A third class I major histocompatibility complex antigen encoded by a gene in the D region of the H-2^d haplotype recognized by cytotoxic T lymphocytes

The D region of the H-2^d haplotype contains five class I genes: H-2D^d, D2^d, D3^d, D4^d and H-2L^d. Although previous studies have suggested the presence of D-end encoded class I molecules in addition to H-2D^d and H-2L^d, segregation of genes encoding such molecules has not been demonstrated. In this report we have used cãtotoxic T lymphocytes (CTL) to examine the D region of the H-2^d haplotype for the presence of additional class I molecules. CTL generated in (C3H × B6.K1)F₁ (K^kD^k, K^bD^b) mice against the hybrid class I gene product Q10^d/L^d expressed on L cells cross-react with H-2L^d but not H-2D^d molecules, as determined by lysis of transfected cells expressing H-2L^d but not H-2D^d. Although H-2L^d-specific monoclonal antibodies (mAb) completely inhibit H-2L^d-specific CTL from killing B10.A(3R) (K^bD^dL^d) target cells, only partial inhibition of anti-Q10 CTL-mediated lysis was observed, suggesting the presence of an additional D-end molecule as a target for these latter CTL. To identify the region containing the gene encoding the Q10 cross-reactive molecule, we show that anti-Q10 CTL lyse target cells from a D-region recombinant strain B10.RQDB, which has H-2D^d, D2^d, D3^d, D4^d, and H-2D^b but not the H-2L^dH-2^d, and H-2L^d (including D2^d, D3^d, and D4^d, lacks this anti-Q10 CTL target molecule. Together, these data demonstrate that a class I gene mapping between H-2D^d and H-2L^d encodes an antigen recognozed by anti-Q10 CTL. A likely candidate for this gene is D2^d, D3^d or D4^d.