Analysis of the local and systemic immune responses induced in BALB/c mice by experimental respiratory syncytial virus infection

Preclinical animal models to evaluate therapeutic antiviral antibodies

Despite the availability of effective preventative vaccines and potent small-molecule antiviral drugs, effective non-toxic prophylactic and therapeutic measures are still lacking for many viruses. The use of monoclonal and polyclonal antibodies in an antiviral context could fill this gap and provide effective virus-specific medical interventions. In order to develop these therapeutic antibodies, preclinical animal models are of utmost importance. Due to the variability in viral pathogenesis, immunity and overall characteristics, the most representative animal model for human viral infection differs between virus species. Therefore, throughout the years researchers sought to find the ideal preclinical animal model for each virus. The most used animal models in preclinical research include rodents (mice, ferrets, …) and non-human primates (macaques, chimpanzee, ….). Currently, antibodies are tested for antiviral efficacy against a variety of viruses including different hepatitis viruses, human immunodeficiency virus (HIV), influenza viruses, respiratory syncytial virus (RSV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and rabies virus. This review provides an overview of the current knowledge about the preclinical animal models that are used for the evaluation of therapeutic antibodies for the abovementioned viruses.

Immunity Cell Responses to RSV and the Role of Antiviral Inhibitors: A Systematic Review

Antigen-presenting cells recognize respiratory syncytial virus antigens, and produce cytokines and chemokines that act on immune cells. Dendritic cells play the main role in inflammatory cytokine responses. Similarly, alveolar macrophages produce IFN-β, IFN-α, TNF-α, IL-6, CXCL10, and CCL3, while alternatively activated macrophages differentiate at the late phase, and require IL-13 or IL-4 cytokines. Furthermore, activated NKT cells secrete IL-13 and IL-4 that cause lung epithelial, endothelial and fibroblasts to secrete eotaxin that enhances the recruitment of eosinophil to the lung. CD8⁺ and CD4⁺T cells infection by the virus decreases the IFN-γ and IL-2 production. Despite this, both are involved in terminating virus replication. CD8⁺T cells produce a larger amount of IFN-γ than CD4⁺T cells, and CD8⁺T cells activated under type 2 conditions produce IL-4, down regulating CD8 expression, granzyme and IFN-γ production. Antiviral inhibitors inhibit biological functions of viral proteins. Some of them directly target the virus replication machinery and are effective at later stages of infection; while others inhibit F protein dependent fusion and syncytium formation. TMC353121 reduces inflammatory cytokines, TNF-α, IL-6, and IL-1β and chemokines, KC, IP-10, MCP and MIP1-α. EDP-938 inhibits viral nucleoprotein (N), while GRP-156784 blocks the activity of respiratory syncytial virus ribonucleic acid (RNA) polymerase. PC786 inhibits non-structural protein 1 (NS-1) gene, RANTES transcripts, virus-induced CCL5, IL-6, and mucin increase. In general, it is an immune reaction that is blamed for the disease severity and pathogenesis in respiratory syncytial virus infection. Anti-viral inhibitors not only inhibit viral entry and replication, but also may reduce inflammatory cytokines and chemokines. Many respiratory syncytial virus inhibitors are proposed; however, only palivizumab and ribavirin are approved for prophylaxis and treatment, respectively. Hence, this review is focused on immunity cell responses to respiratory syncytial virus and the role of antiviral inhibitors.

Antigenic Site-Specific Competitive Antibody Responses to the Fusion Protein of Respiratory Syncytial Virus Were Associated With Viral Clearance in Hematopoietic Cell Transplantation Adults

Background: Recent studies of human sera showed that the majority of the respiratory syncytial virus (RSV) neutralizing antibodies are directed against pre-fusion conformation of the fusion (F) protein of RSV and revealed the importance of pre-fusion antigenic site Ø specific antibodies. However, detailed analysis of multiple antigenic site-specific competitive antibody responses to RSV F protein and their contribution to virus clearance in humans are lacking. Methods: We prospectively enrolled a cohort of RSV infected hematopoietic cell transplantation (HCT) adults (n = 40). Serum samples were collected at enrollment (acute, n = 40) and 14 to 60 days post-enrollment (convalescent, n = 40). Antigenic site-specific F protein antibodies were measured against pre-fusion site Ø, post-fusion site I, and sites II and IV present in both the pre-fusion and post-fusion F protein conformations utilizing four different competitive antibody assays developed with biotinylated monoclonal antibodies (mAb) D25, 131-2A, palivizumab, and 101F, respectively. The lower limit of detection were 7.8 and 1.0 μg/mL for the competitive antibody assays that measured site Ø specific response, as well as sites I, II, and IV specific responses, respectively. Neutralizing antibody titers to RSV A and B subgroups was determined by microneutralization assays. Results: The overall findings in RSV infected HCT adults revealed: (1) a significant increase in antigenic site-specific competitive antibodies in convalescent sera except for site Ø competitive antibody (p < 0.01); (2) comparable concentrations in the acute and convalescent serum samples of antigenic site-specific competitive antibodies between RSV/A and RSV/B infected HCT adults (p > 0.05); (3) significantly increased concentrations of the antigenic site-specific competitive antibodies in HCT adults who had genomic RSV detected in the upper respiratory tract for <14 days compared to those for ≥14 days (p < 0.01); and (4) statistically significant correlation between the antigenic site-specific competitive antibody concentrations and neutralizing antibody titers against RSV/A and RSV/B (r ranged from 0.33 to 0.83 for acute sera, and 0.50-0.88 for convalescent sera; p < 0.05). Conclusions: In RSV infected HCT adults, antigenic site-specific antibody responses were induced against multiple antigenic sites found in both the pre-fusion and post-fusion F conformations, and were associated with a more rapid viral clearance and neutralizing antibody activity. However, the association is not necessarily the cause and the consequence.

T Lymphocytes as Measurable Targets of Protection and Vaccination Against Viral Disorders

Continuous epidemiological surveillance of existing and emerging viruses and their associated disorders is gaining importance in light of their abilities to cause unpredictable outbreaks as a result of increased travel and vaccination choices by steadily growing and aging populations. Close surveillance of outbreaks and herd immunity are also at the forefront, even in industrialized countries, where previously eradicated viruses are now at risk of re-emergence due to instances of strain recombination, contractions in viral vector geographies, and from their potential use as agents of bioterrorism. There is a great need for the rational design of current and future vaccines targeting viruses, with a strong focus on vaccine targeting of adaptive immune effector memory T cells as the gold standard of immunity conferring long-lived protection against a wide variety of pathogens and malignancies. Here, we review viruses that have historically caused large outbreaks and severe lethal disorders, including respiratory, gastric, skin, hepatic, neurologic, and hemorrhagic fevers. To observe trends in vaccinology against these viral disorders, we describe viral genetic, replication, transmission, and tropism, host-immune evasion strategies, and the epidemiology and health risks of their associated syndromes. We focus on immunity generated against both natural infection and vaccination, where a steady shift in conferred vaccination immunogenicity is observed from quantifying activated and proliferating, long-lived effector memory T cell subsets, as the prominent biomarkers of long-term immunity against viruses and their associated disorders causing high morbidity and mortality rates.

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.

Respiratory Syncytial Virus: Infection, Detection, and New Options for Prevention and Treatment

Respiratory syncytial virus (RSV) infection is a significant cause of hospitalization of children in North America and one of the leading causes of death of infants less than 1 year of age worldwide, second only to malaria. Despite its global impact on human health, there are relatively few therapeutic options available to prevent or treat RSV infection. Paradoxically, there is a very large volume of information that is constantly being refined on RSV replication, the mechanisms of RSV-induced pathology, and community transmission. Compounding the burden of acute RSV infections is the exacerbation of preexisting chronic airway diseases and the chronic sequelae of RSV infection. A mechanistic link is even starting to emerge between asthma and those who suffer severe RSV infection early in childhood. In this article, we discuss developments in the understanding of RSV replication, pathogenesis, diagnostics, and therapeutics. We attempt to reconcile the large body of information on RSV and why after many clinical trials there is still no efficacious RSV vaccine and few therapeutics.

Ameliorating Effect of Dietary Xylitol on Human Respiratory Syncytial Virus (hRSV) Infection

Human respiratory syncytial virus (hRSV) is the most common cause of bronchiolitis and pneumonia in infants. The lack of proper prophylactics and therapeutics for controlling hRSV infection has been of great concern worldwide. Xylitol is a well-known sugar substitute and its effect against bacteria in the oral cavity is well known. However, little is known of its effect on viral infections. In this study, the effect of dietary xylitol on hRSV infection was investigated in a mouse model for the first time. Mice received xylitol for 14 d prior to virus challenge and for a further 3 d post challenge. Significantly larger reductions in lung virus titers were observed in the mice receiving xylitol than in the controls receiving phosphate-buffered saline (PBS). In addition, fewer CD3(+) and CD3(+)CD8(+) lymphocytes, whose numbers reflect inflammatory status, were recruited in the mice receiving xylitol. These results indicate that dietary xylitol can ameliorate hRSV infections and reduce inflammation-associated immune responses to hRSV infection.

