Interaction of Mycobacterium tuberculosis with human respiratory mucosa

Antibodies as clinical tools for tuberculosis

Tuberculosis (TB) is a leading cause of morbidity and mortality worldwide. Global research efforts to improve TB control are hindered by insufficient understanding of the role that antibodies play in protective immunity and pathogenesis. This impacts knowledge of rational and optimal vaccine design, appropriate diagnostic biomarkers, and development of therapeutics. Traditional approaches for the prevention and diagnosis of TB may be less efficacious in high prevalence, remote, and resource-poor settings. An improved understanding of the immune response to the causative agent of TB, Mycobacterium tuberculosis (Mtb), will be crucial for developing better vaccines, therapeutics, and diagnostics. While memory CD4+ T cells and cells and cytokine interferon gamma (IFN-g) have been the main identified correlates of protection in TB, mounting evidence suggests that other types of immunity may also have important roles. TB serology has identified antibodies and functional characteristics that may help diagnose Mtb infection and distinguish between different TB disease states. To date, no serological tests meet the World Health Organization (WHO) requirements for TB diagnosis, but multiplex assays show promise for improving the sensitivity and specificity of TB serodiagnosis. Monoclonal antibody (mAb) therapies and serum passive infusion studies in murine models of TB have also demonstrated some protective outcomes. However, animal models that better reflect the human immune response to Mtb are necessary to fully assess the clinical utility of antibody-based TB prophylactics and therapeutics. Candidate TB vaccines are not designed to elicit an Mtb-specific antibody response, but evidence suggests BCG and novel TB vaccines may induce protective Mtb antibodies. The potential of the humoral immune response in TB monitoring and control is being investigated and these studies provide important insight into the functional role of antibody-mediated immunity against TB. In this review, we describe the current state of development of antibody-based clinical tools for TB, with a focus on diagnostic, therapeutic, and vaccine-based applications.

From immunology to artificial intelligence: revolutionizing latent tuberculosis infection diagnosis with machine learning

Latent tuberculosis infection (LTBI) has become a major source of active tuberculosis (ATB). Although the tuberculin skin test and interferon-gamma release assay can be used to diagnose LTBI, these methods can only differentiate infected individuals from healthy ones but cannot discriminate between LTBI and ATB. Thus, the diagnosis of LTBI faces many challenges, such as the lack of effective biomarkers from Mycobacterium tuberculosis (MTB) for distinguishing LTBI, the low diagnostic efficacy of biomarkers derived from the human host, and the absence of a gold standard to differentiate between LTBI and ATB. Sputum culture, as the gold standard for diagnosing tuberculosis, is time-consuming and cannot distinguish between ATB and LTBI. In this article, we review the pathogenesis of MTB and the immune mechanisms of the host in LTBI, including the innate and adaptive immune responses, multiple immune evasion mechanisms of MTB, and epigenetic regulation. Based on this knowledge, we summarize the current status and challenges in diagnosing LTBI and present the application of machine learning (ML) in LTBI diagnosis, as well as the advantages and limitations of ML in this context. Finally, we discuss the future development directions of ML applied to LTBI diagnosis.

Clinical manifestations and immune response to tuberculosis

Tuberculosis is a far-reaching, high-impact disease. It is among the top ten causes of death worldwide caused by a single infectious agent; 1.6 million tuberculosis-related deaths were reported in 2021 and it has been estimated that a third of the world's population are carriers of the tuberculosis bacillus but do not develop active disease. Several authors have attributed this to hosts' differential immune response in which cellular and humoral components are involved, along with cytokines and chemokines. Ascertaining the relationship between TB development's clinical manifestations and an immune response should increase understanding of tuberculosis pathophysiological and immunological mechanisms and correlating such material with protection against Mycobacterium tuberculosis. Tuberculosis continues to be a major public health problem globally. Mortality rates have not decreased significantly; rather, they are increasing. This review has thus been aimed at deepening knowledge regarding tuberculosis by examining published material related to an immune response against Mycobacterium tuberculosis, mycobacterial evasion mechanisms regarding such response and the relationship between pulmonary and extrapulmonary clinical manifestations induced by this bacterium which are related to inflammation associated with tuberculosis dissemination through different routes.

