Dendritic cells (DCs) play a central role in the immune system as they drive activation of T lymphocytes by cognate interactions. However, as DCs express high levels of major histocompatibility complex class I, this intimate contact may also result in elimination of DCs by activated cytotoxic T lymphocytes (CTLs) and thereby limit induction of immunity. We show here that immature DCs are indeed susceptible to CTL-induced killing, but become resistant upon maturation with anti-CD40 or lipopolysaccharide. Protection is achieved by expression of serine protease inhibitor (SPI)-6, a member of the serpin family that specifically inactivates granzyme B and thereby blocks CTL-induced apoptosis. Anti-CD40 and LPS-induced SPI-6 expression is sustained for long periods of time, suggesting a role for SPI-6 in the longevity of DCs. Importantly, T helper 1 cells, which mature DCs and boost CTL immunity, induce SPI-6 expression and subsequent DC resistance. In contrast, T helper 2 cells neither induce SPI-6 nor convey protection, despite the fact that they trigger DC maturation with comparable efficiency. Our data identify SPI-6 as a novel marker for DC function, which protects DCs against CTL-induced apoptosis.

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 proteinase inhibitor (PI)9 is the only human protein able to inhibit the activity 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 directed against human PI9 was developed, which specifically stained PI9-transfected COS-7 cells, and was used for immunohistochemistry. Both in primary lymphoid organs and in inflammatory infiltrates, PI9 was present in different subsets of dendritic cells. Also T-lymphocytes in primary and organ-associated lymphoid tissues were PI9 positive. Endothelial cells of small vessels in most organs tested as well as the endothelial layer of large veins and arteries showed strong PI9 staining. Surprisingly, high PI9 protein expression was also found at immune-privileged sites like the placenta, the testis, the ovary, and the eye. These data fit with the hypothesis that PI9 is expressed at sites where degranulation of CTL or NK cells is potentially deleterious.

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In vitro studies suggest that resistance to chemotherapy-induced apoptosis might explain poor response to therapy in fatal cases. Actual execution of apoptosis depends on proper functioning of effector caspases, particularly caspase 3, and on the expression levels of apoptosisregulating proteins, including Bcl-2 and the recently identified granzyme Bspecific protease inhibitor 9 (PI9). Thus, high levels of caspase 3 activation should reflect proper functioning of the apoptosis pathways, resulting in chemotherapysensitive neoplastic cells and a favorable prognosis. We tested this hypothesis by quantifying numbers of tumor cells positive for active caspase 3, Bcl-2, and PI9, respectively, in pretreatment biopsies of systemic anaplastic large cell lymphoma (ALCL) patients and by comparing these numbers with clinical outcome. Activation of caspase 3 in more than 5% of the tumor cells was strongly correlated with a highly favorable outcome. High numbers of Bcl-2-and PI9-positive tumor cells were found to predict unfavorable prognosis. This prognostic effect was strongly related to anaplastic lymphoma kinase (ALK) status: ALK-positive ALCL had significantly higher levels of active caspase 3, while high expression of the antiapoptotic proteins Bcl-2 and PI9 was almost completely restricted to ALK-negative cases. In conclusion, high numbers of active caspase 3-positive tumor cells predict a highly favorable prognosis in systemic ALCL patients. Poor prognosis is strongly related to high numbers of Bcl-2-and PI9-positive neoplastic cells. These data support the notion that a favorable response to chemotherapy depends on an intact apoptosis cascade. Moreover, these data indicate that differences in prognosis between ALK-positive and ALKnegative ALCL might be explained by differences in expression of apoptosisinhibiting proteins. (Blood. 2002;99: 4540-4546)