Targeting PI3Kδ function for amelioration of murine chronic graft‐versus‐host disease

Chronic graft‐versus‐host disease (cGVHD) is a leading cause of morbidity and mortality following allotransplant. Activated donor effector T cells can differentiate into pathogenic T helper (Th)‐17 cells and germinal center (GC)–promoting T follicular helper (Tfh) cells, resulting in cGVHD. Phosphoinositide‐3‐kinase‐δ (PI3Kδ), a lipid kinase, is critical for activated T cell survival, proliferation, differentiation, and metabolism. We demonstrate PI3Kδ activity in donor T cells that become Tfh cells is required for cGVHD in a nonsclerodermatous multiorgan system disease model that includes bronchiolitis obliterans (BO), dependent upon GC B cells, Tfhs, and counterbalanced by T follicular regulatory cells, each requiring PI3Kδ signaling for function and survival. Although B cells rely on PI3Kδ pathway signaling and GC formation is disrupted resulting in a substantial decrease in Ig production, PI3Kδ kinase‐dead mutant donor bone marrow–derived GC B cells still supported BO cGVHD generation. A PI3Kδ‐specific inhibitor, compound GS‐649443, that has superior potency to idelalisib while maintaining selectivity, reduced cGVHD in mice with active disease. In a Th1‐dependent and Th17‐associated scleroderma model, GS‐649443 effectively treated mice with active cGVHD. These data provide a foundation for clinical trials of US Food and Drug Administration (FDA)–approved PI3Kδ inhibitors for cGVHD therapy in patients