Organisation of the equine immunoglobulin constant heavy chain genes: II. Equine cγ genes

The number of immunoglobulin G constant heavy chain genes (cγ genes) varies broadly among mammalian species, reflecting structural and functional differences between expressed immunoglobulin G (IgG) isotypes and allotypes. Up to now equine IgG isotypes have been defined only at the biochemical and serological level. It is still not clear how many IgG isotypes exist in horses and whether there are any allotypes. Here, we describe the isolation and characterisation of equine cγ genes. An equine genomic lambda phage library was screened with a human cγ4 probe. Cross-hybridising equine cγ sequences were cloned twice and characterised by restriction mapping with the human cγ4 and a murine sγ1 probe. Genomic equine DNA probes for both, cγ genes and corresponding switch regions (sγ), were isolated and used for a more detailed BamHI restriction analysis, comparing genomic DNA of various horses. This analysis reveals the existence of at least five, or probably six cγ genes in the equine haploid genome. Beside the porcine system, this is the highest number of cγ genes described for any mammalian species. Moreover, for two of these cγ genes, BamHI restriction fragment length polymorphism became evident.