DPP4 and chronic obstructive pulmonary disease: Pro-inflammatory enzymes cyclooxygenase-2 (COX-2), 5-lipoxygenase (5-LO), dipeptidyl peptidase 4 (DPP4), and leukotriene A4 hydrolase (LTA4H) and anti-inflammatory enzymes histone deacetylase 2 (HDAC2) and poly(ADP-ribose) polymerase-1 (PARP-1) have been demonstrated to be connected to COPD and underlying systemic inflammation [2,4,5,6].