Chronic graft versus host disease (cGVHD), a systemic autoimmune syndrome, remains the major cause of morbidity and mortality of long-term survivors of allogeneic hematopoietic cell transplantation (HCT). During the past funding period, we used murine and humanized murine models of cGVHD and biospecimens from cGVHD patients to demonstrate that CD4+ T cells and B cells and their interactions in non-lymphoid tissues affected by cGVHD such as the liver, lung and skin play a critical role in the pathogenesis and persistence of the disease. The premise of this renewal proposal is that deeper mechanistic understanding of the interactions between non-circulating tissue-resident CD4+ memory T (Trm) and tissue-resident B (Brm) cells in GVHD target tissues will identify new therapeutic targets for prevention or treatment of cGVHD. In Aim 1, we will determine how murine and human Trm subsets such as PSGL1hi and PSGL1loCD4+ Trm cells mediate pathogenesis in target tissues. We will dissect how TCR-signaling triggered by donor- or recipient-type antigen-presenting cells and down-stream signaling and nuclear transcription factors such as STAT3, HOBIT/Blimp-1 and the tissue environmental alarmin IL-33 regulate Trm differentiation, expansion and function. Finally, we will attempt to develop a novel technology to target STAT3 in donor T cells, because we have observed that STAT3 deficiency in donor T cells effectively prevents cGVHD, with a marked reduction in the numbers and function of Trm cells in GVHD target tissues. In Aim 2, we will test whether murine and human Brm cells produce antigen-specific IgG to augment fibrosis in cGVHD target tissues. We will test whether Brm differentiation requires help from PSGL1loCD4+ T cells. We anticipate that targeting STAT3 in donor T cells will simultaneously prevent formation of pathogenic Trm and Brm cells. These studies are of high biological and translational significance and may lead to a paradigm shift in our understating of cGVHD pathogenesis and development of novel approaches for preventing and treating cGVHD.