Project Summary Sarcoidosis is an interstitial lung disease (ILD) of unknown etiology with striking disparities in clinical outcome. About 60% of patients will spontaneously resolve sarcoidosis over time, while the remainder will experience a gradual decline in lung function as a result of granulomatous inflammation transitioning to fibrosis. Although the role of CD4+ T cells in pulmonary sarcoidosis has been appreciated since 1981, the significance of CD8+ T cells has remained undervalued. Previous investigations have implicated the inhibitory receptor PD-1 as well as the costimulatory molecule CD28 in pulmonary sarcoidosis progression. Our preliminary findings revealed elevated frequencies of PD-1-expressing CD8+ T cells in sarcoidosis patients experiencing disease progression coinciding with functional consequences. PD-1 upregulation impaired the ability of sarcoidosis CD8+ T cells to proliferate. PD-1 neutralization restored CD8+ T cell proliferative capacity to healthy control levels. Longitudinal analysis demonstrated increased PD-1+CD8+ T cells among patients experiencing loss of lung function, but normal PD-1 expression and proliferative capacity in patients with spontaneous resolution. Our results also demonstrated significantly reduced CD28 expression in PD-1+CD8+ T cells compared to PD-1- CD8+ T cells. CD28 costimulation is essential for effective T cell immune responses. Loss of CD28 expression on CD8+ T cells has been shown to promote dermal fibrosis, but the impact of CD28 deficiency on CD8+ T cells in pulmonary fibrosis remains unknown. This proposal will identify the signaling mechanisms by which sarcoidosis PD-1+CD8+ T cells promote pulmonary fibrosis. Collectively, these observations support the hypothesis that PD-1 upregulation induces CD8+ T cell dysfunction through CD28 inactivation which in turn promotes lung fibrosis. To identify the mechanisms by which PD-1 enhances fibrosis, we propose the following investigation among sarcoidosis patients with disease resolution, pulmonary disease progression and healthy controls: Aim 1. To identify the mechanisms by which PD-1- mediated inhibition of CD28 promotes CD8+ T cell dysfunction. Aim 2. To assess how PD- 1-mediated inhibition of CD28 on CD8+ T cells promotes pulmonary fibrosis in vivo. Aim 2 will be evaluated using the previously published sarcoidosis murine model.