Antigen-nonspecific recruitment of Th2 cells to the lung as a mechanism for viral infection-induced allergic asthma

Asthma Biologics Across the T2 Spectrum of Inflammation in Severe Asthma: Biomarkers and Mechanism of Action

Airway inflammation, a hallmark feature of asthma, drives many canonical features of the disease, including airflow limitation, mucus plugging, airway remodeling, and hyperresponsiveness. The T2 inflammatory paradigm is firmly established as the dominant mechanism of asthma pathogenesis, largely due to the success of inhaled corticosteroids and biologic therapies targeting components of the T2 pathway, including IL-4, IL-5, IL-13, and thymic stromal lymphopoietin (TSLP). However, up to 30% of patients may lack signatures of meaningful T2 inflammation (ie, T2 low). In T2-low asthma patients, T2 inflammation may be masked due to anti-inflammatory treatments or may be highly variable depending on exposure to common asthma triggers such as allergens, respiratory infections, and smoke or pollution. The epithelium and epithelial cytokines (TSLP, IL-33) are increasingly recognized as upstream drivers of canonical T2 pathways and as modulators of various effector cells, including mast cells, eosinophils, and neutrophils, which impact the pathological manifestations of airway smooth muscle hypertrophy, hypercontractility, and airway hyperresponsiveness. Approved biologics for severe asthma target several distinct mechanisms of action, leading to differential effects on the spectrum of T2 inflammation, inflammatory biomarkers, and treatment efficacy (reducing asthma exacerbations, improving lung function, and diminishing symptoms). The approved anti-asthma biologics primarily target T2 immune pathways, with little evidence suggesting a benefit of targeting non-T2 asthma-associated mediators. Indeed, many negative results challenge current assumptions about the etiology of non-T2 asthma and raise doubts about the viability of targeting popular alternative inflammatory pathways, such as T17. Novel data have emerged from the use of biologics to treat various inflammatory mediators and have furthered our understanding of pathogenic mechanisms that drive asthma. This review discusses inflammatory pathways that contribute to asthma, quantitatively outlines effects of available biologics on biomarkers, and summarizes data and challenges from clinical trials that address non-T2 mechanisms of asthma.

Early-immune development in asthma: A review of the literature

This review presents a comprehensive examination of the various factors contributing to the immunopathogenesis of asthma from the prenatal to preschool period. We focus on the contributions of genetic and environmental components as well as the role of the nasal and gut microbiome on immune development. Predisposing genetic factors, including inherited genes associated with increased susceptibility to asthma, are discussed alongside environmental factors such as respiratory viruses and pollutant exposure, which can trigger or exacerbate asthma symptoms. Furthermore, the intricate interplay between the nasal and gut microbiome and the immune system is explored, emphasizing their influence on allergic immune development and response to environmental stimuli. This body of literature underscores the necessity of a comprehensive approach to comprehend and manage asthma, as it emphasizes the interactions of multiple factors in immune development and disease progression.

T-cell responses in asthma exacerbations

OBJECTIVE

Asthma is a chronic lung disease comprising multiple endotypes and characterized by periodic exacerbations. A diverse array of T cells has been found to contribute to all endotypes of asthma in pathogenic and regulatory roles. Here, we review the contributions of CD4+, CD8+, and unconventional T cells in allergic and nonallergic asthma.

DATA SOURCES

Review of published literature pertaining to conventional and unconventional T-cell types in asthma.

STUDY SELECTIONS

Recent peer-reviewed articles pertaining to T cells in asthma, with additional peer-reviewed studies for context.

RESULTS

Much research in asthma has focused on the roles of CD4+ TH cells. Roles for TH2 cells in promoting allergic asthma pathogenesis have been well-described, and the recent description of pathogenic TH2A cells provides additional insight into these responses. Other TH types, notably TH1 and TH17, have been linked to neutrophilic and steroid-resistant asthma phenotypes. Beyond CD4+ T cells, CD8+ Tc2 cells are also strongly associated with allergic asthma. An emerging area for study is unconventional T-cell types, including γδT, invariant natural killer T, and mucosal-associated invariant T cells. Although data in asthma remain limited for these cells, their ability to bridge innate and adaptive responses likely makes them key players in asthma. A number of asthma therapies target T-cell responses, and, although data are limited, they seem to modulate T-cell populations.

CONCLUSION

Given the diversity and heterogeneity of asthma and T-cell responses, there remain many rich avenues for research to better understand the pathogenesis of asthma. Despite the breadth of T cells in asthma, approved therapeutics remain limited to TH2 networks.

Distinct spatial and temporal roles for Th1, Th2, and Th17 cells in asthma

Immune response in the asthmatic respiratory tract is mainly driven by CD4⁺ T helper (Th) cells, represented by Th1, Th2, and Th17 cells, especially Th2 cells. Asthma is a heterogeneous and progressive disease, reflected by distinct phenotypes orchestrated by τh2 or non-Th2 (Th1 and Th17) immune responses at different stages of the disease course. Heterogeneous cytokine expression within the same Th effector state in response to changing conditions in vivo and interlineage relationship among CD4⁺ T cells shape the complex immune networks of the inflammatory airway, making it difficult to find one panacea for all asthmatics. Here, we review the role of three T helper subsets in the pathogenesis of asthma from different stages, highlighting timing is everything in the immune system. We also discuss the dynamic topography of Th subsets and pathogenetic memory Th cells in asthma.

Evaluation of the Relationships between Intestinal Regional Lymph Nodes and Immune Responses in Viral Infections in Children

Viral infections increase the risk of developing allergies in childhood, and disruption of mucosal homeostasis is presumed to be involved. However, no study has reported a role for viral infections in such disruption. In this study, we clarified the mechanism of immunoglobulin A (IgA) overproduction in viral infections. Autopsies were performed on 33 pediatric cases, IgA and interferon (IFN)β levels were measured, and histopathological and immunohistochemical examinations were conducted. Furthermore, we cultured human cells and measured IFNβ and IgA levels to examine the effect of viral infections on IgA production. Blood IgA levels in viral infections were higher than in bacterial infections. Moreover, IFNβ levels in most viral cases were below the detection limit. Cell culture revealed increased IgA in gastrointestinal lymph nodes, especially in Peyer’s patches, due to enhanced IFNβ after viral stimulation. Conversely, respiratory regional lymph nodes showed enhanced IgA with no marked change in IFNβ. Overproduction of IgA, identified as an aberration of the immune system and resulting from excessive viral infection-induced IFNβ was observed in the intestinal regional lymph nodes, particularly in Peyer’s patches. Further, increased IgA without elevated IFNβ in the respiratory system suggested the possibility of a different mechanism from the gastrointestinal system.

Human TH1 and TH2 cells targeting rhinovirus and allergen coordinately promote allergic asthma

BACKGROUND

Allergic asthmatic subjects are uniquely susceptible to acute wheezing episodes provoked by rhinovirus. However, the underlying immune mechanisms and interaction between rhinovirus and allergy remain enigmatic, and current paradigms are controversial.

