Second, the pH in primary HTEpCs is often mildly acidic; however, endogenous proteases may support viral infection of HTEpCs because tracheal proteases, such as transmembrane protease serine 2 (TMPRSS2) [83] and human airway trypsin-like protease (HAT) [84], which are required to process released virions, allow for multiple cycles of viral infection [85]. Here, TMPRSS2 is linked to viral infectious disease.