Neisserial binding to CEACAM1 arrests the activation and proliferation of CD4+ T lymphocytes

Diversion of the immune response to Neisseria gonorrhoeae from Th17 to Th1/Th2 by treatment with anti-transforming growth factor β antibody generates immunological memory and protective immunity

The immune response to Neisseria gonorrhoeae is poorly understood, but its extensive antigenic variability and resistance to complement are thought to allow it to evade destruction by the host’s immune defenses. We propose that N. gonorrhoeae also avoids inducing protective immune responses in the first place. We previously found that N. gonorrhoeae induces interleukin-17 (IL-17)-dependent innate responses in mice and suppresses Th1/Th2-dependent adaptive responses in murine cells in vitro through the induction of transforming growth factor β (TGF-β). In this study using a murine model of vaginal gonococcal infection, mice treated with anti-TGF-β antibody during primary infection showed accelerated clearance of N. gonorrhoeae, with incipient development of Th1 and Th2 responses and diminished Th17 responses in genital tract tissue. Upon secondary reinfection, mice that had been treated with anti-TGF-β during primary infection showed anamnestic recall of both Th1 and Th2 responses, with the development of antigonococcal antibodies in sera and secretions, and enhanced resistance to reinfection. In mouse knockout strains defective in Th1 or Th2 responses, accelerated clearance of primary infection due to anti-TGF-β treatment was dependent on Th1 activity but not Th2 activity, whereas resistance to secondary infection resulting from anti-TGF-β treatment during primary infection was due to both Th1- and Th2-dependent memory responses. We propose that N. gonorrhoeae proactively elicits Th17-driven innate responses that it can resist and concomitantly suppresses Th1/Th2-driven specific adaptive immunity that would protect the host. Blockade of TGF-β reverses this pattern of host immune responsiveness and facilitates the emergence of protective antigonococcal immunity.

Pathogen-host interactions during infectious disease are conventionally thought of as two-way reactions, that of the host against the pathogen and vice versa, with the outcome dependent on which one ultimately prevails. We propose that Neisseria gonorrhoeae, a pathogen that has become extremely well adapted to its exclusive human host, proactively directs the manner in which the host responds in ways that are beneficial to its own survival but detrimental to the host. Gonorrhea is a widely prevalent sexually transmitted infection, and naturally occurring gonococcal strains are becoming resistant to most available antibiotics, yet no effective vaccine has been developed. These new insights into the immune response to N. gonorrhoeae should lead to novel therapeutic strategies and facilitate new approaches to vaccine development.

The Pathobiology of Neisseria gonorrhoeae Lower Female Genital Tract Infection

Infection and disease associated with Neisseria gonorrhoeae, the gonococcus, continue to be a global health problem. Asymptomatic and subclinical gonococcal infections occur at a high frequency in females; thus, the true incidence of N. gonorrhoeae infections are presumed to be severely underestimated. Inherent to this asymptomatic/subclinical diseased state is the continued prevalence of this organism within the general population, as well as the medical, economic, and social burden equated with the observed chronic, disease sequelae. As infections of the lower female genital tract (i.e., the uterine cervix) commonly result in subclinical disease, it follows that the pathobiology of cervical gonorrhea would differ from that observed for other sites of infection. In this regard, the potential responses to infection that are generated by the female reproductive tract mucosa are unique in that they are governed, in part, by cyclic fluctuations in steroid hormone levels. The lower female genital tract has the further distinction of being able to functionally discriminate between resident commensal microbiota and transient pathogens. The expression of functionally active complement receptor 3 by the lower, but not the upper, female genital tract mucosa; together with data indicating that gonococcal adherence to and invasion of primary cervical epithelial cells and tissue are predominately aided by this surface-expressed host molecule; provide one explanation for asymptomatic/subclinical gonococcal cervicitis. However, co-evolution of the gonococcus with its sole human host has endowed this organism with variable survival strategies that not only aid these bacteria in successfully evasion of immune detection and function but also enhance cervical colonization and cellular invasion. To this end, we herein summarize current knowledge pertaining to the pathobiology of gonococcal infection of the human cervix.

Potential of recombinant opa proteins as vaccine candidates against hyperinvasive meningococci

Neisseria meningitidis causes half a million cases of septicemia and meningitis globally each year. The opacity (Opa) integral outer membrane proteins from N. meningitidis are polymorphic and highly immunogenic. Particular combinations of Opa proteins are associated with the hyperinvasive meningococcal lineages that have caused the majority of serogroup B and C meningococcal disease in industrialized countries over the last 60 years. For the first time, this genetic structuring of a diverse outer membrane protein family has been used to select a novel combination of representative antigens for immunogenicity testing. Fourteen recombinant Opa variants were produced and used in murine immunizations inducing an increase in specific antimeningococcal total IgG levels. All 14 Opa proteins elicited bactericidal antibodies against at least one hyperinvasive meningococcal isolate, and most isolates from each hyperinvasive lineage were killed by at least one Opa antiserum at a titer of 1:16 or greater. Cross-reactive bactericidal antibody responses were observed among clonal complexes. A theoretical coverage of 90% can be achieved by using a particular combination of 6 Opa proteins against an isolate collection of 227 recent United Kingdom disease cases. This study indicates the potential of Opa proteins to provide broad coverage against multiple meningococcal hyperinvasive lineages.

New concepts in immunity to Neisseria gonorrhoeae: innate responses and suppression of adaptive immunity favor the pathogen, not the host

It is well-known that gonorrhea can be acquired repeatedly with no apparent development of protective immunity arising from previous episodes of infection. Symptomatic infection is characterized by a purulent exudate, but the host response mechanisms are poorly understood. While the remarkable antigenic variability displayed by Neisseria gonorrhoeae and its capacity to inhibit complement activation allow it to evade destruction by the host's immune defenses, we propose that it also has the capacity to avoid inducing specific immune responses. In a mouse model of vaginal gonococcal infection, N. gonorrhoeae elicits Th17-driven inflammatory–immune responses, which recruit innate defense mechanisms including an influx of neutrophils. Concomitantly, N. gonorrhoeae suppresses Th1- and Th2-dependent adaptive immunity, including specific antibody responses, through a mechanism involving TGF-β and regulatory T cells. Blockade of TGF-β alleviates the suppression of specific anti-gonococcal responses and allows Th1 and Th2 responses to emerge with the generation of immune memory and protective immunity. Genital tract tissues are naturally rich in TGF-β, which fosters an immunosuppressive environment that is important in reproduction. In exploiting this niche, N. gonorrhoeae exemplifies a well-adapted pathogen that proactively elicits from its host innate responses that it can survive and concomitantly suppresses adaptive immunity. Comprehension of these mechanisms of gonococcal pathogenesis should allow the development of novel approaches to therapy and facilitate the development of an effective vaccine.

