Local triggering of the ICOS coreceptor by CD11c⁺ myeloid cells drives organ inflammation in lupus

The inducible Tcell costimulator (ICOS) is a potent promoter of organ inflammation in murine lupus. ICOS stimulates T follicular helper cell differentiation in lymphoid tissue, suggesting that it might drive autoimmunity by enhancing autoantibody production. Yet the pathogenic relevance of this mechanism remains unclear. It is also unknown whether other ICOS-induced processes might contribute to lupus pathology. Here we show that selective ablation of ICOS ligand (ICOSL) in CD11c⁺ cells, but not in B cells, dramatically ameliorates kidney and lung inflammation in lupus-prone MRL.Fas^lpr mice. Autoantibody formation was largely unaffected by ICOSL deficiency in CD11c⁺ cells. However, ICOSL display by CD11c⁺ cells in inflamed organs had a nonredundant role in protecting invading Tcells from apoptosis by elevating activity of the PI3K-Akt signaling pathway, thereby facilitating Tcell accrual. These findings reveal a mechanism that locally sustains organ inflammation in lupus.