Comparative analysis of the mucosal adjuvanticity of the type II heat-labile enterotoxins LT-IIa and LT-IIb

Immunomodulatory regulation by heat-labile enterotoxins and potential therapeutic applications

Introduction: Heat-labile enterotoxins (HLTs) and their cognate ganglioside receptors have been extensively studied because of their therapeutic potential. Gangliosides play arole in modulating effector cells of the immune system, and HLTs provide a novel means for stimulating ganglioside-mediated responses in immunocompetent cells.Areas covered: To evaluate the mechanisms of HLT adjuvanticity, a systemic literature review was performed using relevant keyword searches of the PubMed database, accessing literature published as recently as late 2020. Since HLTs bind to specific ganglioside receptors on immunocytes, they can act as regulators via stimulation or tapering of immune responses from associated signal transduction events. Binding of HLTs to gangliosides can increase proliferation of T-cells, increase cytokine release, augment mucosal/systemic antibody responses, and increase the effectiveness of antigen presenting cells. Subunit components also independently stimulate certain immune responses. Mutant forms of HLTs have potent immunomodulatory effects without the toxicity associated with holotoxins.Expert opinion: HLTs have been the subject of abundant research exploring their use as vaccine adjuvants, in the treatment of autoimmune conditions, in cancer therapy, and for weight loss, proving that these molecules are promising tools in the field of immunotherapy.

LT adjuvant modulates epitope specificity and improves the efficacy of murine antibodies elicited by sublingual vaccination with the N-terminal domain of Streptococcus mutans P1

In this study, we evaluated the immunogenicity, protective efficacy and peptide-based immune signatures of antibodies raised in mice after sublingual immunization with a recombinant form of the P1 (aka AgI/II, PAc) adhesin (P1_39-512) of Streptococcus mutans, a major etiological agent of dental caries. Sublingual administration of P1_39-512 in combination with the mucosal adjuvant LTK4R (a derivative of heat-labile LT toxin) induced strong and long-lasting systemic and mucosal immune responses. Incorporation of the adjuvant resulted in an enhancement of the anti-adhesive and anti-colonization activity against S. mutans as evaluated both under in vitro and in vivo conditions. Incorporation of the adjuvant to the vaccine formulation also changed the epitope specificity of the induced antibodies as determined by immunological signatures of sera collected from vaccinated mice. Use of a peptide microarray library led to the identification of peptide targets recognized by antibodies in serum samples with enhanced anti-adhesive effects. Altogether, the results presented herein showed that the sublingual administration of a P1-based subunit vaccine represents a promising approach for the prevention of dental caries caused by S. mutans. In addition, the present study disclosed the role of adjuvants on the epitope specificity and functionality of antibodies raised by subunit vaccines.

Immunomodulation with Enterotoxins for the Generation of Secretory Immunity or Tolerance: Applications for Oral Infections

The heat-labile enterotoxins, such as cholera toxin (CT), and the labile toxins types I and II (LT-I and LT-II) of Escherichia coli have been extensively studied for their immunomodulatory properties, which result in the enhancement of immune responses. Despite superficial similarity in structure, in which a toxic A subunit is coupled to a pentameric binding B subunit, different toxins have different immunological properties. Administration of appropriate antigens admixed with or coupled to these toxins by oral, intranasal, or other routes in experimental animals induces mucosal IgA and circulating IgG antibodies that have protective potential against a variety of enteric, respiratory, or genital infections. These include the generation of salivary antibodies that may protect against colonization with mutans streptococci and the development of dental caries. However, exploitation of these adjuvants for human use requires an understanding of their mode of action and the separation of their desirable immunomodulatory properties from their toxicity. Recent findings have revealed that adjuvant action is not critically dependent upon the enzymic activity of the A subunits, and that the isolated B subunits may exert different effects on cells of the immune system than do the intact toxins. Interaction of the toxins with immunocompetent cells is not exclusively dependent upon their conventional ganglioside receptors. Immunomodulatory effects have been observed on dendritic cells, macrophages, CD4+ and CD8+ T-cells, and B-cells. Numerous factors—including the precise form of the toxin adjuvant, properties of the antigen, whether and how they are coupled, route of administration, and species of animal model—affect the outcome, whether this is enhanced humoral and cellular immunity, or specific induced tolerance toward the antigen.

Novel Strategy To Protect against Influenza Virus-Induced Pneumococcal Disease without Interfering with Commensal Colonization

Streptococcus pneumoniae commonly inhabits the nasopharynx as a member of the commensal biofilm. Infection with respiratory viruses, such as influenza A virus, induces commensal S. pneumoniae to disseminate beyond the nasopharynx and to elicit severe infections of the middle ears, lungs, and blood that are associated with high rates of morbidity and mortality. Current preventive strategies, including the polysaccharide conjugate vaccines, aim to eliminate asymptomatic carriage with vaccine-type pneumococci. However, this has resulted in serotype replacement with, so far, less fit pneumococcal strains, which has changed the nasopharyngeal flora, opening the niche for entry of other virulent pathogens (e.g., Streptococcus pyogenes, Staphylococcus aureus, and potentially Haemophilus influenzae). The long-term effects of these changes are unknown. Here, we present an attractive, alternative preventive approach where we subvert virus-induced pneumococcal disease without interfering with commensal colonization, thus specifically targeting disease-causing organisms. In that regard, pneumococcal surface protein A (PspA), a major surface protein of pneumococci, is a promising vaccine target. Intradermal (i.d.) immunization of mice with recombinant PspA in combination with LT-IIb(T13I), a novel i.d. adjuvant of the type II heat-labile enterotoxin family, elicited strong systemic PspA-specific IgG responses without inducing mucosal anti-PspA IgA responses. This response protected mice from otitis media, pneumonia, and septicemia and averted the cytokine storm associated with septic infection but had no effect on asymptomatic colonization. Our results firmly demonstrated that this immunization strategy against virally induced pneumococcal disease can be conferred without disturbing the desirable preexisting commensal colonization of the nasopharynx.

Enhancement of humoral immunity by the type II heat-labile enterotoxin LT-IIb is dependent upon IL-6 and neutrophils

LT-IIb, a type II heat-labile enterotoxin produced by Escherichia coli, is a potent intradermal adjuvant that enhances immune responses to coadministered antigens. Although the immune mechanisms that promote this augmented immune response have not been well defined, prior intradermal immunization experiments suggested that early cellular and immunomodulatory events at the site of immunization modulated the augmentation of antigen-specific immune responses by LT-IIb. To investigate that hypothesis, mice were intradermally immunized with a recombinant ricin vaccine, a prospective toxin subunit antigen, in the presence and absence of LT-IIb. Analysis of tissue-fluid collection, coupled with histologic sections from the site of intradermal immunization, revealed that a single dose of LT-IIb induced local production of interleukin 6 and promoted a regional infiltration of neutrophils. The adjuvant effects of LT-IIb were abrogated in interleukin 6-deficient mice and when mice were depleted of neutrophils by pretreatment with anti-Ly6G. Overall, these data firmly demonstrated that LT-IIb, when used as an intradermal adjuvant, recruits neutrophils and is a potent rapid inducer of interleukin 6.

