Respiratory syncytial virus persistence in macrophages alters the profile of cellular gene expression

Responding to Higher-Than-Expected Infant Mortality Rates from Respiratory Syncytial Virus (RSV): Improving Treatment and Reporting Strategies

Respiratory syncytial virus (RSV) has a major role in respiratory infections in young infants around the world. However, substantial progress has been made in recent years in the field of RSV. A wide variety of observational studies and clinical trials published in the past decade provide a thorough idea of the health and economic burden of RSV disease in the developing world. In this review, we discuss the impact of RSV burden of disease, major gaps in disease estimations, and challenges in generating new therapeutic options and an immune response against the virus, and briefly describe next generation technologies that are being evaluated.

Transmission of Classical Swine Fever Virus in Cohabitating Piglets with Various Immune Statuses Following Attenuated Live Vaccine

Classical swine fever (CSF) is a systemic hemorrhagic disease affecting domestic pigs and wild boars. The modified live vaccine (MLV) induces quick and solid protection against CSF virus (CSFV) infection. Maternally derived antibodies (MDAs) via colostrum could interfere with the MLV's efficacy, leading to incomplete protection against CSFV infection for pigs. This study investigated CSFV transmission among experimental piglets with various post-MLV immune statuses. Nineteen piglets, 18 with MDAs and 1 specific-pathogen-free piglet infected with CSFV that served as the CSFV donor, were cohabited with piglets that had or had not been administered the MLV. Five-sixths of the piglets with MDAs that had been administered one dose of MLV were fully protected from contact transmission from the CSFV donor and did not transmit CSFV to the piglets secondarily exposed through cohabitation. Cell-mediated immunity, represented by the anti-CSFV-specific interferon-γ-secreting cells, was key to viral clearance and recovery. After cohabitation with a CSFV donor, the unvaccinated piglets with low MDA levels exhibited CSFV infection and spread CSFV to other piglets through contact; those with high MDA levels recovered but acted as asymptomatic carriers. In conclusion, MLV still induces solid immunity in commercial herds under MDA interference and blocks CSFV transmission within these herds.

Human Respiratory Syncytial Virus Infection in a Human T Cell Line Is Hampered at Multiple Steps

Human respiratory syncytial virus (HRSV) is the most frequent cause of severe respiratory disease in children. The main targets of HRSV infection are epithelial cells of the respiratory tract, and the great majority of the studies regarding HRSV infection are done in respiratory cells. Recently, the interest on respiratory virus infection of lymphoid cells has been growing, but details of the interaction of HRSV with lymphoid cells remain unknown. Therefore, this study was done to assess the relationship of HRSV with A3.01 cells, a human CD4⁺ T cell line. Using flow cytometry and fluorescent focus assay, we found that A3.01 cells are susceptible but virtually not permissive to HRSV infection. Dequenching experiments revealed that the fusion process of HRSV in A3.01 cells was nearly abolished in comparison to HEp-2 cells, an epithelial cell lineage. Quantification of viral RNA by RT-qPCR showed that the replication of HRSV in A3.01 cells was considerably reduced. Western blot and quantitative flow cytometry analyses demonstrated that the production of HRSV proteins in A3.01 was significantly lower than in HEp-2 cells. Additionally, using fluorescence in situ hybridization, we found that the inclusion body-associated granules (IBAGs) were almost absent in HRSV inclusion bodies in A3.01 cells. We also assessed the intracellular trafficking of HRSV proteins and found that HRSV proteins colocalized partially with the secretory pathway in A3.01 cells, but these HRSV proteins and viral filaments were present only scarcely at the plasma membrane. HRSV infection of A3.01 CD4⁺ T cells is virtually unproductive as compared to HEp-2 cells, as a result of defects at several steps of the viral cycle: Fusion, genome replication, formation of inclusion bodies, recruitment of cellular proteins, virus assembly, and budding.

Respiratory Syncytial Virus's Non-structural Proteins: Masters of Interference

Respiratory Syncytial Virus (RSV) is a highly prevalent virus that affects the majority of the population. The virus can cause severe disease in vulnerable populations leading to high hospitalization rates from bronchiolitis or secondary bacterial infections leading to pneumonia. Two early and non-structural proteins (Ns1 and Ns2), strongly over-ride the antiviral innate system but also diminish the adaptive response as well. This review will cover interactions of Ns1 and Ns2 with the host antiviral response with a focus on alterations to signaling pathways, cytokine gene expression, and effects of the Ns proteins on mitochondria.

