Blocking inhibitory immune checkpoints holds promise to treat multiple myeloma (MM) patients. However, currently available checkpoint inhibitors have not shown significant clinical benefits for MM patients, warranting the need for alternative checkpoint blockers. The immune checkpoint TIGIT was recently shown to be the most upregulated immune inhibitory receptor on CD8+ T cells in MM patients' bone marrow (BM), compared to other checkpoints (Guillerey C., Blood. 2018). Preclinical models demonstrated the dominant effects of TIGIT blockade or depletion, by significantly improving mice survival, reducing myeloma cell numbers and exhausted T cell hallmarks (Minnie S., Blood. 2018). As a result, several clinical trials using anti-TIGIT monoclonal antibodies have been recently initiated in MM patients.
The DNAM-1 family, in addition to TIGIT, also includes the inhibitory receptor PVRIG, that competes with the co-activating receptor DNAM-1 for the binding to the shared ligand PVRL2, similarly to the TIGIT/PVR/DNAM-1 interaction. Accordingly, TIGIT and PVRIG co-blockade were shown to synergize in enhancing T cell activity and anti-tumor activity in preclinical models (Whelan S., Cancer Immunol. Res. 2019). PVRL2 together with PVR (ligand of TIGIT) were shown to be highly expressed on plasma cells and on CD14+ cells in BM of MM patients (Lozano E., Clin. Cancer Res. 2020). This study aimed at evaluating DNAM-1 axis receptors expression in MM patients' BM.
Fresh BM aspirates were collected from 21 MM patients with progressive disease (PD) or in complete response (CR) after obtaining IRB approval. BM mononuclear cells were isolated and single cell suspensions were obtained followed by staining with anti-human antibodies to evaluate DNAM-1 axis members and PD-1 expression. BM biopsies from 6 MM patients (each patient had 4 core on the Tissue Micro-Array T291 USBiomax) were stained for PVRL2 expression by immuno-histochemistry (IHC).
Flow cytometry analysis of PD-1 and DNAM-1 axis receptors revealed a significant lower fraction of PD1+ cells among cell populations examined compared with other receptors. TIGIT expression was the highest on NK, CD8+ and NKT cells compared to CD4+ T cells, which is in line with previous published data (Lozano E. Clin. Cancer Res. 2020). In contrast, DNAM-1 was expressed on CD8+ T, NK and NKT cells with prominent high expression on CD4+ T cells (Fig 1A). The highest expression among the receptors was of PVRIG on all lymphoid populations, except CD4+ where DNAM-1 was more highly expressed. Importantly, 50% of CD8+ T cells co-expressed TIGIT and PVRIG, supporting a combinatorial therapeutic approach (Fig. 1B).
Additionally, the expression of the PVRL2 ligand on MM plasma and endothelial cells was demonstrated by IHC. FACS analysis further supported PVRL2 expression on plasma cells in MM BM (Fig 2).
A higher expression of PVRIG, TIGIT and PD-1 was present on DNAM-1 negative CD8+ T cells (Fig 3A, B), suggesting accumulation of exhausted cells in MM tumor microenvironment (TME) as previously described (Minnie S., Blood. 2018). PVRIG had significantly higher expression on DNAM+ cells, compared to PD-1 and TIGIT (Fig 3C), suggesting the potential of its blockade to enhance DNAM-1 activation and subsequent proliferation of earlier differentiated memory cells in MM TME. Finally, CR patients had a trend for higher DNAM-1 expression on CD8+ T cells compared to those with PD (Fig 3D). This is consistent with other reports in mice showing that the expression of DNAM-1 negatively correlates with BM myeloma cell numbers (Minnie S., Blood. 2018).
DNAM-1 axis receptors are dominantly expressed on lymphocytes in BM of MM patients, with PVRIG exhibiting the most prominent expression. The reduced expression of DNAM-1 in PD patients' TME, compared to CR patients, suggests a link between DNAM-1 axis and clinical outcomes. Recent data suggest TIGIT is an attractive target for blockade in MM. Our new findings highlight for the first time the dominant expression of PVRIG, as well as TIGIT, and suggest that combined blockade of TIGIT with PVRIG may potentially benefit MM patients, placing the DNAM-1 axis as a dominant pathway in MM therapy.
Frenkel: Compugen Ltd.: Current Employment, Other: in the event of frontal participation, I will be reimbursed for my travel expenses by Compugen Ltd.. Alteber: Compugen Ltd.: Current Employment. Cojocaru: Compugen Ltd.: Current Employment. Ophir: Compugen Ltd.: Current Employment.