Serpin A9
Serpin A9, commonly called proteinase inhibitor 9 (PI-9), is the principal intracellular inhibitor of granzyme B, the cytotoxic serine protease that killer lymphocytes inject into target cells to trigger apoptosis. PI-9 is unusual in the family for residing mainly in the cytoplasm and nucleus rather than being secreted, and its reactive center loop contains a rare P1 glutamate suited to granzyme B recognition. It is expressed in cytotoxic lymphocytes, dendritic cells, and placenta to protect these cells from bystander granzyme B damage, and tumors exploit PI-9 to escape immune killing. Recombinant Serpin A9 is used in granzyme B inhibition assays and cytotoxicity and immune-evasion research.