The effect of dietary bovine colostrum on respiratory syncytial virus infection and immune responses following the infection in the mouse

Human respiratory syncytial virus (hRSV) is the most common cause of respiratory tract infection among young children because of immature T cell immunity of them against hRSV. CD8 T cells play a pivotal role in clearing hRSV and preventing subsequent infection. We examined the effects of dietary bovine colostrum on virus infection and CD8 T cell responses following hRSV infection in the mouse model. Mice received bovine colostrum for 14 days prior to hRSV challenge, and lung indexes (severity of symptom) and lung virus titers were analyzed. In addition, the activation of CD8 T cells in the bronchoalveolar lavage fluids (BALFs) of mice receiving bovine colostrum were compared with those in the BALFs of mice receiving phosphate-buffered saline (PBS) or ribavirin, post virus challenge. The severity of infection and lung virus titers were reduced in the mice receiving bovine colostrum, compared to those receiving PBS. Moreover CD8 T cell responses were selectively enhanced in the former. Our results suggest that dietary bovine colostrum exerts the effects to inhibit hRSV and ameliorate the symptom by hRSV infection, and enhances the CD8 T cell response during the hRSV infection.

Advances in and the potential of vaccines for respiratory syncytial virus

Respiratory syncytial virus (RSV) is the leading cause of serious lower respiratory track illness causing bronchiolitis and some mortality in infants and the elderly. Despite decades of research there is no licensed RSV vaccine. To enable the development of RSV vaccines, several major obstacles must be overcome including immature and waning immunity to RSV infection, the capacity of RSV to evade immunity and the failure of RSV infection to induce robust enduring immunity. Since the failure of the formalin-inactivated RSV vaccine trial, more cautious and deliberate progress has been made toward RSV vaccine development using a variety of experimental approaches. The scientific rational and the state of development of these approaches are reviewed in this article.

Tidal volume drives inflammation during mechanical ventilation for viral respiratory infection

OBJECTIVE

Respiratory syncytial virus lower respiratory tract infection is the most frequent cause of respiratory insufficiency necessitating mechanical ventilation in infants during the winter season. Recently, we presented a new animal model to show that mechanical ventilation aggravates respiratory syncytial virus-induced pulmonary inflammation by distinct mechanisms. We now use this model to study whether low tidal volume mechanical ventilation causes less ventilator-induced lung injury in the presence of respiratory syncytial virus lower respiratory tract infection.

DESIGN

Randomized controlled experimental study.

SETTING

University Medical Center animal laboratory.

SUBJECTS

Male BALB/c mice, 6-8 weeks old and weighing 20-28 g.

INTERVENTIONS

Mice were inoculated with respiratory syncytial virus or mock virus on day 0 and ventilated on day 1 or 5 with high (12 mL/kg) or low (6 mL/kg) tidal volume for 5 hours.

MEASUREMENTS AND MAIN RESULTS

Total and differential cell counts as well as cytokine concentrations were determined in bronchoalveolar lavage fluid. Compared with nonventilated respiratory syncytial virus-infected mice, high tidal volume ventilation of respiratory syncytial virus-infected mice on day 5 enhanced bronchoalveolar lavage fluid total cell count (0.35 vs 0.99 × 10e6/mL; p < 0.01), neutrophils (0.02 vs 0.17 × 10e6/mL; p < 0.01), interleukin-6 (58 vs 250 pg/mL; p < 0.01), and keratinocyte-derived chemokine (95 vs 335 pg/mL; p < 0.01) levels. In low tidal volume ventilation of respiratory syncytial virus-infected mice, no significant difference in cell counts or cytokine concentrations was observed compared with spontaneous breathing respiratory syncytial virus-infected controls on both days.

CONCLUSIONS

Low tidal volume mechanical ventilation causes less ventilation-induced cellular and cytokine influx into the bronchoalveolar space during respiratory syncytial virus lower respiratory tract infection.

The respiratory syncytial virus fusion protein and neutrophils mediate the airway mucin response to pathogenic respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of death due to a viral etiology in infants. RSV disease is characterized by epithelial desquamation, neutrophilic bronchiolitis and pneumonia, and obstructive pulmonary mucus. It has been shown that infection of BALB/cJ mice with RSV clinical isolate A2001/2-20 (2-20) results in a higher early viral load, greater airway necrosis, and higher levels of interleukin-13 (IL-13) and airway mucin expression than infection with RSV laboratory strain A2. We hypothesized that the fusion (F) protein of RSV 2-20 is a mucus-inducing viral factor. In vitro, the fusion activity of 2-20 F but not that of A2 F was enhanced by expression of RSV G. We generated a recombinant F-chimeric RSV by replacing the F gene of A2 with the F gene of 2-20, generating A2-2-20F. Similar to the results obtained with the parent 2-20 strain, infection of BALB/cJ mice with A2-2-20F resulted in a higher early viral load and higher levels of subsequent pulmonary mucin expression than infection with the A2 strain. A2-2-20F infection induced greater necrotic airway damage and neutrophil infiltration than A2 infection. We hypothesized that the neutrophil response to A2-2-20F infection is involved in mucin expression. Antibody-mediated depletion of neutrophils in RSV-infected mice resulted in lower tumor necrosis factor alpha levels, fewer IL-13-expressing CD4 T cells, and less airway mucin production in the lung. Our data are consistent with a model in which the F and attachment (G) glycoprotein functional interaction leads to enhanced fusion and F is a key factor in airway epithelium infection, pathogenesis, and subsequent airway mucin expression.

Benefit and harm from immunity to respiratory syncytial virus: implications for treatment

PURPOSE OF REVIEW

Human respiratory syncytial virus (RSV) infection is a major cause of morbidity in children and of morbidity and mortality in elderly or immunocompromised adults. Given prophylactically, antibody can protect against infection, but natural levels are poorly protective. Vaccination may enhance disease, and there is no well tolerated and effective vaccine or antiviral treatment. Despite over 50 years of research, therapy remains nonspecific and supportive.

RECENT FINDINGS

Experimental human challenge in adult volunteers is beginning to elucidate the dynamics of viral shedding and causes of disease, but investigations of naturally infected children remain logistically challenging. RSV was known to bind several surface ligands, but the recent demonstration that nucleolin acts as a receptor for the RSV fusion protein was unexpected. Recent studies increasingly emphasize the relevance of innate immune responses and the dysregulation of inflammation as key factors in causing the pathological effects of infection. Studies in both human infants and mice indicate that interleukin-17 plays a role in some forms of RSV disease and regulatory T cells may be important in controlling inflammation.

SUMMARY

Improved understanding of the human immune response to RSV infection continues to be needed in order to accelerate the development of vaccines and new treatments for bronchiolitis.

Differential pathogenesis of respiratory syncytial virus clinical isolates in BALB/c mice

Airway mucus is a hallmark of respiratory syncytial virus (RSV) lower respiratory tract illness. Laboratory RSV strains differentially induce airway mucus production in mice. Here, we tested the hypothesis that RSV strains differ in pathogenesis by screening six low-passage RSV clinical isolates for mucogenicity and virulence in BALB/cJ mice. The RSV clinical isolates induced variable disease severity, lung interleukin-13 (IL-13) levels, and gob-5 levels in BALB/cJ mice. We chose two of these clinical isolates for further study. Infection of BALB/cJ mice with RSV A2001/2-20 (2-20) resulted in greater disease severity, higher lung IL-13 levels, and higher lung gob-5 levels than infection with RSV strains A2, line 19, Long, and A2001/3-12 (3-12). Like the line 19 RSV strain, the 2-20 clinical isolate induced airway mucin expression in BALB/cJ mice. The 2-20 and 3-12 RSV clinical isolates had higher lung viral loads than laboratory RSV strains at 1 day postinfection (p.i.). This increased viral load correlated with higher viral antigen levels in the bronchiolar epithelium and greater histopathologic changes at 1 day p.i. The A2 RSV strain had the highest peak viral load at day 4 p.i. RSV 2-20 infection caused epithelial desquamation, bronchiolitis, airway hyperresponsiveness, and increased breathing effort in BALB/cJ mice. We found that RSV clinical isolates induce variable pathogenesis in mice, and we established a mouse model of clinical isolate strain-dependent RSV pathogenesis that recapitulates key features of RSV disease.

