Mutational analysis of human CEACAM1: the potential of receptor polymorphism in increasing host susceptibility to bacterial infection

Exploring the evolutionary and pathogenic role of Acinetobacter baumannii biofilm-associated protein (Bap) through in silico structural modeling

Acinetobacter species encode for extracellularly secreted Biofilm-associated protein (Bap), a multi-domain protein with variable molecular weights reaching several hundred kilodaltons. Bap is crucial for the development of multi-dimensional structures of mature biofilms. In our investigation, we analyzed 7338 sequences of A. baumannii from the NCBI database and found that Bap or Bap-like protein (BLP) was present in 6422 (87.52%) isolates. Further classification revealed that 12.12% carried Type-1 Bap, 68.44% had Type-2, 6.91% had Type-3, 0.05% had Type-6 or SDF-Type, and 12.51% lacked Bap or BLP. The majority of isolates with Type-1, Type-2, and Type-3 Bap belonged to ST1, ST2, and ST25, respectively. Phylogenetic analysis suggested that Type-1 Bap is the most ancient, while Type-3 and SDF-Type have evolved recently. Studying the interaction of predicted Bap structures with human CEACAM-1 and PIgR showed that Bap with its BIg13 and BIg6 domains interact with the N-terminal domain of CEACAM-1, involving Arg43 and Glu40, involved in CEACAM-1 dimerization. Also, we found that recently evolved Type-3 and SDF-Type Bap showed greater interaction with CEACAM-1 and PIgR. It can be asserted that the evolution of Bap has conferred enhanced virulence characteristics to A. baumannii with increased interaction with CEACAM-1 and PIgR. Using in silico approaches, this study explores the evolutionary, physicochemical, and structural features of A. baumannii Bap and unravels its crucial role in mediating interaction with human CEACAM-1 and PIgR through detailed structure modelling. These findings advance our understanding of A. baumannii Bap and highlight its role in pathogenesis.

Human CEACAM1 is targeted by a Streptococcus pyogenes adhesin implicated in puerperal sepsis pathogenesis

Life-threatening bacterial infections in women after childbirth, known as puerperal sepsis, resulted in classical epidemics and remain a global health problem. While outbreaks of puerperal sepsis have been ascribed to Streptococcus pyogenes, little is known about disease mechanisms. Here, we show that the bacterial R28 protein, which is epidemiologically associated with outbreaks of puerperal sepsis, specifically targets the human receptor CEACAM1. This interaction triggers events that would favor the development of puerperal sepsis, including adhesion to cervical cells, suppression of epithelial wound repair and subversion of innate immune responses. High-resolution structural analysis showed that an R28 domain with IgI3-like fold binds to the N-terminal domain of CEACAM1. Together, these findings demonstrate that a single adhesin-receptor interaction can drive the pathogenesis of bacterial sepsis and provide molecular insights into the pathogenesis of one of the most important infectious diseases in medical history.

CEACAM Gene Family Mutations Associated With Inherited Breast Cancer Risk - A Comparative Oncology Approach to Discovery

Introduction

Recent studies comparing canine mammary tumors (CMTs) and human breast cancers have revealed remarkable tumor similarities, identifying shared expression profiles and acquired mutations. CMTs can also provide a model of inherited breast cancer susceptibility in humans; thus, we investigated breed-specific whole genome sequencing (WGS) data in search for novel CMT risk factors that could subsequently explain inherited breast cancer risk in humans.

Methods

WGS was carried out on five CMT-affected Gold Retrievers from a large pedigree of 18 CMT-affected dogs. Protein truncating variants (PTVs) detected in all five samples (within human orthlogs) were validated and then genotyped in the 13 remaining CMT-affected Golden Retrievers. Allele frequencies were compared to canine controls. Subsequently, human blood-derived exomes from The Cancer Genome Atlas breast cancer cases were analyzed and allele frequencies were compared to Exome Variant Server ethnic-matched controls.

