Group A Streptococcus Secreted Esterase Hydrolyzes Platelet-Activating Factor to Impede Neutrophil Recruitment and Facilitate Innate Immune Evasion

Mengyao Liu,Wenchao Feng,Jonathan Hilmer,Brian Bothner,Mark T Quinn,Mauro M Teixeira,Luciana P Tavares,Benfang Lei,Jinquan Li,Arthur W Layton,Cristiana C Garcia,Liliya N Kirpotina,Hui Zhu,Michael R Wessels

doi:10.1371/journal.ppat.1002624

Abstract

The innate immune system is the first line of host defense against invading organisms. Thus, pathogens have developed virulence mechanisms to evade the innate immune system. Here, we report a novel means for inhibition of neutrophil recruitment by Group A Streptococcus (GAS). Deletion of the secreted esterase gene (designated sse) in M1T1 GAS strains with (MGAS5005) and without (MGAS2221) a null covS mutation enhances neutrophil ingress to infection sites in the skin of mice. In trans expression of SsE in MGAS2221 reduces neutrophil recruitment and enhances skin invasion. The sse deletion mutant of MGAS5005 (Δsse MGAS5005) is more efficiently cleared from skin than the parent strain. SsE hydrolyzes the sn-2 ester bond of platelet-activating factor (PAF), converting biologically active PAF into inactive lyso-PAF. KM and k cat of SsE for hydrolysis of 2-thio-PAF were similar to those of the human plasma PAF acetylhydrolase. Treatment of PAF with SsE abolishes the capacity of PAF to induce activation and chemotaxis of human neutrophils. More importantly, PAF receptor-deficient mice significantly reduce neutrophil infiltration to the site of Δsse MGAS5005 infection. These findings identify the first secreted PAF acetylhydrolase of bacterial pathogens and support a novel GAS evasion mechanism that reduces phagocyte recruitment to sites of infection by inactivating PAF, providing a new paradigm for bacterial evasion of neutrophil responses.

Highlights

Neutrophils are one of the first responders of innate inflammatory cells to migrate towards the site of infecting agents
platelet-activating factor (PAF) was incubated with SsE, and the reaction was analyzed by thin layer chromatography (TLC), which could resolve PAF and lyso-PAF because PAF migrates much faster (Figure 1)
Our work adds a new virulence factor to the large array of Group A Streptococcus (GAS) virulence factors that interfere with the bactericidal function of neutrophils. Another conclusion of this work is that SsE contributes to the enhanced inhibition of neutrophil recruitment as a result of the null covS mutation in MGAS5005

Summary

Introduction

Neutrophils are one of the first responders of innate inflammatory cells to migrate towards the site of infecting agents. Evasion of the neutrophil microbicidal response is critical for survival, dissemination, and infectability of bacterial pathogens. Bacterial pathogens evade the neutrophil responses by multiple mechanisms, including inhibition of neutrophil infiltration, antiphagocytosis, and killing of neutrophils. The success of GAS as a pathogen is based, in part, on its ability to evade the innate immune system. GAS expresses extracellular peptidases ScpA and SpyCEP/ScpC to inhibit neutrophil recruitment by degrading the chemotactic C5a peptide and IL-8/CXC chemokines, respectively [2,3,4,5]. The hyaluronic acid capsule and surface M protein made by GAS confer resistance to opsonophagocytosis and phagocytosis by neutrophils [6,7]. Streptolysin S and streptolysin O kill and induce apoptosis of neutrophils [11,12]

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Discussion

Conclusion