Molecular and cellular characterization of the porcine CD1 family

The CD1 family of major histocompatibility complex (MHC) class Ic molecules is encoded by a species-specific number of genes that map outside of the MHC. Rather than conventional peptide antigens, human (h) CD1b and CD1c molecules can present lipid and glycolipid antigens, adding considerable intrigue to the CD1 family. We constructed a porcine (p) cosmid library, screened with a CD1 α3 domain probe, and isolated eight CD1⁺ genomic clones. Genetic analysis using DNA probes from pCD1⁺ clones revealed that the pig genome harbors at least five CD1 genes. That one CD1⁺ cosmid encodes two pCD1 genes demonstrates that pig CD1 genes, like human, are closely linked. One CD1 gene, which was expressed most highly in the thymus by Northern blot analysis, was sequenced and identified as pCD1A. Subsequent RT-PCR analysis demonstrated that pCD1A is expressed by macrophages and dendritic cells in numerous tissues but not by peripheral blood mononuclear cells. The size of the encoding region and exon-intron intervals are well conserved between human and pig CD1A loci. Sequence comparisons of pCD1A and hCD1A revealed 81% nucleotide and 78% amino acid (AA) similarity. Predicted AA similarities between pCD1A and hCD1A were greater in the α1 (83%) and α2 (82%) domains than in the α3 domain (74%) or the combined transmembrane/cytoplasmic regions (73%) of the molecule. In contrast to hCD1a, the predicted pCD1a protein has a longer cytoplasmic tail and contains two serine residues similar to that found often in MHC class Ia molecules. A pCD1B-encoding cosmid has also been isolated. Northern blot and RT-PCR analyses indicate that this pCD1B locus is expressed primarily in the thymus. Together, these data demonstrate evolutionary conservation of CD1 genes but also indicate that pCD1 loci exhibit unique features meaning that the pig must be included to fully understand this lineage of antigen presentation molecules.