OBJECTIVE

We sought to perform a comprehensive analysis of type 1 and type 2 innate and adaptive responses in allergic asthmatic subjects infected with rhinovirus.

METHODS

Circulating virus-specific T_H1 cells and allergen-specific T_H2 cells were precisely monitored before and after rhinovirus challenge in allergic asthmatic subjects (total IgE, 133-4692 IU/mL; n = 28) and healthy nonallergic controls (n = 12) using peptide/MHCII tetramers. T cells were sampled for up to 11 weeks to capture steady-state and postinfection phases. T-cell responses were analyzed in parallel with 18 cytokines in the nose, upper and lower airway symptoms, and lung function. The influence of in vivo IgE blockade was also examined.

RESULTS

In uninfected asthmatic subjects, higher numbers of circulating virus-specific PD-1⁺ T_H1 cells, but not allergen-specific T_H2 cells, were linked to worse lung function. Rhinovirus infection induced an amplified antiviral T_H1 response in asthmatic subjects versus controls, with synchronized allergen-specific T_H2 expansion, and production of type 1 and 2 cytokines in the nose. In contrast, T_H2 responses were absent in infected asthmatic subjects who had normal lung function, and in those receiving anti-IgE. Across all subjects, early induction of a minimal set of nasal cytokines that discriminated high responders (G-CSF, IFN-γ, TNF-α) correlated with both egress of circulating virus-specific T_H1 cells and worse symptoms.

CONCLUSIONS

Rhinovirus induces robust T_H1 responses in allergic asthmatic subjects that may promote disease, even after the infection resolves.

T cells in severe childhood asthma

Severe asthma in children is a debilitating condition that accounts for a disproportionately large health and economic burden of asthma. Reasons for the lack of a response to standard anti-inflammatory therapies remain enigmatic. Work in the last decade has shed new light on the heterogeneous nature of asthma, and the varied immunopathologies of severe disease, which are leading to new treatment approaches for the individual patient. However, most studies to date that explored the immune landscape of the inflamed lower airways have focused on adults. T cells are pivotal to the inception and persistence of inflammatory processes in the diseased lungs, despite a contemporary shift in focus to immune events at the epithelial barrier. This article outlines current knowledge on the types of T cells and related cell types that are implicated in severe asthma. The potential for environmental exposures and other inflammatory cues to condition the immune environment of the lung in early life to favour pathogenic T cells and steroid resistance is discussed. The contributions of T cells and their cytokines to inflammatory processes and treatment resistance are also considered, with an emphasis on new observations in children that argue against conventional type 1 and type 2 T cell paradigms. Finally, the ability for new technologies to revolutionize our understanding of T cells in severe childhood asthma, and to guide future treatment strategies that could mitigate this disease, is highlighted.

Dendritic Cells and T Cells Interact Within Murine Afferent Lymphatic Capillaries

Afferent lymphatic vessels contribute to immunity by transporting antigen and leukocytes to draining lymph nodes (LNs) and are emerging as new players in the regulation of peripheral tolerance. Performing intravital microscopy in inflamed murine ear skin we found that migrating dendritic cells (DCs) and antigen-experienced effector T cells spend considerable time arresting or clustering within afferent lymphatic capillaries. We also observed that intralymphatic T cells frequently interacted with DCs. When imaging polyclonal T cells during an ongoing contact-hypersensitivity response, most intralymphatic DC-T cell interactions were short-lived. Conversely, during a delayed-type-hypersensitivity response, cognate antigen-bearing DCs engaged in long-lived MHCII-(I-A/I-E)-dependent interactions with antigen-specific T cells. Long-lived intralymphatic DC-T cell interactions reduced the speed of DC crawling but did not delay overall DC migration to draining LNs. While further consequences of these intralymphatic interactions still need to be explored, our findings suggest that lymphatic capillaries represent a unique compartment in which adaptive immune interaction and modulation occur.

Intestinal helminth infection promotes IL-5- and CD4+ T cell-dependent immunity in the lung against migrating parasites

The ability of helminths to manipulate the immune system of their hosts to ensure their own survival is often credited with affecting responses to other pathogens. We undertook co-infection experiments in mice to determine how infection with the intestinal helminth Heligmosomoides polygyrus affected the parasitological, immunological and physiological outcomes of a primary infection with a distinct species of helminth; the lung migratory parasite Nippostrongylus brasiliensis. We found that migrating N. brasiliensis larvae were killed in the lungs of H. polygyrus-infected mice by a process involving IL-33-activated CD4⁺ T cells that released IL-5 and recruited activated eosinophils. The lung pathology normally associated with N. brasiliensis larval migration was also reduced. Importantly, lung immunity remained intact in mice cleared of prior H. polygyrus infection and also occurred during infection with another entirely enteric helminth, Trichuris muris. This study identifies a cross-mucosal immune mechanism by which intestinal helminths may protect their hosts against co-infection by a different parasite at a distal site, via circulation of activated CD4⁺ T cells that can be triggered to release effector cytokines and mount inflammatory responses by tissue damage-associated alarmins, such as IL-33.

Small Animal Models of Respiratory Viral Infection Related to Asthma

Respiratory viral infections are strongly associated with asthma exacerbations. Rhinovirus is most frequently-detected pathogen; followed by respiratory syncytial virus; metapneumovirus; parainfluenza virus; enterovirus and coronavirus. In addition; viral infection; in combination with genetics; allergen exposure; microbiome and other pathogens; may play a role in asthma development. In particular; asthma development has been linked to wheezing-associated respiratory viral infections in early life. To understand underlying mechanisms of viral-induced airways disease; investigators have studied respiratory viral infections in small animals. This report reviews animal models of human respiratory viral infection employing mice; rats; guinea pigs; hamsters and ferrets. Investigators have modeled asthma exacerbations by infecting mice with allergic airways disease. Asthma development has been modeled by administration of virus to immature animals. Small animal models of respiratory viral infection will identify cell and molecular targets for the treatment of asthma.

Association of specific viral infections with childhood asthma exacerbations

Introduction

Asthma exacerbations may occur due to a variety of triggers including respiratory viruses. The aim of this study was to determine the role of particular viral infections in asthma exacerbations in children.

Materials and methods

The study was performed at Dr. Daneshvari Hospital Pediatric Emergency Department, Shahid Beheshti University of Medical Sciences, Tehran, Iran between 2014 and 2015. A nasopharyngeal aspirate or swab was obtained from each patient during admission. All samples were maintained at 4 °C until submission to the virology laboratory and were tested for respiratory viruses by nucleic acid testing.

