The granzyme B inhibitor, PI-9, is present in endothelial and mesothelial cells, suggesting that it protects bystander cells during immune responses

Proteinase inhibitor 9 (PI-9) is a 42-kDa human intracellular serpin presen t in cytotoxic lymphocytes (CLs). PI-9 is an extremely efficient inhibitor of the pro-apoptotic CL granule proteinase granzyme B and is thought to fun ction in the cytosol of CLs to protect against apoptosis induced by endogen ously expressed or released granzyme B, particularly during target cell kil ling. Here we show by immunohistochemistry that PI-9 is also present in end othelial cells, in every tissue examined. Cultured endothelial cells expres s functional PI-9 (as assessed by binding to recombinant granzyme B) locali zed to the cytoplasm and nucleus. Immunohistochemistry also showed PI-9 in mesothelial cells, and this was confirmed by analysis of primary cells cult ured from pleural and serous effusions. Granzyme B expression was not detec ted in either endothelial or mesothelial cells. In both cell types, PI-9 is up-regulated at the mRNA and protein level by exposure to the phorbol este r PMA, consistent with a response to inflammatory stimuli. We postulate tha t PI-9 is present in these lining cell types to protect against misdirected , free granzyme B released during a local immune response. (C) 2001 Academi c Press.