A pathway for entry of reoviruses into the host through M cells of the respiratory tract

Genetic characterization and pathogenicity in a mouse model of newly isolated bat-originated mammalian orthoreovirus in South Korea

Mammalian orthoreoviruses (MRVs) infect a wide range of hosts, including humans, livestock, and wildlife. In the present study, we isolated a novel Mammalian orthoreovirus from the intestine of a microbat (Myotis aurascens) and investigated its biological and pathological characteristics. Phylogenetic analysis indicated that the new isolate was serotype 2, sharing the segments with those from different hosts. Our results showed that it can infect a wide range of cell lines from different mammalian species, including human, swine, and non-human primate cell lines. Additionally, media containing trypsin, yeast extract, and tryptose phosphate broth promoted virus propagation in primate cell lines and most human cell lines, but not in A549 and porcine cell lines. Mice infected with this strain via the intranasal route, but not via the oral route, exhibited weight loss and respiratory distress. The virus is distributed in a broad range of organs and causes lung damage. In vitro and in vivo experiments also suggested that the new virus could be a neurotropic infectious strain that can infect a neuroblastoma cell line and replicate in the brains of infected mice. Additionally, it caused a delayed immune response, as indicated by the high expression levels of cytokines and chemokines only at 14 days post-infection (dpi). These data provide an important understanding of the genetics and pathogenicity of mammalian orthoreoviruses in bats at risk of spillover infections.IMPORTANCEMammalian orthoreoviruses (MRVs) have a broad range of hosts and can cause serious respiratory and gastroenteritis diseases in humans and livestock. Some strains infect the central nervous system, causing severe encephalitis. In this study, we identified BatMRV2/SNU1/Korea/2021, a reassortment of MRV serotype 2, isolated from bats with broad tissue tropism, including the neurological system. In addition, it has been shown to cause respiratory syndrome in mouse models. The given data will provide more evidence of the risk of mammalian orthoreovirus transmission from wildlife to various animal species and the sources of spillover infections.

Immune Responses Induced by Recombinant Bacillus subtilis Expressing the PEDV Spike Protein Targeted at Microfold Cells

Bacillus subtilis (B. subtilis), a probiotic bacterium and feeding additive, is widely used for heterologous antigen expression and protective immunisation. Porcine epidemic diarrhoea virus (PEDV) invades swine via mucosal tissue. To enhance the mucosal immune response to PEDV, we modified B. subtilis to express a PEDV antigen and used it as a mucosal vaccine delivery system. Initially, we constructed a recombinant B. subtilis strain (B.s-RCL) that expressed the PEDV spike protein and L-Lectin-β-GF, with the goal of inducing mucosal secretory immunoglobulin A (sIgA) and anti-PEDV serum immunoglobulin G (IgG) production, as well as to increase the number of microfold cells (M cells). Following the oral administration of B.s-RCL to mice, the small intestinal PEDV-specific sIgA expression levels significantly increased, as well as the increased number of B.s-RCL adhered to M cells. Moreover, we found that mice administered B.s-RCL exhibited markedly higher percentages of CD4+ and CD8+ T cells in the mesenteric lymph nodes and spleen compared to the control mice. Furthermore, we found that intestinal mucosa sIgA and serum anti-PEDV IgG levels were higher in mice orally immunised with B.s-RCL, suggesting that the mice could be more resistant to PEDV. In this study, we developed a novel oral vaccine to prevent porcine diarrhoea epidemics.

An Unusual Aspartic Acid Cluster in the Reovirus Attachment Fiber σ1 Mediates Stability at Low pH and Preserves Trimeric Organization

The reovirus attachment protein σ1 mediates cell attachment and receptor binding and is thought to undergo conformational changes during viral disassembly. σ1 is a trimeric filamentous protein with an α-helical coiled-coil tail, a triple-β-spiral body, and a globular head. At the trimer interface, the head domain features an unusual and conserved aspartic acid cluster, which forms the only significant intratrimer interactions in the head and must be protonated to allow trimer formation. To define the role of pH on σ1 stability and conformation, we tested its domains over a wide range of pH values. We show that all domains of σ1 are remarkably thermostable, even at the low pH of the stomach. We determined the optimal pH for stability to be between pHs 5 and 6, a value close to the pH of the endosome and of the jejunum. The σ1 head is stable at acidic and neutral pH but detrimerizes at basic pH. When Asp³⁴⁵ in the aspartic acid cluster is mutated to asparagine (D345N), the σ1 head loses stability at low pH and is more prone to detrimerize. Although the D345N mutation does not affect σ1 binding affinity for the JAM-A receptor, the overall binding stoichiometry is reduced by one-third. The additional replacement of the neighboring His³⁴⁹ with alanine disrupts inner trimer surface interactions, leading to a less thermostable and monomeric σ1 D345N head that fails to bind the JAM-A receptor. When the body is expressed together with the head domain, the thermostability is restored and the stoichiometry of the binding to JAM-A receptor is preserved. Our results confirm a fundamental role of the aspartic acid cluster as a pH-dependent molecular switch controlling trimerization and enhancing thermostability of σ1, which represent essential requirements to accomplish reovirus infection and entry and might be common mechanisms among other enteric viruses. IMPORTANCE Enteric viruses withstand the highly acidic environment of the stomach during transmission, and many of them use low pH as a trigger for conformational changes associated with entry. For many nonenveloped viruses, the structural basis of these effects is not clear. We have investigated the stability of the reovirus attachment protein σ1 over a range of pHs and find it to be remarkably thermostable, especially at low pH. We identify a role for the aspartic acid cluster in maintaining σ1 thermostability, trimeric organization, and binding to JAM-A receptor especially at the gastric pH reovirus has to withstand while passing the stomach. The understanding of monomer-trimer dynamics within σ1 enhances our knowledge of reovirus entry and has implications for stability and transmission of other enteric viruses.

