Identification of commensal Escherichia coli genes involved in biofilm resistance to pathogen colonization

Protection provided by host bacterial microbiota against microbial pathogens is a well known but ill-understood property referred to as the barrier effect, or colonization resistance. Despite recent genome-wide analyses of host microbiota and increasing therapeutic interest, molecular analysis of colonization resistance is hampered by the complexity of direct in vivo experiments. Here we developed an in vitro-to-in vivo approach to identification of genes involved in resistance of commensal bacteria to exogenous pathogens. We analyzed genetic responses induced in commensal Escherichia coli upon entry of a diarrheagenic enteroaggregative E. coli or an unrelated Klebsiella pneumoniae pathogen into a biofilm community. We showed that pathogens trigger specific responses in commensal bacteria and we identified genes involved in limiting colonization of incoming pathogens within commensal biofilm. We tested the in vivo relevance of our findings by comparing the extent of intestinal colonization by enteroaggregative E. coli and K. pneumoniae pathogens in mice pre-colonized with E. coli wild type commensal strain, or mutants corresponding to identified colonization resistance genes. We demonstrated that the absence of yiaF and bssS (yceP) differentially alters pathogen colonization in the mouse gut. This study therefore identifies previously uncharacterized colonization resistance genes and provides new approaches to unravelling molecular aspects of commensal/pathogen competitive interactions.

The AfaR small RNA controls expression of the AfaD-VIII invasin in pathogenic Escherichia coli strains

Pathogenic Escherichia coli strains carrying the afa-8 gene cluster are frequently associated with extra-intestinal infections in humans and animals. The afa-8 A to E genes determine the formation of an afimbrial adhesive sheath consisting of the AfaD-VIII invasin and the AfaE-VIII adhesin at the bacterial cell surface. This structure is thought to be required for host colonization. We characterized a new gene encoding the small RNA AfaR, which is transcribed in cis from the complementary strand of the 3' untranslated region of the afaD messenger RNA, within the afaD-afaE intercistronic region. AfaR is a trans-acting Hfq-dependent antisense small RNA that binds the 5' untranslated region of the afaD messenger RNA, initiating several ribonuclease E-dependent cleavages, thereby downregulating production of the AfaD-VIII invasin. AfaR transcription is dependent on σ(E), a member of the stress response family of extracytoplasmic alternative sigma factors. We found that the AfaR-dependent regulatory pathway was controlled by temperature, allowing the production of the AfaD-VIII invasin at temperatures above 37 °C. Our findings suggest that the entry of afa-8-positive pathogenic E. coli strains into epithelial cells is tightly regulated by the AfaR small RNA.

An in silico model for identification of small RNAs in whole bacterial genomes: characterization of antisense RNAs in pathogenic Escherichia coli and Streptococcus agalactiae strains

Characterization of small non-coding ribonucleic acids (sRNA) among the large volume of data generated by high-throughput RNA-seq or tiling microarray analyses remains a challenge. Thus, there is still a need for accurate in silico prediction methods to identify sRNAs within a given bacterial species. After years of effort, dedicated software were developed based on comparative genomic analyses or mathematical/statistical models. Although these genomic analyses enabled sRNAs in intergenic regions to be efficiently identified, they all failed to predict antisense sRNA genes (asRNA), i.e. RNA genes located on the DNA strand complementary to that which encodes the protein. The statistical models enabled any genomic region to be analyzed theorically but not efficiently. We present a new model for in silico identification of sRNA and asRNA candidates within an entire bacterial genome. This model was successfully used to analyze the Gram-negative Escherichia coli and Gram-positive Streptococcus agalactiae. In both bacteria, numerous asRNAs are transcribed from the complementary strand of genes located in pathogenicity islands, strongly suggesting that these asRNAs are regulators of the virulence expression. In particular, we characterized an asRNA that acted as an enhancer-like regulator of the type 1 fimbriae production involved in the virulence of extra-intestinal pathogenic E. coli.

