5 PUTATIVE SUBCLASSES OF SWINE IGG IDENTIFIED FROM THE CDNA SEQUENCESOF A SINGLE ANIMAL

We report the sequences of more than 40 partial and complete swine C g amma cDNAs obtained by PCR cloning of first strand cDNA, and from a cD NA expression library, all from a single animal. These seem to represe nt five IgG subclasses, that can be grouped into two clusters; one con tains IgG1 and IgG3 and the other, IgG2a, IgG2b and IgG4. IgG2a and Ig G2b differ by only three amino acids, but single strand conformational polymorphism analyses of PCR-amplified IgG2-specific segments in anim als of different breeds, argues for their putative subclass status. Ma jor subclass differences are found in the hinge and C gamma 3, but dif ferences in upper hinge length, associated with segmental flexibility in the IgGs of other species, are absent. All subclasses have identica l middle hinge segments that can accommodate three interheavy chain di sulfide bridges. The putative swine IgG subclasses have their greatest similarity with those of the human except for the near absence of hin ge region variation. Swine subclasses such as ruminant IgG2a, have a l ower hinge deletion which in human IgG1, contains one of the motifs be lieved necessary for interaction with Fc gamma Rs. IgG1 was the most f requently encountered subclass cDNA (25 of 43) and the single swine-mo use hybridoma tested had a sequence identical to IgG1. Partial sequenc e analyses of genomic clones identified one clone identical with the I gG1/IgG3 subclass cluster, two identical to the IgG2b/IgG4 subclass gr oup, and two identical to each other but different from any of the exp ressed sequences reported here. Genomic blots suggest that up to eight C gamma genes are present in the genome.