Type 1 diabetes (T1D) is caused by T cell mediated destruction of the insulin-producing β cells. CD4 T cell responses play a central role in β-cell destruction but the identity of the epitopes recognized by pathogenic CD4 T cells remains unknown. To address this we used a panel of diabetes triggering CD4 T cell clones isolated from non-obese diabetic (NOD) mice. Here we show that these pathogenic CD4 T cells target peptide ligands that are formed by covalent crosslinking of proinsulin peptides to other peptides present in β-cell secretory granules. These hybrid insulin peptides (HIPs) are highly antigenic for CD4 T cells and can be detected by mass spectrometry in β-cells. CD4 T cells from the residual pancreatic islets of two organ donors who had T1D also recognize HIPs. The discovery that autoreactive T cells target hybrid peptides may explain how immune tolerance is broken in T1D.

Autoreactive CD4+ T cells are involved in the pathogenesis of many autoimmune diseases, but the antigens that stimulate their responses have been difficult to identify and in most cases are not well defined. In the nonobese diabetic (NOD) mouse model of type 1 diabetes (T1D), we have identified a peptide WE14 from chromogranin A (ChgA) as the antigen for highly diabetogenic CD4+ T cell clones. Truncation and extension analysis showed that WE14 binds to the NOD mouse MHCII molecule, I-Ag7, in an atypical manner, occupying only the C-terminal half of the I-Ag7 peptide-binding groove. This finding extends the list of T cell antigens in T1D and supports the idea that autoreactive T cells respond to unusually presented self-peptides.

h Galectin-9 is a pleiotropic immune modulator affecting numerous cell types of innate and adaptive immunity. Patients with chronic infection with either hepatitis C virus (HCV) or HIV have elevated circulating levels. Limited data exist on the regulation of natural killer (NK) cell function through interaction with galectin-9. We found that galectin-9 ligation downregulates multiple immune-activating genes, including eight involved in the NK cell-mediated cytotoxicity pathway, impairs lymphokine-activated killing, and decreases the proportion of gamma interferon (IFN-␥)-producing NK cells that had been stimulated with interleukin-12 (IL-12)/IL-15. We demonstrate that the transcriptional and functional changes induced by galectin-9 are independent of Tim-3. Consistent with these results for humans, we find that the genetic absence of galectin-9 in mice is associated with greater IFN-␥ production by NK cells and enhanced degranulation. We also show that in the setting of a short-term (4-day) murine cytomegalovirus infection, terminally differentiated NKs accumulate in the livers of galectin-9 knockout mice, and that hepatic NKs spontaneously produce significantly more IFN-␥ in this setting. Taken together, our results indicate that galectin-9 engagement impairs the function of NK cells, including cytotoxicity and cytokine production.