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

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 (SsEM1) reduces neutrophil recruitment by targeting platelet-activating factor (PAF). SsEM1 and SsE produced by serotype M28 GAS (SsEM28) have a 37% sequence difference. This study aims at determining whether SsEM28 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 SsEM1, SsEM28, SeE, the SsE homologue in Streptococcus equi, and human plasma PAF-AH (hpPAF-AH). PAF incubated with SsEM28 or SeE was converted into lyso-PAF. SsEM1 and SsEM28 had kcat 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 SsEM1, SsEM28, and hpPAF-AH in kcat and Km in hydrolyzing triglycerides, acetyl esters, and PAF indicates that the SsE proteins are more potent hydrolases against PAF and have high affinity for PAF. SsEM28 possesses much lower esterase activities against triglycerides and other esters than SsEM1 but have similar potency with SsEM1 in PAF hydrolysis. Deletion of sseM28 in a covS deletion mutant of GAS increased neutrophil recruitment and reduced skin infection, whereas in trans expression of SsEM28 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.