Comparative Adjuvant Effects of Type II Heat-Labile Enterotoxins in Combination with Two Different Candidate Ricin Toxin Vaccine Antigens

Type II heat-labile enterotoxins (HLTs) constitute a promising set of adjuvants that have been shown to enhance humoral and cellular immune responses when coadministered with an array of different proteins, including several pathogen-associated antigens. However, the adjuvant activities of the four best-studied HLTs, LT-IIa, LT-IIb, LT-IIb(T13I), and LT-IIc, have never been compared side by side. We therefore conducted immunization studies in which LT-IIa, LT-IIb, LT-IIb(T13I), and LT-IIc were coadministered by the intradermal route to mice with two clinically relevant protein subunit vaccine antigens derived from the enzymatic A subunit (RTA) of ricin toxin, RiVax and RVEc. The HLTs were tested with low and high doses of antigen and were assessed for their abilities to stimulate antigen-specific serum IgG titers, ricin toxin-neutralizing activity (TNA), and protective immunity. We found that all four HLTs tested were effective adjuvants when coadministered with RiVax or RVEc. LT-IIa was of particular interest because as little as 0.03 μg when coadministered with RiVax or RVEc proved effective at augmenting ricin toxin-specific serum antibody titers with nominal evidence of local inflammation. Collectively, these results justify the need for further studies into the mechanism(s) underlying LT-IIa adjuvant activity, with the long-term goal of evaluating LT-IIa's activity in humans.

Heat-Labile Enterotoxins

Heat-labile enterotoxins (LTs) of Escherichia coli are closely related to cholera toxin (CT), which was originally discovered in 1959 in culture filtrates of the gram-negative bacterium Vibrio cholerae. Several other gram-negative bacteria also produce enterotoxins related to CT and LTs, and together these toxins form the V. cholerae-E. coli family of LTs. Strains of E. coli causing a cholera-like disease were designated enterotoxigenic E. coli (ETEC) strains. The majority of LTI genes (elt) are located on large, self-transmissible or mobilizable plasmids, although there are instances of LTI genes being located on chromosomes or carried by a lysogenic phage. The stoichiometry of A and B subunits in holotoxin requires the production of five B monomers for every A subunit. One proposed mechanism is a more efficient ribosome binding site for the B gene than for the A gene, increasing the rate of initiation of translation of the B gene independently from A gene translation. The three-dimensional crystal structures of representative members of the LT family (CT, LTpI, and LTIIb) have all been determined by X-ray crystallography and found to be highly similar. Site-directed mutagenesis has identified many residues in the CT and LT A subunits, including His44, Val53, Ser63, Val97, Glu110, and Glu112, that are critical for the structures and enzymatic activities of these enterotoxins. For the enzymatically active A1 fragment to reach its substrate, receptor-bound holotoxin must gain access to the cytosol of target cells.

Intradermal administration of the Type II heat-labile enterotoxins LT-IIb and LT-IIc of enterotoxigenic Escherichia coli enhances humoral and CD8+ T cell immunity to a co-administered antigen

Vaccinations are extremely effective at combating infectious diseases. Many conserved antigen (Ag) targets, however, are poorly immunogenic. Protein subunit vaccines frequently elicit only humoral immune responses and fail to confer protection against serious intracellular pathogens. These barriers to vaccine development are often overcome by the use of appropriate adjuvants. Heat-labile enterotoxins (HLT) produced by enterotoxigenic strains of Escherichia coli are potent adjuvants when administered by mucosal or systemic routes. The efficacy of the type II HLT, however, has not been well-defined when administered by the intradermal (ID) route. Using a murine ID immunization model, the adjuvant properties of LT-IIb and LT-IIc, two type II HLTs, were compared with those of LT-I, a prototypical type I HLT. While all three HLT adjuvants enhanced Ag-specific humoral responses to similar levels, LT-IIb and LT-IIc, in contrast to LT-I, induced a more vigorous Ag-specific CD8⁺ T cell response and proffered faster clearance of Listeria monocytogenes in a challenge model. Additionally, LT-IIb and LT-IIc induced distinct differences in the profiles of the Ag-specific CD8⁺ T cell responses. While LT-IIc stimulated a robust and rapid primary CD8⁺ T cell response, LT-IIb exhibited slower CD8⁺ T cell expansion and contraction kinetics with the formation of higher percentages of effector memory cells. In comparison to LT-I and LT-IIc, LT-IIb evoked better long-term protection after immunization. Furthermore, LT-IIb and LT-IIc enhanced the total number of dendritic cells (DC) in the draining lymph node (DLN) and expression of costimulatory molecules CD80, CD86, and CD40 on DCs. In contrast to LT-I, LT-IIb and LT-IIc induced less edema, cellular infiltrates, and general inflammation at the site of ID injection. Thus, LT-IIb and LT-IIc are attractive comprehensive ID adjuvants with unique characteristic that enhance humoral and cellular immunity to a co-administered protein Ag.

Immune Adjuvant Effect of Molecularly-defined Toll-Like Receptor Ligands

Vaccine efficacy is optimized by addition of immune adjuvants. However, although adjuvants have been used for over a century, to date, only few adjuvants are approved for human use, mostly aimed at improving vaccine efficacy and antigen-specific protective antibody production. The mechanism of action of immune adjuvants is diverse, depending on their chemical and molecular nature, ranging from non-specific effects (i.e., antigen depot at the immunization site) to specific activation of immune cells leading to improved host innate and adaptive responses. Although the detailed molecular mechanism of action of many adjuvants is still elusive, the discovery of Toll-like receptors (TLRs) has provided new critical information on immunostimulatory effect of numerous bacterial components that engage TLRs. These ligands have been shown to improve both the quality and the quantity of host adaptive immune responses when used in vaccine formulations targeted to infectious diseases and cancer that require both humoral and cell-mediated immunity. The potential of such TLR adjuvants in improving the design and the outcomes of several vaccines is continuously evolving, as new agonists are discovered and tested in experimental and clinical models of vaccination. In this review, a summary of the recent progress in development of TLR adjuvants is presented.

LT-IIb(T13I), a non-toxic type II heat-labile enterotoxin, augments the capacity of a ricin toxin subunit vaccine to evoke neutralizing antibodies and protective immunity

Currently, there is a shortage of adjuvants that can be employed with protein subunit vaccines to enhance protection against biological threats. LT-IIb(T13I) is an engineered nontoxic derivative of LT-IIb, a member of the type II subfamily of heat labile enterotoxins expressed by Escherichia coli, that possesses potent mucosal adjuvant properties. In this study we evaluated the capacity of LT-IIb(T13I) to augment the potency of RiVax, a recombinant ricin toxin A subunit vaccine, when co-administered to mice via the intradermal (i.d.) and intranasal (i.n.) routes. We report that co-administration of RiVax with LT-IIb(T13I) by the i.d. route enhanced the levels of RiVax-specific serum IgG antibodies (Ab) and elevated the ratio of ricin-neutralizing to non-neutralizing Ab, as compared to RiVax alone. Protection against a lethal ricin challenge was also augmented by LT-IIb(T13I). While local inflammatory responses elicited by LT-IIb(T13I) were comparable to those elicited by aluminum salts (Imject®), LT-IIb(T13I) was more effective than aluminum salts at augmenting production of RiVax-specific serum IgG. Finally, i.n. administration of RiVax with LT-IIb(T13I) also increased levels of RiVax-specific serum and mucosal Ab and enhanced protection against ricin challenge. Collectively, these data highlight the potential of LT-IIb(T13I) as an effective next-generation i.d., or possibly i.n. adjuvant for enhancing the immunogenicity of subunit vaccines for biodefense.