Results

Carcinoembryonic Antigen-related Cell Adhesion Molecule 24 (CEACAM24) c.247dupG;p.(Val83Glyfs^∗48) was the only validated variant and had a frequency of 66.7% amongst the 18 Golden Retrievers with CMT. This was significant compared to the European Variation Archive (p-value 1.52 × 10^–8) and non-Golden Retriever American Kennel Club breeds (p-value 2.48 × 10^–5). With no direct ortholog of CEACAM24 in humans but high homology to all CEACAM gene family proteins, all human CEACAM genes were investigated for PTVs. A total of six and sixteen rare PTVs were identified in African and European American breast cancer cases, respectively. Single variant assessment revealed five PTVs associated with breast cancer risk. Gene-based aggregation analyses revealed that rare PTVs in CEACAM6, CEACAM7, and CEACAM8 are associated with European American breast cancer risk, and rare PTVs in CEACAM7 are associated with breast cancer risk in African Americans. Ultimately, rare PTVs in the entire CEACAM gene family are associated with breast cancer risk in both European and African Americans with respective p-values of 1.75 × 10^–13 and 1.87 × 10^–04.

Conclusion

This study reports the first association of inherited CEACAM mutations and breast cancer risk, and potentially implicates the whole gene family in genetic risk. Precisely how these mutations contribute to breast cancer needs to be determined; especially considering our current knowledge on the role that the CEACAM gene family plays in tumor development, progression, and metastasis.

Vibrio cholerae toxin coregulated pilus provokes inflammatory responses in Coculture model of Caco-2 and peripheral blood mononuclear cells (PBMC) leading to increased colonization

The aim of this study was to assess the modulatory effect of TcpA in the expression of CEACAM1 adhesin molecule and IL-1, IL-8, and TNF-α pro-inflammatory cytokines in the Coculture model of Caco-2/PBMC (peripheral blood mononuclear cell) that can mimic the intestinal milieu. The TcpA gene from Vibrio cholerae ATCC14035 was cloned in pET-28a and transformed into Escherichia coli Bl-21. The recombinant TcpA-His6 protein was expressed and purified using Ni-column chromatography. The sequencing of transformed plasmid and Western blot analysis of purified protein confirmed the identity of rTcp. The cytotoxicity of different concentrations of recombinant protein for human colon carcinoma cell line (human colorectal adenocarcinoma cell [Caco-2 cell]) was assessed by MTT assay and showed viability of 92%, 82%, and 70%, for 10 µg/mL of TcpA after 24, 48, and 72 h, respectively. Co-cultures of Caco-2 and PBMCs were used to mimic the intestinal milieu and treated with different concentrations of rTcpA (1, 5, 10, and 50 µg/mL). Our data showed about 2.04-, 3.37-, 3.68-, and 42.7-fold increase in CEACAM1 gene expression, respectively, compared with the nontreated Caco-2/PBMC Coculture. Moreover, the expression of IL-1, IL-8, and TNF-α genes was significantly increased up to 15.75-, 7.04-, and 80.95-folds, respectively. In conclusion, V. cholerae TcpA induces statistically significant dose-dependent stimulatory effect on TNF-α, IL-,1, and IL-8 pro-inflammatory cytokines expression. Of these, TNF-α was much more affected which, consequently, elevated the CEACAM1 expression level in IECs. This suggests that TcpA protein is a critical effector as an inducer of increased adhesion potential of V. cholera as well as inflammatory responses of host intestinal tissue.

Bacterial protein domains with a novel Ig-like fold target human CEACAM receptors

Streptococcus agalactiae, also known as group B Streptococcus (GBS), is the major cause of neonatal sepsis in humans. A critical step to infection is adhesion of bacteria to epithelial surfaces. GBS adhesins have been identified to bind extracellular matrix components and cellular receptors. However, several putative adhesins have no host binding partner characterised. We report here that surface-expressed β protein of GBS binds to human CEACAM1 and CEACAM5 receptors. A crystal structure of the complex showed that an IgSF domain in β represents a novel Ig-fold subtype called IgI3, in which unique features allow binding to CEACAM1. Bioinformatic assessment revealed that this newly identified IgI3 fold is not exclusively present in GBS but is predicted to be present in adhesins from other clinically important human pathogens. In agreement with this prediction, we found that CEACAM1 binds to an IgI3 domain found in an adhesin from a different streptococcal species. Overall, our results indicate that the IgI3 fold could provide a broadly applied mechanism for bacteria to target CEACAMs.

