The granzyme B inhibitor, protease inhibitor 9, is mainly expressed by dendritic cells and at immune-privileged sites

Granzyme B is released from CTLs and NK cells and an important mediator of CTL/NK-induced apoptosis in target cells. The human intracellular serpin pr oteinase inhibitor (PI)9 is the only human protein able to inhibit the acti vity of granzyme B. As a first step to elucidate the physiological role of PI9, PI9 protein expression in various human tissues was studied. A mAb dir ected against human PI9 was developed, which specifically stained PI9-trans fected COS-7 cells, and was used for immunohistochemistry. Both in primary lymphoid organs and in inflammatory infiltrates, PI9 was present in differe nt subsets of dendritic cells. Also T-lymphocytes in primary and organ-asso ciated lymphoid tissues were PI9 positive. Endothelial cells of small vesse ls in most organs tested as well as the endothelial layer of large veins an d arteries showed strong PI9 staining. Surprisingly, high PI9 protein expre ssion was also found at immune-privileged sites like the placenta, the test is, the ovary, and the eye. These data fit with the hypothesis that PI9 is expressed at sites where degranulation of CTL or NR cells is potentially de leterious.