CD28 Supports Normal Plasma Cell Survival and Interactions with Microenvironment

The long term generation of protective antibodies (Abs) requires the continuous survival of long-lived plasma cells that are maintained within specialized bone marrow niches by complex interactions that remain largely uncharacterized. Previous studies have shown that the T cell costimulatory receptor CD28 is expressed on normal and transformed (murine plasmacytoma, human multiple myeloma) plasma cells – however, its role in the B cell lineage remained unclear. We have recently shown that CD28 expressed on transformed human plasma cells (multiple myeloma cells) directly delivers pro-survival signals to the myeloma cells and protects them against intrinsically and extrinsically induced death (Bahlis et al, 2007). Furthermore, myeloma cells directly interact with dendritic cells (DC, both in vitro and in patient bone marrow biopsies), and the DC provide the ligands (i.e. CD80 and CD86) for myeloma-CD28. Others studies utilizing competitive bone marrow reconstitution have indirectly suggest a role for CD28 in the function and/or survival of normal murine plasma cells (Delogu et al, 2006). These observations led us to directly investigate the role of CD28 in normal plasma cell survival as well as cell-cell interactions with CD80/CD86⁺ bone marrow derived dendritic cells (BMDC). In vitro serum starvation experiments, direct activation of CD28 by an agonistic anti-CD28 mAb increased survival of serum-starved PC by 63% (p<0.001). Addition of BMDC improved the survival of PC by 20% over that seen with media alone, and resulted in a significant increase in IgG production (p<0.01). We and others have shown that CD28 binding to CD80/CD86 on DC also “backsignals” to the DC to produce the PC survival factor IL-6. We found that co-culture with the murine plasmacytoma cell line S194 induced 155 pg/ml of IL-6 from BMDC (p<0.01 vs. BMDC alone and S194 alone), and primary plasma cells isolated from bone marrow induced 290 pg/ml of IL-6 from BMDC (p<0.001 vs. BMDC alone). Induction of BMDC production of IL-6 by both primary and transformed PC was significantly inhibited (p<0.05) by antibody blockade of CD80 and CD86. Our data demonstrates that signaling through CD28 directly supports the survival of normal bone marrow plasma cells, and that “backsignaling” through PC-CD28 engagement of DC-CD80/CD86 induces DC to secrete the pro-survival cytokine IL-6. These findings suggest that CD28 is a key molecular bridge that connect normal plasma cells to the supportive microenvironment.