Complement is viewed as a critical serum-operative component of innate immunity, with processing of its key component, C3, into activation fragments C3a and C3b confined to the extracellular space. We report here that C3 activation also occurred intracellularly. We found that the T cell-expressed protease cathepsin L (CTSL) processed C3 into biologically active C3a and C3b. Resting T cells contained stores of endosomal and lysosomal C3 and CTSL and substantial amounts of CTSL-generated C3a. While "tonic" intracellular C3a generation was required for homeostatic T cell survival, shuttling of this intracellular C3-activation-system to the cell surface upon T cell stimulation induced autocrine proinflammatory cytokine production. Furthermore, T cells from patients with autoimmune arthritis demonstrated hyperactive intracellular complement activation and interferon-γ production and CTSL inhibition corrected this deregulated phenotype. Importantly, intracellular C3a was observed in all examined cell populations, suggesting that intracellular complement activation might be of broad physiological significance.

Cathepsins were originally identified as proteases that act in the lysosome. Recent work has uncovered nontraditional roles for cathepsins in the extracellular space as well as in the cytosol and nucleus. There is strong evidence that subspecialized and compartmentalized cathepsins participate in many physiologic and pathophysiologic cellular processes, in which they can act as both digestive and regulatory proteases. In this review, we discuss the transcriptional and translational control of cathepsin expression, the regulation of intracellular sorting of cathepsins, and the structural basis of cathepsin activation and inhibition. In particular, we highlight the emerging roles of various cathepsin forms in disease, particularly those of the cardiac and renal systems.

Multiple types of degradative enzymes, including cathepsins of the cysteine protease family, have been implicated in the regulation of angiogenesis and invasion during cancer progression. Several cysteine cathepsins are up-regulated in a mouse model of pancreatic islet cell carcinogenesis (RIP1-Tag2), and tumor progression is impaired following their collective pharmacologic inhibition. Using null mutations of four of the implicated cysteine cathepsins, we have now dissected their individual roles in cancer development. Mutants ofcathepsins BorSimpaired tumor formation and angiogenesis, whilecathepsin BorLknockouts retarded cell proliferation and tumor growth. Absence of any one of these three genes impaired tumor invasion. In contrast, removal ofcathepsin Chad no effect on either tumor formation or progression. We have identified E-cadherin as a target substrate of cathepsins B, L, and S, but not cathepsin C, potentially explaining their differential effects on tumor invasion. Furthermore, we detected analogous increases in cathepsin expression in human pancreatic endocrine neoplasms, and a significant association between increased levels of cathepsins B and L and tumor malignancy. Thus individual cysteine cathepsin genes make distinctive contributions to tumorigenesis.

Autodigestion of the pancreas by its own prematurely activated digestive proteases is thought to be an important event in the onset of acute pancreatitis. The mechanism responsible for the intrapancreatic activation of digestive zymogens is unknown, but a recent hypothesis predicts that a redistribution of lysosomal cathepsin B (CTSB) into a zymogen-containing subcellular compartment triggers this event. To test this hypothesis, we used CTSB-deficient mice in which the ctsb gene had been deleted by targeted disruption. After induction of experimental secretagogue–induced pancreatitis, the trypsin activity in the pancreas of ctsb^–/– animals was more than 80% lower than in ctsb^+/+ animals. Pancreatic damage as indicated by serum activities of amylase and lipase, or by the extent of acinar tissue necrosis, was 50% lower in ctsb^–/– animals. These experiments provide the first conclusive evidence to our knowledge that cathepsin B plays a role in intrapancreatic trypsinogen activation and the onset of acute pancreatitis.