Kallikrein-related peptidases: bridges between immune functions and extracellular matrix degradation.

Kallikrein-related peptidases (KLKs) constitute a family of 15 highly conserved serine proteases encoded by the largest uninterrupted cluster of protease-encoding genes within the human genome. Recent studies, mostly relying on in vitro proteolysis of recombinant proteins, have suggested that KLK activities are regulated by proteolytic activation cascades that can operate in a tissue-specific manner, such as the semen liquefaction and skin desquamation cascades. The validity of KLK activation cascades in vivo largely remains to be demonstrated. Here, we focus on recent investigations showing that KLKs represent interesting players in the broader field of immunology based on their ability to bridge their inherent ability to degrade the extracellular matrix with major functions of the immune system. More specifically, KLKs assist in the infiltration of immune cells through the skin and the blood brain barrier, whereas they catalyze the generation of antimicrobial peptides by proteolytic activation and further processing of protein precursors. In an attempt to integrate current knowledge, we propose KLK-mediated pathways that are putatively involved in inflammation associated with skin wounding and central nervous system disorders, including multiple sclerosis. Finally, we present evidence of KLK participation in autoimmune diseases and allergies.