CpaA Is a Glycan-Specific Adamalysin-like Protease Secreted by Acinetobacter baumannii That Inactivates Coagulation Factor XII.

Ursula Waack,Andrew Yee,Harry L T Mobley,David Ginsburg,Zachary Huttinger,Sara Smith,Maria Sandkvist,Daniel A Lawrence,Mark Warnock,Ayush Kumar,Alban Deroux,Samuel I Miller

doi:10.1128/mbio.01606-18

Abstract

Antibiotic-resistant Acinetobacter baumannii is increasingly recognized as a cause of difficult-to-treat nosocomial infections, including pneumonia, wound infections, and bacteremia. Previous studies have demonstrated that the metalloprotease CpaA contributes to virulence and prolongs clotting time when added to human plasma as measured by the activated partial thromboplastin time (aPTT) assay. Here, we show that CpaA interferes with the intrinsic coagulation pathway, also called the contact activation system, in human as well as murine plasma, but has no discernible effect on the extrinsic pathway. By utilizing a modified aPTT assay, we demonstrate that coagulation factor XII (fXII) is a target of CpaA. In addition, we map the cleavage by CpaA to two positions, 279-280 and 308-309, within the highly glycosylated proline-rich region of human fXII, and show that cleavage at the 308-309 site is responsible for inactivation of fXII. At both sites, cleavage occurs between proline and an O-linked glycosylated threonine, and deglycosylation of fXII prevents cleavage by CpaA. Consistent with this, mutant fXII (fXII-Thr309Lys) from patients with hereditary angioedema type III (HAEIII) is protected from CpaA inactivation. This raises the possibility that individuals with HAEIII who harbor this mutation may be partially protected from A. baumannii infection if CpaA contributes to human disease. By inactivating fXII, CpaA may attenuate important antimicrobial defense mechanisms such as intravascular thrombus formation, thus allowing A. baumannii to disseminate.IMPORTANCE Ventilator-associated pneumonia and catheter-related bacteremia are the most common and severe infections caused by Acinetobacter baumannii Besides the capsule, lipopolysaccharides, and the outer membrane porin OmpA, little is known about the contribution of secreted proteins to A. baumannii survival in vivo Here we focus on CpaA, a potentially recently acquired virulence factor that inhibits blood coagulation in vitro We identify coagulation factor XII as a target of CpaA, map the cleavage sites, and show that glycosylation is a prerequisite for CpaA-mediated inactivation of factor XII. We propose adding CpaA to a small, but growing list of bacterial proteases that are specific for highly glycosylated components of the host defense system.

Highlights

Antibiotic-resistant Acinetobacter baumannii is increasingly recognized as a cause of difficult-to-treat nosocomial infections, including pneumonia, wound infections, and bacteremia
We confirmed that culture supernatant from the WT AB031 strain, but not from the isogenic ΔcpaA mutant, increases the clotting time in an activated partial thromboplastin time (aPTT) assay in human plasma (Fig. 1A)
To the ΔcpaA mutant, culture supernatant from the T2SS mutant ΔgspD strain was lacking CpaA (Fig. S2A and B) and had no effect on clotting (Fig. 1A), and while full complementation was not obtained with plasmidencoded gspD due to only partial restoration of CpaA secretion (Fig. S2C), the increase in clotting time in the presence of pgspD compared to the clotting time of the empty mbio.asm.org 3

Summary

Introduction

Antibiotic-resistant Acinetobacter baumannii is increasingly recognized as a cause of difficult-to-treat nosocomial infections, including pneumonia, wound infections, and bacteremia. Survival in the bloodstream and subsequent colonization by A. baumannii in the spleen and liver is supported in part by the type II secretion system (T2SS) [8], a multiprotein complex that transports a variety of enzymes, including lipases and proteases, across the outer membranes of Acinetobacter and other bacterial pathogens [8, 10,11,12,13]. This was demonstrated using a neutropenic murine model for bacteremia, in which the T2SS mutant ΔgspD strain was outcompeted by the wild-type (WT) A. baumannii strain. A recent study has identified a direct role of autocrine fXII in upregulation of neutrophil function that promotes neutrophil trafficking [22]

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