Somatic diversification of chicken immunoglobulin light chains by point mutations.

Abstract

The light-chain locus of chicken has 1 functional VAl gene, 1 J gene, and 25 pseudo-VA-genes (where V = variable and J = joining). A major problem is which somatic mechanisms expand this extremely limited germ-line information to generate many different antibodies. Weill's group [Reynaud, C. A., Anquez, V., Grimal, H. & Weill, J. C. (1987) Cell 48, 379-388] has shown that the pseudo-VA-genes diversify the rearranged VAl by gene conversion. Here we demonstrate that chicken light chains are further diversified by somatic point mutations and by VA1-J flexible joining. Somatic point mutations were identified in the J and 3' noncoding DNA of rearranged light-chain genes of chicken. These regions were analyzed because point mutations in VAl are obscured by gene conversion; the J and 3' noncoding DNA are presented in one copy per haploid genome and are not subject to gene conversion. In rodents point mutations occur as frequently in the V-J coding regions as in the adjacent flanking DNA. Therefore, we conclude that somatic point mutations diversify the VAl of chicken. The frequency (0-1%) and distribution of the mutations (decreasing in number with increased distance from the VAl segment) in chicken were as observed in rodents. Sequence variability at the VA1-J junctions could be attributed to imprecisejoining of the VAJ and J genes. The modification by gene conversion of rearranged VAJ genes in the bursa was similar in chicken aged 3 months (9.5%) or 3 weeks (9.1%)-i.e., gene conversion that generates the preimmune repertoire in the bursa seems to level off around 3 weeks of age. This preimmune repertoire can be further diversified by somatic point mutations that presumably lead to the formation of antibodies with increased affinity. A segment with structural features of a matrix association region [(A+T)-rich and four topoisomerase H binding sites] was identified in the middle of the J-CA intron (where C = constant).The antibody repertoire of chicken is fairly high and is estimated to be around 106, only one order of magnitude lower than the antibody repertoire of mouse (1). The contribution of the germ-line element for the diversification of chicken immunoglobulin light (L) and heavy (H) chains is rather limited, because their variable (V) regions are encoded by only one VL subgroup (2, 3) and one VH subgroup (4). Furthermore, the entire L-chain repertoire is derived from a single V-J rearrangement (where J = joining) because there is 1 J gene and only 1 functional VAl gene, and the remaining 25 are pseudo-VA-genes that serve as donor sequences to diversify the rearranged VA1 gene by gene hyperconversion (2,5,6). Here we report that chicken L chains are further diversified by somatic point mutations and by imprecise joining of the V and J gene segments. The time course of diversification by gene conversion and by somatic point mutations in relation to the generation of the preimmune repertoire and to increased affinity of antibodies during the immune response are discussed. MATERIALS AND METHODSA single ch...

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