Protection of piglets against enteric colibacillosis by intranasal immunization with K88ac (F4ac) fimbriae and heat labile enterotoxin of Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is an important diarrheal agent of young domestic animals. Currently, there are no commercially available non-living vaccines to protect weaned pigs from the disease and no major veterinary biologics company markets a postweaning ETEC vaccine of any kind. While efforts have been made to develop a non-living postweaning ETEC vaccine for pigs, studies have been limited to the assessment of immune responses to experimental immunogens. In the present study, we describe a reproducible gnotobiotic piglet model of post-weaning ETEC diarrhea and efficacy tests in that model of subunit vaccines consisting of K88 (F4) fimbriae and/or heat labile enterotoxin (LT) delivered by the intranasal route. We also report antibody responses to the vaccine antigens. Piglets vaccinated with both antigens mounted a substantial immune response with serum and cecal antibody titers to K88 antigen significantly greater than those of controls. Serum anti-LT antibody titers were also significantly greater than those of controls. Piglets vaccinated with both antigens remained healthy following challenge with ETEC. At least some pigs vaccinated with either antigen alone, and most of the control piglets developed dehydrating diarrhea and suffered significant weight loss. The results of this study suggest that an intranasal vaccine consisting of both antigens is highly protective against a vigorous experimental challenge of pigs with K88+ ETEC, while that against either antigen alone is not. The current study provides a system whereby various ETEC antigens and/or combinations of antigens can be tested in exploring strategies for the development of vaccines for ETEC.

Mucosal vaccine design and delivery

Mucosal surfaces are a major portal of entry for many human pathogens that are the cause of infectious diseases worldwide. Vaccines capable of eliciting mucosal immune responses can fortify defenses at mucosal front lines and protect against infection. However, most licensed vaccines are administered parenterally and fail to elicit protective mucosal immunity. Immunization by mucosal routes may be more effective at inducing protective immunity against mucosal pathogens at their sites of entry. Recent advances in our understanding of mucosal immunity and identification of correlates of protective immunity against specific mucosal pathogens have renewed interest in the development of mucosal vaccines. Efforts have focused on efficient delivery of vaccine antigens to mucosal sites that facilitate uptake by local antigen-presenting cells to generate protective mucosal immune responses. Discovery of safe and effective mucosal adjuvants are also being sought to enhance the magnitude and quality of the protective immune response.

TLR2-dependent modulation of dendritic cells by LT-IIa-B5, a novel mucosal adjuvant derived from a type II heat-labile enterotoxin

A host of human pathogens invades the body at mucosal surfaces. Yet, strong, protective mucosal immune responses directed against those pathogens routinely cannot be induced without the use of adjuvants. Although the strongest mucosal adjuvants are members of the family of HLTs, the inherent toxicities of HLT holotoxins preclude their clinical use. Herein, it is shown that LT-IIa-B(5) enhances mucosal immune responses by modulating activities of DCs. i.n. immunization of mice with OVA in the presence of LT-IIa-B(5) recruited DCs to the NALT and significantly increased uptake of OVA by those DCs. Furthermore, LT-IIa-B(5) increased expression of CCR7 by DCs, which mediated enhanced migration of the cells from the NALT to the draining CLNs. LT-IIa-B(5) also enhanced maturation of DCs, as revealed by increased surface expression of CD40, CD80, and CD86. Ag-specific CD4(+) T cell proliferation was augmented in the CLNs of mice that had received i.n. LT-IIa-B(5). Finally, when used as an i.n. adjuvant, LT-IIa-B(5) dramatically increased the levels of OVA-specific salivary IgA and OVA-specific serum IgG. Strikingly, each of the activities induced by LT-IIa-B(5) was strictly TLR2-dependent. The data strongly suggest that the immunomodulatory properties of LT-IIa-B(5) depend on the productive modulation of mucosal DCs. Notably, this is the first report for any HLT to demonstrate in vivo the elicitation of strong, TLR2-dependent modulatory effects on DCs with respect to adjuvanticity.

Distinctive immunomodulatory and inflammatory properties of the Escherichia coli type II heat-labile enterotoxin LT-IIa and its B pentamer following intradermal administration

The type I and type II heat-labile enterotoxins (LT-I and LT-II) are strong mucosal adjuvants when they are coadministered with soluble antigens. Nonetheless, data on the parenteral adjuvant activities of LT-II are still limited. Particularly, no previous study has evaluated the adjuvant effects and induced inflammatory reactions of LT-II holotoxins or their B pentameric subunits after delivery via the intradermal (i.d.) route to mice. In the present report, the adjuvant and local skin inflammatory effects of LT-IIa and its B subunit pentamer (LT-IIaB(5)) were determined. When coadministered with ovalbumin (OVA), LT-IIa and, to a lesser extent, LT-IIaB(5) exhibited serum IgG adjuvant effects. In addition, LT-IIa but not LT-IIaB(5) induced T cell-specific anti-OVA responses, particularly in respect to induction of antigen-specific cytotoxic CD8(+) T cell responses. LT-IIa and LT-IIaB(5) induced differential tissue permeability and local inflammatory reactions after i.d. injection. Of particular interest was the reduced or complete lack of local reactions, such as edema and tissue induration, in mice i.d. inoculated with LT-IIa and LT-IIaB(5,) respectively, compared with mice immunized with LT-I. In conclusion, the present results show that LT-IIa and, to a lesser extent, LT-IIaB(5) exert adjuvant effects when they are delivered via the i.d. route. In addition, the low inflammatory effects of LT-IIa and LT-IIaB(5) in comparison to those of LT-I support the usefulness of LT-IIa and LT-IIaB(5) as parenterally delivered vaccine adjuvants.

Characterization of antigen-presenting cells induced by intragastric immunization with recombinant chimeric immunogens constructed from Streptococcus mutans AgI/II and type I or type II heat-labile enterotoxins

Intragastric (i.g.) immunization with recombinant chimeric proteins constructed from the saliva-binding region (SBR) of Streptococcus mutans surface antigen AgI/II and the A2/B subunits of enterobacterial heat-labile enterotoxins has been successfully used to induce salivary and circulating antibodies against S. mutans that have protective potential against dental caries. To investigate the mode of action of these vaccine constructs, mice were immunized i.g. with chimeric proteins constructed from SBR and cholera toxin (CT) or the type II enterotoxins of Escherichia coli, LT-IIa and LT-IIb. Antigen-presenting cells (APC) in Peyer's patches (PP) and mesenteric lymph nodes (MLN) were characterized by flow cytometry. Compared with immunization with SBR alone, chimeric proteins SBR-LTIIaA2/B and SBR-LTIIbA2/B increased the number of B cells and macrophages in PP and diminished B cell numbers in MLN, whereas SBR-CTA2/B diminished the numbers of B cells and macrophages in PP and MLN. Immunization with all three chimeric proteins led to upregulation of MHC class II molecules and co-stimulatory receptors CD40, CD80, and CD86 especially on dendritic cells in PP and also on APC in MLN. The results provide a molecular basis for the enhanced immune responses induced by chimeric proteins compared with uncoupled antigen, and for differential responses to chimeric proteins based on CT or type II enterotoxins.

