Vibrio cholerae, classification, pathogenesis, immune response, and trends in vaccine development

Abstract

Vibrio cholerae is the causative agent of cholera, a highly contagious diarrheal disease affecting millions worldwide each year. Cholera is a major public health problem, primarily in countries with poor sanitary conditions and regions affected by natural disasters, where access to safe drinking water is limited. In this narrative review, we aim to summarize the current understanding of the evolution of virulence and pathogenesis of V. cholerae as well as provide an overview of the immune response against this pathogen. We highlight that V. cholerae has a remarkable ability to adapt and evolve, which is a global concern because it increases the risk of cholera outbreaks and the spread of the disease to new regions, making its control even more challenging. Furthermore, we show that this pathogen expresses several virulence factors enabling it to efficiently colonize the human intestine and cause cholera. A cumulative body of work also shows that V. cholerae infection triggers an inflammatory response that influences the development of immune memory against cholera. Lastly, we reviewed the status of licensed cholera vaccines, those undergoing clinical evaluation, and recent progress in developing next-generation vaccines. This review offers a comprehensive view of V. cholerae and identifies knowledge gaps that must be addressed to develop more effective cholera vaccines.

Keywords: Vibrio cholerae; cholera; cholera toxin; diarrhea; next-generation vaccines; oral vaccine.

Figure 1

Classification and evolution of V. cholerae. (A) V. cholerae is classified into serogroups based on the composition of the O antigen of LPS. Strains belonging to the O1 serogroup are further divided into three serotypes, namely Ogawa, Hikojima, and Inaba. The LPS of these three serotypes is schematically represented, showing the approximate percentage of methylation of the terminal perosamine. Serogroup O1 is also classified into the Classical and El Tor biotypes, based on phenotypic and genetic markers. Over the past two decades, there has been a growing number of reports on V. cholerae strains that possess genetic features from both the Classical and El Tor biotypes, leading to the emergence of hybrid or variant strains. These strains have been linked to several cholera outbreaks worldwide and have contributed significantly to the global burden of this disease. (B) Timeline of the history of cholera pandemics. (C) A schematic representation of the evolutionary process underlying the development of virulence in serogroup O1. This process is mainly driven by the acquisition of mobile genetic elements, including bacteriophages, genomic islands, integrative and conjugative elements, among others.

Figure 2

Pathogenesis of toxigenic V. cholerae. (A) Toxigenic V. cholerae produces several virulence factors that contribute to its pathogenesis. The precise pathogenic mechanism is not yet fully understood, but it is widely accepted that it involves the combination of these virulence factors and the ability to colonize and persist in the small intestine. (B) Upon ingestion, V. cholerae survives the low pH of the stomach via an acid tolerance response. In the small intestine, V. cholerae uses its flagellum to propel through the mucus layer and reach the epithelial surface. Meanwhile, V. cholerae must overcome host immunity and the colonization resistance mechanisms of the gut microbiota. To colonize the small intestine, it expresses virulence factors such as toxin-coregulated pilus (TCP) and cholera toxin (CTX). During infection, other factors such as HapA, GbpA, and NanH are also expressed. For more details on the roles of these virulence factors, please refer to the text. This figure was created using BioRender.com.

Figure 3

Mechanism of action of cholera toxin. (A) The crystal structure of CTX (PDB accession number 1XTC) was determined by Zhang et al. (136). CTX is composed of a heterodimeric CTX-A subunit, which consists of two polypeptide chains, CTX-A₁ (22 kDa) and CTX-A₂ (5 kDa), linked by a single disulfide bond. The CTX-A₂ helical peptide links the CTX-A1 chain to the pentameric CTX-B subunit, which is composed of five identical polypeptide chains (11.6 kDa). (B) The CTX-B pentamer specifically binds to GM1 gangliosides (primary receptor) or histo-blood group antigens (HBGAs; secondary binding site) present on the apical side of intestinal epithelial cells, promoting the endocytosis of the toxin. (C) The internalization of CTX may occur through clathrin-dependent as well as caveolae- and clathrin-independent endocytosis. Regardless of the mechanism of endocytosis, the CTX is internalized to the early endosomal compartment, trafficked to the Golgi, and then onto the endoplasmic reticulum (ER), where it dissociates into a CTX-A₁ and a CTX-A₂/CTX-B complex. Next, the CTX-A₁ is exported out of the ER to the cytosol, where it is activated by ADP ribosylation factor 6 (ARF6). The ARF6-bound, activated CT-A₁ subunit, in turn, activates adenylyl cyclase (AC) by catalyzing ADP ribosylation of a G protein-coupled receptor (GPCR). The AC then catalyzes the conversion of ATP to cyclic adenosine monophosphate (cAMP), increasing the intracellular cAMP concentration. This leads to the activation of protein kinase A (PKA), which phosphorylates the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel proteins, ultimately resulting in the release of electrolytes (Cl⁻, HCO₃⁻, Na⁺, K⁺) and water into the intestinal lumen, causing the secretory diarrhea characteristic of cholera (137, 138). The figure was created with BioRender.com.

Figure 4

Immune response against cholerae infection. Intestinal epithelial cells (IECs) serve as a physical barrier that limits bacteria to the intestinal lumen. They detect PAMPs such as LPS, flagellin, CTX and OmpU, triggering the secretion of proinflammatory cytokines that recruit innate immune cells such as macrophages, dendritic cells (DCs), and neutrophils. Activated neutrophils increase the inflammation of the intestinal lumen through metabolites such as lactoferrin (LF), myeloperoxidase (MPO), and nitric oxide (NO). M cells take up and transport vibrios from the intestinal lumen to the subepithelial dome (SED) region in Peyer’s patches, where DCs engulf them. Activated DCs migrate to mesenteric lymph nodes, where they produce Th17 or Th1-driving cytokines. Macrophages can also contribute to Th17 or Th1 differentiation through the secretion of IL-23 and IL-6 or IFNγ, respectively. Th1, Th17, and Tfh cells induce B-cell differentiation and expansion. Mucosal-associated invariant T (MAIT) cells are present and highly activated in the lamina propria of the duodenum of cholera patients, but their exact role in the protection against cholera remains to be determined. Secretory antibodies (sIgA and sIgM) prevent vibrios from attaching to the epithelium, blocking their access to the epithelial surface and facilitating their removal through peristaltic activities. Some IgG antibodies could enter the intestinal lumen via passive leakage through a damaged and leaky epithelium. The figure was created with BioRender.com.