Cholera toxin activates nonconventional adjuvant pathways that induce protective CD8 T-cell responses after epicutaneous vaccination

Migratory CD103+CD11b+ cDC2s in Peyer's patches are critical for gut IgA responses following oral immunization

Induction and regulation of specific intestinal immunoglobulin (Ig)A responses critically depend on dendritic cell (DC) subsets and the T cells they activate in the Peyer's patches (PP). We found that oral immunization with cholera toxin (CT) as an adjuvant resulted in migration-dependent changes in the composition and localization of PP DC subsets with increased numbers of cluster of differentiation (CD)103- conventional DC (cDC)2s and lysozyme-expressing DC (LysoDCs) in the subepithelial dome and of CD103+ cDC2s that expressed CD101 in the T cell zones, while oral ovalbumin (OVA) tolerization was instead associated with greater accumulation of cDC1s and peripherally induced regulatory T cells (pTregs) in this area. Decreased IgA responses were observed after CT-adjuvanted immunization in huCD207DTA mice lacking CD103+ cDC2s, while oral OVA tolerization was inefficient in cDC1-deficient Batf3-/- mice. Using OVA transgenic T cell receptor CD4 T cell adoptive transfer models, we found that co-transferred endogenous wildtype CD4 T cells can hinder the induction of OVA-specific IgA responses through secretion of interleukin-10. CT could overcome this blocking effect, apparently through a modulating effect on pTregs while promoting an expansion of follicular helper T cells. The data support a model where cDC1-induced pTreg normally suppresses PP responses for any given antigen and where CT's oral adjuvanticity effect is dependent on promoting follicular helper T cell responses through induction of CD103+ cDC2s.

ADP-ribosylating adjuvant reveals plasticity in cDC1 cells that drive mucosal Th17 cell development and protection against influenza virus infection

Migratory dendritic cells expressing CD103 are the targets for mucosal vaccines. These belong to either of two lineage-restricted subsets, cDC1 or cDC2 cells, which have been linked to priming of functionally distinct CD4 T cells. However, recent studies have identified plasticity in cDC2 cells with overlapping functions with cDC1 cells, while the converse has not been reported. We genetically engineered a vaccine adjuvant platform that targeted the cholera toxin A1 (CTA1) ADP-ribosylating enzyme to CD103⁺ cDC1 and cDC2 cells using a single-chain antibody (scFv) to CD103. Unexpectedly, intranasal immunization with the CTA1-svFcCD103 adjuvant modified cDC1 cells to effectively prime Th17 cells, a function previously limited to cDC2 cells. In fact, cDC2 cells were dispensible, while cDC1 cells, lacking in Batf3-/- mice, were critical. Following intranasal immunizations isolated cDC1 cells from mLN exclusively promoted Rorgt⁺ T cells and IL-17, IL-21, and IL-22 production. Strong CD8 T cell responses through antigen cross presentation by cDC1 cells were also observed. Single-cell RNAseq analysis revealed upregulation of Th17-promoting gene signatures in sorted cDC1 cells. Gene expression in isolated cDC2 cells was largely unaffected. Our finding represents a major shift of paradigm as we have documented functional plasticity in cDC1 cells.

Bacteria-Inspired Nanomedicine

The natural world has provided a host of materials and inspiration for the field of nanomedicine. By taking design cues from naturally occurring systems, the nanoengineering of advanced biomimetic platforms has significantly accelerated over the past decade. In particular, the biomimicry of bacteria, with their motility, taxis, immunomodulation, and overall dynamic host interactions, has elicited substantial interest and opened up exciting avenues of research. More recently, advancements in genetic engineering have given way to more complex and elegant systems with tunable control characteristics. Furthermore, bacterial derivatives such as membrane ghosts, extracellular vesicles, spores, and toxins have proven advantageous for use in nanotherapeutic applications, as they preserve many of the features from the original bacteria while also offering distinct advantages. Overall, bacteria-inspired nanomedicines can be employed in a range of therapeutic settings, from payload delivery to immunotherapy, and have proven successful in combatting both cancer and infectious disease.

