Lack of T-cell mediated cytotoxicity in M-MSV system depending on H-2 haplotypes

A Large Number of T Lymphocytes Recognize Moloney-Murine Leukemia Virus-Induced Antigens, but a Few Mediate Long-Lasting Tumor Immunosurveillance

The CD8(+) T cell response to Moloney-murine leukemia virus (M-MuLV)-induced Ags is almost entirely dominated by the exclusive expansion of lymphocytes that use preferential TCRVbeta chain rearrangements. In mice lacking T cells expressing these TCRVbeta, we demonstrate that alternative TCRVbeta can substitute for the lack of the dominant TCRVbeta in the H-2-restricted M-MuLV Ag recognition. We show that, at least for the H-2(b)-restricted response, the shift of TCR usage is not related to a variation of the immunodominant M-MuLV epitope recognition. After virus immunization, all the potentially M-MuLV-reactive lymphocytes are primed, but only the deletion of dominant Vbeta rescues the alternative Vbeta response. The mechanism of clonal T cell "immunodomination" that guides the preferential Vbeta expansion is likely the result of a proliferative advantage of T cells expressing dominant Vbeta, due to differences in TCR affinity and/or cosignal requirements. In this regard, a CD8 involvement is strictly required for the virus-specific cytotoxic activity of CTL expressing alternative, but not dominant, Vbeta gene rearrangements. The ability of T cells expressing alternative TCRVbeta rearrangements to mediate tumor protection was evaluated by a challenge with M-MuLV tumor cells. Although T cells expressing alternative Vbeta chains were activated and expanded, they were not able to control tumor growth in a long-lasting manner due to their incapacity of conversion and accumulation in the T central memory pool.

Three spontaneous H-2Db mutants are generated by genetic micro-recombination (gene conversion) events. Impact on the H-2-restricted immune responsiveness

Sequence analysis of the mutant Dbm13, Dbm14, and Dbm24 genes indicate that they differ from the parental Db gene by 4, 1, and 8 nucleotides, respectively. The mutant sequences substituted into Dbm13 and Dbm24 are identical to those found in the Kb gene, at the homologous positions. Thus, similar to the Kb gene, the Db gene is able to undergo micro-recombination (gene conversion) events with other class I genes. Such data suggest that micro-recombination events could be an important mechanism for the diversification of all H-2 genes. The Db mutant products share a common theme: the alterations in all occur at amino acid residues whose side chains in the homologous class I HLA-A2 molecule project into the postulated peptide antigen-binding cleft, and hence, would be expected to alter the binding of foreign or self peptides. Due to such changes, the bm14 mouse has become a nonresponder in the CTL response to Moloney murine leukemia virus (M-MuLV), as the alteration of one amino acid residue at position 70 (a Gln to His) is sufficient to entirely abrogate the cell-mediated response to the virus. On the other hand, the bm13 mouse has shifted the major part of its M-MuLV restriction to Kb, a profound alteration in CTL responsiveness due to the alteration of three amino acids (Leu to Gln at 114, Phe to Tyr at 116, and Glu to Asp at 119) in a peptide stretch of beta-pleated sheet structure lining the bottom of the antigen-binding cleft. Thus, study of these mutants reveals that, in one step, micro-recombination at the genetic level has resulted at the protein level in profound changes in the immune response to viral infection. Such a mechanism operating at the population level can be a driving force during evolution for modulating the character of CTL immunity.

Leukemia-cell rejection due to T-region encoded antigens

The role of H-2- and T-region products in determining allogeneic cell rejection was evaluated in H-2 congenic and recombinant mice by transplanting A1ATH and A6ATL leukemia cell lines induced in A.TH and A.TL strains, respectively, by Moloney murine leukemia virus.--In K- or D-region incompatible hosts transplant failure was observed, while in I + T-region incompatible hosts either rejection or prolonged survival was seen. In mice preimmunized with spleen cells from I- and/or T-region incompatible donors, leukemia cells were rejected by mice immune only to T-region products, and accepted by mice immune only to I-region products.--Cell-mediated cytotoxicity studies confirmed in vivo results. Secondary CTLs specifically directed against I-region products did not lyse the A1ATH and A6ATL cells, and secondary CTLs from A.TH and A.TL mice sensitized against A6ATL and A1ATH cells respectively exerted a lytic action specific for T-region products, while no activity was observed against I determinants.--The data suggest that tumor-transplant rejection may also be governed by histocompatibility antigens encoded in the T region.

Cytotoxic T cell response against lymphoblasts infected with Moloney (Abelson) murine leukemia virus. Methodological aspects and H-2 requirements

A method for infection of lymphocytes with Moloney(Abelson) murine leukemia virus [M(A)-MuLV] is described. Only lymphoblasts obtained after stimulation of normal spleen cells by the B cell mitogen lipopolysaccharide (LPS) were satisfactory targets for virus-specific, secondary cytotoxic T lymphocytes (CTL), whereas spleen cells stimulated by the T cell mitogen concanavalin A were not. The secondary CTL response against M(A)-MuLV could be efficiently measured using M(A)-MulV-infected LPS blasts as stimulating cells for secondary in vitro restimulation and as target cells for virus-specific destruction. Cold target inhibition demonstrated virus specificity of CTL. The T cell character of the cytotoxic cells was demonstrated by their sensitivity to anti-Thy-1.2 treatment. Using syngeneic virus-infected LPS blasts as target and stimulator, CTL responses were measured with effector cells from C57BL mice of the H-2b haplotype and of recombinant haplotypes sharing either K or D alleles with H-2b. In analogy with previous studies on Moloney virus-specific CTL, it was observed that C57BL/6 (H-2b) effector cells predominantly lysed Db-compatible, virus-infected target cells; B10.A(5R), (KbDd) effector cells showed a poor CTL response against syngeneic, virus-infected target cells. The combined findings indicate the existence of an Ir gene in the H-2D region regulating the CTL response against Moloney leukemia virus.