Long Term Humoral Immunity Is Dependent on CD28 Expression In Plasma Cells

Abstract 1737

Protective immunity against infection requires sustained antibody production by long-lived plasma cells (LLPC) that survive for years/decades within specialized niches. What regulates/supports this survival remains largely unknown. However, is has been shown normal and transformed (human multiple myeloma) LLPC are critically dependent on the bone marrow microenvironment including cell-to-cell interactions. Leading us to rationalize, modulating this interaction could either enhance antibody production for cancer vaccine development or conversely compromise the survival of transformed/normal LLPC in the bone marrow microenvironment. We have shown the T cell costimulatory receptor CD28 expressed on both normal and transformed LLPC plays an essential role. While LLPC and short-lived plasma cells (SLPC) both express CD28, its activation in vitro only significantly increases the survival and IgG production of LLPC. These observations led us to directly investigate the role of CD28 in LLPC survival as well as cell-cell interactions with CD80/CD86⁺ bone marrow derived dendritic cells (BMDC). Utilizing normal murine bone marrow and splenic PC as our model system we further investigated the role of CD28 in LLPC function and survival. We have previously shown, in vitro serum starvation experiments, direct activation of CD28 increased survival of LLPC by 12-fold (p<0.05), whereas CD28 activation of SLPC did not induce survival. Addition of BMDC improved the survival of LLPC 2-fold over that seen with media alone, and resulted in a significant increase in IgG production (p<0.001). In contrast, CD28^-/- PC had no increase in survival when cocultured with BMDC, suggesting a direct role for CD28 in PC-DC interaction. Consistent with these findings we now show that in vivo, vaccinated bone marrow CD28^-/-:μMT chimeras had significantly reduces long-term antibody titers and LLPC (but not SLPC) survival from t_1/2 of 426 to 63 days. Additionally, LLPC CD28 modulates the microenvironment by inducing CD80/CD86⁺ stromal cell production of the supportive cytokine IL-6 (p<0.001 vs. BMDC/PC alone), which was abrogated by blocking CD80 and CD86 (p<0.05). From the above experiments we hypothesized IL-6 was playing a significant role in the survival of LLPC, however to our surprise LLPC cocultured with WT or IL-6^-/- BMDC maintained equivalent LLPC numbers, interestingly however LLPC cocultures with BMDC showed a 3-fold increase of IgG compared to LLPC cocultured with IL-6^-/- BMDC (p<0.001). These data suggest CD28 is a key molecular component in LLPC survival, whereas IL-6 contributes to Ig production. Our data demonstrates that signaling through CD28 directly supports the survival of LLPC, sustaining long term protective antibody titers. These findings suggest CD28 plays an important role in maintaining the quality of protective durable humoral immunity. Strategies to augment CD28 signaling may lead to greater LLPC survival and persistent antibody titers in cancer vaccine development. Conversely, blocking CD28 signaling could compromise the survival of transformed myeloma cells which are critically dependent on the bone marrow microenvironment.