The effects of microenvironment and internal programming on plasma cell survival

Two populations of plasma cells (PCs) are formed after immunization. A short-lived population in the spleen and lymph nodes provides rapid protection. A long-lived population, mainly in the bone marrow, provides lasting immunity. The mechanisms responsible for the differences in PCs life span remain largely unknown. The goal of the current study was to compare the intrinsic survival capacity of isolated short-lived (spleen) versus long-lived (bone marrow) PCs. We approached this question by using a previously established in vitro model that measures PC survival in a supportive stromal environment. Regardless of the tissue source or isolation time point after immunization, the two PC populations showed similar intrinsic ability to survive in vitro. To test differences in the stromal microenvironments, stromal cells from marrow, spleen or lymph nodes were evaluated for ability to support PCs survival. Survival of isolated PC was always greater when co-cultured with marrow stromal cells compared with those from spleen (or lymph node) despite the finding that IL-6, necessary for PC survival in culture, was secreted by all three stromal cell sources. Additionally, low expression of B-cell-activating factor belonging to the tumor necrosis factor-family was detected in all three stromal isolates. In contrast, marrow stromal cells were distinguished by cell-surface phenotype and CXC chemokine ligand (CXCL)12, IL-7 and stem cell factor expression. Although CXCL12 has been suggested as a possible survival factor for PC, addition or neutralization of CXCL12 had minimal effect on PC survival. We conclude the mechanisms regulating PC longevity appear extrinsically driven and marrow favored, but the factors that give marrow stromal cells a unique advantage remain unknown.