Polarized Entry of Human Parechoviruses in the Airway Epithelium

Human parechoviruses (HPeVs), a poorly studied genus within the Picornaviridae family, are classified into 19 genotypes of which HPeV1 and HPeV3 are the most often detected. HPeV1 VP1 C terminus contains an arginine-glycine-aspartic acid (RGD) motif and has been shown to depend on the host cell surface αV integrins (αV ITGs) and heparan sulfate (HS) for entry. HPeV3 lacks this motif and the receptors remain unknown. HPeVs can be detected in patient nasopharyngeal and stool samples, and infection is presumed to occur after respiratory or gastro-intestinal transmission. HPeV pathogenesis is poorly understood as there are no animal models and previous studies have been conducted in immortalized monolayer cell cultures which do not adequately represent the characteristics of human tissues. To bridge this gap, we determined the polarity of infection, replication kinetics, and cell tropism of HPeV1 and HPeV3 in the well-differentiated human airway epithelial (HAE) model. We found the HAE cultures to be permissive for HPeVs. Both HPeV genotypes infected the HAE preferentially from the basolateral surface while the progeny virus was shed toward the apical side. Confocal microscopy revealed the target cell type to be the p63⁺ basal cells for both viruses, αV ITG and HS blocking had no effect on the replication of either virus, and transcriptional profiling suggested that HPeV3 infection induced stronger immune activation than HPeV1. Genotype-specific host responses may contribute to the differences in pathogenesis and clinical outcomes associated with HPeV1 and HPeV3.

Detection of mammalian orthoreovirus type-3 (Reo-3) infections in mice based on serotype-specific hemagglutination protein sigma-1

Background

Reovirus type-3 infections cause severe pathologies in young mice and thus influence animal experiments in many ways. Therefore, the Federation of Laboratory Animal Science Associations (FELASA) recommends an annual screening in laboratory mice as part of a thorough health monitoring program. Based on the high protein sequence homology among the different reovirus serotypes, immunofluorescence antibody assay and other indirect methods relying on the whole virus are presumably cross-reactive to antibodies triggered by mammalian orthoreovirus infections independent of the serotype.

Methods

The serotype-specific protein σ-1 was expressed in Escherichia coli with an N-terminal Strep-tag and a C-terminal His-tag. The purified Strep-rσ-1-His-construct was used to develop an indirect ELISA by testing defined positive and negative sera obtained by experimental infection of mice as well as field sera.

Results

The Strep-rσ-1-His-ELISA provided high sensitivity and specificity during validation. Notably, a high selectivity was also observed for sera positively tested for other relevant FELASA-listed pathogens. Screening of field samples indicated that a commercial reovirus type-3-based ELISA might be cross-reactive to other murine reovirus serotypes and thus produces false-positive results.

Conclusions

The prevalence of reovirus type-3 might be overestimated in German animal facilities and most likely in other countries as well. The occurrence of other reovirus serotypes, however, raises the question if murine health monitoring programs should be extended to these pathogens.

Electronic supplementary material

The online version of this article (10.1186/s12985-018-1021-8) contains supplementary material, which is available to authorized users.

Impaired airway mucociliary function reduces antigen-specific IgA immune response to immunization with a claudin-4-targeting nasal vaccine in mice

Vaccine delivery is an essential element for the development of mucosal vaccine, but it remains to be investigated how physical barriers such as mucus and cilia affect vaccine delivery efficacy. Previously, we reported that C-terminal fragment of Clostridium perfringens enterotoxin (C-CPE) targeted claudin-4, which is expressed by the epithelium associated with nasopharynx-associated lymphoid tissue (NALT), and could be effective as a nasal vaccine delivery. Mice lacking tubulin tyrosine ligase-like family, member 1 (Ttll1-KO mice) showed mucus accumulation in nasal cavity due to the impaired motility of respiratory cilia. Ttll1-KO mice nasally immunized with C-CPE fused to pneumococcal surface protein A (PspA-C-CPE) showed reduced PspA-specific nasal IgA responses, impaired germinal center formation, and decreased germinal center B-cells and follicular helper T cells in the NALT. Although there was no change in the expression of claudin-4 in the NALT epithelium in Ttll1-KO mice, the epithelium was covered by a dense mucus that prevented the binding of PspA-C-CPE to NALT. However, administration of expectorant N-acetylcysteine removed the mucus and rescued the PspA-specific nasal IgA response. These results show that the accumulation of mucus caused by impaired respiratory cilia function is an interfering factor in the C-CPE-based claudin-4-targeting nasal vaccine.

