In a rat model of COPD, cigarette smoke exposure for 3.5 months elevated IL-4 and MMP-12, decreased interferon (IFN)-γ, increased the IL-4/IFN-γ ratio and MMP-12/tissue inhibitor of the metalloproteinases (TIMP-1) ratio, and worsened lung functions. Here, IFNG is linked to chronic obstructive pulmonary disease.