Molecular cloning and characterization of a novel CD1 gene from the pig

Much effort is underway to define the immunological functions of the CD1 multigene family, which encodes a separate lineage of Ag presentation molecules capable of presenting lipid and glycolipid Ags. To identify porcine CD1 homologues, a cosmid library was constructed and screened with a degenerate CD1 alpha3 domain probe. One porcine CD1 gene (pCD1.1) was isolated and fully characterized. The pCD1.1 gene is organized similarly to MHC class I and other CD1 genes and contains an open reading frame of 1020 bp encoding 339 amino acids. Expression of pCD1.1 mRNA was observed in CD3- thymocytes, B lymphocytes, and tissue macrophages and dendritic cells. The pCD1.1 cDNA was transfected into Chinese hamster ovary cells, and subsequent FACS analysis demonstrated that mAb 76-7-4, previously suggested to be a pig CD1 mAb, recognizes cell surface pCD1.1. Structurally, the pCD1.1 alpha1 and alpha2 domains are relatively dissimilar to those of other CD1 molecules, whereas the alpha3 domain is conserved. Overall, pCD1.1 bears the highest similarity with human CD1a, and the ectodomain sequences characteristically encode a hydrophobic Ag-binding pocket. Distinct from other CD1 molecules, pCD1.1 contains a putative serine phosphorylation motif similar to that found in human, pig, and mouse MHC class Ia molecules and to that found in rodent, but not human, MHC class-I related (MR1) cytoplasmic tail sequences. Thus, pCD1.1 encodes a molecule with a conventional CD1 ectodomain and an MHC class I-like cytoplasmic tail. The unique features of pCD1.1 provoke intriguing questions about the immunologic functions of CD1 and the evolution of Ag presentation gene families.