Exploring Host-Commensal Interactions in the Respiratory Tract

Commensal microbes are currently in the limelight in biomedical research because they play an important role in health and disease. Humans harbor an enormous diversity of commensals in various parts of the body, including the gastrointestinal and respiratory tracts. Advancement in metagenomic and other omic approaches, and development of suitable animal models have provided an unprecedented appreciation into the diversity of commensals, and the intricacies of their intimate communication with the host immune system. Most studies have focused on the host–commensal interaction in the gut, while less is known on this relationship in other sites of the body, such as the respiratory tract. In this article, we review emerging data from human and animal studies on the host responses to respiratory commensals, immune cross-reactivity between commensals and pathogens, and use of commensals as a vaccine delivery system. A better understanding of the delicate interplay between commensals and host may aid in efforts to develop effective vaccines and therapeutics.

The Orchestra of Reovirus Cell Entry

Purpose of Review

The ability of viruses to infect host cells is dependent on several factors including the availability of cell-surface receptors, antiviral state of cells, and presence of host factors needed for viral replication. Here, we review findings from in vitro and in vivo studies using mammalian orthoreovirus (reovirus) that have identified an intricate group of molecules and mechanisms used by the virus to attach and enter cells.

Recent Findings

Recent findings provide an improved mechanistic understanding of reovirus cell entry. Of special note is the identification of a cellular mediator of cell entry in neuronal and non-neuronal cells, the effect of cell entry on the outcome of infection and cytopathic effects on the host cell, and an improved understanding of the components that promote viral penetration of cellular membranes.

Summary

A mechanistic understanding of the interplay between host and viral factors has enhanced our view of how viruses usurp cellular processes during infection.

Mechanisms of reovirus bloodstream dissemination

Many viruses cause disease within an infected host after spread from an initial portal of entry to sites of secondary replication. Viruses can disseminate via the bloodstream or through nerves. Mammalian orthoreoviruses (reoviruses) are neurotropic viruses that use both bloodborne and neural pathways to spread systemically within their hosts to cause disease. Using a robust mouse model and a dynamic reverse genetics system, we have identified a viral receptor and a viral nonstructural protein that are essential for hematogenous reovirus dissemination. Junctional adhesion molecule-A (JAM-A) is a member of the immunoglobulin superfamily expressed in tight junctions and on hematopoietic cells that serves as a receptor for all reovirus serotypes. Expression of JAM-A is required for infection of endothelial cells and development of viremia in mice, suggesting that release of virus into the bloodstream from infected endothelial cells requires JAM-A. Nonstructural protein σ1s is implicated in cell cycle arrest and apoptosis in reovirus-infected cells but is completely dispensable for reovirus replication in cultured cells. Surprisingly, a recombinant σ1s-null reovirus strain fails to spread hematogenously in infected mice, suggesting that σ1s facilitates apoptosis of reovirus-infected intestinal epithelial cells. It is possible that apoptotic bodies formed as a consequence of σ1s expression lead to reovirus uptake by dendritic cells for subsequent delivery to the mesenteric lymph node and the blood. Thus, both host and viral factors are required for efficient hematogenous dissemination of reovirus. Understanding mechanisms of reovirus bloodborne spread may shed light on how microbial pathogens invade the bloodstream to disseminate and cause disease in infected hosts.

Respiratory infection of mice with mammalian reoviruses causes systemic infection with age and strain dependent pneumonia and encephalitis

BACKGROUND

Because mammalian reoviruses are isolated from the respiratory tract we modeled the natural history of respiratory infection of adult and suckling mice with T1 Lang (T1L) and T3 Dearing (T3D) reoviruses.

METHODS

Adult and suckling Balb/c mice were infected by the intranasal route and were assessed for dose response of disease as well as viral replication in the lung and other organs. Viral antigen was assessed by immunofluorescence and HRP staining of tissue sections and histopathology was assessed on formalin fixed, H + E stained tissue sections.

RESULTS

Intranasal infection of adult mice resulted in fatal respiratory distress for high doses (10(7) pfu) of T1L but not T3D. In contrast both T1L and T3D killed suckling mice at moderate viral dosages (10(5) pfu) but differed in clinical symptoms where T1L induced respiratory failure and T3D caused encephalitis. Infections caused transient viremia that resulted in spread to peripheral tissues where disease correlated with virus replication, and pathology. Immunofluorescent staining of viral antigens in the lung showed reovirus infection was primarily associated with alveoli with lesser involvement of bronchiolar epithelium. Immunofluorescent and HRP staining of viral antigens in brain showed infection of neurons by T3D and glial cells by T1L.

CONCLUSIONS

These mouse models of reovirus respiratory infection demonstrated age and strain dependent disease that are expected to be relevant to understanding and modulating natural and therapeutic reovirus infections in humans.

