Abrogation of ICOS/ICOS ligand costimulation in NOD mice results in autoimmune deviation toward the neuromuscular system

NOD mice spontaneously develop insulin‐dependent diabetes around 10–40 wk of age. Numerous immune gene variants contribute to the autoimmune process. However, genes that direct the autoimmune response toward β cells remain ill defined. In this study, we provide evidence that the Icos and Icosl genes contribute to the diabetes process. Protection from diabetes in ICOS−/− and ICOSL−/− NOD mice was unexpectedly associated with the development of an autoimmune disorder of the neuro‐muscular system, characterized by myositis, sensory ganglionitis and, to a reduced extent, inflammatory infiltrates in the CNS. This syndrome was reproduced upon adoptive transfer of CD4+ and CD8+ T cells from diseased donors to naïve NOD.scid recipients. Our data further show that protection from diabetes results from defective activation of autoimmune diabetogenic effector T cells in ICOS−/− NOD mice, whereas acceleration of diabetes in BDC2.5 ICOS−/− NOD mice is induced by a dominant defect in Treg. Taken together, our findings indicate that costimulation signals play a key role in regulating immune tolerance in peripheral tissues and that the ICOS/ICOSL costimulatory pathway influences the balance between Treg and diabetogenic effector T cells.