Porcine IgG : structure, genetics, and evolution.

Eleven genomic porcine Cgamma gene sequences are described that represent six putative subclasses that appear to have originated by gene duplication and exon shuffle. The genes previously described as encoding porcine IgG1 and IgG3 were shown to be the IgG1a and IgG1b allelic variants of the IgHG1 gene, IgG2a and IgG2b are allelic variants of the IGHG2 gene, while "new" IgG3 is monomorphic, has a exteded hinge, is structurally unique, and appears to encode the most evolutionarily conserved procine IgG. IgG5b differs most from its putative allele, and its CH1 domain shares sequence homology with the CH1 of IgG3. Four animals were identified that lacked either IgG4 or IgG6. Alternative splice variants were also recovered, some lacking the CH1 domain and potentially encoding heavy chain only antibodies. Potentially, swine can transcribe <20 different Cgamma chains. A comparison of mammalian Cgamma gene sequences revealed that IgG diversified into subclasses after speciation. Thus, the effector functions for the IgG subclasses of each species should not be extrapolated from "same name subclasses" in other species. Sequence analysis identified motifs likely to interact with Fcgamma receptors, FcRn, protein A, protein G and Clq. These revealed IgG3 to be most likely to activate complement and find FcgammaRs. All except IgG5a and IgG6a should bind to FcgammaRs, while all except IgG6a and the putative IgG5 subclass proteins should bind well to porcine FcRn, protein A, and protein G.

Publication year:2009

Keywords:IgG subclasses, evolution, genetics, Fc receptors, comparative immunology