The rearrangements of Ig genes is known to be regulated by the production of H and κ L chains. To determine whether λ L chains have a similar effect, transgenic mice were produced with a λ2 gene. It was necessary to include the H chain enhancer, since a λ gene without the added enhancer did not result in transgene expression. The λ2 transgene with the H enhancer was expressed in lymphoid cells only. The majority of the B cells of newborn transgenic mice produced λ, whereas κ+ cells were reduced. Concomitantly, serum levels of κ and κ mRNA were diminished. By 2 wk after birth the proportion of κ-expressing cells was dramatically increased. Adults had reduced proportions of B cells that produced λ only, but the levels of λ were still higher than in normal littermates. Also, κ+ cells were still lower than in normal mice. Analysis of hybridomas revealed that reduction of κ gene rearrangement was the basis for the decreased frequency of κ+ cells. Furthermore, many cells also contained and unrearranged H chain allele. It was concluded that feedback inhibition by the λ2 together with endogenous H protein may have inhibited recombinase activity in early pre-B cells, leading to inhibition of both H chain and κ gene rearrangement. Thus, λ2 can replace κ in a feedback complex. The levels of serum λ1 and, to a lesser degree, of spleen λ1 mRNA were reduced in the λ2 transgenic mice. However, the proportion of hybridomas with endogenous λ gene rearrangement was at least as high as in normal mice. It was therefore concluded that the suppression of functional λ1 may be a consequence of decreased selection of endogenous λ-producing cells because of the excess of transgenic λ. The escape of κ-producing cells from feedback inhibition may be the result of several mechanisms that operate to varying degrees, among them: (a) κ rearrangement during a period in which the recombinase is still active after appearance of a λ2/μ stop signal; (b) a B cell lineage that is not feedback inhibited at the pre-B cell stage; (c) subthreshold levels of transgenic λ2 in some pre-B cells; and (d) loss of the λ2 transgenes in rare pre-B cells.
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