B lymphocyte subpopulations in the mouse spleen. A study of the differentiation pathway using free flow electrophoretically separated subpopulations of direct PFC progenitor cells.

Free-flow electrophoretic separation of mouse spleen cells provides three distinct progenitor cells of direct PFC, showing high, medium and low electrophoretic mobility. All progenitor cells possess surface immunoglobulin and mouse B-lymphocyte specific antigen. The progenitor cells of high electrophoretic mobility show high cycling turnover, a spleen seeking capacity of 16%, provide PFC with a maximum 8 days after transfer and reveal an isometrical increase of the PFC dose response line as a function of the graft size. The progenitor cells of medium electrophoretic mobility are low cycling, 16% home to the spleen, a maximum of PFC is developed eight days after transfer and the PFC dose response line increases allometrically. The progenitor cells of low EPM show low cycling activity, 20% home to the spleen, a maximum of PFC is attained six days after transfer and the PFC dose response line rises isometrically. These results suggest that the electrokinetically different PFC progenitors represent biologically distinct subsets. In double transfer experiments, some evidence was obtained that progenitor cells of low electrophoretic mobility are derived from progenitors of higher electrophoretic mobility. The same observation accounts also for the formation of B lymphocytes of low EPM. Since it seemed likely that the PFC progenitor cells represent virgin cells of a single lineage, the results were discussed in the terms of differentiation pathways of B lymphocytes. A model is considered in which a progenitor of medium electrophoretic mobility provides those of high electrophoretic mobility which after passing a transient cycling stage finally produce mature resting B lymphocytes of low electrophoretic mobility.