Apical expression of human full-length hCEACAM1-4L protein renders the Madin Darby Canine Kidney cells responsive to lipopolysaccharide leading to TLR4-dependent Erk1/2 and p38 MAPK signalling

CEACAM1 expressed by granulocytes and epithelial cells is recognized as a membrane-associated receptor by some Gram-negative pathogens. Here we report a previously unsuspected role of human CEACAM1-4L (hCEACAM1-4L) in polarized epithelial cells. We find that in contrast with non-transfected cells, Madin Darby Canine Kidney strain II (MDCK) engineered for the apical expression of the long cytoplasmic chain protein hCEACAM1-4L showed a serum-independent increase in the phosphorylation of the extracellular signal-regulated kinase 1/2 (Erk1/2) and p38 mitogen-activated protein kinases (MAPKs) after treatment with lipopolysaccharide (LPS) of wild-type, diffusely adhering Afa/Dr Escherichia coli (Afa/Dr DAEC) strain IH11128. Aggregates of FITC-LPS bind the apical domain of MDCK-hCEACAM1-4L cells colocalizing with the apically expressed hCEACAM1-4L protein and do not bind MDCK-pCEP cells, and surface plasmon resonance analysis shows that LPS binds to the extracellular domain of the CEACAM1-4L protein. We showed that cell polarization and lipid rafts positively control the LPS-IH11128-induced phosphorylation of Erk1/2 in MDCK-hCEACAM1-4L cells. Structure-function analysis using mutated hCEACAM1-4L protein shows that the cytoplasmic domain of the protein is needed for LPS-induced MAPK signalling, and that phosphorylation of Tyr-residues is not increased in association with MAPK signalling. The hCEACAM1-4L-dependent Erk1/2 phosphorylation develops in the presence of lipid A and does not develop in the presence of penta-acylated LPS. Finally, small interfering RNA (siRNA) silencing of canine TLR4 abolishes the hCEACAM1-4L-dependent, LPS-induced phosphorylation of Erk1/2. Collectively, our results support the notion that the apically expressed, full-length hCEACAM1-4L protein functions as a novel LPS-conveying molecule at the mucosal surface of polarized epithelial cells for subsequent MD-2/TLR4 receptor-dependent MAPK Erk1/2 and p38 signalling.

Neisseria meningitidis has two independent modes of recognizing its human receptor CEACAM1

BACKGROUND

Several human-restricted gram-negative bacteria exploit carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) for host colonization. For example, Neisseria meningitidis engages these human receptors via outer membrane proteins of the colony opacity-associated (Opa) protein family triggering internalization into non-phagocytic cells.

PRINCIPAL FINDINGS

We report that a non-opaque strain of N. meningitidis selectively interacts with CEACAM1, but not other CEACAM family members. Using functional assays of bacterial adhesion and internalisation, microscopic analysis, and a panel of CEACAM1 deletion mutants we demonstrate that the engagement of CEACAM1 by non-opaque meningococci occurs in a manner distinct from Opa protein-mediated association. In particular, the amino-terminal domain of CEACAM1 is necessary, but not sufficient for Opa protein-independent binding, which requires multiple extracellular domains of the human receptor in a cellular context. Knock-down of CEACAM1 interferes with binding to lung epithelial cells, whereas chemical or pharmacological disruption of host protein glycosylation does not abrogate CEACAM1 recognition by non-opaque meningococci. The previously characterized meningococcal invasins NadA or Opc do not operate in a CEACAM1-dependent manner.

CONCLUSIONS

The results demonstrate a mechanistically distinct, Opa protein-independent interaction between N. meningitidis and human CEACAM1. Our functional investigations suggest the presence of a second CEACAM1-binding invasin on the meningococcal surface that associates with the protein backbone and not the carbohydrate structures of CEACAM1. The redundancy in meningococcal CEACAM1-binding factors further highlights the important role of CEACAM recognition in the biology of this human-adapted pathogen.

Opa proteins and CEACAMs: pathways of immune engagement for pathogenic Neisseria

Neisseria meningitidis and Neisseria gonorrhoeae are globally important pathogens, which in part owe their success to their ability to successfully evade human immune responses over long periods. The phase-variable opacity-associated (Opa) adhesin proteins are a major surface component of these organisms, and are responsible for bacterial adherence and entry into host cells and interactions with the immune system. Most immune interactions are mediated via binding to members of the carcinoembryonic antigen cell adhesion molecule (CEACAM) family. These Opa variants are able to bind to different receptors of the CEACAM family on epithelial cells, neutrophils, and T and B lymphocytes, influencing the innate and adaptive immune responses. Increased epithelial cell adhesion creates the potential for prolonged infection, invasion and dissemination. Furthermore, Opa proteins may inhibit T-lymphocyte activation and proliferation, B-cell antibody production, and innate inflammatory responses by infected epithelia, in addition to conferring increased resistance to antibody-dependent, complement-mediated killing. While vaccines containing Opa proteins could induce adhesion-blocking and bactericidal antibodies, the consequence of CEACAM binding by a candidate Opa-containing vaccine requires further investigation. This review summarizes current knowledge of the immunological consequences of the interaction between meningococcal and gonococcal Opa proteins and human CEACAMs, considering the implications for pathogenesis and vaccine development.

Tolerance and protection against infection in the genital tract

The genital tract is a unique immunological environment that must support the reproductive function and resist infection. Particularly in the female tract, immunoregulatory and immunosuppressive activities that permit the growth of the fetus create an environment that can readily be exploited by microbes that have become well-adapted to this location. Cellular and molecular mediators of immune responses differ from those found at other mucosal surfaces. Mechanisms of immune response induction and delivery, as well as immune effector functions at the genital mucosae need to be considered in the development of vaccines against infections of the genital tract.

Generation of human CEACAM1 transgenic mice and binding of Neisseria Opa protein to their neutrophils

BACKGROUND

Human CEACAM1 is a cell-cell adhesion molecule with multiple functions including insulin clearance in the liver, vasculogenesis in endothelial cells, lumen formation in the mammary gland, and binding of certain human pathogens.

PRINCIPAL FINDINGS

Three genomic BAC clones containing the human CEACAM1 gene were microinjected into pronuclei of fertilized FVB mouse oocytes. The embryos were implanted in the oviducts of pseudopregnant females and allowed to develop to term. DNA from newborn mice was evaluated by PCR for the presence of the human CEACAM1 gene. Feces of the PCR positive offspring screened for expression of human CEACAM1. Using this assay, one out of five PCR positive lines was positive for human CEACAM1 expression and showed stable transmission to the F1 generation with the expected transmission frequency (0.5) for heterozygotes. Liver, lung, intestine, kidney, mammary gland, and prostate were strongly positive for the dual expression of both murine and human CEACAM1 and mimic that seen in human tissue. Peripheral blood and bone marrow granulocytes stained strongly for human CEACAM1 and bound Neisseria Opa proteins similar to that in human neutrophils.

CONCLUSION

These transgenic animals may serve as a model for the binding of human pathogens to human CEACAM1.

Opacity proteins increase Neisseria gonorrhoeae fitness in the female genital tract due to a factor under ovarian control

The neisserial opacity (Opa) proteins are a family of antigenically distinct outer membrane proteins that undergo phase-variable expression. Opa(+) variants of Neisseria gonorrhoeae strain FA1090 are selected in a cyclical pattern from the lower genital tract of estradiol-treated mice. Here we show that cyclical recovery of Opa(+) gonococci does not occur in ovariectomized mice; therefore, the reproductive cycle plays a role in the selection kinetics in vivo. As predicted by the selection pattern shown by wild-type gonococci, we demonstrated that a constitutive Opa-expressing strain was more fit than an Opa-deficient mutant in the early and late phases of infection. We found no evidence that Opa-mediated colonization selects for Opa(+) variants during murine infection based on adherence assays with cultured murine epithelial cells. We also tested the hypothesis that complement selects for Opa protein expression during infection. Although some Opa(+) variants of a serum-sensitive derivative of strain FA1090 were more resistant to the bactericidal activity of normal human serum, selection for Opa expression was not abrogated in C3-depleted mice. Finally, as previously reported, Opa(+) gonococci were more sensitive to serine proteases. Thus, proteases or protease inhibitors may contribute to the observed in vivo selection pattern. We concluded that Opa proteins promote persistence of N. gonorrhoeae in the female genital tract and that opa gene phase variation allows gonococci to evade or capitalize upon unidentified host factors of the mammalian reproductive cycle. This work revealed an intimate interaction between pathogen and host and provides evidence that hormonally related factors shape bacterial adaptation.

