A colony assay system that detects B cell progenitors in fresh and cultured bone marrow

This report describes a colony assay system, based on methods used to grow myeloid precursors in semisolid medium, in which B cell progenitors can be grown. The formation of these B cell progenitor colonies is dependent upon soluble mediators from a stromal cell line known to support B lymphopoiesis. In initial experiments a double layer culture system was employed in which target cells in methylcellulose medium were separated from an adherent layer of S17 stromal cells by an agar interface. Target cells were harvested from Dexter type long-term bone marrow cultures at a time after transfer to the lymphoid Whitlock-Witte conditions, when myeloid progenitors were depleted and mature B cells had not yet appeared. On day 15 of culture a colony could be identified that contained several hundred tightly clustered lymphoid cells. There was a linear relationship between the number of cells plated and the number of colonies that developed. Identically appearing colonies were also observed in agar using fresh bone marrow cells as targets with either an underlayer of S17 cells or S17 conditioned medium to potentiate colony growth. Lymphoid colonies derived from fresh bone marrow appeared on days 6 and 14 of growth. A proportion of the cells from the fresh or cultured marrow derived colonies expressed the B220 antigen and cytoplasmic mu heavy chains, but surface IgM was never observed. Cell depletion experiments on antibody coated plates demonstrated the colony forming unit to be B220 antigen positive, surface IgM negative, and replating experiments indicated the colonies were lymphoid restricted in their differentiative potential.