Results

A total of 60 patients with asthma exacerbations were recruited for this study. Of the 60 samples collected from the patients with acute asthma exacerbations, rhinovirus was detected in 12 patients (20%), respiratory syncytial virus in 5 (8%), adenovirus in 5 (8%), and influenza virus in 1 (1.6%). Respiratory pathogens were not detected in 37 (61%) samples. All the samples investigated showed single viral infection.

Conclusions

To conclude, the most common viruses detected were rhinovirus followed by respiratory syncytial virus (RSV) and adenovirus. RSV was more commonly associated with more severe attacks. Both the study design (e.g., time of sampling, age of the patients, etc.) and also the method used for viral detection influence the frequency of detection of the respiratory viruses.

Allergic airway inflammation: key players beyond the Th2 cell pathway

Allergic asthma is characterized by eosinophilic airway inflammation, mucus hyperproduction, and airway hyperreactivity, causing reversible airway obstruction. Accumulating evidence indicates that antigen-specific Th2 cells and their cytokines such as IL-4, IL-5, and IL-13 orchestrate these pathognomonic features of asthma. However, over the past decade, the understanding of asthma pathogenesis has made a significant shift from a Th2 cell-dependent, IgE-mediated disease to a more complicated heterogeneous disease. Recent studies clearly show that not only Th2 cytokines but also other T cell-related cytokines such as IL-17A and IL-22 as well as epithelial cell cytokines such as IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) are involved in the pathogenesis of asthma. In this review, we focus on the roles of these players beyond Th2 pathways in the pathogenesis of asthma.

The immunology of the allergy epidemic and the hygiene hypothesis

The immunology of the hygiene hypothesis of allergy is complex and involves the loss of cellular and humoral immunoregulatory pathways as a result of the adoption of a Western lifestyle and the disappearance of chronic infectious diseases. The influence of diet and reduced microbiome diversity now forms the foundation of scientific thinking on how the allergy epidemic occurred, although clear mechanistic insights into the process in humans are still lacking. Here we propose that barrier epithelial cells are heavily influenced by environmental factors and by microbiome-derived danger signals and metabolites, and thus act as important rheostats for immunoregulation, particularly during early postnatal development. Preventive strategies based on this new knowledge could exploit the diversity of the microbial world and the way humans react to it, and possibly restore old symbiotic relationships that have been lost in recent times, without causing disease or requiring a return to an unhygienic life style.

Effects of Allergic Sensitization on Antiviral Immunity: Allergen, Virus, and Host Cell Mechanisms

PURPOSE OF REVIEW

Multiple clinical and epidemiological studies demonstrate links between allergic sensitization and virus-induced atopic disease exacerbations. This review summarizes the recent findings regarding allergen, viral, and host cellular mechanisms relevant to these observations.

RECENT FINDINGS

Recent studies have focused on the molecular pathways and genetic influences involved in allergen-mediated inhibition of innate antiviral immune responses. Multiple tissue and cell types from atopic individuals across the atopy spectrum exhibit deficient interferon responses to a variety of virus infections. Impairment in barrier function, viral RNA and DNA recognition by intracellular sensing molecules, and dysregulation of signaling components are broadly affected by allergic sensitization. Finally, genetic predisposition by numerous nucleotide polymorphisms also impacts immune pathways and potentially contributes to virus-associated atopic disease pathogenesis. Allergen-virus interactions in the setting of atopy involve complex tissue and cellular mechanisms. Future studies defining the pathways underlying these interactions could uncover potential therapeutic targets. Available data suggest that therapies tailored to restore specific components of antiviral responses will likely lead to improved clinical outcomes in allergic disease.

T Cell Trafficking through Lymphatic Vessels

T cell migration within and between peripheral tissues and secondary lymphoid organs is essential for proper functioning of adaptive immunity. While active T cell migration within a tissue is fairly slow, blood vessels and lymphatic vessels (LVs) serve as speedy highways that enable T cells to travel rapidly over long distances. The molecular and cellular mechanisms of T cell migration out of blood vessels have been intensively studied over the past 30 years. By contrast, less is known about T cell trafficking through the lymphatic vasculature. This migratory process occurs in one manner within lymph nodes (LNs), where recirculating T cells continuously exit into efferent lymphatics to return to the blood circulation. In another manner, T cell trafficking through lymphatics also occurs in peripheral tissues, where T cells exit the tissue by means of afferent lymphatics, to migrate to draining LNs and back into blood. In this review, we highlight how the anatomy of the lymphatic vasculature supports T cell trafficking and review current knowledge regarding the molecular and cellular requirements of T cell migration through LVs. Finally, we summarize and discuss recent insights regarding the presumed relevance of T cell trafficking through afferent lymphatics.

Genes associated with RSV lower respiratory tract infection and asthma: the application of genetic epidemiological methods to understand causality

Infants with respiratory syncytial virus (RSV) lower respiratory tract infections (LRIs) are at increased risk for childhood asthma. The objectives of this article are to review the genes associated with both RSV LRI and asthma, review analytic approaches to assessing shared genetic risk and propose a future perspective on how these approaches can help us to understand the role of infant RSV infection as both an important risk factor for asthma and marker of shared genetic etiology between the two conditions. The review of shared genes and thus pathways associated with severity of response to RSV infection and asthma risk can help us to understand mechanisms of disease and ultimately propose new and novel targets for primary prevention of both diseases.

Viral infections and the development of asthma in children

Viral aetiology, host susceptibility (in particular allergic predisposition and sensitization), and illness severity, timing and frequency all appear to contribute as synergistic factors to the risk of developing asthma. Experimental models have shown both innate and adaptive immune responses contribute to this risk with lung inflammatory cells showing marked differences in phenotype and function in young compared with older animals, and these differences are further enhanced following virus infection. Findings to date strongly suggest that the impact of infant and preschool viral infections on the maturing immune system and developing lung that subsequently result in an asthma phenotype occur during a critical susceptibility period, and in a genetically susceptible host. There are currently no therapeutic strategies that allow primary or secondary prevention of asthma following early life viral respiratory infections in high-risk children, thus a focus on understanding the mechanisms of progression from viral wheezing in infants and preschool children to asthma development are urgently needed. This review summarizes the data reporting the role of the two most common viruses, that is, respiratory syncytial virus and human rhinovirus, that result in asthma development, comparing risk factors for disease progression, and providing insight into strategies that might be adopted to prevent asthma development.

Role of viruses in the development of atopic disease in pediatric patients

The prevalence of atopic diseases continues to rise in modernized countries, without a clear explanation for this increase. One potential cause identified from epidemiologic studies of children is respiratory RNA viral infections leading to development of recurrent wheezing, asthma, and allergic sensitization. We review human epidemiologic data that both support and refute the role of viruses in this process. Exploring recent murine models, we document possible immunologic mechanisms that could translate a viral infection into atopic disease. We further discuss evidence for a post-viral "atopic cycle" that could explain the development of multiple allergen sensitization, and we explore available data to suggest a connection between viral infections of the gastrointestinal tract with the development of food allergy. Taken together, this review documents evidence to support the "viral hypothesis", and, in particular, the role of RNA viruses in the development of atopic disease.