Differential regulation of GM1 and asialo-GM1 expression by T cells and natural killer (NK) cells in respiratory syncytial virus infection

We previously reported that respiratory syncytial virus (RSV) infection increases lung CD8(+) T cell GM1 expression. The related lipid asialo-GM1 (ASGM1) is expressed by T cells in viral infection and by natural killer (NK) cells. The in vivo co-expression of GM1 and ASGM1 by immune cells is not defined. Here we analyzed lung lymphocyte GM1 and ASGM1 expression in RSV-infected mice. GM1 and ASGM1 were coordinately upregulated by activated CD8(+) T cells in RSV-infected BALB/c and C57BL/6 mice. In contrast, RSV infection had no effect on constitutively high NK cell GM1 expression, while increasing NK cell ASGM1 expression. GM1 and ASGM1 co-localized in lipid raft structures in NK and CD8(+) T cells sorted from the lungs of RSV-infected mice. Anti-ASGM1 Ab treatment of RSV-infected BALB/c mice depleted GM1/ASGM1-expressing NK cells and GM1/ASGM1-expressing T cells, reduced lung IFN-gamma levels, increased viral load, delayed viral clearance, and reduced illness. STAT1(-/-) mice are more susceptible to RSV replication and disease than wild-type mice. In RSV-infected STAT1(-/-) mice, anti-ASGM1 Ab altered cytokine levels, but in contrast to BALB/c mice, antibody treatment had no effect on viral load or illness. Taken together, GM1 and ASGM1 expression are differentially regulated by T and NK cells in RSV infection. Also, GM1/ASGM1-expressing cells are important for control of RSV in BALB/c mice, whereas STAT1(-/-) mice clear RSV by an alternative pathway.

Impairment of the CD8+ T cell response in lungs following infection with human respiratory syncytial virus is specific to the anatomical site rather than the virus, antigen, or route of infection

Background

A subset of the virus-specific CD8+ cytotoxic T lymphocytes (CTL) isolated from the lungs of mice infected with human respiratory syncytial virus (RSV) is impaired in the ability to secrete interferon γ (IFNγ), a measure of functionality. It was suggested that the impairment specifically suppressed the host cellular immune response, a finding that could help explain the ability of RSV to re-infect throughout life.

Results

To determine whether this effect is dependent on the virus, the route of infection, or the type of infection (respiratory, disseminated, or localized dermal), we compared the CTL responses in mice following intranasal (IN) infection with RSV or influenza virus or IN or intradermal (ID) infection with vaccinia virus expressing an RSV CTL antigen. The impairment was observed in the lungs after IN infection with RSV, influenza or vaccinia virus, and after a localized ID infection with vaccinia virus. In contrast, we observed a much higher percentage of IFNγ secreting CD8+ lymphocytes in the spleens of infected mice in every case.

Conclusion

The decreased functionality of CD8+ CTL is specific to the lungs and is not dependent on the specific virus, viral antigen, or route of infection.

Host transcription profiles upon primary respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is a common cause of severe lower respiratory tract infection in children. Severe RSV disease is related to an inappropriate immune response to RSV resulting in enhanced lung pathology which is influenced by host genetic factors. To gain insight into the early pathways of the pathogenesis of and immune response to RSV infection, we determined the transcription profiles of lungs and lymph nodes on days 1 and 3 after infection of mice. Primary RSV infection resulted in a rapid but transient innate, proinflammatory response, as exemplified by the induction of a large number of type I interferon-regulated genes and chemokine genes, genes involved in inflammation, and genes involved in antigen processing. Interestingly, this response is much stronger on day 1 than on day 3 after infection, indicating that the strong transcriptional response in the lung precedes the peak of viral replication. Surprisingly, the set of down-regulated genes was small and none of these genes displayed strong down-regulation. Responses in the lung-draining lymph nodes were much less prominent than lung responses and are suggestive of NK cell activation. Our data indicate that at time points prior to the peak of viral replication and influx of inflammatory cells, the local lung response, measured at the transcriptional level, has already dampened down. The processes and pathways induced shortly after RSV infection can now be used for the selection of candidate genes for human genetic studies of children with severe RSV infection.

The NS2 protein of human respiratory syncytial virus suppresses the cytotoxic T-cell response as a consequence of suppressing the type I interferon response

The NS1 and NS2 proteins of human respiratory syncytial virus (HRSV) have been shown to antagonize the type I interferon (IFN) response, an effect subject to host range constraints. We have now found that the HRSV NS2 protein strongly controls IFN induction in mouse cells in vitro, validating the use of the mouse model to study the consequences of these gene deletions on host immunity. We evaluated the effects of deleting the NS1 and/or NS2 gene on the induction of HRSV-specific pulmonary cytotoxic T lymphocytes (CTL) in BALB/c and 129S6 mice in response to intranasal infection with HRSV lacking the NS1 and/or NS2 gene and subsequent challenge with wild-type (wt) HRSV. In mice infected with HRSV lacking the NS2 gene (DeltaNS2) or lacking the NS2 gene in combination with the NS1 gene (DeltaNS1/2 HRSV), the magnitude of the pulmonary CTL response was substantially elevated compared to that of mice infected with wt HRSV or the DeltaNS1 mutant, whether measured by binding of CD8(+) cells to an HRSV-specific major histocompatibility complex class I tetramer, by measurement of CD8(+) cells secreting gamma interferon (IFN-gamma) in response to specific in vitro stimulation, or by a standard chromium release cell-killing assay. In contrast, in STAT1 knockout mice, which lack responsiveness to type I IFN, the level of IFN-gamma-secreting CD8(+) cells was not significantly different for HRSV lacking the NS2 gene, suggesting that the increase in CTL observed in IFN-responsive mice is type I IFN dependent. Thus, the NS2 protein of HRSV suppresses the CTL component of the adaptive immune response, and this appears to be a consequence of its suppression of type I IFN.

Suppression of pattern-recognition receptor TLR4 sensing does not alter lung responses to pneumovirus infection

Toll-like receptors (TLR) are an important component in the innate immune response to a wide variety of pathogens. Recently, a series of studies has addressed the hypothesis that TLR4 also participates in the host innate response against respiratory syncytial virus (RSV), the leading cause of lower respiratory tract infections in infants and young children. In most of the studies available, RSV, which is not a natural pathogen of mice, has been systematically used in mouse models of human bronchiolitis, with conflicting results. Pneumonia virus of mice (PVM), a member of the pneumovirus genus, shares many similarities with RSV. The serological and structural relationships that exist between them suggest that the immune response to these viruses may be similar in their respective natural hosts. To determine the role of TLR4 in host defense against PVM, TLR4-competent and TLR4-deficient mice were intranasally infected with PVM. Variation of body weight, pulmonary function values, histopathology, and pulmonary viral loads were analyzed. None of the investigated clinical, functional, histological and virological parameters was different between strains, which demonstrates that the sensitivity of the mouse to its natural pneumovirus infection is independent of the presence or absence of TLR4 sensing.

Hyperresponsiveness to inhaled but not intravenous methacholine during acute respiratory syncytial virus infection in mice

Background

To characterise the acute physiological and inflammatory changes induced by low-dose RSV infection in mice.

Methods

BALB/c mice were infected as adults (8 wk) or weanlings (3 wk) with 1 × 10⁵ pfu of RSV A2 or vehicle (intranasal, 30 μl). Inflammation, cytokines and inflammatory markers in bronchoalveolar lavage fluid (BALF) and airway and tissue responses to inhaled methacholine (MCh; 0.001 – 30 mg/ml) were measured 5, 7, 10 and 21 days post infection. Responsiveness to iv MCh (6 – 96 μg/min/kg) in vivo and to electrical field stimulation (EFS) and MCh in vitro were measured at 7 d. Epithelial permeability was measured by Evans Blue dye leakage into BALF at 7 d. Respiratory mechanics were measured using low frequency forced oscillation in tracheostomised and ventilated (450 bpm, flexiVent) mice. Low frequency impedance spectra were calculated (0.5 – 20 Hz) and a model, consisting of an airway compartment [airway resistance (Raw) and inertance (Iaw)] and a constant-phase tissue compartment [coefficients of tissue damping (G) and elastance (H)] was fitted to the data.

Results

Inflammation in adult mouse BALF peaked at 7 d (RSV 15.6 (4.7 SE) vs. control 3.7 (0.7) × 10⁴ cells/ml; p < 0.001), resolving by 21 d, with no increase in weanlings at any timepoint. RSV-infected mice were hyperresponsive to aerosolised MCh at 5 and 7 d (PC₂₀₀ Raw adults: RSV 0.02 (0.005) vs. control 1.1 (0.41) mg/ml; p = 0.003) (PC₂₀₀ Raw weanlings: RSV 0.19 (0.12) vs. control 10.2 (6.0) mg/ml MCh; p = 0.001). Increased responsiveness to aerosolised MCh was matched by elevated levels of cysLT at 5 d and elevated VEGF and PGE₂ at 7 d in BALF from both adult and weanling mice. Responsiveness was not increased in response to iv MCh in vivo or EFS or MCh challenge in vitro. Increased epithelial permeability was not detected at 7 d.

