A subpopulation of small pre-B cells in rabbit bone marrow expresses kappa light chains and exhibits allelic exclusion of b locus allotypes.

The expression of immunoglobulin heavy and light chains by pre-B cells and B lymphocytes was examined by two-color immunoflourescence in heterozygous b5b9 rabbits. Allelic exclusion of b5 and b9 kappa light chain allotypes was observed for both surface immunoglobulin-negative pre-B cells and surface immunoglobulin-positive B lymphocytes. In newborn bone marrow, pre-B cells and immature B lymphocytes expressing b9 were as numerous as those expressing b5. In contrast, circulating B cells and bone marrow plasma cells expressing the b5 marker outnumber b9+ cells by 2 to 1 in adult b5b9 animals. Whereas most B lymphocytes expressed kappa light chain b allotypes, approximately 80% of the micro heavy chain-positive pre-B cells did not. The pre-B cells that expressed detectable light chains were relatively small lymphocytes. A model is presented which includes a "transitional" pre-B cell that expresses both micro chains and kappa chains.