IgM specific to lipopolysaccharide of Vibrio cholerae is a surrogate antibody isotype responsible for serum vibriocidal activity

Mucosal and systemic immune responses to Vibrio cholerae infection and oral cholera vaccines (OCVs) in humans: a systematic review

INTRODUCTION

Cholera is an enteric disease caused by Vibrio cholerae, a water-borne pathogen, and characterized by severe diarrhea. Vaccines have been recommended for use by the WHO in resource-limited settings. Efficacies of the currently licensed cholera vaccines are not optimal in endemic settings and low in children below the age of five, a section of the population most susceptible to the disease. Development of next generation of cholera vaccines would require a detailed understanding of the required protective immune responses.

AREA COVERED

In this review, we revisit clinical trials which are focused on the early transcriptional mucosal responses elicited during Vibrio cholerae infection and upon vaccination along with summarizing various components of the effector immune response against Vibrio cholerae.

EXPERT OPINION

The inability of currently licensed killed/inactivated vaccines to elicit key inflammatory pathways locally may explain their restricted efficacy in endemic settings. More studies are required to understand the immunogenicity of the live attenuated cholera vaccine in these regions. Various extrinsic and intrinsic factors influence anti-cholera immunity and need to be considered to develop region-specific next generation vaccines.

Characterization of the Immune Response to Vibrio cholerae Infection in a Natural Host Model

The gram-negative bacterium Vibrio cholerae causes the life-threatening diarrheal disease cholera, which is spread through the ingestion of contaminated food or water. Cholera epidemics occur largely in developing countries that lack proper infrastructure to treat sewage and provide clean water. Numerous vertebrate fish species have been found to be natural V. cholerae hosts. Based on these findings, zebrafish (Danio rerio) have been developed as a natural host model for V. cholerae. Diarrheal symptoms similar to those seen in humans are seen in zebrafish as early as 6 hours after exposure. Our understanding of basic zebrafish immunology is currently rudimentary, and no research has been done to date exploring the immune response of zebrafish to V. cholerae infection. In the present study, zebrafish were infected with either pandemic El Tor or non-pandemic, environmental V. cholerae strains and select immunological markers were assessed to determine cellular immunity and humoral immunity. Significant increases in the gene expression of two transcription factors, T-bet and GATA3, were observed in response to infection with both V. cholerae strains, as were levels of mucosal related antibodies. Additionally, the cytokine IL-13 was shown to be significantly elevated and paralleled the mucin output in zebrafish excretions, strengthening our knowledge of IL-13 induced mucin production in cholera. The data presented here further solidify the relevancy of the zebrafish model in studying V. cholerae, as well as expanding its utility in the field of cholera immunology.

Immunomodulatory effects of mesenchymal stem cell-conditioned media on lipopolysaccharide of Vibrio cholerae as a vaccine candidate

BACKGROUND

Vibrio cholerae is the causative agent of cholera, which is commonly associated with high morbidity and mortality, and presents a major challenge to healthcare systems throughout the world. Lipopolysaccharide (LPS) is required for full protection against V. cholerae but can induce inflammation and septic shock. Mesenchymal stem cells (MSCs) are currently used to treat infectious and inflammatory diseases. Therefore, this study aimed to evaluate the immune-modulating effects of the LPS-MSC-conditioned medium (CM) on V. cholerae LPS immunization in a murine model.

METHODS

After preconditioning MSCs with LPS, mice were immunized intraperitoneally on days 0 and 14 with the following combinations: LPS + LPS-MSC-CM; detoxified LPS (DLPS) + MSC-CM; LPS + MSC sup; LPS; LPS-MSC-CM; MSC supernatant (MSC sup); and PBS. The mouse serum and saliva samples were collected to evaluate antibody (serum IgG and saliva IgA) and cytokine responses (TNF-α, IL-10, IL-6, TGF-β, IL-4, IL-5, and B-cell activating factor (BAFF)).

RESULTS

The LPS + LPS-MSC-CM significantly increased total IgG and IgA compared to other combinations (P < 0.001). TNF-α levels, in contrast to IL-10 and TGF-β, were reduced significantly in mice receiving the LPS + LPS-MSC-CM compared to mice receiving only LPS. IL-4, IL-5, and BAFF levels significantly increased in mice receiving increased doses of LPS + LPS-MSC-CM compared to those who received only LPS. The highest vibriocidal antibody titer (1:64) was observed in LPS + LPS-MSC-CM-immunized mice and resulted in a significant improvement in survival in infant mice infected by V. cholerae O1.

