Immune monitoring of islet and pancreas transplant recipients.

Type 1 diabetes (T1D) is an autoimmune disease in which the insulin-producing beta-cells are destroyed. Islet or pancreas transplantation can restore insulin secretion and are established therapies for subgroups of T1D patients. Long-term insulin-independence is, however, hampered by recurrent autoimmunity and rejection. Accurate monitoring of these immune events is therefore of critical relevance for the timely detection of deleterious immune responses. The identification of relevant immune biomarkers of allo- and autoreactivity has allowed a more accurate monitoring of disease progression and responses to therapy at early stages, allowing proper therapeutic intervention, and possibly improvements in the success rate of islet and pancreas transplantation. This review describes the tools established and validated to monitor immune correlates of auto- and alloreactivity that associate with clinical outcome and identifies challenges that current immunosuppression strategies trying to preserve islet graft function face.

CD8 T cell autoreactivity to preproinsulin epitopes with very low human leucocyte antigen class I binding affinity.

Beta cells presenting islet epitopes are recognized and destroyed by autoreactive CD8 T cells in type 1 diabetes. These islet-specific T cells are believed to react with epitopes binding with high affinity to human leucocyte antigen (HLA) expressed on beta cells. However, this assumption might be flawed in case of islet autoimmunity. We evaluated T cell recognition of the complete array of preproinsulin (PPI) peptides with regard to HLA binding affinity and T cell recognition. In a comprehensive approach, 203 overlapping 9-10mer PPI peptides were tested for HLA-A2 binding and subjected to binding algorithms. Subsequently, a high-throughput assay was employed to detect PPI-specific T cells in patient blood, in which conditional HLA ligands were destabilized by ultraviolet irradiation and HLA molecules refolded with arrays of PPI peptides, followed by quantum-dot labelling and T cell staining. Analysis of patient blood revealed high frequencies of CD8 T cells recognizing very low HLA binding peptides. Of 28 peptides binding to HLA-A2, a majority was predicted not to bind. Unpredicted peptides bound mainly with low affinities. HLA binding affinity and immunogenicity may not correlate in autoimmunity. Algorithms used to predict high-affinity HLA peptide binders discount the majority of low-affinity HLA binding epitopes. Appreciation that peptides binding HLA with very low affinity can act as targets of autoreactive T cells may help to understand loss of tolerance and disease pathogenesis and possibly point to tissue-specific immune intervention targets.