Human Mast Cells Activate Fibroblasts: Tryptase is a Fibrogenic Factor Stimulating Collagen Messenger Ribonucleic Acid Synthesis and Fibroblast Chemotaxis

The effect of human mast cells on fibroblast activity was studied using an organotypic skin-equivalent culture system. Human mast cell-1 (HMC-1) cells were embedded in a collagen gel with neonatal dermal fibroblasts at a ratio of 1:4; keratinocytes then were allowed to stratify above this composite culture. Analysis of type a1(I) procollagen mRNA synthesis by in situ hybridization revealed a substantial increase in mRNA levels in the presence of mast cells and especially following degranulation, induced by calcium ionophore A23187. Tryptase, a major product of human mast cells, could substitute for mast cells in this culture system, up-regulating procollagen mRNA synthesis. Tryptase pretreated with the specific protease inhibitor bis(5-amidino-2-benzimidazo-lyl)methane (BABIM) markedly attenuated the collagen mRNA up-regulation. Further studies revealed HMC-1 cell sonicates stimulated fibroblast chemotaxis and procollagen mRNA synthesis. Inhibition of HMC-1 sonicates with either BABIM or a neutralizing mAb against tryptase resulted in significant reduction of fibroblast chemotaxis and procollagen mRNA, implying that tryptase accounted for the majority of HMC-1 sonicate activity. Tryptase directly stimulated fibroblast chemotaxis with optimal concentrations between 10 pM and 1 nM. The maximal response of optimal concentrations of tryptase was comparable with the known fibrogenic factor, TGF-β. Inhibition of tryptase with BABIM resulted in ∼50% reduction in chemotactic activity. Additional studies revealed that tryptase (0.3-3 nM) stimulated procollagen mRNA synthesis in confluent monolayers of dermal fibroblasts.