Conclusion

Infection with 1 × 10⁵ pfu RSV induced extreme hyperresponsiveness to aerosolised MCh during the acute phase of infection in adult and weanling mice. The route-specificity of hyperresponsiveness suggests that epithelial mechanisms were important in determining the physiological effects. Inflammatory changes were dissociated from physiological changes, particularly in weanling mice.

Expression of interleukin-4 by recombinant respiratory syncytial virus is associated with accelerated inflammation and a nonfunctional cytotoxic T-lymphocyte response following primary infection but not following challenge with wild-type virus

The outcome of a viral infection or of immunization with a vaccine can be influenced by the local cytokine environment. In studies of experimental vaccines against respiratory syncytial virus (RSV), an increased stimulation of Th2 (T helper 2) lymphocytes was associated with increased immunopathology upon subsequent RSV infection. For this study, we investigated the effect of increased local expression of the Th2 cytokine interleukin-4 (IL-4) from the genome of a recombinant RSV following primary infection and after a challenge with wild-type (wt) RSV. Mice infected with RSV/IL-4 exhibited an accelerated pulmonary inflammatory response compared to those infected with wt RSV, although the wt RSV group caught up by day 8. In the first few days postinfection, RSV/IL-4 was associated with a small but significant acceleration in the expansion of pulmonary T lymphocytes specific for an RSV CD8(+) cytotoxic T-lymphocyte (CTL) epitope presented as a major histocompatibility complex class I tetramer. However, by day 7 the response of tetramer-positive T lymphocytes in the wt RSV group caught up and exceeded that of the RSV/IL-4 group. At all times, the CTL response of the RSV/IL-4 group was deficient in the production of gamma interferon and was nonfunctional for in vitro cell killing. The accelerated inflammatory response coincided with an accelerated accumulation and activation of pulmonary dendritic cells early in infection, but thereafter the dendritic cells were deficient in the expression of B7-1, which governs the acquisition of cytolytic activity by CTL. Following a challenge with wt RSV, there was an increase in Th2 cytokines in the animals that had previously been infected with RSV/IL-4 compared to those previously infected with wt RSV, but the CD8(+) CTL response and the amount of pulmonary inflammation were not significantly different. Thus, a strong Th2 environment during primary pulmonary immunization with live RSV resulted in early inflammation and a largely nonfunctional primary CTL response but had a minimal effect on the secondary response.

Oral efficacy of a respiratory syncytial virus inhibitor in rodent models of infection

BMS-433771 is a potent inhibitor of respiratory syncytial virus (RSV) replication in vitro. Mechanism of action studies have demonstrated that BMS-433771 halts virus entry through inhibition of F protein-mediated membrane fusion. BMS-433771 also exhibited in vivo efficacy following oral administration in a mouse model of RSV infection (C. Cianci, K. Y. Yu, K. Combrink, N. Sin, B. Pearce, A. Wang, R. Civiello, S. Voss, G. Luo, K. Kadow, E. Genovesi, B. Venables, H. Gulgeze, A. Trehan, J. James, L. Lamb, I. Medina, J. Roach, Z. Yang, L. Zadjura, R. Colonno, J. Clark, N. Meanwell, and M. Krystal, Antimicrob. Agents Chemother. 48:413-422, 2004). In this report, the in vivo efficacy of BMS-433771 against RSV was further examined in the BALB/c mouse and cotton rat host models of infection. By using the Long strain of RSV, prophylactic efficacy via oral dosing was observed in both animal models. A single oral dose, administered 1 h prior to intranasal RSV inoculation, was as effective against infection as a 4-day b.i.d. dosing regimen in which the first oral dose was given 1 h prior to virus inoculation. Results of dose titration experiments suggested that RSV infection was more sensitive to inhibition by BMS-433771 treatment in the BALB/c mouse host than in the cotton rat. This was reflected by the pharmacokinetic and pharmacodynamic analysis of the efficacy data, where the area under the concentration-time curve required to achieve 50% of the maximum response was approximately 7.5-fold less for mice than for cotton rats. Inhibition of RSV by BMS-433771 in the mouse is the result of F1-mediated inhibition, as shown by the fact that a virus selected for resistance to BMS-433771 in vitro and containing a single amino acid change in the F1 region was also refractory to treatment in the mouse host. BMS-433771 efficacy against RSV infection was also demonstrated for mice that were chemically immunosuppressed by cyclophosphamide treatment, indicating that compound inhibition of the virus did not require an active host immune response.

NK T cells contribute to expansion of CD8(+) T cells and amplification of antiviral immune responses to respiratory syncytial virus

CD1d-deficient mice have normal numbers of T lymphocytes and natural killer cells but lack Valpha14(+) natural killer T cells. Respiratory syncytial virus (RSV) immunopathogenesis was evaluated in 129xC57BL/6, C57BL/6, and BALB/c CD1d(-/-) mice. CD8(+) T lymphocytes were reduced in CD1d(-/-) mice of all strains, as shown by cell surface staining and major histocompatibility complex class I tetramer analysis, and resulted in strain-specific alterations in illness, viral clearance, and gamma interferon (IFN-gamma) production. Transient activation of NK T cells in CD1d(+/+) mice by alpha-GalCer resulted in reduced illness and delayed viral clearance. These data suggest that early IFN-gamma production and efficient induction of CD8(+)-T-cell responses during primary RSV infection require CD1d-dependent events. We also tested the ability of alpha-GalCer as an adjuvant to modulate the type 2 immune responses induced by RSV glycoprotein G or formalin-inactivated RSV immunization. However, immunized CD1-deficient or alpha-GalCer-treated wild-type mice did not exhibit diminished disease following RSV challenge. Rather, some disease parameters, including cytokine production, eosinophilia, and viral clearance, were increased. These findings indicate that CD1d-dependent NK T cells play a role in expansion of CD8(+) T cells and amplification of antiviral responses to RSV.

Granulocyte-macrophage colony-stimulating factor expressed by recombinant respiratory syncytial virus attenuates viral replication and increases the level of pulmonary antigen-presenting cells

An obstacle to developing a vaccine against human respiratory syncytial virus (RSV) is that natural infection typically does not confer solid immunity to reinfection. To investigate methods to augment the immune response, recombinant RSV (rRSV) was constructed that expresses murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) from a transcription cassette inserted into the G-F intergenic region. Replication of rRSV/mGM-CSF in the upper and lower respiratory tracts of BALB/c mice was reduced 23- to 74- and 5- to 588-fold, respectively, compared to that of the parental rRSV. Despite this strong attenuation of replication, the level of RSV-specific serum antibodies induced by rRSV/mGM-CSF was comparable to, or marginally higher than, that of the parental rRSV. The induction of RSV-specific CD8(+) cytotoxic T cells was moderately reduced during the initial infection, which might be a consequence of reduced antigen expression. Mice infected with rRSV/mGM-CSF had elevated levels of pulmonary mRNA for gamma interferon (IFN-gamma) and interleukin 12 (IL-12) p40 compared to animals infected by wild-type rRSV. Elevated synthesis of IFN-gamma could account for the restriction of RSV replication, as was observed previously with an IFN-gamma-expressing rRSV. The accumulation of total pulmonary mononuclear cells and total CD4(+) T lymphocytes was accelerated in animals infected with rRSV/mGM-CSF compared to that in animals infected with the control virus, and the level of IFN-gamma-positive or IL-4-positive pulmonary CD4(+) cells was elevated approximately twofold. The number of pulmonary lymphoid and myeloid dendritic cells and macrophages was increased up to fourfold in mice infected with rRSV/mGM-CSF compared to those infected with the parental rRSV, and the mean expression of major histocompatibility complex class II molecules, a marker of activation, was significantly increased in the two subsets of dendritic cells. Enhanced antigen presentation likely accounts for the maintenance of a strong antibody response despite reduced viral replication and would be a desirable property for a live attenuated rRSV vaccine.

Blocking intercellular adhesion molecule-1 on human epithelial cells decreases respiratory syncytial virus infection

The respiratory syncytial virus (RSV) causes potentially fatal lower respiratory tract infection in infants. The molecular mechanism of RSV infection is unknown. Our data show that RSV colocalizes with intercellular adhesion molecule-1 (ICAM-1) on the HEp-2 epithelial cell surface. Furthermore, a neutralizing anti-ICAM-1 mAb significantly inhibits RSV infection and infection-induced secretion of proinflammatory chemokine RANTES and mediator ET-1 in HEp-2 cells. Similar decrease in RSV infection is also observed in A549, a type-2 alveolar epithelial cell line, and NHBE, the normal human bronchial epithelial cell line when pretreated with anti-ICAM-1 mAb prior to RSV infection. Incubation of virus with soluble ICAM-1 also significantly decreases RSV infection of epithelial cells. Binding studies using ELISA indicate that RSV binds to ICAM-1, which can be inhibited by an antibody to the fusion F protein and also the recombinant F protein can bind to soluble ICAM-1, suggesting that RSV interaction with ICAM-1 involves the F protein. It is thus concluded that ICAM-1 facilitates RSV entry and infection of human epithelial cells by binding to its F protein, which is important to viral replication and infection and may lend itself as a therapeutic target.

