RT1.P, RAT CLASS IB GENES RELATED TO MOUSE TL - EVIDENCE THAT CD1 MOLECULES BUT NOT AUTHENTIC TL ANTIGENS ARE EXPRESSED BY RAT THYMUS

CD1 and TL were once thought to be genetic homologues because of their thymus-specific expression. We investigated their equivalents in the rat to clarify whether their structure and pattern of expression are c onserved in rodents. Two rat class Ib genes, containing 3' sequences v ery similar to mouse TL, were identified and designated RT1.P. Neither of them, however, can encode ordinary class I molecules due to the ac cumulation of harmful mutations in the 5' regions that are unique to R T1.P, while the 3' TL-like regions still retain protein-coding capacit y. Comparison of the structural organization of three types of TL fami ly genes, which include mouse T3/T18-encoding TL antigens, mouse T1/T1 6, and rat RT1.P1/P2 pseudogenes, revealed the presence of a clear dem arcation between the type-specific and TL-specific sequences at intron 3. This finding suggests that recombination plays an important role i n creating the TL family genes in rodents. Characteristic features of TL, such as a low level of polymorphism and linkage to the major histo compatibility complex, were also observed in the rat. On the other han d, rat CD1 molecules were expressed at a high level on the surface of thymocytes. Absence of authentic TL antigens and thymic expression of CD id molecules in the rat suggest the plasticity and conservation of class Ib genes in rodent evolution. Functions of TL may be substituted with CD1 or other class Ib molecules expressed by rat thymus.