Glandular kallikrein in immunoregulation

Abstract

Glandular Kallikrein (GK) is an enzyme of the serine protease family capable of generatingbiologically active peptides by partially degrading various substrates. It is found in several tissues with particularly high concentrations in salivary glands, pancreas, kidney and the prostate gland. The physiological functions of this enzyme appear to vary according to the tissue in which it acts and the substrate(s) available in such tissues. Our interest in GK arose when we demonstrated that an immunoregulatory factor isolated from the salivary submandibular (SM) gland of rats, was rat GK (rGK). When added to cultures of lymphoid tissues, rGK induced a significant increase in lymphocyte proliferation manifested either in unstimulated cell cultures or in cultures of cells stimulated with suboptimal amounts of T cell mitogens. When injected subcutaneously, rGK induced a marked, albeit short lived, immunosuppressive effect in various experimental models including contact hyper-sensitivity to picryl chloride, allograft rejection, the production of antibody plaque-forming cells in the spleen of animals immunized with sheep red blood cells, and colagen-induced arthritis. All of the above effects were species-nonspecific inasmuch as murine GK and pig GK induced identical effects to rGK either in mice or in rats. Moreover, for these effects to take place, the enzymatic activity of GK had to be preserved. In the presence of inhibitors of proteolytic activity, both the in vivo and the in vitro immunoregulatory effects were abolished. This indicated that GK acts on a protein substrate to generate immunoregulatory peptides. If this is the case, the apparent conflict between in vitro stimulation and the suppreesion induced by subcutaneously injected GK can be explained as due to the accumulation of lymphocyte stimulating peptides in the in vitro cultures, while in vivo injection probably results in the rapid dispersal and degradation of such peptides leaving the organism depleted of substrate and unable to produce amounts of active peptides sufficient for a normal immune response. The effects described up to this point refer to in vitro phenomena or to the subcutaneousadministration of GK. The fact that GK is found in high concentration in salivary glands and is secreted in saliva suggests that a significant physiological function of GK may occur following external salivary secretion. For this reason, we also tested the effect of orally administered GK in rats. Oral GK was shown to be required for the induction of tolerance to orally administered antigens. Rats subjected to the surgical removal of the SM gland (SMX) did not develop tolerance to oral collagen given in doses that induced significant tolerance in normal controls. The oral administration of GK in the SMX animals fully restored the capacity to develop oral tolerance. This suggests that salivary GK is responsible for homeostasis in the mucosa associated lymphoid tissue which is responsible for the development of tolerance to oral antigens. The observation that GK may be used to enhance the induction of oral tolerance holds promises for the therapuetic application of oral GK in autoimmune diseases.