CD28-Mediated Regulation of Multiple Myeloma Cell Proliferation and Survival.

Although interactions with bone marrow stromal cells are essential for multiple myeloma (MM) cell survival, the specific molecular and cellular elements involved are largely unknown due to the complexity of the bone marrow microenvironment. The CD28 receptor, which costimulates survival signals in T cells, is also expressed on normal plasma cells and myeloma cells. In MM, CD28 expression correlates significantly with disease progression, also suggesting a pro-survival function. In contrast to T cells however, activation and function of CD28 in myeloma and plasma cells is almost entirely undefined. We found that direct activation of myeloma cell CD28 by anti-CD28 mAb alone induced activation of NFkappaB, suppressed MM cell proliferation and protected against serum starvation and dexamethasone-induced cell death. We hypothesized that the specific CD80/CD86 expressing stromal cell partner of this interaction is a professional antigen presenting cells, in particular dendritic cells. Histological studies demonstrated DC were extensively interdigitated throughout the myeloma infiltrates in patient bone marrow biopsies. In vitro coculture with DC also elicited CD28-mediated effects on MM survival and proliferation, and could be blocked by CD28Ig. Our findings suggest a previously undescribed myeloma:DC cell-cell interaction involving CD28 that may play an important role in myeloma cell survival within the bone marrow stroma. These data also suggest that CD28 may represent a therapeutic target in the treatment of multiple myeloma.