Novel strategies to enhance the therapeutic efficacy of chimeric antigen receptor T cell therapy against solid cancer

© 2018 Dr. Sherly MardianaAdoptive cell therapy using chimeric antigen receptor (CAR) T cells has shown great success in haematological malignancies, leading to recent approval of two CD19-specific CAR T cell products by the Food and Drug Administration (FDA) for the treatment of B cell Acute Lymphoblastic Leukaemia (ALL) and non-Hodgkin lymphoma. However, despite this high success in CD19+ malignancies, clinical trials for solid cancers have yielded only limited efficacy to date. This discrepancy is thought to be due to, in part, the immunosuppressive tumour microenvironment and antigen heterogeneity in solid tumours. Therefore strategies to overcome these challenges are necessary for CAR T cells to be effective in solid malignancies. In this thesis, we have investigated two approaches aimed at augmenting CAR T cell therapy in a solid cancer setting. The first approach involved the activation of a costimulatory pathway known as CD137 (4-1BB). Targeting this pathway using an agonist α-4- 1BB antibody has been reported to induce robust co-stimulatory signals for diversifying and enhancing T cell responses, which led us to hypothesise that combining CAR T cells and α-4- 1BB antibody would result in improved therapeutic responses. Herein, we utilised a human- Her2 self-antigen mouse model to assess the therapeutic efficacy of Her2-specific CAR T cells against Her2-expressing solid tumours. We report that the use of an α-4-1BB antibody significantly increased CAR T cell anti-tumour responses against two established Her2+ solid tumours. Our analysis revealed an increase in expression of IFNγ and the proliferation marker Ki67 in intratumoural CAR T cells following treatment with α-4-1BB antibody. Strikingly, we also demonstrated that treatment with an α-4-1BB antibody greatly decreased host immunosuppressive cells at the tumour site, including regulatory T cells and myeloidderived suppressor cells, which correlated with an increased therapeutic response. Interestingly, further investigation in immunocompromised RAG-/- mice showed reduced synergistic effects following the combination therapy, suggesting a contribution of endogenous T cells in mediating these effects. Furthermore, we observed an increased frequency of dendritic cells (DCs) and the endogenous T cells exhibiting an effector CD44hi CD62Llow phenotype in tumour-draining lymph nodes following α-4-1BB antibody stimulation, further supporting the potential involvement of the endogenous immune cells. Taken together, these data indicate an important contribution of the endogenous immune system in enhancing overall CAR T cell efficacy, and thus formed the basis for our second approach that aimed to improve CAR T cell therapy by harnessing the endogenous DC population. Herein, we genetically modified Her2-specific CAR T cells to constitutively secrete a cytokine Flt3L, that is known to be critical for the differentiation of an important subset of DCs, the cross-presenting Batf3-dependent XCR1+ IRF8+ CD103+ DCs. We hypothesised that inducing the differentiation of these CD103+ DCs would increase the overall therapeutic efficacy of CAR T cells. We successfully generated Flt3L-secreting CAR T cells, and demonstrated that CAR T cell-derived Flt3L was able to induce expansion of CD103+ DCs and enhance their T cell stimulatory capacity. Adoptive transfer of Flt3L-secreting CAR T cells led to a significant enhancement of tumour growth inhibition, and subsequently enhanced mouse survival compared to the control CAR T cells. Similarly, using a TCR-based adoptive cellular therapy approach, namely the transfer of OT-I T cells against an OVAexpressing tumour, led to superior anti-tumour responses, particularly with the addition of poly I:C adjuvant that resulted in the most significant suppression of tumour growth. In summary, results presented herein demonstrate the promising potential of combining CAR T cells with immunomodulatory agents for enhancing the efficacy of CAR T cells against solid tumours. Our results indicate that in addition to directly augmenting CAR T cell activity, immune-modulating agents are able to enhance the therapeutic efficacy of CAR T cells by modulating the endogenous immune response. Our studies have established a strong rationale for combining CAR T cells and immunomodulatory agents in the treatment of solid cancer, and therefore warrant clinical investigation of these combination therapies