Intestinal colonization by enteroaggregative Escherichia coli supports long-term bacteriophage replication in mice

Bacteriophages have been known to be present in the gut for many years, but studies of relationships between these viruses and their hosts in the intestine are still in their infancy. We isolated three bacteriophages specific for an enteroaggregative O104:H4 Escherichia coli (EAEC) strain responsible for diarrhoeal diseases in humans. We studied the replication of these bacteriophages in vitro and in vivo in a mouse model of gut colonization. Each bacteriophage was able to replicate in vitro in both aerobic and anaerobic conditions. Each bacteriophage individually reduced biofilms formed on plastic pegs and a cocktail of the three bacteriophages was found to be more efficient. The cocktail was also able to infect bacterial aggregates formed on the surface of epithelial cells. In the mouse intestine, bacteriophages replicated for at least 3 weeks, provided the host was present, with no change in host levels in the faeces. This model of stable and continuous viral replication provides opportunities for studying the long-term coevolution of virulent bacteriophages with their hosts within a mammalian polymicrobial ecosystem.

Repression of invasion genes and decreased invasion in a high-level fluoroquinolone-resistant Salmonella typhimurium mutant

Background

Nalidixic acid resistance among Salmonella Typhimurium clinical isolates has steadily increased, whereas the level of ciprofloxacin resistance remains low. The main objective of this study was to characterize the fluoroquinolone resistance mechanisms acquired in a S. Typhimurium mutant selected with ciprofloxacin from a susceptible isolate and to investigate its invasion ability.

Methodology/Principal Findings

Three different amino acid substitutions were detected in the quinolone target proteins of the resistant mutant (MIC of ciprofloxacin, 64 µg/ml): D87G and G81C in GyrA, and a novel mutation, E470K, in ParE. A protein analysis revealed an increased expression of AcrAB/TolC and decreased expression of OmpC. Sequencing of the marRAB, soxRS, ramR and acrR operons did not show any mutation and neither did their expression levels in a microarray analysis. A decreased percentage of invasion ability was detected when compared with the susceptible clinical isolate in a gentamicin protection assay. The microarray results revealed a decreased expression of genes which play a role during the invasion process, such as hilA, invF and the flhDC operon. Of note was the impaired growth detected in the resistant strain. A strain with a reverted phenotype (mainly concerning the resistance phenotype) was obtained from the resistant mutant.

Conclusions/Significance

In conclusion, a possible link between fluoroquinolone resistance and decreased cell invasion ability may exist explaining the low prevalence of fluoroquinolone-resistant S. Typhimurium clinical isolates. The impaired growth may appear as a consequence of fluoroquinolone resistance acquisition and down-regulate the expression of the invasion genes.

Organised genome dynamics in the Escherichia coli species results in highly diverse adaptive paths

The Escherichia coli species represents one of the best-studied model organisms, but also encompasses a variety of commensal and pathogenic strains that diversify by high rates of genetic change. We uniformly (re-) annotated the genomes of 20 commensal and pathogenic E. coli strains and one strain of E. fergusonii (the closest E. coli related species), including seven that we sequenced to completion. Within the ∼18,000 families of orthologous genes, we found ∼2,000 common to all strains. Although recombination rates are much higher than mutation rates, we show, both theoretically and using phylogenetic inference, that this does not obscure the phylogenetic signal, which places the B2 phylogenetic group and one group D strain at the basal position. Based on this phylogeny, we inferred past evolutionary events of gain and loss of genes, identifying functional classes under opposite selection pressures. We found an important adaptive role for metabolism diversification within group B2 and Shigella strains, but identified few or no extraintestinal virulence-specific genes, which could render difficult the development of a vaccine against extraintestinal infections. Genome flux in E. coli is confined to a small number of conserved positions in the chromosome, which most often are not associated with integrases or tRNA genes. Core genes flanking some of these regions show higher rates of recombination, suggesting that a gene, once acquired by a strain, spreads within the species by homologous recombination at the flanking genes. Finally, the genome's long-scale structure of recombination indicates lower recombination rates, but not higher mutation rates, at the terminus of replication. The ensuing effect of background selection and biased gene conversion may thus explain why this region is A+T-rich and shows high sequence divergence but low sequence polymorphism. Overall, despite a very high gene flow, genes co-exist in an organised genome.

Although abundant knowledge has been accumulated regarding the E. coli laboratory strain K-12, little is known about the evolutionary trajectories that have driven the high diversity observed among natural isolates of the species, which encompass both commensal and highly virulent intestinal and extraintestinal pathogenic strains. We have annotated or re-annotated the genomes of 20 commensal and pathogenic E. coli strains and one strain of E. fergusonii (the closest E. coli related species), including seven that we sequenced to completion. Although recombination rates are much higher than mutation rates, we were able to reconstruct a robust phylogeny based on the ∼2,000 genes common to all strains. Based on this phylogeny, we established the evolutionary scenario of gains and losses of thousands of specific genes, identifying functional classes under opposite selection pressures. This genome flux is confined to very few positions in the chromosome, which are the same for every genome. Notably, we identified few or no extraintestinal virulence-specific genes. We also defined a long-scale structure of recombination in the genome with lower recombination rates at the terminus of replication. These findings demonstrate that, despite a very high gene flow, genes can co-exist in an organised genome.

