Multiple sites and mechanisms of diversification in the primary antibody repertoire of cattle

The diversification of the primary repertoire in cattle was studied by genealogical analysis of V lambda sequences in libraries prepared from individual IPP follicles obtained from animals of different ages. Analysis of these libraries established two features of Ig diversification. First, both gene conversion and somatic mutation are generating diversity in the IPP follicles. Second, by 30 weeks, a greater proportion of the mutations are clustered in CDRs and a smaller proportion in the FRs. These observations suggest that both a mutational and a selective mechanism is operating within IPP follicles. To determine early sites of diversification, the expression of Rag-1 and the diversity of V lambda were examined in late first trimester bovine spleen, liver, ileum and bone marrow. V lambda gene expression could be found in all tissues except bone marrow, but Rag-1 expression was found only in spleen and bone marrow. In the tissues examined, only spleen showed extensive diversification. Most of the diversity in the early fetal spleen could be accounted for by gene conversion. These and other data (Lucier, et al. 1997) strongly suggests that, in cattle, Ig diversifies in at least 2 sites, early fetal spleen and the IPP. Diversification takes place by gene conversion and possibly untemplated somatic mutation also contributes.