Protective multi-epitope candidate vaccine for urinary tract infection

Urinary tract infections (UTIs) are induced by exogenous organisms including extraintestinal pathogenic such as Escherichia coli (ExPEC), Proteus mirabilis and Klebsiella pneumonia, which are closely related. These organisms can colonize in the urinary tract and cause UTIs. In this study, a cross-reactive multi-epitope vaccine was designed by two constructs to stimulate the immune system (CD8+ and CD4 + T cells) against ExPEC, Proteus mirabilis and Klebsiella pneumonia strains. Uropathogenic Escherichia coli (UPEC), Proteus mirabilis and Klebsiella pneumoniae are the main bacterial cause of UTI. They were used for designing experimental candidate vaccine, and their immunogenicity and protectivity were assessed. In this study, conserved antigens from their bacterial genomes were considered, and informatics-based immunological vaccine with cross-protective T and B-cells epitopes was designed and evaluated. The vaccine candidate was used as a broad immune system inducer, and its cross-protective immunity and protectivity were confirmed in in vivo experiments.

Adjuvant selection regulates gut migration and phenotypic diversity of antigen-specific CD4+ T cells following parenteral immunization

Infectious diarrheal diseases are the second leading cause of death in children under 5 years, making vaccines against these diseases a high priority. It is known that certain vaccine adjuvants, chiefly bacterial ADP-ribosylating enterotoxins, can induce mucosal antibodies when delivered parenterally. Based on this, we reasoned vaccine-specific mucosal cellular immunity could be induced via parenteral immunization with these adjuvants. Here, we show that, in contrast to the Toll-like receptor-9 agonist CpG, intradermal immunization with non-toxic double-mutant heat-labile toxin (dmLT) from enterotoxigenic Escherichia coli drove endogenous, antigen-specific CD4⁺ T cells to expand and upregulate the gut-homing integrin α4β7. This was followed by T-cell migration into gut-draining lymph nodes and both small and large intestines. We also found that dmLT produces a balanced T-helper 1 and 17 (Th1 and Th17) response, whereas T cells from CpG immunized mice were predominantly Th1. Immunization with dmLT preferentially engaged CD103⁺ dendritic cells (DCs) compared with CpG, and mice deficient in CD103⁺ DCs were unable to fully license antigen-specific T-cell migration to the intestinal mucosae following parenteral immunization. This work has the potential to redirect the design of existing and next generation vaccines to elicit pathogen-specific immunity in the intestinal tract with non-mucosal immunization.

Low doses of cholera toxin and its mediator cAMP induce CTLA-2 secretion by dendritic cells to enhance regulatory T cell conversion

Immature or semi-mature dendritic cells (DCs) represent tolerogenic maturation stages that can convert naive T cells into Foxp3⁺ induced regulatory T cells (iTreg). Here we found that murine bone marrow-derived DCs (BM-DCs) treated with cholera toxin (CT) matured by up-regulating MHC-II and costimulatory molecules using either high or low doses of CT (CT^hi, CT^lo) or with cAMP, a known mediator CT signals. However, all three conditions also induced mRNA of both isoforms of the tolerogenic molecule cytotoxic T lymphocyte antigen 2 (CTLA-2α and CTLA-2β). Only DCs matured under CT^hi conditions secreted IL-1β, IL-6 and IL-23 leading to the instruction of Th17 cell polarization. In contrast, CT^lo- or cAMP-DCs resembled semi-mature DCs and enhanced TGF-β-dependent Foxp3⁺ iTreg conversion. iTreg conversion could be reduced using siRNA blocking of CTLA-2 and reversely, addition of recombinant CTLA-2α increased iTreg conversion in vitro. Injection of CT^lo- or cAMP-DCs exerted MOG peptide-specific protective effects in experimental autoimmune encephalomyelitis (EAE) by inducing Foxp3⁺ Tregs and reducing Th17 responses. Together, we identified CTLA-2 production by DCs as a novel tolerogenic mediator of TGF-β-mediated iTreg induction in vitro and in vivo. The CT-induced and cAMP-mediated up-regulation of CTLA-2 also may point to a novel immune evasion mechanism of Vibrio cholerae.

