Dipeptidyl Peptidase 9 in Human Leukocytes: Novel Inhibitors Induce Caspase-1- and Gasdermin-Dependent Cell Death

Dipeptidyl peptidase 9 (DPP9) is an intracellular serine protease with key roles in the immune response. In human leukocytes, DPP9 interacts with caspase recruitment domain-containing protein 8 (CARD8) to form an inhibitory complex that prevents spontaneous inflammation. Inhibition or depletion of DPP9 activates CARD8 inflammasome signaling and triggers lytic cell death. Here, we examined the effects of two novel DPP9-targeting inhibitors, cpd-42 and cpd-6a, on the viability of human leukocytes. Both compounds induced dose-dependent lytic cell death in myeloid leukemia cell lines without a concurrent increase in IL-1β or IL-18 secretion. We confirmed that DPP9, caspase-1, gasdermin D (GSDMD) and gasdermin E (GSDME) contribute to the cell death observed with cpd-42 and cpd-6a. In addition, both inhibitors triggered lytic cell death in isolated human monocytes, monocyte-derived macrophages, monocyte-derived dendritic cells, and T cells, with activated T cells showing the lowest sensitivity. Examination of DPP activity revealed upregulation of DPP8/9 activity upon dendritic cell differentiation. Lastly, we found that DPP9 can localize to both the cytoplasm and the nucleus of isolated human leukocytes, warranting further evaluation of DPP9's nuclear roles.