252 Novel DNAM-1 axis member, PVRIG, is potentially a dominant checkpoint involved in stem-like memory T cells – dendritic cell interaction

Background T-cell accumulation in tumors is a prerequisite for response to cancer immunotherapy. Recent studies highlighted the importance of an early-memory (stem-like) T-cell sub-population, that can self-renew and differentiate into effector cells, and of dendritic cells (DCs), which are essential for T-cell priming and expansion following checkpoint blockade.1 2 PVRIG is a novel inhibitory receptor that competes with the co-activating receptor DNAM-1, for the binding of a shared ligand, PVRL2. PVRIG expression is induced on T and NK tumor infiltrating cells, whereas PVRL2 is expressed on tumor, endothelial and myeloid cells in the tumor micro-environment (TME).3 We investigated the expression of PVRIG and PVRL2 across TME immune subpopulations. Methods Publicly available TME scRNA sequencing datasets were analyzed for the expression of PVRIG and PVRL2 across immune subsets. Unsupervised principal-component-analysis and hierarchical co-expression pattern among genes known to be expressed on naive, memory, and exhausted CD8+ T-cells was performed. Observations were validated by flow-cytometry and immunohistochemistry analysis across a variety of tumor indications. Proximity Extension Assay (PEA, Olink) was conducted using serums collected at several time-points from COM701 (anti-PVRIG antibody) and nivolumab treated patients in a Phase-1 study (NCT03667716). Results Across scRNA datasets, PVRIG, like TIGIT and PD-1, was expressed by both stem-like (TCF1+PD1+) and exhausted (TIM3+CD39+) CD8+ T-cells. High resolution unsupervised scRNA gene co-expression analysis revealed that while TIGIT is strongly correlated with PD-1, CTLA-4, and other markers of exhausted T-cells, PVRIG uniquely clusters with markers of early memory T-cells (figure 1). Accordingly, PVRIG protein expression was increased on CD28+ early-memory T-cells across indications (figure 2).RNA expression data also revealed that PVRL2 is more abundantly expressed across DC-subtypes compared to PD-L1 and PVR (ligand of TIGIT, figure 3). Flow cytometry confirmed dominant PVRL2 expression on DC subtypes across tumor indications. Immunohistochemistry analysis identified PVRL2 expression in Tertiary Lymphoid Structures (figure 4). Finally, preliminary analysis of serum from COM701+nivolumab treated patients revealed elevated induction of activated-DC markers in two patients that responded clinically (RECIST criteria), compared to non-responders (figure 5). Conclusions PVRIG is co-expressed with PD-1 and TIGIT on stem-like and exhausted T cells but has a unique dominant expression on early memory cells, while PVRL2 is abundantly expressed across DC-types. PVRIG blockade could therefore enhance memory T-cells activation by DCs, resulting in their increased expansion and differentiation. Accordingly, early data shows increased induction of activated DC markers, potentially following efficient T-DC interaction, in serum of two patients responding to COM701+nivolumab. References Jansen CS, Prokhnevska N, Master VA, et al. An intra-tumoral niche maintains and differentiates stem-like CD8 T cells. Nature 2019;576:465–470. Held W, et al. Intratumoral CD8+ T cells with stem cell-like properties: implications for cancer immunotherapy. Sci Transl Med 2019;11(515):eaay6863 Whelan S, Ophir E, Kotturi MF, Levy O, Ganguly S, Leung L, et al. PVRIG and PVRL2 are induced in cancer and inhibit CD8+ T-cell function. Cancer Immunol Res 2019;7:257–68. Ethics Approval Clinical trial identification NCT03667716The study was approved by each site’s ethics board. Consent Clinical trial identification NCT03667716Written informed consent was obtained from the patient for publication of this abstract and any accompanying images. A copy of the written consent is available for review by the Editor of this journal.