The Bacterial Toxin CNF1 Induces Activation and Maturation of Human Monocyte-Derived Dendritic Cells.

Laura Gall-Mas,Michael Givskov,Alessia Fabbri,Martin Namini,Thorbjørn Krejsgaard,Carla Fiorentini

doi:10.3390/ijms19051408

Abstract

Cytotoxic necrotizing factor 1 (CNF1) is a bacterial protein toxin primarily expressed by pathogenic Escherichia coli strains, causing extraintestinal infections. The toxin is believed to enhance the invasiveness of E. coli by modulating the activity of Rho GTPases in host cells, but it has interestingly also been shown to promote inflammation, stimulate host immunity and function as a potent immunoadjuvant. The mechanisms underlying the immunostimulatory properties of CNF1 are, however, poorly characterized, and little is known about the direct effects of the toxin on immune cells. Here, we show that CNF1 induces expression of maturation markers on human immature monocyte-derived dendritic cells (moDCs) without compromising cell viability. Consistent with the phenotypic maturation, CNF1 further triggered secretion of proinflammatory cytokines and increased the capacity of moDCs to stimulate proliferation of allogenic naïve CD4+ T cells. A catalytically inactive form of the toxin did not induce moDC maturation, indicating that the enzymatic activity of CNF1 triggers immature moDCs to undergo phenotypic and functional maturation. As the maturation of dendritic cells plays a central role in initiating inflammation and activating the adaptive immune response, the present findings shed new light on the immunostimulatory properties of CNF1 and may explain why the toxin functions as an immunoadjuvant.

Highlights

Cytotoxic necrotizing factor 1 (CNF1) is a bacterial protein toxin produced by certain strains of pathogenic Escherichia coli. cnf1-positive E. coli have been isolated from children with diarrhea, but are most frequently associated with extraintestinal conditions, such as urinary tract infections, bacteremia and neonatal meningitis [1]
CNF1-C866S had no significant impact on the expression of the analyzed maturation markers, indicating that CNF1 triggers monocyte-derived dendritic cells (moDCs) maturation via its catalytic activity (Figure 1A–C)
We show that CNF1 potently triggers activation and maturation of immature human moDCs

Summary

Introduction

Cytotoxic necrotizing factor 1 (CNF1) is a bacterial protein toxin produced by certain strains of pathogenic Escherichia coli. cnf1-positive E. coli have been isolated from children with diarrhea, but are most frequently associated with extraintestinal conditions, such as urinary tract infections, bacteremia and neonatal meningitis [1]. Endosomal acidification and cleavage of CNF1 leads to translocation of the active part of the toxin into the cytoplasm where it deamidates a specific glutamine residue in Rho GTPase proteins. This modification impairs the intrinsic GTP hydrolyzing activity of Rho GTPases and thereby locks them in their active state. CNF1 induces rearrangements of the actin cytoskeleton in target cells, leading to morphological and functional changes These rearrangements can facilitate bacterial internalization into host cells, and substantial evidence supports that CNF1 contributes to the invasiveness of pathogenic E. coli by manipulating the epithelial and endothelial barriers [3,8,9]

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