LT-IIc, a new member of the type II heat-labile enterotoxin family, exhibits potent immunomodulatory properties that are different from those induced by LT-IIa or LT-IIb

A plethora of human pathogens invade and/or colonize mucosal surfaces. Elaboration of strong, protective immune responses against those pathogens by mucosal vaccination, however, is hampered by endogenous regulatory systems in the mucosae that dampen responses to foreign antigens (Ags). To overcome those natural barriers, mucosal adjuvants must be employed. Using a mouse mucosal immunization model and AgI/II, a weak immunogen from Streptococcus mutans, LT-IIc, a new member of the type II subgroup of the heat-labile enterotoxin family, was shown to have potent mucosal adjuvant properties. In comparison to mice intranasally immunized only with AgI/II, co-administration of AgI/II with LT-IIc enhanced production of Ag-specific IgA antibodies in the saliva and vaginal fluids and Ag-specific IgA and IgG in the serum. Secretion of IL-2, IL-6, IL-17, IFN-γ, and TNF-α was enhanced in cultures of AgI/II-stimulated splenic cells isolated from mice that had received LT-IIc as a mucosal adjuvant. In contrast, secretion of IL-10 was suppressed in those cells. This pattern of cytokine secretion suggested that LT-IIc stimulates both Th1 and Th2 immune responses. In contrast to LT-IIa and LT-IIb, the original members of the type II subgroup that also are mucosal adjuvants, LT-IIc dramatically enhanced secretion of IL-1α and IL-1β in peritoneal macrophages that had been co-cultured with LPS. Furthermore, the B pentameric subunit of LT-IIc augmented uptake of Ag by bone marrow-derived dendritic cells to levels that exceeded those attained by use of LPS or by the B pentamers of LT-IIa or LT-IIb. These data confirmed that LT-IIc is a strong mucosal adjuvant with immunomodulatory properties that are distinguishable from those of LT-IIa and LT-IIb and which has immunomodulatory properties that may be exploitable in vaccine development.

Binding to gangliosides containing N-acetylneuraminic acid is sufficient to mediate the immunomodulatory properties of the nontoxic mucosal adjuvant LT-IIb(T13I)

By use of a mouse mucosal immunization model, LT-IIb(T13I), a nontoxic mutant type II heat-labile enterotoxin, was shown to have potent mucosal and systemic adjuvant properties. In contrast to LT-IIb, which binds strongly to ganglioside receptors decorated with either N-acetylneuraminic acid (NeuAc) or N-glycolylneuraminic acid (NeuGc), LT-IIb(T13I) binds NeuAc gangliosides much less well. Rather, LT-IIb(T13I) binds preferentially to NeuGc gangliosides. To determine if the adjuvant properties of LT-IIb(T13I) are altered in the absence of NeuGc ganglioside receptors, experiments were conducted using a Cmah-null mouse line which is deficient in the synthesis of NeuGc gangliosides. Several immunomodulatory properties of LT-IIb(T13I) were shown to be dependent on NeuGc gangliosides. LT-IIb(T13I) had reduced binding activity for NeuGc-deficient B cells and macrophages; binding to NeuGc-deficient T cells and dendritic cells (DC) was essentially undetectable. Treatment of Cmah-null macrophages with LT-IIb(T13I), however, upregulated the transcription of interleukin-4 (IL-4), IL-6, IL-17, and gamma interferon (IFN-gamma), four cytokines important for promoting immune responses. The production of mucosal IgA and serum IgG against an immunizing antigen was augmented in NeuGc-deficient mice administered LT-IIb(T13I) as a mucosal adjuvant. Notably, NeuGc gangliosides are not expressed in humans. Still, treatment of human monocytes with LT-IIb(T13I) induced the secretion of IL-6, an inflammatory cytokine that mediates differential control of leukocyte activation. These results suggested that NeuAc gangliosides are sufficient to mediate the immunomodulatory properties of LT-IIb(T13I) in mice and in human cells. The nontoxic mutant enterotoxin LT-IIb(T13I), therefore, is potentially a new and safe human mucosal adjuvant.

Enhanced antigen uptake by dendritic cells induced by the B pentamer of the type II heat-labile enterotoxin LT-IIa requires engagement of TLR2

The potent mucosal adjuvant properties of the type II heat-labile enterotoxin LT-IIa of Escherichia coli are dependent upon binding of the B pentamer of the enterotoxin (LT-IIa-B(5)) to ganglioside receptors on immunocompetent cells. To evaluate the immunomodulatory activities of LT-IIa-B(5), in vitro experiments employing bone marrow-derived dendritic cells (BMDC) were performed. Uptake of OVA-FITC, a model antigen (Ag), was enhanced by treatment of BMDC with LT-IIa-B5, but not by treatment of cells with the B pentamer of cholera toxin (CTB). Expression of co-stimulatory molecules (CD40, CD80, CD86, and MHC-II) and cytokines (IL-12p40, TNF-alpha, and IFN-gamma) was increased in BMDC treated with LT-IIa-B(5). The capacity of LT-IIa-B(5) to enhance Ag uptake and to induce expression of co-stimulatory receptors and cytokines by BMDC was dependent upon expression of TLR2 by the cell. Increased Ag uptake induced by LT-IIa-B(5) was correlated with increased Ag-specific proliferation of CD4(+) T cells in an in vitro syngeneic DO11.10 CD4(+) T cell proliferation assay. These experiments confirm that LT-IIa-B(5) exhibits potent immunomodulatory properties which may be exploitable as a non-toxic mucosal adjuvant.

In vivo and in vitro adjuvant activities of the B subunit of Type IIb heat-labile enterotoxin (LT-IIb-B5) from Escherichia coli

The pentameric B subunit of the Escherichia coli LT-IIb enterotoxin (LT-IIb-B(5)) activates TLR2 signaling in macrophages. Herein we demonstrate that LT-IIb-B(5), in contrast to a TLR2-nonbinding point mutant, induces functional activation of bone marrow-derived dendritic cells and stimulates CD4(+) T cell proliferation, activities which suggested that LT-IIb-B(5) might function as an adjuvant in vivo. Indeed, in an intranasal mouse immunization model, LT-IIb-B(5) augmented specific mucosal and serum antibody responses to a co-administered immunogen, at levels which were almost comparable to those induced by intact LT-IIb holotoxin, a potent but toxic adjuvant. Therefore, LT-IIb-B(5) displays useful adjuvant properties which, combined with lack of enterotoxicity and relative stability against degradation, may find application in mucosal vaccines.

