Project Summary/Abstract Rheumatoid arthritis (RA) is an autoimmune disease marked by debilitating inflammation and destruction of synovial joints. Even with targeted therapies such as anti-TNF, few patients achieve remission and there is no cure. The residual disease and recurrent flares exhibited by RA patients suggest that bona fide resolution of synovial inflammation remains impaired. Improved knowledge of the mechanisms that resolve inflammation and restore synovial homeostasis is acutely needed. In this proposal, we study a novel pro-resolving mechanism in RA driven by fibroblasts within the synovial tissue. This differs from known pro-resolving mechanisms in synovium and elsewhere, which are mediated by leukocytes. Both in RA and other inflammatory diseases, stromal fibroblasts have emerged as powerful regulators of local immune responses in addition to modulating tissue architecture and fibrosis. Having previously identified key mechanisms by which fibroblasts control inflammatory activation in RA, we now examined whether fibroblasts play similarly instrumental roles in regulating inflammatory resolution. Through single-cell RNA sequencing analyses, we observed that synovial fibroblasts express higher levels of the AXL receptor tyrosine kinase during RA remission. AXL and other receptors in the TAM (TYRO3/AXL/MERTK) family have pro-resolving functions attributed to leukocytes. However, in synovium, we find that AXL is expressed markedly more highly by fibroblasts than leukocytes. We show that upon AXL activation, fibroblasts downregulate key inflammatory markers and also efferocytose apoptotic cells, contributing to their clearance. Here we will determine whether AXL signaling in synovial fibroblasts represents a novel, leukocyte-independent pathway driving inflammatory resolution. To accomplish this, in Aim 1, we define the effects of AXL on synovial fibroblast inflammatory polarization by identifying the key transcriptional and phosphorylated targets of AXL signaling. In Aim 2, we assess how AXL- mediated binding and uptake of apoptotic cells by synovial fibroblasts facilitates their transcriptional and metabolic reprogramming towards immunoregulatory or pro-resolving states. Finally, in Aim 3, we test whether targeted, conditional depletion of AXL in synovial fibroblasts exacerbates arthritis severity or delays its resolution in mouse models. Together, these studies will advance our knowledge of fibroblast function and regulation during inflammatory resolution, potentially paving the way for novel stromal-targeted, pro-resolving therapies that foster long-lasting remission in multiple inflammatory diseases.

Project Narrative Fibroblasts are cells that maintain the structural framework of most tissues, and many studies show that they can also promote and suppress tissue inflammation. This project will study how fibroblasts in rheumatoid arthritis develop the ability to suppress joint inflammation and enable joint healing when they are activated by specific signals. Our findings could aid in devising new therapies for rheumatoid arthritis and many other inflammatory diseases involving fibroblasts.