Treatment options for tumors have expanded significantly in recent years, driven by an enhanced understanding of the immunologic mechanisms underlying these diseases (1, 2). These options now include traditional surgical treatments and chemoradiotherapy, as well as innovative approaches like immune checkpoint inhibitors, chimeric antigen receptor T-cells, adoptive cell immunotherapy, monoclonal antibodies, nanoantibodies, and other forms of immunotherapy (3–7). While surgical resection remains the most effective treatment for cancer and can substantially relieve symptoms, it is often not viable for patients in advanced stages of the disease. Moreover, surgery often leads to distant metastasis and local recurrence post-operatively (8). Chemoradiotherapy can decelerate cancer growth and prolong survival, however, it poses severe risks due to its damaging effects on normal cells while targeting cancer cells (9, 10). Immunotherapies, despite their potential, benefit only a select group of patients due to various immunosuppressive factors such as immune system suppression, deficiencies in cytokine types, reduced activity of tumor-infiltrating lymphocytes, impaired function of antigen-presenting cells, and weakened effector T-cell activity (11–13). Oncolytic viruses have emerged as one of the most promising treatments to address these challenges (14). These viruses are capable of selectively replicating within tumor cells, delivering multiple eukaryotic transgene payloads, inducing immunogenic cell death, enhancing antitumor immunity, and exhibiting a safety profile that generally does not overlap with other cancer treatments (15, 16). Recent clinical trials of oncolytic virus therapy (OVT) have shown minimal severe adverse reactions, with only local injection site reactions and low-grade systemic symptoms noted (17–20). In 2005, Chinese regulators approved the first genetically modified oncolytic adenovirus for the treatment of nasopharyngeal carcinoma in combination with chemotherapy, marking the world’s first oncolytic virus approved for clinical cancer treatment (21).

Oncolytic viruses, either genetically modified or naturally occurring, have the capability to activate immune cells that specifically target and destroy cancer cells while sparing healthy cells (22). These viruses selectively infect tumor cells harboring mutations associated with malignancies (23), and often have modifications in or deletions of certain viral genes essential for replication in normal cells but not in tumor cells (24). This selective infection is facilitated by the viruses’ ability to bind specifically to certain receptors on tumor cells, enhanced by altering the virus’s tropism (25). The viral genes are controlled by tumor-specific and/or tissue-specific gene promoters, enhancing the safety by confining viral replication to tumor cells (26, 27). Tumor cells frequently exhibit dysfunctional immune response signaling pathways, which support the virus’s replication and proliferation within them (28, 29). Oncolytic viruses inhibit the production of host cellular products while enhancing viral product synthesis within the tumor cells. When oncolytic viruses lyse tumor cells, they release large quantities of tumor antigens that are then processed by antigen-presenting cells, triggering T-cells to target both infected and uninfected tumor cells (30, 31). Upon infecting tumor cells, the viruses can either directly affect immune cells or stimulate the tumor cells to produce more cytokines, thereby enhancing the immune system to eliminate any remaining tumor cells by phagocytosing them (32). Oncolytic viruses induce various cell death pathways in cancer cells, including necroptosis, pyroptosis, apoptosis, and autophagy ( Figure 1 ) (33–36). For example, the N-terminal gasdermin domain (GSDMNT), when delivered into tumor cells via a recombinant adeno-associated virus, induces pyroptosis, offering significant therapeutic potential (34). Similarly, the oncolytic vaccinia virus, when armed with the aphrocallistes vastus lectin gene, can alter the metabolism of hepatocellular carcinoma cells, increase reactive oxygen species (ROS) production, and promote apoptosis, all contributing to its antitumor effects (37). OVT works by killing cancer cells using viruses that have a selective replication function (1, 38). There are variations among oncolytic viruses in terms of how they interact with immune cells and the tumor microenvironment (TME) during treatment (39, 40). In this paper, we provide a detailed overview of the immune cells, TME, and oncolytic viruses, and discuss the variations in TME and immune responses observed with current OVTs.

