Recent advances in tumor immunology suggest that the expression of the histocompatibility antigens, encoded by the major histocompatibility complex, is important in controlling the metastatic growth of certain murine tumors. The anomalous expression of histocompatibility antigens in many neoplasms appears to be associated with the ability of these cells to evade the immune system and progress to metastasis. This review examines some of the underlying molecular and immunobiological interactions that might determine the metastatic outcome of cellular transformation.

SUMMARY The C3H UV‐induced fibrosarcoma, 1591, is highly immunogenic and, therefore, is readily rejected when transplanted into immunocompetent syngeneic recipients. Previous analysis of 1591 with tumour‐specific or H‐2‐reactive monoclonal antibodies revealed that this antigenicity might be due to the expression of two novel class I major histocompatibility complex (MHC) antigens. In this report we describe the molecular cloning and initial characterization of three genes which account for all of the unique serological class I reactivities observed on this tumour. These include two distinct, but highly conserved, H‐2L‐like genes, and a third gene the product of which bears determinants which are characteristic of both the tumour and of class I products of the H‐2k haplotype. Moreover, each of these genes contains a polymorphic restriction enzyme fragment which is detected in the class I sequences of 1591 relative to normal C3H tissue. Since the expression of these polymorphic class I sequences is relevant to the immunogenicity of 1591, the mutational events by which these genes were generated may be significant to the immunobiology of this tumour.

We have isolated cytotoxic T lymphocytes (CTL) preferentially reactive with the al external domain of the H-2Ld antigen by selecting for T cells capable of recognizing a variant major histocompatibility complex (MHC) class I antigen sharing al sequences with H-2Ld. Using these CTL, we demonstrate that a synthetic al peptide corresponding to one of the helices derived from the H-2Ld molecule can be presented by a class I restriction element to reconstitute a CTL determinant borne by intact H-2Ld. Moreover, several other H-2L-reactive CTL generated independently were also able to recognize H-2Ld either as an intact alloantigen or as a peptide in coiijunction with appropriate class I restriction elements. These data demonstrate that an H-2 peptide can reconstitute a CTL target structure and suggest that some alloreactive T cells in fact might be directed against allogeneic class I peptides in the context of a class I framework.

The UV-induced, C3H fibrosarcoma, 1591, expresses at least three unique MHC class I antigens not found on normal C3H tissue. Here we report the complete DNA sequence of the three novel class I genes encoding these molecules, and describe in detail the recognition of the individual products by tumor-reactive and allospecific CTL. Remarkably, although C3H does not appear to express H-2L locus information, this C3H tumor expresses two distinct antigens, termed A149 and A166, which are extremely homologous to each other and to the H-2Ld antigen from BALB/c. The gene encoding the third novel class I antigen from 1591, A216, is quite homologous to H-2Kk) throughout its 3' end. Since all three of these genes account for polymorphic restriction fragments not found in C3H, it is likely that they were derived by recombination from the endogenous class I genes of C3H. The DNA sequence homology of A149, A166, and H-2Ld is especially significant given the functional conservation observed between the products of these genes. Limited sequence substitutions appear to correlate with some of the discrete serological differences observed between these molecules. In addition, both A149 and A166 crossreact, but to differing extents, with H-2Ld at the level of T cell recognition. Our results are consistent with the view that CTL recognize complex conformational determinants on class I molecules, but extend previous observations by comparing a set of antigens with discrete and overlapping structural and functional differences.

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