Local autoantigen expression as essential gatekeeper of memory T-cell recruitment to islet grafts in diabetic hosts

It is generally believed that inflammatory cues can attract noncognate, "bystander" T-cell specificities to sites of inflammation. We have shown that recruitment of naive and in vitro activated autoreactive CD8⁺ T cells into endogenous islets requires local autoantigen expression. Here, we demonstrate that absence of an autoantigen in syngeneic extrapancreatic islet grafts in diabetic hosts renders the grafts "invisible" to cognate memory (and naive) T cells. We monitored the recruitment of islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP)₂₀₆₋₂₁₄-reactive CD8⁺ T cells into IGRP₂₀₆₋₂₁₄-competent and IGRP₂₀₆₋₂₁₄-deficient islet grafts in diabetic wild-type or IGRP₂₀₆₋₂₁₄(-/-) nonobese diabetic hosts (harboring either naive and memory T cells or only naive IGRP₂₀₆₋₂₁₄-specific T-cells, respectively). All four host-donor combinations had development of recurrent diabetes within 2 weeks. Wild-type hosts recruited IGRP₂₀₆₋₂₁₄-specific T cells into IGRP₂₀₆₋₂₁₄(+/+) but not IGRP₂₀₆₋₂₁₄(-/-) grafts. In IGRP₂₀₆₋₂₁₄(-/-) hosts, there was no recruitment of IGRP₂₀₆₋₂₁₄-specific T cells, regardless of donor type. Graft-derived IGRP₂₀₆₋₂₁₄ activated naive IGRP₂₀₆₋₂₁₄-specific T cells, but graft destruction invariably predated their recruitment. These results indicate that recurrent diabetes is exclusively driven by autoreactive T cells primed during the primary autoimmune response, and demonstrate that local antigen expression is a sine qua non requirement for accumulation of memory T cells into islet grafts. These findings underscore the importance of tackling autoreactive T-cell memory after β-cell replacement therapy.

Guarding the perimeter: protection of the mucosa by tissue-resident memory T cells

Mucosal tissues are continually bombarded with infectious agents seeking to gain entry into the body. The absence of a tough physical exterior layer surrounding these tissues creates a unique challenge for the immune system, which manages to provide broad protection against a plethora of different organisms with the aid of special adaptations that augment immunity at these vulnerable sites. For example, specialized populations of memory T lymphocytes reside at initial sites of pathogen entry into the body, where they provide an important protective barrier. Similar anatomically-confined populations of pathogen-specific CD8 T cells can be found near the outer margins of the body following recovery from a variety of local infections, where they share very similar phenotypic characteristics. How these tissue-resident T cells are retained in a single anatomic location where they can promote immunity is beginning to be defined. Here, we will review current knowledge of the mechanisms that help establish and maintain these regional lymphocytes in the mucosal tissues and discuss relevant data that enhance our understanding of the contribution of these lymphocyte populations to protective immunity against infectious diseases.

The effect of antihelminthic treatment on subjects with asthma from an endemic area of schistosomiasis: a randomized, double-blinded, and placebo-controlled trial

This is a prospective, double-blinded, and placebo-controlled trial evaluating the influence of antihelminthic treatments on asthma severity in individuals living in an endemic area of schistosomiasis. Patients from group 1 received placebo of Albendazole or of Praziquantel and from group 2 received Albendazole and Praziquantel. Asthma severity was assessed by clinical scores and by pulmonary function test. There was no significant difference in the asthma scores from D0 to D1–D7 after Albendazole or Praziquantel and from D0 to D30–90 after Albendazole or Praziquantel in both, group 1 and 2. It was observed, however, a clinical worsening of the overall studied population after 6 months and 12 months of antihelminthic treatments. Additionally, we observed increased frequency of forced expiratory volume in 1 second (FEV1) <80% on 12 and 18 months after treatment. The worsening of asthma severity after repeated antihelminthic treatments is consistent with the hypothesis of the protective role conferred by helminths in atopic diseases.

A mechanistic role for type III IFN-λ1 in asthma exacerbations mediated by human rhinoviruses

RATIONALE

Human rhinoviruses (HRV) are the leading cause of upper respiratory infections and have been postulated to trigger asthma exacerbations. However, whether HRV are detected during crises because upper respiratory infections often accompany asthma attacks, or because they specifically elicit exacerbations, is unclear. Moreover, although several hypotheses have been advanced to explain virus-induced exacerbations, their mechanism remains unclear.

OBJECTIVES

To determine the role of HRV in pediatric asthma exacerbations and the mechanisms mediating wheezing.

METHODS

We prospectively studied 409 children with asthma presenting with upper respiratory infection in the presence or absence of wheezing. Candidate viral and immune mediators of illness were compared among children with asthma with different degrees of severity of acute asthma.

MEASUREMENTS AND MAIN RESULTS

HRV infections specifically associated with asthma exacerbations, even after adjusting for relevant demographic and clinical variables defined a priori (odds ratio, 1.90; 95% confidence interval, 1.21-2.99; P = 0.005). No difference in virus titers, HRV species, and inflammatory or allergic molecules was observed between wheezing and nonwheezing children infected with HRV. Type III IFN-λ(1) levels were higher in wheezing children infected with HRV compared with nonwheezing (P < 0.001) and increased with worsening symptoms (P < 0.001). Moreover, after adjusting for IFN-λ(1), children with asthma infected with HRV were no longer more likely to wheeze than those who were HRV-negative (odds ratio, 1.18; 95% confidence interval, 0.57-2.46; P = 0.66).

CONCLUSIONS

Our findings suggest that HRV infections in children with asthma are specifically associated with acute wheezing, and that type III IFN-λ(1) responses mediate exacerbations caused by HRV. Modulation of IFN- λ(1) should be studied as a therapeutic target for exacerbations caused by HRV.

Tissue exit: a novel control point in the accumulation of antigen-specific CD8 T cells in the influenza a virus-infected lung

Memory/effector T cells efficiently migrate into extralymphoid tissues and sites of infection, providing immunosurveillance and a first line of defense against invading pathogens. Even though it is a potential means to regulate the size, quality, and duration of a tissue infiltrate, T cell egress from infected tissues is poorly understood. Using a mouse model of influenza A virus infection, we found that CD8 effector T cells egressed from the infected lung in a CCR7-dependent manner. In contrast, following antigen recognition, effector CD8 T cell egress decreased and CCR7 function was reduced in vivo and in vitro, indicating that the exit of CD8 T cells from infected tissues is tightly regulated. Our data suggest that the regulation of T cell egress is a mechanism to retain antigen-specific effectors at the site of infection to promote viral clearance, while decreasing the numbers of bystander T cells and preventing overt inflammation.