Allergen-Induced CD4+ T Cell Cytokine Production within Airway Mucosal Dendritic Cell-T Cell Clusters Drives the Local Recruitment of Myeloid Effector Cells

Allergic asthma develops in the mucosal tissue of small bronchi. At these sites, local cytokine production by Th2/Th17 cells is believed to be critical for the development of tissue eosinophilia/neutrophilia. Using the mouse trachea as a relevant model of human small airways, we performed advanced in vivo dynamic and in situ static imaging to visualize individual cytokine-producing T cells in the airway mucosa and to define their immediate cellular environment. Upon allergen sensitization, newly recruited CD4⁺ T cells formed discrete Ag-driven clusters with dendritic cells (DCs). Within T cell-DC clusters, a small fraction of CD4⁺ T cells produced IL-13 or IL-17 following prolonged Ag-specific interactions with DCs. As a result of local Th2 cytokine signaling, eosinophils were recruited into these clusters. Neutrophils also infiltrated these clusters in a T cell-dependent manner, but their mucosal distribution was more diffuse. Our findings reveal the focal nature of allergen-driven responses in the airways and define multiple steps with potential for interference with the progression of asthmatic pathology.

Evaluation of protective efficacy induced by virus-like particles containing a Trichinella spiralis excretory-secretory (ES) protein in mice

BACKGROUND

The frequent outbreaks of human trichinellosis globally underscore the need to develop effective vaccine. We hypothesized that a novel vaccine could improve vaccine efficacy against Trichinella spiralis.

METHODS

In this study, we developed virus-like particles (VLPs) containing the 53 KDa excretory/secretory (ES) protein of T. spiralis and the influenza matrix protein 1 (M1) as a core protein, and investigated the protective efficacy of the VLPs alone or with cholera toxin (CT) in a mouse model.

RESULTS

Intramuscular immunization induced T. spiralis-specific IgG, IgG1 and IgG2a antibody responses before and after challenge infections in the sera. These antibody responses were significantly enhanced in mice immunized with adjuvanted VLPs. Upon challenge infection, vaccinated mice showed significantly reduced worm burden in the diaphragm. Protective immune responses and efficacy of protection were significantly improved by immunization with VLPs together with CT adjuvant.

CONCLUSIONS

Our results are informative for a better understanding of the protective immunity induced by T. spiralis VLPs, and will provide insight into designing safe and effective vaccines.

Cholera toxin adjuvant promotes a balanced Th1/Th2/Th17 response independently of IL-12 and IL-17 by acting on Gsα in CD11b⁺ DCs

Despite an extensive literature on the mechanism of action of cholera toxin (CT), we still lack critical information about how the toxin acts as an adjuvant and, especially, which dendritic cells (DCs) are the target cells. Although a T helper type 2 (Th2)-skewing effect of CT is most commonly reported, effective priming of Th17 cells as well as suppression of Th1 responses are well documented. However, the ability of CT to block interferon regulatory factor 8 (IRF8) function and interleukin (IL)-12 production in DCs, which blocks CD8α DC and Th1 cell development, is inconsistent with priming of Th1 and CD8 T cells in many other reports. This prompted us to investigate the adjuvant effect of CT in wild-type, IL-12p40-/-, Batf3-/-, and IL-17A-/- mice and in mice that selectively lack the Gsα target protein for CT adenosine diphosphate (ADP)-ribosylation in DCs. We found that CT promoted Th1 priming independently of IL-12, and whereas Th2 and also Th17 responses were augmented, the gut IgA responses did not require IL-17A. Adjuvanticity was intact in Batf3-/- mice, lacking CD8α(+) DCs, but completely lost in mice with Gsα-deficient CD11c cells. Thus, our data demonstrate that the adjuvant effect requires Gsα expression in CD11b(+) DCs, and that priming of mucosal IgA and CD4 T cells appears unbiased and is independent of IL-12 and IL-17A.