Respiratory syncytial virus exhibits differential tropism for distinct human placental cell types with Hofbauer cells acting as a permissive reservoir for infection

BACKGROUND

Respiratory syncytial virus (RSV) is capable of transient viremia and extrapulmonary dissemination. Recently, this virus has been identified in fetal cord blood, suggesting the possibility of in utero acquisition in humans. Here, we assess permissivity and kinetics of RSV replication in primary human placental cells, examine their potential to transfer this infection to neighboring cells, and measure the inflammatory response evoked by the virus.

METHODS AND FINDINGS

Human placental villus tissue was collected immediately upon delivery and processed for isolation of placental cytotrophoblast, fibroblast, and macrophage (Hofbauer) cells. Isolated cells were infected with a recombinant RSV-A2 strain (rrRSV) expressing red fluorescent protein (RFP) and analyzed by fluorescence microscopy, Western blot, and quantitative PCR (qPCR). Based on RFP expression, rrRSV exhibited differential tropism for the three major placental cell types. Placental fibroblasts and Hofbauer cells were permissive and supported productive rrRSV replication. While infected cytotrophoblast cells expressed viral glycoprotein (G protein), only limited RSV replication was detected. Importantly, qPCR and fluorescence-focused unit assay revealed that the viral progeny remains trapped within infected Hofbauer cells for up to 30 days, with no release into surrounding media. Yet, Hofbauer cells passed the infection onto overlaid naïve 16HBE cells, suggesting contact-dependent trans-infection. Lastly, a significant increase in proinflammatory cytokines, particularly IL-6, TNF-alpha, and IFN-gamma was measured in the supernatant of infected Hofbauer cells by multiplex cytokine assay and conventional ELISA.

CONCLUSIONS

This study demonstrates that RSV can replicate in human placenta, exhibits differential tropism for distinct placental cell types, can be stored and transferred to neighboring cells by Hofbauer cells, and elicits an inflammatory response. It also supports the hypothesis that this respiratory virus can be vertically transferred to the fetus and potentially affect its development and the outcome of pregnancies.

Differences in Susceptibility of Human and Mouse Macrophage Cell Lines to Respiratory Syncytial Virus Infection

OBJECTIVES

Differences have been observed in the susceptibility of macrophage cell lines to respiratory syncytial virus (RSV) infection. In this study, we evaluated whether the type of macrophage cell line and RSV strain used have an influence on the infectivity and production of progeny virus.

METHODS

Both human and murine macrophage-like cell lines were infected with different RSV strains, both lab strains as well as clinical isolates. The infection was evaluated after 24 and 72 h by immunofluorescence staining and microscopic analysis, and the production of new virus particles was determined by plaque assay.

RESULTS

Susceptibility of macrophages to RSV was influenced by the RSV strain used but was mostly dependent on the macrophage cell line. Numbers of infected cells and virus production were generally very low or absent in murine cell lines. In human cell lines, clear infection was observed associated with production of new virus particles.

CONCLUSION

Differences in susceptibility of macrophage cell lines to RSV infection are primarily related to the species of origin of the cell line but are also influenced by the RSV strain.

The BeWo cell line derived from a human placental choriocarcinoma is permissive for respiratory syncytial virus infection

The respiratory syncytial virus (RSV) is the main pathogen associated with upper respiratory tract infections during early childhood. Vertical transmission of this virus has been suggested in humans, based on observations recorded during animal studies that revealed an association of RSV with persistent structural and functional changes in the developing lungs of the offspring. However, human placentas have not yet been evaluated for susceptibility to RSV infection. In this study, we examined the capacity of RSV to infect a human trophoblast model, the BeWo cell line. Our results suggest that BeWo cells are susceptible to RSV infection since they allow RNA viral replication, viral protein translation, leading to the production of infectious RSV particles. In this report, we demonstrate that a human placenta model system, consisting of BeWo cells, is permissive to RSV infection. Thus, the BeWo cell line may represent a useful model for studies that aim to characterize the events of a possible RSV infection at the human maternal-fetal interface.

