The sheep Ig variable region repertoire consists of a single VH family

Nine germ-line Ig heavy chain variable (VH) segments (including three pseudogenes) were isolated from a genomic DNA library, and the other six were obtained by PCR, using 5'and 3' primers deduced from the first three. They appear to belong to a homogeneous VH gene family, with >80% sequence identity. This sheep VH gene family is related to the human VH4 family and to the murine VH1 subgroup (clan II). Southern blot analysis shows a maximum of 10 positive restriction fragments; therefore, the nine VH genes isolated probably constitute the major part of the repertoire. Thirty-one expressed mu variable regions (and one gamma 1 variable region) were obtained from adult spleen by either cDNA cloning or anchored reverse transcriptase-PCR; they are >80% similar to each other (in their leader to framework 3 regions) and to the germ-line sequences as well. The sheep VH repertoire thus seems to derive from a small (approximately 10 members) germ-line gene family, and its diversification must rely chiefly on junctional (D and/or N regions) diversity and somatic hypermutations.