Date Published: June 28, 2019
Publisher: Public Library of Science
Author(s): Xue Xia, Yue Liu, Andrea Hodgson, Dongqing Xu, Wenxuan Guo, Hongbing Yu, Weifeng She, Chenxing Zhou, Lei Lan, Kai Fu, Bruce A. Vallance, Fengyi Wan, John M. Leong.
Attaching/Effacing (A/E) bacteria include human pathogens enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli (EHEC), and their murine equivalent Citrobacter rodentium (CR), of which EPEC and EHEC are important causative agents of foodborne diseases worldwide. While A/E pathogen infections cause mild symptoms in the immunocompetent hosts, an increasing number of studies show that they produce more severe morbidity and mortality in immunocompromised and/or immunodeficient hosts. However, the pathogenic mechanisms and crucial host-pathogen interactions during A/E pathogen infections under immunocompromised conditions remain elusive. We performed a functional screening by infecting interleukin-22 (IL-22) knockout (Il22-/-) mice with a library of randomly mutated CR strains. Our screen reveals that interruption of the espF gene, which encodes the Type III Secretion System effector EspF (E. coli secreted protein F) conserved among A/E pathogens, completely abolishes the high mortality rates in CR-infected Il22-/- mice. Chromosomal deletion of espF in CR recapitulates the avirulent phenotype without impacting colonization and proliferation of CR, and EspF complement in ΔespF strain fully restores the virulence in mice. Moreover, the expression levels of the espF gene are elevated during CR infection and CR induces disruption of the tight junction (TJ) strands in colonic epithelium in an EspF-dependent manner. Distinct from EspF, chromosomal deletion of other known TJ-damaging effector genes espG and map failed to impede CR virulence in Il22-/- mice. Hence our findings unveil a critical pathophysiological function for EspF during CR infection in the immunocompromised host and provide new insights into the complex pathogenic mechanisms of A/E pathogens.
The human pathogens enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) are the leading causative agents for foodborne diseases, which lead to severe economic burden, morbidity, and mortality worldwide [1,2,3,4]. EHEC, EPEC and their murine equivalent Citrobacter rodentium (CR) cause attaching and effacing (A/E) lesions, characterized by the attachment of the bacteria to host epithelial cells and the localized destruction of brush-border microvilli [5,6]. Notably, CR shares 66.7% of its genes and most pathogenic mechanisms with EPEC and EHEC [5,7,8,9,10,11]. Since human pathogens EPEC and EHEC do not infect mice well , CR infection in mice has been widely used as an animal model to study the pathogenic mechanisms of A/E pathogens. There are approximately 5,000 genes in each genome of EHEC, EPEC, and CR. However, only a miniscule portion of the encoded genes, including the best-characterized gene cluster locus of enterocyte effacement (LEE) pathogenicity island, has been functionally related to A/E pathogenicity [12,13]. It is therefore believed that a substantial amount of essential host-pathogen interactions during A/E pathogen infections in vivo remain poorly understood.
Gastrointestinal infections caused by A/E pathogens EPEC and EHEC, which are associated with diarrheal diseases, impose significant economic burdens and remain a major public health issue [1,2,3]. Understanding the pathogenic mechanisms of EPEC/EHEC will greatly facilitate new discoveries for treatment against EPEC/EHEC infections in humans. Unfortunately, EPEC and EHEC do not infect mice well under specific pathogen free conditions, which substantially impedes disease modeling and discoveries of pathogenic mechanisms, especially the host-pathogen interactions in pathophysiological settings. The majority of the pathogen-encoded genes and pathogenic strategies are shared among EPEC, EHEC, and their murine equivalent CR, which makes CR infection in mice a widely used small animal model to investigate the EPEC/EHEC pathogenicity, although a substantial amount of pathophysiological pathogen-host interactions are still elusive . A/E pathogens are known to cause a self-limited infection with transient inflammation and mild symptoms in immunocompetent hosts . A recent study demonstrates that CR infection triggers a strikingly elevated mortality in Il22-/- mice, but not wild-type C57Bl/6 animals . Moreover, the Global Enteric Multicenter Study reveals that EPEC is a leading cause of lethality associated with diarrhea among children less than 12 months of age [20,23,39]. The low circulating IL-22 levels in human neonates was speculated to be a significant risk factor for serious infections in the neonatal period . These mouse model and epidemiological studies underscore the severe symptoms and outcomes of A/E pathogen infections in immunocompromised and/or immunodeficient hosts. However, studies of A/E pathogens in immunocompromised and/or immunodeficient settings significantly lag behind those of immunocompetent settings. Hence there is an urgent need to identify the crucial bacterial virulence proteins in A/E pathogens that are important for the infection-associated severe symptoms in the immunocompromised conditions.