Date of Award

January 2020

Document Type

Thesis

Degree Name

Medical Doctor (MD)

Department

Medicine

First Advisor

Marcus W. Bosenberg

Abstract

Checkpoint inhibitors such as anti-PD1 and anti-CTLA4 have revolutionized treatment of metastatic melanoma. However, a large subset of patients receiving such treatments fail to respond due to mechanisms such as PD-L1 upregulation within the tumor and T cell exhaustion in the tumor microenvironment, which can be provoked by many mechanisms, one of which is upregulation of prostaglandin E2 (PGE2). Thus, new combination immunotherapeutic regimens that target such cancer-promoting pathways are greatly desired to improve response rates to checkpoint inhibition. Furthermore, many of patients on checkpoint inhibitors experience autoimmune toxicities, also called immune-related adverse events (irAEs), that can be fatal or necessitate drug discontinuation. Although irAEs are typically treated with short courses of high- dose steroids when they occur, concerns are emerging regarding whether the extensive immunosuppressive effect of steroids ultimately interferes with efficacy of checkpoint inhibition. Additionally, many irAEs are refractory to steroids, and there is currently minimal data regarding intelligent selection of next-line immunosuppressives. In this work, we used the YUMMER1.7 (Yale University Mouse Melanoma Exposed to Radiation) model to explore both of these questions: we evaluated the efficacy of PGE2 inhibition via ibuprofen on enhancing the antitumor response to anti-PD1 therapy, and also characterized the effect of steroids and IL-6 inhibition on the antitumor response provoked by checkpoint inhibition when given for irAEs.

In order to explore these two questions, Kaplan-Meier survival curves from mice implanted with subcutaneous YUMMER1.7 tumors, flow cytometry, immunohistochemistry, cytokine analysis, single cell RNA sequencing analysis, and additional in vitro studies were performed. We found that PGE2 inhibition is a powerful way of restoring function of exhausted T cells when given concurrently with anti-PD1, likely through repression of tumor-upregulated PD-L1, which normally promotes T-cell exhaustion. This thereby resulted in enhanced T cell cytotoxic function and translated into markedly increased survival in mice receiving the ibuprofen/anti-PD1 combination regimen compared to mice receiving anti-PD1 treatment alone. With regards to our question about the effect of immunosuppressive agents for irAEs on the efficacy of checkpoint inhibition, we found that steroids reduced efficacy of checkpoint inhibitor therapy in some models but not others, especially when given prior to tumor implantation, likely through inhibition of differentiation of naïve T cells into cytotoxic effector T cells. We also demonstrated that targeted blockade of IL-6, which has been implicated in the pathogenesis of several autoimmune disorders as well as multiple irAEs, was similarly effective to steroids at reducing checkpoint inhibitor-induced inflammation without compromising the antitumor response. Taken together, this work has demonstrated 1) promising preclinical rationale for commonly-used PGE2 inhibitors such as ibuprofen and celecoxib to advance to clinical studies in conjunction with aPD1 in patients with melanoma, and 2) that steroids do have the potential to inhibit efficacy of checkpoint inhibitors, and that more selective targeting of immunostimulating pathways responsible for irAEs, such as IL-6, may similarly ameliorate irAEs without compromising the antitumor response.

Comments

This thesis is restricted to Yale network users only. This thesis is permanently embargoed from public release.

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