Differential Role of Anti-Viral Sensing Pathway for the Production of Type I Interferon β in Dendritic Cells and Macrophages Against Respiratory Syncytial Virus A2 Strain Infection

Respiratory syncytial virus (RSV) is a major cause of respiratory infectious disease in infants and young children. Dendritic cells (DCs) and macrophages (MACs) are known to play important roles in RSV recognition, and in the production of type I interferons (IFNs) and pro-inflammatory cytokine in RSV infection. Toll-like receptor 7 (TLR7), myeloid differentiation primary response 88 (MyD88), and mitochondrial antiviral-signaling protein (MAVS) are known to be important for the RSV sensing pathway in DCs and MACs. However, despite the critical roles of type I IFNs in the anti-RSV immune response, the pattern recognition receptors (PRRs) that are required for RSV sensing in DCs and MACs remain unclear. Here, we investigate the pathway activated by RSV A2 strain infection using an IFN-β/YFP reporter mouse model to visualize IFN-β-producing cells and in vitro RSV infection in bone marrow-derived DCs (BM-DCs) and macrophages (BM-DMs). We present our finding that MyD88, but not TLR7, are important for RSV recognition and type I IFN and pro-inflammatory production in DCs and MACs. MAVS-deficient BM-DCs and BM-DMs show impaired induction of IFN-β production upon RSV stimulation, and this effect is RSV replication-dependent. Our study provides information on cell type-specific PRR requirements in innate immune responses against RSV infection.

Influence of Immunological Maturity on Respiratory Syncytial Virus-Induced Morbidity in Young Children

Respiratory syncytial virus (RSV) is a very frequent viral respiratory pathogen of the young (<5 years old) with a significant portion of young toddlers having been infected before 2 years of age. Although we understand that some of the morbidity associated with RSV in neonates is due to immunological maturation that favors immunosuppression over antiviral innate and/or adaptive immune responses, the rapid development of the immune system right after birth suggests that each age group (newborn, early infant, older infant, toddler, and older) may respond to the virus in different ways. In this study, we summarize the morbidity associated with infection in young children in the context of immunological maturation of monocytes/macrophages and the ramifications for poor innate control of viral pathogenesis. We also summarize key mechanisms that contribute to the diminished antiviral innate immune responses of these young children.

Current and Future Therapeutic Strategies for Lentiviral Eradication from Macrophage Reservoirs

Macrophages, one of the most abundant populations of leukocytes in the body, function as the first line of defense against pathogen invaders. Human Immunodeficiency virus 1 (HIV-1) remains to date one of the most extensively studied viral infections. Naturally occurring lentiviruses in domestic and primate species serve as valuable models to investigate lentiviral pathogenesis and novel therapeutics. Better understanding of the role macrophages play in HIV pathogenesis will aid in the advancement towards a cure. Even with current efficacy of first- and second-line Antiretroviral Therapy (ART) guidelines and future efficacy of Long Acting Slow Effective Release-ART (LASER-ART); ART alone does not lead to a cure. The major challenge of HIV eradication is viral latency. Latency Reversal Agents (LRAs) show promise as a possible means to eradicate HIV-1 from the body. It has become evident that complete eradication will need to include combinations of various effective therapeutic strategies such as LASER-ART, LRAs, and gene editing. Review of the current literature indicates the most promising HIV eradication strategy appears to be LASER-ART in conjunction with viral and receptor gene modifications via the CRISPR/Cas9 system. Graphical abstract A multimodal approach to HIV treatment including gene editing, LASER-ART, and latency reversal agents may provide a means to achieve HIV eradication.

