Immature B cells from human and mouse bone marrow can change their surface light chain expression

The capacity of bone marrow-derived surface immunoglobulin-positive (sIg(+)) human and mouse immature B cells, generated either in vitro or in vivo, to change their light (L) chain expression, has been assayed by the number of cells which change in vitro from one type of L chain to the other type, or to no sig at all. Immature sIg(+) B cells were generated in vitro from sig precursor cells from human or mouse bone marrow. The immature sIg(+) cells expressed RAG-1, Human sIg(+) cells expressed kappa and lambda L chains in ratios between 1:1 and 3:1, whereas in mouse cells, this ratio ranged from 10:1 to 20:1. Upon reculture of the human and mouse kappa(+) sIg(+) cells, about half of them remained kappa(+), a quarter became lambda(+), and another quarter became sIg(-). Between 1 and 3% expressed both kappa and lambda chains. Of the human lambda(+) cells, about two-thirds remained lambda(+), only 1 to 2% became kappa(+), while the other third became sIg(-). Again, between 1 and 3% expressed both kappa, and lambda L chains. These results indicate that expression of sIgM in the B cell membrane does not terminate L chain gene rearrangement, and that some order exists in kappa versus lambda gene rearrangements. Hence, human and mouse kappa(+) immature B cells can become lambda(+), but very few of the lambda(+) cells can become kappa(+), and both can become sIg(-). Further, human CD10(+)/sIg(+) kappa(+) and lambda(+) cells and mouse B220(low)/sIg(low) kappa(+) cells enriched from bone marrow, i.e. immature B cells differentiated in vivo, changed their Ig phenotype upon in vitro culture, but in lower frequencies. By contrast, human and mouse mature B cells did not change their L chain or Ig phenotype. Hence, at least a part of the sIg(+) immature B cells in bone marrow retain the capacity to change their L chain and Ig phenotype, and this capacity is lost when they become mature, peripheral B cells.