Surface immunoglobulin-negative B-cell precursors detected by formation of antibody-secreting colonies in agar

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Abstract
The development of semi-solid in vitro cloning assays has helped distinguish the different stages in early haematopoietic differentiation. The progenitors of erythrocytes, granulocytes, macrophages and megakaryocytes have been quantitated and characterized in such systems but until now, similar assays for progenitors of antibody-producing B lymphocytes have not been established despite many reports describing the properties of B-cell precursors which proliferate and differentiate either in vivo in adoptive hosts or in in vitro liquid culture systems. A semi-solid agar assay is described here which permits a murine B-cell precursor to develop into a colony containing antibody-secreting cells after 7-11 days in culture. The precursor cells were found in fetal liver and could be clearly distinguished from mature clonable B cells. This assay thus provides a method to quantitate functional B-cell precursors and establish the requirements for the generation of B lymphocytes.