CONCLUSIONS

The LPS-MSC-CM modulates the immune response to V. cholerae LPS by regulating inflammatory and anti-inflammatory responses and inducing vibriocidal antibodies, which protect neonate mice against V. cholerae infection.

Recent advances in lipopolysaccharide-based glycoconjugate vaccines

INTRODUCTION

The public health burden caused by pathogenic Gram-negative bacteria is increasingly prominent due to antimicrobial resistance. The surface carbohydrates are potential antigens for vaccines against Gram-negative bacteria. The enhanced immunogenicity of the O-specific polysaccharide (O-SP) moiety of LPS when coupled to a carrier protein may protect against bacterial pathogens. However, because of the toxic lipid A moiety and relatively high costs of O-SP isolation, LPS has not been a popular vaccine antigen until recently.

AREAS COVERED

In this review, we discuss the rationales for developing LPS-based glycoconjugate vaccines, principles of glycoconjugate-induced immunity, and highlight the recent developments and challenges faced by LPS-based glycoconjugate vaccines.

EXPERT OPINION

Advances in LPS harvesting, LPS chemical synthesis, and newer carrier proteins in the past decade have propelled LPS-based glycoconjugate vaccines toward further development, through to clinical evaluation. The development of LPS-based glycoconjugates offers a new horizon for vaccine prevention of Gram-negative bacterial infection.

Longitudinal analysis of human humoral responses after vaccination with a live attenuated V. cholerae vaccine

Vibrio cholerae is a bacterial pathogen which causes the severe acute diarrheal disease cholera. Given that a symptomatic incident of cholera can lead to long term protection, a thorough understanding of the immune response to this pathogen is needed to identify parameters critical to the generation and durability of immunity. To approach this, we utilized a live attenuated cholera vaccine to model the response to V. cholerae infection in 12 naïve subjects. We found that this live attenuated vaccine induced durable vibriocidal antibody titers that were maintained at least one year after vaccination. Similar to what we previously reported in infected patients from Bangladesh, we found that vaccination induced plasmablast responses were primarily specific to the two immunodominant antigens lipopolysaccharide (LPS) and cholera toxin (CT). Interestingly, the magnitude of the early plasmablast response at day 7 predicted the serological outcome of vaccination at day 30. However, this correlation was no longer present at later timepoints. The acute responses displayed preferential immunoglobulin isotype usage, with LPS specific cells being largely IgM or IgA producing, while cholera toxin responses were predominantly IgG. Finally, CCR9 was highly expressed on vaccine induced plasmablasts, especially on IgM and IgA producing cells, suggesting a role in migration to the gastrointestinal tract. Collectively, these findings demonstrate that the use of a live attenuated cholera vaccine is an effective tool to examine the primary and long-term immune response following V. cholerae exposure. Additionally, it provides insight into the phenotype and specificity of the cells which likely return to and mediate immunity at the intestinal mucosa. A thorough understanding of these properties both in peripheral blood and in the intestinal mucosae will inform future vaccine development against both cholera and other mucosal pathogens. Trial Registration: NCT03251495.

A Salmonella Typhi Controlled Human Infection Study for Assessing Correlation between Bactericidal Antibodies and Protection against Infection Induced by Typhoid Vaccination

Vi-polysaccharide conjugate vaccines are efficacious against typhoid fever in children living in endemic settings, their recent deployment is a promising step in the control of typhoid fever. However, there is currently no accepted correlate of protection. IgG and IgA antibodies generated in response to Vi conjugate or Vi plain polysaccharide vaccination are important but there are no definitive protective titre thresholds. We adapted a luminescence-based serum bactericidal activity (SBA) for use with S. Typhi and assessed whether bactericidal antibodies induced by either Vi tetanus toxoid conjugate (Vi-TT) or Vi plain polysaccharide (Vi-PS) were associated with protection in a controlled human infection model of typhoid fever. Both Vi-PS and Vi-TT induced significant increase in SBA titre after 28 days (Vi-PS; p < 0.0001, Vi-TT; p = 0.003), however higher SBA titre at the point of challenge did not correlate with protection from infection or reduced symptom severity. We cannot eliminate the role of SBA as part of a multifactorial immune response which protects against infection, however, our results do not support a strong role for SBA as a mechanism of Vi vaccine mediated protection in the CHIM setting.

