The CXC chemokine MIG/CXCL9 is important in innate immunity against Streptococcus pyogenes

Pharyngitis caused by Streptococcus pyogenes is one of the most common bacterial infections in humans and is also a starting point for invasive S. pyogenes infection. Here, we describe that tonsil fluid from patients with streptococcal pharyngitis contains high amounts of the interferon ( IFN)-dependent CXC chemokine known as monokine induced by IFN-gamma ( MIG)/CXCL9. Also in vitro, inflamed pharyngeal epithelium produced large amounts of MIG/CXCL9 in the presence of bacteria. The CXC chemokines MIG/CXCL9, IFN-inducible protein-10/CXCL10, and IFN-inducible T cell alpha-chemoattractant/CXCL11 all showed antibacterial activity against S. pyogenes, and inhibition of MIG/CXCL9 expression reduced the antibacterial activity at the surface of inflamed pharyngeal cells. S. pyogenes of the clinically important M1 serotype secrets the protein streptococcal inhibitor of complement ( SIC), which inhibited the antibacterial activity of the chemokines. As exemplified by S. pyogenes pharyngitis, the data identify a novel innate defense mechanism against invasive bacteria on epithelial surfaces.