The therapeutic outcome of OVT is shaped by the dynamic interplay between the tumor, immune system, and OVs. This triad determines whether OVs are neutralized, persist long enough to cause oncolysis, or successfully trigger systemic antitumor immunity. Upon infection, OVs induce immunogenic cell death (ICD), releasing tumor-associated antigens (TAAs), damage-associated molecular patterns (DAMPs), and pro-inflammatory cytokines that activate antigen-presenting cells (APCs) and prime cytotoxic T lymphocytes (CTLs), initiating abscopal effects that target metastatic lesions (23).

However, this therapeutic potential is often limited by antiviral immunity, tumor heterogeneity, and the immunosuppressive TME (24). Host defenses including PRR-mediated antiviral responses, type I IFN signaling, and neutralizing antibodies can limit OV replication and accelerate their clearance (25, 26). Simultaneously, tumors evade immune detection through MHC downregulation, checkpoint molecule upregulation (e.g., PD-L1, CTLA-4), impaired antigen presentation, and secretion of suppressive cytokines such as TGF-β and IL-10 (27, 28).

Paradoxically, the same immunosuppressive features can make tumors more permissive to OV infection. Many cancer cells harbor defects in antiviral sensing and apoptotic pathways, which help them evade immune surveillance but also create vulnerabilities that OVs can exploit. Factors such as hypoxia, metabolic stress, and acidity further impair IFN signaling, enhancing OV replication, particularly in “cold” tumors like pancreatic ductal adenocarcinoma (PDAC), where VSV and Coxsackievirus A21 thrive (29, 30).

In contrast, healthy cells detect viral infections via pattern recognition receptors (PRRs) such as TLRs and RLRs (31), triggering interferon-stimulated genes (ISGs) and pro-apoptotic pathways that restrict viral spread. Tumor cells often lose this capacity due to mutations or epigenetic silencing in key components like JAK-STAT, PKR, and OAS-RNase L, enabling selective OV replication (31–33). Additionally, overexpression of viral entry receptors [(e.g., Coxsackievirus and Adenovirus Receptor (CAR), nectin-1, EGFR)] enhances OV tropism (34). For example, glioblastoma and lung tumors with high CAR expression are particularly susceptible to AdV-based OVs, while PDAC with impaired IFN signaling supports VSV and Coxsackievirus A21 infection (34).

Navigating this triad requires balancing viral persistence and immune activation. While innate sensing and IFN responses are critical for systemic antitumor immunity, they can also prematurely eliminate therapeutic viruses. To overcome these barriers, next-generation OVs are engineered to evade or modulate immune detection, resist antiviral pathways, and express immunostimulatory payloads. These strategies convert “cold” tumors into “hot,” inflamed lesions that recruit and activate effector immune cells. Combination approaches further enhance efficacy by transiently suppressing innate antiviral defenses during OV administration and subsequently reactivating immune responses to promote tumor clearance (35–37).

From the earliest stages of malignancy, cancer cells establish an immunosuppressive TME characterized by chronic inflammation, cell exhaustion and weak immunogenicity. While these conditions hinder immune surveillance, they paradoxically support selective OV infection and replication, and localized immune activation (38). As discussed in Section 2.1, the tumor-OV-immune triad shapes therapeutic outcomes. Here, we focus specifically on how the immunosuppressive TME and its inherent heterogeneity modulates to OV permissiveness and influences the outcome of OVT.

Tumors heterogeneity further influences immune engagement, giving rise to distinct immune phenotypes, immune-inflamed, immune-excluded, and immune-desert, each presenting unique challenges and opportunities for OVT (38). The TME contains immunosuppressive stromal cells, such as cancer-associated fibroblasts (CAFs), lymphatic endothelial cells as well as infiltrating immunosuppressive cells, such as MDSCs, regulator T cells (Tregs), and tumor-associated macrophages (TAMs), all of which restrict CTL infiltration and function. CAFs secrete immunosuppressive cytokines (e.g., TGF-β, IL-10) and upregulate checkpoint molecules (e.g., PD-L1 and CTLA-4) ( Figure 1 ), reinforcing immune escape. Although they express MHC-II and can present antigens to CD4⁺ T cells, their dominant role in the TME is suppressive. MHC-II expression is regulated by IFN-γ via Janus kinase/signal transducers and activators of transcription (JAK-STAT) pathway and the class II trans-activator (CIITA) (39). Lymphatic endothelial cells, while facilitating immune cell trafficking, also promote Treg expansion and local immune tolerance. These suppressive features impair IFN signaling and antiviral responses, creating metabolic stress that facilitates OV propagation. Engineered OVs expressing ICIs (e.g., anti-PD-1, anti-CTLA-4) or immunostimulatory cytokines (e.g., GM-CSF) can counteract these signals, reinvigorate T-cell function, and enhance both viral oncolysis and immune-mediated tumor clearance (40, 41).

PD-L1 [also known as cluster of different (CD274) or B7 homology 1 (B7-H1)], expressed on APCs, macrophages, and tumor cells bind PD-1 on T cells, recruiting SHP-1 and SHP-2 phosphatases. This inhibits T cell receptor (TCR) signaling by dephosphorylating CD3 and Zeta-chain-associated protein kinase 70 (ZAP70) and suppress IL-2 secretion (42). This cascade not only blunts T-cell activation but also promotes regulatory T cell (Treg) differentiation and polarization of TAMs toward the immunosuppressive M2 phenotype. These M2-TAMs secrete pro-tumorigenic cytokines such as vascular endothelia growth factor (VEGF), fibroblast growth factor, and TNF-α, supporting angiogenesis and metastasis (43). PD-1 expression on NK cells also suppresses their cytotoxic activity, further aiding immune evasion.

