Class I genes of MHC expressed in the rat thymus - An analysis of the transmembrane portions of MHC Class I genes expressed in the rat thymus and an establishment of the novel gene family RT1. N

Thymus is a major site of T cell development where selection of conventional T cells occurs by interaction between T cell receptors and MHC molecules. However, the functions of class I-related genes and their products (referred to as class Ib) remain unclear. Our previous results suggested that the thymus-specific class Ib genes homologous to mouse TL and human CD1 existed in the rat genome but were not expressed in the thymus. We therefore conducted a study to characterize class Ib genes expressed in rat thymocytes. At least eight genes were functionally transcribed and most of them showed high similarity to classical class I genes with over 80% similarity in exon 5, which encodes the transmembrane portion. The only one exception was clone RH-104 which has very low sequence similarity to other class I genes. The RH-104 mRNA was preferentially transcribed in thymus and to a lesser extent in the spleen. DNA data base analysis revealed that RH-104 was highly similar to RT1. N genes located in the growth and reproduction complex (grc-G/C region) of the rat MHC. Genomic cloning analysis revealed that RH-104 was derived from a novel gene, RT1. N4. The products of this novel gene family may act as ligand or antigen-presenting molecules which are required for normal T cell development in the rat.