DPP4 and chronic obstructive pulmonary disease: The key search terms were as follows: “DPP4 or DPPIV or CD26”, “DPP4 inhibitor or Vildagliptin or Saxagliptin or Sitagliptin or Alogliptin,” “respiratory disease or lung disease,” “lung cancer,” “pulmonary fibrosis,” “pulmonary hypertension,” “asthma,” “chronic obstructive pulmonary disease or COPD,” “SARS-CoV2 or COVID-19,” “Middle East Respiratory Syndrome or MERS”.