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PorB is a well-characterized outer membrane protein that is common among Neisseria species and is required for survival. A vaccine candidate, PorB induces antibody responses that are directed against six variable surface-exposed loops that differ in sequence depending on serotype. Although Neisseria meningitidis is naturally competent and porB genetic mosaicism provides evidence for strong positive selection, the sequences of PorB serotypes commonly associated with invasive disease are often conserved, calling into question the interaction of specific PorB loop sequences in immune engagement. In this report, we provide evidence that antibody binding to a PorB epitope can be altered by sequence mutations in non-epitope loops. Through the construction of hybrid PorB types and PorB molecular dynamics simulations, we demonstrate that loops both adjacent and non-adjacent to the epitope loop can enhance or diminish antibody binding, a phenotype that correlates with serum bactericidal activity. We further examine the interaction of PorB with outer membrane-associated proteins, including PorA and RmpM. Deletion of these proteins alters the composition of PorB-containing native complexes and reduces antibody binding and serum killing relative to the parental strain, suggesting that both intramolecular and intermolecular PorB interactions contribute to host adaptive immune evasion., (© 2017 John Wiley & Sons Ltd.)

The variable efficacy of tuberculosis (TB) vaccines and the emergence of drug-resistant strains of Mycobacterium tuberculosis (Mtb) emphasize the urgency for not only generating new and more effective vaccines against TB but also understanding the underlying mechanisms that mediate vaccine-induced protection. We demonstrate that mucosal adjuvants, such as type II heat labile enterotoxin (LT-IIb), delivered through the mucosal route induce pulmonary Mtb-specific T helper type 17 (Th17) responses and provide vaccine-induced protection against Mtb infection. Importantly, protection is interferon-γ (IFNγ)-independent but interleukin-17 (IL-17)-dependent. Our data show that IL-17 mediates C-X-C motif chemokine ligand 13 (CXCL13) induction in the lung for strategic localization of proinflammatory cytokine-producing CXCR5+ (C-X-C motif chemokine receptor 5-positive) T cells within lymphoid structures, thereby promoting early and efficient macrophage activation and the control of Mtb. Our studies highlight the potential value of targeting the IL-17-CXCL13 pathway rather than the IFNγ pathway as a new strategy to improve mucosal vaccines against TB.

Bacterial heat-labile (LT) enterotoxins signal through tightly regulated interactions with host cell gangliosides. LT-IIa and LT-IIb of Escherichia coli bind preferentially to gangliosides with a NeuAcα2-3Galβ1-3GalNAc terminus, with key distinctions in specificity. LT-IIc, a newly discovered E. coli LT, is comprised of an A polypeptide with high homology, and a B polypeptide with moderate homology, to LT-IIa and LT-IIb. LT-IIc is less cytotoxic than LT-IIa and LT-IIb. We theorized that LT-IIc-host cell interaction is regulated by specific structural attributes of immune cell ganglioside receptors and designed experiments to test this hypothesis. Overlay immunoblotting to a diverse array of neural and macrophage gangliosides indicated that LT-IIc bound to a restrictive range of gangliosides, each possessing a NeuAcα2-3Galβ1-3GalNAc with a requisite terminal sialic acid. LT-IIc did not bind to GM1a with short-chain fatty acyl ceramides. Affinity overlay immunoblots, constructed to a diverse array of known ganglioside structures of murine peritoneal macrophages, established that LT-IIc bound to GM1a comprised of long-chain fatty acyl ceramides. Findings were confirmed with LT-IIc also binding to GM1a of RAW264.7 cells, comprised of a long-chain fatty acyl ceramide. Thus, LT-IIc-ganglioside binding differs distinctly from that of LT-IIa and LT-IIb. LT-IIc binding is not just dependent on carbohydrate composition, but also upon the orientation of the oligosaccharide portion of GM1a by the ceramide moiety. These studies are the first demonstration of LT-ganglioside dependence upon ceramide composition and underscore the contribution of long-chain fatty acyl ceramides to host cell interactions.

