Next-generation sequencing reveals new insights about gene usage and CDR-H3 composition in the horse antibody repertoire

•IGH predominant genes are IGHV2S3, IGHD18S1, and IGHJ1S5 in non-immunized horses.•IGL predominant genes are IGLV8S1 and IGLJ5S1 in adult non-immunized horses.•CDR-H3 composition by positions is different in horse compared to others mammals.•CDR-H3 extended form seems be predominant in horse CDR-H3 loops. Horse serum antibodies have been used for greater than a century for the treatment and prophylaxis of infectious diseases and envenomations. Little is known, however, about the immunogenetic diversity that produces horse serum antibodies. Here, we employed next-generation sequencing for a first-in-kind comprehensive analysis of the equine B-cell repertoire. Nearly 45,000 and 30,000 clonotypes were obtained for the heavy-chain (IGH) and lambda light-chain (IGL) loci, respectively. We observed skewed use of the common subgroups IGHV2 (92.49%) and IGLV8 (82.50%), consistent with previous reports, but also novel use of the rare genes IGHV6S1 and IGLV4S2. CDR-H3 amino acid composition revealed different amino acid patterns at positions 106 and 116 compared to human, rabbit, and mouse, suggesting that an extended conformation predominates among horse CDR-H3 loops. Our analysis provides new insights regarding the mechanisms employed to generate antibody diversity in the horse, and could be applicable to the optimized design of synthetic antibodies intended for future therapeutic use.