Chronic nasopharyngeal and otitis media tuberculosis: diagnosis and management difficulties

Nasopharyngeal and otitis media tuberculosis are rare extrapulmonary manifestations of Mycobacterium tuberculosis infection. We present a case of a middle-aged woman with manifestations of both conditions along with a description of the anatomical and temporal evolution of the disease. This case also highlights the difficulty of diagnosis and management of this condition, requiring a multidisciplinary approach. Extrapulmonary tuberculosis must be considered in the differential diagnosis of multiple head and neck conditions, including refractory chronic rhinosinusitis and otitis.

Mycobacterial Adhesion: From Hydrophobic to Receptor-Ligand Interactions

Adhesion is crucial for the infective lifestyles of bacterial pathogens. Adhesion to non-living surfaces, other microbial cells, and components of the biofilm extracellular matrix are crucial for biofilm formation and integrity, plus adherence to host factors constitutes a first step leading to an infection. Adhesion is, therefore, at the core of pathogens’ ability to contaminate, transmit, establish residency within a host, and cause an infection. Several mycobacterial species cause diseases in humans and animals with diverse clinical manifestations. Mycobacterium tuberculosis, which enters through the respiratory tract, first adheres to alveolar macrophages and epithelial cells leading up to transmigration across the alveolar epithelium and containment within granulomas. Later, when dissemination occurs, the bacilli need to adhere to extracellular matrix components to infect extrapulmonary sites. Mycobacteria causing zoonotic infections and emerging nontuberculous mycobacterial pathogens follow divergent routes of infection that probably require adapted adhesion mechanisms. New evidence also points to the occurrence of mycobacterial biofilms during infection, emphasizing a need to better understand the adhesive factors required for their formation. Herein, we review the literature on tuberculous and nontuberculous mycobacterial adhesion to living and non-living surfaces, to themselves, to host cells, and to components of the extracellular matrix.

Heme Oxygenase-1 as a Pharmacological Target for Host-Directed Therapy to Limit Tuberculosis Associated Immunopathology

Excessive inflammation and tissue damage are pathological hallmarks of chronic pulmonary tuberculosis (TB). Despite decades of research, host regulation of these clinical consequences is poorly understood. A sustained effort has been made to understand the contribution of heme oxygenase-1 (HO-1) to this process. HO-1 is an essential cytoprotective enzyme in the host that controls inflammation and oxidative stress in many pathological conditions. While HO-1 levels are upregulated in animals and patients infected with Mycobacterium tuberculosis (Mtb), how it regulates host responses and disease pathology during TB remains unclear. This lack of clarity is due in part to contradictory studies arguing that HO-1 induction contributes to both host resistance as well as disease progression. In this review, we discuss these conflicting studies and the role of HO-1 in modulating myeloid cell functions during Mtb disease progression. We argue that HO-1 is a promising target for host-directed therapy to improve TB immunopathology.

Interaction of Mycobacterium avium subsp. paratuberculosis with bovine sperm

Mycobacterium avium subsp. paratuberculosis (MAP) is responsible for Paratuberculosis mainly affecting domestic ruminants. The interaction between MAP and sperm and/or germ cells has not yet been established, however the adherence between MAP and the host cell surface is associated to the 85 complex proteins that bind to the host cell's fibronectin. Therefore, this study aimed to evaluate the binding of MAP to bovine sperm and to verify changes in these cells by the presence of MAP before and after sperm cryopreservation. Polyclonal antibodies to MAP 85 complex proteins were produced and utilized in the analyzes. Two Nelore bulls were used for semen collection and MAP dilutions (10³-10⁸ CFU/mL) were inoculated in the samples; sperm motility and vigor were evaluated using light microscopy at different times before and after cryopreservation and in the presence and absence of the antibodies 85A and 85B. Interaction of MAP and sperm, interaction of MAP and sperm in the presence of Ab 85A and in the presence of Ab 85B were analyzed by scanning electron microscopy. The viability of MAP after sperm cryopreservation were evaluated by plating the samples after thawing. It was observed that sperm in the presence of MAP shows a decrease in motility and vigor, and that the higher the MAP concentration, the lower the sperm performance. It was possible to determine the viability of MAP after cryopreservation in samples of higher concentrations, which demonstrates the potential of transmission of this pathogen through artificial insemination. The interaction of MAP with bovine sperm occurs mainly in the midpiece and may be linked to the proteins 85A and 85B present in the MAP membrane.

Functional and structural investigations of fibronectin-binding protein Apa from Mycobacterium tuberculosis

BACKGROUND

Alanine and proline-rich protein (Apa) is a secreted antigen of Mycobacterium spp. which involves in stimulating immune responses and adhering to host cells by binding to fibronectin (Fn). Here, we report the crystal structure of Apa from Mycobacterium tuberculosis (Mtb) and its Fn-binding characteristics.

