Lymphomas are sensitive to perforin-dependent cytotoxic pathways despite expression of PI-9 and overexpression of bcl-2

There is considerable interest in immunotherapeutic approaches for lymphoma. The expression of proteinase inhibitor 9 (PI-9), a molecule that inactivates granzyme B, is considered an immune escape mechanism in lymphoma. Further, lymphomas frequently overexpress the antiapoptotic molecule bcl-2, which is able to inhibit perforin-dependent cytotoxic pathways. In this study, the impact of PI-9 and bcl-2 expression on the sensitivity of lymphomas to T- and natural killer (NK) cell-mediated cytotoxicity was analyzed. We found PI-9 expression in 10 of 18 lymphoma cell lines and in 9 of 14 primary lymphomas. Overexpression of bcl-2 was found in 8 of 18 cell lines and in 12 of 14 primary lymphomas. All lymphoma cells were sensitive to cytolysis by specific T cells and cytokine-activated NK cells, and no difference in sensitivity was observed with respect to PI-9 or bcl-2 expression. Cytolysis was mediated predominantly through perforin-dependent pathways despite expression of PI-9 and bcl-2. Interestingly, the majority of lymphoma cells were resistant to cytolysis by resting allogeneic NK cells. This was due to the failure of lymphomas to induce degranulation of resting NK cells. These results show that resistance to perforin-dependent pathways is not a relevant immune escape mechanism in lymphoma and therefore is unlikely to impair clinical outcome of immunotherapeutic approaches.