Neisseria gonorrhoeae enhances HIV-1 infection of primary resting CD4+ T cells through TLR2 activation

Sexually transmitted infections increase the likelihood of HIV-1 transmission. We investigated the effect of Neisseria gonorrheae (gonococcus [GC]) exposure on HIV replication in primary resting CD4(+) T cells, a major HIV target cell during the early stage of sexual transmission of HIV. GC and TLR2 agonists, such as peptidylglycan (PGN), Pam(3)CSK(4), and Pam(3)C-Lip, a GC-derived synthetic lipopeptide, but not TLR4 agonists including LPS or GC lipooligosaccharide enhanced HIV-1 infection of primary resting CD4(+) T cells after viral entry. Pretreatment of CD4(+) cells with PGN also promoted HIV infection. Anti-TLR2 Abs abolished the HIV enhancing effect of GC and Pam(3)C-Lip, indicating that GC-mediated enhancement of HIV infection of resting CD4(+) T cells was through TLR2. IL-2 was required for TLR2-mediated HIV enhancement. PGN and GC induced cell surface expression of T cell activation markers and HIV coreceptors, CCR5 and CXCR4. The maximal postentry HIV enhancing effect was achieved when PGN was added immediately after viral exposure. Kinetic studies and analysis of HIV DNA products indicated that GC exposure and TLR2 activation enhanced HIV infection at the step of nuclear import. We conclude that GC enhanced HIV infection of primary resting CD4(+) T cells through TLR2 activation, which both increased the susceptibility of primary CD4(+) T cells to HIV infection as well as enhanced HIV-infected CD4(+) T cells at the early stage of HIV life cycle after entry. This study provides a molecular mechanism by which nonulcerative sexually transmitted infections mediate enhancement of HIV infection and has implication for HIV prevention and therapeutics.

Coevolution of activating and inhibitory receptors within mammalian carcinoembryonic antigen families

BACKGROUND

Most rapidly evolving gene families are involved in immune responses and reproduction, two biological functions which have been assigned to the carcinoembryonic antigen (CEA) gene family. To gain insights into evolutionary forces shaping the CEA gene family we have analysed this gene family in 27 mammalian species including monotreme and marsupial lineages.

RESULTS

Phylogenetic analysis provided convincing evidence that the primordial CEA gene family in mammals consisted of five genes, including the immune inhibitory receptor-encoding CEACAM1 (CEA-related cell adhesion molecule) ancestor. Our analysis of the substitution rates within the nucleotide sequence which codes for the ligand binding domain of CEACAM1 indicates that the selection for diversification is, perhaps, a consequence of the exploitation of CEACAM1 by a variety of viral and bacterial pathogens as their cellular receptor. Depending on the extent of the amplification of an ancestral CEACAM1, the number of CEACAM1-related genes varies considerably between mammalian species from less than five in lagomorphs to more than 100 in bats. In most analysed species, ITAM (immunoreceptor tyrosine-based activation motifs) or ITAM-like motif-containing proteins exist which contain Ig-V-like, ligand binding domains closely related to that of CEACAM1. Human CEACAM3 is one such protein which can function as a CEACAM1 decoy receptor in granulocytes by mediating the uptake and destruction of specific bacterial pathogens via its ITAM-like motif. The close relationship between CEACAM1 and its ITAM-encoding relatives appears to be maintained by gene conversion and reciprocal recombination. Surprisingly, secreted CEACAMs resembling immunomodulatory CEACAM1-related trophoblast-specific pregnancy-specific glycoproteins (PSGs) found in humans and rodents evolved only in a limited set of mammals. The appearance of PSG-like genes correlates with invasive trophoblast growth in these species.

CONCLUSIONS

These phylogenetic studies provide evidence that pathogen/host coevolution and a possible participation in fetal-maternal conflict processes led to a highly species-specific diversity of mammalian CEA gene families.

Homophilic adhesion and CEACAM1-S regulate dimerization of CEACAM1-L and recruitment of SHP-2 and c-Src

Carcinoembryonic antigen (CEA)-related cell adhesion molecule 1 (CAM1 [CEACAM1]) mediates homophilic cell adhesion and regulates signaling. Although there is evidence that CEACAM1 binds and activates SHP-1, SHP-2, and c-Src, knowledge about the mechanism of transmembrane signaling is lacking. To analyze the regulation of SHP-1/SHP-2/c-Src binding, we expressed various CFP/YFP-tagged CEACAM1 isoforms in epithelial cells. The supramolecular organization of CEACAM1 was examined by cross-linking, coclustering, coimmunoprecipitation, and fluorescence resonance energy transfer. SHP-1/SHP-2/c-Src binding was monitored by coimmunoprecipitation and phosphotyrosine-induced recruitment to CEACAM1-L in cellular monolayers. We find that trans-homophilic CEACAM1 binding induces cis-dimerization by an allosteric mechanism transmitted by the N-terminal immunoglobulin-like domain. The balance of SHP-2 and c-Src binding is dependent on the monomer/dimer equilibrium of CEACAM1-L and is regulated by trans-binding, whereas SHP-1 does not bind under physiological conditions. CEACAM1-L homodimer formation is reduced by coexpression of CEACAM1-S and modulated by antibody ligation. These data suggest that transmembrane signaling by CEACAM1 operates by alteration of the monomer/dimer equilibrium, which leads to changes in the SHP-2/c-Src-binding ratio.

Adhesive organelles of Gram-negative pathogens assembled with the classical chaperone/usher machinery: structure and function from a clinical standpoint

This review summarizes current knowledge on the structure, function, assembly and biomedical applications of the superfamily of adhesive fimbrial organelles exposed on the surface of Gram-negative pathogens with the classical chaperone/usher machinery. High-resolution three-dimensional (3D) structure studies of the minifibers assembling with the FGL (having a long F1-G1 loop) and FGS (having a short F1-G1 loop) chaperones show that they exploit the same principle of donor-strand complementation for polymerization of subunits. The 3D structure of adhesive subunits bound to host-cell receptors and the final architecture of adhesive fimbrial organelles reveal two functional families of the organelles, respectively, possessing polyadhesive and monoadhesive binding. The FGL and FGS chaperone-assembled polyadhesins are encoded exclusively by the gene clusters of the γ3- and κ-monophyletic groups, respectively, while gene clusters belonging to the γ1-, γ2-, γ4-, and π-fimbrial clades exclusively encode FGS chaperone-assembled monoadhesins. Novel approaches are suggested for a rational design of antimicrobials inhibiting the organelle assembly or inhibiting their binding to host-cell receptors. Vaccines are currently under development based on the recombinant subunits of adhesins.

Functional characterization of antibodies against Neisseria gonorrhoeae opacity protein loops

Background

The development of a gonorrhea vaccine is challenged by the lack of correlates of protection. The antigenically variable neisserial opacity (Opa) proteins are expressed during infection and have a semivariable (SV) and highly conserved (4L) loop that could be targeted in a vaccine. Here we compared antibodies to linear (Ab_linear) and cyclic (Ab_cyclic) peptides that correspond to the SV and 4L loops and selected hypervariable (HV₂) loops for surface-binding and protective activity in vitro and in vivo.

Methods/Findings

Ab_{SV cyclic} bound a greater number of different Opa variants than Ab_{SV linear}, including variants that differed by seven amino acids. Antibodies to the 4L peptide did not bind Opa-expressing bacteria. Ab_SVcyclic and Ab_HV2cyclic, but not Ab_SVlinear or Ab_{HV2 linear} agglutinated homologous Opa variants, and Ab_HV2BDcyclic but not Ab_HV2BDlinear blocked the association of OpaB variants with human endocervical cells. Only Ab_HV2BDlinear were bactericidal against the serum resistant parent strain. Consistent with host restrictions in the complement cascade, the bactericidal activity of Ab_HV2BDlinear was increased 8-fold when rabbit complement was used. None of the antibodies was protective when administered vaginally to mice. Antibody duration in the vagina was short-lived, however, with <50% of the antibodies recovered 3 hrs post-administration.

