UC San Diego

Group A Streptococcus (GAS) is a leading human pathogen infecting millions annually. Colonization is mediated through bacterial attachment to host epithelium. Internalization of epithelial cells may offer the bacteria protection from immune cells and antibiotics, and allow for penetration of epithelial barriers. We investigated the role of surface-anchored protein serum opacity factor (SOF) in the invasion of epithelial cells and virulence. Our data demonstrate that SOF plays a critical role in invasion of epithelial cells, mediated through its activity domain, independent of its fibronectin binding capacity. To this end, elimination of SOF in GAS produced smaller lesions and yielded significantly less bacteria per lesion than wild-type (WT) in a mouse model of necrotizing fasciitis. To cause invasive disease GAS must circumvent host immune defenses. Neutrophils circulate in the bloodstream until chemokine signals recruit them to sites of infections. GAS produce a protease, SpyCEP, which cleaves and inactivates one of the most potent neutrophil chemoattractants interleukin-8 (IL-8). We observe that this leads to decreased neutrophil migration compared to a GAS mutant lacking the SpyCEP gene. In addition, we demonstrate that SpyCEP inhibis the formation of neutrophil NETs and contributes to GAS-induced neutrophil apoptosis. The SpyCEP mutant was significantly attenuated for virulence in a mouse model of necrotizing fasciitis. Macrophages are necessary defenders against GAS infections. We have found that live GAS trigger significant accelerated caspase-dependent macrophage apoptosis upon phagocytosis. This apoptosis disrupts mitochondrial integrity and promotes membrane remodeling, leading to the release of cytochrome c into the cytosol. Apoptosis-induction is dependent upon the GAS pore-forming cytolysin streptolysin O (SLO), which is necessary and sufficient for this phenotype. Accelerated apoptosis hampers the ability of macrophages to kill the bacteria and release cytokines, blunting immune activation. An SLO mutant was attenuated in a mouse systemic infection model, with less bacteria surviving in the blood and reduced killing of mice. This work demonstrates the complexity of GAS interactions with the host during disease and the multitude of virulence mechanisms employed to escape immune defenses. A deeper understanding of GAS disease pathogenesis can lead to novel therapeutic strategies in the treatment of this disease