METHODS

The crystal structure of Mtb Apa was determined at resolutions of 1.54 Å. The dissociation constants (K_D) of Apa and individual modules of Fn were determined by surface plasmon resonance and enzyme-linked immunosorbent assay. Site-directed mutagenesis was performed to investigate the putative Fn-binding motif of Apa.

RESULTS

Mtb Apa folds into a large seven-stranded anti-parallel β-sheet which is flanked by three α-helices. The binding affinity of Mtb Apa to individual Fn modules was assessed and the results indicated that the Mtb Apa binds to FnIII-4 and FnIII-5 of Fn CBD segment. Notably, structure analysis suggested that the previously proposed Fn-binding motif ²⁵⁸RWFV²⁶¹ is buried within the protein and may not be accessible to the binding counterpart.

CONCLUSIONS

The structural and Fn-binding characteristics we reported here provide molecular insights into the multifunctional protein Mtb Apa. FnIII-4 and FnIII-5 of CBD are the only two modules contributing to Apa-Fn interaction.

GENERAL SIGNIFICANCE

This is the first study to report the structure and Fn-binding characteristics of mycobacterial Apa. Since Apa plays a central role in stimulating immune responses and host cells adhesion, these results are of great importance in understanding the pathogenesis of mycobacterium. This information shall provide a guidance for the development of anti-mycobacteria regimen.

Immune Response to Mycobacterium tuberculosis: A Narrative Review

The encounter between Mycobacterium tuberculosis (Mtb) and the host leads to a complex and multifaceted immune response possibly resulting in latent infection, tubercular disease or to the complete clearance of the pathogen. Macrophages and CD4⁺ T lymphocytes, together with granuloma formation, are traditionally considered the pillars of immune defense against Mtb and their role stands out clearly. However, there is no component of the immune system that does not take part in the response to this pathogen. On the other side, Mtb displays a complex artillery of immune-escaping mechanisms capable of responding in an equally varied manner. In addition, the role of each cellular line has become discussed and uncertain further than ever before. Each defense mechanism is based on a subtle balance that, if altered, can lean to one side to favor Mtb proliferation, resulting in disease progression and on the other to the host tissue damage by the immune system itself. Through a brief and complete overview of the role of each cell type involved in the Mtb response, we aimed to highlight the main literature reviews and the most relevant studies in order to facilitate the approach to such a complex and changeable topic. In conclusion, this narrative mini-review summarizes the various immunologic mechanisms which modulate the individual ability to fight Mtb infection taking in account the major host and pathogen determinants in the susceptibility to tuberculosis.

The Role of Functional Amyloids in Bacterial Virulence

Amyloid fibrils are best known as a product of human and animal protein misfolding disorders, where amyloid formation is associated with cytotoxicity and disease. It is now evident that for some proteins, the amyloid state constitutes the native structure and serves a functional role. These functional amyloids are proving widespread in bacteria and fungi, fulfilling diverse functions as structural components in biofilms or spore coats, as toxins and surface-active fibers, as epigenetic material, peptide reservoirs or adhesins mediating binding to and internalization into host cells. In this review, we will focus on the role of functional amyloids in bacterial pathogenesis. The role of functional amyloids as virulence factor is diverse but mostly indirect. Nevertheless, functional amyloid pathways deserve consideration for the acute and long-term effects of the infectious disease process and may form valid antimicrobial targets.

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Functional amyloids are widespread in bacteria, pathogenic and non-pathogenic.

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Bacterial biofilms most commonly function as structural support in the extracellular matrix of biofilms or spore coats, and in cell–cell and cell-surface adherence.

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The amyloid state can be the sole structured and functional state, or can be facultative, as a secondary state to folded monomeric subunits.

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Bacterial amyloids can enhance virulence by increasing persistence, cell adherence and invasion, intracellular survival, and pathogen spread by increased environmental survival.

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Bacterial amyloids may indirectly inflict disease by triggering inflammation, contact phase activation and possibly induce or aggravate human pathological aggregation disorders.

The innate immune response in human tuberculosis

Mycobacterium tuberculosis (Mtb) infection can be cleared by the innate immune system before the initiation of an adaptive immune response. This innate protection requires a variety of robust cell autonomous responses from many different host immune cell types. However, Mtb has evolved strategies to circumvent some of these defences. In this mini-review, we discuss these host-pathogen interactions with a focus on studies performed in human cells and/or supported by human genetics studies (such as genome-wide association studies).

