TLR9 and bacterial pneumonia: However, our data indicates that impaired IL-17 production in TLR4 and TLR9 mutant mice may be attributable to reduced IL-23 expression by DC and possibly other proximal cells, as IL-23 is known to be a major paracrine inducer of IL-17 in bacterial pneumonia [13], [15], [16] and we observed substantial defects in lung IL-23 production from TLR4/9 double mutant mice compared to infected WT animals.