Central Role of the NF-κB Pathway in the Scgb1a1-Expressing Epithelium in Mediating Respiratory Syncytial Virus-Induced Airway Inflammation

Lower respiratory tract infection with respiratory syncytial virus (RSV) produces profound inflammation. Despite an understanding of the role of adaptive immunity in RSV infection, the identity of the major sentinel cells initially triggering inflammation is controversial. Here we evaluate the role of nonciliated secretoglobin (Scgb1a1)-expressing bronchiolar epithelial cells in RSV infection. Mice expressing a tamoxifen (TMX)-inducible Cre recombinase-estrogen receptor fusion protein (CreERTM) knocked into the Scgb1a1 locus were crossed with mice that harbor a RelA conditional allele (RelA^fl ), with loxP sites flanking exons 5 to 8 of the Rel homology domain. The Scgb1a1^CreERTM/+ × RelA^fl/fl mouse is a RelA conditional knockout (RelA^CKO) of a nonciliated epithelial cell population enriched in the small bronchioles. TMX-treated RelA^CKO mice have reduced pulmonary neutrophilic infiltration and impaired expression and secretion of NF-κB-dependent cytokines in response to RSV. In addition, RelA^CKO mice had reduced expression levels of interferon (IFN) regulatory factor 1/7 (IRF1/7) and retinoic acid-inducible gene I (RIG-I), components of the mucosal IFN positive-feedback loop. We demonstrate that RSV replication induces RelA to complex with bromodomain-containing protein 4 (BRD4), a cofactor required for RNA polymerase II (Pol II) phosphorylation, activating the atypical histone acetyltransferase (HAT) activity of BRD4 required for phospho-Ser2 Pol II formation, histone H3K122 acetylation, and cytokine secretion in vitro and in vivo TMX-treated RelA^CKO mice have less weight loss and reduced airway obstruction/hyperreactivity yet similar levels of IFN-γ production despite higher levels of virus production. These data indicate that the nonciliated Scgb1a1-expressing epithelium is a major innate sensor for restricting RSV infection by mediating neutrophilic inflammation and chemokine and mucosal IFN production via the RelA-BRD4 pathway.IMPORTANCE RSV infection is the most common cause of infant hospitalizations in the United States, resulting in 2.1 million children annually requiring medical attention. RSV primarily infects nasal epithelial cells, spreading distally to produce severe lower respiratory tract infections. Our study examines the role of a nonciliated respiratory epithelial cell population in RSV infection. We genetically engineered a mouse that can be selectively depleted of the NF-κB/RelA transcription factor in this subset of epithelial cells. These mice show an impaired activation of the bromodomain-containing protein 4 (BRD4) coactivator, resulting in reduced cytokine expression and neutrophilic inflammation. During the course of RSV infection, epithelial RelA-depleted mice have reduced disease scores and airway hyperreactivity yet increased levels of virus replication. We conclude that RelA-BRD4 signaling in nonciliated bronchiolar epithelial cells mediates neutrophilic airway inflammation and disease severity. This complex is an attractive target to reduce the severity of infection.

MicroRNA 146-5p, miR-let-7c-5p, miR-221 and miR-345-5p are differentially expressed in Respiratory Syncytial Virus (RSV) persistently infected HEp-2 cells

Many viruses can establish non-cytolytic, chronic infections in host cells. Beyond the intrinsically interesting questions of how this long-term parasitism is achieved, persistently infected cells can be useful to study virus-host interactions. MicroRNAs (miRNAs) are a class of noncoding RNAs transcribed from the genomes of all multicellular organisms and some viruses. Individual miRNAs may regulate several hundred genes. In this research we have studied the expression of a selective group of host-cell encoded miRNAs, as expressed in a Respiratory Syncytial Virus (RSV) persistently infected HEp-2 cell line (HEp-2 + RSV-GFP). The RSV is a virus that does not encode miRNAs in its genome. Our study shows that Dicer is down regulated, miRNA's 146a-5p is strongly up-regulated and miRNAs 345-5p, let-7c-5p and miRNA's-221 are down-regulated in HEp-2 + RSV-GFP cells. Correspondingly, changes in the miRNA 146a-5p and he sequences of the reference genes are miRNA 345-5p respective miRNAs target proteins: HSP-70 and p21, were observed. Thus, RSV persistent viral infection induces unique patterns of miRNA's expression with relevance to how the virus regulates the host cell response to infection.

