Genetics of the rat CT system: its apparent complexity is a consequence of cross-reactivity between the distinct MHC class I antigens RT1.C and RT1.A.

The rat CT antigens are a system of medial histocompatibility antigens linked to RT1, the rat major histocompatibility complex (MHC). They have aroused interest firstly because, despite their extreme serological weakness, they are targets for 'unrestricted' cytotoxic T lymphocytes (CTL); and secondly because they have appeared to represent a complex genetic system in terms both of the number of genetic loci involved and the number of distinguishable antigenic specificities expressed. The CT system was originally defined by the reactions of LEW anti-F344 (RT1l anti-RT1lv1) secondary in vitro CTL. These CTL reacted strongly on DA(RT1av1) targets, but much more weakly on AUG or PVG (RT1c) targets. We have used the recently derived RT1 recombinant rat strains PVG.R19 (RT1.Aav1Iav1Cc) and PVG.R20 (RT1.AcIcCav1) to investigate the genetic control of this system. Contrary to previous interpretations, the results are consistent with a model in which CT is a single locus, which maps to the RT1.C region. In addition, our results demonstrate that there is cross-reactivity of anti-RT1C CTLs on RT1A products, and we suggest that the earlier placement of a CT locus in the RT1.A region was probably incorrect and a consequence of this cross-reactivity.