Both immunisation with a formalin-inactivated respiratory syncytial virus (RSV) vaccine and a mock antigen vaccine induce severe lung pathology and a Th2 cytokine profile in RSV-challenged mice

Respiratory syncytial virus (RSV) is the most important cause of bronchiolitis and pneumonia in infants and young children. Immunopathology may play a role in RSV-induced disease and a severe RSV infection may also be associated with an increased risk of developing asthma. Vaccination with formalin-inactivated RSV (FI-RSV) prior to infection resulted both in human and in the mouse model in extensive lung pathology. In the mouse model, it has been shown that this aggravation of disease was associated with a shift in the balance between Th1 and Th2 cytokines towards a Th2-type response. The aim of the present study was to characterise the immunological and inflammatory responses in BALB/c mice upon RSV infection with or without prior vaccination with aluminium-adjuvanted FI-RSV or control antigens (FI-Mock). As previously reported by others, we also observed that a primary RSV infection in BALB/c mice resulted in a predominant Th1-type cytokine response, which was associated with slight bronchiolitis and alveolitis. FI-RSV vaccination prior to RSV challenge prevented virus replication and was associated with an aggravation of pulmonary histopathology and a shift towards a Th2-type response. Vaccination with FI-Mock did not prevent RSV replication in the lung but resulted in an even more pronounced Th2 response after infection while these mice were not sensitised to specific viral antigens. Thus, viral replication in a Th2 responding animal (induced by aluminium-adjuvanted mock vaccine) appears to boost the Th2 response upon RSV infection.

Critical roles for interleukin-4 and interleukin-5 during respiratory syncytial virus infection in the development of airway hyperresponsiveness after airway sensitization

In mice, respiratory syncytial virus (RSV) infection can enhance the consequences of allergic airway sensitization, resulting in lung eosinophilia and the development of airway hyperresponsiveness (AHR) to inhaled methacholine (MCh). To delineate a role for interleukin-5 (IL-5), interleukin-4 (IL-4), and interferon gamma (IFN-gamma) in mediating the effects of RSV infection on subsequent allergic sensitization, we treated BALB/c mice with anti-IL-5 during acute RSV infection but not during subsequent exposure to ovalbumin (OVA). IL-5-deficient and IL-4-deficient mice were also treated with IL-5 either during acute RSV infection or during the sensitization period. Airway responsiveness to inhaled MCh was assessed and numbers of lung eosinophils were monitored. Anti-IL-5 treatment during RSV infection reduced AHR and lung eosinophilia after subsequent exposure to allergen. In IL-5-deficient or IL-4-deficient mice lung eosinophilia and AHR after RSV infection and allergen exposure were also markedly reduced. IL-5 administration during RSV infection restored the responses to allergen in both IL-5- and IL-4-deficient mice. However, IL-5 administration only during sensitization restored these responses in IL-4-deficient but not in IL-5-deficient animals. IFN-gamma-deficient mice developed AHR and some lung eosinophilia after allergen exposure alone and when RSV infection preceded allergen, these responses were enhanced. We conclude that both IL-5, particularly during acute infection, and IL-4 are critical in mediating the effects of RSV infection on allergic airway sensitization, resulting in the development of AHR and lung eosinophilia.

Differential cytokine mRNA expression in Dermatophagoides farinae allergen-sensitized and respiratory syncytial virus-infected mice

The interaction between mite allergen sensitization and respiratory syncytial virus (RSV) infection at the level of cytokine mRNA expression was examined in a murine model in the present study. Primary RSV infection enhances expression of inflammatory cytokines such as IL-6, IFN-gamma, and eotaxin in the lung and upregulates the expression of Th2-like cytokines IL-10 and IL-13 in the spleen in BALB/c mice. Mite antigen-sensitized and RSV-infected (ASRSV) mice show enhanced (P < 0.05) total serum IgE compared to antigen-sensitized mice. However, the levels of viral mRNA in the lung tissues are comparable between RSV-infected and ASRSV mice. It is concluded that compartmentalization of cytokine expression following RSV infection plays a role in the augmentation of Th2-like and IgE antibody response to RSV.

Influenza and respiratory syncytial virus. Update on infection, management, and prevention

This article is an overview of the epidemiology and spectrum of clinical disease attributed to influenza and respiratory syncytial virus in children. It separately discusses the pathogenesis of the two diseases and the host responses to the viruses to emphasize each infection's significance and need for a vaccine. Updates on current preventive measures and a preview of potential future vaccine developments are presented.

Absence of lung immunopathology following respiratory syncytial virus (RSV) challenge in mice immunized with a recombinant RSV G protein fragment

The relative immunopathogenic potential of a recombinant fusion protein incorporating residues 130-230 of respiratory syncytial virus (RSV-A) G protein (BBG2Na), formalin-inactivated RSV-A (FI-RSV), and phosphate-buffered saline (PBS) was investigated in mice after immunization and RSV challenge. FI-RSV priming resulted in massive infiltration of B cells and activated CD4(+) and CD8(+) T lymphocytes in mediastinal lymph nodes (MLN) and lungs, where eosinophilia and elevated IFN-gamma, IL-2, -4, -5, -10, and -13 mRNA transcripts were also detected. PBS-primed mice showed only elevated pulmonary IL-2 and IFN-gamma mRNAs, while an activated CD8(+) T cell peak was detected in MLN and lungs. Cell infiltration also occurred in MLN of BBG2Na-immunized mice. However, there was no evidence of T cell, B cell, or granulocyte infiltration or activation in lungs, while transient transcription of Th1-type cytokine genes was evident. The absence of pulmonary infiltration is unlikely due to insufficient viral antigen. Thus, this recombinant fusion RSV G fragment does not prime for adverse pulmonary immunopathologic responses.

Respiratory syncytial virus infection: immune response, immunopathogenesis, and treatment

Respiratory syncytial virus (RSV) is the single most important cause of lower respiratory tract infection during infancy and early childhood. Once RSV infection is established, the host immune response includes the production of virus-neutralizing antibodies and T-cell-specific immunity. The humoral immune response normally results in the development of anti-RSV neutralizing-antibody titers, but these are often suboptimal during an infant's initial infection. Even when the production of RSV neutralizing antibody following RSV infection is robust, humoral immunity wanes over time. Reinfection during subsequent seasons is common. The cellular immune response to RSV infection is also important for the clearance of virus. This immune response, vital for host defense against RSV, is also implicated in the immunopathogenesis of severe lower respiratory tract RSV bronchiolitis. Many details of the immunology and immunopathologic mechanisms of RSV disease known at present have been learned from rodent models of RSV disease and are discussed in some detail. In addition, the roles of immunoglobulin E, histamine, and eosinophils in the immunopathogenesis of RSV disease are considered. Although the treatment of RSV bronchiolitis is primarily supportive, the role of ribavirin is briefly discussed. Novel approaches to the development of new antiviral drugs with promising anti-RSV activity in vitro are also described.

CD8 T Cells Are Essential in the Development of Respiratory Syncytial Virus-Induced Lung Eosinophilia and Airway Hyperresponsiveness

Viral respiratory infections can cause bronchial hyperresponsiveness and exacerbate asthma. In mice, respiratory syncytial virus (RSV) infection results in airway hyperresponsiveness (AHR) and eosinophil influx into the airways. The immune cell requirements for these responses to RSV infection are not well defined. To delineate the role of CD8 T cells in the development of RSV-induced AHR and lung eosinophilia, we tested the ability of mice depleted of CD8 T cells to develop these symptoms of RSV infection. BALB/c mice were depleted of CD8 T cells using anti-CD8 Ab treatment before intranasal administration of infectious RSV. Six days postinfection, airway responsiveness to inhaled methacholine was assessed by barometric body plethysmography, and numbers of lung eosinophils and levels of IFN-γ, IL-4, and IL-5 in bronchoalveolar lavage fluid were monitored. RSV infection resulted in airway eosinophilia and AHR in control mice, but not in CD8-depleted animals. Further, whereas RSV-infected mice secreted increased amounts of IL-5 into the airways as compared with noninfected controls, no IL-5 was detectable in both bronchoalveolar lavage fluid and culture supernatants from CD8-depleted animals. Treatment of CD8-depleted mice with IL-5 fully restored both lung eosinophilia and AHR. We conclude that CD8 T cells are essential for the influx of eosinophils into the lung and the development of AHR in response to RSV infection.