Early target cells of measles virus after aerosol infection of non-human primates

Measles virus (MV) is highly infectious, and has long been thought to enter the host by infecting epithelial cells of the respiratory tract. However, epithelial cells do not express signaling lymphocyte activation molecule (CD150), which is the high-affinity cellular receptor for wild-type MV strains. We have generated a new recombinant MV strain expressing enhanced green fluorescent protein (EGFP), based on a wild-type genotype B3 virus isolate from Khartoum, Sudan (KS). Cynomolgus macaques were infected with a high dose of rMV^KSEGFP by aerosol inhalation to ensure that the virus could reach the full range of potential target cells throughout the entire respiratory tract. Animals were euthanized 2, 3, 4 or 5 days post-infection (d.p.i., n = 3 per time point) and infected (EGFP⁺) cells were identified at all four time points, albeit at low levels 2 and 3 d.p.i. At these earliest time points, MV-infected cells were exclusively detected in the lungs by fluorescence microscopy, histopathology and/or virus isolation from broncho-alveolar lavage cells. On 2 d.p.i., EGFP⁺ cells were phenotypically typed as large mononuclear cells present in the alveolar lumen or lining the alveolar epithelium. One to two days later, larger clusters of MV-infected cells were detected in bronchus-associated lymphoid tissue (BALT) and in the tracheo-bronchial lymph nodes. From 4 d.p.i. onward, MV-infected cells were detected in peripheral blood and various lymphoid tissues. In spite of the possibility for the aerosolized virus to infect cells and lymphoid tissues of the upper respiratory tract, MV-infected cells were not detected in either the tonsils or the adenoids until after onset of viremia. These data strongly suggest that in our model MV entered the host at the alveolar level by infecting macrophages or dendritic cells, which traffic the virus to BALT or regional lymph nodes, resulting in local amplification and subsequent systemic dissemination by viremia.

Measles remains an important vaccine-preventable cause of morbidity and mortality in developing countries. The causative agent, measles virus (MV), is one of the most contagious viruses known. Measles has an incubation time of approximately two weeks, and surprisingly little is known about the early events after MV infection. Epithelial cells in the upper respiratory tract have long been considered as early target cells, but more recently alveolar macrophages (AM) and dendritic cells (DC) have been proposed as alternatives. We have infected cynomolgus macaques with a high dose of a recombinant EGFP-expressing MV strain via aerosol inhalation, to ensure that the virus had access to the entire respiratory tract. At 2 days post-infection, MV-infected mononuclear cells were detectable in the alveolar lumen but not in the upper respiratory tract. These infected cells migrated through the bronchus-associated lymphoid tissue to the draining tracheo-bronchial lymph node at 3 days post-infection. Systemic infection was initiated from this point, as observed in macaques euthanized 4 or 5 days post-infection. Thus, even though the aerosolized virus had access to epithelial cells and lymphoid tissues along the entire respiratory tract, AM and DC in the lungs were the first cells that sustained MV replication.

Expression of endothelia and lymphocyte adhesion molecules in bronchus-associated lymphoid tissue (BALT) in adult human lung

Background

Bronchus-associated lymphoid tissue (BALT) is the secondary lymphoid tissue in bronchial mucosa and is involved in the development of bronchopulmonary immune responses. Although migration of lymphocytes from blood vessels into secondary lymphoid tissues is critical for the development of appropriate adaptive immunity, the endothelia and lymphocyte adhesion molecules that recruit specific subsets of lymphocytes into human BALT are not known. The aim of this study was to determine which adhesion molecules are expressed on lymphocytes and high endothelial venules (HEVs) in human BALT.

Methods

We immunostained frozen sections of BALT from lobectomy specimens from 17 patients with lung carcinoma with a panel of monoclonal antibodies to endothelia and lymphocyte adhesion molecules.

Results

Sections of BALT showed B cell follicles surrounded by T cells. Most BALT CD4⁺ T cells had a CD45RO⁺ memory phenotype. Almost all BALT B cells expressed α₄ integrin and L-selectin. In contrast, 43% of BALT T cells expressed α₄ integrin and 20% of BALT T cells expressed L-selectin. Almost all BALT lymphocytes expressed LFA-1. HEVs, which support the migration of lymphocytes from the bloodstream into secondary lymphoid tissues, were prominent in BALT. All HEVs expressed peripheral node addressin, most HEVs expressed vascular cell adhesion molecule-1, and no HEVs expressed mucosal addressin cell adhesion molecule-1.

Conclusion

Human BALT expresses endothelia and lymphocyte adhesion molecules that may be important in recruiting naive and memory/effector lymphocytes to BALT during protective and pathologic bronchopulmonary immune responses.

In vivo importance of heparan sulfate-binding glycoproteins for murid herpesvirus-4 infection

Many herpesviruses bind to heparan sulfate (HS). Murid herpesvirus-4 (MuHV-4) does so via its envelope glycoproteins gp70 and gH/gL. MuHV-4 gp150 further regulates an HS-independent interaction to make that HS-dependent too. Cell binding by MuHV-4 virions is consequently strongly HS-dependent. Gp70 and gH/gL show some in vitro redundancy: an antibody-mediated blockade of HS binding by one is well tolerated, whereas a blockade of both severely impairs infection. In order to understand the importance of HS binding for MuHV-4 in vivo, we generated mutants lacking both gL and gp70. As expected, gL(-)gp70(-) MuHV-4 showed very poor cell binding. It infected mice at high dose but not at low dose, indicating defective host entry. But once entry occurred, host colonization, which for MuHV-4 is relatively independent of the infection dose, was remarkably normal. The gL(-)gp70(-) entry deficit was much greater than that of gL(-) or gp70(-) single knockouts. And gp150 disruption, which allows HS-independent cell binding, largely rescued the gL(-)gp70(-) cell binding and host entry deficits. Thus, it appeared that MuHV-4 HS binding is important in vivo, principally for efficient host entry.