Quantity and Quality of Inhaled Dose Predicts Immunopathology in Tuberculosis

Experimental animal models of tuberculosis (TB) have convincingly demonstrated that inhaled dose predicts immunopathology and survival. In contrast, the importance of inhaled dose has generally not been appreciated in TB epidemiology, clinical science, or the practice of TB control. Infectiousness of TB patients has traditionally been assessed using microscopy for acid-fast bacilli in the sputum, which should be considered only a risk factor. We have recently demonstrated that cough aerosol cultures from index cases with pulmonary TB are the best predictors of new infection among household contacts. We suggest that cough aerosols of M. tuberculosis are the best surrogates of inhaled dose, and we hypothesize that the quantity of cough aerosols is associated with TB infection versus disease. Although several factors affect the quality of infectious aerosols, we propose that the particle size distribution of cough aerosols is an important predictor of primary upper airway disease and cervical lymphadenitis and of immune responses in exposed hosts. We hypothesize that large droplet aerosols (>5 μ) containing M. tuberculosis deposit in the upper airway and can induce immune responses without establishing infection. We suggest that this may partially explain the large proportion of humans who never develop TB disease in spite of having immunological evidence of M. tuberculosis infection (e.g., positive tuberculin skin test or interferon gamma release assay). If these hypotheses are proven true, they would alter the current paradigm of latent TB infection and reactivation, further demonstrating the need for better biomarkers or methods of assessing TB infection and the risk of developing disease.

Pili of Mycobacterium tuberculosis: current knowledge and future prospects

Many pathogenic bacteria express filamentous appendages, termed pili, on their surface. These organelles function in several important bacterial processes, including mediating bacterial interaction with, and colonization of the host, signalling events, locomotion, DNA uptake, electric conductance, and biofilm formation. In the last decade, it has been established that the tuberculosis-causing bacterium, Mycobacterium tuberculosis, produces two pili types: curli and type IV pili. In this paper, we review studies on M. tuberculosis pili, highlighting their structure and biological significance to M. tuberculosis pathogenesis, and discuss their potential as targets for therapeutic intervention and diagnostic test development.

Interaction of Bordetella bronchiseptica and Its Lipopolysaccharide with In Vitro Culture of Respiratory Nasal Epithelium

The nasal septa of fetal rabbits at 26 days of gestation were harvested by cesarean section of the does while under anesthesia and then exposed to Bordetella bronchiseptica or its lipopolysaccharide (LPS) for periods of 2 and 4 hours. A total of 240 explants were used. The tissues were examined using the Hematoxylin & Eosin technique. Then, semithin sections (0.5 μm) were stained with toluidine blue and examined with indirect immunoperoxidase (IPI) and lectin histochemistry. The most frequent and statistically significant findings were as follows: (1) cell death and increased goblet cell activity when exposed to bacteria and (2) cell death, cytoplasmic vacuolation and infiltration of polymorphonuclear leukocytes when exposed to LPS. The lesions induced by the bacterium were more severe than with LPS alone, except for the cytoplasmic vacuolation in epithelial cells. IPI stained the ciliated border of the epithelium with the bacterium more intensely, while LPS lectin histochemistry preferentially labeled the cytoplasm of goblet cell. These data indicate that B. bronchiseptica and its LPS may have an affinity for specific glycoproteins that would act as adhesion receptors in both locations.

Elastin, a novel extracellular matrix protein adhering to mycobacterial antigen 85 complex

The antigen 85 complex (Ag85) consists of three predominantly secreted proteins (Ag85A, Ag85B, and Ag85C), which play a key role in the mycobacterial pathogenesis and also possess enzymatic mycolyltransferase activity involved in cell wall synthesis. Ag85 is not only considered to be a virulence factor because its expression is essential for intracellular survival within macrophages, but also because it contributes to adherence, invasion, and dissemination of mycobacteria in host cells. In this study, we report that the extracellular matrix components, elastin and its precursor (tropoelastin) derived from human aorta, lung, and skin, serve as binding partners of Ag85 from Mycobacterium tuberculosis. The binding affinity of M. tuberculosis Ag85 to human tropoelastin was characterized (K(D) = 0.13 ± 0.006 μm), and a novel Ag85-binding motif, AAAKAA(K/Q)(Y/F), on multiple tropoelastin modules was identified. In addition, the negatively charged Glu-258 of Ag85 was demonstrated to participate in an electrostatic interaction with human tropoelastin. Moreover, binding of Ag85 on elastin siRNA-transfected Caco-2 cells was significantly reduced (34.3%), implying that elastin acts as an important ligand contributing to mycobacterial invasion.

