Distinctive Developmental Origins and Specificities of Murine CD5+ B Cells

CD5⁺ B cells constitute a small fraction of cells in the spleen of adult mice that exhibit numerous features serving to distinguish them from the bulk of IgD⁺⁺CD5^?“conventional” B cells. In this review we focus on two major questions relating to this population: 1) the relationship of CD5⁺ B cells to other B cells; and 2) the distinctive enrichment of particular autoreactive specificities in this subset. The nature of their origins is clarified by a thorough analysis of intermediate stages of early B-cell development in both fetal and adult tissues. The reactivity to bromeliad-treated mouse red blood cells serves as a prototype system for the investigation of biased specificities in CD5⁺ B cells. These lines of investigation lead us to propose that CD5⁺ B cells in the adult are the remnant of a distinct fetal B-cell differentiation pathway wherein selection of cells from this fetal/neonatal population into the adult long-lived pool results in the over-expression of certain germline-encoded autoreactivities.