Using gammadelta T cells as a potent adoptive cell therapy platform in models of pediatric sarcoma

New treatment approaches for high-risk pediatric sarcomas are desperately needed. Immunotherapy advances with adoptive cell therapy in hematologic malignancies have not yet been translated to pediatric solid tumors including sarcomas. Hallmarks of the immune microenvironment in pediatric sarcomas include the presence of immunosuppressive immune cells and cytokines, and a lack of potent T cell activation. We are seeking ways to overcome these barriers to effective adoptive cell therapy in pediatric sarcomas. gammadelta T cells are known to have innate anti-tumor activity across many cancers but their activity against pediatric sarcomas remains poorly characterized. Here we detail the anti-tumor activity of gammadelta T cells against a panel of human and murine models of pediatric sarcomas including human and murine osteosarcoma (OS), human and murine rhabdomyosarcoma (RMS), and human Ewings sarcoma (EWS). Human and murine gammadelta T cells demonstrated varied anti-tumor activity across the different models of pediatric sarcomas. Co-cultures of varying effector to target ratios (E:T) of gammadelta T cells to tumor demonstrated strong anti-tumor effects against our models of OS and EWS. Interestingly, RMS, especially the human fusion-positive RMS cell line Rh30 showed significant resistance to gammadelta T cell killing. We further assessed the gammadelta T cell potency against pediatric sarcomas in vivo. In our syngeneic and human tumor xenograft models of sarcoma we observed temporary tumor growth control following adoptive transfer with ten million activated gammadelta T cells highlighting the need to improve in vivo function. We explored the addition of intratumoral STING (stimulator of interferon genes) agonist with the goal of increasing gammadelta T cell trafficking to the tumor. We show that STING agonist delivery prior to adoptive transfer results in increased numbers of gammadelta T cells in the tumor. In addition, we are exploring the introduction of a novel synthetic co-receptor, CD8a:MyD88, shown previously to augment alphabeta T cell effector function, to determine if gammadelta T cell function can be improved as well. We transduced gammadelta T cells ex vivo to express the CD8a:MyD88 synthetic co-receptor resulting in improved in vitro tumor lytic function and increased interferon gamma production when co-cultured with osteosarcoma cells compared to control vector-transduced T cells. Ongoing and future work is investigating the in vivo effectiveness of the CD8a:MyD88 expressing gammadelta T cells in our panel of sarcoma models. These studies further define the obstacles and potential solutions to successful adoptive T cell therapy for pediatric sarcomas.