Reovirus preferentially infects the basolateral surface and is released from the apical surface of polarized human respiratory epithelial cells

Mammalian reoviruses infect respiratory and gastrointestinal epithelia and cause disease in neonates. Junctional adhesion molecule-A (JAM-A) is a serotype-independent receptor for reovirus. JAM-A localizes to tight junctions and contributes to paracellular permeability in polarized epithelia. To investigate the mechanisms of reovirus infection of polarized epithelial cells, we assessed reovirus replication, release, and spread after apical and basolateral adsorption to primary human airway epithelial cultures. Reovirus infection of human airway epithelia was more efficient after adsorption to the basolateral surface than after adsorption to the apical surface, and it was dependent on JAM-A. Reovirus binding to carbohydrate coreceptor sialic acid inhibited apical infection, which was partially ameliorated by treatment of the cultures with neuraminidase. Despite the preference for basolateral infection, reovirus was released from the apical surface of respiratory epithelia and did not disrupt tight junctions. These results establish the existence of an infectious circuit for reovirus in polarized human respiratory epithelial cells.

Regulation of polymeric immunoglobulin receptor expression by reovirus

Polymeric immunoglobulin receptor (pIgR) transcytoses dimeric IgA and IgA-coated immune complexes from the lamina propria across epithelia and into secretions. The effect of reovirus infection on regulation of pIgR expression in the human intestinal epithelial cell line HT-29 was characterized in this report. Both replication-competent and UV-inactivated reovirus at m.o.i. equivalents of 1-100 p.f.u. per cell upregulated pIgR mRNA by 24 h post-infection and intracellular pIgR protein was increased at 48 h following exposure to UV-inactivated virus. Binding of virus to HT-29 cells was required, as pre-incubating virus with specific antiserum, but not non-immune serum, inhibited reovirus-mediated pIgR upregulation. Endosomal acidification leading to uncoating of virus is a required step for pIgR upregulation, as ammonium chloride or bafilomycin A1 pre-treatment inhibited virus-induced pIgR upregulation. Inhibition experiments using the calpain inhibitor N-acetyl-leucyl-leucyl-norleucinal suggested that calpains are involved in reovirus-mediated pIgR upregulation. Upregulation of pIgR following virus infection appears to be an innate immune response against invading pathogens that could help the host clear infection effectively. Signalling induced by microbes and their products may serve to augment pIgR-mediated transcytosis of IgA, linking the innate and acquired immune responses to viruses.

Neutrophil elastase, an acid-independent serine protease, facilitates reovirus uncoating and infection in U937 promonocyte cells

BACKGROUND

Mammalian reoviruses naturally infect their hosts through the enteric and respiratory tracts. During enteric infections, proteolysis of the reovirus outer capsid protein sigma3 is mediated by pancreatic serine proteases. In contrast, the proteases critical for reovirus replication in the lung are unknown. Neutrophil elastase (NE) is an acid-independent, inflammatory serine protease predominantly expressed by neutrophils. In addition to its normal role in microbial defense, aberrant expression of NE has been implicated in the pathology of acute respiratory distress syndrome (ARDS). Because reovirus replication in rodent lungs causes ARDS-like symptoms and induces an infiltration of neutrophils, we investigated the capacity of NE to promote reovirus virion uncoating.

RESULTS

The human promonocyte cell line U937 expresses NE. Treatment of U937 cells with the broad-spectrum cysteine-protease inhibitor E64 [trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane] and with agents that increase vesicular pH did not inhibit reovirus replication. Even when these inhibitors were used in combination, reovirus replicated to significant yields, indicating that an acid-independent non-cysteine protease was capable of mediating reovirus uncoating in U937 cell cultures. To identify the protease(s) responsible, U937 cells were treated with phorbol 12-myristate 13-acetate (PMA), an agent that induces cellular differentiation and results in decreased expression of acid-independent serine proteases, including NE and cathepsin (Cat) G. In the presence of E64, reovirus did not replicate efficiently in PMA-treated cells. To directly assess the role of NE in reovirus infection of U937 cells, we examined viral growth in the presence of N-Ala-Ala-Pro-Val chloromethylketone, a NE-specific inhibitor. Reovirus replication in the presence of E64 was significantly reduced by treatment of cells with the NE inhibitor. Incubation of virions with purified NE resulted in the generation of infectious subviron particles that did not require additional intracellular proteolysis.

CONCLUSION

Our findings reveal that NE can facilitate reovirus infection. The fact that it does so in the presence of agents that raise vesicular pH supports a model in which the requirement for acidic pH during infection reflects the conditions required for optimal protease activity. The capacity of reovirus to exploit NE may impact viral replication in the lung and other tissues during natural infections.