CTLA-4, another key checkpoint, competes with CD28 for B7 binding, limiting T-cell co-stimulation and metabolic fitness. It also impairs APC function and nutrient uptake. LAG-3 exacerbates T-cell dysfunction by disrupting calcium signaling during TCR activation (44–46). In preclinical models, deletion of PD-1 or SHP-2 enhances antigen presentation and T-cell activation, underscoring the therapeutic potential of targeting these pathways to restore antitumor immunity.

In the TME, various cell types, including tumor cells, CAFs, TAMs, endothelial cells, and DCs, overexpress viral entry receptors, which can be exploited for OVT. Tumor cells frequently upregulate receptors such as CAR, heparan sulfate proteoglycans, and EGFR, due to oncogenic mutations or loss of tumor suppressors like p53 and RB. These changes are driven by tumor-intrinsic signaling, inflammatory cues, and epigenetic regulation. Similarly, CAFs express neuropilin-1 (NRP1) and integrins; TAMs may express DC-SIGN and ACE2; endothelial cells lining tumor vasculature express ACE2, NRP1, and integrins; and APCs like DCs express DC-SIGN (CD209) and L-SIGN, collectively enhancing OV tropism and entry (47–51).

Mutations in p53 and RB further shape the TME. p53 loss promotes immune evasion by downregulating antigen presentation, increasing secretion of immunosuppressive cytokines, and recruiting Tregs and MDSCs (52). It also weakens ISG expression in chemotherapy-resistant tumors like GBM, enabling oHSV-1 replication (53). RB inactivation, common in HPV-associated cancers, enhances E2F activity and S-phase entry, creating conditions favorable for replication of S-phase-dependent OVs like AdVs and HSV (53, 54). Overexpression of anti-apoptotic proteins such as Bcl-2 also impairs OV-induced cell death, but this can be countered by engineering OVs to express pro-apoptotic transgenes or inhibitors of survival signaling pathways in tumor cells, thereby enhancing therapeutic oncolysis (55).

Chronic inflammation remodels the TME, promotes recruitment of suppressive population (Tregs, MDSCs, TAMs), and enhances OV infection by weakening antiviral defenses (56–60). Elevated cytokines such as TNF-α and IL-6 increase vascular permeability, facilitating OV delivery, but also inhibit immune cell function and upregulate immune checkpoints, further dampening antitumor immunity (56, 57).

Overall, the immunosuppressive, metabolically dysregulated, and chronically inflamed TME, while a barrier to immune surveillance, offers a strategic opportunity for OVT. By targeting impaired antiviral sensing, overexpressed viral entry receptors, and metabolic vulnerabilities, rationally engineered OVs can achieve enhanced selectivity and therapeutic efficacy. As our understanding of these mechanisms deepens, next-generation OVs are being designed with greater precision, reinforcing their role in the evolving landscape of cancer immunotherapy.

DCs play a central role by capturing tumor antigens released during OV-induced ICD. DAMPs and proinflammatory cytokines promote DC maturation, especially when combined with type I IFNs or innate agonists like STING or TLR ligands. Mature DCs migrate to lymph nodes, present antigens to naïve T cells, and initiate CTL responses limiting viral persistence. Plasmacytoid dendritic cells (pDCs), found in blood, lymphoid organs, and bone marrow, are specialized antiviral sentinels that detect viral RNA and CpG DNA via TLR7 and TLR9 ( Figure 2 ). Upon activation, they secrete large quantities of type I IFNs that suppress OV replication and activate CTLs (61, 62). In addition to IFN production, DCs migrate to lymph nodes, present viral antigens via MHC I and II, and initiate adaptive immune responses by activating CD4⁺ helper T cells, CD8⁺ CTLs, and B cells, leading to antibody production and virus-specific cytotoxicity.

While these responses are essential for controlling natural infections, they can limit OV persistence and therapeutic efficacy, especially after systemic administration. To address this, recent strategies aim to transiently suppress pDC activation or reprogram their function, either to extend the window for OV replication or to harness their antigen-presenting capacity to enhance systemic antitumor immunity (63, 64). Other innate immune cells also restrict OV efficacy. NK cells rapidly recognize virus-infected tumor cells via stress ligands and missing-self signals, releasing cytotoxic granules and IFN-γ to recruit additional immune effectors (65). Macrophages, including Kupffer cells and splenic macrophages, efficiently clear circulating viral particles, while tissue-resident macrophages release antiviral cytokines like TNF-α and type I IFNs. Neutrophils further restrict viral spread through reactive oxygen species and neutrophil extracellular traps (66). The complement system adds another layer of defense. Activation of classical and alternative pathways leads to opsonization and lysis of viral particles, particularly in patients with pre-existing antiviral antibodies, posing a challenge for intravenous OV delivery (67).