Uropathogenic Escherichia coli AL511 requires flagellum to enter renal collecting duct cells

Escherichia coli is the leading cause of urinary tract infections, but the mechanisms governing renal colonization by this bacterium remain poorly understood. We investigated the ability of 13 E. coli strains isolated from the urine of patients with pyelonephritis and cystitis and normal stools to invade collecting duct cells, which constitute the first epithelium encountered by bacteria ascending from the bladder. The AL511 clinical isolate adhered to mouse collecting duct mpkCCD(cl4) cells, used as a model of renal cell invasion, and was able to enter and persist within these cells. Previous studies have shown that bacterial flagella play an important role in host urinary tract colonization, but the role of flagella in the interaction of E. coli with renal epithelial cells remains unclear. An analysis of the ability of E. coli AL511 mutants to invade renal cells showed that flagellin played a key role in bacterial entry. Both flagellum filament assembly and the motor proteins MotA and MotB appeared to be required for E. coli AL511 uptake into collecting duct cells. These findings indicate that pyelonephritis-associated E. coli strains may invade renal collecting duct cells and that flagellin may act as an invasin in this process.

Renal collecting duct epithelial cells react to pyelonephritis-associated Escherichia coli by activating distinct TLR4-dependent and -independent inflammatory pathways

TLR4 plays a central role in resistance to pyelonephritis caused by uropathogenic Escherichia coli (UPEC). It has been suggested that renal tubule epithelial cells expressing TLRs may play a key role in inflammatory disorders and in initiating host defenses. In this study we used an experimental mouse model of ascending urinary tract infection to show that UPEC isolates preferentially adhered to the apical surface of medullary collecting duct (MCD) intercalated cells. UPEC-infected C3H/HeJ (Lps(d)) mice carrying an inactivating mutation of tlr4 failed to clear renal bacteria and exhibited a dramatic slump in proinflammatory mediators as compared with infected wild-type C3H/HeOuJ (Lps(n)) mice. However, the level of expression of the leukocyte chemoattractants MIP-2 and TNF-alpha still remained greater in UPEC-infected than in naive C3H/HeJ (Lps(d)) mice. Using primary cultures of microdissected Lps(n) MCDs that expressed TLR4 and its accessory molecules MD2, MyD88, and CD14, we also show that UPECs stimulated both a TLR4-mediated, MyD88-dependent, TIR domain-containing adaptor-inducing IFN-beta-independent pathway and a TLR4-independent pathway, leading to bipolarized secretion of MIP-2. Stimulation by UPECs of the TLR4-mediated pathway in Lps(n) MCDs leads to the activation of NF-kappaB, and MAPK p38, ERK1/2, and JNK. In addition, UPECs stimulated TLR4-independent signaling by activating a TNF receptor-associated factor 2-apoptosis signal-regulatory kinase 1-JNK pathway. These findings demonstrate that epithelial collecting duct cells are actively involved in the initiation of an immune response via several distinct signaling pathways and suggest that intercalated cells play an active role in the recognition of UPECs colonizing the kidneys.

The solution structure of the invasive tip complex from Afa/Dr fibrils

Afa/Dr family of adhesins are produced by pathogenic Escherichia coli strains that are especially prevalent in chronic diarrhoeal and recurrent urinary tract infections. Most notably, they are found in up to 50% of cystitis cases in children and 30% of pyelonephritis in pregnant women. Afa/Dr adhesins are capped surface fibrils that mediate recognition of the host and subsequent bacterial internalization. Using the newly solved three-dimensional structure of the minimal invasive complex (AfaDE) combined with biochemical and cellular assays, we reveal the architecture of the fibrillar cap and identify a novel mode of synergistic integrin recognition.

