The histocompatibility (H) system comprises a large number of loci responsible for acceptance and rejection of both neoplastic and normal tissue. The H-2 locus is the mouse major histocompatibility complex (MHC). H-2 class I and class II alloantigens exhibit high antigenic strength, while the remaining H loci are characterized by weaker immunological rejection and they are, therefore, named minor H loci (reviewed by Roopenian 1992). More than 40 minor H loci have been positioned on the mouse chromosomal map by tissue rejection analysis as independently segregating genetic units ).
The BALB.B and C57BL/6 mouse strains are both homozygous for the b haplotype of H-2, while the two strains differ in the alleles they carry of many minor histocompatibility determinants (Bailey 1975). Nevertheless, the cellular immune response generated by C57BL/6 mice challenged with BALB.B splenocytes is dominated by a small number of antigenic determinants (Wettstein 1986). We have named them cytotoxic T-cell target (CTT) antigens.
Competition studies suggested that CTT-1 antigenic determinant was shared by BALB.B and C3H.SW mice. However, while typing studies of CXB (BALB x B6) recombinant inbred (RI) mice showed perfect concordance between inheritance of CTT-1 alleles and inheritance of other markers of chromosome (Chr) 1, studies of six BXH (B6 x C3H) RI strains showed concordance with markers of Chrs 4 and 7 . The limited genetic data available thus raised the possibility that the CTT-1 phenotype might reflect the action of closely related genes mapping to independently segregating loci in the two inbred strains.