IL-5 and Eosinophils Are Essential for the Development of Airway Hyperresponsiveness Following Acute Respiratory Syncytial Virus Infection

Viral respiratory infections can cause bronchial hyperresponsiveness and exacerbate asthma. In mice, respiratory syncytial virus (RSV) infection, which induces an immune response dominated by IFN-γ, results in airway hyperresponsiveness (AHR) and eosinophil influx into the airways, both of which are prevented by pretreatment with anti-IL-5 Ab. To delineate the role of IL-5, IL-4, and IFN-γ in the development of RSV-induced AHR and lung eosinophilia, we tested the ability of mice deficient in each of these cytokines to develop these symptoms of RSV infection. Mice deficient in either IL-5, IL-4, or IFN-γ were administered infectious RSV intranasally, and 6 days later, airway responsiveness to inhaled methacholine was assessed by barometric body plethysmography, and numbers of lung eosinophils and production of IFN-γ, IL-4, and IL-5 by mononuclear cells from peribronchial lymph nodes were monitored. RSV infection resulted in airway eosinophilia and AHR in both IL-4- and IFN-γ-deficient mice, but not in IL-5-deficient mice. Reconstitution of IL-5-deficient mice with IL-5 restored these responses and enhanced the responses in IL-4-deficient mice. Anti-VLA-4 (very late Ag-4) treatment prevented lung eosinophilia and AHR following RSV infection and IL-5 reconstitution. We conclude that in response to RSV, IL-5 is essential for the influx of eosinophils into the lung and that eosinophils in turn are critical for the development of AHR. IFN-γ and IL-4 are not essential for these responses to RSV infection.

Phenotypic analysis of local cellular responses in calves infected with bovine respiratory syncytial virus

Changes in lymphocyte subsets in the trachea, pulmonary tissue, bronchoalveolar lavage (BAL), peripheral blood and bronchial lymph node (BLN) of gnotobiotic calves infected with bovine respiratory syncytial virus (BRSV) were analysed by flow cytometry. Following BRSV infection, virus titres in the nasopharynx reached a peak between days 5 and 7 and infection was resolving from day 10. Although calves did not develop signs of clinical respiratory disease, there was evidence of gross pneumonia and histological changes typical of BRSV bronchiolitis, which were most extensive from day 710 of infection. Following BRSV infection there was a recruitment of CD8+ T cells into the trachea and lung, which peaked on day 10 after infection. Thus, there were approximately equal numbers of CD8+ and CD4+ T cells in the lung and trachea of uninfected calves, whereas by day 10 of infection, CD8+ cells outnumbered CD4+ cells by 3:1 in the lungs and 6:1 in the trachea of the infected calves. Although the increase in CD4+ T cells into the lungs was less marked than that of CD8+ T cells, changes in expression of CD45R, CD45RO, L-selectin and interleukin-2 receptors all suggested that CD4+ T cells were activated during BRSV infection. Changes in gamma delta T cells were not observed in BRSV-infected calves. There was a marked increase in B cells in the BLN after infection and BLN CD4+ T cells changed from the majority expressing L-selectin and CD45R in uninfected calves to a predominance of L-selectin- CD45R- CD45RO+ phenotype, 10 days after infection. In conclusion, CD8+ T cells constitute the major lymphocyte subpopulation in the respiratory tract of calves recovering from BRSV infection.

Development of human antibodies against linear antigenic and immunogenic regions of respiratory syncytial virus (RSV) nucleocapsid and phospho-proteins shows the site-directed characteristics

BACKGROUND

The successful development of an RSV vaccine requires a better understanding of the pathogenesis of primary infection, susceptibility to reinfection, and the immunopathology of enhanced illness in children immunized with a non-replicating RSV candidate vaccine. The exact role of different immune parameters in RSV pathogenesis remains controversial.

OBJECTIVES

To study the contribution of antibodies directed to the linear antigenic and immunogenic regions of the N and P proteins in the titer rise and avidity maturation of total anti-RSV antibodies.

STUDY DESIGN

The occurrence of antibodies directed against three linear antigenic and immunogenic regions in each of the nucleocapsid (N): N3 (Thr11 to Gly30), N25 (Ser231 to Ala250) and N39 (Thr371 to Leu391), and the phospho-(P) proteins of respiratory syncytial virus (RSV), subgroup A: P49 (Pro91 to Asp110), P56 (Ser161 to Lys180) and P62 (Glu221 to Phe241), were analyzed in ELISA with (a) 32 paired sera from humans with recent or previous RSV subgroup A and/or B infection diagnosed by conventional ELISA, detection of antigen in nasopharyngeal aspirates and measurement of antibody avidity change; and (b) 40 RSV antibody-positive sera (HCS) obtained from patients during their convalescence from RSV infection and possessing clearly demonstrable titers of RSV IgG in conventional enzyme immunoassays (EIA) based on whole RSV antigen.

RESULTS

The titer rise of antibodies directed to the combined three peptides representing the RSV N protein was well correlated with the rise in anti-RSV antibodies measured in whole antigen ELISA. Surprisingly, the rise in antibodies against a truncated main C-terminal epitope (Gln381 to Leu391) of the N protein (represented by subgroup A specific sequence of the N39/1 peptide) was inversely correlated with the titer rise of total anti-RSV antibodies. The titer rise of antibodies to the C-terminal linear site of the RSV N (N39/1) protein was subgroup-specific during the course of primary RSV infection. A titer rise in antibodies to the C-terminal linear sites of RSV N (i.e. N39/1) and P (P62) proteins had a dominating appearance in sera from newborn infants (6-7 months) and from patients with RSV reinfections. Anti-RSV antibody titers of late convalescent sera correlated with the titers of antibodies directed to the C-terminal linear site of RSV P (P62) protein. The avidity maturation of the anti-RSV immune response followed the titer rise of anti-P62 antibodies during the course of primary or secondary RSV infection.

Respiratory syncytial virus infection results in airway hyperresponsiveness and enhanced airway sensitization to allergen

Viral respiratory infections can predispose to the development of asthma by mechanisms that are presently undetermined. Using a murine model of respiratory syncytial virus (RSV) infection, acute infection is associated with airway hyperresponsiveness as well as enhanced responses to subsequent sensitization to allergen. We demonstrate that acute viral infection results in increased airway responsiveness to inhaled methacholine and pulmonary neutrophilic and eosinophilic inflammation. This response is associated with predominant production of Th-1-type cytokines in peribronchial lymph node cells in vitro. Mice sensitized to ovalbumin via the airways after RSV infection developed increased airway responsiveness to methacholine and pulmonary eosinophilic and neutrophilic inflammation, associated with the predominant production of Th-2-type cytokines. Treatment of the mice with anti-IL-5 antibody abolished airway hyperresponsiveness and eosinophilic but not neutrophilic inflammation in both acutely infected mice and mice sensitized after infection. We conclude that RSV infection results in airway hyperresponsiveness in the acute phase and leads to changes in immune function that can enhance the effects of airway sensitization to antigen after infection. In both situations, airway hyperresponsiveness is closely associated with pulmonary eosinophilic inflammation. This model provides a means for further analyzing the influence of viral respiratory infections on airway sensitization and the development of altered airway responsiveness.

Immune responses of lambs to the fusion (F) glycoprotein of bovine respiratory syncytial virus expressed on insect cells infected with a recombinant baculovirus

A group of lambs was immunized with the F protein of bovine respiratory syncytial virus expressed in a baculovirus (Bac-F) and their humoral and cellular immune responses to bovine RSV studied before and after challenge with infectious bovine RSV. Immunization with Bac-F resulted in significant humoral immune responses as measured by virus neutralization and cellular immune responses as measured by lympho-proliferation against inactivated bovine RSV and specific cytotoxicity against autologous targets. Challenge infection with bovine RSV was characterized by significant anamnestic responses in Bac-F immunized lambs and virus shedding in nasal secretions was significantly lower in Bac-F immunized lambs than in control lambs.

Comparison of the T helper cell response induced by respiratory syncytial virus and its fusion protein in BALB/c mice

Specific proliferative T-cell responses were induced in the lymph node cells (LNC) of mice immunised with a sucrose density gradient purified preparation of respiratory syncytial (RS) virus or an immunoaffinity purified preparation of the F glycoprotein. Inhibition studies and flow cytometric analysis showed that the responding cell population were CD4+ T cells. The cytokines produced by virus-specific and F-specific cells were assessed using the CTLL cell line. Peak quantities of cytokine were consistently detected in the supernatants of stimulated cultures 24 h prior to maximum proliferation. The proportion of IL-2 released was determined by blocking IL-2 activity with an anti-IL-2 monoclonal antibody. In cultures of RS virus primed LNC challenged with whole virus there was a switch of cytokine production from 70% IL-2 at day 3 to 80% IL-4 by 6 days of culture. In contrast, LNC cultures from mice immunised with F protein secreted 75-100% IL-2 throughout the culture period. These data suggest that after 6 days of challenge with viral antigen, the RS virus-primed LNC response consists of T helper cells which are predominantly of the Th2 subset, secreting IL-4, whilst F protein-primed LNC secrete large quantities of IL-2 and can therefore be classified as predominantly of the Th1 subset.

