NIR laser irradiation, this nanoplatform induced HMGB1 release in mouse models of colorectal cancer, indicating ICD induction. Additionally, it promoted T-cell recruitment and activation. When combined with a PD-L1 inhibitor, this nanoplatform inhibited the growth of distant tumors in mouse models.

Leveraging Chemotherapy-Induced ICD for Nanomaterials-Based ISV

Certain chemotherapy drugs, including doxorubicin, oxaliplatin, and bleomycin, are well-established ICD inducers [223,224,225,226]. Although their mechanisms of ICD induction vary, Wang et al. reported that these chemotherapy agents generally induce ICD through ROS generation and by promoting immunogenicity via ER stress [227]. Consequently, many researchers have developed nanoplatforms that incorporate these chemotherapy agents.

Doxorubicin is a widely used chemotherapeutic agent that inhibits topoisomerases—enzymes essential for DNA replication and transcription—leading to DNA strand breaks and ICD induction [228]. Han et al. developed a nanoplatform utilizing PEI to encapsulate cis-aconityl-doxorubicin and CpG oligodeoxynucleotides through electrostatic interactions [229]. Cis-aconityl-doxorubicin, sensitive to the acidic TME, releases doxorubicin within the tumor, thereby inducing ICD. When used in the ISV setting, this nanoplatform significantly increased CRT exposure, CD8 + T-cell recruitment, DC maturation, and the expression levels of IFN-γ and TNF-α in mouse models of melanoma compared with the control group. Wang et al. developed another doxorubicin-based nanoplatform that linked CpG-silver nanoclusters with doxorubicin via manganese dioxide [230]. This platform induced the expression of CRT, ATP, and HMGB1, stimulating a robust immune response. Additionally, doxorubicin inhibited Tregs and enhanced the activity of effector T cells, further strengthening anti-tumor immunity.

Oxaliplatin, another widely used chemotherapy drug, belongs to the platinum family and inhibits DNA replication and transcription [231]. Numerous studies have confirmed that oxaliplatin functions as an ICD inducer [232, 233]. Lu et al. developed an oxaliplatin-based nanoplatform using mesoporous silica nanoparticles to encapsulate oxaliplatin and the indoleamine 2,3-dioxygenase (IDO) inhibitor indoximod [234]. As with other ICD inducers, this nanoplatform increased CD8 + T-cell infiltration and decreased Tregs in a mouse model of pancreatic cancer. Indoximod further enhanced the ICD effect and strengthened the immune response.

Similarly, Jin et al. developed a paclitaxel-based nanoplatform incorporating paclitaxel, the TLR7/8 activator resiquimod, and the IDO inhibitor epacadostat [235]. Paclitaxel, which disrupts tumor cell mitosis, can also induce ICD [236, 237]. This platform reversed the immunosuppressive TME by epacadostat, increasing CD8 + T-cell infiltration and reducing Treg populations. Consistent with other studies, paclitaxel induced the expression of CRT and HMGB1, confirming ICD induction. Although the mechanisms of these chemotherapy drugs differ, most are able to induce ICD. As a well-established and widely utilized anti-tumor treatment, chemotherapy holds considerable promise as an ISV strategy.

Enhancing Radiotherapy-Induced ICD with Nanomaterials-Based ISV

Radiotherapy, a primary anti-tumor treatment, has a robust ability to eliminate tumors by inducing double-strand DNA damage [238]. As previously discussed, the “abscopal effect” observed in radiotherapy has been linked to ICD, where damaged tumor cells release DAMPs and thus stimulate an immune response. Leveraging this mechanism, researchers have developed various nanomaterials to amplify the ICD effect induced by radiotherapy.

Janic et al. demonstrated that Au nanoparticles, functioning as radiosensitizers, enhance radiotherapy-induced ICD in a mouse model of breast cancer [239]. Au nanoparticles significantly increased CRT expression and macrophage infiltration. Similarly, Huang et al. developed a gadolinium (Gd)-based nanoplatform incorporating Gd, guanosine monophosphate, and hemin, which exhibits peroxidase-mimic catalytic activity [240]. After radiotherapy, this nanoplatform enhanced radiosensitization and reduced tumor size in a mouse model of colon carcinoma by increasing the levels of CRT, HMGB1, and ATP expression. Additionally, it increased the proportion of CD8 + T cells in the TME, an effect not achieved with radiotherapy alone, and exhibited a synergistic effect with PD-L1 inhibitors. The study also confirmed that this nanoplatform amplified the “abscopal effect” of radiation therapy.