The pentameric B subunit of the type II heat-labile enterotoxin of Escherichia coli (LT-IIb-B(5)) is a potent signaling molecule capable of modulating innate immune responses. It has previously been shown that LT-IIb-B(5), but not the LT-IIb-B(5) Ser74Asp variant [LT-IIb-B(5)(S74D)], activates Toll-like receptor (TLR2) signaling in macrophages. Consistent with this, the LT-IIb-B(5)(S74D) variant failed to bind TLR2, in contrast to LT-IIb-B(5) and the LT-IIb-B(5) Thr13Ile [LT-IIb-B(5)(T13I)] and LT-IIb-B(5) Ser74Ala [LT-IIb-B(5)(S74A)] variants, which displayed the highest binding activity to TLR2. Crystal structures of the Ser74Asp, Ser74Ala and Thr13Ile variants of LT-IIb-B(5) have been determined to 1.90, 1.40 and 1.90 Å resolution, respectively. The structural data for the Ser74Asp variant reveal that the carboxylate side chain points into the pore, thereby reducing the pore size compared with that of the wild-type or the Ser74Ala variant B pentamer. On the basis of these crystallographic data, the reduced TLR2-binding affinity of the LT-IIb-B(5)(S74D) variant may be the result of the pore of the pentamer being closed. On the other hand, the explanation for the enhanced TLR2-binding activity of the LT-IIb-B(5)(S74A) variant is more complex as its activity is greater than that of the wild-type B pentamer, which also has an open pore as the Ser74 side chain points away from the pore opening. Data for the LT-IIb-B(5)(T13I) variant show that four of the five variant side chains point to the outside surface of the pentamer and one residue points inside. These data are consistent with the lack of binding of the LT-IIb-B(5)(T13I) variant to GD1a ganglioside.

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., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Two families of bacterial heat-labile enterotoxins (HLTs) have been described: the type I HLTs are comprised of cholera toxin (CT) of Vibrio cholerae, LT-I of Escherichia coli, and several related HLTs; the type II HLTs are comprised of LT-IIa and LT-IIb. Herein, we report LT-IIc, a new type II HLT encoded from an enterotoxigenic E. coli (ETEC) strain isolated from an avian host. Using a mouse Y1 adrenal cell bioassay, LT-IIc was shown to be less cytotoxic than CT, LT-IIa, or LT-IIb. Cytotoxicity of LT-IIc was partially neutralized by antisera recognizing LT-IIa or LT-IIb but not by anti-CT antiserum. Genes encoding putative A polypeptide and B polypeptides of LT-IIc were arranged in an operon which was flanked by potential prophage sequences. Analysis of the nucleotide and predicted amino acid sequences demonstrated that the A polypeptide of LT-IIc has moderate homology to the A polypeptides of CT and LT-I and high homology to the A polypeptides of LT-IIa and LT-IIb. The B polypeptide of LT-IIc exhibited no significant homology to the B polypeptides of CT and LT-I and only moderate homology to the B polypeptides of LT-IIa and LT-IIb. The binding pattern of LT-IIc for gangliosides was distinctive from that of either LT-IIa or LT-IIb. The data suggest that other types of the type II HLT subfamily are circulating in the environment and that host specificity of type II HLT is likely governed by changes in the B polypeptide which mediate binding to receptors.

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.

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.