Cholera toxin, LT-I, LT-IIa and LT-IIb: the critical role of ganglioside binding in immunomodulation by type I and type II heat-labile enterotoxins

The heat-labile enterotoxins expressed by Vibrio cholerae (cholera toxin) and Escherichia coli (LT-I, LT-IIa and LT-IIb) are potent systemic and mucosal adjuvants. Coadministration of the enterotoxins with a foreign antigen produces an augmented immune response to that antigen. Although each enterotoxin has potent adjuvant properties, the means by which the enterotoxins induce various immune responses are distinctive for each adjuvant. Various mutants have been engineered to dissect the functions of the enterotoxins required for their adjuvanticity. The capacity to strongly bind to one or more specific ganglioside receptors appears to drive the distinctive immunomodulatory properties associated with each enterotoxin. Mutant enterotoxins with ablated or altered ganglioside-binding affinities have been employed to investigate the role of gangliosides in enterotoxin-dependent immunomodulation.

In vitro induction of immunoglobulin A (IgA)- and IgM-secreting plasma blasts by cholera toxin depends on T-cell help and is mediated by CD154 up-regulation and inhibition of gamma interferon synthesis

Cholera toxin (CT) and the type II heat-labile enterotoxins (LT-IIa and LT-IIb) are potent immunological adjuvants which are hypothesized to enhance the production of antibody (Ab)-secreting cells, although their mechanisms of action are not fully understood. The treatment of splenic cells with concanavalin A (ConA) plus CT enhanced the production of immunoglobulin A (IgA) and IgM by dividing cells that expressed high levels of major histocompatibility complex class II (MHC-II), CD19, and CD138 and low levels of B220 a phenotype characteristic of plasma blasts. LT-IIa or LT-IIb moderately enhanced IgA and IgM production without enhancing plasma blast differentiation. CT up-regulated CD25, CD69, CD80, CD86, and MHC-II in isolated B cells but failed to induce proliferation or differentiation. The treatment of unfractionated splenic cells with ConA plus CT induced B-cell proliferation and differentiation, but the elimination of CD4(+) T cells inhibited this effect. CT treatment of ConA-activated CD4(+) T cells up-regulated CD134 and CD154, whereas the blockage of CD40-CD154 interactions inhibited the induction of plasma blasts and Ig synthesis. The treatment of unfractionated splenic cells with CT, LT-IIa, or LT-IIb enhanced the production of interleukin-6 (IL-6) and IL-10, whereas the production of gamma interferon was inhibited in both CD4(+) and CD8(+) T cells mostly by CT. Thus, major regulatory effects of CT on lymphocytes are likely exerted early during the induction of immune responses when B and T cells initially encounter antigen. Neither LT-IIa or LT-IIb had these effects, indicating that type II enterotoxins augment Ab responses by other mechanisms.

Mutants of type II heat-labile enterotoxin LT-IIa with altered ganglioside-binding activities and diminished toxicity are potent mucosal adjuvants

The structure and function LT-IIa, a type II heat-labile enterotoxin of Escherichia coli, are closely related to the structures and functions of cholera toxin and LT-I, the type I heat-labile enterotoxins of Vibrio cholerae and enterotoxigenic Escherichia coli, respectively. While LT-IIa is a potent systemic and mucosal adjuvant, recent studies demonstrated that mutant LT-IIa(T34I), which exhibits no detectable binding activity as determined by an enzyme-linked immunosorbent assay, with gangliosides GD1b, GD1a, and GM1 is a very poor adjuvant. To evaluate whether other mutant LT-IIa enterotoxins that also exhibit diminished ganglioside-binding activities have greater adjuvant activities, BALB/c mice were immunized by the intranasal route with the surface adhesin protein AgI/II of Streptococcus mutans alone or in combination with LT-IIa, LT-IIa(T14S), LT-IIa(T14I), or LT-IIa(T14D). All three mutant enterotoxins potentiated strong mucosal immune responses that were equivalent to the response promulgated by wt LT-IIa. All three mutant enterotoxins augmented the systemic immune responses that correlated with their ganglioside-binding activities. Only LT-IIa and LT-IIa(T14S), however, enhanced expression of major histocompatibility complex class II and the costimulatory molecules CD40, CD80, and CD86 on splenic dendritic cells. LT-IIa(T14I) and LT-IIa(T14D) had extremely diminished toxicities in a mouse Y1 adrenal cell bioassay and reduced abilities to induce the accumulation of intracellular cyclic AMP in a macrophage cell line.

Potent protective cellular immune responses generated by a DNA vaccine encoding HSV-2 ICP27 and the E. coli heat labile enterotoxin

A mouse model was employed to evaluate protective cellular immune responses induced by an immediate early antigen of HSV-2. Particle-mediated DNA vaccination of mice with a DNA plasmid-encoding ICP27 resulted in the induction of ICP27-specific IFN-gamma and TNF-alpha production in Balb/c mice, but little protection to intranasal challenge with wild type HSV-2. However, when the DNA vaccine was supplemented with as little as 50ng of a vector encoding the A and B subunits of the Escherichia coli heat labile enterotoxin (LT), animals were profoundly protected from morbidity and mortality. The ICP27+LT-mediated protection was correlated with a large increase in ICP27-specific IFN-gamma and TNF-alpha production but cytokine-specific monoclonal antibody treatment at the time of challenge showed that protection was mediated predominantly by IFN-gamma. Furthermore, depletion of T cell subsets prior to infectious challenge demonstrated that removal of either CD8+ or CD4+ T cells impaired protection with CD8+ T cells appearing to play a direct effector role. These data demonstrate that augmented cellular immune responses resulting from LT vector plus antigen vector administration to the skin are biologically significant, leading to enhanced protection against mucosal pathogenic challenge.

Oral immunogenicity of a plant-made, subunit, tuberculosis vaccine

Transgenic plants are a novel way to produce and deliver oral vaccines. Arabidopsis thaliana material shown previously to express the tuberculosis (TB) antigen ESAT-6 fused to the B subunit of Escherichia coli heat-labile enterotoxin (LTB) was fed to mice and the resulting immune response investigated. The plant-made LTB-ESAT-6 fusion protein induced antigen-specific responses from CD4+ cells and increased IFN-gamma production, indicating a Th1 response. In addition, a Th2 response was induced in the Peyer's patch. This is the first report of an orally delivered, subunit, tuberculosis vaccine priming an antigen-specific, Th1 response.

Enterotoxin-based mucosal adjuvants alter antigen trafficking and induce inflammatory responses in the nasal tract

The safety of nasal vaccines containing enterotoxin-based mucosal adjuvants has not been studied in detail. Previous studies have indicated that native cholera toxin (nCT) can alter antigen trafficking when applied nasally. In this study, we determined the enterotoxin-based variables that alter antigen trafficking. To measure the influence of enterotoxin-based mucosal adjuvants on antigen trafficking in the nasal tract, native and mutant enterotoxins were coadministered with radiolabeled tetanus toxoid (TT). The nCT and heat-labile enterotoxin type 1 (LTh-1) redirected TT into the olfactory neuroepithelium (ON/E). Antigen redirection occurred mainly across the nasal epithelium without subsequent transport along olfactory neurons into the olfactory bulbs (OB). Thus, no significant accumulation of the vaccine antigen TT was observed in the OB when coadministered with nCT. In contrast, neither mutant CT nor mutant LTh-1, which lack ADP-ribosyltransferase activity, redirected TT antigen into the ON/E. Thus, ADP-ribosyltransferase activity was essential for antigen trafficking across the olfactory epithelium. Accumulation of TT in the ON/E was also due to B-subunit binding to GM1 gangliosides, as was demonstrated (i) by redirection of TT by LTh-1 in a dose-dependent manner, (ii) by ganglioside inhibition of the antigen redirection by LTh-1 and nCT, and (iii) by the use of LT-IIb, a toxin that binds to gangliosides other than GM1. Redirection of TT into the ON/E coincided with elevated production of interleukin 6 (IL-6) but not IL-1beta or tumor necrosis factor alpha in the nasal mucosa. Thus, redirection of TT is dependent on ADP-ribosyltransferase activity and GM1 binding and is associated with production of the inflammatory cytokine IL-6.

