DOMINANT EXPRESSION OF A 1.3 MB HUMAN IG-KAPPA LOCUS REPLACING MOUSE LIGHT-CHAIN PRODUCTION

Expression studies of multigene families, such as the immunoglobulin ( Ig) loci, are difficult because of their large size and the necessity to introduce germline configured regions into an animal. Antibody dive rsity from Ig gene miniloci is limited by the number of variable (V) r egion genes and the need for distal regulatory elements to control exp ression, Here, we show germline transfer into mice of a 1300 kb human Ig kappa light chain locus on a yeast artificial chromosome that resul ted in early DNA rearrangement and highly efficient human light chain expression, The human locus was assembled from a 300 kb authentic regi on using contig extension by addition of cosmid multimers to supplemen t the variable gene cluster. This resulted in the addition of about 10 0 V region genes in germline configuration from different families. In transgenic animals with Ig kappa disruption, this large human kappa l ocus replaced the endogenous locus, and subsequent down-regulation of Ig lambda light chain contribution Zed to a dominant expression of the rearranged human genes. Contrary to expectation, rather than providin g a solely selective advantage for ensuring repertoire formation contr olled by the sheer number of introduced genes, the lambda/kappa ratio in serum appears to be the result of competition for early surface Ig expression maintained in the developing B cell.