Conclusions

We conclude that an SV loop-specific cyclic peptide can be used to induce antibodies that recognize a broad spectrum of antigenically distinct Opa variants and have agglutination abilities. HV₂ loop-specific cyclic peptides elicited antibodies with agglutination and adherence blocking abilities. The use of human complement when testing the bactericidal activity of vaccine-induced antibodies against serum resistant gonococci is also important.

Chemokine expression patterns in the systemic and genital tract compartments are associated with HIV-1 infection in women from Benin

INTRODUCTION

Understanding the genital mucosal immunity and the factors involved in linking innate to adaptive immunity is crucial for the design of efficient preventive strategies against human immunodeficiency virus (HIV)-1.

METHODS

Levels of both genital mucosal and blood chemokines were compared between 58 HIV-1-uninfected and 50 HIV-1-infected female commercial sex workers (CSWs) as well as 53 HIV-1-uninfected non-CSW control women at low risk for exposure, recruited in Cotonou, Benin.

RESULTS

HIV-1-infected CSWs had significantly higher blood and genital levels of monocyte chemotactic protein (MCP-3/CCL7) and monokine induced by gamma interferon (MIG/CXCL9) compared with those in both the HIV-1-uninfected CSW and non-CSW groups. In the HIV-1-infected group, levels of MCP-3 and MIG were significantly higher in the genital mucosa than in the blood. However, the blood levels of macrophage inflammatory protein (MIP-1a/CCL3) and MIP-1b/CCL4 were higher in HIV-1-uninfected CSWs compared with those in the other groups.

CONCLUSION

Increased production of chemokines in the genital tract may favour the recruitment of HIV-1 target cells causing a mucosal environment that promotes viral replication and dissemination, whereas higher expression of beta-chemokines at the systemic level is associated with protection from HIV-1 infection.

Variation of Neisseria gonorrhoeae lipooligosaccharide directs dendritic cell-induced T helper responses

Gonorrhea is one of the most prevalent sexually transmitted diseases in the world. A naturally occurring variation of the terminal carbohydrates on the lipooligosaccharide (LOS) molecule correlates with altered disease states. Here, we investigated the interaction of different stable gonoccocal LOS phenotypes with human dendritic cells and demonstrate that each variant targets a different set of receptors on the dendritic cell, including the C-type lectins MGL and DC-SIGN. Neisseria gonorrhoeae LOS phenotype C constitutes the first bacterial ligand to be described for the human C-type lectin receptor MGL. Both MGL and DC-SIGN are locally expressed at the male and female genital area, the primary site of N. gonorrhoeae infection. We show that targeting of different C-type lectins with the N. gonorrhoeae LOS variants results in alterations in dendritic cell cytokine secretion profiles and the induction of distinct adaptive CD4⁺ T helper responses. Whereas N. gonorrhoeae variant A with a terminal N-acetylglucosamine on its LOS was recognized by DC-SIGN and induced significantly more IL-10 production, phenotype C, carrying a terminal N-acetylgalactosamine, primarily interacted with MGL and skewed immunity towards the T helper 2 lineage. Together, our results indicate that N. gonorrhoeae LOS variation allows for selective manipulation of dendritic cell function, thereby shifting subsequent immune responses in favor of bacterial survival.

Neisseria gonorrhoeae is a species of Gram-negative bacteria responsible for the sexually transmitted disease gonorrhea. Although effective antibiotic treatments are available, little is known about the host immune response to this pathogen. Here, we analyzed three well-characterized gonococcal variants that only differ in the outer sugar present on the lipooligosaccharide of the bacteria. We found that human dendritic cells use different receptors, including the C-type lectins MGL and DC-SIGN, to detect the three N. gonorrhoeae phenotypes. Dendritic cells carrying the MGL and DC-SIGN receptor were present in the human genital tissues, the site of gonoccocal invasion. DC activation with the gonococcal variants resulted in different cytokine secretion profiles and alterations in the subsequent adaptive T cell response. In particular, LOS containing a terminal N-acetylgalactosamine induced more T helper 2-type responses, which are unfavorable for clearing the bacteria. Our data provide new insights into the pathogenesis of N. gonorrhoeae and suggest that variation of lipooligosaccharide glycosylation enables to bacteria to selectively subvert host immune defense mechanisms.

Opa+ and Opa- isolates of Neisseria meningitidis and Neisseria gonorrhoeae induce sustained proliferative responses in human CD4+ T cells

T cells may interact with a number of bacterial surface antigens, an encounter which has the potential to downmodulate host immune responses. Neisseria meningitidis, a human colonizer and an agent of septicemia and meningitis, expresses Opa proteins which interact with the CEACAM1 receptor expressed on activated T cells. Since CEACAM1 can act as an inhibitory receptor and T cells in subepithelial tissues may encounter whole bacteria, which often express Opa proteins in vivo, this study assessed primarily if Opa proteins expressed on meningococci affect T-cell functions. In addition, Opa-containing outer membrane vesicles (OMV) have been used as vaccine antigens, and therefore Opa+ and Opa- OMV were also studied. While Opa+ bacteria adhered to CEACAM-expressing T cells, both the Opa+ and Opa- phenotypes induced no to a small transient depression, followed by a prolonged increase in proliferation as well as cytokine production. Such responses were also observed with heat-killed bacteria or OMV. In addition, while anti-CEACAM antibodies alone inhibited proliferation, on coincubation of T cells with bacteria and the antibodies, bacterial effects predominated and were Opa independent. Thus, while Opa proteins of N. meningitidis can bind to T-cell-expressed CEACAM1, this is not sufficient to overcome the T-cell recognition of bacterial factors, which results in a proliferative and cytokine response, an observation consistent with the ability of the host to establish lasting immunity to Opa-expressing meningococci that it frequently encounters. The data also imply that Opa-proficient vaccine preparations may not necessarily inhibit T-cell functions via CEACAM1 binding.

Carcinoembryonic antigen-related cell adhesion molecule 1 modulates experimental autoimmune encephalomyelitis via an iNKT cell-dependent mechanism

Carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1) is a CEA family member that has been reported to have an important role in the regulation of Th1-mediated colitis. In this study, we examined the role of CEACAM1 in an animal model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). Treatment of C57BL/6J mice with CEACAM1-Fc fusion protein, a homophilic ligand of CEACAM1, inhibited the severity of EAE and reduced myelin oligodendrocyte glycoprotein-derived peptide (MOG(35-55))-reactive interferon-gamma and interleukin-17 production. In contrast, treatment of these animals with AgB10, an anti-mouse CEACAM1 blocking monoclonal antibody, generated increased severity of EAE in association with increased MOG(35-55)-specific induction of both interferon-gamma and interleukin-17. These results indicated that the signal elicited through CEACAM1 ameliorated EAE disease severity. Furthermore, we found that there was both a rapid and enhanced expression of CEACAM1 on invariant natural killer T cells after activation. The effect of CEACAM1-Fc fusion protein and anti-CEACAM1 mAb on both EAE and MOG(35-55)-reactive cytokine responses were abolished in invariant natural killer T cell-deficient Jalpha18(-/-) mice. Taken together, the ligation of CEACAM1 negatively regulates the severity of EAE by reducing MOG(35-55)-specific induction of both interferon-gamma and interleukin-17 via invariant natural killer T cell-dependent mechanisms.

