Fc and complement receptors on newly formed marrow small lymphocytes with that of surface IgM and to examine the receptor status of proliferating large lymphoid cells, as shown by the rosetting

Many small lymphocytes in mouse bone marrow display surface IgM molecules and receptors for Fc and complement (1-3). Cytocentrifuge rosetting studies demonstrate well-defined Fc and complement receptors on certain marrow small lymphocytes, but the incidence of these cells relative to IgM-bearing lymphocytes is lower than in the peripheral lymphoid tissues (1, 2). These findings suggest that the Fc and complement receptor-bearing cells may form part of the population of young, B small lymphocytes 1 produced continuously in the marrow (4). If so, the timing and sequence of receptor development on small marrow lymphocytes would define stages in the maturation of primary B lymphocytes, allowing a correlation to be made with the onset of functional responsiveness. In addition, in the light of the kinetic and functional heterogeneity of marrow lymphocytes (4, 5), some Fc and complement receptor-bearing cells may represent lymphocyte subtypes other than immature B lymphocytes. A majority of marrow small lymphocytes are newly formed cells, renewed rapidly by local cell production (4-7). After a postmitotic delay, many of these cells develop readily detectable surface IgM molecules increasing in apparent density with time (8). Other young, marrow small lymphocytes, so-called null cells, lack both surface IgM and Thy.-1 antigen (2-5, 8). It remains to be verified whether all or only some of the null marrow lymphocytes actually belong to the B lineage, destined to develop surface IgM and other B-lymphocyte receptors as they mature. Newly formed marrow small lymphocytes in general migrate rapidly to the spleen and lymph nodes (9, 10) where their maturation may be completed (4, 11), In contrast, a minority of small lymphocytes in the marrow are long-lived slowly renewed cells, mainly recirculating immigrants, which increase in numbers throughout postnatal life (4-6). Their receptor subtypes are unknown. Numerically, many of the observed Fc and complement receptor-bearing lymphocytes in the marrow could be long-lived recirculating B lymphocytes, rather than newly formed indigenous cells. Double-laheling experiments have now been performed to relate the surface receptor status of individual marrow lymphocytes to cell age and renewal, combining surface rosetting, and radioautographic DNA-labeling techniques. The aims of the studies were, (a) to compare the development of Fc and complement receptors on newly formed marrow small lymphocytes with that of surface IgM and to examine the receptor status of proliferating large lymphoid cells, as shown by the rosetting