Additional innate effectors, such as conventional dendritic cells (cDCs) and natural killer T (NKT) cells, bridge innate and adaptive immunity. cDCs detect viral PAMPs via TLRs and RLRs, produce type I IFNs and pro-inflammatory cytokines, and present antigens to T cells. NKT cells recognize lipid antigens via CD1d and amplify early cytokine responses that restrict viral propagation (68). Central to these defenses is type I IFN signaling. IFN-α/β binding activates the JAK-STAT pathway, inducing antiviral effectors like PKR, OAS-RNase L, and Mx proteins that degrade viral RNA, inhibit protein synthesis, and create a hostile environment for viral replication (69). While these mechanisms protect against opportunistic infections, they also limit the therapeutic window for OVs.

To overcome these barriers, innovative strategies are being developed, including transient immunosuppression, polymeric shielding of viral particles, and the use of carrier cells to deliver OVs to tumors. Balancing these approaches, by exploiting tumor-intrinsic vulnerabilities while fine-tuning host innate responses, offers a promising path to improving the durability and potency of oncolytic virotherapy (63, 64).

Despite the immune system’s capacity to detect and eliminate pathogens, many viruses have evolved sophisticated mechanisms to evade immune surveillance. These natural strategies form the basis for engineering OVs that can persist within tumors, avoid premature immune neutralization, and enhance therapeutic efficacy. RNA viruses like VSV, measles virus, and Newcastle Disease Virus naturally mutate their surface proteins to escape antibody recognition (70). Engineered OVs mimic this adaptability through immune cloaking strategies such as PEGylation, CD47 coating, or envelopment in extracellular vesicles. For example, CD47-coated AdVs, extracellular vesicle-shielded VacV, and measles virus with mutated envelope proteins evade phagocytic clearance and prolong persistence in the TME (71).

Critically, many tumors exhibit defects in innate antiviral defenses, particularly in the 2′-5′-OAS-RNase L and PKR pathways, key components of type I IFN-mediated responses. These sensors detect viral double-stranded RNA (dsRNA) and initiate RNA degradation, translational arrest, or apoptosis to suppress viral replication (72, 73). However, dysfunction in these pathways allows OVs to evade immune detection and selectively replicate in malignant cells (73, 74).

To exploit this IFN-related vulnerabilities, introduced in section 2.1., OVs have evolved mechanisms to inhibit these antiviral sensors. Some encode proteins that directly block RNase L (e.g., HSV-1’s ICP34.5 and SKIV2L), while others produce decoy RNAs that bind OAS, preventing the synthesis of 2′-5′-linked oligoadenylates required for RNase L activation. Structural modifications in viral RNA can also reduce recognition by OAS, and some OVs minimize dsRNA production to avoid triggering both OAS-RNase L and PKR pathways. For instance, VacV uses E3 and K3 proteins to inhibit OAS and PKR, AdVs deploy VA RNA to block PKR activation, and Newcastle Disease Virus expresses a V protein that prevents PKR recognition of viral RNA (75–78). These adaptations enhance OV replication, tumor lysis, and immune activation, particularly in immune-cold tumors where antiviral defenses are already suppressed (79, 80). Tumors with impaired IFN signaling and MHC downregulation are particularly permissive to viruses like HSV and cytomegalovirus, which naturally evade immune detection. This principle has guided the development of engineered HSV strains (e.g., G207, G47Δ) with deletions in ICP34.5, enabling selective replication in IFN-deficient tumor cells while sparing healthy tissue (79, 81). Some OVs are further engineered to transiently suppress antigen presentation, delaying immune clearance and enhancing intratumoral replication (81).

OVs represent a promising strategy to enhance T cell function and metabolism, particularly in tumors that are resistant to conventional therapies. By reshaping the metabolic landscape of T cells, OVs contribute to sustained antitumor immune responses and amplify the efficacy of existing immunotherapies (111).

A key mechanism involves metabolic reprogramming of tumor-infiltrating T cells. OVs interact with glycolytic enzymes in infected cells, increasing energy availability to support T cell expansion and function (112). Additionally, OV-induced ICD and release of tumor antigens and DAMPs activate and metabolically reprogram T cells to sustain a potent antitumor immune response (113). Some OVs are engineered to express metabolic regulators, such as leptin, to further enhance T cell fitness and tumor eradication (21).

OVs also improves T cell infiltration and activation within the TME. By disrupting immunosuppressive barriers, they enhance the trafficking and function of CAR T cells and endogenous T cells (114). When combined with immune checkpoint inhibitors (ICIs) or adoptive T cell therapies, OVs create synergistic effects that overcome the limitations of monotherapies and promote durable antitumor responses (102). Engineered OVs frequently express cytokines such as IL-2, IL-12, and TNF-α, which are essential for T cell proliferation, survival, and metabolic activation (115). IL-2 promotes clonal expansion, IL-12 enhances cytotoxicity, and TNF-α increases tumor vasculature permeability, facilitating deeper immune infiltration. These cytokines also stimulate glycolysis and oxidative phosphorylation, supporting sustained CTL activity.

In parallel, OVs enhance antigen presentation by upregulating MHC molecules and recruiting DCs, which prime naïve T cells and reinforce metabolic reprogramming. This coordinated activation equips T cells with the energy and signaling support needed for long-term tumor control (41, 116). This effect is further strengthened by DC recruitment and activation, leading to increased T cell priming and metabolic reprogramming, equipping T cells with the necessary energy reserves to sustain a long-term antitumor response.

