Structural analysis of the rat homologue of CD1: Evidence for evolutionary conservation of the CD1D class and widespread transcription by rat cells

The cDNA encoding the rat homologue of CD1 was isolated and the complete nucleotide sequence was determined. It contained an open reading frame of 1008 bp that was capable of encoding a polypeptide with 336 amino acids composed of hydrophobic leader and transmembrane sequences, three extracellular domains, and 5' and 3' untranslated sequences. Comparison of the amino acid sequence of rat CD1 with those of other species revealed that it showed the highest similarity to mouse CD1, which belongs to the CD1D class of the CD1 system and is distinct from the classic CD1 class including CD1a, CD1b, and CD1c expressed primarily on human thymocytes and some dendritic cells. Widespread transcription of rat CD1 was readily detected by Northern blot analysis in nonlymphoid organs, including the liver, kidney, and heart, as well as in lymphoid organs, including the thymus, lymph node, and spleen. Intestinal expression was also demonstrated by the more sensitive reverse transcription-PCR method. Immunoprecipitation with a rabbit anti-rat CD1 Ab showed that rat CD1 was expressed on the cell surface as a β₂- microglobulin-associated heterodimer. Southern blot analysis of inbred rat strains suggested that rat CD1 shows limited polymorphism and that only one CD1 gene is detectable in the F344 rat genome. These results provide evidence for the conservation of CD1D class through mammalian evolution and an apparent lack of the classic CD1 class genes in rodents. Functional similarity of rodent CD1 is implied.