Abstract 4905: Effects of MAPK pathway inhibitors in the tumor immune microenvironment
Authors: Belvin Marcia P. , Williams Erin , Luoh Shiuh-Ming , Cheung Jeanne , Orr Christine , Chan Emily , Ebert Peter , Mellman Ira , Kim Jeong , Merchant Mark 
Published: July 2016

Abstract
Immunotherapeutic agents have shown great promise in the clinic in recent years and this has led to their approval as single agents or as immune doublet combinations in melanoma and lung cancer. In order to increase the extent of benefit from these agents and to extend immunotherapies to additional patients, combinations are being evaluated of immunotherapeutic agents with chemotherapy and targeted agents.
Inhibitors of the mitogen-activated kinase protein kinase (MAPK) pathway, including BRAF and MEK inhibitors, have been approved in melanoma, and are being evaluated in additional indications. We evaluated the effects of MEK and ERK inhibitors on human T cells cultured in vitro as well as on the tumor microenvironment in preclinical syngeneic mouse models. In studies with human T cells in vitro, we find that MEK or ERK inhibition reduced proliferation of naïve T cells, but had little effect on the proliferation of central memory cells, suggesting a differential requirement on MAPK signaling for proliferation of these T cell subsets. In vivo, MEK and ERK inhibition resulted in an increase in CD8+ infiltration in the tumor, as well as a decrease of markers of T cell exhaustion such as PD-1 and EOMES. The combination of MEK or ERK inhibitors plus anti-PD-L1 resulted in increased CD8+ effector function as measured by increased interferon gamma (IFNg) levels. In some cases, the MEK and ERK inhibitors showed differences in their pharmacodynamic effects as single agents; however, in combination with anti-PD-L1, the effects were similar. For example, single agent MEK inhibition but not single agent ERK inhibition increased the number of CD4+ helper and regulatory T cells (Tregs) in the tumor. However, both of these CD4+ subsets, as well as CD4+ IFNg levels, were increased with MEK or ERK inhibitors combined with anti-PD-L1. Similarly, single agent MEK inhibition decreased the number of infiltrating CD11b+Ly6G+ myeloid cells whereas single agent ERK inhibition increased the number of infiltrating CD11b+Ly6C+ myeloid cells. However, in combination with anti-PD-L1, these changes in myeloid populations were less apparent.
Taken together, these data show that MAPK pathway suppression can modulate multiple immune cell subtypes within the tumor, with many of these changes expected to activate the immune infiltrate. Despite the increase in Tregs in response to ERK and MEK inhibition in combination with anti-PD-L1, these combinations were efficacious. This suggests that the other immune stimulatory effects of these treatments are sufficient to drive efficacy despite the presence of increased Tregs. Our data suggest that the combination of MAPK inhibition plus anti-PD-L1 inhibition, such as with atezolizumab, is a promising hypothesis to test in the clinic.
Citation Format: Marcia P. Belvin, Erin Williams, Shiuh-Ming Luoh, Jeanne Cheung, Christine Orr, Emily Chan, Peter Ebert, Ira Mellman, Jeong Kim, Mark Merchant. Effects of MAPK pathway inhibitors in the tumor immune microenvironment. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4905.