Coordination between T helper cells, iNKT cells, and their follicular helper subsets in the humoral immune response against Clostridium difficile toxin B

Host Immunity and Immunization Strategies for Clostridioides difficile Infection

Clostridioides difficile infection (CDI) represents a significant challenge to public health. C. difficile-associated mortality and morbidity have led the U.S. CDC to designate it as an urgent threat. Moreover, recurrence or relapses can occur in up to a third of CDI patients, due in part to antibiotics being the primary treatment for CDI and the major cause of the disease. In this review, we summarize the current knowledge of innate immune responses, adaptive immune responses, and the link between innate and adaptive immune responses of the host against CDI. The other major determinants of CDI, such as C. difficile toxins, the host microbiota, and related treatments, are also described. Finally, we discuss the known therapeutic approaches and the current status of immunization strategies for CDI, which might help to bridge the knowledge gap in the generation of therapy against CDI.

A subset of follicular helper-like MAIT cells can provide B cell help and support antibody production in the mucosa

[Figure: see text].

Use of a Clostridioides difficile Murine Immunization and Challenge Model to Evaluate Single and Combination Vaccine Adjuvants Consisting of Alum and NKT Cell-Activating Ligands

Adjuvant combinations may enhance or broaden the expression of immune responses to vaccine antigens. Information on whether established Alum type adjuvants can be combined with experimental CD1d ligand adjuvants is currently lacking. In this study, we used a murine Clostridioides difficile immunization and challenge model to evaluate Alum (Alhydrogel™), α-galactosylceramide (α-GC), and one of its analogs 7DW8-5 singly and in combination as vaccine adjuvants. We observed that the Alum/α-GC combination caused modest enhancement of vaccine antigen-specific IgG1 and IgG2b responses, and a broadening to include IgG2c that did not significantly impact overall protection. Similar observations were made using the Alum/7DW8-5 combination. Examination of the impact of adjuvants on NKT cells revealed expansion of invariant NKT (iNKT) cells with modest expansion of their iNKTfh subset and little effect on diverse NKT (dNKT) cells. Side effects of the adjuvants was determined and revealed transient hepatotoxicity when Alum/α-GC was used in combination but not singly. In summary these results showed that the Alum/α-GC or the Alum/7DW8-5 combination could exert distinct effects on the NKT cell compartment and on isotype switch to produce Th1-driven IgG subclasses in addition to Alum/Th2-driven subclasses. While Alum alone was efficacious in stimulating IgG-mediated protection, and α-GC offered no apparent additional benefit in the C. difficile challenge model, the work herein reveals immune response features that could be optimized and harnessed in other vaccine contexts.

Host Immune Responses to Clostridioides difficile: Toxins and Beyond

Clostridioides difficile is often resistant to the actions of antibiotics to treat other bacterial infections and the resulting C. difficile infection (CDI) is among the leading causes of nosocomial infectious diarrhea worldwide. The primary virulence mechanism contributing to CDI is the production of toxins. Treatment failures and recurrence of CDI have urged the medical community to search for novel treatment options. Strains that do not produce toxins, so called non-toxigenic C. difficile, have been known to colonize the colon and protect the host against CDI. In this review, a comprehensive description and comparison of the immune responses to toxigenic C. difficile and non-toxigenic adherence, and colonization factors, here called non-toxin proteins, is provided. This revealed a number of similarities between the host immune responses to toxigenic C. difficile and non-toxin proteins, such as the influx of granulocytes and the type of T-cell response. Differences may reflect genuine variation between the responses to toxigenic or non-toxigenic C. difficile or gaps in the current knowledge with respect to the immune response toward non-toxigenic C. difficile. Toxin-based and non-toxin-based immunization studies have been evaluated to further explore the role of B cells and reveal that plasma cells are important in protection against CDI. Since the success of toxin-based interventions in humans to date is limited, it is vital that future research will focus on the immune responses to non-toxin proteins and in particular non-toxigenic strains.

α-Galactosylceramide-Reactive NKT Cells Increase IgG1 Class Switch against a Clostridioides difficile Polysaccharide Antigen and Enhance Immunity against a Live Pathogen Challenge

