Group A Streptococcus Prevents Initiation of the Oxidative Burst Response in Immune Cells

Abstract

Group A Streptococcus (GAS) is a gram‐positive bacterium that produces infections ranging from mild self‐limiting infections to more severe infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. In response to pathogens such as GAS, the host innate immune system produces antimicrobial factors such as reactive oxygen species that target microbes directly via oxidative stress, or indirectly via the initiation of other innate immune responses. Some bacteria, such as Lactococcus lactis and Staphylococcus aureus, contain pigment or catalase to detoxify these oxygen radicals. While GAS lacks these enzymes, it is still able to survive in innate immune cells, indicating that GAS may inhibit the oxidative burst response altogether. To examine this possibility, we infected macrophages with M1 and M49 serotype GAS, Staphylococcus aureus, Lactococcus lactis, and Group B Streptococcus and measured hydrogen peroxide production. Cells infected with either GAS serotypes produced significantly less peroxide compared with cells infected with other bacterial species, indicating that GAS suppresses the oxidative burst response. The GAS pore‐forming toxin streptolysin O (SLO) has similarity to listerolysin O (LLO), which prevents the assembly of the multi‐subunit NADPH oxidase on phagosome membranes. NADPH oxidase assembly is necessary to initiate the oxidative burst response. Peroxide generation was restored when cells were infected with GAS mutant bacteria of both M1 and M49 serotypes lacking SLO. We are currently examining the mechanism by which GAS uses SLO to prevents the oxidative burst response. Activation of the NADPH oxidase p47 subunit was verified Western blot for phosphorylated p47. Assembly of the NADPH oxidase is currently being assessed by cell fractionation of phagosomes and mass spectrometry. Preliminary data indicate that several NADPH oxidase subunits are missing from the phagosome. Thus, similar to LLO, SLO may inhibit the proper assembly of the NADPH oxidase, and prevent the generation of harmful reactive oxygen species.