To further amplify T cell responses, OVs can be engineered to express costimulatory molecules such as OX40L, CD40L, CD137, ICAM-1, and GITR (117–120). For example, VALO-D102, an AdV encoding CD40L and OX40L, induces robust CD8⁺ T cell infiltration and tumor control in melanoma models, with enhanced efficacy when combined with anti-PD-1 therapy (119). Similarly, LOAd703, encoding CD40L and CD137, activates cytotoxic T cells and upregulates immunostimulatory molecules (CD80, CD86, CD70), MHC, and adhesion molecules like ICAM-1, improving tumor immunogenicity and therapeutic outcomes in multiple myeloma (120).

By integrating oncolysis, metabolic support, antigen presentation, costimulatory signaling, and checkpoint inhibition, OVs offer a powerful strategy for enhancing T cell-based cancer immunotherapy. These synergistic mechanisms collectively drive more effective and durable antitumor responses, offering renewed hope for patients with treatment-refractory malignancies.

TGF-β is a key regulator of immune homeostasis, but in cancer, it fosters an immunosuppressive TME by inhibiting T cell activation, proliferation, and cytotoxicity. This allows tumors to evade immune surveillance. Blocking TGF-β signaling within tumors is a promising strategy to restore T cell function and enhance cancer immunotherapy. OVs engineered to deliver TGF-β inhibitors offer a localized and potent approach to achieve this, converting immune-excluded tumors into highly inflamed tumors that are more responsive to immunotherapy (121).

Genetically modified OVs can express TGF-β inhibitors, including small molecules, decoy receptors, or neutralizing antibodies, ensuring targeted blockade of TGF-β signaling within the TME while minimizing systemic toxicity. TGF-β signaling suppresses CTLs and enhances the function of Tregs and MDSCs, all of which contribute to immune evasion (122). By disrupting this pathway, OVs enhance T cell metabolism, cytokine production, and effector function, while reducing immunosuppressive cell populations and promoting sustained antitumor immunity (123).

Several OV platforms incorporating TGF-β blockade are under investigation. Seneca Valley virus (SVV-001) induces tumor lysis and reprograms the immune microenvironment to enhance T cell infiltration. AdAPT-001, an oncolytic adenovirus (oAdV) expressing a TGF-β trap, has shown promising results in a Phase 1 trial, particularly when combined with ICIs, yielding durable responses in refractory tumors such as sarcomas and triple-negative breast cancer (124, 125). Jurona virus (JURV), in combination with anti-PD-1 therapy, improved survival and immune activation in hepatocellular carcinoma models, with favorable tolerability (14).

VacV engineered to express TGF-β receptor (TGF-βR) inhibitors represents another potent approach. By delivering these inhibitors directly into tumors, VacV suppresses TGF-β-mediated immune evasion, enhances T cell recruitment and cytotoxicity, and significantly improves ICI efficacy in preclinical models (126, 127).

Although early findings are encouraging, further research is needed to optimize delivery, specificity, and immune modulation. Ongoing clinical trials will be critical in determining the safety and efficacy of TGF-β-blocking OVs. Their ability to reshape the TME, synergize with ICIs (e.g., anti-PD-1 and anti-CTLA-4 therapies), and directly enhance T cell function positions them as powerful tools in the next generation of cancer immunotherapy (40, 128). While checkpoint blockade alleviates T cell exhaustion, OVs enhance T cell metabolic fitness and functional capacity, overcoming multiple layers of tumor-mediated immune suppression. This combination strategy has demonstrated enhanced tumor regression and prolonged survival in preclinical models, underscoring its potential for clinical translation (124, 125). In a Phase 1 clinical trial, AdAPT-001 exhibited encouraging responses, particularly when combined with ICIs, yielding a high objective response rate and durable clinical benefit in patients with refractory tumors, such as sarcomas and triple-negative breast cancer (124, 125). Jurona virus (JURV), a novel OV, in combination with anti-PD-1 antibody, enhanced immune response and improved survival in HCC models (14). The therapy demonstrated favorable tolerability, with most adverse events being mild and transient, further supporting its clinical potential.

To overcome challenges of OVT as monotherapy, such as tumor heterogeneity, immune evasion, and the immunosuppressive TME as discussed in section 2, combination strategies have emerged as a powerful means to potentiate OVT. The integration of OVs with ICIs, chemotherapy, radiotherapy, or targeted agents, has amplified antitumor responses, improved viral delivery, and sustained immune activation, thus enhancing therapeutic outcomes of OVT across diverse cancer types. Combination of Tumor Treating Fields (TTFields) and radiation therapy with OVT offers an innovative therapeutic synergy to improve tumor targeting, viral spread, and immune engagement.

The combination of OVs with drugs, nutraceuticals, and their synthetic analogs, has emerged as a promising strategy in cancer therapy. Over the past decade, these small molecules have been explored for their ability to modulate key signaling pathways, enhance immune responses, and reshape the immunosuppressive TME (191). Their potential to augment OVT has opened new avenues for combination therapies, offering enhanced therapeutic efficacy and improved patient outcomes. They work synergistically with OVs by complementing and enhancing OV-mediated tumor destruction (192). They can modulate intracellular signaling, increase viral replication, improve tumor selectivity, and promote ICD, thereby maximizing the antitumor effects of OVs. Immune-modulating agents can also counteract the immunosuppressive TME, making it more susceptible to OV infection and immune activation. Additionally, these agents may sensitize tumor cells to OV-induced apoptosis or inhibit pathways that allow cancer cells to evade viral oncolysis.

