Peptides of endogenous retrovirus Gag gene variants are not agonistic ligands for autoreactive T cells in non-obese diabetic mice

Abstract Endogenous retroviruses (ERV) have been isolated from the pancreatic islets of non-obese diabetic (NOD) mice. We previously demonstrated that a murine leukemia retrovirus–like ERV Gag protein was secreted together with exosomes by an insulinoma and by primary islet mesenchymal stem cells. Here, we further demonstrated that Gag is an effective antigen to stimulate IFN-gamma production by autoreactive T cells using recombinant Gag protein or virus-like particles (VLPs) expressing the Gag gene. However, candidate peptides of this Gag protein were not effective in stimulating the T cells. Upon analyses of ERV Gag genes expressed in the islets of prediabetic NOD mice, we were surprised to find multiple Gag gene variants with complete open-reading frames. These results suggested the expression of different endogenous proviruses, or alternatively, the detection of point mutations incurred during viral replication. A new batch of altered peptides was synthesized reflecting the variations among the Gag gene variants, and a few of them induced detectable amount of cytokine secretion from the T cells. But, the levels of IFN-gamma induced by the peptides were much lower than that induced by the Gag protein and the VLPs, suggesting a low affinity activation. This weak T-cell-stimulating activity of the Gag peptides was further confirmed by testing diabetogenic T-cell clones, as well as by using MHC tetramers loaded with candidate peptides. The data suggest that ERV Gag or its gene variants may trigger autoimmunity via a pathway that could be independent from direct presentation of viral peptides to autoreactive T cells.