Mycobacterium tuberculosis cell wall glycolipids directly inhibit CD4+ T-cell activation by interfering with proximal T-cell-receptor signaling

Immune evasion is required for Mycobacterium tuberculosis to survive in the face of robust adaptive CD4(+) T-cell responses. We have previously shown that M. tuberculosis can indirectly inhibit CD4(+) T cells by suppressing the major histocompatibility complex class II antigen-presenting cell function of macrophages. This study was undertaken to determine if M. tuberculosis could directly inhibit CD4(+) T-cell activation. Murine CD4(+) T cells were purified from spleens by negative immunoaffinity selection followed by flow sorting. Purified CD4(+) T cells were activated for 16 to 48 h with CD3 and CD28 monoclonal antibodies in the presence or absence of M. tuberculosis and its subcellular fractions. CD4(+) T-cell activation was measured by interleukin 2 production, proliferation, and expression of activation markers, all of which were decreased in the presence of M. tuberculosis. Fractionation identified that M. tuberculosis cell wall glycolipids, specifically, phosphatidylinositol mannoside and mannose-capped lipoarabinomannan, were potent inhibitors. Glycolipid-mediated inhibition was not dependent on Toll-like receptor signaling and could be bypassed through stimulation with phorbol 12-myristate 13-acetate and ionomycin. ZAP-70 phosphorylation was decreased in the presence of M. tuberculosis glycolipids, indicating that M. tuberculosis glycolipids directly inhibited CD4(+) T-cell activation by interfering with proximal T-cell-receptor signaling.

Neisserial Opa Proteins: Impact on Colonization, Dissemination and Immunity

The pathogenic Neisseria sp. encode a family of phase-variable and antigenically distinct Opa proteins that allow bacterial attachment to virtually every cell type encountered during infection. Some Opa variants bind cell surface-expressed heparan sulfate proteoglycans, including members of the syndecan family of receptors, and extracellular matrix proteins such as fibronectin and vitronectin. Other variants bind members of the carcinoembryonic antigen family of cellular adhesion molecules. Depending on the Opa variant(s) expressed, these receptor interactions can allow neisserial entry and transcellular transcytosis across polarized epithelial cell monolayers, entry into endothelial cells, suppression of lymphocyte function and/or bacterial engulfment and killing by neutrophils. Recent advances in our understanding of how these Opa protein-mediated interactions influence the host cellular response are discussed in the context of their impact on various stages of neisserial infection.

An African perspective on mucosal immunity and HIV-1

HIV prevention mandates an understanding of the mechanisms of mucosal immunity with attention to some unique features of the epidemic and mucosal environment in the developing world. An effective vaccine will have to induce mucosal protection against a highly diverse virus, which is equipped with a number of immune evasion strategies. Its development will require assessment of mucosal immune responses, and it will have to protect a mucosal environment where inflammation and altered immune responses are common because of the presence of other mucosal infections, such as sexually transmitted infections and parasites, and where nutritional status may also be compromised. Ideally, not only prevention methods would protect adults but also provide cover against gastrointestinal transmission through maternal milk. Prevention might also be complemented by microbicides and circumcision, two alternative approaches to mucosal protection. It seems unlikely that a single solution will work in all instances and intervention might have to act at multiple levels and be tailored to local circumstances. We review here some of the mucosal events associated with HIV infection that are most relevant in an African setting.

Challenges and progress in the development of a serogroup B meningococcal vaccine

Serogroup B meningococci cause the majority of the meningococcal disease burden in developed countries. Production of an effective and safe vaccine for serogroup B organisms has been hampered by the poor immunogenicity of the capsular polysaccharide that defines this group of bacteria. Previous efforts have focused on outer membrane vesicle vaccines, which have been implemented successfully during clonal outbreaks. However, the search for a universal vaccine against endemic polyclonal serogroup B meningococcal disease continues. In this review, we have highlighted recent development of outer membrane vesicle vaccines and progress in the evaluation of recombinant outer membrane protein vaccines.

Meningococcal interactions with the host

Neisseria meningitidis interacts with host tissues through hierarchical, concerted and co-ordinated actions of a number of adhesins; many of which undergo antigenic and phase variation, a strategy that helps immune evasion. Three major structures, pili, Opa and Opc predominantly influence bacterial adhesion to host cells. Pili and Opa proteins also determine host and tissue specificity while Opa and Opc facilitate efficient cellular invasion. Recent studies have also implied a role of certain adhesin-receptor pairs in determining increased host susceptibility to infection. This chapter examines our current knowledge of meningococcal adhesion and invasion mechanisms particularly related to human epithelial and endothelial cells which are of primary importance in the disease process.

Neisseria gonorrhoeae activates the proteinase cathepsin B to mediate the signaling activities of the NLRP3 and ASC-containing inflammasome

Neisseria gonorrhoeae is a common sexually transmitted pathogen that significantly impacts female fertility, neonatal health, and transmission of HIV worldwide. N. gonorrhoeae usually causes localized inflammation of the urethra and cervix by inducing production of IL-1beta and other inflammatory cytokines. Several NLR (nucleotide-binding domain, leucine-rich repeat) proteins are implicated in the formation of pro-IL-1beta-processing complexes called inflammasomes in response to pathogens. We demonstrate that NLRP3 (cryopyrin, NALP3) is the primary NLR required for IL-1beta/IL-18 secretion in response to N. gonorrhoeae in monocytes. We also show that N. gonorrhoeae infection promotes NLRP3-dependent monocytic cell death via pyronecrosis, a recently described pathway with morphological features of necrosis, including release of the strong inflammatory mediator HMBG1. Additionally, N. gonorrhoeae activates the cysteine protease cathepsin B as measured by the breakdown of a cathepsin B substrate. Inhibition of cathepsin B shows that this protease is an apical controlling step in the downstream activities of NLRP3 including IL-1beta production, pyronecrosis, and HMGB1 release. Nonpathogenic Neisseria strains (Neisseria cinerea and Neisseria flavescens) do not activate NLRP3 as robustly as N. gonorrhoeae. Conditioned medium from N. gonorrhoeae contains factors capable of initiating the NLRP3-mediated signaling events. Isolated N. gonorrhoeae lipooligosaccharide, a known virulence factor from this bacterium that is elaborated from the bacterium in the form of outer membrane blebs, activates both NLRP3-induced IL-1beta secretion and pyronecrosis. Our findings indicate that activation of NLRP3-mediated inflammatory response pathways is an important venue associated with host response and pathogenesis of N. gonorrhoeae.

Cofactor infections and HIV epidemics in developing countries: implications for treatment

This article shows that the burden of certain tropical disease infections, after controlling for other factors, is positively correlated with HIV prevalence. Using cross-national data and multivariate linear regression analysis, we investigate the determinants of HIV prevalence in low- and middle-income countries. We begin with social and economic variables used in other cross-national studies and then incorporate data on parasitic and infectious diseases endemic in poor populations, which are found to be strongly and significantly correlated with--and are potent predictors of--HIV prevalence. The paper concludes by arguing that treating tropical diseases may be a cost-effective add-on to HIV-prevention and -treatment programs, thus slowing the spread of HIV in disease-burdened populations.

Regulatory T cells are locally induced during intravaginal infection of mice with Neisseria gonorrhoeae

Neisseria gonorrhoeae is a gram-negative diplococcus that in human beings produces gonorrhea. Much clinical evidence has led to the conclusion that gonococcus has important mechanisms to evade host immune functions; however, these mechanisms are only now beginning to be elucidated. In this study, we determined that the BALB/c mouse is a good animal model to study gonococcus infection and examined the immune response against the bacteria. We determined that after intravaginal inoculation of mice with Neisseria gonorrhoeae, the bacteria reached and invaded the upper female reproductive tissues and elicited a T-cell-specific immune response associated with a very weak humoral response, altogether resembling gonococcus infection and disease in women. Remarkably, in the draining lymph nodes of the genital tracts of infected mice, we found an increase of regulatory T lymphocytes, namely, transforming growth factor beta1-positive CD4(+) T cells and CD4(+) CD25(+) Foxp3(+) T cells. Altogether, results indicate that N. gonorrhoeae induces regulatory T cells, which might be related to the local survival of the pathogen and the establishment of a chronic asymptomatic infection.