The structure of anticancer drugs directly contributes to their capacity to enhance OVT by targeting complementary pathways in cancer cells and the TME ( Table 1 ). HDAC inhibitors such as valproic acid and trichostatin A use their zinc-chelating functional groups to disrupt chromatin condensation and restore expression of viral entry receptors and immunostimulatory genes, sensitizing tumors to viral infection and immune-mediated clearance (193, 194). Multi-kinase inhibitors like sunitinib, whose indolinone scaffold allows ATP-site binding across receptor tyrosine kinases, suppress angiogenesis and impair antiviral signaling pathways such as IFN responses, thereby promoting viral replication within tumors (182, 183, 195). Lenalidomide’s phthalimide core enables targeted degradation of transcription factors critical for tumor survival and immune evasion, indirectly boosting antiviral immunity and modulating the TME to favor OVT (196–199). Bortezomib’s boronic acid moiety blocks proteasomal degradation of pro-apoptotic proteins, increasing virus-induced cell death. Doxorubicin, through its planar anthracycline core intercalation and free radical generation, compromises DNA integrity and heightens susceptibility to viral oncolysis (200). Everolimus leverages its macrocyclic lactone structure to inhibit mTORC1, a pathway critical both for tumor growth and for type I interferon production, thereby enhancing viral replication and spread (201–203). Platinum-based drugs like cisplatin and carboplatin, with their reactive platinum coordinated to reactive ligands that form intra- and interstrand DNA crosslinks ultimately triggering apoptosis and synergizing with viral lytic activity (204–206). Taxanes and camptothecin derivatives such as paclitaxel, docetaxel, and irinotecan, possess complex diterpenoid cores that stabilize microtubules and prevent their depolymerization, arresting mitosis or topoisomerase function, sensitizing dividing cancer cells to virus-induced apoptosis. Antimetabolites like 5-fluorouracil and gemcitabine, by depleting nucleotide pools and incorporating into nucleic acids, create replication stress that increases tumor permissiveness to viral infection (207, 208). Collectively, the unique structural features of these agents not only define their primary anticancer mechanisms but also strategically prime tumors to respond more robustly to oncolytic viruses, offering a rational basis for combination therapies that exploit vulnerabilities in cancer cell survival, antiviral defense, and immune regulation.

Recent studies have identified several successful OV-small molecule combinations, highlighting both therapeutic potential and challenges (209). As cancer immunotherapy evolves, the strategic development of these small molecule as next-generation immunotherapeutic may redefine cancer treatment paradigms (210). The synergy between OVs and pharmacological small molecules can be categorized into distinct mechanisms that enhance tumor targeting and immune activation ( Figure 3 ). However, further research and clinical trials are essential to optimize dosing regimens, identify predictive biomarkers, and address potential resistance mechanisms, ensuring this combinatorial strategy achieves its full therapeutic potential in oncology.

As the field of OVT advances toward precision medicine, there is a growing need for systematic frameworks that can stratify tumors based on their molecular characteristics. Recent studies have highlighted the importance of genomic and epigenomic alterations in shaping tumor susceptibility to OV infection, replication, and immune activation. This section introduces a molecular classification system that categorizes tumors according to their genetic, epigenetic, and immunologic profiles, providing a foundation for tailoring OVT strategies to individual tumor vulnerabilities. By aligning therapeutic design with tumor-intrinsic features, this approach aims to enhance treatment efficacy and expand the clinical applicability of OVT.

Tumors with specific genetic alterations, such as loss-of-function mutations in tumor suppressor genes like p53 or RB1, exhibit impaired antiviral responses, rendering them highly permissive to OV replication. For instance, GBM, frequently harboring these mutations, represent ideal candidates for OVs engineered to exploit such vulnerabilities, particularly when paired with synthetic lethal agents that further sensitize tumor cells to oncolysis (116, 237, 238). Tumors characterized by chromosomal instability, such as ovarian cancer, often display defective DNA repair mechanisms. These tumors are highly responsive to OVs armed with interferon-stimulatory transgenes, which trigger innate immune activation. The therapeutic efficacy of these OVs is further amplified when combined with ICIs, promoting durable antitumor immunity. This genetic-based classification system offers a structured framework for designing personalized OVT, optimizing treatment by targeting the unique immune, metabolic, and genetic traits of tumors. Ultimately this approach aims to improve patient outcomes and drive greater clinical success.

In contrast, epigenetically driven tumors, such as hepatocellular carcinoma, exhibit widespread DNA hypermethylation and histone modifications that repress immune-related gene expression. In such tumors, OV responsiveness can be enhanced through epigenetic reprogramming using hypomethylating agents or histone deacetylase inhibitors, which restore immune surveillance pathways. Moreover, CRISPR-based genome editing strategies offer promising avenues to augment OV replication and restore tumor suppressor activity, further improving therapeutic outcomes.

This molecularly guided classification system allows for precision-matched OVT, tailored to the genetic, metabolic, and immunologic profiles of individual tumors. By integrating OVs with co-therapies that address the specific vulnerabilities of each tumor subtype, this strategy holds the potential to revolutionize personalized cancer immunotherapy and significantly improve patient prognosis.