Neisseria gonorrhoeae infects the heterogeneous epithelia of the human cervix using distinct mechanisms

Sexually transmitted infections are a critical public health issue. However, the mechanisms underlying sexually transmitted infections in women and the link between the infection mechanism and the wide range of clinical outcomes remain elusive due to a lack of research models mimicking human infection in vivo. We established a human cervical tissue explant model to mimic local Neisseria gonorrhoeae (GC) infections. We found that GC preferentially colonize the ectocervix by activating integrin-β1, which inhibits epithelial shedding. GC selectively penetrate into the squamocolumnar junction (TZ) and endocervical epithelia by inducing β-catenin phosphorylation, which leads to E-cadherin junction disassembly. Epithelial cells in various cervical regions differentially express carcinoembryonic antigen-related cell adhesion molecules (CEACAMs), the host receptor for GC opacity-associated proteins (Opa_CEA). Relatively high levels were detected on the luminal membrane of ecto/endocervical epithelial cells but very low levels intracellularly in TZ epithelial cells. CEACAM-Opa_CEA interaction increased ecto/endocervical colonization and reduced endocervical penetration by increasing integrin-β1 activation and inhibiting β-catenin phosphorylation respectively, through CEACAM downstream signaling. Thus, the intrinsic properties of cervical epithelial cells and phase-variation of bacterial surface molecules both play a role in controlling GC infection mechanisms and infectivity, preferential colonization or penetration, potentially leading to asymptomatic or symptomatic infection.

Gonorrhea, caused by Neisseria gonorrhoeae (GC), is a common sexually transmitted infection and has become a public health crisis due to a steady increase in antibiotic-resistant cases. How GC infect the female reproductive tract (FRT) and cause various clinical outcomes is still unknown. This study used human cervical explants to examine the mechanism by which GC infect the heterogeneous mucosa of the human cervix, the gate of the FRT. We show that GC preferentially colonize stratified epithelial cells in the vaginal-cervical region by enhancing epithelial cell adherence. GC selectively penetrate into tissues of the squamocolumnar junction and cervical-endometrial regions but not the vaginal-cervical region by loosening epithelial cell-cell junctions. The expression of opacity-associated proteins on GC regulates the infection patterns depending on the expression of the host receptors on various types of epithelia and receptor downstream signaling. These data suggest that both intrinsic properties of cervical epithelial cells and phase-variation of bacterial surface molecules play a role in determining GC infectivity. Our findings can explain how GC infection leads to asymptomatic or symptomatic infection.

Fusobacterium spp. target human CEACAM1 via the trimeric autotransporter adhesin CbpF

Neisseria meningitidis, Haemophilus influenzae, and Moraxella catarrhalis are pathogenic bacteria adapted to reside on human respiratory mucosal epithelia. One common feature of these species is their ability to target members of the carcinoembryonic antigen-related cell adhesion molecule (CEACAM) family, especially CEACAM1, which is achieved via structurally distinct ligands expressed by each species. Beside respiratory epithelial cells, cells at the dentogingival junction express high levels of CEACAM1. It is possible that bacterial species resident within the oral cavity also utilise CEACAM1 for colonisation and invasion of gingival tissues. From a screen of 59 isolates from the human oral cavity representing 49 bacterial species, we identified strains from Fusobacterium bound to CEACAM1. Of the Fusobacterium species tested, the CEACAM1-binding property was exhibited by F. nucleatum (Fn) and F. vincentii (Fv) but not F. polymorphum (Fp) or F. animalis (Fa) strains tested. These studies identified that CEACAM adhesion was mediated using a trimeric autotransporter adhesin (TAA) for which no function has thus far been defined. We therefore propose the name CEACAM binding protein of Fusobacterium (CbpF). CbpF was identified to be present in the majority of unspeciated Fusobacterium isolates confirming a subset of Fusobacterium spp. are able to target human CEACAM1.