Mucosal Neisseria gonorrhoeae coinfection during HIV acquisition is associated with enhanced systemic HIV-specific CD8 T-cell responses

BACKGROUND

The host immune response against mucosally acquired pathogens may be influenced by the mucosal immune milieu during acquisition. As Neisseria gonorrhoeae can impair dendritic cell and T-cell immune function, we hypothesized that coinfection during HIV acquisition would impair subsequent systemic T-cell responses.

METHODS

Monthly screening for sexually transmitted infections was performed in high risk, HIV seronegative Kenyan female sex workers as part of an HIV prevention trial. Early HIV-specific CD8 T-cell responses and subsequent HIV viral load set point were assayed in participants acquiring HIV, and were correlated with the presence of prior genital infections during HIV acquisition.

RESULTS

Thirty-five participants acquired HIV during follow-up, and 16 out of 35 (46%) had a classical sexually transmitted infection at the time of acquisition. N. gonorrhoeae coinfection was present during HIV acquisition in 6 out of 35 (17%), and was associated with an increased breadth and magnitude of systemic HIV-specific CD8 T-cell responses, using both interferon-gamma gamma and MIP-1 beta as an output. No other genital infections were associated with differences in HIV-specific CD8 T-cell response, and neither N. gonorrhoeae nor other genital infections were associated with differences in HIV plasma viral load at set point.

CONCLUSION

Unexpectedly, genital N. gonorrhoeae infection during heterosexual HIV acquisition was associated with substantially enhanced HIV-specific CD8 T-cell responses, although not with differences in HIV viral load set point. This may have implications for the development of mucosal HIV vaccines and adjuvants.

Local and humoral immune responses against primary and repeat Neisseria gonorrhoeae genital tract infections of 17beta-estradiol-treated mice

The 17beta-estradiol-treated mouse model is the only small animal model of gonococcal genital tract infection. Here we show gonococci localized within vaginal and cervical tissue, including the lamina propria, and high numbers of neutrophils and macrophages in genital tissue from infected mice. Infection did not induce a substantial or sustained increase in total or gonococcal-specific antibodies. Mice could be reinfected with the same strain and repeat infection did not boost the antibody response. However, intravaginal immunization of estradiol-treated mice induced gonococcal-specific primary and secondary serum antibody responses. We conclude that similar to human infection, experimental murine infection induces local inflammation but not an acquired immune response or immunological memory.

Differences in immunoregulatory cytokine expression patterns in the systemic and genital tract compartments of HIV-1-infected commercial sex workers in Benin

Initial exposure to human immunodeficiency virus type 1 (HIV-1) during heterosexual transmission occurs in the genital tract. Although much of the literature on the immune response to HIV-1 infection is based on studies performed at the systemic level, our understanding of tissue-specific immunity is lacking. Levels of both genital mucosal and blood interleukin (IL)-2, IL-4, IL-6, IL-10, tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma production were compared between 57 HIV-1-uninfected and 52 HIV-1-infected female commercial sex workers (CSWs) as well as 73 HIV-1-uninfected non-CSW control women at low risk for exposure. HIV-1-infected CSWs had significantly higher genital mucosal levels of TNF-alpha and IFN-gamma compared with those in both the HIV-uninfected CSW and non-CSW groups. In contrast, the serum levels of all the cytokines tested were lower in HIV-1-infected CSWs compared with those in the other groups. The increased production of genital mucosal pro-inflammatory cytokines in HIV-1-infected CSWs possibly reflects susceptibility to HIV-1 infection and disease progression/perpetuation at the initial site of exposure.

CEACAM1 dynamics during neisseria gonorrhoeae suppression of CD4+ T lymphocyte activation

Neisseria gonorrhoeae colony opacity-associated (Opa) proteins bind to human carcinoembryonic antigen cellular adhesion molecules (CEACAM) found on host cells including T lymphocytes. Opa binding to CEACAM1 suppresses the activation of CD4(+) T cells in response to a variety of stimuli. In this study, we use primary human CD4(+) T cells isolated from peripheral blood to define the molecular events occurring subsequent to Opa-CEACAM1 binding. We establish that, in contrast to other cell types, T cells do not engulf N. gonorrhoeae upon CEACAM1 binding. Instead, the bacteria recruit CEACAM1 from intracellular stores and maintain it on the T cell surface. Upon TCR ligation, the co-engaged CEACAM1 becomes phosphorylated on tyrosine residues within the ITIMs apparent in the cytoplasmic domain. This allows the recruitment and subsequent activation of the src homology domain 2-containing tyrosine phosphatases SHP-1 and SHP-2 at the site of bacterial attachment, which prevents the normal tyrosine phosphorylation of the CD3zeta-chain and ZAP-70 kinase in response to TCR engagement. Combined, this dynamic response allows the bacteria to effectively harness the coinhibitory function of CEACAM1 to suppress the adaptive immune response at its earliest step.

Dynamic expression of protective CEACAM1 on melanoma cells during specific immune attack

An efficient immune response against tumours depends on a well-orchestrated function of integrated components, but is finally exerted by effector tumour-infiltrating lymphocytes (TIL). We have previously reported that homophilic CEACAM1 interactions inhibit the specific killing and interferon-gamma (IFN-gamma) release activities of natural killer cells and TIL. In this study a model for the investigation of melanoma cells surviving long coincubation with antigen-specific TIL is reported. It is demonstrated that the surviving melanoma cells increase their surface CEACAM1 expression, which in turn confers enhanced resistance against fresh TIL. Furthermore, it is shown that this is an active process, driven by specific immune recognition and is at least partially mediated by lymphocyte-derived IFN-gamma. Similar results were observed with antigen-restricted TIL, either autologous or allogeneic, as well as with natural killer cells. The enhanced CEACAM1 expression depends, however, on the presence of IFN-gamma and sharply drops within 48 hr. This may be a mechanism that allows tumour cells to transiently develop a more resistant phenotype upon recognition by specific lymphocytes. Therefore, this work substantiates the melanoma-promoting role of CEACAM1 and marks it as an attractive target for novel immunotherapeutic interventions.

Carcinoembryonic antigen-related cell adhesion molecule 1 inhibits proximal TCR signaling by targeting ZAP-70

The long cytoplasmic tail (CT) isoforms of carcinoembryonic Ag-related cell adhesion molecule 1 (CEACAM1) are expressed on activated human T cells and possess two ITIM motifs in the CT. These isoforms of CEACAM1 are inhibitory for T cell responses initiated by the TCR/CD3 complex with the inhibition dependent upon the ITIMs of CEACAM1 and Src homology 2 domain-containing phosphatase 1 (SHP-1). However, the mechanism by which this inhibition occurs in T cells is unknown. We demonstrate here that the Src family kinase, Lck, and the ability of CEACAM1 to bind homophilically are required for the ITIM phosphorylation of CEACAM1 that is a prerequisite for CEACAM1 association with SHP-1. We further show that CEACAM1 associates with and recruits SHP-1 to the TCR/CD3 complex leading to decreased phosphorylation of CD3-zeta and ZAP-70 and consequently decreased activation of the elements downstream of ZAP-70. This is physiologically relevant because extinction of SHP-1 expression or blockade of homophilic binding by CEACAM1 using a Fab that specifically recognizes the homophilic binding region of human CEACAM1 increases the cytolytic function initiated by the TCR/CD3 complex. These studies show that long CT isoforms of CEACAM1 orchestrate an inhibitory program that abrogates extremely proximal events downstream of the TCR/CD3 complex by focusing on the activation of ZAP-70.

