REARRANGEMENT OF UPSTREAM D-H AND V-H GENES TO A REARRANGED IMMUNOGLOBULIN VARIABLE REGION GENE INSERTED INTO THE DQ52-J(H) REGION OF THE IMMUNOGLOBULIN HEAVY-CHAIN LOCUS

We investigated gene rearrangements in the mutant IgH locus of a mouse strain generated by insertion of a rearranged heavy chain variable re gion gene (V-T15) into the DQ52-J(H) region through gene targeting. In more than half of the B cells of heterozygous mutant mice, the mutant IgH locus was silenced by the rearrangement of an endogenous D-H or D -H and V-H gene to the inserted V-T15 gene. In these cases, a function al V(H)D(H)J(H) gene was present on the wild-type allele. The silencin g rearrangement appeared to be mediated by recombination signal sequen ce (RSS)-like elements present in the ''recipient'' V-T15 gene. Among the many such elements on the inserted V-T15 gene, which apparently me t the requirement for an RSS with respect to nucleotide sequence, only two were observed in the actual rearrangements. This indicates that t argeting of the recombination machinery involves sequences in addition to the RSS motifs as they have been characterized so far. In homozygo us mutant mice, most B cells appeared to carry the intact V-T15 gene o n both mutant IgH alleles, although single-cell polymerase chain react ion revealed that silencing rearrangements occured frequently in B cel l progenitors in the bone marrow. This observation indicates that once silencing rearrangements are initiated in a cell, they involve both V -T15 genes in most cases, reminiscent of normal D-H-J(H), rearrangemen t. B cells which did not initiate such rearrangements develop to popul ate the peripheral B cell compartment.