Differential binding of Escherichia coli enterotoxins LT-IIa and LT-IIb and of cholera toxin elicits differences in apoptosis, proliferation, and activation of lymphoid cells

Cholera toxin (CT), LT-IIa, and LT-IIb are potent adjuvants which induce distinct T-helper (Th)-cell cytokine profiles and immunoglobulin G (IgG) subclass and IgA antibody responses. To determine if the distinct immune regulatory effects observed for LT-IIa, LT-IIb, and CT are elicited by binding of the enterotoxins to their cognate ganglioside receptors, the lineages of lymphoid cells that interact with the three enterotoxins and their effects on various lymphocyte responses in vitro were evaluated. Binding patterns of LT-IIa, LT-IIb, and CT to several lymphoid cell populations were distinctive for each enterotoxin. LT-IIa and CT, but not LT-IIb, induced apoptosis in CD8(+) T cells. LT-IIa(T34I), a mutant with no detectable binding to gangliosides, did not induce apoptosis. Blockade of GM(1) on the surface of CD8(+) T cells by LT-IIa(T14I), a mutant that binds only to GM(1) but does not induce apoptosis, did not inhibit induction of apoptosis by LT-IIa. Mitogen-induced proliferation of CD8(+) T cells was abrogated by treatment with CT, while resting CD8(+) T cells which were sensitive to LT-IIa-induced apoptosis became more resistant to apoptosis after mitogen activation. Exposure to CT, but not to LT-IIa or LT-IIb, inhibited mitogen-driven CD4(+) T-cell proliferation and expression of CD25 and CD69. In mitogen-stimulated B cells, CT, but not LT-IIa or LT-IIb, enhanced expression levels of CD86, while only CT induced B-cell differentiation into plasma cells. Thus, LT-IIa, LT-IIb, and CT exhibit distinguishable immunomodulatory properties which are likely dependent upon their capacities to recognize different ganglioside receptors on lymphocytes.

Expression systems and developments in plant-made vaccines

Delivery of vaccines to mucosal surfaces can elicit humoral and cell-mediated responses of the mucosal and systemic immune systems, evoke less pain and discomfort than parenteral delivery, and eliminate needle-associated risks. Transgenic plants are an ideal means by which to produce oral vaccines, as the rigid walls of the plant cell protect antigenic proteins from the acidic environment of the stomach, enabling intact antigen to reach the gut associated lymphoid tissue. In the past few years, new techniques (such as chloroplast transformation and food processing) have improved antigen concentration in transgenic plants. In addition, adjuvants and targeting proteins have increased the immunogenicity of mucosally administered plant-made vaccines. These studies have moved plant-made vaccines closer to the development phase.

Mucosal adjuvant properties of mutant LT-IIa and LT-IIb enterotoxins that exhibit altered ganglioside-binding activities

LT-IIa and LT-IIb, the type II heat-labile enterotoxins of Escherichia coli, are closely related in structure and function to cholera toxin and LT-I, the type I heat-labile enterotoxins of Vibrio cholerae and E. coli, respectively. Recent studies from our group demonstrated that LT-IIa and LT-IIb are potent systemic and mucosal adjuvants. To determine whether binding of LT-IIa and LT-IIb to their specific ganglioside receptors is essential for adjuvant activity, LT-IIa and LT-IIb enterotoxins were compared with their respective single-point substitution mutants which have no detectable binding activity for their major ganglioside receptors [e.g., LT-IIa(T34I) and LT-IIb(T13I)]. Both mutant enterotoxins exhibited an extremely low capacity for intoxicating mouse Y1 adrenal cells and for inducing production of cyclic AMP in a macrophage cell line. BALB/c female mice were immunized by the intranasal route with the surface adhesin protein AgI/II of Streptococcus mutans alone or in combination with LT-IIa, LT-IIa(T34I), LT-IIb, or LT-IIb(T13I). Both LT-IIa and LT-IIb potentiated strong mucosal and systemic immune responses against AgI/II. Of the two mutant enterotoxins, only LT-IIb(T13I) had the capacity to strongly potentiate mucosal anti-AgI/II and systemic anti-AgI/II antibody responses. Upon boosting with AgI/II, however, both LT-IIa(T34I) and LT-IIb(T13I) enhanced humoral memory responses to AgI/II. Flow cytometry demonstrated that LT-IIa(T34I) had no affinity for cervical lymph node lymphocytes. In contrast, LT-IIb(T13I) retained binding activity for T cells, B cells, and macrophages, indicating that this immunostimulatory mutant enterotoxin interacts with one or more unknown lymphoid cell receptors.

Toll-like receptor 2 mediates cellular activation by the B subunits of type II heat-labile enterotoxins

The type II heat-labile enterotoxins (LT-IIa and LT-IIb) of Escherichia coli have an AB5 subunit structure similar to that of cholera toxin (CT) and other type I enterotoxins, despite significant differences in the amino acid sequences of their B subunits and different ganglioside receptor specificities. LT-II holotoxins and their nontoxic B subunits display unique properties as immunological adjuvants distinct from those of CT and its B subunits. In contrast to type II holotoxins, the corresponding pentameric B subunits, LT-IIaB and LT-IIbB, stimulated cytokine release in both human and mouse cells dependent upon Toll-like receptor 2 (TLR2). Induction of interleukin-1beta (IL-1beta), IL-6, IL-8, or tumor necrosis factor alpha in human THP-1 cells by LT-IIaB or LT-IIbB was inhibited by anti-TLR2 but not by anti-TLR4 antibody. Furthermore, transient expression of TLR1 and TLR2 in human embryonic kidney 293 cells resulted in activation of a nuclear factor-kappaB-dependent luciferase gene in response to LT-IIaB or LT-IIbB. Moreover, peritoneal macrophages from TLR2-deficient mice failed to respond to LT-IIaB or LT-IIbB, in contrast to wild-type or TLR4-deficient cells. These results demonstrate that besides their established binding to gangliosides, the B subunits of type II enterotoxins also interact with TLR2. Although a ganglioside-nonbinding mutant (T34I) of LT-IIaB effectively induced cytokine release, a phenotypically similar point mutation (T13I) in LT-IIbB abrogated cytokine induction, suggesting a variable requirement for gangliosides as coreceptors in TLR2 agonist activity. TLR2-dependent activation of mononuclear cells by type II enterotoxin B subunits appears to be a novel mechanism whereby these molecules may exert their immunomodulatory and adjuvant activities.

