Abstract 4720: TIGIT blockade enhances cytolytic function in antigen-specific CTLs in a manner non-redundant to PD1 blockade
Authors: Malhotra Deepali , Moody Gordon , Overstreet Michael , Mumm John 
Published: August 2018

Abstract
Background Many mechanisms contribute to the dysfunction of CTLs in the tumor microenvironment, rendering these cells unable to kill. TIGIT is a co-inhibitory receptor on T cells that is notably expressed at high levels by antigen-specific PD1-expressing CTLs in tumors. The ligands for TIGIT, CD155 and CD112, are in turn expressed by tumor cells and tumor-infiltrating immune stroma. TIGIT-mediated suppression of T cell function results from T cell intrinsic competition with the co-stimulatory molecule CD226 for engagement of CD155 and CD112, or enhancement of cell intrinsic repressive function of TIGIT downstream of the T cell receptor (TCR). To dissect the release of TIGIT-mediated suppression of polyclonal antigen-specific CTLs, we 1) examined the expression of TIGIT, CD226, and their shared ligands CD112 and CD155 in primary human tumor disaggregates and 2) studied the functional consequences of TIGIT blockade in an assay in which human polyclonal antigen-specific CD8+ CTLs were expanded and co-cultured with human tumor cell lines that endogenously express their cognate antigen.
Methods Primary human tumor disaggregates were examined for expression of TIGIT, CD226, CD155 and CD112 by flow cytometry. To further examine the repressive function of TIGIT, CTLs from donor human PBMCs were expanded in the presence of a specific major histocompatibility class I (MHC I) peptide antigen and subsequently screened for enrichment of activation and exhaustion markers to confirm expansion. CTLs were then co-cultured with CD155+ and CD112+ tumor cell lines endogenously expressing the MHC I-binding peptide of interest, in the presence and absence of TIGIT or PD1 blocking antibodies. To further investigate the mechanism of cytolysis, inhibitors of TCR signaling, cytoskeletal rearrangement, and cytokine receptor signaling were investigated.
Summary of Results TIGIT blockade induced a rapid, potent, and reproducible cytolytic response in co-culture experiments. The maximal effect was similar to that observed with PD1 inhibition, and moreover the combination of PD1 and TIGIT blockade was greater than either monotherapy, implying non-redundancy of mechanism. Inhibitors of cytoskeletal function and signal transduction downstream of the TCR appeared to block TIGIT mediated cytotoxicity.
Conclusion TIGIT has a clearly defined role in the repression of cytotoxic function in antigen-specific CTLs, in a manner non-redundant with PD1. The enhancement in cytotoxicity subsequent to TIGIT blockade is dependent on cytoskeletal function as well as additional signal transduction downstream of TCR engagement. Strategies to enhance these signal transduction events may be of further interest to potentiate TIGIT blocking therapy.
Citation Format: Deepali Malhotra, Gordon Moody, Michael Overstreet, John Mumm. TIGIT blockade enhances cytolytic function in antigen-specific CTLs in a manner non-redundant to PD1 blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4720.