Streptococcal pyrogenic exotoxin B inhibits apoptotic cell clearance by macrophages through protein S cleavage.

Chia-Ling Chen,Yee-Shin Lin,Chih-Feng Kuo,Chiu-Yueh Chen,Chia-Li Han,Pei-Jane Tsai,Yueh-Ying Wu,Jiunn-Jong Wu,Ching-Chuan Liu,Shuying Wang,Woei-Jer Chuang,Chiou-Feng Lin

doi:10.1038/srep26026

Abstract

Clearance of apoptotic cells by macrophages plays an important role in maintaining tissue homeostasis. Previous study indicated that streptococcal pyrogenic exotoxin B (SPE B) reduces phagocytic activity in group A streptococcus (GAS) infection. Here, we demonstrate that SPE B causes an inhibitory effect on protein S-mediated phagocytosis. In the presence of SPE B, serum- and purified protein S-mediated phagocytosis of apoptotic cells were significantly inhibited. The binding abilities of protein S to apoptotic cells were decreased by treatment with SPE B. Bacterial culture supernatants from GAS NZ131 strain also caused a reduction of protein S binding to apoptotic cells, but speB mutant strain did not. SPE B directly cleaved protein S in vitro and in vivo, whereas a lower level of cleavage occurred in mice infected with a speB isogenic mutant strain. SPE B-mediated initial cleavage of protein S caused a disruption of phagocytosis, and also resulted in a loss of binding ability of protein S-associated C4b-binding protein to apoptotic cells. Taken together, these results suggest a novel pathogenic role of SPE B that initiates protein S degradation followed by the inhibition of apoptotic cell clearance by macrophages.

Highlights

Streptococcus pyogenes, called group A streptococcus (GAS), is an important human pathogen that causes various diseases with a wide range of severity, including pharyngitis, impetigo, scarlet fever, acute rheumatic fever, post-streptococcal glomerulonephritis, necrotizing fasciitis, and streptococcal toxic shock syndrome[1]
Flow cytometric analysis showed that the percentages of fetal calf serum (FCS)-mediated phagocytosis were significantly reduced when FCS was pretreated with streptococcal pyrogenic exotoxin B (SPE B) but not C192S, a SPE B mutated protein lacking protease activity (Fig. 1b)
We provide evidence of a novel pathogenic effect of SPE B which involves cleavage of serum-derived vitamin K-dependent protein S followed by inhibition of apoptotic cell clearance by macrophages

Summary

Introduction

Streptococcus pyogenes, called group A streptococcus (GAS), is an important human pathogen that causes various diseases with a wide range of severity, including pharyngitis, impetigo, scarlet fever, acute rheumatic fever, post-streptococcal glomerulonephritis, necrotizing fasciitis, and streptococcal toxic shock syndrome[1]. A number of factors and receptors expressed on the surface of phagocytes or present in serum are involved in mechanisms of apoptotic cell clearance[22,23,24]. SPE B directly triggers macrophages to undergo apoptosis thereby reducing macrophage phagocytosis[17] These studies suggest an anti-phagocytic role of SPE B in GAS infection. We demonstrate a novel pathogenic action of SPE B on macrophage-mediated clearance of apoptotic cells through protein S degradation. Theses results indicate that SPE B-regulated protein S degradation leads to a decrease of efferocytosis and loses the protection of C4BP on apoptotic cells from complement attack, which may trigger secondary necrosis and severe inflammatory responses

Methods

Results

Conclusion