Tumors exhibit distinct immune profiles that influence OV efficacy. Immune-hot tumors, such as melanoma and non-small cell lung cancer, feature high T-cell infiltration and an inflamed TME, responding well to OVs, especially when combined with ICI and immune-stimulatory cytokines (204, 239). Immune-cold tumors, like pancreatic adenocarcinoma, have low T-cell infiltration and abundant immunosuppressive cells (240). OVs targeting these tumors should be engineered to reprogram the TME into an immune-permissive state, with combination therapies to modulate immune cell recruitment and activity (240). Immune-excluded tumors, such as colorectal cancer, present additional challenge as T cells is confined to the stroma, limiting tumor infiltration (33). OVs for these tumors should incorporate matrix-degrading enzymes to break down physical barriers, and VEGF blockade may further enhance penetration and therapeutic effectiveness (241).

Tumor metabolism significantly impacts OV replication and therapeutic efficacy. Glycolytic tumors, like TNBC, rely on aerobic glycolysis, creating an acidic, hypoxic environment (124, 125). OVs targeting these tumors should include pH-adaptive envelope proteins to survive in acidic conditions, with metabolic inhibitors to disrupt glycolytic pathways (207, 208). Oxidative tumors, such as ovarian cancer, primarily depend on oxidative phosphorylation and exhibit high ROS levels (187–190). OVs can exploit this oxidative stress to enhance replication, potentially combined with NADPH oxidase inhibitors to increase tumor susceptibility. Metabolically plastic tumors, like colorectal cancer, can switch between glycolysis and oxidative phosphorylation, complicating treatment. OVs targeting these tumors should disrupt metabolic flexibility, potentially combined with dual metabolic inhibitors to restrict the tumor’s ability to adapt to metabolic stress.

To overcome the dense ECMs, hypovascularity, and high interstitial fluid pressure that restrict intratumoral spread of OVs integration of tumor-penetrating peptides, enzymatic ECM degradation (e.g., hyaluronidase co-expression), and stromal remodeling agents to improve viral distribution within solid tumors, become pertinent (13). Nanoparticle- and hydrogel-based delivery platforms may also enhance OV retention and diffusion at tumor sites. immune cloaking techniques using polymer coatings or ex vivo OV-loaded immune cells, such as monocytes or DCs, to stealthily deliver the virus to tumors could be used to overcome pre-existing antiviral immunity in the host. Moreover, tumor-tropic mesenchymal stem cells (MSCs) have shown promise in shielding OVs from immune detection and improving delivery to immune-privileged sites like the brain. Sites such as the brain, eyes, and testes present natural obstacles to immune access. Nanoparticle-based delivery (for example encapsulating viruses in nanoliposomes), modular capsid engineering, and BBB-penetrating vectors can enhance OV access to these areas. Additionally, ultrasound-mediated delivery and convection-enhanced delivery are promising strategies to physically drive OVs into solid masses or across tight barriers (242, 243). To personalize OVT, robust biomarkers are needed to predict treatment response. Promising candidates include PD-L1 expression, tumor mutational burden, and deficiencies in antiviral pathways (e.g., STING, JAK) that facilitate OV replication. The development of imaging-based viral tracking, liquid biopsies, and RNA signatures may further guide patient selection and treatment monitoring.

Live vaccines represent a groundbreaking approach in cancer immunovirotherapy, repurposing viral vaccines as powerful tools to combat cancer. Originally developed to prevent infectious diseases and some cancers, live vaccines are now being explored for their ability to activate both immune responses, directly targeting and eliminating cancer cells (243). This approach leverages the unique immunogenic properties of attenuated or engineered viruses, stimulating broad immune responses capable of targeting tumor cells throughout the body.

Oncolytic virus live vaccines are also used preventively to protect high-risk individuals from cancers driven by oncogenic pathogens. Unlike therapeutic live viruses, which are administered after malignancy develops, to convert tumors into in situ vaccines, preventive vaccines prime the immune system to recognize and eradicate cancer-causing infections before tumorigenesis occurs. They deliver viral antigens or virus-like particles that elicit neutralizing antibodies and durable memory responses. The HPV vaccine, which prevents cervical, anal, and oropharyngeal cancers by targeting high-risk HPV strains, and the hepatitis B virus (HBV) vaccine, which lowers hepatocellular carcinoma risk by preventing chronic HBV infection, are among the most successful examples. These interventions have demonstrated high efficacy in reducing infection rates and associated cancer incidence, underscoring their pivotal role in global cancer prevention strategies (244, 245).

However, the concept of prophylactic administration of OVs to prevent non-virus-induced cancers remains clinically unexploited. Also, in the absence of tumor tissue, OVs lack a target substrate for infection and lysis, precluding the release of TAAs necessary to prime an effective immune response. Furthermore, non-virus-driven malignancies do not share uniform pre-existing antigens that could be targeted in advance. Administering OVs to healthy individuals therefore poses safety risks without conferring the needed immunologic benefit. Consequently, OVs such as T-VEC and VV are reserved for therapeutic use in established cancers, where they convert tumors into in situ vaccines and synergize with other immunotherapies to drive robust antitumor immunity (244, 245).