Diffusely adherent Escherichia coli strains expressing Afa/Dr adhesins (Afa/Dr DAEC): hitherto unrecognized pathogens

Diffusely adherent Escherichia coli (DAEC) strains are currently considered to constitute a putative sixth group of diarrheagenic E. coli. However, on the basis of their diffuse adherence to HEp-2 and HeLa cells, the detection of afa/dra/daa-related operons encoding this adherence phenotype, and the mobilization of decay-accelerating factor, both commensal and pathogenic strains can be classified as Afa/Dr DAEC isolates. Furthermore, strains associated with diarrheal diseases and strains causing extra-intestinal infections can also be identified as Afa/Dr DAEC strains. Although several cell signaling events that occur after epithelial cells have been infected by Afa/Dr DAEC have been reported, the pathophysiological processes that allow intestinal and extra-intestinal infections to develop are not fully understood. This review focuses on the genetic organization of the afa/dra/daa-related operons and on the virulence factors that trigger cellular responses, some of which are deleterious for the host cells. Finally, this review suggests future lines of research that could help to elucidate these questions.

Adhesins and invasins of pathogenic Escherichia coli

Pathogenic E. coli cause both intestinal and extra-intestinal infections in humans and animals. Bacteria must be able to adhere to host cells if they are to colonize and to invade their hosts. Numerous E. coli adhesins with different morphological features and receptor specificities have been identified. Many bacteria produce several adhesins with different receptor specificities. Although not all adhesin receptors have been identified yet, it appears that adhesins generally behave as lectins, recognizing oligosaccharide residues of glycoproteins or glycolipids. This review summarizes recent advances concerning host tissue colonization properties, providing new insights into adhesive organelle biogenesis in pathogenic E. coli and into the development of reservoirs of pathogenic bacteria in the host. To limit the length of this review, I will use examples of structural characteristics and invasive properties of a few bacterial adherence factors: type 1 pili, Afa adhesive sheath and some outer membrane adhesins.

High Resolution Studies of the Afa/Dr Adhesin DraE and Its Interaction with Chloramphenicol

Pathogenic Escherichia coli expressing Afa/Dr adhesins are able to cause both urinary tract and diarrheal infections. The Afa/Dr adhesins confer adherence to epithelial cells via interactions with the human complement regulating protein, decay accelerating factor (DAF or CD55). Two of the Afa/Dr adhesions, AfaE-III and DraE, differ from each other by only three residues but are reported to have several different properties. One such difference is disruption of the interaction between DraE and CD55 by chloramphenicol, whereas binding of AfaE-III to CD55 is unaffected. Here we present a crystal structure of a strand-swapped trimer of wild type DraE. We also present a crystal structure of this trimer in complex with chloramphenicol, as well as NMR data supporting the binding position of chloramphenicol within the crystal. The crystal structure reveals the precise atomic basis for the sensitivity of DraE-CD55 binding to chloramphenicol and demonstrates that in contrast to other chloramphenicol-protein complexes, drug binding is mediated via recognition of the chlorine "tail" rather than via intercalation of the benzene rings into a hydrophobic pocket.

Use of deoxyribose by intestinal and extraintestinal pathogenic Escherichia coli strains: a metabolic adaptation involved in competitiveness

We showed that the deoK operon, which confers the ability to use deoxyribose as a carbon source, is more common among pathogenic than commensal Escherichia coli strains. The expression of the deoK operon increases the competitiveness of clinical isolates, suggesting that this biochemical characteristic plays a role in host infectivity.

An Atomic Resolution Model for Assembly, Architecture, and Function of the Dr Adhesins

Pathogenic bacteria possess adhesion protein complexes that play essential roles in targeting host cells and in propagating infection. Although each family of adhesion proteins is generally associated with a specific human disease, the Dr family from Escherichia coli is a notable exception, as its members are associated with both diarrheal and urinary tract infections. These proteins are reported to form both fimbrial and afimbrial structures at the bacterial cell surface and target a common host cell receptor, the decay-accelerating factor (DAF or CD55). Using the newly solved three-dimensional structure of AfaE, we have constructed a robust atomic resolution model that reveals the structural basis for assembly by donor strand complementation and for the architecture of capped surface fibers.

A specific genetic background is required for acquisition and expression of virulence factors in Escherichia coli