Nasal-associated lymphoid tissue is a mucosal inductive site for virus-specific humoral and cellular immune responses

Peyer's patches are known as mucosal inductive sites for humoral and cellular immune responses in the gastrointestinal tract. In contrast, functionally equivalent structures in the respiratory tract remain elusive. It has been suggested that nasal-associated lymphoid tissue (NALT) might serve as a mucosal inductive site in the upper respiratory tract. However, typical signs of mucosal inductive sites like development of germinal center reactions after Ag stimulation and isotype switching of naive B cells to IgA production have not been directly demonstrated. Moreover, it is not known whether CTL can be generated in NALT. To address these issues, NALT was structurally and functionally analyzed using a model of intranasal infection of C3H mice with reovirus. FACS and histological analyses revealed development of germinal centers in NALT in parallel with generation and expansion of IgA(+) and IgG2a(+) B cells after intranasal reovirus infection. Reovirus-specific IgA was produced in both the upper respiratory and the gastrointestinal tract, whereas production of reovirus-specific IgG2a was restricted to NALT, submandibular, and mesenteric lymph nodes. Moreover, virus-specific CTL were detected in NALT. Limiting dilution analysis showed a 5- to 6-fold higher precursor CTL frequency in NALT compared with a cervical lymph node. Together these data provide direct evidence that NALT is a mucosal inductive site for humoral and cellular immune responses in the upper respiratory tract.

Respiratory reovirus 1/L induction of diffuse alveolar damage: a model of acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a clinical syndrome that is characterized by diffuse alveolar damage usually secondary to an intense host inflammatory response of the lung to a pulmonary or extrapulmonary infectious or noninfectious insult. In this report we describe a unique animal model in which CBA/J mice infected with reovirus serotype 1, strain Lang develop ARDS. This model recapitulates the histopathological changes observed in human ARDS, which consists of the overlapping phases of exudation including the formation of hyaline membranes, regeneration, and healing via resolution and/or repair with fibrosis. While the consequences of a number of infectious and noninfectious insults in various animal systems have been developed as models of human ARDS, they are models of acute lung injury and are of short-term duration. Therefore, they do not recapitulate all of the clinical and pathological phases observed in human ARDS. Thus, study of the cellular and molecular factors involved in these distinct phases of the disease have been limited. Reovirus 1/L infection of CBA/J mice will allow investigations of the pathophysiology of ARDS as it progresses from the initial stages of edema and neutrophilia to fibrotic lesion development in late stages.

Epithelial M cells: differentiation and function

M cells are distinctive epithelial cells that occur only in the follicle-associated epithelia that overlie organized mucosa-associated lymphoid tissues. They are structurally and functionally specialized for transepithelial transport, delivering foreign antigens and microorganisms to organized lymphoid tissues within the mucosae of the small and large intestines, tonsils and adenoids, and airways. M cell transport is a double-edged sword: Certain pathogens exploit the features of M cells that are intended to promote uptake for the purpose of immunological sampling. Eludication of the molecular architecture of M cell apical surfaces is important for understanding the strategies that pathogens use to exploit this pathway and for utilizing M cell transport for delivery of vaccines to the mucosal immune system. This article reviews the functional and biochemical features that distinguish M cells from other intestinal cell types. In addition it synthesizes the available information on development and differentiation of organized lymphoid tissues and the specialized epithelium associated with these immune inductive sites.

Ultrastructural study on the follicle-associated epithelium of nasal-associated lymphoid tissue in specific pathogen-free (SPF) and conventional environment-adapted (SPF-CV) rats

Membranous (M) cells in follicle-associated epithelium (FAE) play an important role in the mucosal immunity through transport of a variety of foreign antigens to the underlying mucosa-associated lymphoid tissue (MALT). We aimed to investigate the ultrastructure of M cells in the FAE covering nasal-associated lymphoid tissue (NALT) both in specific pathogen-free (SPF) rats and in conventional environment-adapted (SPF-CV) rats aged 8-38 wk. In NALT of both SPF and SPF-CV rats, FAE included the nonciliated microvillous cell, which appears to be an analogue of M cell previously described in other MALT. In SPF rats, M cells increased in number only slightly with age, and they maintained morphological uniformity irrespective of age. In SPF-CV rats, M cells selectively increased in number resulting in prominent expansion of FAE surface area in parallel with the duration of maintenance in a conventional environment. In addition, M cells in SPF-CV rats showed heterogeneity in their surface morphology such as the length and number of microvilli and cell surface area and outline. In addition, the FAE was stratified by various subtypes of M cells, which were characterised by several subcellular alterations including the presence of many keratin filaments, homogeneous dark bodies and extensive cytoplasmic interfoliation with wide intercellular spaces filled with amorphous proteinaceous material. These characteristics of M cells in SPF-CV rat were intimately related with a preferential influx of immunocompetent cells into the FAE, which was not seen or was very rare in SPF rats irrespective of age. The results suggest the possibility that NALT may effectively carry out the mucosal immune response against antigenic stimuli of different magnitude through the unique dynamics of M cells which seem to be influenced by the infiltration of immunocompetent cells.

