Maturation of bone marrow lymphocytes. III. Genesis of Fc receptor-bearing "null" cells and B lymphocytes subtypes defined by concomitant expression of surface IgM, Fc, and complement receptors.

Lymphocyte subtypes in mouse bone marrow have been analyzed according to the combination of three surface membrane markers, IgM molecules, Fc, and complement receptors (FcR, CR), expressed simultaneously on individual cells. Marrow cell suspensions were depleted of IgM-, FcR-, and CR-bearing cells, respectively, by differential centrifugation after rosetting with appropriately sensitized erythrocytes. After rerosetting, the FcR-depleted marrow fraction showed many IgM + ve but no CR + ve small lymphocytes, the CR-depleted fraction contained both IgM + ve and FcR + ve small lymphocytes, while the IgM-depleted fraction showed many FcR + ve but few CR + ve small lymphocytes. Radioautography after [³H]thymidine labeling for 1 and 4 days in vivo demonstrated an active turnover of the various lymphocyte subtypes, particularly rapid for (IgM ? ve, FcR + ve) cells. The results demonstrate the presence of three subtypes of marrow small lymphocytes which correspond with three proposed stages in the maturation of newly formed primary B lymphocytes; (a) null cells (IgM ? ve, FcR ? ve, CR ? ve), (b) IgM + ve, FcR ? ve, CR ? ve, and (c) IgM + ve, FcR + ve, CR + ve. In addition, the turnover of a sizeable population of null small lymphocytes which bear FcR, without IgM and CR, suggests the genesis of a distinct marrow lymphocyte lineage, not previously described.