Inhibition of B Lymphopoiesis by Adipocytes and Myeloid-Derived Suppressor Cells.

다국어 초록 (Multilingual Abstract)

B lymphopoiesis declines with age in humans, mice, and rabbits. Impaired B lymphopoiesis correlates with increased fat in the bone marrow (BM), suggesting that adipocytes negatively regulate this process. In fact, adipocyte factors were found to inhibit B cell development in BM cultures.
Our goal was to understand the mechanism by which adipocytes inhibit B cell development. Through culturing mouse BM cells on OP9 stromal cells in the presence of adipocyte-conditioned medium (ACM), we found that adipocytes promote the accumulation of CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs). These cells were not simply bystanders, as we report for the first time that MDSCs potently inhibit B cell development.
ACM-generated MDSCs express high levels of arginase and iNos, which are important for suppressing T cells. However, these effector molecules did not mediate the loss of B lymphopoiesis. By cytokine array analysis of MDSC-CM, we found that ACM-generated MDSCs produce IL-1. Further, neutralization of IL-1 in BM cultures containing MDSCs restored B lymphopoiesis, suggesting that MDSCs inhibit via IL-1. Inhibition by IL-1 did not directly block B lineage development, but instead acted at the MPP stage of hematopoietic development to drive myelopoiesis at the expense of B lymphopoiesis.
In contrast to humans and mice, where B lymphopoiesis declines in mid-to-late life, B lymphopoiesis arrests at two-to-four months of age in rabbits. Characterization of rabbit BM showed an increased number of adipocytes, an expanded myeloid compartment, and increased expression of inflammatory factors when B lymphopoiesis is arrested. This reduction in B lymphopoiesis and increase in myeloid cells was recapitulated in BM cultures treated with BM fat-CM. These data coupled with the identification of an inhibitory myeloid population, suggest that the BM microenvironment is responsible for the arrest of B lymphopoiesis in rabbits.
Our study has uncovered potential targets for therapies aimed at boosting B lymphopoiesis in scenarios with fatty BM, such as aging and obesity. For example, blocking the NLRP3 inflammasome in ACM-treated BM cultures prevented MDSC accumulation and enhanced B lymphopoiesis. We envision this observation; along with our other findings will provide insight into mechanisms that negatively regulate B cell development in fatty BM.