Abstract 610: The expression of the PVRIG/TIGIT pathway is dominant in the bone marrow of patients with multiple myeloma
Authors: Frenkel Masha , Alteber Zoya , Xu Ning , Li Mingjie , Chen Haiming , Cojocaru Gady , Berenson James , Ophir Eran 
Published: June 2022

Abstract
Blocking inhibitory immune receptors has shown limited clinical benefit in patients with multiple myeloma (MM), warranting the discovery of alternative immune inhibitory pathways. Blockade of the immune checkpoint TIGIT was shown to enhance anti-tumor immunity in MM pre-clinical models. In addition to TIGIT, the DNAM1 axis includes the novel inhibitory receptor PVRIG. Both TIGIT and PVRIG deliver inhibitory signals to T and NK cells and compete with the co-activating receptor DNAM1 for binding to PVR and PVRL2, respectively. Accordingly, TIGIT and PVRIG co-blockade was shown to synergize in enhancing anti-tumor immunity in preclinical models. PVRL2 and PVR were shown to be expressed on MM plasma cells in the bone marrow (BM). In this study, we evaluated DNAM1 axis receptor expression in the BM of patients with MM. BM mononuclear cells derived from 21 patients with MM were analyzed for the expression of PD1 and DNAM1 axis molecules by flow cytometry. Patients classified with progressed disease (PD), or complete response (CR) were treated with multiple lines of therapies including targeted therapies, chemotherapies, proteasome inhibitors. PVRIG demonstrated the highest expression among all evaluated receptors on NK (88%), NKT (81%) and CD8+ T cells (79%), significantly higher than PD1 (p&lt;0.0001) and TIGIT (P&lt;0.001). TIGIT showed substantially increased expression on NK (64%, P&lt;0.0001) and CD8+ T cells (58%, p&lt;0.05) compared to PD1 (expressed on 12% of NK cells and 42% of CD8+ T cells). Importantly, 50% and 60% of CD8+ and NK cells, respectively, co-expressed TIGIT and PVRIG, and all examined cell populations showed increased levels of DNAM1 (&gt;50%). Additionally, the expression of PVRIG ligand, PVRL2, on BM plasma cells from 6 patients with MM was demonstrated by immuno-histochemistry and flow cytometry. A fraction of CD8+ T cells were DNAM1- and positive for PVRIG/TIGIT/PD1 (22%), suggesting the accumulation of an exhausted CD8+ T cell population in the MM tumor microenvironment. PVRIG showed significantly higher (p&lt;0.0001) expression on DNAM+ CD8+ T cells (81%), compared to TIGIT (43%) and PD1 (34%), supporting the potential of PVRIG blockade to enhance DNAM1 signaling and subsequent CD8+ T cell activation. Finally, patients with CR had a trend towards higher DNAM1 expression on CD8+ T cells (75%) compared to PD patients (54%, p=0.057). To conclude, DNAM1 axis receptors are dominantly expressed on MM BM lymphocytes, with PVRIG exhibiting the most prominent expression. The reduced expression of DNAM1 in patients with PD suggests a link between DNAM1 axis and clinical outcome. Our findings highlight for the first time the dominant expression of PVRIG, as well as TIGIT and suggest that combined blockade of these checkpoints may potentially benefit patients with MM, placing the DNAM1 axis as a promising therapeutic pathway in MM therapy.
Citation Format: Masha Frenkel, Zoya Alteber, Ning Xu, Mingjie Li, Haiming Chen, Gady Cojocaru, James Berenson, Eran Ophir. The expression of the PVRIG/TIGIT pathway is dominant in the bone marrow of patients with multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 610.