Efficient antibody diversification by gene conversion in vivo in the absence of selection for V(D)J-encoded determinants

Antibody diversification in the bursa of Fabricius occurs by gene conversion: pseudogene-derived sequences replace homologous sequences in rearranged immunoglobulin genes. Bursal cells expressing a truncated immunoglobulin mu heavy chain, introduced by retroviral gene transfer, bypass normal requirements for endogenous surface immunoglobulin expression. Immunoglobulin light chain rearrangements in such cells undergo gene conversion under conditions where the products are not selected based on their ability to encode a functional protein. The efficiency with which gene conversion maintains a productive reading frame exceeds 97% under such non-selective conditions. By analysis of donor pseudogene usage we demonstrate that bursal cell development is not driven by a restricted set of antigenic specificities. We further demonstrate that gene conversion can restore a productive reading frame to out-of-frame VJ(L) junctions, providing a rationale for the elimination of cells containing non-productive VJL rearrangements prior to the onset of gene conversion in normal bursal cell development.