CD84: A Potential Novel Therapeutic Target in Multiple Myeloma

Introduction: Multiple Myeloma (MM) cell survival strictly depends on the interaction with multiple cell types in the bone marrow (BM), collectively referred to as the MM microenvironment (MM-ME). CD84 (SLAMF5) belongs to the signaling lymphocyte activation molecule family of immunoreceptors; previous data from our group have shown that CD84 mediates malignant B cells and their ME (Marom et al. 2017); however, its role within the MM-ME has not yet been investigated.

Results: Using the MMRF CoMMpass IA9 dataset, we found that CD84 mRNA expression is low or absent in CD138+MM cells isolated from 660 newly diagnosed patients. In agreement with these data, flow analysis showed an absence of CD84 expression in all MM cell lines tested (n=9) and minimal surface expression (5.5-13%) in primary cells (n=3). However, a significantly higher expression of CD84 was detected on the surface of BM and peripheral blood (PB) monocytic fractions (CD14+) (76-97%) compared to that of matched CD14 negative fractions (2-9%) obtained from 16 different MM patients (p<0.001). Since it was recently reported that CD14+ monocytic myeloid derived suppressor cells (Mo-MDSCs) play a pivotal role in supporting MM growth by creating an immunosuppressive ME, we investigated CD84 expression in this population. Using primary PB (n=9) and BM (n=3) samples obtained from MM patients with active disease, we found that the Mo-MDSC population is significantly over-represented (0.34-2.88%) in MM patients compared to the PB from healthy donors (n=7) (0.04-0.45%) (p<0.05) and that CD84 is also significantly up-regulated in this population in MM patients (43-92%) compared to healthy donors (7-21%) (p<0.001).

Next, to understand whether the expansion of Mo-MDSC and associated CD84 up-regulation is directly dependent on the MM cells, we investigated Mo-MDSC expansion and CD84 expression in two different MM mouse models. Specifically, 5TGM1 murine MM cells were IV injected into syngeneic immune-competent KaLwRijHsd mice (n=7), and human MM.1S cells were IV injected into immune-deficient NSG mice (n=10). Our data show that MM progression leads to significant Mo-MDSC expansion (p<0.0001) and a subsequent increase in CD84 levels (p<0.05) on this population in both mouse models.

Since T cell checkpoint inhibitors PD-1/PD-L1 are tumor immune escape receptors that play a pivotal role in supporting an immunosuppressive MM-ME, we also investigated the expression of PD-L1 on the CD14+ fraction in MM patients. BM-CD14+ cells (n=3) were compared with matched CD14 negative cells. We found that the CD14+ fraction had a higher PD-L1 expression in the BM of MM patients compared to the CD14 negative fraction (60-96% versus 3-8%). Hence, we investigated whether CD84-mediated cell-cell contact may increase the level of PD-L1 expression on Mo-MDSCs and whether MM cells can regulate CD84 and PD-L1 expression on CD14+ cells. Co-culture assays were performed using several human MM cell lines (MM1S, U266 and KMS11) with PB CD14+ cells derived from healthy donors (n=3). Our data show a 1.9-fold increase from the basal level of CD84 (27±16% to 50±9.4%) and a 2.9-fold increase from the basal level of PD-L1 (18±8.5% to 52.3±5.7%).

Our recently generated mouse anti-human CD84 blocking antibody (Marom et al. 2017) was used to determine whether CD84 direct targeting can affect MM-induced CD84 and PD-L1 expression on the surface of CD14+cells. Co-culture experiments show that targeting CD84 lowered CD84 (1.6-fold decrease) and PD-L1 expression (1.8-fold decrease) on a CD14+ population co-cultured with MM cells. Moreover, when T cells were included in the co-culture experiments, we observed reduced expression of exhaustion markers PD-1 (1.1-fold decrease) and CTLA4 (1.4-fold decrease) in the presence of the CD84 blocking antibody.

Conclusion: We show here that CD84 is expressed on Mo-MDSCs in MM and that the expansion of this myeloid population is strictly associated with MM progression. Disruption of CD84 contacts with a blocking antibody decreases the expression of PD-L1 on CD14+ cells and exhaustion markers on T cells. Thus, our results reveal a novel role for CD84 in regulating PD-L1 in the MM-ME and provides the scientific rationale to investigate whether targeting CD84 expression on Mo-MDSCs can restore T cell functions. To further confirm the role of CD84 regulating T cell activity and Mo-MDSC, in vivo mouse studies are ongoing and results will be presented at the meeting.