Evidence for a causal relationship between respiratory syncytial virus infection and asthma

Respiratory syncytial virus (RSV) infects all children early in life, is the most common cause of infant lower respiratory tract infections, and causes disease exacerbations in children with asthma. Episodes of lower respiratory tract infection in early life are associated with asthma development. Whether RSV infection early in life directly causes asthma or simply identifies infants who are genetically predisposed to develop subsequent wheezing is debatable. Recent studies suggest that these two explanations are not mutually exclusive, and are likely both important in asthma development. An open-label study of RSV immunoprophylaxis administered to preterm infants reduced recurrent wheezing by 50%. Clinical trials of infant RSV prevention, delay or severity reduction on the outcome of childhood asthma would confirm the causal relationship between RSV infection and asthma, and offer a primary prevention strategy.

Ara h 1-reactive T cells in individuals with peanut allergy

BACKGROUND

Effective immunotherapy for peanut allergy is hampered by a lack of understanding of peanut-reactive CD4(+) T cells.

OBJECTIVE

To identify, characterize, and track Ara h 1-reactive cells in subjects with peanut allergy by using Ara h 1-specific class II tetramers.

METHODS

Tetramer-guided epitope mapping was used to identify the antigenic peptides within the peanut allergen Ara h 1. Subsequently, HLA class II/Ara h 1-specific tetramers were used to determine the frequency and phenotype of Ara h 1-reactive T cells in subjects with peanut allergy. Cytokine profiles of Ara h 1-reactive T cells were also determined.

RESULTS

Multiple Ara h 1 epitopes with defined HLA restriction were identified. Ara h 1-specific CD4(+) T cells were detected in all of the subjects with peanut allergy tested. Ara h 1-reactive T cells in subjects with allergy expressed CCR4 but did not express CRTH2. The percentage of Ara h1-reactive cells that expressed the β7 integrin was low compared with total CD4(+) T cells. Ara h 1- reactive cells that secreted IFN-γ, IL-4, IL-5, IL-10, and IL-17 were detected.

CONCLUSION

In individuals with peanut allergy, Ara h 1-reactive T cells occurred at moderate frequencies, were predominantly CCR4(+) memory cells, and produced IL-4. Class II tetramers can be readily used to detect Ara h 1-reactive T cells in the peripheral blood of subjects with peanut allergy without in vitro expansion and would be effective for tracking peanut-reactive CD4(+) T cells during immunotherapy.

Pathogenesis of airway inflammation in bronchial asthma

Bronchial asthma is a chronic disorder characterized by airway inflammation, reversible airway obstruction, and airway hyperresponsiveness. Eosinophils are believed to play important roles in the pathogenesis of asthma through the release of inflammatory mediators. In refractory eosinophilic asthma, anti-IL-5 mAb reduces exacerbations and steroid dose, indicating roles of eosinophils and IL-5 in the development of severe eosinophilic asthma. Even in the absence of IL-5, it is likely that the "Th2 network", including a cascade of vascular cell adhesion molecule-1/CC chemokines/GM-CSF, can sufficiently maintain eosinophilic infiltration and degranulation. Cysteinyl leukotrienes can also directly provoke eosinophilic infiltration and activation in the airways of asthma. Therefore, various mechanisms would be involved in the eosinophilic airway inflammation of asthma. In the pathogenesis of severe asthma, not only eosinophils but also mast cells or neutrophils play important roles. Mast cells are much infiltrated to smooth muscle in severe asthma and induce airway remodeling by release of inflammatory mediators such as amphiregulin. Treatment with anti-IgE Ab, which neutralizes circulating IgE and suppresses mast cell functions, reduces asthma exacerbations in severe asthmatic patients. Furthermore, infiltration of neutrophils in the airway is also increased in severe asthma. IL-8 plays an important role in the accumulation of neutrophils and is indeed upregulated in severe asthma. In the absence of chemoattractant for eosinophils, neutrophils stimulated by IL-8 augment the trans-basement membrane migration of eosinophils, suggesting that IL-8-stimulated neutrophils could lead eosinophils to accumulate in the airways of asthma. In view of these mechanisms, an effective strategy for controlling asthma, especially severe asthma, should be considered.

Development of atopy by severe paramyxoviral infection in a mouse model

BACKGROUND

Atopic diseases have been increasing in prevalence, yet the initial inciting events that lead to atopy are not understood. Paramyxoviral infections have been suggested to play a role; however, much of these data are correlative.

OBJECTIVE

To determine whether exposure to a nonviral antigen during a paramyxoviral infection is sufficient to drive IgE production against the bystander antigen and whether clinical disease against this antigen would result.

METHODS

Wild-type C57BL6 mice or mice deficient in FcεRIα (FcεRIα(-/-)) or IgE (IgE(-/-)) were inoculated with Sendai virus (SeV) or UV-inactivated SeV (UV-SeV) and subsequently exposed to ovalbumin (OVA) intranasally. Mice were further challenged 3 times with intranasal OVA on days 20 to 22 after inoculation with SeV, and airway hyperreactivity and mucous cell metaplasia were determined.

RESULTS

Exposure to OVA during SeV infection led to significant OVA specific IgE production (median, 548 vs 0 ng/mL; P = .03; SeV vs UV-SeV). This induction of OVA specific IgE production depended on FcεRI because FcεRIα(-/-) mice produced significantly less IgE (112 ng/mL; P = .03; vs wild-type mice). Furthermore, in wild-type mice OVA exposure and challenge significantly enhanced SeV-induced airway hyperreactivity and mucous cell metaplasia, but this failed to occur in either FcεRIα(-/-) or IgE(-/-) mice.

CONCLUSION

A single exposure to a bystander allergen during a paramyxoviral infection is sufficient to drive allergen specific IgE production in a partial FcεRI-dependent mechanism. These data begin to provide mechanistic insight into how viral infections might drive development of atopic disease.

Pathogens and immunologic memory in asthma: what have we learned?

Animal models and clinical studies of asthma have generated important insights into the first effector phase leading to the development of allergic airway disease and bronchial hyper-reactivity. In contrast, mechanisms related to asthma chronicity or persistence are less well understood. The CD4(+) T-helper 2 lymphocytes are known initiators of the inflammatory response associated with asthma. There is now increasing evidence that memory T-cells, sensitized against allergenic, occupational or viral antigens, are also involved in the persistence of asthma. Additionally, the role of pathogens in asthma has been linked to both the initial susceptibility to and flares of this disease. This review will discuss the potential links between infection and asthma, the role of the memory T-cells in asthma, and the potential mechanisms by which these factors interact to lead to the development and/or persistence of asthma.

