CAR19 Tregs treat murine chronic graft-versus-host disease through immune suppression without measurable B cell cytolysis.

Chronic graft-versus-host disease (cGVHD) remains a major cause of morbidity and mortality after allogeneic hematopoietic transplantation. cGVHD pathophysiology involves cooperation between T follicular helper cells (TFHs) and germinal center B cells (GCBs), allo- and autoantibody depositions in cGVHD tissues, and fibrosis. We evaluated human CD19-directed chimeric antigen receptor (CAR19) T cell therapy in a clinically relevant murine cGVHD model with bronchiolitis obliterans syndrome (BOS). Although CD8+ CAR19 T cells effectively reduced peripheral B cell and GCB frequencies, pulmonary function was unimproved. In contrast, a single infusion of CAR19 CD4+ regulatory T cells (Tregs) mitigated ongoing pulmonary disease and modulated germinal centers (GCs) associated with reduced TFH frequencies compared with control Tregs but without measurable B cell depletion. Compared with EGFR Treg infusion, mice receiving CAR19 Tregs exhibited enhanced suppression of B cell activation and preserved splenic architecture, and CAR19 Treg infusion provided greater opportunities for interaction with CD19+ B cells at the B cell follicle boundary zones. Taken together with the absence of detectable B cell cytolysis, these findings were most consistent with GC suppression rather than B cell depletion as the dominant mechanism. Overall, our findings suggest that CAR19 Tregs represent a promising and safe cGVHD/BOS therapeutic strategy, offering immunosuppressive benefits and improved disease outcomes that may be more limited with CD8+ CAR19 T cell treatment.