Human Respiratory Syncytial Virus: Infection and Pathology

The human respiratory syncytial virus (hRSV) is by far the major cause of acute lower respiratory tract infections (ALRTIs) worldwide in infants and children younger than 2 years. The overwhelming number of hospitalizations due to hRSV-induced ALRTI each year is due, at least in part, to the lack of licensed vaccines against this virus. Thus, hRSV infection is considered a major public health problem and economic burden in most countries. The lung pathology developed in hRSV-infected individuals is characterized by an exacerbated proinflammatory and unbalanced Th2-type immune response. In addition to the adverse effects in airway tissues, hRSV infection can also cause neurologic manifestations in the host, such as seizures and encephalopathy. Although the origins of these extrapulmonary symptoms remain unclear, studies with patients suffering from neurological alterations suggest an involvement of the inflammatory response against hRSV. Furthermore, hRSV has evolved numerous mechanisms to modulate and evade the immune response in the host. Several studies have focused on elucidating the interactions between hRSV virulence factors and the host immune system, to rationally design new vaccines and therapies against this virus. Here, we discuss about the infection, pathology, and immune response triggered by hRSV in the host.

Measles virus nucleocapsid protein modulates the Signal Regulatory Protein-β1 (SIRPβ1) to enhance osteoclast differentiation in Paget's disease of bone

Paget's disease of bone (PDB) is a chronic localized bone disorder in an elderly population. Environmental factors such as paramyxovirus are implicated in PDB and measles virus nucleocapsid protein (MVNP) has been shown to induce pagetic osteoclasts (OCLs). However, the molecular mechanisms underlying MVNP stimulation of OCL differentiation in the PDB are unclear. We therefore determined the MVNP regulated gene expression profiling during OCL differentiation. Agilent microarray analysis of gene expression identified high levels of SIRPβ1 (353-fold) expression in MVNP transduced human bone marrow mononuclear cells stimulated with RANKL. Real-time PCR analysis further confirmed that MVNP alone upregulates SIRPβ1 mRNA expression in these cells. Also, bone marrow mononuclear cells derived from patients with PDB showed high levels of SIRPβ1 mRNA expression compared to normal subjects. We further show that MVNP increases SIRPβ1 interaction with DAP12 adaptor protein in the presence and absence of RANKL stimulation. shRNA knockdown of SIRPβ1 expression in normal human bone marrow monocytes decreased the levels of MVNP enhanced p-Syk and c-Fos expression. In addition, SIRPβ1 knockdown significantly decreased MVNP stimulated dendritic cell-specific transmembrane protein (DC-STAMP) and connective tissue growth factor (CTGF) mRNA expression during OCL differentiation. Furthermore, we demonstrated the contribution of SIRPβ1 in MVNP induced OCL formation and bone resorption. Thus, our results suggest that MVNP modulation of SIRPβ1 provides new insights into the molecular mechanisms which control high bone turnover in PDB.

A Preliminary Assessment of the Role of Ambient Nitric Oxide Exposure in Hospitalization with Respiratory Syncytial Virus Bronchiolitis

Some in vitro studies have indicated a possible link between respiratory syncytial virus (RSV) infection and exposure to Nitric Oxide (NO). However, these studies used much higher NO concentrations than normally found in the ambient environment. This preliminary study explored whether an association was present with short-term exposure to NO in the environment. RSV-related admission data between November 2011 and February 2012 were obtained from Sheffield Children's Hospital. The dates of admission were linked to contemporaneous ambient NO derived from sentinel air monitors. The case-crossover design was used to study the relationship between daily RSV admissions and NO, controlling for temperature and relative humidity. We found little evidence of association between daily RSV admission rates and exposure to ambient NO at different lags or average exposure across several lags. The findings should, however, be viewed with caution due to the low number of events observed during the time frame. It is possible that the apparent lack of association may be accounted for by the timing of the seasonal RSV epidemic in relation to peaks in NO concentrations. A larger study incorporating a wider range of RSV and NO peaks would determine whether said peaks enhanced the number of RSV hospitalizations in children.