All clinical Clostridioides difficile strains identified to date express a surface capsule-like polysaccharide structure known as polysaccharide II (PSII). The PSII antigen is immunogenic and, when conjugated to a protein carrier, induces a protective antibody response in animal models. Given that CD1d-restricted natural killer T (NKT) cells promote antibody responses, including those against carbohydrates, we tested the hypothesis that immunization with PSII and a CD1d-binding glycolipid adjuvant could lead to enhanced protection against a live C. difficile challenge. We purified PSII from a clinical isolate of C. difficile and immunized B6 mice with PSII alone or PSII plus the CD1d-binding glycolipid α-galactosylceramide (α-GC). PSII-specific IgM and IgG titers were evident in sera from immunized mice. The inclusion of α-GC had a modest influence on isotype switch but increased the IgG1/IgG2c ratio. Enhanced protection against C. difficile disease was achieved by inclusion of the α-GC ligand and was associated with reduced bacterial numbers in fecal pellets. In contrast, NKT-deficient Traj18^-/- mice were not protected by the PSII/α-GC immunization modality. Absence of NKT cells similarly had a modest effect on isotype switch, but ratios of IgG1/IgG2c decreased. These results indicate that α-GC-driven NKT cells move the humoral immune response against C. difficile PSII antigen toward Th2-driven IgG1 and may contribute to augmented protection. This study suggests that NKT activation represents a pathway for additional B-cell help that could be used to supplement existing efforts to develop vaccines against polysaccharides derived from C. difficile and other pathogens.

IL-16 and BCA-1 Serum Levels Are Associated with Disease Severity of C. difficile Infection

Clostridioides difficile infection (CDI) is associated with a high risk for complications and death, which requires identifying severe patients and treating them accordingly. We examined the serum level of six cytokines and chemokines (IL-16, IL-21, IL-23, IL-33, BCA-1, TRAIL) and investigated the association between them and patients' disease severity. Concentrations of six cytokines and chemokines were measured using the MILLIPLEX®MAP kit (Billerica, MA, USA) in serum samples attained from CDI patients within 24-48 h after laboratory confirmation of C. difficile presence. Demographic and clinical data were collected from medical records. The disease severity score was determined according to guidelines of the "Society for Healthcare Epidemiology of America and the Infectious Diseases Society of America" (SHEA-IDSA). Out of 54 patients, 20 (37%) had mild to moderate disease and 34 (63%) had severe disease. IL-16 (p = 0.005) and BCA-1 (p = 0.012) were associated with a more severe disease. In conclusion, IL-16 and BCA-1, along with other cytokines and chemokines, may serve as biomarkers for the early prediction of CDI severity in the future. An improved and more accessible assessment of CDI severity will contribute to the adjustment of the medical treatment, which will lead to a better patient outcome.

Morbid Obesity Increases 30-Day Readmission and Morbidity in Clostridiodes difficile Infection

BACKGROUND

Readmission for patients hospitalized with Clostridioides difficile infections (CDIs) carries high morbidity, mortality, and burden on healthcare resources. This study aims to determine if morbid obesity is risk factor for 30-day readmission in patients hospitalized with CDI.

METHODS

This retrospective cohort study used the 2014 National Readmission Database. Included patients had a principal diagnosis of CDI and excluded if younger than 18 years of age or admitted during the month of December. Readmissions to any hospital for non-trauma diagnoses within 30 days of the index admission were included. The primary outcome was 30-day all cause readmission. Secondary outcomes were (1) in-hospital mortality; (2) morbidity, initiation of dialysis, or total parenteral nutrition; and (3) resource utilization during index admissions.

RESULTS

A total of 91,265 subjects were included in this study, 4388 of whom were morbidly obese. Morbid obesity was associated with significantly higher odds of readmission and was associated with higher adjusted mean total hospitalization charges and costs, higher odds of PMV, and acute renal failure requiring dialysis in individuals that develop CDI.

CONCLUSIONS

Morbid obesity is an independent risk factor for 30-day readmission in patients hospitalized for CDI. Morbidly obese patients admitted for CDI experienced higher morbidity and increased resource utilization.

The adaptive immune response to Clostridioides difficile: A tricky balance between immunoprotection and immunopathogenesis

Clostridioides difficile (C. difficile) is the major cause of hospital-acquired gastrointestinal infections in individuals following antibiotics treatment. The pathogenesis of C. difficile infection (CDI) is mediated mainly by the production of toxins that induce tissue damage and host inflammatory responses. While innate immunity is well characterized in human and animal models of CDI, adaptive immune responses remain poorly understood. In this review, the current understanding of adaptive immunity is summarized and its influence on pathogenesis and disease outcome is discussed. The perspectives on what we believe to be the main pending questions and the focus of future research are also provided. There is no doubt that the innate immune response provides a first line of defense to CDI. But, is the adaptive immune response a friend or a foe? Probably it depends on the course of the disease. Adaptive immunity is essential for pathogen eradication, but may also trigger uncontrolled or pathological inflammation. Most of the understanding of the role of T cells is based on findings from experimental models. While they are a very valuable tool for research studies, more studies in human are needed to translate these findings into human disease. Another main challenge is to unravel the role of the different T cell populations on protection or induction of immunopathogenesis.

