On the structure and function of granzymes, the deadly squad of cytotoxic lymphocytes

Granzymes are a family of trypsin-like serine proteinases (TLSP) produced by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. They play critical roles in immune defense by inducing diverse forms of programmed cell death (e.g., apoptosis, pyroptosis, and necroptosis) in infected or abnormal cells. Here, we review current information on the structural mechanisms underlying the activation, substrate specificity, and inhibition of the five human granzymes (GzmA, GzmB, GzmH, GzmK, and GzmM). We highlight how specific variations of the canonical trypsin fold allow these five proteinases to display entirely distinct substrate specificities. We emphasize the role of allosteric mechanisms, such as exosites and dimerization, in expanding their substrate recognition profiles far beyond simple P1 cleavage site requirements. We discuss the rapidly growing evidence on the contribution of dysregulated granzyme activity to various inflammatory and autoimmune diseases. Finally, we explore emerging therapeutic strategies targeting granzymes, aiming to either enhance their beneficial cytotoxic effects (e.g., in cancer immunotherapy and infection) or inhibit their pathological roles in chronic inflammation and autoimmunity. Despite significant progress, open questions remain regarding the full spectrum of physiological granzyme substrates, their precise synergistic actions in vivo, and the translational challenges of applying in vitro findings to complex clinical contexts.