The Type II heat-labile enterotoxins LT-IIa and LT-IIb and their respective B pentamers differentially induce and regulate cytokine production in human monocytic cells

The type II heat-labile enterotoxins, LT-IIa and LT-IIb, exhibit potent adjuvant properties. However, little is known about their immunomodulatory activities upon interaction with innate immune cells, unlike the widely studied type I enterotoxins that include cholera toxin (CT). We therefore investigated interactions of LT-IIa and LT-IIb with human monocytic THP-1 cells. We found that LT-II enterotoxins were inactive in stimulating cytokine release, whereas CT induced low levels of interleukin-1beta (IL-1beta) and IL-8. However, all three enterotoxins potently regulated cytokine induction in cells activated by bacterial lipopolysaccharide or fimbriae. Induction of proinflammatory (tumor necrosis factor alpha [TNF-alpha]) or chemotactic (IL-8) cytokines was downregulated, whereas induction of cytokines with anti-inflammatory (IL-10) or mucosal adjuvant properties (IL-1beta) was upregulated by the enterotoxins. These effects appeared to depend on their A subunits, because isolated B-pentameric subunits lacked regulatory activity. Enterotoxin-mediated inhibition of proinflammatory cytokine induction in activated cells was partially attributable to synergism for endogenous production of IL-10 and to an IL-10-independent inhibition of nuclear factor kappaB (NF-kappaB) activation. In sharp contrast to the holotoxins, the B pentamers (LT-IIaB and, to a greater extent, LT-IIbB) stimulated cytokine production, suggesting a link between the absence of the A subunit and increased proinflammatory properties. In this regard, the ability of LT-IIbB to activate NF-kappaB and induce TNF-alpha and IL-8 was antagonized by the LT-IIb holotoxin. These findings support distinct immunomodulatory roles for the LT-II holotoxins and their respective B pentamers. Moreover, the anti-inflammatory properties of the holotoxins may serve to suppress innate immunity and promote the survival of the pathogen.

Colonization of gnotobiotic mice with human gut microflora at birth protects against Escherichia coli heat-labile enterotoxin-mediated abrogation of oral tolerance

Previous work has shown that the indigenous gut microflora in mice plays a protective role against Escherichia coli heat-labile enterotoxin (LT)-mediated abrogation of oral tolerance to an unrelated co-ingested protein. To assess potential protection by human gut microflora, we studied the effect of human gut microflora in a murine model. Oral tolerance was studied in adult gnotobiotic mice (i.e. ex-germ-free mice) colonized with the entire human fecal microflora and orally administered once with LT and ovalbumin. Systemic suppression of IgG, IgG1, IgG2a, and IgE antibody responses was assessed by ELISA. Both specific IgG subclasses and IgE hyporesponsiveness was induced in LT + ovalbumin-fed gnotobiotic mice, indicating that the human gut microflora can protect against the LT-mediated abrogation of oral tolerance. However, as confirmed with mouse gut microflora, this protective effect only occurs when the gut microflora is associated from birth on. Colonization of germ-free mice with a single bacterial strain, E. coli, predominant in the human and mouse gut microflora in the neonatal period, showed that this strain alone did not induce protection. These results supported the hypothesis that the natural establishment of the gut microflora in neonates crucially influenced resistance to LT-mediated abrogation of oral tolerance by reinforcing suppression of both T helper type 1- and T helper type 2-controlled responses, and suggested that sequential bacterial colonization of the gut rather than a single bacterial species may be involved in this phenomenon.

Microbial/host interactions: mechanisms involved in host responses to microbial antigens

The indigenous oral microflora and the host are normally in a state of equilibrium; however, the introduction of a pathogen can result in innate and adaptive immune responses that either contribute to the development of the disease or lead to host immunity. The interactions between the microorganisms and the host are very dynamic, thus allowing the complex interplay between host molecules and bacterial antigens. In this article, we focus on the mechanisms involved in the pathogenesis of and host responses to two oral pathogens: the Gram-negative bacterium Porphyromonas gingivalis, implicated in the etiology of periodontal disease, and the Gram-positive Streptococcus mutans, a primary agent involved in dental caries formation. Furthermore, we address mechanisms involved in the ability of select adjuvants and delivery systems to potentiate mucosal and systemic immune responses to microbial vaccine antigens.

Dental caries vaccines: prospects and concerns

Dental caries remains one of the most common infectious diseases of mankind. Cariogenic micro-organisms enter the dental biofilm early in life and can subsequently emerge, under favorable environmental conditions, to cause disease. In oral fluids, adaptive host defenses aroused by these infections are expressed in the saliva and gingival crevicular fluid. This review will focus on methods by which mucosal host defenses can be induced by immunization to interfere with dental caries caused by mutans streptococci. The natural history of mutans streptococcal colonization is described in the context of the ontogeny of mucosal immunity to these and other indigenous oral streptococci. Molecular targets for dental caries vaccines are explored for their effectiveness in intact protein and subunit (synthetic peptide, recombinant and conjugate) vaccines in pre-clinical studies. Recent progress in the development of mucosal adjuvants and viable and non-viable delivery systems for dental caries vaccines is described. Finally, the results of clinical trials are reviewed, followed by a discussion of the prospects and concerns of human application of the principles presented.

Immunization for protection of the reproductive tract: a review

PROBLEM

Local application of non-replicating antigens to the female reproductive tract is ineffective in stimulating the common mucosal immune system, and induces only weak genital antibody responses. Studies of immune responses to genital infections such as gonorrhea also support the concept that, lacking mucosal immune inductive sites, the reproductive tract is ill-equipped to mount effective immune responses.

METHOD OF STUDY

Intranasal (i.n.) and intravaginal (i.vag.) routes of immunization of mice with a protein antigen coupled to cholera toxin (CT) B subunit, or genetically engineered as chimeric proteins with the A2/B sunbunits of CT or type II heat-labile enterotoxin, were compared for their ability to induce specific antibody responses in vaginal fluids, saliva, and serum.

RESULTS

Mice immunized i.n. developed substantially stronger vaginal immunoglobulin A (IgA) and immunoglobulin G (IgG) and serum IgG and IgA antibodies, than those immunized i.vag. which also failed to develop salivary antibodies. Vaginal antibody responses induced i.n. persisted for at least 1 year, and were recallable by booster immunization after a prolonged period.

CONCLUSIONS

Such alternative strategies for inducing potent genital antibody responses offer the prospect of prophylactic immunization against genital infections. Further studies are required to evaluate their applicability to humans, and to comprehend the cellular and molecular mechanisms involved in delivering effective immune responses to the reproductive tracts.

Plasmid vectors encoding cholera toxin or the heat-labile enterotoxin from Escherichia coli are strong adjuvants for DNA vaccines

Two plasmid vectors encoding the A and B subunits of cholera toxin (CT) and two additional vectors encoding the A and B subunits of the Escherichia coli heat-labile enterotoxin (LT) were evaluated for their ability to serve as genetic adjuvants for particle-mediated DNA vaccines administered to the epidermis of laboratory animals. Both the CT and the LT vectors strongly augmented Th1 cytokine responses (gamma interferon [IFN-gamma]) to multiple viral antigens when codelivered with DNA vaccines. In addition, Th2 cytokine responses (interleukin 4 [IL-4]) were also augmented by both sets of vectors, with the effects of the LT vectors on IL-4 responses being more antigen dependent. The activities of both sets of vectors on antibody responses were antigen dependent and ranged from no effect to sharp reductions in the immunoglobulin G1 (IgG1)-to-IgG2a ratios. Overall, the LT vectors exhibited stronger adjuvant effects in terms of T-cell responses than did the CT vectors, and this was correlated with the induction of greater levels of cyclic AMP by the LT vectors following vector transfection into cultured cells. The adjuvant effects observed in vivo were due to the biological effects of the encoded proteins and not due to CpG motifs in the bacterial genes. Interestingly, the individual LT A and B subunit vectors exhibited partial adjuvant activity that was strongly influenced by the presence or absence of signal peptide coding sequences directing the encoded subunit to either intracellular or extracellular locations. Particle-mediated delivery of either the CT or LT adjuvant vectors in rodents and domestic pigs was well tolerated, suggesting that bacterial toxin-based genetic adjuvants may be a safe and effective strategy to enhance the potency of both prophylactic and therapeutic DNA vaccines for the induction of strong cellular immunity.