Antibody-Dependent Enhancement of Bacterial Disease: Prevalence, Mechanisms, and Treatment

Antibody-dependent enhancement (ADE) of viral disease has been demonstrated for infections caused by flaviviruses and influenza viruses; however, antibodies that enhance bacterial disease are relatively unknown. In recent years, a few studies have directly linked antibodies with exacerbation of bacterial disease. This ADE of bacterial disease has been observed in mouse models and human patients with bacterial infections. This antibody-mediated enhancement of bacterial infection is driven by various mechanisms that are disparate from those found in viral ADE. This review aims to highlight and discuss historic evidence, potential molecular mechanisms, and current therapies for ADE of bacterial infection. Based on specific case studies, we report how plasmapheresis has been successfully used in patients to ameliorate infection-related symptomatology associated with bacterial ADE. A greater understanding and appreciation of bacterial ADE of infection and disease could lead to better management of infections and inform current vaccine development efforts.

Transcriptional profiling of Vibrio cholerae O1 following exposure to human anti- lipopolysaccharide monoclonal antibodies

Following an episode of cholera, a rapidly dehydrating, watery diarrhea caused by the Gram-negative bacterium, Vibrio cholerae O1, humans mount a robust anti-lipopolysaccharide (LPS) antibody response that is associated with immunity to subsequent re-infection. In neonatal mouse and rabbit models of cholera, passively administered anti-LPS polyclonal and monoclonal (MAb) antibodies reduce V. cholerae colonization of the intestinal epithelia by inhibiting bacterial motility and promoting vibrio agglutination. Here we demonstrate that human anti-LPS IgG MAbs also arrest V. cholerae motility and induce bacterial paralysis. A subset of those MAbs also triggered V. cholerae to secrete an extracellular matrix (ECM). To identify changes in gene expression that accompany antibody exposure and that may account for motility arrest and ECM production, we subjected V. cholerae O1 El Tor to RNA-seq analysis after treatment with ZAC-3 IgG, a high affinity MAb directed against the core/lipid A region of LPS. We identified > 160 genes whose expression was altered following ZAC-3 IgG treatment, although canonical outer membrane stress regulons were not among them. ompS (VCA1028), a porin associated with virulence and indirectly regulated by ToxT, and norR (VCA0182), a σ54-dependent transcription factor involved in late stages of infection, were two upregulated genes worth noting.

Immune responses to O-specific polysaccharide (OSP) in North American adults infected with Vibrio cholerae O1 Inaba

Background

Antibodies targeting O-specific polysaccharide (OSP) of Vibrio cholerae may protect against cholera; however, little is known about this immune response in infected immunologically naïve humans.

Methodology

We measured serum anti-OSP antibodies in adult North American volunteers experimentally infected with V. cholerae O1 Inaba El Tor N16961. We also measured vibriocidal and anti-cholera toxin B subunit (CtxB) antibodies and compared responses to those in matched cholera patients in Dhaka, Bangladesh, an area endemic for cholera.

Principal findings

We found prominent anti-OSP antibody responses following initial cholera infection: these responses were largely IgM and IgA, and highest to infecting serotype with significant cross-serotype reactivity. The anti-OSP responses peaked 10 days after infection and remained elevated over baseline for ≥ 6 months, correlated with vibriocidal responses, and may have been blunted in blood group O individuals (IgA anti-OSP). We found significant differences in immune responses between naïve and endemic zone cohorts, presumably reflecting previous exposure in the latter.

Conclusions

Our results define immune responses to O-specific polysaccharide in immunologically naive humans with cholera, find that they are largely IgM and IgA, may be blunted in blood group O individuals, and differ in a number of significant ways from responses in previously humans. These differences may explain in part varying degrees of protective efficacy afforded by cholera vaccination between these two populations.

Trial registration number

ClinicalTrials.gov NCT01895855.

Cholera is an acute, secretory diarrheal disease caused by Vibrio cholerae O1. There is a growing body of evidence that immune responses targetting the O-specific polysaccharide (OSP) of V. cholerae are associated with protecton against cholera. Despite this, little is known about immune responses targeting OSP in immunologically naive individals. Cholera affects populations in severely resource-limited areas. To address this, we assessed anti-OSP immune responses in North American volunteers experimentally infected with wild type V. cholerae O1 El Tor Inaba strain N16961. We found that antibody responses were largely IgM and IgA, cross-reacted to both Inaba and Ogawa serotypes, and correlated with vibriocidal responses. We found no association of responses to severity of disease, but did find that blood group O individuals mounted lower IgA fold-changes to OSP than did non-blood group O individuals. Individuals with blood group O are at particular risk for severe cholera, and are less well protected against cholera following oral vaccination. We also compared anti-OSP responses in previously unexposed individuals to responses in matched endemic zone patients, and found a number of significant differences. Such differences may explain in part the varying degrees of protective efficacy afforded by cholera vaccination between these two populations.