Protease–proteoglycan complexes of mouse and human mast cells and importance of their β‐tryptase–heparin complexes in inflammation and innate immunity

Summary:  Approximately 50% of the weight of a mature mast cell (MC) consists of varied neutral proteases stored in the cell’s secretory granules ionically bound to serglycin proteoglycans that contain heparin and/or chondroitin sulfate E/diB chains. Mouse MCs express the exopeptidase carboxypeptidase A3 and at least 15 serine proteases [designated as mouse MC protease (mMCP) 1–11, transmembrane tryptase/tryptase γ/protease serine member S (Prss) 31, cathepsin G, granzyme B, and neuropsin/Prss19]. mMCP‐6, mMCP‐7, mMCP‐11/Prss34, and Prss31 are the four members of the chromosome 17A3.3 family of tryptases that are preferentially expressed in MCs. One of the challenges ahead is to understand why MCs express so many different protease–proteoglycan macromolecular complexes. MC‐like cells that contain tryptase–heparin complexes in their secretory granules have been identified in the Ciona intestinalis and Styela plicata urochordates that appeared approximately 500 million years ago. Because sea squirts lack B cells and T cells, it is likely that MCs and their tryptase–proteoglycan granule mediators initially appeared in lower organisms as part of their innate immune system. The conservation of MCs throughout evolution suggests that some of these protease–proteoglycan complexes are essential to our survival. In support of this conclusion, no human has been identified that lacks MCs. Moreover, transgenic mice lacking the β‐tryptase mMCP‐6 are unable to combat a Klebsiella pneumoniae infection effectively. Here we summarize the nature and function of some of the tryptase–serglycin proteoglycan complexes found in mouse and human MCs.