Modulation of the granzyme B inhibitor proteinase inhibitor 9 (PI‐9) by activation of lymphocytes and monocytes in vitro and by Epstein–Barr virus and bacterial infection

Summary Proteinase inhibitor 9 (PI‐9) is an intracellular serpin expressed in lymphocytes and monocyte‐derived cells. It is the only known endogenous natural antagonist of granzyme B (GrB), and its proposed function is protection of cells from misdirected GrB. We have studied the regulation of PI‐9 in primary peripheral blood mononuclear cells (PBMCs) following ex‐vivo stimulation, and in PBMCs from patients suffering from viral or bacterial infections. By intracellular flow cytometry, we found identical PI‐9 expression in all lymphocyte subsets, lower levels in monocytes and none in granulocytes. PI‐9 was stable for 48 h in the presence of cycloheximide, indicating slow protein turnover. Incubation of PBMCs with several stimuli including lipopolysaccharide (LPS) led to up‐regulation in the monocyte, but not the lymphocyte fraction, within 48 h, inhibitable by the NF‐κB inhibitor pyrrolidin dithiocarbamate (PTDC). Up‐regulation of PI‐9 was observed in lymphocytes and monocytes of patients with acute Epstein–Barr virus (EBV), but not bacterial infection. Preterm infants had similar PI‐9 expression as adults in monocytes, but lower in lymphocytes, decreasing during bacterial infection. Taken together, our data indicate that PI‐9 is rapidly up‐regulated upon stimulation of monocytes, but not lymphocytes. By protecting monocytes and macrophages from misdirected GrB in the inflammatory process, PI‐9 might be involved in the regulation of antigen presentation.