PIGR and chronic obstructive pulmonary disease: This is corroborated by previous studies that showed decreased PIGR levels and impaired host defense upon chronic smoke exposure [65], development of COPD-like pathology in polymeric immunoglobulin receptor (pIgR) deficient mice [66] and decreased pIgR levels in COPD patients, importantly in incompletely differentiated and squamous metaplasia dominated airway epithelial regions [24,67].