Our group has investigated the potential of repurposing the live attenuated trivalent measles, mumps, and rubella (MMR) vaccine as a cost-effective cancer immunotherapy (33). In murine models of HCC and colorectal cancer, intratumoral MMR vaccine injections resulted in significant tumor growth inhibition and improved survival. The vaccine activated a robust CTL response and reduced TAMs, enhancing the antitumor immune response and offering a promising option for cancer immunotherapy. Additionally, the recombinant HSV-1 vaccine vector has shown promise for personalized cancer vaccines (151). A recombinant engineered oHSV-1 called VC2-OVA, expressing a fragment of ovalbumin fused with the VP26 viral capsid protein, demonstrated specific antitumor immunity in a murine melanoma model, significantly reducing tumor cell colonization in the lungs and extending survival (228). The effects of HSV-1 vaccine strain VC2 in a mouse model of stage four breast cancer was investigated, and VC2 treatment resulted in significant intratumor infiltration of functionally active T cells, inhibited tumor metastasis and reduced the expression of pro-tumor genes VEGF and PD-L1 (246). Similarly, intramuscular injection of mice with live attenuated VC2 vaccine strain protected against ocular challenges, which was associated with enhanced intracorneal infiltration of γδ T cells, which helped control ocular immunopathogenesis (246). These live vaccines mimic viral infections, stimulating immune responses that target tumor cells directly and generating systemic antitumor effects. They activate CTL, NK cells, and other immune effectors, which infiltrate the TME, promote tumor cell destruction and can address distant metastases. Notably, live vaccines can generate long-lasting immune memory, providing protection against tumor recurrence (233).

One of the key advantages of repurposing live vaccines for cancer is their ability to be readily available for cancer treatment without the need for extensive production or toxicology testing (247). Certain live vaccines, including those based on VSV, reoviruses, and engineered measles viruses, exhibit natural oncolytic activity. These viruses selectively infect and replicate within tumor cells, exploiting the unique characteristics of cancer, such as defective in antiviral responses and altered cellular machinery. This replication process causes tumor cell death, releasing viral particles that spread to adjacent cancer cells, amplifying the therapeutic effect.

In addition to their oncolytic properties, live vaccines also enhance immune responses against tumors that are typically resistant to immune attack. The viral infection itself acts as a “danger signal” that alerts the immune system to the presence of tumor cells, converting “cold” tumors (poorly infiltrated by immune cells) into “hot” tumors that are more receptive to immune-based therapies, including ICIs (248).

Several live vaccines are currently undergoing clinical trials for cancer treatment. The modified VacV used in the oncolytic vaccine Pexa-Vec has shown promise in treating melanoma and colorectal cancer. Similarly, engineered oncolytic measles viruses have demonstrated efficacy in tumor regression across multiple cancers, including GBM and ovarian cancer. The reovirus, with its selective oncolytic properties, has also shown potential when combined with chemotherapy and ICIs.

Despite these promising results, challenges remain, particularly concerning safety. Although engineered to minimize risks, live viral vectors may still pose potential threats, particularly for immunocompromised patients. However, OVs are often modified to attenuate their effects or selectively target cancer cells, reducing these risks. Rigorous monitoring during clinical application is crucial to detect and manage potential side effects, such as viral shedding or immune-related reactions (247).

One of the significant advantages of live vaccines is their ability to enhance other cancer therapies. When combined with ICIs, live vaccines have shown synergistic effects by boosting T-cell activity and improving immune cell infiltration, making tumors more susceptible to checkpoint blockade. Additionally, live vaccines can be combined with targeted therapies or chemotherapy, offering a multifaceted approach to overcoming tumor heterogeneity and resistance mechanisms.

Looking forward, live vaccines are poised to play an increasingly critical role in cancer treatment. As research deepens our understanding of immune responses and the TME, and as new viral vectors and delivery systems, e.g. are developed, live vaccine-based therapies will likely become a cornerstone of cancer immunotherapy. The potential of these vaccines to induce lasting immune responses, treat a broad range of malignancies, and prevent cancer recurrence offers new hope for patients facing challenging cancers. Ongoing research and clinical trials will continue to refine their applications, making live vaccine-based therapies a promising avenue for future cancer treatments.

Nutraceuticals, bioactive food-derived compounds, are emerging as promising adjuncts to OVT, particularly through their ability to reprogram immunosuppressive cells within the TME. By modulating immune pathways, these compounds can reduce populations of Tregs and MDSCs, shift TAMs from a pro-tumor M2 phenotype to an antitumor M1 phenotype, and enhance the infiltration and activity of CTLs and NK cells, all of which are critical for effective OVT. Curcumin, exhibit potent immunomodulatory and anti-inflammatory properties. It enhances both innate and adaptive immunity by activating CTLs and NK cells, inhibiting immunosuppressive cytokines and enzymes via NF-κB suppression, reprogramming M2-like TAMs into M1-like antitumor phenotypes, and reducing Treg and MDSC activity.

By modulating multiple biological pathways, nutraceuticals can improve immune responses, modify the TME, and increase cancer cells’ susceptibility to viral infection, thus improving the potency of OVT. Combining nutraceuticals with OVs holds great promise in optimizing treatment outcomes, overcoming resistance mechanisms, and improving patient survival (40). A primary mechanism through which nutraceuticals support OVs is by boosting the immune responses, as OVs rely heavily on the host’s immune system to effectively target and eliminate tumor cells.

