Early stages of human marrow lymphocyte differentiation: induction in vitro by thymopoietin and ubiquitin.

To study early stages of human lymphocyte differentiation, bone marrow cells were physically separated according to their density and size by gradient centrifugation and then velocity sedimentation. The isolated cell fractions were incubated with putative inducing agents and then assayed for their expression of an array of surface differentiation markers. The inducing agents used were two polypeptides, thymopoietin (Tp) and ubiquitin (Ub), and the cyclic nucleotide, dibutyril cyclic 3'5' adenosine monophosphate (cAMP). Tp, Ub, and cAMP each induced the ability to form sheep erythrocyte rosettes by small lymphocytes, which may thus represent T cell precursors. Ub and Tp induced rosette formation with mouse erythrocytes on lymphocytes of more heterogenous size, which may be "early" B cell precursors. Ub alone could induce surface IgM expression on small lymphocytes, which might be "late" B cell precursors. Both Tp and Ub induced Fc receptors on small lymphocytes. Complement receptors could not be induced on marrow lymphocytes by Tp, Ub, or cAMP. A number of lymphocyte precursors can thus be identified by their physical characteristics and their ability to respond to particular soluble factors with the expression of new differentiation markers.