Anti-Gag cytolytic T lymphocytes specific for an alternative translational reading frame-derived epitope and resistance versus susceptibility to retrovirus-induced murine AIDS in F(1) mice.

Murine AIDS (MAIDS) develops in susceptible mouse strains after infection with the LP-BM5 murine leukemia virus complex that contains causative defective, and ecotropic helper, retroviruses. We previously demonstrated that the MAIDS-resistant H-2(d) strains BALB/cByJ and C57BL/KsJ generate MHC class I (K(d)) restricted virus-specific CD8(+) cytolytic T lymphocytes (CTLs) that lyse cells expressing either defective or ecotropic gag proteins. In contrast, the congenic BALB.B and closely related C57BL/6J MAIDS-susceptible H-2(b) strains were unable to serve as a source of gag-specific CTLs (Schwarz and Green, 1994), suggesting that anti-gag CTLs might provide a basis for resistance to MAIDS. Although its susceptibility to MAIDS was unknown, the (BALB/c x C57BL/6J) F(1) (CBY6F(1)) strain could also produce H-2(d)-, but not H-2(b)-, restricted, anti-gag CTLs (Schwarz and Green, 1994). Because of this correlation between anti-gag CTLs and resistance to MAIDS, it was important to provide more direct evidence in support of CTL-mediated protection and to determine both the fine specificity of CByB6F(1) anti-gag CTLs, in comparison with the resistant C57BL/Ks and BALB/c strains, and the susceptibility of this F(1) strain to LP-BM5-induced MAIDS. We report here that no symptoms of MAIDS were observed in CBY6F(1) (H-2(dxb)) mice. For F(2) mice, in contrast to the high susceptibility of H-2(b/b) mice, 77% of H-2(d/d) and 81% of H-2(b/d) F(2) mice did not exhibit MAIDS after LP-BM5 infection. These results are in contrast to other published studies that concluded that susceptibility, rather than resistance, is dominant in F(1) (resistant x susceptible or susceptible x resistant) mice. We also show that CBY6F(1) anti-gag CTLs exhibit a fine specificity shared by the MAIDS-resistant BALB/c and C57BL/Ks strains, that is, the immunodominant gag epitope, SYNTGRFPPL, encoded by an alternative open reading frame. Together with our direct demonstration here that in vivo monoclonal antibody (mAb) depletion of CD8(+) T cells converts genetically resistant mice to MAIDS susceptibility, these data on the ability to mount anti-ORF2/SYNTGRFPPL, gag-specific CTL responses strongly suggest that CTLs are a primary factor in determining MAIDS resistance. Accordingly, given the K(d)-restricted nature of the CTLs, the main genetic determinant of resistance appeared to be the codominant expression of the resistant H-2(d) haplotype. Interestingly, however, 19% of H-2(d/b) and 23% of the H-2(d/d) F(2) mice had at least one clinical aspect of MAIDS, suggesting that a non-MHC genetic determinant(s) can negatively influence T-cell protection and thus disease outcome

An alternative translational reading frame encodes an immunodominant retroviral CTL determinant expressed by an immunodeficiency-causing retrovirus.

Recognition of virus-infected or transformed cells by CD8+ CTL requires a trimolecular complex composed of MHC class I, beta2-microglobulin, and a specific foreign peptide composed of 8 to 10 linear amino acids. The generation of such CTL epitopes has traditionally been thought to be from the primary open reading frame encoding the viral or tumor-associated proteins. In this report it is demonstrated that a viral CTL epitope can also be generated from an alternative reading frame. Using a combination of synthetic peptides and Sindbis or vaccinia expression systems, MHC class I Kd-restricted BALB/cByJ CTL directed against defective gag gene constructs of the LP-BM5 virus complex that causes murine AIDS were shown to have specificity for the antigenic peptide SYNTGRFPPL. This epitope is generated in a novel fashion from the second open reading frame (ORF2) of both the defective and ecotropic helper virus components of LP-BM5. Importantly, lysis of target cells expressing BM5 ecotropic helper, and/or defective viral gag, demonstrated that the SYNTGRFPPL epitope is generated during the course of a normal retroviral infection. Furthermore, MAIDS-resistant BALB/cByJ mice also generated secondary restimulated CTL specific for SYNTGRFPPL following in vivo priming with the LP-BM5 retroviral complex. These data suggest that retroviruses, and potentially other viruses and foreign genes, are capable of expressing T cell epitopes from alternative open reading frames. If one considers the influence of self peptides on T cell development, these "alternative reading frame-derived" peptides could provide an important additional influence on the functional T cell repertoire.