Several nutraceuticals, including curcumin from turmeric, possess immunomodulatory properties that can activate immune cells such as CTLs and NK cells, key players in immune-mediated tumor destruction. Curcumin has been shown to enhance both innate and adaptive immunity by promoting immune cell activation, reducing immunosuppressive signaling or cytokines and enzymes by inhibiting NF-κB binding, and creating a more favorable environment for viral replication within the TME (256). Additionally, curcumin possesses anti-inflammatory and immunomodulatory qualities (249). Moreover, curcumin’s potential to enhance chemotherapy and ICIs suggests that it could increase the effectiveness of OVT. Curcumin has been shown to influence the proliferation and function of different immune cells, such as CTL and NK cells, potentially enhancing the immune system’s response to the tumor (249). Yet, curcumin might also suppress certain antiviral mechanisms that typically restrict OV replication. It can reduce the expression of ISGs, a critical component of the body’s antiviral defense (250). Furthermore, curcumin can trigger apoptosis in cancer cells, rendering them more vulnerable to OV-induced infection and destruction (251). Curcumin can inhibit angiogenesis within tumors, potentially starving them of nutrients and enhancing their vulnerability to OVT. Preclinical studies have shown that curcumin enhances cancer therapy (18), however, further clinical trials are needed to validate that curcumin could be a beneficial supplement to OVT, partly addressing some of the limitations of single use of OVs.

Other nutraceuticals, like resveratrol (from grapes and berries), epigallocatechin-3-gallate, a polyphenol in green tea), have both demonstrated the ability to boost NK cell activity and promote immune responses, thereby enhancing viral-induced tumor regression (18, 252). These compounds can improve OVT efficacy by modulating immune checkpoint molecules and promoting a more favorable immune environment within the TME.

In addition to modulating the immune system, nutraceuticals can help overcome TME barriers that limit OVT efficacy. The TME is often characterized by hypoxia, dense extracellular matrices, and immunosuppressive factors, all of which hinder viral replication and spread. Nutraceuticals, such as sulforaphane, found in cruciferous vegetables, can downregulate factors like VEGF and MMPs. However, upregulation of VEGF increases angiogenesis which facilitate the entry of oncolytic viruses like HSV-1, in cancer cells (253). Antioxidants such as vitamins C and E, known for reducing oxidative stress, help optimize the TME, promoting viral replication and immune function. These antioxidants also activate immune cells, fostering a more favorable environment and enhancing the overall efficacy of OVTs. Clinical trials exploring the use of vitamins C and E as adjuncts to cancer therapy have shown promising results, particularly in melanoma and head and neck cancers, indicating improved viral replication and immune response (254).

Nutraceuticals can also sensitize cancer cells to OVs by modulating cellular pathways critical for viral entry and replication. For example, quercetin, a flavonoid abundantly found in apples and onions, enhances oAdV therapy by influencing autophagy and apoptosis, which regulate cancer cells’ responses to viral infection (255). While autophagy facilitates viral particle degradation, apoptosis promotes viral-induced cell death. In liver cancer models, quercetin improved OVT efficacy, resulting in more effective tumor cell lysis (114). Similarly, berberine, an alkaloid derived from various herbs, has been shown to exhibit direct antiviral properties, complementing OV activity by enhancing viral replication and tumor cell killing (256). In combination with HSV-1, berberine has been shown to enhance viral replication and tumor cell killing in pancreatic and liver cancer models by modulating cellular stress responses and promoting viral entry (256).

Certain nutraceuticals also possess inherent antiviral properties that can boost OVT efficacy. For example, garlic extract, rich in sulfur compounds such as allicin, has demonstrated antiviral activity that supports viral replication and spread within tumors (257). In combination with oncolytic VSV, garlic extract could improve viral particle production and oncolytic activity in lung cancer models. By stimulating innate immune responses and inhibiting viral clearance, garlic extract can complement OV therapies. Omega-3 fatty acids from fish oil, known for their anti-inflammatory properties (257), can modify the TME to promote OV activity by suppressing inflammatory pathways like NF-κB and promoting resolution molecules such as resolvins and protectins, which reduce inflammation and boost OV efficacy (258).

Despite the promise of combining nutraceuticals with OVs, challenges remain in optimizing this strategy. The timing, dosage, and selection of nutraceuticals must be tailored to maximize their synergistic effects with OVs. Also, not all nutraceuticals are suitable for every cancer type or OVT. Moreover, patient-specific factors such as genetic background, tumor type, and disease stage must be considered to ensure the best therapeutic outcomes.

Integrating nutraceuticals into OVT represents a novel approach to overcoming barriers in cancer treatment ( Figure 4 ). By enhancing the immune responses, modifying the TME, and boosting viral replication and sensitivity, nutraceuticals and other small molecular scaffolds inspired from the latter can significantly amplify OVs efficacy. Ongoing preclinical and clinical studies continue to explore these combinations, with promising early results suggesting that nutraceuticals may overcome some limitations of monotherapy. Combining nutraceuticals with OVs is not without challenges. The timing, dosage, and selection of nutraceuticals must be tailored to maximize their synergistic effects with OVs. Also, not all nutraceuticals are suitable for every cancer type or OVT. Moreover, patient-specific factors such as genetic background, tumor type, and disease stage must be considered to ensure the best therapeutic outcomes. As research advances, this integrated approach has the potential to transform cancer therapy, providing a more robust and personalized treatment strategy for patients with hard-to-treat cancers.