Table 1. The search strategy summary.

Items	Specification
Date of search	2022.10.01
Databases searched	PubMed
Search terms used	(DPP4) AND (Lung Diseases)
	(DPP9) AND (Lung Diseases)
Timeframe	1990–2022
Inclusion and exclusion criteria	Inclusion: mainly in English; DPP4/9 articles related to respiratory diseases, including COPD, asthma, pneumonia, lung cancer, PF, tuberculosis, and PH
	Exclusion: thoracic surgery-type diseases, no restrictions on article types
Selection process	Primarily selected for DPP4/9 association with lung disease, independently performed

DPP, dipeptidyl peptidase; COPD, chronic obstructive pulmonary disease; PF, pulmonary fibrosis; PH, pulmonary hypertension.