In addition to inorganic nanomaterials, nanoplatforms incorporating chemotherapeutic agents can be combined with radiotherapy. For example, Qin et al. developed a bismuth (Bi)-based nanoplatform composed of upconversion nanophosphors containing Bi ions and doxorubicin [241]. After radiation therapy, this nanoplatform induced greater ATP production compared with radiation alone, signifying ICD induction. Beyond the promotion of ICD, this nanoplatform recruited NK cells, activated effector T cells, and increased the expression levels of M1 macrophage-associated cytokines and chemokines, including IL-3, IL-5, IFN-γ, and TNF-α. Moreover, it converted M2 macrophages to M1 macrophages, effectively reversing the immunosuppressive TME. Through these combined mechanisms, this nanomedicine significantly inhibited lung tumor growth in mouse models.

Nanomaterials have also been investigated in combination with radiotherapy and immunotherapy. For instance, Choi et al. developed a nanoplatform composed of snowflake-like Au nanocarriers, silver (Ag), and a PD-L1 inhibitor [242]. After radiation therapy, this nanoplatform strongly induced ICD by increasing CRT and HMGB1 expression, upregulating PD-L1, reducing Treg differentiation, and promoting DC maturation in a mouse model of prostate cancer. These findings highlight the potential for radiotherapy integration with immunotherapy and nanomaterials to enhance anti-tumor immune responses.

Although the combination of radiotherapy with nanomaterials demonstrates robust potential, several critical questions remain, including the optimal timing between ISV and radiotherapy, the ideal frequency of radiotherapy sessions, and the most effective radiation dose. Further research is required to fully establish clinical applications of nanomaterials in the ISV setting to complement radiotherapy.

Enhancing PDT-Induced ICD with Nanomaterials-Based ISV

PDT and photothermal therapy are related but distinct treatment modalities. Whereas photothermal therapy eradicates tumor cells through heat produced from light energy, PDT utilizes light energy to stimulate photochemical reactions of PSs, generating ROS that induce tumor cell death [243, 244]. PDT involves two types of photochemical reactions: one in which the excited triplet state of PS (³PS) directly interacts with substrates to produce organic radicals, and another in which ³PS transfers energy to molecular oxygen, thereby generating singlet oxygen [245]. Both reactions can occur simultaneously; the dominant pathway depends on PS type, substrate concentration, and oxygen availability. Certain PSs, such as hematoporphyrin derivatives, have received clinical approval for treating bladder, esophageal, and lung cancers [246,247,248].

However, PDT faces some challenges, particularly in the hypoxic TME, which limits its efficacy [249, 250]. Additionally, PDT-induced oxygen consumption can further exacerbate hypoxia, potentially promoting tumor migration. Nanomaterials capable of releasing oxygen can mitigate this issue and enhance PDT’s anti-tumor effects. Another limitation is the aggregation-caused quenching effect at high PS concentrations, which reduces PDT efficacy [251, 252]. By lowering the required PS dose, nanomaterials used as ISV agents offer a promising solution when combined with PDT.

Liang et al. developed a core-shell Au nanocage coated with manganese dioxide (MnO₂) [253]. In the acidic TME, MnO₂ decomposes, releasing oxygen to mitigate hypoxia. After NIR light irradiation, this nanoplatform significantly increased CRT exposure and upregulated the expression levels of ATP and HMGB1, indicating enhanced ICD. It also elevated ROS levels in the TME, thereby improving PDT efficacy. Furthermore, this platform promoted DC maturation and activated CD8 + T cells. Similarly, Liu et al. designed a MnO₂-based nanoplatform composed of calcium carbonate (CaCO₃) and MnO₂, loaded with ICG and PD-L1-targeting siRNA [254]. After NIR light irradiation, this platform activated DCs and increased CD8 + T-cell infiltration in a mouse model of lung cancer. Through oxygen release, MnO₂ enhanced ROS production, thereby improving the anti-tumor effects of PDT and PD-L1 immunotherapy. Furthermore, PDT combined with photothermal therapy produces synergistic effects [243].

