Dual pathway of B lymphocyte differentiation in vitro

Direct visualization of the events resulting from LPS stimulation of mouse spleen cells in vitro was achieved by characterizing the cells during four days of culture for morphology, Ig and theta surface markers and autoradiography after [3H] thymidine uptake. The changes observed were related to biochemical parameters such as incorporation of [3H] thymidine into DNA, Ig biosynthesis and secretion. Two pathways of B lymphocyte differentiation were observed: a) the generation of a large number of small B lymphocytes with high density of surface Ig but no internal pool detectable by immunofluorescence, and b) the maturation of a very small proportion of cells with a large intracellular pool and the ability to secrete Ig. Both cell types arise from dividing blast cells, either physically separated or traced by pulse chase experiments with [3H] thymidine. We discuss whether this duality is caused by the triggering of different B cell subpopulations at different developmental stages, preprogramed to one or the other pathway or whether the final direction of development depends on the microenvironment of individual dividing cells.