Recognition of interspecies hybrid class I histocompatibility antigens by antigen-specific cytolytic T lymphocytes

Two reciprocal interspecies hybrid class I histocompatibility genes have been constructed between genomic clones of human HLA-A2 and murine K-2K^b. The proteins encoded by these genes have been designated A2¹⁺²/K^b, where the polymorphic domains, α1 and α2, of HLA-A2 are linked to the carboxyl-terminal domains (α3, transmembrane, and intracytoplasmic domains) of H-2K^b, and K^b1+2/A2, where the α1 and α2 domains of the H-2K^b antigen are linked to the carboxyl-terminal domains of HLA-A2. These genes have been transfected and expressed in recipient mouse L cells and human RD (rhabdomyosarcoma) cells. Both hybrid antigens were found to be serologically intact when tested with a panel of antigen-specific monoclonal antibodies. The monoclonal antibody W6/32, which recognizes a monomorphic determinant on all HLA-A, -B, and -C antigens, recognizes the α1 and/or the α2 domain, rather than the more conserved α3 domain. Human cytolytic T lymphocytes (CTL) specific for the HLA-A2 antigen recognized the A2 and A2¹⁺²/K^b proteins only when expressed in human cells and not when expressed in mouse cells, even when surface antigen levels were 10-fold greater on the mouse cells than on the human cells. In contrast, a long-term, murine anti-H-2^b CTL line not only lysed mouse L-cell lines that expressed the parental K^b and hybrid K^b1+2/A2 antigens but also lysed the K^b and K^b1+2/A2 human cell RD transformants as well. In both cases, the level of CTL recognition and lysis of the transformants that expressed the native antigen K^b was greater than of those transformants that expressed the hybrid antigen K^b1+2/A2. These data suggest that the carboxyl-terminal domains play some role in CTL allorecognition. The lack of human CTL recognition of HLA molecules expressed in mouse L cells, however, cannot be explained by the presence of a xenogeneic carboxyl terminus. Since murine CTL can recognize their target antigen when expressed on the surface of human cells, the possibility remains either that a ligand necessary for other molecular interactions of human CTL may be absent on mouse target cells or that murine and human CTL differ in affinity of binding to target antigens in the absence of accessory-molecule interactions.