Use of a single VH family and long CDR3s in the variable region of cattle Ig heavy chains

Abstract

We have analyzed transcripts encoding the variable regions of Ig heavy chains from adult and fetal bovine splenocytes and bovine x mouse heterohybridomas. The 13 adult, seven fetal and two heterohybridomas transcripts as well as the six genes that were sequenced had > 83% identity to each other in the VH-encoded regions (FRs 1-3 and CDRs 1 and 2). By this criterion, all the bovine sequences were assigned to one family, which corresponds to the bovine homolog of the murine Q52 family. Southern blot analysis of genomic DNA demonstrated that homologs of other murine VH families such as 7183, S107 and 36-60 were present in the genome, but transcripts from these families were not detected in rapid amplification of cDNA ends (RACE)-PCR amplified products or in individual clones. The sequences of the adult transcripts using the mu isotype showed extensive somatic mutation indicating that the process of somatic hypermutation begins earlier in development of the bovine B cell. The length of CDR3 from V(D)J rearrangements averaged 21 amino acids, which is larger than other mammalian CDR3s. Analysis of CDR3s from 23 fetal transcripts revealed a preference for a reading frame in the putative D genes which is rich in glycine and tyrosine, and is also extensively mutated in adults. The bovine immune system appears to utilize Ig VH genes of a single family, but generates antibody diversity by extensive somatic mutation and long CDR3s which are subsequently hypermutated.