MicroRNA-155 Is Required for Clearance of Streptococcus pneumoniae

Pneumonia caused by Streptococcus pneumoniae is a major cause of death and an economic burden worldwide. S. pneumoniae is an intermittent colonizer of the human upper respiratory tract, and the ability to control asymptomatic colonization deter-mines the likelihood of developing invasive disease. Recognition of S. pneumoniae by resident macrophages via Toll-like recep-tor 2 (TLR-2) and the macrophage receptor with collagenous structure (MARCO) and the presence of interleukin-17 (IL-17)-secreting CD4 T cells are required for macrophage recruitment and bacterial clearance. Despite the fact that the primary cellular effectors needed for bacterial clearance have been identified, much of the underlying regulatory mechanisms are un-known. Herein, we demonstrate that the small, noncoding RNAmicroRNA-155 (mir-155) is critical for the effective clearance of S. pneumoniae. Our studies show that mir-155-deficient mice maintain the ability to prevent acute invasive pneumococcal infec-tion but have significantly higher bacterial burdens following colonization, independently of macrophage recognition by TLR-2, MARCO expression, or bactericidal capacity. The observed defects in bacterial clearance parallel reduced IL-17A and gamma interferon CD4 T-cell responses in vivo, lower IL-17AmRNA levels in the nasopharynx, and a reduced capacity to induce Th17 cell polarization. Given that knockout mice are also limited in the capacity to generate high-titer S. pneumoniae-specific anti-bodies, we conclude that mir-155 is a critical mediator of the cellular effectors needed to clear primary and secondary S. pneu-moniae colonizations. Pneumonia caused by Streptococcus pneumoniae infection is amajor cause of death and an economic burden in both devel