Distribution of intranasal instillations in mice: effects of volume, time, body position, and anesthesia

Intranasal instillation techniques are used to deliver various substances to the upper and lower respiratory tract (URT and LRT) in mice. Here, we quantify the relative distribution achieved with intranasal delivery of a nonabsorbable tracer, (99m)Tc-labeled sulfide-colloid. Relative distribution was determined by killing mice after instillation and quantifying the radioactivity in dissected tissues using gamma scintigraphy. A significant effect of delivery volume on relative distribution was observed when animals were killed 5 min after instillation delivered under gas anesthesia. With a delivery volume of 5 microl, no radiation was detected in the LRT; this increased to a maximum of 55.7 +/- 2.5% distribution to the LRT when 50 microl were delivered. The majority of radiation not detected in the LRT was found in the URT. Over the course of the following 1 h, radiation in the LRT remained constant, while that in the URT decreased and appeared in the gastrointestinal tract. Instillation of 25 microl into anesthetized mice resulted in 30.1 +/- 6.9% distribution to the LRT, while only 5.3 +/- 1.5% (P < 0.05) of the same volume was detected in the LRT of awake mice. Varying the body position of mice did not affect relative distribution. When using intranasal instillation, the relative distribution between the URT and LRT and the gastrointestinal tract is heavily influenced by delivery volume and level of anesthesia.

Oxidised mannan as a novel adjuvant inducing mucosal IgA production

Mannan, oxidatively coupled to recombinant protein antigens, has here been tested as a possible adjuvant for the production of antibody on the mucosa. Given intranasally, but not intraperitoneally, mannan markedly enhanced the production of IgA, IgG1 and IgG2a in the serum, and IgA locally in the lung and at remote mucosal sites, including tears, vaginal and salivary secretions. Oxidative coupling was critical to its action, since neither mannan simply mixed with protein nor mannan-protein conjugates which had been reduced by treatment with sodium borohydride, acted as adjuvants. Oxidatively coupled mannan was compared with the widely studied mucosal adjuvant, cholera toxin (CT). The use of oxidised mannan as an adjuvant induced better responses than CT judged by the induction of IgA in serum, vaginal washings and saliva. Thus, oxidised mannan, which is non-toxic and can be administered without injection, is a suitable adjuvant coupled with protective antigens for vaccinating against a number of infections that occur via the mucous membranes.

Distinct cytokine regulation by cholera toxin and type II heat-labile toxins involves differential regulation of CD40 ligand on CD4(+) T cells

Cholera toxin (CT) and the type II heat-labile enterotoxins (HLT) LT-IIa and LT-IIb act as potent systemic and mucosal adjuvants and induce distinct T-helper (Th)-cell cytokine profiles. In the present study, CT and the type II HLT were found to differentially affect cytokine production by anti-CD3-stimulated human peripheral blood mononuclear cells (PBMC), and the cellular mechanisms responsible were investigated. CT suppressed interleukin-2 (IL-2), tumor necrosis factor alpha (TNF-alpha), and IL-12 production by PBMC cultures more than either LT-IIa or LT-IIb. CT but not LT-IIa or LT-IIb reduced the expression of CD4(+) T-cell surface activation markers (CD25 and CD69) and subsequent proliferative responses of anti-CD3-stimulated T cells. CT but not LT-IIa or LT-IIb significantly reduced the expression of CD40 ligand (CD40L) on CD4(+) T cells. In a coculture system, CT-treated CD4(+) T cells induced significantly less TNF-alpha and IL-12 p70 production by both autologous monocytes and monocyte-derived dendritic cells than either LT-IIa- or LT-IIb-treated CD4(+) T cells. These findings demonstrate that CT, LT-IIa, and LT-IIb differentially affect CD40-CD40L interactions between antigen-presenting cells and T cells and help explain the distinct cytokine profiles observed with type I and type II HLT when used as mucosal adjuvants.

Recombinant antigen-enterotoxin A2/B chimeric mucosal immunogens differentially enhance antibody responses and B7-dependent costimulation of CD4(+) T cells

The ADP-ribosylating enterotoxins, cholera toxin (CT) and the Escherichia coli heat-labile toxin (LT-IIa), have been shown to enhance mucosal and systemic antibody (Ab) responses to coadministered antigens. The purpose of the present study was to compare the ability of the nontoxic A2/B subunits of these toxins, which have distinct targeting properties, to augment the immunogenicity of a genetically coupled protein antigen. Structurally similar chimeric proteins were generated by genetically replacing the toxic A1 subunit of CT or LT-IIa with the saliva-binding region (SBR) from the streptococcal adhesin AgI/II. Intranasal immunization of BALB/c mice with either chimeric protein induced significantly higher plasma and mucosal anti-SBR immunoglobulin A (IgA) and IgG Ab responses than SBR alone. Moreover, compared to SBR-LT-IIaA2/B, SBR-CTA2/B elicited significantly higher levels of plasma IgG1 and salivary IgA anti-SBR Ab responses. Ex vivo and in vitro experiments revealed that SBR-CTA2/B selectively up-regulated B7-2 expression on murine B cells isolated from both the nasal associated lymphoid tissue, cervical lymph nodes, and spleen. In contrast, SBR-LT-IIaA2/B had little effect on B7-1 or B7-2 expression on B220(+), CD11b(+), or CD11c(+) cells. Analysis of the functional costimulatory activity of SBR-CTA2/B-treated B cells revealed a significant enhancement in anti-CD3-stimulated CD4(+) T-cell proliferative responses, and this proliferation was significantly reduced by treatment with anti-B7-2 but not with anti-B7-1 or isotype control Abs. Thus, SBR-CTA2/B and SBR-LT-IIaA2/B exhibit distinct patterns of antibody responses associated with differential effects on B7-2 expression and subsequent costimulatory effects on CD4(+) T cells.

Strategies of immunization against mucosal infections

The presence of secretory (S-) IgA in middle-ear fluid and localization of IgA-secreting cells in its mucosae suggest that the middle ear is an effector site of the mucosal immune system. Several strategies have been devised to induce potent, long-lasting, and recallable mucosal S-IgA antibodies, as well as circulating IgG antibodies and Th1- or Th2-type help, according to the most appropriate responses for a particular infection. Application of immunogens to inductive sites in the upper respiratory tract may be most effective for generating responses in the middle ear and nasopharynx for protection against the organisms responsible for otitis media.