Costimulation pathways are described as central in T cell activation and the control of autoimmune responses. We previously reported that NOD mice that are deficient for the icosl gene are protected from diabetes, but instead develop a spontaneous autoimmune neuromyopathy. The general phenotype of the neuromyopathy observed in ICOSL-/- NOD mice is globally similar to that observed in ICOS-/- and ICOS-/-ICOSL-/- double knockout NOD mice. The neuromyopathy is observed in 100% of female mice by the age of 35 weeks. The neuropathy remains limited to the peripheral nerve tissue. The disease is characterized by an infiltration of immune cells: CD4+ T cells, CD8+ T cells, dendritic cells and B lymphocytes, but does not extend to the central nervous system. A similar infiltrate is seen in muscles. Autoimmune neuromyopathy can be transfer to naive recipients by T lymphocytes. Transfer is achieved in NOD.scid recipient mice by CD4+ T-cells, although not by CD8+ T-cells, isolated from 35 week old ICOSL-/- NOD. The predominant role of CD4+T-cells is further demonstrated in this model by the observation that CIITA-/-ICOSL-/- NOD mice do not developed the neuromyopathy. By contrast, ȕ2m-/-ICOSL-/- NOD mice develop a neuromyopathy. We obtained evidence (in chimeric mice) that the interaction between antigen-presenting cells (APC) and T lymphocytes via ICOS/ICOSL is a prerequisite to the development of diabetes, while the loss of the interaction between T lymphocytes and APC play a key role in the development of nervous and muscular autoimmunity. Finally, the spectrum analysis of antibodies specificity in mouse ICOSL-/- with Western blot and mass spectrometry indicated the antigenic targets of myopathy. Altogether, our data indicate that the deviation of autoimmunity in NOD mice from the pancreas to muscles and the peripheral nervous system in the absence of ICOS/ICOSL signal is dependent on the loss of the physiological interaction between T cells and APC
