KDELR2 and chronic obstructive pulmonary disease: Using ELISA and immunofluorescence staining, it was seen that downregulation of KDELR2 caused a reduction in MUC5AC levels in both bronchial epithelium and BALF (Figure 4B,C,E), which supported the hypothesis that KDELR2 might participate in MUC5AC hypersecretion in COPD.