Mechanisms governing inflammatory and invasive fibroblast pathology

Abstract

Fibroblasts are mesenchymal cells abundantly present in the connective tissue stroma of most organs. Though classically regarded as passive bystander cells providing structural support to tissues, fibroblasts have increasingly emerged as active orchestrators of both tissue homeostasis and disease. From regulating immunity and organizing tissue microarchitecture to guiding the differentiation of neighboring cells, fibroblasts assume myriad roles in controlling normal tissue development, maintenance, and repair. By comparison, during diseases such as autoimmunity, cancer, and fibrosis, fibroblasts acquire aberrant inflammatory, fibrogenic, tissue degradative, or hyperplastic phenotypes that drive pathology. The persistence of pathologic fibroblast states in diseased tissues has been linked to treatment nonresponse and poor clinical outcomes, both in cancers and in chronic inflammatory diseases including rheumatoid arthritis (RA). Understanding the mechanisms governing pathologic fibroblast behavior will enable us to develop improved, fibroblast-targeted therapeutics. The joint synovium, a connective tissue predominantly composed of fibroblasts, represents a valuable context in which to study fibroblast behavior and function. During RA, synovial fibroblasts dramatically expand in number and become pathologically activated to drive both chronic joint inflammation and the invasion and destruction of surrounding cartilage and bone. Critically, disease-associated fibroblasts are enriched in the synovia of the nearly 50% of RA patients who fail to achieve remission with conventional therapies. There is a sparse understanding of the mechanisms driving pathologic fibroblast activation and function. In this thesis, we applied a combination of experimental, computational, and functional genomics approaches to define new regulators of inflammatory and invasive fibroblast behavior.

First, we found that the Wnt signaling pathway, widely regarded as a developmental morphogen, functions as an unexpected and potent driver of inflammatory fibroblast activation in RA. Inhibition of Wnt signaling attenuates inflammatory disease in a murine model of arthritis and may represent a promising fibroblast-directed therapeutic strategy for RA. Next, we examined how transcription factors regulate the inflammatory and invasive functions of fibroblasts. We observed that rather than exhibiting maximal inflammatory and invasive states simultaneously, synovial fibroblasts in RA are polarized to become either more inflammatory or more invasive. We discovered that the transcription factor ARID5B drives this divergence by blunting the inflammatory activation of fibroblasts while augmenting their invasiveness. Finally, we performed a genetic deletion screen in primary synovial fibroblasts derived from RA patients to identify new regulators of pathologic fibroblast behavior. We characterized the roles of one integrin and two transcription factors in modulating key inflammatory and degradative factors produced by pathologically activated fibroblasts.

Together, our work presents a set of key insights into inflammatory and invasive fibroblast regulation that may inform the development of fibroblast-targeting therapies for RA and other chronic inflammatory diseases.