Post-translational modification of self-proteins has been shown to provoke adaptive immune responses in rheumatoid arthritis and celiac disease. Accumulating evidence suggests that similar mechanisms are also relevant in type 1 diabetes. Using HLA class II tetramers, we have identified several post-translationally modified peptides derived from known beta cell antigens, including GAD65, IA-2, and IAPP, which are recognized with significantly higher affinity by CD4+ T cells than their unmodified counterparts. Enhanced recognition of these peptides occurs by two complimentary mechanisms. Enzymatic citrullination and transglutamination of amino acids at anchor positions significantly enhances their binding to diabetes susceptible HLA class II proteins (including DRB1*04:01 and DQB1*03:02) creating neo-epitopes. Corresponding modifications of amino acids at T cell contact positions modulate their recognition by self-reactive T cell receptors creating altered-self epitopes. Modification at both an anchor position and a TCR contact position can occur simultaneously in the same peptide. Using HLA class II tetramers, we have verified that CD45RO+ memory T cells specific for modified epitopes can be detected directly ex vivo in the peripheral blood of subjects with type 1 diabetes. Naïve and memory T cells specific for modified epitopes can be also be detected in healthy subjects but these occur at significantly lower frequencies. Some post-translationally modified epitopes coincide with peptides that are recognized in their unmodified form, but distinct subsets of T cells recognize different forms of these epitopes. Therefore, it appears that post-translational modifications, which tend to occur under conditions of inflammation and ER stress, contribute to the progression of autoimmune diabetes by eliciting T cell responses to new epitope specificities that circumvent tolerance mechanisms.
This work is supported by JDRF award 17-2012-121