In situ recognition of autoantigen as an essential gatekeeper in autoimmune CD8+ T cell inflammation

A current paradigm states that non-antigen-specific inflammatory cues attract noncognate, bystander T cell specificities to sites of infection and autoimmune inflammation. Here we show that cues emanating from a tissue undergoing spontaneous autoimmune inflammation cannot recruit naive or activated bystander T cell specificities in the absence of local expression of cognate antigen. We monitored the recruitment of CD8(+) T cells specific for the prevalent diabetogenic epitope islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP)(206-214) in gene-targeted nonobese diabetic (NOD) mice expressing a T cell "invisible" IGRP(206-214) sequence. These mice developed islet inflammation and diabetes with normal incidence and kinetics, but their inflammatory lesions could recruit neither naive (endogenous or exogenous) nor ex vivo-activated IGRP(206-214)-reactive CD8(+) T cells. Conversely, IGRP(206-214)-reactive, but not nonautoreactive CD8(+) T cells rapidly homed to and accumulated in the inflamed islets of wild-type NOD mice. Our results indicate that CD8(+) T cell recruitment to a site of autoimmune inflammation results from an active process that is strictly dependent on local display of cognate pMHC and suggest that CD8(+) T cells contained in extralymphoid autoimmune lesions are largely autoreactive.

Schistosoma mansoni antigens modulate the allergic response in a murine model of ovalbumin-induced airway inflammation

Schistosoma mansoni infection has been associated with protection against allergies. The mechanisms underlying this association may involve regulatory cells and cytokines. We evaluated the immune response induced by the S. mansoni antigens Sm22.6, PIII and Sm29 in a murine model of ovalbumin (OVA)-induced airway inflammation. BALB/c mice were sensitized with subcutaneously injected OVA-alum and challenged with aerolized OVA. Mice were given three doses of the different S. mansoni antigens. Lung histopathology, cellularity of bronchoalveolar lavage (BAL) and eosinophil peroxidase activity in lung were evaluated. Immunoglobulin (Ig)E levels in serum and cytokines in BAL were also measured. Additionally, we evaluated the frequency of CD4+forkhead box P3 (FoxP3)+ T cells in cultures stimulated with OVA and the expression of interleukin (IL)-10 by these cells. The number of total cells and eosinophils in BAL and the levels of OVA-specific IgE were reduced in the immunized mice. Also, the levels of IL-4 and IL-5 in the BAL of mice immunized with PIII and Sm22.6 were decreased, while the levels of IL-10 were higher in mice immunized with Sm22.6 compared to the non-immunized mice. The frequency of CD4+FoxP3+ T cells was higher in the groups of mice who received Sm22.6, Sm29 and PIII, being the expression of IL-10 by these cells only higher in mice immunized with Sm22.6. We concluded that the S. mansoni antigens used in this study are able to down-modulate allergic inflammatory mediators in a murine model of airway inflammation and that the CD4+FoxP3+ T cells, even in the absence of IL-10 expression, might play an important role in this process.

Harnessing regulatory T cells to suppress asthma: from potential to therapy

Regulatory T cells (Tregs) play an essential role in maintaining the homeostatic balance of immune responses. Asthma is an inflammatory condition of the airways that is driven by dysregulated immune responses toward normally innocuous antigens. Individuals with asthma have fewer and less functional Tregs, which may lead to uncontrolled effector cell responses and promote proasthmatic responses of T helper type 2, T helper 17, natural killer T, antigen-presenting, and B cells. Tregs have the capacity to either directly or indirectly suppress these responses. Hence, the induced expansion of functional Tregs in predisposed or individuals with asthma is a potential approach for the prevention and treatment of asthma. Infection by a number of micro-organisms has been associated with reduced prevalence of asthma, and many infectious agents have been shown to induce Tregs and reduce allergic airways disease in mouse models. The translation of the regulatory and therapeutic properties of infectious agents for use in asthma requires the identification of key modulatory components and the development and trial of effective immunoregulatory therapies. Further translational and clinical research is required for the induction of Tregs to be harnessed as a therapeutic strategy for asthma.

Adjuvant modulation of the cytokine balance in Mycobacterium tuberculosis subunit vaccines; immunity, pathology and protection

It is known that protection against tuberculosis is mediated primarily by T helper type 1 (Th1) cells but the influence of the Th1/Th2 balance of a vaccination response on the subsequent protection and pathology during infection has not been studied in detail. We designed a panel of Ag85B-ESAT-6 subunit vaccines based on adjuvants with different Th1/Th2-promoting activities and studied cellular responses, bacterial replication and pathology in the lungs of mice infected with Mycobacterium tuberculosis. All vaccines induced cell-mediated and humoral responses but with markedly different interferon-gamma : interleukin-5 (IFN-gamma : IL-5) and immunoglobulin G1 (IgG1) : IgG2 ratios. The vaccines promoted different levels of control of bacterial replication with the most efficient protection being exerted by cationic liposomes containing monophosphoryl lipid A and low to completely absent immunity with conventional aluminium. The level of protection correlated with the amount of IFN-gamma produced in response to the vaccine whereas there was no inverse correlation with the level of IL-5. Characterizing a protective response was an accelerated recruitment of IL-17 and IFN-gamma-producing lymphocytes resulting in the early formation of granulomas containing clustered inducible nitric oxide synthase-activated macrophages. In comparison, non-protected mice exhibited a different inflammatory infiltrate rich in neutrophil granulocytes. This study indicates that the adjuvant component of a tuberculosis vaccine may be crucial in determining the kinetics by which effective granulomas, pivotal in controlling bacterial growth, are formed.

Induction of high-affinity IgE receptor on lung dendritic cells during viral infection leads to mucous cell metaplasia

Respiratory viral infections are associated with an increased risk of asthma, but how acute Th1 antiviral immune responses lead to chronic inflammatory Th2 disease remains undefined. We define a novel pathway that links transient viral infection to chronic lung disease with dendritic cell (DC) expression of the high-affinity IgE receptor (FcepsilonRIalpha). In a mouse model of virus-induced chronic lung disease, in which Sendai virus triggered a switch to persistent mucous cell metaplasia and airway hyperreactivity after clearance of replicating virus, we found that FceRIa(-/-) mice no longer developed mucous cell metaplasia. Viral infection induced IgE-independent, type I IFN receptor-dependent expression of FcepsilonRIalpha on mouse lung DCs. Cross-linking DC FcepsilonRIalpha resulted in the production of the T cell chemoattractant CCL28. FceRIa(-/-) mice had decreased CCL28 and recruitment of IL-13-producing CD4(+) T cells to the lung after viral infection. Transfer of wild-type DCs to FceRIa(-/-) mice restored these events, whereas blockade of CCL28 inhibited mucous cell metaplasia. Therefore, lung DC expression of FcepsilonRIalpha is part of the antiviral response that recruits CD4(+) T cells and drives mucous cell metaplasia, thus linking antiviral responses to allergic/asthmatic Th2 responses.