Animal models for cutaneous vaccine delivery

Main challenges in skin vaccination are overcoming the stratum corneum (SC) barrier and targeting the antigen presenting cells (APC) in the epidermis and the dermis. For this purpose many delivery techniques are being developed. In vivo immunogenicity and safety studies in animals are mandatory before moving to clinical trials. However, the results obtained in animals may or may not be predictive for humans. Knowledge about differences and similarities in skin architecture and immunology within a species and between species is crucial. In this review, we discuss variables, including skin morphology, skin barrier function, mechanical properties, site of application and immunology, which should be taken into account when designing animal studies for vaccination via the skin in order to support the translation to clinical trial outcomes.

Intranasal immunization with influenza antigens conjugated with cholera toxin subunit B stimulates broad spectrum immunity against influenza viruses

Frequent mutation of influenza viruses keep vaccinated and non-vaccinated populations vulnerable to new infections, causing serious burdens to public health and the economy. Vaccination with universal influenza vaccines would be the best way to effectively protect people from infection caused by mismatched or unforeseen influenza viruses. Presently, there is no FDA approved universal influenza vaccine. In this study, we expressed and purified a fusion protein comprising of influenza matrix 2 protein ectodomain peptides, a centralized influenza hemagglutinin stem region, and cholera toxin subunit B. Vaccination of BALB/c mice with this novel artificial antigen resulted in potent humoral immune responses, including induction of specific IgA and IgG, and broad protection against infection by multiple influenza viruses. Furthermore, our results demonstrated that when used as a mucosal antigen, cholera toxin subunit B improved antigen-stimulated T cell and memory B cell responses.

CD11b+ migratory dendritic cells mediate CD8 T cell cross-priming and cutaneous imprinting after topical immunization

Topical antigen application is a focus of current vaccine research. This immunization route mimics natural antigen exposure across a barrier tissue and generates T cells imprinted for skin-selective homing. Soluble antigens introduced through this route require cross-presentation by DC to generate CD8 T cell responses. Here we have explored the relative contribution of various skin-derived DC subsets to cross-priming and skin-selective imprinting. In our model, DC acquire soluble Ag in vivo from immunized murine skin for cross-presentation to naïve CD8 T cells ex vivo. We find CD11b⁺ migratory DC to be the relevant cross-priming DC in this model. Both Langerin⁺ and Langerin^- CD11b⁺ migratory DC can cross-present antigen in our system, but only the Langerin⁺ subset can induce expression of the skin-selective addressin E-selectin ligand. Thus, the CD11b⁺ Langerin⁺ migratory DC population, comprised primarily of Langerhans cells, both cross-primes naïve CD8 T cells and imprints them with skin-homing capabilities.

Toxicity and immunogenicity of Enterotoxigenic Escherichia coli heat-labile and heat-stable toxoid fusion 3xSTa(A14Q)-LT(S63K/R192G/L211A) in a murine model

Diarrhea is the second leading cause of death to young children. Enterotoxigenic Escherichia coli (ETEC) are the most common bacteria causing diarrhea. Adhesins and enterotoxins are the virulence determinants in ETEC diarrhea. Adhesins mediate bacterial attachment and colonization, and enterotoxins including heat-labile (LT) and heat-stable type Ib toxin (STa) disrupt fluid homeostasis in host cells that leads to fluid hyper-secretion and diarrhea. Thus, adhesins and enterotoxins have been primarily targeted in ETEC vaccine development. A recent study reported toxoid fusions with STa toxoid (STa_P13F) fused at the N- or C-terminus, or inside the A subunit of LT_R192G elicited neutralizing antitoxin antibodies, and suggested application of toxoid fusions in ETEC vaccine development (Liu et al., Infect. Immun. 79:4002-4009, 2011). In this study, we generated a different STa toxoid (STa_A14Q) and a triple-mutant LT toxoid (LT_{S63K/R192G/L211A}, tmLT), constructed a toxoid fusion (3xSTa_A14Q-tmLT) that carried 3 copies of STa_A14Q for further facilitation of anti-STa immunogenicity, and assessed antigen safety and immunogenicity in a murine model to explore its potential for ETEC vaccine development. Mice immunized with this fusion antigen showed no adverse effects, and developed antitoxin antibodies particularly through the IP route. Anti-LT antibodies were detected and were shown neutralizing against CT in vitro. Anti-STa antibodies were also detected in the immunized mice, and serum from the IP immunized mice neutralized STa toxin in vitro. Data from this study indicated that toxoid fusion 3xSTa_A14Q-tmLT is safe and can induce neutralizing antitoxin antibodies, and provided helpful information for vaccine development against ETEC diarrhea.

