Enhancing efficacy of oncolytic myxoma virus and adoptive T cell therapy against tumors

Immunotherapy in general and adoptive cell transfer in particular have recently been in the spotlight of cancer research. Recent FDA approval of the first immunotherapeutic drugs and promising results in clinical trials suggest that this therapeutic modality holds great promise in fighting cancers. Another attractive strategy is oncolytic viral therapy, the use of cancer-specific viruses to target and kill cancer cells. Combining immunotherapies and viral therapies has shown synergistic effect in the past.;Most of our work focused on myxoma virus, a rabbit poxvirus, which can efficiently infect various types of mouse and human cancer cells. It is a strict rabbit-specific pathogen, and is thought to be safe as a therapeutic agent in all non-rabbit hosts tested including mice and humans.;Chapter 1 introduces concepts of immune system and cancer interaction, gives an overview of current immunotherapy strategies, describes the current state of oncolytic viral therapy, and ends with discussion on myxoma virus as a gene delivery agent for cancer therapy.;In chapter 2, we describe engineering a new recombinant myxoma virus (vMyx-IL15Ralpha-tdTr), which expresses an IL15Ralpha-IL15 fusion protein as well as tdTomato red fluorescent reporter protein, and the antitumor effect of the virus. Interleukin-15 (IL15) is an immunomodulatory cytokine with significant potential for stimulating anti-tumor T lymphocytes and NK cells. Co-expression of IL15 with the alpha subunit of IL15 receptor (IL15Ralpha) greatly enhances IL15 stability and bioavailability. Our findings demonstrated successful expression of the introduced proteins in cells infected by the modified virus in culture. In the animal tumor models, the novel IL15Ralpha-IL15 fusion protein expressing virus showed increased antitumor activity compared to non-modified virus and other controls in both RAG1-/- mice that lack B and T cells and in fully immunocompetent C57BL/6 mice. We showed by immunostaining that both natural killer (NK) cells and T cells likely play a role in the increased antitumor effect of vMyx-IL15Ralpha-tdTr.;In chapter 3, potential combination therapy of the oncolytic myxoma virus and adoptive T cell therapy was further evaluated. Combination therapy of previously characterized vMyx-IL15Ralpha-tdTr and naive or tumor-antigen specific activated T cells conveyed some antitumor protection, but the effect of combination treatment was not statistically significant. Increasing the levels of tumor-associated antigens in the context of dying cancer cells sensitizes supporting cells in the tumor to reverse immunosuppression and prime antitumor adaptive immune response. We hypothesized that myxoma virus with its specificity for cancer cells could be a delivery vehicle for increasing the tumor-associated antigens at the tumor site. In order to test that hypothesis, we conducted initial steps in generating a new recombinant myxoma virus, vMyx-SIY-tdTr, for exploring possibilities of combination therapy with the 2C T cell -- SIY tumor antigen model system.;In chapter 4, we further evaluated the nature of the side effect of the adoptive T cell therapy in our preclinical model. It was noticed earlier that some tumor bearing mice treated with genetically engineered CD4+ T cells developed graft-versus-host disease (GVHD) symptoms. Occurrence of these side effects is a clinically relevant phenomenon, and understanding their basis is important for improving the safety of adoptive T cell therapies. We investigated the incidence and cellular mechanism of these symptoms in experimental mice.;Overall, our work showed the feasibility of using an oncolytic/oncotropic virus as a delivery agent to additionally boost innate and adaptive immune system against cancer. Novel viral engineering strategies and therapy combinations may improve the viral as well as the immune antitumor effect. Our work has contributed to understanding the efficacy and safety of clinically relevant therapeutic approaches.