Lymphocytes from b5/b9 rabbits were stained in suspension with fluorescent antiallotype antibody reagents to selectively label with fluorescent molecules those cells bearing membrane immunoglobulin (Ig) of the b5 or b9 allotype. After staining, the cells were separated by the fluorescence-activated cell sorter into populations markedly enriched in cells bearing b5 or b9 membrane Ig or totally depleted of cells with detectable membrane Ig. The potential of these separated cells to give rise to Ig-synthesizing plasma cells either in vivo after transfer into irradiated recipients or in vitro during culture in the presence of phytohemagglutinin or pokeweed mitogen was assessed by immunofluorescence. The relative proportion of b5 and b9 cytoplasmic Ig-stained cells (CSC) arising from the separated cells was determined to test directly whether B lymphocytes and their progeny are committed to the synthesis of Ig of one allotype.

It was found that b5- and b9-bearing cells gave rise almost exclusively to b5- and b9-producing plasma cells, respectively, in both the in vivo and in vitro assay systems. Most of these CSC were probably not derived from previously existing CSC but arose as the result of the differentiation of lymphocytes with membrane Ig. When cell populations totally depleted of Ig-bearing lymphocytes were cultured, very few CSC were found, indicating that the majority of immediate precursors of CSC have membrane Ig. These results suggest that individual B cell clones are phenotypically restricted to the expression of immunoglobulin genes on one chromosome; the significance of this clonal allelic exclusion is discussed.

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