Antagonistic Pleiotropy in the Bifunctional Surface Protein FadL (OmpP1) during Adaptation of Haemophilus influenzae to Chronic Lung Infection Associated with Chronic Obstructive Pulmonary Disease

Tracking bacterial evolution during chronic infection provides insights into how host selection pressures shape bacterial genomes. The human-restricted opportunistic pathogen nontypeable Haemophilus influenzae (NTHi) infects the lower airways of patients suffering chronic obstructive pulmonary disease (COPD) and contributes to disease progression. To identify bacterial genetic variation associated with bacterial adaptation to the COPD lung, we sequenced the genomes of 92 isolates collected from the sputum of 13 COPD patients over 1 to 9 years. Individuals were colonized by distinct clonal types (CTs) over time, but the same CT was often reisolated at a later time or found in different patients. Although genomes from the same CT were nearly identical, intra-CT variation due to mutation and recombination occurred. Recurrent mutations in several genes were likely involved in COPD lung adaptation. Notably, nearly a third of CTs were polymorphic for null alleles of ompP1 (also called fadL), which encodes a bifunctional membrane protein that both binds the human carcinoembryonic antigen-related cell adhesion molecule 1 (hCEACAM1) receptor and imports long-chain fatty acids (LCFAs). Our computational studies provide plausible three-dimensional models for FadL's interaction with hCEACAM1 and LCFA binding. We show that recurrent fadL mutations are likely a case of antagonistic pleiotropy, since loss of FadL reduces NTHi's ability to infect epithelia but also increases its resistance to bactericidal LCFAs enriched within the COPD lung. Supporting this interpretation, truncated fadL alleles are common in publicly available NTHi genomes isolated from the lower airway tract but rare in others. These results shed light on molecular mechanisms of bacterial pathoadaptation and guide future research toward developing novel COPD therapeutics.IMPORTANCE Nontypeable Haemophilus influenzae is an important pathogen in patients with chronic obstructive pulmonary disease (COPD). To elucidate the bacterial pathways undergoing in vivo evolutionary adaptation, we compared bacterial genomes collected over time from 13 COPD patients and identified recurrent genetic changes arising in independent bacterial lineages colonizing different patients. Besides finding changes in phase-variable genes, we found recurrent loss-of-function mutations in the ompP1 (fadL) gene. We show that loss of OmpP1/FadL function reduces this bacterium's ability to infect cells via the hCEACAM1 epithelial receptor but also increases its resistance to bactericidal fatty acids enriched within the COPD lung, suggesting a case of antagonistic pleiotropy that restricts ΔfadL strains' niche. These results show how H. influenzae adapts to host-generated inflammatory mediators in the COPD airways.

Helicobacter pylori adhesin HopQ disrupts trans dimerization in human CEACAMs

The human gastric pathogen Helicobacter pylori is a major causative agent of gastritis, peptic ulcer disease, and gastric cancer. As part of its adhesive lifestyle, the bacterium targets members of the carcinoembryonic antigen-related cell adhesion molecule (CEACAM) family by the conserved outer membrane adhesin HopQ. The HopQ-CEACAM1 interaction is associated with inflammatory responses and enables the intracellular delivery and phosphorylation of the CagA oncoprotein via a yet unknown mechanism. Here, we generated crystal structures of HopQ isotypes I and II bound to the N-terminal domain of human CEACAM1 (C1ND) and elucidated the structural basis of H. pylori specificity toward human CEACAM receptors. Both HopQ alleles target the β-strands G, F, and C of C1ND, which form the trans dimerization interface in homo- and heterophilic CEACAM interactions. Using SAXS, we show that the HopQ ectodomain is sufficient to induce C1ND monomerization and thus providing H. pylori a route to influence CEACAM-mediated cell adherence and signaling events.

