TNF and susceptibility to pneumonia measurement: In this study, IL-18 could be the superior candidate to prime hUC-MSCs to enhance the therapeutic efficacy of severe H1N1-induced pneumonia in mice.Many studies have found that cytokines that emerge in inflammatory microenvironments are typically used to prime MSCs to enhance specific properties, including IFN-γ, TNF-α, IL-1β, IL-17A, and IL-25 [13, 16–26].