A comprehensive analysis of germline and expressed immunoglobulin repertoire in the horse

Based on the recently released horse genome, we have characterized the genomic organization of the horse Ig gene loci. The horse IgH locus in genomic scaffold Un0011 contains 40 DH segments, 8 JH segments and 50 VH segments. The Igκ locus contains only a single Cκ gene, 5 Jκ segments and a 60 Vκ segments, whereas the Igλ locus contains 7 Cλ genes each preceded by a Jλ gene segment. A total of 110 Vλ segments with the same transcriptional polarity as Jλ–Cλ were identified upstream of the Jλ–Cλ cluster. However, 34 Vλ segments locating downstream of the Jλ–Cλ cluster showed an opposite transcriptional polarity. Our results reveal that the horse germline V repertoires were more complex than previously estimated. By analyzing the cloned IgH/L cDNA, we further showed that several selected V subgroups were utilized in the expressed VH, Vκ, or Vλ and a high frequency of nucleotide deletions and insertions were introduced by somatic hypermutation in these expressed V genes.