Subcutaneous cholera toxin exposure induces potent CD103⁺ dermal dendritic cell activation and migration

CD103⁺ dermal dendritic cells (dDCs) are a recently described DC subset of the skin shown to be the principal migratory DCs capable of efficiently cross-presenting antigens and activating CD8⁺ T cells. Harnessing their activity would promote vaccine efficacy, but it has been unclear how this can be achieved. We tested a panel of adjuvants for their ability to affect dDCs. In comparison to the other adjuvants tested, the capacity of cholera toxin (CT) to induce the migration of dDCs was unique. Within 24 h of CT injection, large numbers of highly activated dDCs (including CD103⁺ dDCs) migrated to the draining lymph nodes and cross-presented coinjected antigens, potently activating naïve CD8⁺ T cells. Peptide vaccines adjuvanted with CT induced T-cell responses uniquely characterized by dynamic cytokine responses including the production of IL-2, and such vaccines were protective in situations reliant on CD8⁺ T-cell responses, including liver-stage Plasmodium challenge, or tumor challenge. This study is the first to examine the effects of adjuvants on CD103⁺ dDCs and identifies CT as a prototypical adjuvant for the activation of CD103⁺ dDCs, opening the way to development of vaccines and adjuvants that specifically target dDCs and generate effective CD8⁺ T-cell responses.

Site-specific C-terminal and internal loop labeling of proteins using sortase-mediated reactions

Methods for site-specific modification of proteins should be quantitative and versatile with respect to the nature and size of the biological or chemical targets involved. They should require minimal modification of the target, and the underlying reactions should be completed in a reasonable amount of time under physiological conditions. Sortase-mediated transpeptidation reactions meet these criteria and are compatible with other labeling methods. Here we describe the expression and purification conditions for two sortase A enzymes that have different recognition sequences. We also provide a protocol that allows the functionalization of any given protein at its C terminus, or, for select proteins, at an internal site. The target protein is engineered with a sortase-recognition motif (LPXTG) at the place where modification is desired. Upon recognition, sortase cleaves the protein between the threonine and glycine residues, facilitating the attachment of an exogenously added oligoglycine peptide modified with the functional group of choice (e.g., fluorophore, biotin, protein or lipid). Expression and purification of sortase takes ∼3 d, and sortase-mediated reactions take only a few minutes, but reaction times can be extended to increase yields.

Transcutaneous antigen delivery system

Transcutaneous immunization refers to the topical application of antigens onto the epidermis. Transcutaneous immunization targeting the Langerhans cells of the skin has received much attention due to its safe, needle-free, and noninvasive antigen delivery. The skin has important immunological functions with unique roles for antigen-presenting cells such as epidermal Langerhans cells and dermal dendritic cells. In recent years, novel vaccine delivery strategies have continually been developed; however, transcutaneous immunization has not yet been fully exploited due to the penetration barrier represented by the stratum corneum, which inhibits the transport of antigens and adjuvants. Herein we review recent achievements in transcutaneous immunization, focusing on the various strategies for the enhancement of antigen delivery and vaccination efficacy. [BMB Reports 2013; 46(1): 17-24]

Antigen presentation by Langerhans cells

Langerhans cells and other skin-resident dendritic cells (DC) are required for the development of cutaneous adaptive immune responses. In vivo experiments using mice with selective DC-subset deficiencies and ex vivo experiments using isolated DC suggests that each subset makes a unique contribution to the adaptive response. This review focuses on the functional outcome of antigen presentation by Langerhans cells. Special attention is given to their ability to promote CD4 T cell differentiation in a variety of inflammatory contexts and whether this subset has the capacity to cross-prime CD8 T cells.