Dipeptidyl peptidase that cleaves off N-terminal dipeptides from proteins having a Pro or Ala residue at position 2 (PubMed:24223149). Acts as a key inhibitor of caspase-1-dependent monocyte and macrophage pyroptosis in resting cells by preventing activation of NLRP1 and CARD8 (PubMed:27820798, PubMed:29396289). Sequesters the cleaved C-terminal part of NLRP1 and CARD8, which respectively constitute the active part of the NLRP1 and CARD8 inflammasomes, in a ternary complex, thereby preventing their oligomerization and activation (By similarity). The dipeptidyl peptidase activity is required to suppress NLRP1 and CARD8; however, neither NLRP1 nor CARD8 are bona fide substrates of DPP9, suggesting the existence of substrate(s) required for NLRP1 and CARD8 inhibition (By similarity). Release of an N-terminal dipeptide, Xaa-Yaa-|-Zaa-, from a polypeptide, preferentially when Yaa is Pro, provided Zaa is neither Pro nor hydroxyproline. Inhibited by the serine proteinase inhibitor 4-(2-aminoethyl)benzenesulphonyl fluoride (AEBSF), and by di-isopropylfluorophosphate (By similarity). Inhibited by Val-boroPro (Talabostat, PT-100), a non-selective inhibitor, which triggers pyroptosis in monocytes and macrophages (PubMed:27820798, PubMed:29396289). Val-boroPro inhibits activity by binding to the active site, mimicking a substrate-bound state, thereby displacing the C-terminal fragment of NLRP1, leading to activation of the NLRP1 inflammasome. In contrast, Val-boroPro does not directly displaces CARD8: it acts by promoting degradation of the N-terminal part of CARD8, leading to indirect disruption of the ternary complex (By similarity). Homodimer. Forms a ternary complex with NLRP1, composed of a DPP9 homodimer, one full-length NLRP1 protein, and one cleaved C-terminus of NLRP1 (NACHT, LRR and PYD domains-containing protein 1, C-terminus). Forms a ternary complex with CARD8, composed of a DPP9 homodimer, one full-length NLRP1 protein, and one cleaved C-terminus of CARD8 (Caspase recruitment domain-containing protein 8, C-terminus). In the ternary complex, only one subunit of the DPP9 homodimer is bound to NLRP1 or CARD8. Detected in kidney, skin, brain, thymus and liver (at protein level). Detected in hypoglossal cord and first branchial arch at 10.75 dpc (at protein level). Belongs to the peptidase S9B family. DPPIV subfamily.