Selection for B cells with productive IgL gene rearrangements occurs in the bursa of Fabricius during chicken embryonic development.

Abstract

The vast majority of immunoglobulin-expressing mature chicken B lymphocytes contain one functionally rearranged and one unrearranged allele of the immunoglobulin light chain (IgL) gene. Therefore, nearly all IgL V-J rearrangements present in mature chickens are in-frame. In contrast, the Ig genes of mature mammalian B cells contain a high proportion of out-of-frame V-J joints. To investigate the basis for this difference, gene rearrangement at the chicken IgL lOCUS was characterized during embryonic development and in mature B-cell lines. Joining of the single functional variable (V,) segment with the single joining (JL) segment occurs in cells in multiple tissues during a transient period of chicken embryogenesis. Only one-third of the V-J joints cloned from days 10-12 of development are in-frame. An increasing proportion of in-frame V-J joints is observed within the bursa of Fabricius at successively later stages of development. Our data suggest that the bursa of Fabricius serves during embryonic development as a site of selective amplification of cells that have undergone productive v-J joining, such that nearly all v-J joints present in postembryonic B cells are in-frame. The high frequency of rearranged alleles joined in-frame that is found in posthatching bursal cells and mature B-cell lines appears to result from a low frequency with which cells undergo Ig, rearrangement at both alleles, rather than from an increase in the precision of V-J joining in avian species.

Murine lambda gene rearrangements: the stochastic model prevails over the ordered model.

“…Indeed, three main hypotheses have been considered (Durdik et al, 1984;Persiani et al, 1987;Muller et al, 1988): (i) there is only a chance correlation between RS expression and X locus activation without a cause-effect relation, (ii) the RS recombination allows the activation of a transpositive factor (in this case, only one RS recombination is sufficient), (iii) the RS recombination inhibits a transrepressing factor (implying the deletion of the repressor on both alleles). However, RS recombination has only been studied in a few independent murine X-expressing B cell clones (Durdik et al, 1984;Persiani et al, 1987;Muller et al, 1988 (Blackwell et al, 1989 (Reynaud et al, 1985;Thompson and Nieman, 1987;McCormack et al, 1989). Moreover, the recent description of primitive vertebrate Ig light chains, shows that the amino acid sequences are closely related to mammalian X chains (Shamblott et al, 1989).…”

“…However, only one third of the VJ joins are in frame in the B cells from embryonic days 10-12 of development, suggesting that cells which contain a non-functional rearrangement on one allele cannot rearrange the other. Thus, B cells may present a VJ recombinase activity for only a short time (McCormack et al, 1989). We did not consider in this work a 'pure stochastic model' where the probability of initiation of rearrangements at the X locus is particularly low, because the ordered scheme H -L, implies that the X light chains are also activated.…”

Development of B cells expressing surface immunoglobulin molecules that lack V(D)J-encoded determinants in the avian embryo bursa of Fabricius

“…This has led to considerable speculation that the specificity encoded by the prediversified sIg receptor plays a critical role in B cell development (2,3,(15)(16)(17)(18)(19).…”

“…Although Ig gene rearrangement is not intrinsically biased toward productive rearrangement, only those B cells that have undergone productive rearrangement are selected within the bursa (15). Because there is no detectable clonal expansion of B cell precursors in bursal follicles before the expression of sIg (13), transit across the basement membrane and subsequent precursor expansion in epithelial buds require sIg expression and represent a critical checkpoint in B lymphopoiesis.…”

A new view of V(D)J recombination

“…Broken coding ends are rare but can also be detected; most of them are hairpins, but some open coding ends that have lost bases were also seen. In a general sense, hairpin coding ends are likely intermediates in V(D)J recombination, because of the frequent appearance of short selfcomplementary additions (P nucleotides) in coding joints (Lafaille et al 1989;McCormack et al 1989). These most probably derive from the off-centre cutting of hairpins.…”