Dipeptidyl Peptidase I Is Enriched in Granules of in Vitro- and in Vivo-Activated Cytotoxic T Lymphocytes

Recent studies have suggested that dipeptidyl peptidase I (DPPI) is the major post-translational processing enzyme responsible for generating activated myeloid and lymphoid granule serine proteases. The current studies assessed the relative levels of DPPI and granzyme A (BLT esterase) in B6 anti-H-2^d-specific CTL generated in mixed lymphocyte cultures (in vitro-activated CTL), by infusion of B6 spleen cells into irradiated H-2^d mice (graft-vs-host, GVH CTL) or by 1° and 2° peritoneal immunization of B6 mice with P815 (H-2^d) cells (PE CTL). In contrast to low levels of DPPI activity in unstimulated CD4⁺ spleen T cells, both unstimulated CD8⁺ spleen T cells and in vitro-activated CTL populations were several-fold enriched in DPPI activity, while PE CTL and GVH CTL expressed even higher levels of DPPI. Depletion of DPPI-enriched cells by treatment with Leu-Leu-OMe resulted in loss of cytolytic effector function from each CTL population. However, PE CTL and GVH CTL were more sensitive to the toxicity of Leu-Leu-OMe than were in vitro-activated CTL. While standard BLT esterase assays detected much higher levels of this serine protease activity in GVH CTL or in vitro-activated CTL than in PE CTL, levels of BLT esterase activity significantly above the basal levels present in unstimulated CD8⁺ or CD4⁺ T lymphocytes were found in association with immunoreactive granzyme A in lysates of PE CTL. In both PE CTL and in vitro-activated CTL, DPPI and BLT esterase activity co-localized in the granule fraction of cell lysates, and similar percentages of total cellular BLT esterase and DPPI were exocytosed upon cross-linking of surface CD3. Thus, both in vivo- and in vitro-activated CTL were found to possess functional granules containing readily detectable albeit somewhat different levels of DPPI and granzyme A activity.