Mast Cell Influence in the Progression of Post-burn Hypertrophic Scar Pathophysiology

Painful, motion-limiting hypertrophic scars (HTS) form after protracted wound healing in patients with severe full-thickness burns and pose difficult treatment challenges. Despite substantial reduction in postburn mortality, long-term outcomes of HTS are the most serious therapeutic challenge following severe burn injury. Investigations into cellular influences in postburn scar pathophysiology are constrained to dermal fibroblast activity in the wound and signaling among inflammatory cell populations such as neutrophils, macrophages, and T-lymphocytes. However, mast cells are known to initiate fibroproliferative signaling in other fibrotic pathologies and, their influence in burn wounds is poorly understood. In the present investigation, we show increased mast cell density and dissemination throughout pediatric postburn scars concomitant with elevated serum tryptase concentrations over time in pediatric burn patients.
Using in vitro approaches, we demonstrate tryptase’s ability to activate proliferation in primary post-burn HTS fibroblasts compared to that of non-burned skin fibroblasts. Tryptase, an abundant mast cell protease, directly cleaves and activates the protease-activator receptor-2 (PAR2) and activation has proliferative and fibrotic consequences. We demonstrate that PAR2 is highly expressed in post-burn HTSs and, activation sustains fibroproliferative signaling. Co-culture with post-burn HTS fibroblasts and human mast cells elevated expression of collagen-1 and α-smooth muscle actin, facilitating a myofibrotic shift. Most importantly, PAR2 blockade or PAR2 mRNA knockdown significantly attenuated mast cell-induced fibrotic phenotype in post-burn HTS fibroblasts. We also demonstrate that mast cell stabilization minimizes HTS formation in a post-burn in vivo model. Cromolyn sodium (CS) is an FDA-approved mast cell stabilizer that significantly inhibits degranulation and has been used to successfully to relieve detrimental symptoms of mast cell activation. Here, we applied a 4% topical emulsification of CS to post-burn HTSs of red Duroc pigs. CS significantly reduced mast cell density and scar severity over time. Scar height, volume, and collagen deposition were all lessened considerably compared to vehicle treatment alone or autologous split-thickness skin grafts. Additionally, we show that mast cell stabilization decreases fibroproliferation by impeding tryptase-induced PAR2 signaling. Together, this evidence suggests that localized mast cell stabilization, in combination with current therapies, may be an effective approach to reduce pathologic scarring following a severe burn.