Project Summary/Abstract Graft versus host disease (GVHD) is an immune-mediated condition affecting many organs. It is categorized into acute and chronic subtypes. Eyes are affected in 60-90% of the patients with chronic GVHD resulting in significant visual morbidity. About half of the ocular GVHD (oGVHD) patients develop steroid- refractory conjunctival and lacrimal fibrosis and severe dry eye, sometimes with ocular perforation and loss of globe. Pathophysiology of oGVHD remains incompletely understood. The current proposal will investigate the role of host and donor macrophages, their phenotypic changes, and their crosstalk with fibroblasts and goblet cells as mechanisms underlying oGVHD-associated ocular surface damage, fibrosis, and dry eye. The long- term goal is to understand the immune-mediated pathophysiological basis of oGVHD for its prevention and better therapeutic management. This proposal will use two mouse models of allogeneic bone marrow transplantation to induce oGVHD. These models recapitulate many features of GVHD-associated ocular surface damage, including a decrease in tear volume, signs of corneal keratopathy, conjunctival fibrosis, and loss of goblet cells. Aim 1 will investigate whether irradiation, host macrophage activation, and the influx of donor marrow-derived macrophages initiate and perpetuate ocular surface injury in oGVHD. Aim 2 will identify whether profibrotic mediators released by macrophages cause transdifferentiation of conjunctival and lacrimal gland fibroblasts to elicit oGVHD-associated fibrosis. Aim 2 will also test the role of the fibroblast-mediated release of macrophage colony-stimulating factor-1 (CSF-1) and other chemokines in recruiting donor macrophages to the ocular surface. Furthermore, aim 2 will evaluate whether pexidartinib, a CSF-1 receptor inhibitor, can attenuate oGVHD by depleting conjunctival and lacrimal gland macrophages. Aim 3 will investigate whether rescue of goblet cell loss by IL-13 can mitigate oGVHD-associated dry eye. Aim 3 will also identify the macrophage receptors involved in mediating the biological effects of goblet cell mucins. The results of this study will provide data on the role of host-donor macrophages, their interaction with fibroblasts and goblet cells as underlying mechanisms in oGVHD pathology and will potentially identify novel therapeutic approaches for the management of oGVHD.