For nearly a century, researchers have been studying viruses. The first viruses identified were the foot-and-mouth disease virus from animals and the contagium vivium fluidum from plants, both discovered in 1898 (41, 42). Additionally, the yellow fever virus was first identified in 1901 and associated with human disease (43, 44). Advances in technology and an enhanced understanding of virus morphology and virulence have enabled ex vivo virus replication. This has facilitated the linking of several diseases, including rabies and influenza, to specific viruses (45, 46). Subsequent research has provided a detailed understanding of viral species, including their structure and biological characteristics (47). Simultaneously, researchers discovered that the virus may be utilized to cure tumors in addition to causing infectious diseases (48, 49).

Studies have documented cases of tumor regression following viral infection (49, 50). However, these remissions were typically short-lived, generally lasting only one to two months. In 1922, Levaditi and colleagues observed that the vaccinia virus could inhibit various cancers in mice and rats (51). In 1950, Pack reported remissions in patients with metastatic melanoma who were treated with the rabies virus. Additionally, patients with hematological malignancies such as leukemia or lymphoma experienced remission following infections with chickenpox or influenza (52, 53). Concurrent regressions of leukemia, Hodgkin’s disease, and Burkitt’s lymphoma have also been noted during measles infections, suggesting that under the right circumstances, certain viruses can target malignancies effectively without endangering the patient (54, 55). For instance, in a study of juvenile diffuse intrinsic pontine glioma treated with the oncolytic DNX-2401 virus, magnetic resonance imaging showed a reduction in tumor volume in 75% of the cases, and 66.7% of the patients achieved stable disease (56). Furthermore, a study involving 19 patients with recurrent glioblastoma treated with the oncolytic virus DNX-2401 demonstrated its safety and feasibility; notably, one patient achieved complete regression and was still alive eight years later (57).

Various viruses have been evaluated for their oncolytic properties in human tumor cell lines prior to their progressive utilization in clinical trials and other therapeutic contexts (58). In models of KRAS-mutated colorectal cancer, such as HCT116 colon cancer cells and patient-derived peripheral blood mononuclear cells, the oncolytic reovirus (pelareorep) exploits host autophagic machinery to enhance its proliferation and achieve selective oncolysis (59). In a phase 1 dose-escalation study, patients with newly diagnosed high-grade glioma who were treated with neural stem cell-delivered engineered oncolytic adenovirus showed promising safety and efficacy (60). Originally, hepatitis viruses were used to treat hematological malignancies and leukemia symptoms remitted in patients infected with adenovirus or EB virus (61). Subsequently, Alice Moore solidified the existence of an oncolytic virus in 1950 through in vivo tumor models and clinical and preclinical research (62). Oncolytic viruses, defined as naturally occurring or genetically modified viruses, selectively replicate in tumor cells, destroying them without harming healthy tissues and can also stimulate systemic or localized anti-tumor immunity (14, 63). These viruses target specific cell surface receptors overly expressed by cancer cells and naturally prefer cell surface proteins, enabling them to bind these receptors and penetrate the cells (64, 65). Oncolytic viruses are divided into two main categories: RNA and DNA viruses, including single-stranded (ssRNA and ssDNA) and double-stranded (dsRNA and dsDNA) viruses ( Figure 2 ) (66).The most common RNA viruses used are measles and coxsackie virus group B, while commonly used DNA viruses include adenovirus, herpes simplex virus 1, and vaccinia virus. Except for vaccinia, DNA viruses have longer replication cycles than RNA viruses and replicate in the nuclei of infected cells (67, 68). Oncolytic viruses can also be classified into genetically engineered and naturally occurring strains. Engineered strains exhibit decreased pathogenicity, enhanced tumor expression, and increased lethality compared to wild-type strains (69). Mutations in the presence or lack of certain viral genes that are necessary for the virus to multiply in healthy cells but not in tumor cells (24). Genetically engineered oncolytic viruses are also designed to improve targeting selectivity for tumor cells and host cells with cancer-related mutations (70). Oncolytic viruses are intended to identify tumor-upregulated receptors, allowing the virus for an improved fidelity (71).To increase safety and limit viral replication to tumor cells, their principal genes are cloned into tumor-specific or tissue-specific promoters (26, 27). Besides their direct oncolytic activity of preferentially replicating within and destroying cancer cells, oncolytic viruses are highly effective in triggering immune responses against both the tumor cells and themselves. They are capable of inducing both local and systemic anti-tumor immunity, utilizing immune evasion strategies employed by cancer cells (72).