Identification and characterization of gonococcal iron transport systems as potential vaccine antigens

Gonorrhea is the second most commonly reported infectious disease in the USA, and incidence has been increasing in recent years. Antibiotic resistance among clinical isolates has reached a critical point at which the CDC currently recommends only a single class of antibiotic for treatment. These developments have hastened the search for a vaccine to protect against gonococcal infections. Vaccine efforts have been thwarted by the ability of the gonococcus to antigenically vary most surface structures. The transferrin-iron transport system is not subject to high-frequency phase or antigenic variation and is expressed by all pathogenic Neisseria. Vaccine formulations comprised of epitopes of the transferrin-binding proteins complexed with inactivated cholera toxin generated antibodies with potentially protective characteristics. These antigens, and others predicted from genome sequence data, could be developed into a vaccine that protects against neisserial infections.

Opa protein repertoires of disease-causing and carried meningococci

The meningococcal Opa proteins play an important role in pathogenesis by mediating invasion of human cells. The aim of this investigation was to determine whether carried and disease-associated meningococci possess different Opa repertoires and whether the diversity of these proteins is associated with clinical severity of disease. Opa repertoires in 227 disease-associated meningococci, isolated in the United Kingdom over a period of 6 years, were compared to the repertoires in 190 asymptomatically carried meningococci isolated in the United Kingdom from a contemporary, nonepidemic period. Multidimensional scaling (MDS) was employed to investigate the association between Opa repertoires and multilocus sequence typing (MLST) genotypes. Associations with clinical severity were also analyzed statistically. High levels of diversity were observed in opa alleles, variable regions, and repertoires, and MDS revealed that MLST genotypes were strongly associated with particular Opa repertoires. Individual Opa proteins or repertoires were not associated with clinical severity, though there was a trend toward an association with the opaD locus. Meningococcal Opa repertoire is strongly linked to MLST genotype irrespective of epidemiological sampling and therefore correlates with invasiveness. It is not, however, strongly associated with severity of meningococcal disease.

Direct interaction of tumor suppressor CEACAM1 with beta catenin: identification of key residues in the long cytoplasmic domain

CEACAM1-4L (carcinoembryonic antigen cell adhesion molecule 1, with 4 extracellular Ig-like domains and a long, 71 amino acid cytoplasmic domain) is expressed in epithelial cells and activated T-cells, but is down-regulated in most epithelial cell cancers and T-cell leukemias. A highly conserved sequence within the cytoplasmic domain has ca 50% sequence homology with Tcf-3 and -4, transcription factors that bind beta-catenin, and to a lesser extent (32% homology), with E-cadherin that also binds beta-catenin. We show by quantitative yeast two-hybrid, BIAcore, GST-pull down, and confocal analyses that this domain directly interacts with beta-catenin, and that H-469 and K-470 are key residues that interact with the armadillo repeats of beta-catenin. Jurkat cells transfected with CEACAM1-4L have 2-fold less activity in the TOPFLASH reporter assay, and in MCF7 breast cancer cells that fail to express CEACAM1, transfection with CEACAM1 and growth in Ca2+ media causes redistribution of beta-catenin from the cytoplasm to the cell membrane, demonstrating a functional role for the long cytoplasmic domain of CEACAM1 in regulation of beta-catenin activity.

hCEACAM1-4L downregulates hDAF-associated signalling after being recognized by the Dr adhesin of diffusely adhering Escherichia coli

Human decay accelerating factor (hDAF, CD55) and members of the carcinoembryonic-antigen-related cell-adhesion molecules (hCEACAMs) family are recognized as receptors by Gram-negative, diffusely adhering Escherichia coli (DAEC) strains expressing Afa/Dr adhesins. We report here that hCEACAM1-4L has a key function in downregulating the protein tyrosine Src kinase associated with hDAF signalling. After infecting HeLa epithelial cells stably transfected with hCEACAM1-4L cDNA with Dr adhesin-positive E. coli, the amount of the pTyr(416)-active form of the Src protein decreased, whereas that of the pTyr(527)-inactive form of Src protein did not increase. This downregulation of the Src protein implies that part of the hCEACAM1-4L protein had been translocated into lipid rafts, the protein was phosphorylated at Tyr residues in the cytoplasmic domain, and it was physically associated with the protein tyrosine phosphatase, SHP-2. Finally, we found that the hCEACAM1-4L-associated SHP-2 was not phosphorylated and lacked phosphatase activity, suggesting that the downregulation of Src protein associated with hDAF signalling results from the absence of dephosphorylation of the pTyr(527)-inactive form necessary for Src kinase activation.

Haplotypic diversity in human CEACAM genes: effects on susceptibility to meningococcal disease

Adhesion between the opacity-associated adhesin (Opa) proteins of Neisseria meningitidis and human carcino-embryonic antigen cell adhesion molecule (CEACAM) proteins is an important stage in the pathogenesis of meningococcal disease, a globally important bacterial infection. Most disease is caused by a small number of meningococcal genotypes known as hyperinvasive lineages. As these are also carried asymptomatically, acquisition of them alone cannot explain why only some hosts develop meningococcal disease. Our aim was to determine whether genetic diversity in CEACAM is associated with susceptibility to meningococcal disease. Frequency distributions of alleles, genotypes and haplotypes were compared in four CEACAM genes in 384 case samples and 190 controls. Linkage disequilibrium among polymorphic sites, haplotype structures and relationships were also analysed. A number of polymorphisms were observed in CEACAM genes but the diversity of CEACAM1, to which most Opa proteins bind, was lower, and a small number of high-frequency haplotypes were detected. Dose-dependent associations of three CEACAM haplotypes with meningococcal disease were observed, with the effect of carrying these haplotypes amplified in homozygous individuals. Two haplotypes were protective while one haplotype in CEACAM6 was associated with a twofold increase in disease susceptibility. These data imply that human CEACAM may be one determinant of human susceptibility to meningococcal disease.

Species-specific evolution of immune receptor tyrosine based activation motif-containing CEACAM1-related immune receptors in the dog

BACKGROUND

Although the impact of pathogens on the evolution of the mammalian immune system is still under debate, proteins, which both regulate immune responses and serve as cellular receptors for pathogens should be at the forefront of pathogen-driven host evolution. The CEA (carcinoembryonic antigen) gene family codes for such proteins and indeed shows tremendous species-specific variation between human and rodents. Since little is known about the CEA gene family in other lineages of placental mammals, we expected to gain new insights into the evolution of the rapidly diverging CEA family by analyzing the CEA family of the dog.

RESULTS

Here we describe the complete CEA gene family in the dog. We found that the gene coding for the ITIM-bearing immunoregulatory molecule CEACAM1 gave rise to a recent expansion of the canine CEA gene family by gene duplication, similar to that previously found in humans and mice. However, while the murine and human CEACAMs (carcinoembryonic antigen-related cell adhesion molecules) are predominantly secreted and GPI-anchored, respectively, in the dog, most of the CEACAMs represent ITAM-bearing transmembrane proteins. One of these proteins, CEACAM28, exhibits nearly complete sequence identity with the ligand-binding N domain of CEACAM1, but antagonizing signaling motifs in the cytoplasmic tail. Comparison of nonsynonymous and synonymous substitutions indicates that the CEACAM28 N domain is under the strongest purifying selection of all canine CEACAM1-related CEACAMs. In addition, CEACAM28 shows a similar expression pattern in resting immune cells and tissues as CEACAM1. However, upon activation CEACAM28 mRNA and CEACAM1 mRNA are differentially regulated.

CONCLUSION

Thus, CEACAM1 and CEACAM28 are the first paired immune receptors identified within the CEA gene family, which are expressed on T cells and are most likely involved in the fine-tuning of T cell responses. The direction of gene conversion accompanied by purifying selection and expression in immune cells suggests the possibility that CEACAM28 evolved in response to selective pressure imposed by species-specific pathogens.

