DPP4 and pulmonary fibrosis: Examples of pro-inflammatory pathways implicated by the GWAS results involve the activation of cytokine receptor signalling (TYK2), receptors that facilitate chemotaxis of immune cells to the infected tissue (CCR1, CCR2, XCR1, CXCR6 and CCR9) and inflammasome activation (DPP9); this dipeptidyl peptidase has also been associated with lung fibrosis and belongs to the DPP4 family, where DPP4 is the entry receptor for MERS [22, 26, 29, 31, 32].