1.2. HSV Vectors

    Herpes simplex virus 1 and 2 (HSV-1 and HSV-2), also known as human herpesvirus 1 and 2 (HHV-1 and HHV-2), are two members of the herpesvirus family, Herpesviridae, that infect humans. Both HSV-1 (which produces most cold sores) and HSV-2 (which produces most genital herpes) are ubiquitous and contagious. They can be spread when an infected person is producing and shedding the virus.

The use of  HSV cancer vectors for prophylaxis against viral infections and tumors requires genetically stable mutants, incapable of replicating in CNS and of spreading in immunocompromised individuals, unable to reactivate, not transmissible from immunized vaccinated individuals to contacts; however, the recombinant viral vectors have to retain their immunogenicity in order to elicit protective immunity against diseases in their hosts. A large number of defective viruses have been proposed for immunization on the ground that although they cannot produce infectious progeny and fundamentally make proteins only in the set of initially infected cells, the presentation of viral antigens and/or transgene, synthesized ex novo in these cells, to the host’s immune system is able to elicit long-lived and potent cellular and humoral immune responses.

Herpes simplex virus currently used for biological therapy is mainly derived from herpes simplex virus type 1 (HSV-1). The HSV-1 genome is 248 CD long and consists of 8 interconnected long-segment (L) and short-segment (S) linear double-stranded DNA molecules. At least 89 genes have been identified, of which about half are non-essential gene. Because of its advantages of establishing latent infections in neuronal cells, insertion of exogenous fragments, high infection efficiency, wide range of host cells, infection of resting and non-resting cells, its application has become a treatment for nervous system diseases.

 Herpes simplex virus for biotherapy can be divided into three categories: category 3 is the HSV amplicon vector, only the origin of replication of HSV and the packaging signal sequence are inserted into the plasmid, when transfected into packaging cells and HSV. After the super-infection of the helper virus, the pseudo-virus containing the amplicon can be obtained; the second type is the recombinant replication-deficient herpes simplex virus, which eliminates the essential genes and non-essential genes associated with replication to reduce cytotoxicity. It is mostly used for long-term expression of exogenous therapeutic genes in host neuronal cells; the third type is reproducible herpes simplex virus, which eliminates non-essential genes and retains replication-related genes, because it has the characteristics of lysing cells, mainly as Oncolytic viruses to selectively kill tumor cells.

1.2.1 HSV carries therapeutic genes

Tumor gene mutations have inspired people to start gene therapy research. Gene therapy is to repair, enhance or inhibit specific gene expression by introducing exogenous genes. Exogenous genes include suicide genes, cytokines, chemokines, and immunity, which stimulate molecules or tumor suppressor genes. Viruses are effective exogenous gene vectors. Gene-carrying viruses have stronger anti-tumor capabilities. HSV-1 can carry macromolecular therapeutic genes, such as immunomodulatory molecules: IL-12, IL-24, IL-4, CD80, IL-18 and IFN-γ. These exogenous genes inhibit tumor growth by stimulating local inflammatory response or host immune response. This gene therapy method deserves further study for clinical application.

Interleukin-12 (IL-12) has a strong anti-cancer effect. IL-12 has a stimulating effect on CTLs, helper T lymphocytes and natural killer cells. IL-12 is an important regulator of immune response, involved in the recruitment and activation of natural killer cells and T cells. In addition to immune regulation, IL-12 can stimulate the secretion of interferon-γby helper T lymphocytes to inhibit angiogenesis. Carrying IL-12 herpes simplex virus inhibits angiogenesis and significantly inhibits tumors. In squamous cell carcinoma in animal models of liver metastasis, NV1024 expressing IL-12 showed stronger killing ability than NV1023 which did not express IL-12 and NV1034 which expressed GM-CSF. Immunohistochemistry showed that infiltration of T cells, macrophages, CD4+ cells and CD8+ cells were present in tumor after NV1024 treatment. Lymphocyte infiltration helps the virus to kill tumor cells. However, immune cell infiltration also causes the virus to be cleared and damage to normal tissues. 

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is secreted by activated lymphocytes and macrophages and can cause a variety of immune-activating effects, including recruitment of antigen-presenting cells, such as macrophages and dendritic cells, and promoting them, differentiation and activation. GM-CSF is involved in the initiation phase of the immune response. Oncovex GM-CSF is called oncolytic vaccine or oncolytic immunotherapy. The virus exhibits anti-tumor effects in various cell lines and animal experiments. GM-CSF may Cytotoxicity was inhibited by activation of T cells. The removal of T cells inhibited the oncolytic effect of NV1034, and NV1034, NV1023 oncolytic was not statistically significant. High doses of GM-CSF were reported to be toxic and reduce host immunity.

Interleukin-24 (IL-24) and interleukin-4 (IL-4) interferon (IFN) selectively induce tumor cell apoptosis by inhibiting tumor angiogenesis, regulating mature T helper cells, or altering the tumor microenvironment. Exogenous genes can produce anti-tumor effects. However, oncolytic viruses can only carry a specific exogenous gene, so the anti-tumor effect is limited, and the expression of exogenous genes is difficult to control.

…to be continued in part three.

Reference

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