A comprehensive phylogenetic and structural analysis of the carcinoembryonic antigen (CEA) gene family

The carcinoembryonic antigen (CEA) gene family belongs to the immunoglobulin (Ig) superfamily and codes for a vast number of glycoproteins that differ greatly both in amino acid composition and function. The CEA family is divided into two groups, the carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) and the pregnancy-specific glycoproteins. The CEA family members are implicated in pleiotropic (patho)physiological functions including cell-cell adhesion, pregnancy, immunity, neovascularization, regulation of insulin homeostasis, and carcinogenesis. In general, the CEA-encoded proteins are composed of an extracellular region with Ig variable and constant-like domains and a cytoplasmic region containing signaling motifs. Of particular interest, the well-studied human and mouse CEA genes are arranged in clusters in a single chromosome. Taking into account this characteristic, we made an effort to reconstruct the evolutionary history of the CEA gene family. Toward this end, the publicly available genomes were searched extensively for CEA homologs. The domain organization of the retrieved protein sequences was analyzed, and, subsequently, comprehensive phylogenetic analyses of the entire length CEA homologous proteins were performed. A series of evolutionarily conserved amino acid residues, functionally important, were identified. The relative positioning of these residues on the modeled tertiary structure of novel CEA protein domains revealed that they are, also, spatially conserved. Furthermore, the chromosomal arrangement of CEA genes was examined, and it was found that the CEA genes are preserved in terms of position, transcriptional orientation, and number in all species under investigation.

Absence of platelet endothelial cell adhesion molecule 1, PECAM-1/CD31, in vivo increases resistance to Salmonella enterica serovar Typhimurium in mice

PECAM-1/CD31 is known to regulate inflammatory responses and exhibit pro- and anti-inflammatory functions. This study was designed to determine the functional role of PECAM-1 in susceptibility to murine primary in vivo infection with Salmonella enterica serovar Typhimurium and in in vitro inflammatory responses of peritoneal macrophages. Lectin profiling showed that cellular PECAM-1 and recombinant human PECAM-1-Ig chimera contain high levels of mannose sugars and N-acetylglucosamine. Consistent with this carbohydrate pattern, both recombinant human and murine PECAM-1-Ig chimeras were shown to bind S. Typhimurium in a dose-dependent manner in vitro. Using oral and fecal-oral transmission models of S. Typhimurium SL1344 infection, PECAM-1(-/-) mice were found to be more resistant to S. Typhimurium infection than wild-type (WT) C57BL/6 mice. While fecal shedding of S. Typhimurium was comparable in wild-type and PECAM-1(-/-) mice, the PECAM-1-deficient mice had lower bacterial loads in systemic organs such as liver, spleen, and mesenteric lymph nodes than WT mice, suggesting that extraintestinal dissemination was reduced in the absence of PECAM-1. This reduced bacterial load correlated with reduced tumor necrosis factor (TNF), interleukin-6 (IL-6), and monocyte chemoattractant protein (MCP) levels in sera of PECAM-1(-/-) mice. Following in vitro stimulation of macrophages with either whole S. Typhimurium, lipopolysaccharide (LPS) (Toll-like receptor 4 [TLR4] ligand), or poly(I·C) (TLR3 ligand), production of TNF and IL-6 by PECAM-1(-/-) macrophages was reduced. Together, these results suggest that PECAM-1 may have multiple functions in resistance to infection with S. Typhimurium, including binding to host cells, extraintestinal spread to deeper tissues, and regulation of inflammatory cytokine production by infected macrophages.