Respiratory mucosal immunization with reovirus serotype 1/L stimulates virus-specific humoral and cellular immune responses, including double-positive (CD4(+)/CD8(+)) T cells

Respiratory virus infections are a serious health challenge. A number of models that examine the nature of the respiratory immune response to particular pathogens exist. However, many pathogens that stimulate specific immunity in the lung are frequently not effective immunogens at other mucosal sites. A pathogen that is an effective respiratory as well as gastrointestinal immunogen would allow studies of the interaction between the mucosal sites. Reovirus (respiratory enteric orphan virus) serotype 1 is known to be an effective gut mucosal immunogen and provides a potential model for the relationship between the respiratory and the gut mucosal immune systems. In this study, we demonstrate that intratracheal immunization with reovirus 1/Lang (1/L) in C3H mice resulted in high titers of virus in the respiratory tract-associated lymphoid tissue (RALT). High levels of reovirus-specific immunoglobulin A were determined in the RALT fragment cultures. The major responding components of the bronchus-associated lymphoid tissue were the CD8(+) T lymphocytes. Cells from draining lymph nodes also exhibited lysis of reovirus-infected target cells after an in vitro culture. The present study also describes the distribution of transiently present CD4(+)/CD8(+) double-positive (DP) T cells in the mediastinal and tracheobronchial lymph nodes of RALT. CD4(+)/CD8(+) DP lymphocytes were able to proliferate in response to stimulation with viral antigen in culture. Furthermore, these cells exhibited lysis of reovirus-infected target cells after in vitro culture. These results establish reovirus 1/L as a viable model for future investigation of the mucosal immune response in the RALT and its relationship to the common mucosal immune system.

The M cell as a portal of entry to the lung for the bacterial pathogen Mycobacterium tuberculosis

M. tuberculosis accesses the terminal lung and is phagocytosed by alveolar macrophages. Utilizing a mouse intratracheal challenge model, we demonstrate that M. tuberculosis rapidly enters through M cells as well. From there, bacilli are deposited within associated intraepithelial leukocytes and subsequently conveyed to the draining lymph nodes early after infection. Osteopetrotic (Csfm(op)/Csfm(op)) mice, null mutants for macrophage colony-stimulating factor, possess diminished numbers of circulating monocytes and tissue macrophages. Csfm(op)/Csfm(op) mice were highly susceptible to challenge with M. tuberculosis. In contrast to controls, tubercle bacilli were not conveyed to draining lymph nodes early after infection but were instead retained within the mucosa. These results indicate that M cells represent an alternate portal of entry for M. tuberculosis, which may contribute to the rapid development of protective lung immune responses.

M cells at locations outside the gut

Lymphoid tissue associated with mucosal membranes is found not only along the gastrointestinal tract, but also in the tonsils, the upper and lower airways, and the conjunctiva of the eye. The epithelia overlying this mucosa-associated lymphoid tissue (MALT) contain membranous (M) cells which transport antigenic matter across the mucosal membrane to initiate immune responses. Although the morphology and function of intestinal M cells have been thoroughly studied, relatively little is known about the presence and properties of M cells in MALT outside the gut. The available data on ultrastructure, histochemistry, and antigen sampling function of the epithelia in tonsils, nasal-, larynx-, bronchus-, and conjunctiva-associated lymphoid tissue are reviewed and critically discussed. It is concluded that, in principle, the concepts of mucosal immune protection can be applied to these sites of MALT. However, it is questionable whether a separate cell type similar to intestinal M cells exists and performs antigen sampling in the different MALT epithelia. Further studies combining functional and morphological techniques are essential to understand the initiation of immune reaction at the mucosal membranes.

The composition and function of M cell apical membranes: implications for microbial pathogenesis

M cells, an epithelial cell phenotype that occurs only over organized mucosal lymphoid follicles, deliver samples of foreign material by transepithelial transport from the lumen to organized lymphoid tissues within the mucosa of the small and large intestines. The apical membranes of M cells in the intestine are designed to facilitate adherence and uptake of antigens and microorganisms, a prerequisite for immunological sampling. The molecular features of M cell apical surfaces that promote adherence and transport are crucial for understanding the strategies that pathogens use to exploit this pathway.

Prolonged induction of IL-8 gene expression in a human fibroblast cell line infected with reovirus serotype 1 strain Lang

Viruses which infect mucosal surfaces commonly infect these particular anatomical sites based on both the virion structure and the interaction of the virus with a particular microenvironment. We infected a human lung epithelial cell line, a human gut epithelial cell line, and a human lung fibroblast cell line with reovirus 1/L to explore how this natural isolate of both the lung and the gut may interact with mucosal surfaces. While reovirus infection of the gut and lung epithelial cell lines was lytic, a chronic infection was established in the human lung fibroblast cell line. All three cell lines also produced interleukin-8 (IL-8) after infection with reovirus 1/L, and IL-8 production was not dependent upon viral replication. A prolonged production of IL-8 was observed in the chronically infected lung fibroblast cell line, suggesting that this mucosal population may be involved in the generation of inflammatory responses after the resolution of the initial lytic infection of the epithelium. These studies provide an in vitro model system for analyzing the interaction of reovirus 1/L with resident mucosal cell populations.