Formulation of the purified fusion protein of respiratory syncytial virus with the saponin QS-21 induces protective immune responses in Balb/c mice that are similar to those generated by experimental infection

The feasibility of employing a vaccine composed of the purified fraction 21 of Quillaja saponaria (QS-21) and the fusion (F) protein of respiratory syncytial virus (RSV) to induce protective immune responses in the lower respiratory tract of Balb/c mice was examined. Our goal was to compare local and systemic immune responses with those induced following immunization with the protein adsorbed to aluminium hydroxide (F/ALOH) adjuvant or by experimental infection. Sera from mice vaccinated with the QS-21 formulation (F/QS-21) contained elevated anti-F protein IgG antibody titres that were dependent on the dose of QS-21 employed. Similar to the immune responses generated by experimental infection, the sera from mice vaccinated with F/QS-21 possessed greater capacity to neutralize virus infectivity that was associated with the generation of heightened complement-fixing IgG2a antibody titres. In contrast, vaccination with F/ALOH elicited systemic immune responses that were characterized by a predominance of protein-specific antibodies of the IgG1 subclass and lower neutralizing antibody titres. The capacity of F/QS-21 to facilitate local pulmonary immune responses was also examined and found to be similar to those induced by experimental infection. After virus challenge, a 90-fold increase in the number of F protein-specific antibody-secreting cells was observed and associated with the clearance of virus from the infected lungs. Moreover, elevated levels of antigen-dependent killer cell activity were detected and appeared to be mediated by class I major histocompatibility complex restricted CD8+ T cells. Additional characterization of the pulmonary immune response was performed on the cellular infiltrates obtained after bronchoalveolar lavage and on formalin-fixed lung tissue. The local protective immune responses induced after challenge of the groups immunized with F/QS-21 or infectious virus were significantly different from those elicited in naive control mice injected with adjuvant alone, or in mice immunized with F/ALOH. The cellularity of the lavage fluids from the former groups was characterized by a significantly greater percentage of lymphocytes and less neutrophils. In similar fashion histological evaluation of the lungs from mice immunized with F/QS-21 or infectious virus revealed significantly elevated local immune responses after challenge. In conclusion, the results suggest that formulation with F/QS-21 alters the qualitative and quantitative nature of the immune response to the F glycoprotein when compared with the traditional aluminium-based adjuvants.

Interaction of alveolar macrophages and respiratory syncytial virus

Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract illness in infants. However, the mechanisms leading to resolution of RSV infections are poorly understood. Since alveolar macrophages play an important role in defending the respiratory tract against infectious agents we investigated the interactions of RSV with these cells. Murine alveolar macrophages were challenged in vitro with RSV at different multiplicities of infection. The percentage of macrophages expressing viral antigen was determined by staining with monoclonal anti-RSV antibodies and evaluation by fluorescence microscopy or FACS analysis. The ability of macrophages to support virus replication was measured by a plaque forming assay on HEp-2 cells. Cell lysates of macrophages contained only small amounts of viable RSV in comparison to disrupted HEp-2 cells. The amount of viable RSV as well as the percentage of macrophages expressing viral antigen decreased rapidly over time. Activated macrophages had a reduced virus load in comparison to resting macrophages. RSV infected macrophages released biologically active tumor necrosis factor (TNF) in a virus dose dependent manner. In contrast, a high virus inoculum resulted in reduced microbicidal activity and oxygen radical production. Our results suggest that RSV infection influences different functions of alveolar macrophages in various ways. Since TNF is thought to restrict viral replication in several cell types it may play a role in limiting virus replication.

In vivo production of tumour necrosis factor-alpha and interleukin-6 in BALB/c mice inoculated intranasally with a high dose of respiratory syncytial virus

Intranasal administration of an inoculum of 10(7) focus-forming units (FFU) of respiratory syncytial (RS) virus induced disease in BALB/c mice with signs of anorexia, cachexia, ruffled fur, and pneumonia. Mice displayed mild signs of illness on day 1 postinoculation (PI), followed by a transient recovery phase of 3 days. Disease rapidly reappeared on day 5 PI and worsened on subsequent days, with mortalities by day 7 PI. Mice inoculated with 5 x 10(6) FFU exhibited milder signs of disease, while those inoculated with 2 x 10(6) FFU and control mice given only Hep-2c cell suspension exhibited no noticeable signs of illness. High levels of bioactive tumour necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) were detected in both lungs and sera of mice inoculated with 10(7) FFU of virus. Peak levels of both cytokines were detected at day 1 PI but remained detectable throughout the 7 day period studied postinoculation. Cytokine levels were much lower or were undetectable in control mice. These results suggest that the macrophage is stimulated in vivo to produce inflammatory cytokines in response to RS virus infection.

The pulmonary immune response of Balb/c mice vaccinated with the fusion protein of respiratory syncytial virus

We have investigated the efficacy of vaccination with the purified fusion (F) protein of respiratory syncytial virus (RSV) on aluminium hydroxide adjuvant in Balb/c mice. The purpose of the study was to define the role of the local pulmonary mononuclear cell (PMC) infiltrate in the clearance of virus from the lower respiratory tract. Balb/c mice immunized with F protein were able to inhibit the replication of virus in the lungs as early as 4 days after intranasal challenge. In contrast, unimmunized mice required 8 days. Examination of humoral immune mechanisms demonstrated that vaccination with the purified protein induced moderate titres of serum neutralizing antibody. In addition, immunization induced low to moderate levels of antigen-dependent killer cell activity. To examine the immunological events responsible for virus clearance in vivo, PMC infiltrates were isolated after virus challenge and tested directly for protective capacity. After virus challenge, the F protein-immune mice were able to recall the cytolytic cells to the pulmonary tissues. The results further suggested that the local antigen-dependent killer activity was mediated by cytolytic T cells of the CD8 phenotype. Adoptive transfer studies were also conducted to identify further the role the PMC infiltrate had in protective immunity. Adoptive transfer of F protein-educated PMC into naive syngeneic recipients suggested that the pulmonary infiltrates contained the cellular constituents necessary for protective immunity. Both humoral and cellular immune elements were present.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunization of mice with vaccinia virus-M2 recombinant induces epitope-specific and cross-reactive Kd-restricted CD8+ cytotoxic T cells

The M2 protein of respiratory syncytial virus (RSV) is a protective antigen in H-2d, but not H-2b or H-2k mice. None of the other RSV proteins, excluding the surface glycoproteins that induce neutralizing antibodies, is protective in mice bearing these haplotypes. Thus, the M2 protein stands alone as a nonglycoprotein-protective antigen of RSV. The M2 protein is a target for murine Kd-restricted cytotoxic T lymphocytes (CTLs), and the resistance induced by infection with a vaccinia virus-RSV M2 (vac-M2) recombinant is mediated by CD8+ CTLs. Since the nonameric consensus sequence for H-2 Kd-restricted T-cell epitopes and the amino acid sequence of the M2 protein of subgroup A and B strains of RSV are known, the present study sought to identify the specific epitope(s) on the M2 protein recognized by CD8+ CTLs. This was done by examining the ability of four predicted Kd-specific motif peptides present in the M2 amino acid sequence of an RSV subgroup A strain to sensitize target cells for lysis by pulmonary or splenic CTLs obtained from mice infected with RSV or vac-M2. The following observations were made. First, two of the four peptides sensitized target cells for lysis by pulmonary or splenic CTLs induced by infection with either vac-M2 or RSV. Second, one of the two peptides, namely the 82-90 (M2) peptide, sensitized targets at a very low peptide concentration (10(-10) to 10(-12) M). Third, cold-target competition experiments revealed that the predominant CTL population induced by infection with vac-M2 or RSV recognized the 82-90 (M2) peptide, and this CTL population appeared to recognize the 71-79 (M2) peptide in a cross-reactive manner. Fourth, CTL recognition of targets sensitized with either the 71-79 (M2) or the 82-90 (M2) peptide was Kd restricted. Fifth, CTLs induced by infection with RSV subgroup A or B strains recognized the two M2 peptides. The findings suggest that the M2 protein of RSV contains an immunodominant Kd-restricted CTL epitope consisting of amino acid residues 82 to 90 (SYIGSINNI), which are shared by subgroup A and B RSVs.

Identification of T-cell epitopes adjacent to neutralizing antigenic domains on the fusion protein of respiratory syncytial virus

F glycoprotein has been identified as an important target structure in the immunological response following respiratory syncytial virus (RSV) infection. Two sequential B epitopes corresponding to amino acids 200 to 225 and 255 to 278 have already been defined with anti-RSV rabbit serum. The T helper response to peptides which belong to these sequences was investigated in this study. Proliferative T-cell responses to these peptides were analysed in BALB/c mice (H-2d) and others strains: SJL (H-2s), C3H/He (H-2k), B10.BR (H-2k) and C57BL/6 (H-2b). By using various strategies, two T-cell epitopes were identified in the amino acid 200 to 225 and 255 to 278 regions, close to a neutralizing epitope. The T-cell responses to these peptides were H-2- restricted. In addition, the peptide 255-278 was able to stimulate T cells that responded to a subsequent immunization with F glycoprotein in vitro.