Identification of a CD4 T cell epitope in the pneumonia virus of mice glycoprotein and characterization of its role in protective immunity

Pneumonia virus of mice (PVM) causes bronchiolitis and pneumonia in mice. Infection is associated with high levels of viral replication in the lungs and results in the functional inactivation of pulmonary virus-specific CD8 T cells. Due to its similarity to severe human respiratory syncytial virus (RSV) infection, PVM infection in mice has been proposed as an alternative RSV model. Here, we have delineated the minimal requirements for protective T cell immunity in the PVM model. Immunization with a CD8 T cell epitope from the PVM phosphoprotein P, combined with the ovalbumin (OVA) CD4 T cell epitope, did not confer protective immunity against lethal PVM challenge, suggesting a possible role of cognate CD4 T cell immunity. To determine the role of PVM-specific CD4 T cell responses, we mapped a PVM CD4 T cell epitope in the glycoprotein G, using a panel of overlapping peptides. Although immunization with this epitope provided some protection, solid protective immunity was only observed after immunization with a combination of the PVM-specific CD4 and CD8 T cell epitopes. Analysis of post-challenge T cell responses in immunized mice indicated that G-specific pulmonary CD4 T cells displayed a mixed Th1/Th2 phenotype, which was characterized by the presence of both IL-5 and IFN-gamma secreting cells, in the absence of overt pathology.

Interactions between T cells responding to concurrent mycobacterial and influenza infections

CD4(+) T cells are central in mediating granuloma formation and limiting growth and dissemination of mycobacterial infections. To determine whether T cells responding to influenza infection can interact with T cells responding to Mycobacterium bovis bacille Calmette-Guérin (BCG) infection and disrupt granuloma formation, we infected mice containing two monoclonal T cell populations specific for the model Ags pigeon cytochrome c (PCC) and hen egg lysozyme (HEL). These mice were chronically infected with PCC epitope-tagged BCG (PCC-BCG) and acutely infected with HEL epitope-tagged influenza virus (HEL-flu). In these mice, PCC-BCG infection is much more abundant in the liver than the lung, whereas HEL-flu infection is localized to the lung. We observe that both T cells have access to both inflammatory sites, but that PCC-specific T cells dominate the PCC-BCG inflammatory site in the liver, whereas HEL-specific T cells dominate the HEL-flu inflammatory site in the lung. Influenza infection, in the absence of an influenza-specific T cell response, is able to increase the activation state and IFN-gamma secretion of PCC-BCG-specific T cells in the granuloma. Activation of HEL-specific T cells allows them to secrete IFN-gamma and contribute to protection in the granuloma. Ultimately, infection with influenza has little effect on bacterial load, and bacteria do not disseminate. In summary, these data illustrate complex interactions between T cell responses to infectious agents that can affect effector responses to pathogens.

Antimicrobial strategies: an option to treat allergy?

Respiratory infections by bacteria and viruses often trigger symptoms of asthma in both adults and children. This observation and subsequent mechanistic studies have demonstrated important interactions among allergens, microbes and the atopic host. The mechanisms responsible for microbe-induced asthma exacerbations are only incompletely understood. A focal point of current research is the inflammatory response of the host following an encounter with a pathogenic microbe, including variations in chemokine and cytokine production and resulting in changes in bronchial hyper-responsiveness and lung function. Direct bronchial infection, exposure of nerves with resulting neurogenic inflammation and a deviated host immune response are among the mechanisms underlying these functional disorders. Lately, suboptimal innate immune responses, expressed as defective interferon production, have gained attention as they might be amenable to intervention. This review describes the suggested mechanisms involved in the complex interactions between 'asthmagenic' microbes, the immune system and atopy, based on in-vitro and in-vivo experimental models and epidemiological evidence. In addition, it provides a synopsis of potential therapeutic strategies either directly against the microorganisms or in respect to the associated inflammation.

Role of CCL5 (RANTES) in viral lung disease

CCL5/RANTES is a key proinflammatory chemokine produced by virus-infected epithelial cells and present in respiratory secretions of asthmatics. To examine the role of CCL5 in viral lung disease, we measured its production during primary respiratory syncytial virus (RSV) infection and during secondary infection after sensitizing vaccination that induces Th2-mediated eosinophilia. A first peak of CCL5 mRNA and protein production was seen at 18 to 24 h of RSV infection, before significant lymphocyte recruitment occurred. Treatment in vivo with Met-RANTES (a competitive chemokine receptor blocker) throughout primary infection decreased CD4+ and CD8+ cell recruitment and increased viral replication. In RSV-infected, sensitized mice with eosinophilic disease, CCL5 production was further augmented; Met-RANTES treatment again reduced inflammatory cell recruitment and local cytokine production. A second wave of CCL5 production occurred on day 7, attributable to newly recruited T cells. Paradoxically, mice treated with Met-RANTES during primary infection demonstrated increased cellular infiltration during reinfection. We therefore show that RSV induces CCL5 production in the lung and this causes the recruitment of RSV-specific cells, including those making additional CCL5. If this action is blocked with Met-RANTES, inflammation decreases and viral clearance is delayed. However, the exact effects of chemokine modulation depend critically on time of administration, a factor that may potentially complicate the use of chemokine blockers in inflammatory diseases.

Intestinal and pulmonary mucosal T cells: local heroes fight to maintain the status quo

Mucosal immunity in the lung and intestine is controlled by complex multifaceted systems. While mucosal T cells are essential for protection against invading pathogens owing to their proximity to the outside world, powerful systems must also be in place to harness ongoing inflammatory processes. In each site, distinct anatomical structures play key roles in mounting and executing both protective and deleterious mucosal T cell responses. Although analogies can be drawn regarding the immune systems of these two organs, there are substantial dissimilarities necessitated by unique physiologic constraints. Here, we discuss how T cell activation and effector function are generated in the mucosae.

Residual antigen presentation after influenza virus infection affects CD8 T cell activation and migration

Activated virus-specific CD8 T cells remain in the lung airways for several months after influenza virus infection. We show that maintenance of this cell population is dependent upon the route of infection and prolonged presentation of viral antigen in the draining lymph nodes (DLN) of the respiratory tract. The local effects on T cell migration have been examined. We show retention of virus-specific CD8 T cells in the mediastinal lymph node (MLN) and continuing recruitment of blood-borne migrants into the lung airways during antigen presentation. These data show that antigen that is retained after pulmonary influenza virus infection controls the migratory pattern and activation state of virus-specific CD8 T cells near the site of virus amplification.

Immunological wrong turns in the face of multiple viral challenges

To further understand the myriad of ways by which viruses can cause disease, development of new and creative model systems using laboratory mice will be required. Mouse models have been essential tools for the study of viral pathogenesis over the past 50 years, allowing for detailed assessments of viral replication, spread and immune response induction. However, virtually all of these studies were accomplished using a single-challenge approach, in which immunologically naïve adult mice were inoculated with only one pathogen. This approach, while useful for exploring individual pathogen–host interactions, does not parallel the complexity of immune history and potential concurrent immune challenges that occur in humans. This review discusses current progress in viral pathogenesis using the single-challenge approach, followed by an overview of the present understanding of how concurrent immune challenges may influence pathogenesis. It is suggested that the development of more complex mouse models will be a substantial advance, leading to the discovery of previously unappreciated aspects of immune cell recruitment and novel ways by which viruses can, either directly or indirectly, promote illness.

