It is well established that MHC-restricted T cell responses decisively influence the outcome of virus infection (1-3). In the rejection of murine leukemia virus-induced lymphomas, Th and cytotoxic T (Tc) t cells are considered essential effector cells, besides the effects of MuLVspecific antibodies, which can neutralize the virus and inhibit its spread (reviewed in reference 4). Many attempts have been made to characterize the viral antigens that are important in recognition by murine leukemia virus (MuLV)-specific T cells. The presence of viral proteins on the cell surface of MuLV induced tumor cell lines has been documented in a number of serological studies. These cell surface viral proteins, which in processed form are potential target molecules for recognition by Th and Tc cells (5), include the viral env gene-encoded products gPr85env, gp70, and p15(E) and the gag gene-encoded gP95g°g, gP85g°g, p10, pp12, p30, and p15 (reviewed in reference 6). In addition to this, recent data indicate that viral proteins that are not detectable at the surface of cells by serology can also serve as targets for T cells (7). DNA-mediated gene transfer experiments, in which single MuW viral antigens together with particular MHC class I antigens were expressed in heterologous cell lines, and the availability of Tc clones have provided a more subtle analysis of MHC and viral antigen recognition (8, 9). A highly complex picture emerged from these studies, which collectively indicate that both MHC and non-MHC genes determine which viral antigens are recognized in the context of MHC.Recently, substantial progress has been made in the understanding of MHCrestricted recognition ofantigens by T cells. It now appears that T cells usually recognize relatively short antigenic peptides, derived by processing of the native antigen (10, 11). This holds for MHC class II-restricted antigen recognition at the surface of APC by Th cells, but also for MHC class I-restricted antigen recognition by Tc cells, which results in lysis of the antigenic peptide-bearing cells (7,12). The MHCpeptide interaction required for T cell recognition is rather MHC allele specific, which implies that the nature of viral antigens preferentially recognized differs from individual to individual (7,(13)(14)(15)(16)(17) .

We performed a cytogenetic study on 16 murine mature B-cell lymphomas and 10 T-cell lymphomas, using G-banding techniques. All tumors, with the exception of 3 spontaneous B-cell tumors, were induced by various slowly transforming murine leukemia viruses (MuLV). Metaphases were obtained from primary (10 B-cell tumors) and first or second transplant generation lymphomas (6 B-cell and 10 T-cell tumors), all of which were well characterized with respect to phenotypic, histologic and genotypic features. In the T-cell tumors we found relatively simple karyotypic abnormalities, including various numerical aberrations, such as trisomy 15, in line with many earlier reports. However, the majority of B-cell tumors showed a great variety of both structural and numerical chromosomal anomalies. Three B-cell lymphomas had an apparently normal karyotype. No single cytogenetic abnormality occurred commonly in the B-cell lymphomas, but some structural abnormalities were found in more than one stemline, in particular, ins (II) (A1; A2) in 3 tumors, and deletions involving the D-region of chromosome 14 in 3 other lymphomas. These cytogenetic results clearly indicate that the pathogenic mechanisms involved in MuLV-induced (long latency) B-cell lymphomagenesis and (short latency) T-cell lymphomagenesis differ considerably.

The molecular etiology of retrovirally induced T-cell tumors has been shown in many cases to involve proviral integration near a cellular oncogene, c-myc, N-myc, Pim-1 and pvt-1 being frequent targets for insertional activation. Murine B-cell tumors induced by infection with murine leukemia virus have been studied for rearrangements in these and other loci. In contrast to the T-cell lymphomas, tumors of the B-cell lineage, either early B-cell tumors induced in nude mice or late B-cell tumors in immunocompetent mice, did not show disruption of N-myc or Pim-1 in any of the tumors studied, although those lymphomas had acquired many new proviruses. The loci c-abl, bcl-2, fis-1, c-erbB, c-myb, and neu were likewise not involved. Rearrangement of c-myc was seen in 1 out of 71 and rearrangement of the pvt-1 locus in 4 out of 73 (5%) of the B-cell tumors. Thus it appears that mechanistic differences exist in the development of T-cell tumors and B-cell tumors caused by the same etiological agent.

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Three monoclonal antibodies react with the T cell receptor on the tumor line HPB-ALL and in addition with 3 to 13% of human peripheral blood T cells of normal donors. These antibodies are shown to react with an epitope encoded by the V beta 5 family of T cell receptor beta-chain variable region gene segments. Cells expressing V beta 5 gene segments can have cytotoxic or helper function, be of the T4+ or T8+ phenotype, and have specificity for either class I or class II major histocompatibility complex alloantigens. Seven T cell clones were generated, which express V beta 5 and are specific for the HLA-A2 molecule. With the use of these clones, we illustrate how isoelectric focusing can be used to analyze T cell receptor alpha- and beta-chain structure. The seven clones recognize five distinct conformational determinants on HLA-A2. They procure different binding sites by the use of different alpha-chains, J beta sequences, or both.

SUMMARY Neonatal infection of C57BL/10 mice with cloned ecotropic and/or dual‐tropic mink cell focus‐inducing (MCF) mouse leukaemia viruses (MuLV), induces a wide spectrum of different lymphomas of T, B, and non‐T/non‐B cell types. The H‐2 complex has a marked influence on both the development of lymphoma incidence and lymphoma type. A study using the oncogenic MCF 1233 virus and a series of B10 congenic mice enabled the mapping of the following: (a) Resistance to the early development of T cell lymphomas is controlled by the H‐21‐A locus. (b) Susceptibility to early T cell lymphomagenesis is associated with an I‐A‐regulated low anti‐MCF 1233 envelope antibody response and persistent infection of the thymus. (c) B10 (H‐2b) mice, which are resistant to early T cell lymphomagenesis induced by MCF 1233 or other MuLV isolates, have high anti‐MuLV envelope antibody responses which are I‐A‐regulated. These mice develop more B cell lymphomas late in life in contrast to the early development of T cell lymphoma in B10.A (H‐2a mice. The possible response mechanisms which underlie these observations, including: (1) I‐A ‐regulated immunoselection against MuLV antigens expressed by (pre) leukaemic T cells, (2) aberrant expression of class II MHC antigens on some B cell lymphomas and (3) I‐A‐regulated chronic immunostimulation of MuLV‐expressing (pre) leukaemic B cells, are discussed.

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