Immunization-Expanded NKT Follicular Helper Cells Drive IgG1 Isotype Switch against an Exogenous T-Independent Polysaccharide but Do Not Promote Recall Responses

The CD1d-binding glycolipid α-galactosylceramide (α-GC) is a potent adjuvant that activates NKT cells and in turn enhances T-dependent humoral immunity. Very little is known about how NKT cells and the NKT follicular helper (NKTfh) subset influence the immune response to T-independent polysaccharides. In this study, we used a Cre-Lox approach to generate mice devoid of the Bcl6 master transcription factor in CD4 lineage cells and thus devoid of NKTfh cells but not total NKT cells. It was observed that α-GC-driven IgG1 class switch against a polysaccharide Ag was dependent on the NKTfh subset. However, α-GC was unable to stimulate a polysaccharide-specific Ab recall response. It was observed that NKT-derived IL-21 was able to exert limited influence on the IgG1 response and was therefore likely to work in concert with other factors. This work shows that α-GC-driven NKTfh cells can direct polysaccharide-specific B cell responses by promoting IgG1 class switch but do not provide signals needed for generation of polysaccharide-specific B cell memory.

Insights Into the Molecular Mechanisms of T Follicular Helper-Mediated Immunity and Pathology

T follicular helper (Tfh) cells play key role in providing help to B cells during germinal center (GC) reactions. Generation of protective antibodies against various infections is an important aspect of Tfh-mediated immune responses and the dysregulation of Tfh cell responses has been implicated in various autoimmune disorders, inflammation, and malignancy. Thus, their differentiation and maintenance must be closely regulated to ensure appropriate help to B cells. The generation and function of Tfh cells is regulated by multiple checkpoints including their early priming stage in T zones and throughout the effector stage of differentiation in GCs. Signaling pathways activated downstream of cytokine and costimulatory receptors as well as consequent activation of subset-specific transcriptional factors are essential steps for Tfh cell generation. Thus, understanding the mechanisms underlying Tfh cell-mediated immunity and pathology will bring into spotlight potential targets for novel therapies. In this review, we discuss the recent findings related to the molecular mechanisms of Tfh cell differentiation and their role in normal immune responses and antibody-mediated diseases.

Activation and Regulation of B Cell Responses by Invariant Natural Killer T Cells

CD1d-restricted invariant natural killer T (iNKT) cells play central roles in the activation and regulation of innate and adaptive immunity. Cytokine-mediated and CD1d-dependent interactions between iNKT cells and myeloid and lymphoid cells enable iNKT cells to contribute to the activation of multiple cell types, with important impacts on host immunity to infection and tumors and on the prevention of autoimmunity. Here, we review the mechanisms by which iNKT cells contribute to B cell maturation, antibody and cytokine production, and antigen presentation. Cognate interactions with B cells contribute to the rapid production of antibodies directed against conserved non-protein antigens resulting in rapid but short-lived innate humoral immunity. iNKT cells can also provide non-cognate help for the generation of antibodies directed against protein antigens, by promoting the activation of follicular helper T cells, resulting in long-lasting adaptive humoral immunity and B cell memory. iNKT cells can also regulate humoral immunity by promoting the development of autoreactive B cells into regulatory B cells. Depletions and functional impairments of iNKT cells are found in patients with infectious, autoimmune and malignant diseases associated with altered B cell function and in murine models of these conditions. The adjuvant and regulatory activities that iNKT cells have for B cells makes them attractive therapeutic targets for these diseases.

The Influence of Invariant Natural Killer T Cells on Humoral Immunity to T-Dependent and -Independent Antigens

Vaccination with CD1d-binding glycolipid adjuvants and co-administered protein, lipid, and carbohydrate antigens leads to invariant natural killer T (NKT) cell-dependent enhancement of protective B cell responses. NKT cell activation boosts the establishment of protein antigen-specific B cell memory and long-lived plasma cell (LLPC) compartments. NKT cells may exert a similar effect on some carbohydrate-specific B cells, but not lipid-specific B cells. The mechanisms of action of NKT cells on B cell responsiveness and subsequent differentiation into memory B cells and LLPC is dependent on CD1d expression by dendritic cells and B cells that can co-present glycolipids on CD1d and antigen-derived peptide on MHCII. CD1d/glycolipid-activated NKT cells are able to provide help to B cells in a manner dependent on cognate and non-cognate interactions. More recently, a glycolipid-expanded subset of IL-21-secreting NKT cells known as NKT follicular helper cells has been suggested to be a driver of NKT-enhanced humoral immunity. This review summarizes established and recent findings on how NKT cells impact humoral immunity and suggests possible areas of investigation that may allow the incorporation of NKT-activating agents into vaccine adjuvant platforms.