Systemic dissemination and persistence of Th2 and type 2 cells in response to infection with a strictly enteric nematode parasite

Oral infection with the nematode parasite Heligmosomoides polygyrus H. polygyrus is entirely restricted to the small intestine. Although the evoked Th2 response has been extensively studied in secondary lymphoid organs, little is known about the systemic dissemination of Th2 cells or type 2 associated eosinophils and basophils. In this study we use bicistronic 4get IL-4 reporter mice to directly visualize the type 2 response to H. polygyrus infection. We observed that CD4(+)/GFP(+) Th2 cells spread systemically and found that these cells accumulated in nonlymphoid "hot spots" in the liver, the lung airways, and the peritoneal cavity. Interestingly, the total number of Th2 cells in the peritoneal cavity was comparable to those found in the draining mesenteric lymph node or the spleen. Peritoneal Th2 cells were distinguished by an exceptionally low apoptotic potential and high expression of the intestinal homing receptor alpha(4)beta(7) integrin. CD4(+)/GFP(+) Th2 cells from these peripheral sites were fully functional as indicated by rapid IL-4 production upon polyclonal or Ag-specific restimulation. Th2 cells persisted in the intestinal tissue and the peritoneal cavity of drug-cured mice for weeks. The presence of peripheral memory Th2 cells in the intestine might be crucial for immunity to recall infections. These findings have important implications for the design of vaccination strategies because it may be necessary to establish and maintain memory CD4(+) T cells at the potential future site of infection.

A novel role of hepatocyte growth factor as an immune regulator through suppressing dendritic cell function

Hepatocyte growth factor (HGF) plays an important role in many biological events such as angiogenesis, cell proliferation, anti-fibrosis and antiapoptosis. It is well known that HGF promotes tumor progression and suppresses development of fibrosis after tissue injury. In contrast, its role in immune-mediated disorders has not been fully clarified. In the present study, we examined the role of HGF in Ag-specific immune response using in vitro studies and an experimental model of allergic airway inflammation. We first confirmed that dendritic cells (DCs) expressed the receptor for HGF, c-met, which was not expressed in T cells. Treatment with HGF both in vitro and in vivo potently suppressed DC functions such as Ag-presenting capacity, thus down-regulating Ag-induced Th1- and Th2-type immune responses. Exogenous administration of the HGF expression plasmid into Ag-primed mice markedly suppressed the development of airway eosinophilia and airway hyperresponsiveness, which was induced by Ag inhalation, with suppression of the Ag-presenting capacity of DCs in the lung. HGF exhibited these immunosuppressive effects without up-regulation of IL-10 or TGF-beta. We also found that expression of endogenous HGF in the lung significantly increased following Ag sensitization and inhalation challenges. Finally, neutralization of endogenous HGF in vivo significantly increased airway eosinophilia and airway hyperresponsiveness with up-regulation of the Ag-presenting capacity of DCs in the lung. These results demonstrated a novel, significant, and possibly therapeutic role of HGF as a potent regulator in immune-mediated disorders such as asthma.

Antigen-specific and non-specific CD4+ T cell recruitment and proliferation during influenza infection

To track epitope-specific CD4(+) T cells at a single-cell level during influenza infection, the MHC class II-restricted OVA(323-339) epitope was engineered into the neuraminidase stalk of influenza/A/WSN, creating a surrogate viral antigen. The recombinant virus, influenza A/WSN/OVA(II), replicated well, was cleared normally, and stimulated both wild-type and DO11.10 or OT-II TCR transgenic OVA-specific CD4(+) T cells. OVA-specific CD4 T cells proliferated during infection only when the OVA epitope was present. However, previously primed (but not naive) transgenic CD4(+) T cells were recruited to the infected lung both in the presence and absence of the OVA(323-339) epitope. These data show that, when primed, CD4(+) T cells may traffic to the lung in the absence of antigen, but do not proliferate. These results also document a useful tool for the study of CD4 T cells in influenza infection.

Virus-induced airway dysfunction: pathogenesis and biomechanisms

BACKGROUND

Viral respiratory tract infections cause significant morbidity and mortality. Respiratory viruses are suspected to play a role in the inception of asthma early in life. Respiratory syncytial virus (RSV) is the most common cause of infant bronchiolitis, which is associated with the development of childhood wheezing and asthma. However, it is not clear whether this association is "causal" or "circumstantial."

METHODS

Animal models have been pivotal in studying the pathophysiology of viral respiratory infections. Various approaches to assessing airway inflammation and function have been used to define the mechanisms of virus-induced airway dysfunction and to address clinically relevant questions regarding the role of RSV in wheezing and asthma after bronchiolitis.

RESULTS

Viral lower respiratory tract infections alter airway function in humans and animals. The extent and duration of the alterations may depend on the virus itself, host factors and environmental factors. Animal studies demonstrated that viral infection induces airway hyperresponsiveness and enhances this alteration in the allergen-sensitized and exposed host. This altered airway function is mediated by immune and neurogenic inflammatory mechanisms. Recent studies in mice show that neonatal RSV infection sensitizes the newborn to develop an asthma-like phenotype on reinfection, providing further opportunities to investigate the role of RSV in postbronchiolitis wheezing and asthma in this animal model.

CONCLUSIONS

Further studies are needed to fully establish the mechanisms underlying the pathophysiology of viral respiratory tract infections and to clarify their role in the inception and/or progression of chronic airway diseases such as asthma. The results of ongoing therapeutic studies promise to minimize the impact of such viral infections.

CCL5-CCR5 interaction provides antiapoptotic signals for macrophage survival during viral infection

Host defense against viruses probably depends on targeted death of infected host cells and then clearance of cellular corpses by macrophages. For this process to be effective, the macrophage must presumably avoid its own virus-induced death. Here we identify one such mechanism. We show that mice lacking the chemokine Ccl5 are immune compromised to the point of delayed viral clearance, excessive airway inflammation and respiratory death after mouse parainfluenza or human influenza virus infection. Virus-inducible levels of Ccl5 are required to prevent apoptosis of virus-infected mouse macrophages in vivo and mouse and human macrophages ex vivo. The protective effect of Ccl5 requires activation of the Ccr5 chemokine receptor and consequent bilateral activation of G(alphai)-PI3K-AKT and G(alphai)-MEK-ERK signaling pathways. The antiapoptotic action of chemokine signaling may therefore allow scavengers to finally stop the host cell-to-cell infectious process.