NOS2 and chronic obstructive pulmonary disease: Activated STAT1 signaling drives the expression of signature M1 genes such as inducible nitric oxide synthase (iNOS), CD86, monocyte chemoattractant protein 1 (MCP1) and interleukin-1β (IL-1β), which cause pulmonary inflammatory injuries, even apoptosis, exacerbate pulmonary damages of COPD [14].