Serologic and cellular characterization of products of a new major histocompatibility gene region, RT1.C, of the rat; possible homology to mouse H-2 Qa.

The A region of the major histocompatibility system of the rat, RT1, codes for ubiquitously expressed class I antigens and the RT1.B region determines class II (Ia) molecules. A third region, RT1.C, adjacent to B, has been described recently by histogenetic methods. Data are now presented on the analysis of RT1.C region products by the use of alloantisera and cytotoxic T lymphocytes, both prepared in two, C region congenic strain combinations. C region antigens have been detected serologically by complement-dependent cytotoxicity assays on the majority of B and T lymphocytes, but not on erythrocytes or liver and brain tissue by absorption tests. With the typing sera available only three "alleles" could be identified among 13 different rat strains, so that the RT1.C polymorphism appears to be restricted. Cytotoxic T lymphocytes could be induced against C region products after in vivo immunization and in vitro restimulation; killing was not restricted by the A and B regions. The concept that RT1.C is homologous to H-2 Qa is discussed.