Campylobacter jejuni virulence factors: update on emerging issues and trends

Abstract

Campylobacter jejuni is a very common cause of gastroenteritis, and is frequently transmitted to humans through contaminated food products or water. Importantly, C. jejuni infections have a range of short- and long-term sequelae such as irritable bowel syndrome and Guillain Barre syndrome. C. jejuni triggers disease by employing a range of molecular strategies which enable it to colonise the gut, invade the epithelium, persist intracellularly and avoid detection by the host immune response. The objective of this review is to explore and summarise recent advances in the understanding of the C. jejuni molecular factors involved in colonisation, invasion of cells, collective quorum sensing-mediated behaviours and persistence. Understanding the mechanisms that underpin the pathogenicity of C. jejuni will enable future development of effective preventative approaches and vaccines against this pathogen.

Keywords: Campylobacter jejuni; Biofilm; Chemotaxis; Pathogenesis; Polysaccharide capsule; Vaccine.

Fig. 1

C. jejuni virulence mechanisms discussed in this review and potential strategies for their inhibition. Chemotaxis, facilitated by Transducer-like proteins (Tlps) and Chemotaxis (Che) proteins, enables C. jejuni to navigate towards favourable environments for colonisation. Targeting Tlp/Che signaling offers therapeutic potential. While the mechanisms of translocation and invasion of C. jejuni are not fully understood, the protease HtrA has been implicated in mediating paracellular translocation. In addition, type III and type VI secretions system (T3SS, T6SS) have been identified to as key players in invasion, with the T3SS effector CiaI facilitating intracellular survival. Inhibiting HtrA presents a promising strategy to diminish translocation and invasion. Impeding the assembly of T3SS or T6SS, or inactivating their effectors, could be an innovative strategy for future therapeutic interventions. The role of biofilm formation in the environmental survival of C. jejuni is well recognised. While further investigation into the role of quorum sensing and biofilm formation in human campylobacteriosis is still needed, targeting AI-2 sensing and biofilm formation as a way to reduce persistence should be explored in future research. Adhesion is an important virulence mechanism facilitating colonisation and invasion. Development of anti-adhesive agents or blockers of known adhesins such as CadF or FlpA represents a novel therapeutic direction. As the capsule plays a role in adhesion, inhibiting capsule biosynthesis enzymes may reduce virulence, as well as potentially reduce immune evasion. Recent research highlights the complexity, phase variation and diversity of capsule biosynthesis pathways, making this one of the central topics for future C. jejuni investigations. The cytolethal distending toxin (CDT) is a major pro-inflammatory and cytotoxic agent of C. jejuni. Preventing either its secretion in outer membrane vesicles (OMVs) or its assembly into a tripartite holotoxin could attenuate C. jejuni virulence. Identifying other OMV-secreted proteins and blocking their action presents another anti-virulence strategy. Recent research into anti-OMV vaccine has shown promise and warrants further investigation

Fig. 2

The proposed translocation and invasion mechanisms utilised by C. jejuni.1A. C. jejunitranslocation by the transcellular route, whereby C. jejuni enter the intestinal epithelial cells at the apical side, and are able to transmigrate to the basolateral side. 1B. C. jejunitranslocation by the paracellular route, whereby C. jejuni secretes HtrA, which is able to cleave occludin at tight junctions of epithelial cells, and E-cadherin at adherens junctions of epithelial cells. HtrA secretion disrupts the barrier between epithelial cells and allows C. jejuni passage between epithelial cells to the basolateral cell surface. From here, C. jejuni is able to initiate invasion. 2.C. jejuni invasion of intestinal epithelial cells. Invasion is mediated by injection of the Cia effector proteins into the host cell cytosol. Additionally, invasion is initiated by the binding of fibronectin by C. jejuni proteins CadF and FlpA. This initiates a host-cell signalling cascade and initiation of cytoskeletal rearrangements, which enable uptake of C. jejuni intracellularly, into a Campylobacter Containing Vacuole (CCV). With the assistance of some of the Cia antigens, C. jejuni prevents the fusion of the CCV with lysosomes, and is able to persist intracellularly (figure modified and expanded from [23])

Fig. 3

An overview of the steps and factors involved in C. jejuni biofilm formation. C. jejuni biofilm formation is initiated by adhesion, aggregation and microcolony formation, which is followed by biofilm maturation and EPS matrix production. Biofilm formation in C. jejuni is a process regulated by autoinducer 2-mediated gene expression, and has been shown to be enhanced by environmental factors including the presence of chicken exudate, environmental oxygen levels and the presence of other bacterial species in the environment such as P. aeruginosa. The presence of a C. jejuni biofilm enables the release of planktonic cells into the environment, facilitating their further contamination of surfaces in the environment of a poultry processing facility, or enabling colonisation of other niches within a host environment

Fig. 4

PI-IBS, a long-term sequela of C. jejuni infection observed in patients with C. jejuni enterocolitis. Severity of enterocolitis has been linked to increased likelihood of PI-IBS development, and potential mechanisms of PI-IBS development include the development of autoimmunity against commensal bacteria, as well as autoimmunity against the host protein vinculin. These mechanisms subsequently result in the C. jejuni-mediated translocation of commensal bacteria, reduction in membrane integrity, and dysbiosis

Fig. 5

A summary of the progress in vaccine development for C. jejuni. Development of a CPS-based vaccine for human use required synthesis of CPS repeating unit 1 and involved trials of a conjugate CPS-CRM vaccine. The CPS-CRM vaccine has shown promise in animal models, with ALFQ as the preferred adjuvant. OMVs have also recently emerged as a suitable vaccine candidate, showing promise in a mouse model. However, further safety testing is required due to the presence of LOS within the OMVs. Efforts have also been directed towards development of a C. jejuni vaccine for use in chickens to limit C. jejuni colonisation and subsequent transmission to humans. This approach lead to the identification of conserved proteins FliD, PldA, BtuB and CdtB as vaccine antigen candidates. Finally, a reverse vaccinology approach, which includes bioinformatics analysis of sequence conservation, subcellular localisation and predicted immunogenicity, has pinpointed 4 new vaccine candidates, which showed promise in both mouse and chicken trials