Size separation and polyclonal activation to immunoglobulin secretion of early precursors of B lymphocytes.

Abstract

In adult mammals the bone marrow is a major site of B lymphocyte production, which occurs through the differentiation of pluripotent stem cells. In vivo studies have permitted the description of intermediate stages of differentiation. Following the recent development of culture conditions and the discovery of many polyclonal activators, it is now possible to carry out in vitro a finer analysis of B lymphocyte precursors differentiation. Because in vivo studies have shown that precursors vary in size, adult murine bone marrow cells were separated according to size. Fractions were cultured with polyclonal mitogens and tested for their capacity to develop into Ig-secreting cells. With this assay system, up to six populations were detected in adult bone marrow that can differentiate along the B cell lineage. The characterization of those populations was made according to size, mitogen(s) responsiveness, and kinetics of maturation. Precursor frequencies estimated by limiting dilution analysis further suggest that in cultures of some fractions of the gradient, new lymphocytes are produced through differentiation of immature precursors, although in cultures of some other fractions B lymphocytes may arise by self-renewal of mature cells.