Analysis of human islet immunopeptidome reveals presentation of a modified PPI15-24 epitope that is targeted by a clonally distinct subset of CD8+ T cells.

CD8+T cells survey the repertoire of antigenic peptides on beta cells presented in the context of HLA-I. During insulitis, metabolic or inflammatory stress in the vicinity of beta cells induces local enzymatic activities and promotes the generation of post-translationally modified (PTM) neoantigens to which central immune tolerance is absent. PTM of antigens is known to increase the diversity of peptides presented in the periphery and as such can contribute to the loss of T cell tolerance, culminating in autoimmunity. Importantly, the natural repertoire of islet cell presented antigenic peptides has not been studied. Here, we used a reverse immunology strategy in combination with highly sensitive mass spectrometry to characterise the HLA-A*02:01 immunopeptidome of cytokine-conditioned human islets. We identified a PTM PPI_15-24 epitope that is naturally processed and presented by HLA-A*02:01 on human islet cells. The PTM PPI_15-24 epitope is generated by an islet cell resident enzyme, the activity of which can be induced by inflammatory stress. Intriguingly, despite very close structural similarity between A2/PPI_15-24 and A2/(PPI_15-24)^PTM, the highly promiscuous PPI_15-24-specific 1E6 CD8+ T cell clone does not recognise the PTM-PPI_15-24. However, tetramer staining experiments show that PTM-PPI_15-24 is a major target for circulating effector CD8+ T cells from HLA-A*02:01 patients with new onset of type 1 diabetes, indicating that it is a target for pathogenic autoreactivity. Work is ongoing to isolate and characterise patient derived CD8 T cells specific for PTM-PPI_15-24. This study highlights the fact that beta cell specific PTMs generated by an inflammation inducible enzyme can be targeted by autoreactive T cells clonally distinct from those targeting wild type epitopes. A better understanding of these T cells will aid in biomarker development and potentially in the development of therapeutic interventions.