Immunosuppression in the tumor microenvironment, exacerbated by immune checkpoint inhibitors like PD-1/PD-L, diminishes effective neoantigen presentation and hinders anti-cancer T cell responses (141). An engineered oncolytic virus designed to express PD-L1 inhibitors can initiate tumor neoantigen-specific T cell responses. This approach has shown promise in enhancing endogenous T cell reactions against tumor neoantigens in patients with advanced cancers, leading to durable outcomes, including complete responses in various cancer types such as melanoma, and metastatic lung, kidney, and bladder cancers (142, 143).

Oncolytic viruses, capable of inducing anti-tumor immunity both locally and systemically, are either injected directly into the tumor or administered systemically. Once these viruses infect tumor cells, they multiply, often triggering immunogenic cell death, and spread throughout the tumor, stimulating an inflammatory response that can be modulated by macrophages and NK cells of the innate immune system (144). Armed oncolytic viruses also express immunomodulatory transgenes, enhancing immune reactions against the tumor. The immunogenic cell death and inflammation attract DCs to the tumor site, where they initiate a comprehensive immune attack on the tumor by activating T and B cells (145).

Oncolytic viruses can counteract immune evasion strategies utilized by cancer cells. These strategies often involve immuno-inhibitory receptors on tumor cells and in the surrounding microenvironment, which deactivate immune effector cells and promote the secretion of IL-10 and TGF-β. These factors can attract immunosuppressive cells, such as myeloid-derived suppressor cells and tumor-associated macrophages, to the tumor site (146, 147). Oncolytic viruses disrupt this suppressive environment through various mechanisms that alter the cytokine landscape and the composition of immune cells within the TME (148).

Furthermore, several oncolytic viruses possess the capability to express therapeutic genes or alter the function of tumor-associated endothelial cells, enhancing the recruitment of T cells into TMEs that are otherwise immune-deserted or immune-excluded.

There are several challenges with the oncolytic viral therapy approach. Research on oncolytic viruses is still in its infancy. Many oncolytic viruses are currently being explored both theoretically and experimentally, and their potential therapeutic value remains uncertain. Additionally, there are no specific diagnostic criteria to identify patients who might benefit from oncolytic viral therapies.

In conclusion, the function of oncolytic viruses in tumor therapy and the tumor microenvironment is gradually becoming apparent. As either naturally occurring or genetically engineered agents, oncolytic viruses achieve their therapeutic effects by influencing immune cells, tumor cells, and the tumor microenvironment. This review has discussed the history and characteristics of the tumor microenvironment, immune cells, and oncolytic viruses, as well as the potential therapeutic efficacy of oncolytic viruses. These viruses activate various pathways to induce tumor cell death. A significant challenge in cancer treatment is that many oncolytic virus therapies are still in the preclinical trial stage. While a few oncolytic viruses have been approved for clinical use, transitioning these therapies into clinical applications remains a substantial hurdle. Consequently, understanding how oncolytic viruses modify the tumor microenvironment in different tumors will lead to the enhancement and development of oncolytic virus treatments for cancer. Moving forward, continued research into oncolytic viruses is essential.