In bacteria, the evolution of pathogenicity seems to be the result of the constant arrival of virulence factors (VFs) into the bacterial genome. However, the integration, retention, and/or expression of these factors may be the result of the interaction between the new arriving genes and the bacterial genomic background. To test this hypothesis, a phylogenetic analysis was done on a collection of 98 Escherichia coli/Shigella strains representing the pathogenic and commensal diversity of the species. The distribution of 17 VFs associated to the different E. coli pathovars was superimposed on the phylogenetic tree. Three major types of VFs can be recognized: (1) VFs that arrive and are expressed in different genetic backgrounds (such as VFs associated with the pathovars of mild chronic diarrhea: enteroaggregative, enteropathogenic, and diffusely-adhering E. coli), (2) VFs that arrive in different genetic backgrounds but are preferentially found, associated with a specific pathology, in only one particular background (such as VFs associated with extraintestinal diseases), and (3) VFs that require a particular genetic background for the arrival and expression of their virulence potential (such as VFs associated with pathovars typical of severe acute diarrhea: enterohemorragic, enterotoxigenic, and enteroinvasive E. coli strains). The possibility of a single arrival of VFs by chance, followed by a vertical transmission, was ruled out by comparing the evolutionary histories of some of these VFs to the strain phylogeny. These evidences suggest that important changes in the genome of E. coli have occurred during the diversification of the species, allowing the virulence factors associated with severe acute diarrhea to arrive in the population. Thus, the E. coli genome seems to be formed by an "ancestral" and a "derived" background, each one responsible for the acquisition and expression of different virulence factors.

Recognition of the cellular beta1-chain integrin by the bacterial AfaD invasin is implicated in the internalization of afa-expressing pathogenic Escherichia coli strains

The afa operons from Escherichia coli associated with extra-intestinal and intestinal infections have been characterized and the AfaD protein has been shown to be involved in the low internalization of laboratory strains expressing the afa-3 operon. The aim of this study was to determine the role of the AfaD invasin during the interaction of pathogenic E. coli with epithelial cells. We show that AfaD is implicated in the entry of a clinical isolate into both HeLa and undifferentiated Caco-2 cells. Once in the cytoplasm of these cells, the bacteria formed inclusions in which they were able to survive for at least 72 h. Internalization assays using polystyrene beads coated with His6-tagged purified AfaD (rAfaD) demonstrated that this invasin mediates entry into cells derived from various tissues (intestine and urothelium) that are targets for afa-positive strains. Consistent with the previous observation that an antibody blockade involving anti-alpha5beta1 integrin abolishes bacterial internalization, we show here that the entry of rAfaD-coated beads was dependent on the production and accessibility of beta1 integrins on the cells. The AfaD proteins belong to a family of invasins that are at least 45% identical. Despite their differences, the recombinant rAfaD-III and rAfaD-VIII proteins both bound to beta1 integrins. Our results suggest that beta1 integrin is a common receptor for AfaD invasins and that additional AfaD-type-specific receptors exist.

Extended virulence genotype of pathogenic Escherichia coli isolates carrying the afa-8 operon: evidence of similarities between isolates from humans and animals with extraintestinal infections

The afimbrial AfaE-VIII adhesin is common among Escherichia coli isolates from calves with intestinal and/or extraintestinal infections and from humans with sepsis or pyelonephritis. The virulence genotypes of 77 Escherichia coli afa-8 isolates from farm animals and humans were compared to determine whether any trait of commonality exists between isolates of the different host species. Over half of the extraintestinal afa-8 isolates were associated with pap and f17Ac adhesin genes and contained virulence genes (pap, hly, and cnf1) which are characteristic of human extraintestinal pathogenic E. coli (ExPEC). PapG, which occurs as three known variants (variants I to III), is encoded by the corresponding three alleles of papG. Among the pap-positive strains, new papG variants (papGrs) that differed from the isolates with genes for the three adhesin classes predominated over isolates with papG allele III, which in turn were more prevalent than those with allele II. The data showed the substantial prevalence of the enteroaggregative E. coli heat-stable enterotoxin gene (east1) among afa-8 isolates. Most of the afa-8 isolates harbored the high-pathogenicity island (HPI) present in pathogenic Yersinia; however, two-thirds of the HPI-positive strains shared a truncated HPI integrase gene. The presence of ExPEC-associated virulence factors (VFs) in extraintestinal isolates that carry genes typical of enteric strains and that express O antigens associated with intestinal E. coli is consistent with transfer of VFs and O-antigen determinants between ExPEC and enteric strains. The similarities between animal and human ExPEC strains support the hypothesis of overlapping populations, with members of certain clones or clonal groups including animal and human strains. The presence of multiple-antibiotic-resistant bovine afa-8 strains among such clones may represent a potential public health risk.

