The mRNA expression of genes involved in Th17 differentiation pathway, including IL-17A, Rorgt, Rora, IRF4, IL-23R, STAT3, Smad2, p38, mTOR, STAT5 and STAT6, were detected in lungs of both IL-9−/− PCP and WT PCP mice as shown in Figure 4B. It was observed that the mRNA expression of IL-17A, RORγt, RORa, IRF4, IL-23R, STAT3, Smad2, STAT5 and STAT6 were significantly increased in IL-9−/− PCP mice compared with WT PCP mice (all P < 0.05). Here, IRF4 is linked to pneumocystosis.