Characterization of Streptococcal Platelet-Activating Factor Acetylhydrolase Variants That Are Involved in Innate Immune Evasion

Abstract

ABSTRACT Human pathogen group A streptococcus (GAS) has developed mechanisms to subvert innate immunity. We recently reported that the secreted esterase produced by serotype M1 GAS (SsE M1 ) reduces neutrophil recruitment by targeting platelet-activating factor (PAF). SsE M1 and SsE produced by serotype M28 GAS (SsE M28 ) have a 37% sequence difference. This study aims at determining whether SsE M28 is also a PAF acetylhydrolase and participates in innate immune evasion. We also examined whether SsE evolved to target PAF by characterizing the PAF acetylhydrolase (PAF-AH) activity and substrate specificity of SsE M1 , SsE M28 , SeE, the SsE homologue in Streptococcus equi , and human plasma PAF-AH (hpPAF-AH). PAF incubated with SsE M28 or SeE was converted into lyso-PAF. SsE M1 and SsE M28 had k cat values of 373 s −1 and 467 s −1 , respectively, that were ≥30-fold greater than that of hpPAF-AH (12 s −1 ). The comparison of SsE M1 , SsE M28 , and hpPAF-AH in k cat and K m in hydrolyzing triglycerides, acetyl esters, and PAF indicates that the SsE proteins are more potent hydrolases against PAF and have high affinity for PAF. SsE M28 possesses much lower esterase activities against triglycerides and other esters than SsE M1 but have similar potency with SsE M1 in PAF hydrolysis. Deletion of sse M28 in a covS deletion mutant of GAS increased neutrophil recruitment and reduced skin infection, whereas in trans expression of SsE M28 in GAS reduced neutrophil infiltration and increased skin invasion in subcutaneous infection of mice. These results suggest that the SsE proteins evolved to target PAF for enhancing innate immune evasion and skin invasion.