Bronchus-associated lymphoid tissue (BALT) and larynx-associated lymphoid tissue (LALT) are found at different frequencies in children, adolescents and adults

The lung in 98 and the larynx in 51 consecutive autopsies (age: 17th gestational week to 99 years) were studied for the presence of organized lymphoid tissue in the epiglottis and in the wall of larger bronchi. Bronchus-associated lymphoid tissue (BALT) was seen in about 40% of patients younger than 20 years of age but in older patients only in exceptional cases. In the wall of the epiglottis, however, larynx-associated lymphoid tissue (LALT) was found at a frequency of approximately 80% in patients younger than 20 years and in 56% of the patients older than 20 years. The clinical relevance of LALT as a physiological entry site for antigens or for vaccination protocols using aerosols needs to be studied in further experiments.

Antigen sampling across epithelial barriers and induction of mucosal immune responses

Epithelial barriers on mucosal surfaces at different sites in the body differ dramatically in their cellular organization, and antigen sampling strategies at diverse mucosal sites are adapted accordingly. In stratified and pseudostratified epithelia, dendritic cells migrate to the outer limit of the epithelium, where they sample antigens for subsequent presentation in local or distant organized lymphoid tissues. In simple epithelia, specialized epithelial M cells (a phenotype that occurs only in the epithelium over organized lymphoid follicles) deliver samples of foreign material by transepithelial transport from the lumen to organized lymphoid tissues within the mucosa. Certain pathogens exploit the M cell transport process to cross the epithelial barrier and invade the mucosa. Here we review the features of M cells that determine antigen and pathogen adherence and transport into mucosal lymphoid tissues.

Reovirus infection in rat lungs as a model to study the pathogenesis of viral pneumonia

We undertook the present study to elucidate the pathogenesis of the pathologic response to reovirus infection in the lungs and further understand the interactions of reoviruses with pulmonary cells. We found that reoviruses were capable of causing acute pneumonia in 25- to 28-day-old Sprague-Dawley rats following intratracheal inoculation with the reoviruses type 1 Lang (T1L) and type 3 Dearing (T3D). The onset of the pneumonia was rapid, marked by type I alveolar epithelial cell degeneration, type II alveolar epithelial cell hyperplasia, and the infiltration of leukocytes into the alveolar spaces. More neutrophils were recruited into the lungs during T3D infection than during T1L infection, and the serotype difference in the neutrophil response was mapped to the S1 gene of reovirus. Viral replication in the lungs was required for the development of pneumonia due to T1L and T3D infections, and replication occurred in type I alveolar epithelial cells. T1L grew to higher titers in the lungs than did either T3D or type 3 clone 9, and the S1 gene was found to play a role in determining the level of viral replication. We propose that experimental reovirus infection in the lungs can serve as a model for the pathogenesis of viral pneumonia in which pulmonary inflammation results following direct infection of lung epithelial cells.

M cells in Peyer's patches of the intestine

M cells are specialized epithelial cells of the mucosa-associated lymphoid tissues. A characteristic of M cells is that they transport antigens from the lumen to cells of the immune system, thereby initiating an immune response or tolerance. Soluble macromolecules, small particles, and also entire microorganisms are transported by M cells. The interactions of these substances with the M cell surface, their transcytosis, and the role of associated lymphoid cells are reviewed in detail. The ultrastructure and several immuno- and lectin-histochemical properties of M cells vary according to species and location along the intestine. We present updated reports on these variations, on identification markers, and on the origin and differentiation of M cells. The immunological significance of M cells and their functional relationship to lymphocytes and antigenpresenting cells are critically reviewed. The current knowledge on M cells in mucosa-associated lymphoid tissues outside the gut is briefly outlined. Clinical implications for drug deliver, infection, and vaccine development are discussed.

Lymphocytes in the lung: an often neglected cell. Numbers, characterization and compartmentalization

The lung is continuously in contact with inhaled particles, some of which are of microbial origin. This requires adequate defence mechanisms in the form of immune reactions. These can be subdivided into the afferent and efferent limb. Specific immune reactions depend on the interactions between lymphoid and accessory cells. Therefore, the local histotopographic localization of lymphocyte subsets has to be known to understand pulmonary immune reactions. As lymphocytes have often not been mentioned when cells in the respiratory tract have been characterized, their compartmentalization, number and subset composition in the lung are outlined here. Lymphocytes are found in the epithelium and lamina propria of the bronchi with different subset compositions. In some species, like the rabbit, bronchus-associated lymphoid tissue (BALT) is found as follicle-like aggregations with lymphocytes infiltrating the epithelium, which shows specialized epithelial cells. BALT, however, is not a constitutive structure in all species, e.g. in humans. Nevertheless, certain (probably) microbial stimuli can induce BALT in adult humans. In contrast to many other organs, the lung vascular bed contains large numbers of lymphocytes. Little is known about the adhesion molecules that make this margination possible. In the lung interstitium about 10 x 10(9) lymphocytes have been calculated for healthy adults. The most easily accessible pool of lymphocytes in the human lung are those recovered by bronchoalveolar lavage. The vast majority of such lymphocytes express markers typical for "memory lymphocytes". The intrapulmonary migratory routes of lymphocytes and the integration of the lung in the common mucosal immune system are described.(ABSTRACT TRUNCATED AT 250 WORDS)