A carcinoembryonic antigen-related cell adhesion molecule 1 homologue plays a pivotal role in nontypeable Haemophilus influenzae colonization of the chinchilla nasopharynx via the outer membrane protein P5-homologous adhesin

In vitro studies suggest an important role for CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1) in infection by multiple gram-negative bacteria. However, in vivo evidence supporting this role is lacking, largely because the bacterial adhesins involved in this host-microbe association do not bind to murine-derived CEACAM1. One of several adhesins expressed by nontypeable Haemophilus influenzae (NTHI), the outer membrane protein P5-homologous adhesin (or P5), is essential for colonization of the chinchilla nasopharynx and infection of the middle ear. Here we reveal that NTHI P5 binds to the chinchilla homologue of CEACAM1 and that rabbit anti-human carcinoembryonic antigen blocks NTHI colonization of the chinchilla nasopharynx, providing the first demonstration of a role for CEACAM receptor binding by any bacterial pathogen in vivo.

FGL chaperone-assembled fimbrial polyadhesins: anti-immune armament of Gram-negative bacterial pathogens

This review summarizes the current knowledge on the structure, function, assembly, and biomedical applications of the family of adhesive fimbrial organelles assembled on the surface of Gram-negative pathogens via the FGL chaperone/usher pathway. Recent studies revealed the unique structural and functional properties of these organelles, distinguishing them from a related family, FGS chaperone-assembled adhesive pili. The FGL chaperone-assembled organelles consist of linear polymers of one or two types of protein subunits, each possessing one or two independent adhesive sites specific to different host cell receptors. This structural organization enables these fimbrial organelles to function as polyadhesins. Fimbrial polyadhesins may ensure polyvalent fastening of bacteria to the host cells, aggregating their receptors and triggering subversive signals that allow pathogens to evade immune defense. The FGL chaperone-assembled fimbrial polyadhesins are attractive targets for vaccine and drug design.

Neisserial outer membrane vesicles bind the coinhibitory receptor carcinoembryonic antigen-related cellular adhesion molecule 1 and suppress CD4+ T lymphocyte function

Pathogenic Neisseria bacteria naturally liberate outer membrane "blebs," which are presumed to contribute to pathology, and the detergent-extracted outer membrane vesicles (OMVs) from Neisseria meningitidis are currently employed as meningococcal vaccines in humans. While the composition of these vesicles reflects the bacteria from which they are derived, the functions of many of their constituent proteins remain unexplored. The neisserial colony opacity-associated Opa proteins function as adhesins, the majority of which mediate bacterial attachment to human carcinoembryonic antigen-related cellular adhesion molecules (CEACAMs). Herein, we demonstrate that the Opa proteins within OMV preparations retain the capacity to bind the immunoreceptor tyrosine-based inhibitory motif-containing coinhibitory receptor CEACAM1. When CD4(+) T lymphocytes were exposed to OMVs from Opa-expressing bacteria, their activation and proliferation in response to a variety of stimuli were effectively halted. This potent immunosuppressive effect suggests that localized infection will generate a "zone of inhibition" resulting from the diffusion of membrane blebs into the surrounding tissues. Moreover, it demonstrates that OMV-based vaccines must be developed from strains that lack CEACAM1-binding Opa variants.

Conserved regions from Neisseria gonorrhoeae pilin are immunosilent and not immunosuppressive

PilE is the primary subunit of type IV pili from Neisseria gonorrhoeae and contains a surface-exposed hypervariable region thought to be one feature of pili that has prevented development of a pilin-based vaccine. We have created a three-dimensional structure-based antigen by replacing the hypervariable region of PilE with an aspartate-glutamine linker chosen from the sequence of Pseudomonas aeruginosa PilA. We then characterized murine immune responses to this novel protein to determine if conserved PilE regions could serve as a vaccine candidate. The control PilE protein elicited strong T-cell-dependent B-cell responses that are specific to epitopes in both the hypervariable deletion and control proteins. In contrast, the hypervariable deletion protein was unable to elicit an immune response in mice, suggesting that in the absence of the hypervariable region, the conserved regions of PilE alone are not sufficient for antibody production. Further analysis of these PilE proteins with suppressor cell assays showed that neither suppresses T- or B-cell responses, and flow cytometry experiments suggested that they do not exert suppressor effects by activating T regulatory cells. Our results show that in the murine model, the hypervariable region of PilE is required to activate immune responses to pilin, whereas the conserved regions are unusually nonimmunogenic. In addition, we show that both hypervariable and conserved regions of pilin are not suppressive, suggesting that PilE does not cause the decrease in T-cell populations observed during gonococcal cervicitis.

The granulocyte receptor carcinoembryonic antigen-related cell adhesion molecule 3 (CEACAM3) directly associates with Vav to promote phagocytosis of human pathogens

The human granulocyte-specific receptor carcinoembryonic antigen-related cell adhesion molecule (CEACAM)3 is critically involved in the opsonin-independent recognition of several bacterial pathogens. CEACAM3-mediated phagocytosis depends on the integrity of an ITAM-like sequence within the cytoplasmic domain of CEACAM3 and is characterized by rapid stimulation of the GTPase Rac. By performing a functional screen with CEACAM3-expressing cells, we found that overexpression of a dominant-negative form of the guanine nucleotide exchange factor Vav, but not the dominant-negative versions SWAP70, Dock2, or ELMO1 interfered with CEACAM3-initiated phagocytosis. Moreover, small interfering RNA-mediated silencing of Vav reduced uptake and abrogated the stimulation of Rac in response to bacterial CEACAM3 engagement. In Vav1/Vav2-deficient cells, CEACAM3-mediated internalization was only observed after re-expression of Vav. Vav colocalized with CEACAM3 upon bacterial infection, coimmunoprecipitated in a complex with CEACAM3, and the Vav Src homology 2 domain directly associated with phosphorylated Tyr(230) of CEACAM3. In primary human granulocytes, TAT-mediated transduction of dominant-negative Vav, but not SWAP70, severely impaired the uptake of CEACAM3-binding bacteria. These data support the view that, different from canonical ITAM signaling, the CEACAM3 ITAM-like sequence short-wires bacterial recognition and Rac stimulation via a direct association with Vav to promote rapid phagocytosis and elimination of CEACAM-binding human pathogens.

The specific innate immune receptor CEACAM3 triggers neutrophil bactericidal activities via a Syk kinase-dependent pathway

The human-restricted pathogens Neisseria gonorrhoeae, Neisseria meningitidis, Haemophilus influenzae and Moraxella catarrhalis colonize host tissues via carcinoembryonic antigen-related cellular adhesion molecules (CEACAMs). One such receptor, CEACAM3, acts in a host-protective manner by orchestrating the capture and engulfment of invasive bacteria by human neutrophils. Herein, we show that bacterial binding to CEACAM3 causes recruitment of the cytoplasmic tyrosine kinase Syk, resulting in the phosphorylation of both CEACAM3 and Syk. This interaction is specific for the immunoreceptor tyrosine-based activation motif (ITAM) in the CEACAM3 cytoplasmic domain. While dispensable for the phagocytic uptake of single bacteria by CEACAM3, Syk is necessary for internalization when cargo size increases or when the density of CEACAM-binding ligand on the cargo surface is below a critical threshold. Moreover, Syk engagement is required for an effective bacterial killing response, including the neutrophil oxidative burst and degranulation functions in response to N. gonorrhoeae. These data reveal CEACAM3 as a specific innate immune receptor that mediates the opsonin-independent clearance of CEACAM-binding bacteria via Syk, a molecular trigger for functional immunoreceptor responses of both the adaptive (TCR, BCR, FcR) and innate (Dectin-1, CEACAM3) immune systems.