Enhancing sonodynamic therapy (SDT)-Induced ICD with Nanomaterials-Based ISV

SDT is a non-invasive treatment with minimal side effects [255]. However, the exact mechanism by which ultrasound generates ROS remains under investigation. Ultrasound may induce the formation of gas bubbles in aqueous environments [256, 257]. The collapse of these bubbles releases energy, producing light and heat that activate or degrade sonosensitizers, leading to ROS production. Similar to PDT, ROS generated by SDT induce tumor cell death and promote ICD. Alternatively, bubble vibrations may directly damage tumor cells, triggering the release of DAMPs [258].

Compared with PDT, SDT is more effective in the treatment of deep-seated malignant tumors, making it a promising therapeutic option. However, SDT efficacy is limited by sonosensitizer performance. To address this limitation, nanoparticles can be incorporated to enhance the ICD effect induced by SDT. Yue et al. developed a nanoplatform containing hematoporphyrin monomethyl ether (HMME) and IM encapsulated in liposomes [259]. HMME, an FDA-approved sonosensitizer, increased CRT expression in a mouse model of breast cancer after ultrasound treatment, confirming the occurrence of ICD [260]. This platform also promoted DC maturation and increased the levels of IL-6 and TNF-α expression. Furthermore, the combination of SDT with this nanoplatform and a PD-L1 inhibitor demonstrated synergistic effects, effectively inhibiting both primary and distant tumors.

As observed with PDT, the hypoxic TME limits the ICD effect produced by ROS during SDT [258]. To address this limitation, certain nanoplatforms have been designed to release oxygen, thereby enhancing the anti-tumor effects of SDT. Li et al. developed a nanoplatform composed of meso-tetra(4-carboxyphenyl)porphine (TCPP) combined with catalase and fluorinated chitosan [261]. Catalase facilitated the decomposition of hydrogen peroxide into oxygen, alleviating tumor hypoxia. After SDT, this platform enhanced ROS production by the sonosensitizer TCPP in a mouse model of bladder cancer.

Some studies have shown that certain sonosensitizers, such as porphyrin derivatives and sonnelux, also function as PSs [262, 263]. Consequently, the combination of SDT with PDT can enhance anti-tumor efficacy while minimizing side effects. However, because SDT is particularly effective for deep-seated tumors, it is less commonly combined with ISV, which is more suitable for superficial tumors. An exception is bladder cancer, which, despite being a deep-seated tumor, is accessible for ISV due to its direct communication with the external environment. Consequently, the combination of SDT and ISV holds considerable promise for bladder cancer treatment.

Enhancing chemodynamic therapy (CDT)-Induced ICD with Nanomaterials-Based ISV

CDT utilizes nanocatalysts to catalyze the decomposition of hydrogen peroxide in the TME, generating ROS and inducing ICD [264, 265]. In contrast to PDT or SDT, the efficacy of CDT does not depend on external stimuli but is instead influenced by nanocatalyst presence, hydrogen peroxide level, and TME characteristics. Therefore, regulation of the hypoxic and immunosuppressive TME, enhancement of hydrogen peroxide levels, and improvements to nanocatalyst efficiency are crucial for optimizing CDT.

Yin et al. developed a nanozyme incorporating CpG oligodeoxynucleotides, copper ions (Cu²⁺), and hydroxides [266]. Cu²⁺ facilitate glutathione (GSH) depletion through redox reactions. GSH, the most abundant intracellular antioxidant, neutralizes ROS and promotes tumorigenesis [267, 268]. This nanozyme increased CRT exposure and upregulated the levels of ATP and HMGB1 expression in a mouse model of glioma, confirming ICD induction. Additionally, it promoted DC maturation, converted M2 macrophages to M1 macrophages, and increased CD8 + T-cell infiltration. Through the catalytic activity of this nanoplatform, chemical reactions were successfully induced, effectively inhibiting the growth of distant tumors. He et al. constructed another nanoplatform using PEI and Cu²⁺ to encapsulate lactate oxidase (LOX) [269]. Lactate plays a critical role in tumorigenesis by contributing to an acidic TME and regulating gene expression [269, 270]. LOX decomposes lactate, producing hydrogen peroxide, which is subsequently converted into ROS by Cu²⁺. This nanoplatform also increased the proportions of M1 macrophages and CD8 + T cells, effectively suppressing tumor growth in a mouse model of breast cancer.

