The central role of antigen presentation in islets of Langerhans in autoimmune diabetes.

The islets of Langerhans normally contain resident antigen presenting cells (APCs), which in normal conditions are mostly represented by macrophages, with a few dendritic cells (DC). We present here the features of these islet APCs, making the point that they have a supportive function in islet homeostasis. Islet APCs express high levels of major histocompatibility complexes (MHC) molecules on their surfaces and are highly active in antigen presentation in the autoimmune diabetes of the NOD mouse: they do this by presenting peptides derived from molecules of the β-cells. These APCs also are instrumental in the localization of diabetogenic T cells into islets. The islet APC present exogenous peptides derived from secretory granules of the β-cell, giving rise to unique peptide-MHC complexes (pMHC) that activate those non-conventional T cells that bypass thymus selection.