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Maturation of B lymphocytes. I. Concurrent appearance of increasing Ig, Ia, and mitogen responsiveness.

The relationship between the surface IgM (S-IgM) density and Ia antigens or total S-Ig density and the ability to produce B lymphocyte colonies (BL-C) in agar was examined for small lymphocyte populations in adult CBA bone marrow and spleen. Lymphocyte fractions with various levels of S-IgM sorted by the fluorescence-activated cell sorter (FACS) were relabeled for Ia antigens with a 125I-labeled Protein A sandwich technique. Similar fractions sorted by the FACS on the basis of different S-Ig levels were grown in agar for generating BL-C. Most or all S-IgM-bearing cells, including those with the lowest S-IgM levels, were found to be Ia + ve in both organs. Thus a lag, if any, between S-IgM acquisition and Ia acquisition by the B cells in the adult marrow must be small. Secondly, the Ia density of Ia + ve cells increased in parallel with IgM density in both the organs, indicating a strong functional association between the two surface markers. Most BL-C-forming cells (BL-CFC) were found to be included in the S-Ig + ve class, but the BL-CFC incidence within this class increased with increasing S-Ig density, possibly suggesting a parallel increase in the putative mitogen receptors on B cells. Null (S-Ig - ve, θ - ve) lymphocytes both in the marrow and the spleen showed a small proportion of Ia + ve cells as well as a small incidence of BL-CFC; these subsets remain to be characterized further. These studies reveal that B cell maturation in the adult animal is associated with a concurrent appearance of increasing Ig, Ia, and mitogen responsiveness.