The role of epithelial integrin receptors in recognition of pulmonary pathogens

Integrins are a large family of heterodimeric transmembrane cell adhesion receptors. During the last decade, it has become clear that integrins significantly participate in various host-pathogen interactions involving pathogenic bacteria, fungi, and viruses. Many bacteria possess adhesins that can bind either directly or indirectly to integrins. However, there appears to be an emerging role for integrins beyond simply adhesion molecules. Given the conserved nature of integrin structure and function, and the diversity of the pathogens which use integrins, it appears that they may act as pattern recognition receptors important for the innate immune response. Several clinically significant bacterial pathogens target lung epithelial integrins, and this review will focus on exploring various structures and mechanisms involved in these interactions.

Mycobacterium tuberculosis produces pili during human infection

Mycobacterium tuberculosis is responsible for nearly 3 million human deaths worldwide every year. Understanding the mechanisms and bacterial factors responsible for the ability of M. tuberculosis to cause disease in humans is critical for the development of improved treatment strategies. Many bacterial pathogens use pili as adherence factors to colonize the host. We discovered that M. tuberculosis produces fine (2- to 3-nm-wide), aggregative, flexible pili that are recognized by IgG antibodies contained in sera obtained from patients with active tuberculosis, indicating that the bacilli produce pili or pili-associated antigen during human infection. Purified M. tuberculosis pili (MTP) are composed of low-molecular-weight protein subunits encoded by the predicted M. tuberculosis H37Rv ORF, designated Rv3312A. MTP bind to the extracellular matrix protein laminin in vitro, suggesting that MTP possess adhesive properties. Isogenic mtp mutants lost the ability to produce Mtp in vitro and demonstrated decreased laminin-binding capabilities. MTP shares morphological, biochemical, and functional properties attributed to bacterial pili, especially with curli amyloid fibers. Thus, we propose that MTP are previously unidentified host-colonization factors of M. tuberculosis.

Mycobacterium tuberculosis malate synthase is a laminin-binding adhesin

Mycobacterium tuberculosis (M. tb) uses the glyoxalate bypass for intracellular survival in vivo. These studies provide evidence that the M. tb malate synthase (MS) has adapted to function as an adhesin which binds to laminin and fibronectin. This binding is achieved via the unique C-terminal region of the M. tb MS. The ability to function as an adhesin necessitates extracellular localization. We provide evidence that despite the absence of a Sec-translocation signal sequence the M. tb MS is secreted/excreted, and is anchored on the cell wall by an undefined mechanism. The MS of Mycobacterium smegmatis is cytoplasmic but the M. tb MS expressed in M. smegmatis localizes to the cell wall and enhances the adherence of the bacteria to lung epithelial A549 cells. Antibodies to the C-terminal laminin/fibronectin-binding domain interfere with the binding of the M. tb MS to laminin and fibronectin and reduce the adherence of M. tb to A549 cells. Coupled to the earlier evidence of in vivo expression of M. tb MS during active but not latent infection in humans, these studies show that a housekeeping enzyme of M. tb contributes to its armamentarium of virulence promoting factors.

Endobronchial tuberculosis

Endobronchial tuberculosis is defined as tuberculous infection of the tracheobronchial tree. Although clinical features differ between various types and stages of endobronchial tuberculosis, common symptoms are cough, hemoptysis, sputum production, wheezing, chest pain, fever and dyspnea. Endobronchial tuberculosis is difficult to diagnose, because the lesion is not evident in the chest radiograph. Computerized tomography is very useful in evaluating bronchial lesions such as stenosis or obstruction. The most important goal of treatment in active endobronchial tuberculosis is the eradication of tubercle bacilli. The second most important goal is prevention of bronchial stenosis. Corticosteroid therapy for prevention of bronchial stenosis in endobronchial tuberculosis remains controversial, but the best results are associated with minimal delay in the initiation of steroid treatment. In inactive disease, treatment to restore full patency is appropriate. As steroids or other medication are unable to reverse stenosis from fibrous disease, airway patency must be restored mechanically by surgery or endobronchial intervention. Aerosol therapy with streptomycin and corticosteroids is useful in treatment against active endobronchial tuberculosis. Time to healing of ulcerous lesions is shorter, and bronchial stenosis is less severe in patients on aerosol therapy. Progression to bronchial stenosis may be prevented if the therapy is initiated as soon as possible.