Ins and outs of microbial adhesion

Microbial adhesion is generally a complex process, involving multiple adhesins on a single microbe and their respective target receptors on host cells. In some situations, various adhesins of a microbe may co-operate in an apparently hierarchical and sequential manner whereby the first adhesive event triggers the target cell to express receptors for additional microbial adhesins. In other instances, adhesins may act in concert leading to high avidity interactions, often a prelude to cellular invasion and tissue penetration. Mechanisms used to target the host include both lectin-like interactions and protein-protein interactions; the latter are often highly specific for the host or a tissue within the host. This reflective chapter aims to offer a point of view on microbial adhesion by presenting some experiences and thoughts especially related to respiratory pathogens and explore if there can be any future hope of controlling bacterial infections via preventing adhesion or invasion stages of microbial pathogenesis.

Neisseria gonorrhoeae selectively suppresses the development of Th1 and Th2 cells, and enhances Th17 cell responses, through TGF-β-dependent mechanisms

Infection with Neisseria gonorrhoeae does not induce specific immunity or immune memory. Our previous studies in a murine model of vaginal gonococcal infection showed that innate immunity governed by Th17 cells was a critical aspect of the immune response elicited by this pathogen. Herein we show that N. gonorrhoeae selectively inhibited Th1 and Th2 cells and enhanced Th17 cell development through the induction of TGF-β. Whereas Th17 responses depended on gonococcal lipooligosaccharide acting through TLR4, the inhibitory effect of N. gonorrhoeae on Th1/Th2 responses involved gonococcal Opa proteins. In vitro Th17 responses to N. gonorrhoeae could be diverted to Th1/Th2 by blockade of TGF-β, but not by blockade of IL-17. The results reveal that N. gonorrhoeae suppresses Th1/Th2-mediated adaptive immune response through mechanisms dependent on TGF-β, and that this effect can be manipulated to promote the development of adaptive immunity.

CEACAM1 recognition by bacterial pathogens is species-specific

BACKGROUND

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), an immunoglobulin (Ig)-related glycoprotein, serves as cellular receptor for a variety of Gram-negative bacterial pathogens associated with the human mucosa. In particular, Neisseria gonorrhoeae, N. meningitidis, Moraxella catarrhalis, and Haemophilus influenzae possess well-characterized CEACAM1-binding adhesins. CEACAM1 is typically involved in cell-cell attachment, epithelial differentiation, neovascularisation and regulation of T-cell proliferation, and is one of the few CEACAM family members with homologues in different mammalian lineages. However, it is unknown whether bacterial adhesins of human pathogens can recognize CEACAM1 orthologues from other mammals.

RESULTS

Sequence comparisons of the amino-terminal Ig-variable-like domain of CEACAM1 reveal that the highest sequence divergence between human, murine, canine and bovine orthologues is found in the beta-strands comprising the bacteria-binding CC'FG-face of the Ig-fold. Using GFP-tagged, soluble amino-terminal domains of CEACAM1, we demonstrate that bacterial pathogens selectively associate with human, but not other mammalian CEACAM1 orthologues. Whereas full-length human CEACAM1 can mediate internalization of Neisseria gonorrhoeae in transfected cells, murine CEACAM1 fails to support bacterial internalization, demonstrating that the sequence divergence of CEACAM1 orthologues has functional consequences with regard to bacterial recognition and cellular invasion.

CONCLUSIONS

Our results establish the selective interaction of several human-restricted bacterial pathogens with human CEACAM1 and suggest that co-evolution of microbial adhesins with their corresponding receptors on mammalian cells contributes to the limited host range of these highly adapted infectious agents.

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.

Cellular and molecular biology of Neisseria meningitidis colonization and invasive disease

