Immunoglobulin gene diversification in cattle.

Research in several species has revealed that different types of mammals have evolved divergent molecular and cellular strategies for generating immunoglobulin diversity. Other chapters in this text have highlighted the specific characteristics unique to chicken, rabbit, mouse, human and sheep B lymphocyte development; namely indicating differences in the mechanisms of diversity and the site of primary B cell development. Studies of the bovine system have indicated that like the sheep system, the ileal Peyer's patch (IPP) is a likely chicken bursal equivalent, and is a site of primary B lymphocyte development. Substantial investigation in sheep has indicated that Ig diversity is created by untemplated somatic mutation and intense selective pressure (Reynaud et al., 1991). The frequency of alteration in the sheep Ig light chain gene locus also is characteristic of the bovine system, however, recent evidence from sequencing of bovine lambda light chain genes indicates that one mechanism that contributes to diversity is gene conversion, utilizing several pseudogenes located in the Ig locus (Parng et al., 1996). The mechanism by which this hyperalteration of Ig genes occurs in both sheep and cattle is poorly understood and is thus the focus of considerable investigation. The study of events in the IPP may also have informative ramifications for secondary diversification of the Ig repertoire by somatic hyperalteration in germinal centers.

Gene conversion contributes to Ig light chain diversity in cattle.

In humans and mice, extensive gene rearrangement is the major mechanism of diversification of the primary Ig repertoire. This study shows that cattle depart from this pattern because rearrangement in the light chain locus is sharply limited. Furthermore, in cattle, gene conversion contributes to the diversification of the primary light chain repertoire. Sequencing of germ-line and expressed Vlambda genes revealed three important features. First, the germ line contained a number of Vlambda pseudogenes. In fact, 14 (70%) of the 20 germ-line genes identified and sequenced were pseudogenes, because they had one or more of the following defects: lack of recombination signal sequences at the 3' end, stop codons within the reading frame or truncations, and/or insertions or deletions that resulted in loss of reading frame. Second, Vlambda cDNA from ileal Peyer's patch B cells demonstrated that the light chain repertoire arises from only a small number of V(J) rearrangements. Even though two J genes were identified in the germ line, all of the expressed Vlambda genes examined contained the same J segment, indicating that only a single J gene participates in rearrangement at the lambda locus. Third, a significant number of departures from the germ-line sequences of rearranged Vlambda can be traced to donor sequences of one or more Vlambda pseudogenes. We conclude that a limited number of rearrangements and gene conversion play a role in contributing to the diversification of the primary lambda repertoire. Furthermore, while clear indications of a role for somatic mutation in lambda diversification was seen, V gene rearrangement was not a major factor.