Inhibition of adherence of Mycobacterium avium complex and Mycobacterium tuberculosis to fibronectin on the respiratory mucosa

Mycobacterium species adhere to the respiratory mucosa via mucus and fibronectin of extracellular matrix exposed by damaged epithelium. We have investigated whether inhibiting adherence to fibronectin influences subsequent infection of human respiratory tissue by Mycobacterium avium complex and Mycobacterium tuberculosis. Human respiratory tissue was pretreated with mycobacterial fibronectin attachment proteins prior to infection with M. avium complex and M. tuberculosis and the number of recoverable bacteria over time was compared to untreated controls. Inhibition significantly reduced recovery of M. avium complex at 15min (P= 0.02), 7days (P = 0.04), and 14 days (P= 0.03); whereas recovery of M. tuberculosis was only reduced at 15 min (P = 0.01) and not at later timepoints. We conclude that M. avium complex and M. tuberculosis infection of the mucosa proceeds by different mechanisms, since M. tuberculosis infection is independent of fibronectin adherence.

Interaction between mycobacteria and mucus on a human respiratory tissue organ culture model with an air interface

Mycobacteria adhere specifically to extracellular matrix (ECM) and mucus with a fibrous, but not globular, appearance, in organ cultures of human respiratory mucosa examined by scanning electron microscopy. Previously, light microscopy sections made of tissue infected for 7 days demonstrated mycobacteria associated with mucus on the organ culture surface, and within submucosal glands in areas of damaged epithelium. The authors have now investigated the interactions between Mycobacterium avium complex (MAC), Mycobacterium tuberculosis (MTB), and Mycobacterium smegmatis (MS) and mucus by preincubating bacteria with purified mucins MUC5AC and MUC5B prior to inoculation onto the organ culture mucosal surface. They have also measured mucin production by the organ culture after mycobacterial infection. Mucus did not cause clumping of mycobacteria. There was a significant (P=.03) increase in the amount of fibrous mucus, but not globular mucus, observed on tissue inoculated with mucins compared to controls. The number of bacteria adhering to ECM was markedly reduced after incubation with mucins, which could indicate a protective effect. Mycobacterial infection did not increase mucin production by the organ culture. Mycobacterial adherence to mucins may play a role in the pathogenicity of mycobacteria in diseases such as cystic fibrosis, bronchiectasis, and chronic obstructive pulmonary disease (COPD), in which there are changes in mucus composition and clearance.

Mycobacterium avium subsp. paratuberculosis fibronectin attachment protein facilitates M-cell targeting and invasion through a fibronectin bridge with host integrins

Efficient attachment and ingestion of Mycobacterium avium subsp. paratuberculosis by cultured epithelial cells requires the expression of a fibronectin (FN) attachment protein homologue (FAP-P) which mediates FN binding by M. avium subsp. paratuberculosis. Invasion of Peyer's patches by M. avium subsp. paratuberculosis occurs through M cells, which, unlike other intestinal epithelial cells, express integrins on their luminal faces. We sought to determine if the interaction between FAP-P of M. avium subsp. paratuberculosis and soluble FN enabled targeting and invasion of M cells by M. avium subsp. paratuberculosis in vivo via these surface integrins. Wild-type and antisense FAP-P mutant M. avium subsp. paratuberculosis strains were injected alone or coinjected with blocking peptides or antibodies into murine gut loops, and immunofluorescence microscopy was performed to assess targeting and invasion of M cells by M. avium subsp. paratuberculosis. Nonopsonized M. avium subsp. paratuberculosis preferentially invaded M cells in murine gut loops. M-cell invasion was enhanced 2.6-fold when M. avium subsp. paratuberculosis was pretreated with FN. Invasion of M cells by the antisense FAP-P mutant of M. avium subsp. paratuberculosis was reduced by 77 to 90% relative to that observed for the control strains. Peptides corresponding to the RGD and synergy site integrin recognition regions of FN blocked M. avium subsp. paratuberculosis invasion of M cells by 75 and 45%, respectively, whereas the connecting segment 1 peptide was noninhibitory. Antibodies against the alpha5, alphaV, beta1, and beta3 integrin subunits inhibited M-cell invasion by 52 to 73%. The results indicate that targeting and invasion of M cells by M. avium subsp. paratuberculosis in vivo is mediated primarily by the formation of an FN bridge formed between FAP-P of M. avium subsp. paratuberculosis and integrins on M cells.