The human species is the only natural host of Neisseria meningitidis, an important cause of bacterial meningitis globally, and, despite its association with devastating diseases, N. meningitidis is a commensal organism found frequently in the respiratory tract of healthy individuals. To date, antibiotic resistance is relatively uncommon in N. meningitidis isolates but, due to the rapid onset of disease in susceptible hosts, the mortality rate remains approx. 10%. Additionally, patients who survive meningococcal disease often endure numerous debilitating sequelae. N. meningitidis strains are classified primarily into serogroups based on the type of polysaccharide capsule expressed. In total, 13 serogroups have been described; however, the majority of disease is caused by strains belonging to one of only five serogroups. Although vaccines have been developed against some of these, a universal meningococcal vaccine remains a challenge due to successful immune evasion strategies of the organism, including mimicry of host structures as well as frequent antigenic variation. N. meningitidis express a range of virulence factors including capsular polysaccharide, lipopolysaccharide and a number of surface-expressed adhesive proteins. Variation of these surface structures is necessary for meningococci to evade killing by host defence mechanisms. Nonetheless, adhesion to host cells and tissues needs to be maintained to enable colonization and ensure bacterial survival in the niche. The aims of the present review are to provide a brief outline of meningococcal carriage, disease and burden to society. With this background, we discuss several bacterial strategies that may enable its survival in the human respiratory tract during colonization and in the blood during infection. We also examine several known meningococcal adhesion mechanisms and conclude with a section on the potential processes that may operate in vivo as meningococci progress from the respiratory niche through the blood to reach the central nervous system.

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.

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.

Genetic influence on bloodstream infections and sepsis

Bloodstream infections (BSIs) are a major burden in health care today, associated with considerable morbidity, mortality and costs. They are either caused by direct influx of pathogens via devices into the blood (primary BSI) or by bacterial spillover from infected distant organs (secondary BSI). The recognition of invading microbes by sensing of conserved molecular patterns is pivotal for the host in staging an adequate immune response to eradicate the pathogen. Moreover, a balanced immune response is crucial to avoid over inflammation followed by additional damage to the host. This complex host response pattern is controlled by soluble proteins and cellular receptors, which have recently been found to contain substantial individual genetic variations. Single nucleotide polymorphisms have been shown to affect susceptibility to and the course of numerous diseases. A large number of genes and their products are involved in the host reaction to BSIs, and genetic variation in these molecules alters the frequency and course of these events. Here we summarise recent findings on genetic variations in molecules of the innate immune system and other systems as well as their connection with susceptibility to BSIs and sepsis and the way the host stages a beneficial response to infection.

The Moraxella adhesin UspA1 binds to its human CEACAM1 receptor by a deformable trimeric coiled-coil

Moraxella catarrhalis is a ubiquitous human-specific bacterium commonly associated with upper and lower respiratory tract infections, including otitis media, sinusitis and chronic obstructive pulmonary disease. The bacterium uses an autotransporter protein UspA1 to target an important human cellular receptor carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1). Using X-ray crystallography, we show that the CEACAM1 receptor-binding region of UspA1 unusually consists of an extended, rod-like left-handed trimeric coiled-coil. Mutagenesis and binding studies of UspA1 and the N-domain of CEACAM1 have been used to delineate the interacting surfaces between ligand and receptor and guide assembly of the complex. However, solution scattering, molecular modelling and electron microscopy analyses all indicate that significant bending of the UspA1 coiled-coil stalk also occurs. This explains how UspA1 can engage CEACAM1 at a site far distant from its head group, permitting closer proximity of the respective cell surfaces during infection.

Binding of Dr adhesins of Escherichia coli to carcinoembryonic antigen triggers receptor dissociation

Carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) are host receptors for the Dr family of adhesins of Escherichia coli. To define the mechanism for binding of Dr adhesins to CEACAM receptors, we carried out structural studies on the N-terminal domain of CEA and its complex with the Dr adhesin. The crystal structure of CEA reveals a dimer similar to other dimers formed by receptors with IgV-like domains. The structure of the CEA/Dr adhesin complex is proposed based on NMR spectroscopy and mutagenesis data in combination with biochemical characterization. The Dr adhesin/CEA interface overlaps appreciably with the region responsible for CEA dimerization. Binding kinetics, mutational analysis and spectroscopic examination of CEA dimers suggest that Dr adhesins can dissociate CEA dimers prior to the binding of monomeric forms. Our conclusions include a plausible mechanism for how E. coli, and perhaps other bacterial and viral pathogens, exploit CEACAMs. The present structure of the complex provides a powerful tool for the design of novel inhibitory strategies to treat E. coli infections.