Plasma cells (PCs) are specialized in antibody (Ab) production and they are, therefore, responsible for maintaining humoral immune responses. The human PC compartment is heterogeneous. PCs from inductive secondary lymphoid organs and from peripheral blood (PB) show less capability for prolonged survival and Ab production than bone marrow (BM) PCs, a pool consisting of fully mature cells. The HLDA9 workshop has allowed the use of labeled-monoclonal Abs (moAbs) recognizing a variety of recently identified lymphocyte modulatory surface receptors. In this study, flow cytometry analysis has been used to define the presence of these receptors on human PCs obtained from human tonsil (as an example of inductive organ), from PB and from BM. It was found that human PCs commonly expressed SLAMF1 (CD150), SLAMF2 (CD48), SLAMF3 (CD229), SLAMF6 (CD352) and SLAMF7 (CD319), but not SLAMF4 (CD244). In addition, PCs distinctively showed a low level of SLAMF5 (CD84) and a very high level of SLAMF7 expression in comparison with earlier stages of B cell maturation. All PC subsets exhibited a similar pattern of expression of SLAMF receptors suggesting a stage-dependent role for these proteins. In addition, most circulating PCs clearly expressed TNFRSF14 (CD270), BTLA (CD272), B7-1 (CD80) and B7-2 (CD86), and a substantial fraction of them were also positive for TNFRSF18 (CD357), FCRL1 (CD307a) and LAIR-1 (CD305). In contrast, tonsil and BM PCs only exhibited partial expression of TNFRSF14 and B7-2, a pattern of molecular expression similar to that detected on germinal center (GC) B cells. Present results indicate that human PCs exhibit a common pattern of SLAMF proteins, but differ in the rest of the receptors examined; this difference might be associated with their distinctive homing and functional requirements.
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