A novel dispersin protein in enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) is a diarrheal pathogen defined by its characteristic aggregative adherence (AA) to HEp-2 cells in culture. We have previously shown that EAEC strains secrete a 10-kDa protein that is immunogenic in a human EAEC challenge model. We report here that this protein is encoded by a gene (called aap) lying immediately upstream of that encoding the AggR transcriptional activator, and that aap is under AggR control. The product of aap has a typical signal sequence and is secreted to the extracellular milieu, where it remains noncovalently attached to the surface of the bacterium. EAEC aap mutants aggregate more intensely than the wild-type parent in a number of assays, forming larger aggregates and fewer individual bacteria. Infection of colonic biopsies with wild-type EAEC strain 042 and its aap mutant revealed more dramatic autoagglutination of the mutant compared with the wild-type parent. Our data suggest that the aap gene product participates in formation of a surface coat that acts to disperse the bacteria, thus partially counteracting aggregation mediated by aggregative adherence fimbriae. We have therefore named the aap gene product "dispersin," and we propose that it may be representative of a functional class of colonization factors. Since dispersin is expressed in vivo, is highly immunogenic, and is present in most EAEC strains, it holds considerable promise as an EAEC immunogen.

Chronic diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome associated with HEp-2 adherent Escherichia coli in adults infected with human immunodeficiency virus in Bangui, Central African Republic

In human immunodeficiency virus (HIV)-infected adults from the Central African Republic, the occurrence of chronic diarrhea due to HEp-2 adherent Escherichia coli (EAEC) harboring virulence markers (eaeA, BFP, EAF, astA determinant of EAST/1, positive FAS test, enteropathogenic E. coli O serogroup) was shown to be associated with AIDS. We also show that EAEC that produce verotoxin (Stx2) but do not harbor the genetic markers for classical enterohemorrhagic E. coli are involved in hemorrhagic colitis and hemolytic-uremic syndrome in patients with HIV.

Identification of an aggregative adhesion fimbria (AAF) type III-encoding operon in enteroaggregative Escherichia coli as a sensitive probe for detecting the AAF-encoding operon family

Enteroaggregative Escherichia coli (EAEC) is recognized as an emerging cause of diarrhea in children and adults worldwide, and recent studies have implicated EAEC in persistent diarrhea in patients infected with human immunodeficiency virus (HIV). In this study, we identified aggregative adhesion fimbria type III (AAF-III) in isolate 55989, a typical EAEC strain. Analysis of the sequence of the plasmid-borne agg-3 gene cluster encoding AAF-III showed this cluster to be closely related to the agg and aaf operons and to the afa operons carried by diffusely adherent pathogenic E. coli. We investigated the adhesion properties of a collection of 25 EAEC strains isolated from HIV-infected patients presenting with persistent diarrhea. We found that a minority of strains (36%) carried sequences similar to those of the agg and aaf operons, which encode AAF-I and AAF-II, respectively. We developed PCR assays specific for the agg-3 operon. In our collection, the frequency of AAF-III strains was similar (12%) to that of AAF-I strains (16%) but higher than that of AAF-II isolates (0%). Differences between EAEC strains in terms of the virulence factors present render detection of these strains difficult with the available DNA probes. Based on comparison of the agg, aaf, and agg-3 operons, we defined an AAF probe internal to the adhesion gene clusters and demonstrated that it was efficient for the identification of EAEC strains. We investigated 32 EAEC isolates, of which only 34.4% were detected with the classical CVD432 probe (detecting pAA virulence plasmids) whereas 65.6% were detected with the AAF probe.

Binding of Escherichia coli adhesin AfaE to CD55 triggers cell-surface expression of the MHC class I-related molecule MICA

MICA are distant homologs of MHC class I molecules expressed in the normal intestinal epithelium. They are ligands of the NKG2D activating receptor expressed on most gammadelta T cells, CD8+ alphabeta T cells, and natural killer cells and therefore play a critical role in innate immune responses. We investigated MICA cell-surface expression on infection of epithelial cell lines by enteric bacteria and show here that MICA expression can be markedly increased by bacteria of the diffusely adherent Escherichia coli diarrheagenic group. This effect is mediated by the specific interaction between bacterial adhesin AfaE and its cellular receptor, CD55, or decay-accelerating factor. It is extremely rapid after AfaE binding, consistent with a stress-induced signal. MICA induction on epithelial cells triggered IFN-gamma release by the NKG2D expressing natural killer cell line NKL. This host-bacteria interaction pathway could play a role in the pathogenesis of inflammatory bowel disease, a condition that implicates a bacterial trigger in genetically susceptible individuals. This was supported by the increased MICA expression at the surface of epithelial cells in colonic biopsies from Crohn's disease-affected patients compared with controls.