The status of Ig loci rearrangements in single cells from different stages of B cell development.

Differential expression of c-kit, CD25 (TAC), surrogate L chain and cytoplasmic muH chain, and surface expression of IgM and IgD allows the separation of B220 (CD45+) B cell subpopulations. PCR analyses with DNA of single cells developed by others and by us have been used to monitor the conformation of the Ig H and L chain gene loci in these different B lineage subpopulations. The results of these analyses indicate that B220+/c-kit+/CD25- cells are the precursors of large B220+/CD25+/sIgM- which, in turn, are the precursors of small B220+/CD25+/sIgM- cells. The majority of B220+/c-kit+/CD25- cells are DHJH-rearranged, with L chain loci in germline configuration and are thus pre-B I cells. More than 90% of all large B220+/CD25+/sIgM- cells have at least one H chain locus VHDHJH rearranged; half of them have also the second locus VHDHJH rearranged and are thus large pre-B II cells. Rearrangements of at least one allele of the kappa L chain loci become detectable in 65% of the small B220+/CD25+/sIgM- cells, 67% of the immature B and > 75% of the mature B cells. The ratio of kappa L to lambda L gene rearrangements in all three subpopulations is approximately 10:1, indicating that the kappa L/lambda L ratio is established as soon as rearrangements are made.