CDT can be combined with other therapeutic strategies to enhance its efficacy. For instance, Fu et al. designed a PEGylated cobalt ferrite (CoFe₂O₄) nanoflower (CFP) with multiple enzymatic activities [271]. CoFe₂O₄ facilitated a Fenton-like reaction to generate ROS and release oxygen, thereby reversing the hypoxic TME. Additionally, CFP functioned as a sonosensitizer, producing ROS upon ultrasound stimulation. After SDT, this multifunctional nanozyme induced ATP secretion and upregulated levels of CRT and HMGB1 expression in a mouse model of breast cancer, confirming ICD occurrence. Furthermore, when combined with a PD-L1 inhibitor, CFP increased CD8 + T-cell infiltration and inhibited both primary and distant tumors in mouse models.

Synergistic effects of multimodal therapies

The combination of different therapeutic modalities with nanomaterials-based ISV can greatly enhance ICD and stimulate stronger anti-tumor immune responses. For instance, the integration of photothermal therapy, chemotherapy, and radiotherapy with ISV increases ICD induction by promoting the release of additional DAMPs, thereby amplifying immune activation. These combined approaches enhance tumor targeting and help to overcome the immunosuppressive TME. Photothermal therapy induces localized hyperthermia, triggering ICD and further enhancing immune responses when combined with ISV [272, 273]. Chemotherapeutic agents such as doxorubicin and oxaliplatin, both well-established ICD inducers, further strengthen anti-tumor effects when integrated with ISV [274, 275]. Similarly, radiotherapy synergizes with ISV by inducing the abscopal effect, in which localized radiation triggers systemic immune responses [276, 277]. Collectively, these multimodal strategies create a more favorable environment for the immune system to recognize and eliminate tumor cells, improving both local and systemic tumor control.

Future perspectives and challenges

The use of nanomaterials in ISV development represents an important advancement in cancer therapy. This article has summarized the mechanisms and recent applications of nanomaterials-based ISV.

Whereas current cancer treatments primarily rely on surgery, radiotherapy, and chemotherapy, immunotherapy—particularly through nanomaterials-based ISV—holds robust potential for future clinical applications. When new therapeutic strategies are introduced in clinical settings, they are often first tested in patients with advanced-stage disease who have exhausted all standard treatment options. In these late-stage patients, the high tumor burden typically results in a more immunosuppressive TME. Enhancement of the TME through ISV may complement existing treatments and provide new therapeutic options for patients with extensive metastases. Additionally, nanomaterials-based ISV could serve as a neoadjuvant therapy for patients with early-stage cancer. Considering the ICD mechanism of ISV and its potential to induce the abscopal effect, it is conceivable that nanomaterials-based ISV, when utilized as neoadjuvant therapy, could eliminate distant micrometastases by modulating the immune response. Some studies have demonstrated that immunotherapy can effectively inhibit tumors within 3–8 days after treatment [278, 279]. Therefore, when pathological results confirm a malignant tumor, there is a sufficient window for ISV prior to surgery. In the early stages of disease, micrometastases are more susceptible to immune system activity, making it feasible to enhance the immune response and reduce the risks of metastasis and recurrence by utilizing ISV as a neoadjuvant therapy. Compared with conventional neoadjuvant therapies, ISV offers systemic immune benefits through localized treatment, providing a safer, faster, and more durable response. Thus, similar to other immunotherapies, nanomaterials-based ISV represents a promising avenue for future clinical development as a neoadjuvant therapy. Furthermore, safety remains a critical consideration in drug development. The modification of nanomaterials can minimize adverse effects, enhance the function of immune adjuvants, and enable the controlled release of therapeutic agents. Therefore, advancements in nanomaterial technologies are needed to optimize the development and application of ISV in clinical settings.

Despite promising preclinical results for nanomaterials-based ISV, several key challenges must be addressed to facilitate successful clinical translation. First, although nanomaterials can reduce drug-related adverse effects, some inorganic nanomaterials are not readily metabolized, potentially leading to long-term complications. Further research is required to minimize risks associated with nanomaterial metabolism. Second, most studies regarding nanomaterials-based ISV have been conducted in experimental animal models; no clinical trials have been performed. The efficacies of these treatments in humans must be validated to support their clinical application. Finally, although nanomaterials-based ISV is frequently combined with other therapeutic modalities—such as immunotherapy, photothermal therapy, radiotherapy, and chemotherapy—the optimal timing, dosage, and frequency of administration require further investigation.