The mRNA expression of genes involved in IL-17 signaling pathway, including IL-17A, IL-17RA, Traf6, Nfkb1, Nfkbia, CXCL1, CXCL5, CCL20, IL-6, TNF-α, Csf3, Csf2, lcn2, MMP13, and Fosb, were measured in lungs of both IL-9−/− PCP and WT PCP mice 3-week postinfection, and the results were shown in Figure 4A. The mRNA levels of IL-17A, IL-17RA, Nfkb1, Nfkbia, TNF-α, Csf3, Csf2 and MMP13 were significantly higher in IL-9−/− PCP group than WT PCP group (all P < 0.05). This evidence concerns the gene MMP13 and pneumocystosis.