LT-IIb, a type II heat-labile enterotoxin of Escherichia coli, is a potent immunologic adjuvant with high affinity binding for ganglioside GD1a. Earlier study suggested that LT-IIb bound preferentially to the terminal sugar sequence NeuAcalpha2-3Galbeta1-3GalNAc. However, studies in our laboratory suggested a less restrictive binding epitope. LT-IIb(T13I), an LT-IIb variant, engineered by a single isoleucine-threonine substitution, retains biological activity, but with less robust inflammatory effects. We theorized that LT-IIb has a less restrictive binding epitope than previously proposed and that immunologic differences between LT-IIb and LT-IIb (T13I) correlate with subtle ganglioside binding differences. Ganglioside binding epitopes, determined by affinity overlay immunoblotting and enzymatic degradation of ganglioside components of RAW264.7 macrophages, indicated that LT-IIb bound to a broader array of gangliosides than previously recognized. Each possessed NeuAcalpha2-3Galbeta1-3GalNAc, although not necessarily as a terminal sequence. Rather, each had a requisite terminal or penultimate single sialic acid and binding was independent of ceramide composition. RAW264.7 enterotoxin-binding and non-binding ganglioside epitopes were definitively identified as GD1a and GM1a, respectively, by enzymatic degradation and mass spectroscopy. Affinity overlay immunoblots, constructed to the diverse array of known ganglioside structures of murine peritoneal macrophages, established that LT-IIb bound NeuAc- and NeuGc-gangliosides with nearly equal affinity. However, LT-IIb(T13I) exhibited enhanced affinity for NeuGc-gangliosides and more restrictive binding. These studies further elucidate the binding epitope for LT-IIb and suggest that the diminished inflammatory activity of LT-IIb(T13I) is mediated by a subtle shift in ganglioside binding. These studies underscore the high degree of specificity required for ganglioside-protein interactions.

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.

Noroviruses are the major cause of nonbacterial gastroenteritis in humans. However, little is known regarding the norovirus life cycle, including cell binding and entry. In contrast to human noroviruses, the recently discovered murine norovirus 1 (MNV-1) readily infects murine macrophages and dendritic cells in culture. Many viruses, including the related feline calicivirus, use terminal sialic acids (SA) as receptors for infection. Therefore, we tested whether SA moieties play a role during MNV-1 infection of murine macrophages. Competition with SA-binding lectins and neuraminidase treatment led to a reduction in MNV-1 binding and infection in cultured and primary murine macrophages, suggesting a role for SA during the initial steps of the MNV-1 life cycle. Because SA moieties can be attached to glycolipids (i.e., gangliosides), we next determined whether MNV-1 uses gangliosides during infection. The gangliosides GD1a, GM1, and asialo-GM1 (GA1) are natural components of murine macrophages. MNV-1 bound to ganglioside GD1a, which is characterized by an SA on the terminal galactose, but not to GM1 or asialo-GM1 in an enzyme-linked immunosorbent assay. The depletion of gangliosides using an inhibitor of glycosylceramide synthase (d-threo-P4) led to a reduction of MNV-1 binding and infection in cultured and primary murine macrophages. This defect was specifically rescued by the addition of GD1a. A similar phenotype was observed for MNV field strains WU11 (GV/WU11/2005/USA) and S99 (GV/Berlin/2006/DE). In conclusion, our data indicate that MNV can use terminal SA on gangliosides as attachment receptors during binding to murine macrophages.

The pentameric B subunit of type IIb Escherichia coli enterotoxin (LT-IIb-B(5)), a doughnut-shaped oligomeric protein from enterotoxigenic E. coli, activates the TLR2/TLR1 heterodimer (TLR2/1). We investigated the molecular basis of the LT-IIb-B(5) interaction with TLR2/1 to define the structure-function relationship of LT-IIb-B(5) and, moreover, to gain an insight into how TLR2/1 recognizes large, nonacylated protein ligands that cannot fit within its lipid-binding pockets, as previously shown for the Pam(3)CysSerLys(4) (Pam(3)CSK(4)) lipopeptide. We first identified four critical residues in the upper region of the LT-IIb-B(5) pore. Corresponding point mutants (M69E, A70D, L73E, S74D) were defective in binding TLR2 or TLR1 and could not activate APCs, despite retaining full ganglioside-binding capacity. Point mutations in the TLR2/1 dimer interface, as determined in the crystallographic structure of the TLR2/1-Pam(3)CSK(4) complex, resulted in diminished activation by both Pam(3)CSK(4) and LT-IIb-B(5). Docking analysis of the LT-IIb-B(5) interaction with this apparently predominant activation conformation of TLR2/1 revealed that LT-IIb-B(5) might primarily contact the convex surface of the TLR2 central domain. Although the TLR1/LT-IIb-B(5) interface is relatively smaller, the leucine-rich repeat motifs 9-12 in the central domain of TLR1 were found to be critical for cooperative TLR2-induced cell activation by LT-IIb-B(5). Moreover, the putative LT-IIb-B(5) binding site overlaps partially with that of Pam(3)CSK(4); consistent with this, Pam(3)CSK(4) suppressed TLR2 binding of LT-IIb-B(5), albeit not as potently as self-competitive inhibition. We identified the upper pore region of LT-IIb-B(5) as a TLR2/1 interactive domain, which contacts the heterodimeric receptor at a site that is distinct from, although it overlaps with, that of Pam(3)CSK(4).

