GETTING UNDER THE SKIN OF FIBROSIS: A FUNCTION FOR MAST CELLS IN THE PROGRESSION AND RESOLUTION OF DERMAL FIBROSIS, AND AN ANTI-FIBROTIC FUNCTION FOR KETOTIFEN ON DERMAL FIBROBLASTS

Abstract

Fibrosis is a pathological feature of many chronic inflammatory and fibroproliferative diseases, characterized by an excessive accumulation of extracellular matrix proteins within tissues and organs. This pathological accumulation can lead to loss of function and mortality. Fibrosis is associated with approximately 45% of the deaths in the Western world. The immune system is a key regulator of fibrosis, interacting with fibroblasts in the local microenvironment to modulate fibrotic outcomes. Mast cells are long-lived, tissue resident, sentinel immune cells with notable functions in health and disease, such as allergy. Mast cells are described to have contradictory roles in fibrosis, with mast cell-derived mediators demonstrating pro-fibrotic and anti-fibrotic potential. We evaluated the impact of mast cells on dermal fibrosis using a bleomycin-induced mouse model and importantly, the resolution of dermal fibrosis, which has not been well studied. Using two strains of mast cell-deficient mice and subsequent mast cell reconstitution experiments, we identified localized mast cell contributions promoting dermal fibrosis, but also better fibrosis resolution in the presence of mast cells. Notably, mast cell density was increased during the peak of fibrosis and further elevated during resolution. Using ketotifen to stabilize mast cells, it was determined that degranulation was instrumental in promoting dermal fibrosis. Continued administration of ketotifen during resolution impaired the resolving potential of the skin after cessation of injurious stimuli, demonstrating that degranulation-associated mediators were important in this process. Exogenous addition of prostaglandin D2 improved resolution of dermal fibrosis compared to control resolution conditions. Taken together, these results demonstrate that while mast cells promote dermal fibrosis, under appropriate microenvironmental conditions, mast cells contribute to processes of resolution. Ketotifen treatment demonstrated direct anti-fibrotic effects on human dermal fibroblasts activated with TGFβ1 in vitro, reducing expression of α-smooth muscle actin and other cytoskeletal and contractility-associated genes. Contractility of pro-fibrotic fibroblasts within collagen lattices were reduced with ketotifen treatment compared with control treatment Ketotifen reduced pro-fibrotic YAP/TAZ signaling and demonstrated partial inhibition of AKT phosphorylation. Overall, these findings highlight novel mechanisms of action of mast cells and potential therapeutic approaches for the resolution of fibrosis.