The cytolytic activity of pulmonary CD8+ lymphocytes, induced by infection with a vaccinia virus recombinant expressing the M2 protein of respiratory syncytial virus (RSV), correlates with resistance to RSV infection in mice

Previous studies demonstrated that the pulmonary resistance to respiratory syncytial virus (RSV) challenge induced by immunization with a recombinant vaccinia virus expressing the M2 protein of RSV (vac-M2) was significantly greater 9 days after immunization than at 28 days and was mediated predominantly by CD8+ T cells. In this study, we have extended these findings and sought to determine whether the level of CD8+ cytotoxic T-lymphocyte (CTL) activity measured in vitro correlates with the resistance to RSV challenge in vivo. Three lines of evidence documented an association between the presence of pulmonary CTL activity and resistance to RSV challenge. First, vac-M2 immunization induced pulmonary CD8+ CTL activity and pulmonary resistance to RSV infection in BALB/c (H-2d) mice, whereas significant levels of pulmonary CTL activity and resistance to RSV infection were not seen in BALB.K (H-2k) or BALB.B (H-2b) mice. Second, pulmonary CD8+ CTL activity was not induced by infection with other vaccinia virus-RSV recombinants that did not induce resistance to RSV challenge. Third, the peak of pulmonary CTL activity correlated with the peak of resistance to RSV replication (day 6), with little resistance being observed 45 days after immunization. An accelerated clearance of virus was not observed when mice were challenged with RSV 45 days after immunization with vac-M2. The results indicate that resistance to RSV induced by immunization with vac-M2 is mainly mediated by primary pulmonary CTLs and that this resistance decreases to very low levels within 2 months following immunization. The implications for inclusion of CTL epitopes into RSV vaccines are discussed in the context of these observations.

T cell redistribution kinetics after secondary infection of BALB/c mice with respiratory syncytial virus

BALB/c mice were infected intranasally with live respiratory syncytial virus (RSV) and reinfected 4 weeks later. At regular intervals thereafter groups of animals were killed and T cell subsets were determined in blood, spleen and bronchoalveolar lavage (BAL) with flow cytometry employing T cell subset-specific MoAbs. Total lymphocyte counts in the peripheral blood decreased 1-3 days after infection, returning to preinfection levels on day 8 (P = 0.0111). Simultaneously, a marked increase of lymphocytes was noted in the BAL, reaching a maximum at day 8 (P < 0.0001). Both CD4+ and CD8+ T cells decreased in the blood on day 1-3 (P < 0.0097 and P = 0.003 respectively), and increased in the BAL progressively towards a maximum at day 8 (P < 0.0001). In BAL, CD4+ cells increased 35-fold and CD8+ cells 27-fold during the first week after reinfection. On the other hand, in the spleen a significant decline of CD4+ and CD8+ cells was noted 1 day post-infection (P = 0.0002). It is concluded that a strong T cell redistribution response among systemic and mucosal tissues occurs after reinfection with RSV. The kinetics of this response differ both quantitatively and qualitatively from the T cell response after primary infection. The magnitude of cell traffic is more pronounced in blood, spleen and BAL than after primary infection. CD4+ T cells are more intensively distributed to the lungs than after primary infection.

CD4+ T cells clear virus but augment disease in mice infected with respiratory syncytial virus. Comparison with the effects of CD8+ T cells

Respiratory syncytial (RS) virus-specific T cell lines were derived from the spleens of BALB/c mice primed by intranasal infection with RS virus. The lines were expanded by repeated antigenic stimulation in vitro, and separated into CD4+ and CD8+ T cell-enriched fractions by immunomagnetic adhesion. The effects of passive transfer of these fractions into RS virus infected mice were observed. The most severe immunopathological changes were seen in mice receiving CD4+ cells. Transfer of CD4+, CD8+ or both cell fractions caused RS virus-infected mice to become ill and lose weight. Both cell lines caused an increase in the severity of lung pathology (as monitored by bronchoalveolar lavage) with the appearance of lung haemorrhage and polymorphonuclear cell efflux. In addition, recipients of CD4+ cells developed striking pulmonary eosinophilia. In CD4+ cell recipients, 5 x 10(5) cells were sufficient to decrease lung virus titre, whereas 2 x 10(6) CD8+ cells were needed to produce a similar effect. The unseparated T cell line and the CD4+ cell fraction secreted significant amounts of IL-3, IL-4 and IL-5 (P less than 0.001). High levels of IL-2 were produced only by the unseparated T cell line. The CD8+ cell fraction secreted IL-3 only. The results show that, cell-for-cell, CD4+ cells are more anti-viral and more immunopathogenic than CD8+ cells in RS virus infected mice. Such effects may have contributed to the augmented disease seen in some infants vaccinated against RS virus.

Mucosal T cell distribution during infection with respiratory syncytial virus

Groups of 12-week-old Balb/c mice were inoculated intranasally with respiratory syncytial virus (RSV) and sacrificed at regular intervals after infection. T lymphocyte subset distribution was determined in lung tissue, bronchoalveolar lavage (BAL), peripheral blood, and spleen by means of flow cytometry employing monoclonal antibodies against the T cell membrane antigens Thy1.2 (pan-T), Ly2 (CD8), and L3T4 (CD4). Thy1.2+ cells increased in the lung from 35.4% of total lymphocytes before infection to 47.6% on day 7 after infection. This increase was largely accounted for by an increase in Ly2+ cells, which manifested a rise from 7.8% preinfection to 19.8% on day 7. The level of L3T4+ cells remained constant (27.9% preinfection vs. 25.2% on day 7). The L3T4+/Ly2+ ratio in the lungs reached a nadir 7 days post infection (1.5 vs. 3.5 before infection). The total cell count in BAL increased more than tenfold during the first week after infection. At the same time Thy1.2+ cells in the BAL increased from 41.1% of total lymphocytes on day 1 to 85.3% on day 7. Ly2+ influx was the most important (5.8% on day 1 vs. 41.1% on day 7). L3T4+ cell levels increased from 17.2% on day 1 to 40.1% on day 7. RSV-specific lymphocyte transformation was observed in BAL and blood but not in the lung tissue and spleen on day 7 postinfection. The disappearance of infectious virus in the lung correlated directly to the peak appearance of Ly2+ T cells in the lung tissue and BAL.

Recognition of respiratory syncytial (RS) virus proteins by human and BALB/C CD4+ lymphocytes

Lymphocyte proliferation assays were used to determine the ability of human and BALB/c T-lymphocytes to recognise and respond to in vitro challenge with purified preparations of four respiratory syncytial (RS) virus proteins. Human peripheral blood lymphocytes (PBLs) collected from adult donors as well as primed BALB/c mouse splenocytes each responded specifically to challenge with intact RS virus and preparations of the fusion (F), attachment (G), 23 kilodalton (23K), and 34K phospho- (P) proteins of the virus. F protein was recognised most frequently by human PBLs, and elicited higher levels of response than equivalent concentrations of the other protein preparations examined. The human PBL proliferative responses elicited by in vitro challenge with intact virus antigen as well as with each of the four protein preparations were found to be confined to the CD4+ T-helper (Th) sub-population of lymphocytes. However, proliferative responses to intact virus and F protein were found to be accompanied by only modest and inconsistent production of Interleukin-2 (IL-2). Finally, no evidence was obtained to indicate that any of the challenge antigens employed in this study were intrinsically mitogenic, as neither naive human cord blood lymphocytes, nor un-primed BALB/c mouse splenocytes proliferated when challenged with intact RS virus or with F, G, 23K, or P protein preparations.

Respiratory syncytial virus infection in anti-mu-treated mice

BALB/c mice were depleted of B cells by anti-mu treatment to investigate the pathogenesis of respiratory syncytial virus (RSV) infection in the absence of antibody. Termination of RSV replication after primary infection occurred with the same kinetics in anti-mu-treated mice as in phosphate-buffered saline (PBS)-treated controls. Yet, when rechallenged, anti-mu-treated mice were more permissive to RSV replication than PBS-treated controls. Anti-mu-treated mice also experienced greater illness than PBS-treated controls during both primary infection and rechallenge. Passive transfer of RSV-specific immune serum to anti-mu-treated mice before rechallenge reconstituted complete protection from RSV replication and diminished illness. Thus, RSV-specific antibody is not required to terminate RSV replication in primary infection, but without antibody, only partial immunity against rechallenge is induced. While it is unknown whether the mechanism is a direct effect on RSV titer or modulation of the illness-causing cellular immune response, the presence of RSV-specific antibody reduces illness in both primary RSV infection and rechallenge of mice.