Mast Cells as Sentinels : Role of serglycin and mast cell proteases in infection and inflammation

Abstract: Mast cells (MCs), normally classified into connective tissue MCs and mucosal MCs, are highly granulated cells found in the interface between the interior and the exterior environment of our body, e.g. skin, airways and gastro-intestinal tract. They react to bacteria, parasites, viruses, and allergens by degranulation and release of premade and newly synthesized inflammatory mediators. The MC-proteases (tryptases, chymases and carboxypeptidase A), histamine and serglycin (SG) proteoglycans are premade mediators. Among these, SG is also expressed in a variety of other immune and non-immune cells. Heparin and chondroitin sulphate glycosaminoglycan chains confer highly negative charge to SG, by which MC-proteases are retained in secretory granules. Deletion of SG cause impaired packing and storage of most MC-proteases. During challenge with Toxoplasma gondii the SG-deficient mice showed significant lower inflammatory cytokine levels in comparison to wild-type mice. Results were consistently similar in vitro, bringing forward the importance of SG in inflammatory cytokine and innate immune responses towards T. gondii. Infection with Trichinella spiralis in SG-/- mice caused increased intestinal enteropathy, a tendency of delayed worm expulsion and increased larval burden in the muscle tissue as compared to wild-type animals. An altered TH2 cytokine response was also observed, and all these effects were not repaired by wild-type MC reconstitution of the SG-/- mice. Altogether, our results suggest that SG is important for tissue homeostasis, and that SG expressing cells seem capable of switching from a SG-dependent storage mode to a SG-independent secretory mode upon infection.The chymase (MCPT4) expressed by connective tissue MC has been implicated to have a protective role during infection and in limiting inflammation. We explored a protective role by inducing T. gondii infection in the Mcpt4-null mice, and found MCPT4-mediated recruitment of neutrophils and eosinophils via control of cytokine signaling. Endogenous proteins “alarmins” released by dead cells can trigger tissue and cell damage. We conclusively show that chymase efficiently degrades Hsp70 both in vitro and in vivo and that the degradation of other alarmins, e.g. HMGB1, biglycan and IL-33 may also depend on chymase.