The type IIb heat-labile enterotoxin of Escherichia coli (LT-IIb) and its nontoxic pentameric B subunit (LT-IIb-B(5)) display different immunomodulatory activities, the mechanisms of which are poorly understood. We investigated mechanisms whereby the absence of the catalytically active A subunit from LT-IIb-B(5) renders this molecule immunostimulatory through TLR2. LT-IIb-B(5), but not LT-IIb, induced TLR2-mediated NF-kappaB activation and TNF-alpha production. These LT-IIb-B(5) activities were antagonized by LT-IIb; however, inhibitors of adenylate cyclase or protein kinase A reversed this antagonism. The LT-IIb antagonistic effect is thus likely dependent upon the catalytic activity of its A subunit, which causes elevation of intracellular cAMP and activates cAMP-dependent protein kinase A. Consistent with this, a membrane-permeable cAMP analog and a cAMP-elevating agonist, but not catalytically defective point mutants of LT-IIb, mimicked the antagonistic action of wild-type LT-IIb. The mutants moreover displayed increased proinflammatory activity compared with wild-type LT-IIb. Additional mechanisms for the divergent effects on TLR2 activation by LT-IIb and LT-IIb-B(5) were suggested by findings that the latter was significantly stronger in inducing lipid raft recruitment of TLR2 and interacting with this receptor. The selective use of TLR2 by LT-IIb-B(5) was confirmed in an assay for IL-10, which is inducible by both LT-IIb and LT-IIb-B(5) at comparable levels; TLR2-deficient macrophages failed to induce IL-10 in response to LT-IIb-B(5) but not in response to LT-IIb. These differential immunomodulatory effects by LT-IIb and LT-IIb-B(5) have important implications for adjuvant development and, furthermore, suggest that enterotoxic E. coli may suppress TLR-mediated innate immunity through the action of the enterotoxin A subunit.

Innate recognition and signaling by Toll-like receptors (TLRs) is facilitated by functionally associated coreceptors, although the cooperativity mechanisms involved are poorly understood. As a model we investigated TLR2 interactions with the GD1a ganglioside binding subunit of type IIb Escherichia coli enterotoxin (LT-IIb-B(5)). Both LT-IIb-B(5) and a GD1a binding-defective mutant (LT-IIb-B(5)(T13I)) could modestly bind to TLR2, but only the wild-type molecule displayed a dramatic increase in TLR2 binding activity in the presence of GD1a (although not in the presence of irrelevant gangliosides). Moreover, fluorescence resonance energy transfer experiments indicated that LT-IIb-B(5) induces lipid raft recruitment of TLR2 and TLR1 and their clustering with GD1a, in contrast to the GD1a binding-defective mutant, which moreover fails to activate TLR2 signaling. LT-IIb-B(5)-induced cell activation was critically dependent upon the Toll/IL-1 receptor domain-containing adaptor protein, which was induced to colocalize with TLR2 and GD1a, as shown by confocal imaging. Therefore, GD1a provides TLR2 coreceptor function by enabling the ligand to recruit, bind, and activate TLR2. These findings establish a model of TLR2 coreceptor function and, moreover, suggest novel mechanisms of adjuvanticity by non-toxic derivatives of type II enterotoxins dependent upon GD1a/TLR2 cooperative activity.

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.

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.

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.

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.