Linkage haplotype for allotypic variants of porcine IgA and IgG subclass genes

Six putative subclasses of expressed porcine IgG have been described from gene sequences and allotypic variants for five of these have been proposed. We tested this hypothesis by studying the transcription of these 11 variants in outbred hemizygous farm pigs. Since Cγ subclass genes are closely linked, they are most likely inherited as a haplotype. Since hemizygous pigs can only express genes encoded on one chromosome, identifying the expressed genes can indicate which allelic variants are linked as well as testing whether the putative alleles are indeed alleles or separate subclass genes. The procedure for producing B cell knockout pigs has recently been described; our study examines transcripts from the hemizygous parents and offspring generated by this technology. More than 570 Cγ gene clones from hemizygous animals were identified according to subclass and allotype by a combination of clone hybridization and sequencing. IgG3 accounted for 80% in newborn animals but <5% in adults. IgG1 accounted for ~50% of all clones recovered from adults and IgG4 was the least frequently recovered (4%). Results indicate that IgG1(b), IgG2(a), IgG3, IgG4(a), IgG5(a), and IgG6(a) are linked and also linked to IgA(a). This comprises a haplotype for domesticated swine. For simplicity, we propose that the current nomenclature for the allotypes of IgG1 be reversed so that all genes in the Cγ(a)-Cα(a) haplotype are designated "a".