Lipoxins: nature's way to resolve inflammation

An effective host defense mechanism involves inflammation to eliminate pathogens from the site of infection, followed by the resolution of inflammation and the restoration of tissue homeostasis. Lipoxins are endogenous anti-inflammatory, pro-resolving molecules that play a vital role in reducing excessive tissue injury and chronic inflammation. In this review, the mechanisms of action of lipoxins at the site of inflammation and their interaction with other cellular signaling molecules and transcription factors are discussed. Emphasis has also been placed on immune modulatory role(s) of lipoxins. Lipoxins regulate components of both the innate and adaptive immune systems including neutrophils, macrophages, T-, and B-cells. Lipoxins also modulate levels of various transcription factors such as nuclear factor κB, activator protein-1, nerve growth factor-regulated factor 1A binding protein 1, and peroxisome proliferator activated receptor γ and control the expression of many inflammatory genes. Since lipoxins and aspirin-triggered lipoxins have clinical relevance, we discuss their important role in clinical research to treat a wide range of diseases like inflammatory disorders, renal fibrosis, cerebral ischemia, and cancer. A brief overview of lipoxins in viral malignancies and viral pathogenesis especially the unexplored role of lipoxins in Kaposi’s sarcoma-associated herpes virus biology is also presented.

Differential impact of respiratory syncytial virus and parainfluenza virus on the frequency of acute otitis media is explained by lower adaptive and innate immune responses in otitis-prone children

BACKGROUND

Acute otitis media (AOM) is a leading cause of bacterial pediatric infections associated with viral upper respiratory infections (URIs). We examined the differential impact of respiratory syncytial virus (RSV) and parainfluenza virus URIs on the frequency of AOM caused by Streptococcus pneumoniae (Spn) and nontypeable Haemophilus influenzae (NTHi) in stringently defined otitis-prone (sOP) and non-otitis-prone (NOP) children as a potential mechanism to explain increased susceptibility to AOM.

METHODS

Peripheral blood and nasal washes were obtained from sOP and NOP children (n = 309). Colonization events and antiviral responses consisting of total specific immunoglobulin G (IgG) responses, neutralizing antibody responses, and T-cell responses were determined. Isolated neutrophils were infected with varying multiplicities of infection of both viruses, and opsonophagocytosis potential was measured.

RESULTS

A significant increase was found in frequency of AOM events caused by Spn and NTHi, with a concurrent RSV infection in sOP children. These results correlated with diminished total RSV-specific IgG, higher viral nasal burdens, and lower IgG neutralizing capacity. The sOP children had diminished T-cell responses to RSV that correlated with lower Toll-like receptor 3/7 transcript and decreased expression of HLA-DR on antigen-presenting cells. RSV interfered with the Spn phagocytic capacity of neutrophils in a dose-dependent manner. Parainfluenza virus infections did not differentially affect AOM events in sOP and NOP children.

CONCLUSIONS

Lower innate and adaptive immune responses to RSV in sOP children may slow the kinetics of viral clearance from the nasopharynx and allow for viral interference with antibacterial immune responses, thus contributing to increased frequency of AOMs.

Macrophages are required for dendritic cell uptake of respiratory syncytial virus from an infected epithelium

We have previously shown that the respiratory syncytial virus [RSV] can productively infect monocyte derived dendritic cells [MoDC] and remain dormant within the same cells for prolonged periods. It is therefore possible that infected dendritic cells act as a reservoir within the airways of individuals between annual epidemics. In the present study we explored the possibility that sub-epithelial DCs can be infected with RSV from differentiated bronchial epithelium and that in turn RSV from DCs can infect the epithelium. A dual co-culture model was established in which a differentiated primary airway epithelium on an Air Liquid Interface (ALI) was cultured on a transwell insert and MoDCs were subsequently added to the basolateral membrane of the insert. Further experiments were undertaken using a triple co-culture model in which in which macrophages were added to the apical surface of the differentiated epithelium. A modified RSV [rr-RSV] expressing a red fluorescent protein marker of replication was used to infect either the MoDCs or the differentiated epithelium and infection of the reciprocal cell type was assessed using confocal microscopy. Our data shows that primary epithelium became infected when rr-RSV infected MoDCs were introduced onto the basal surface of the transwell insert. MoDCs located beneath the epithelium did not become infected with virus from infected epithelial cells in the dual co-culture model. However when macrophages were present on the apical surface of the primary epithelium infection of the basal MoDCs occurred. Our data suggests that RSV infected dendritic cells readily transmit infection to epithelial cells even when they are located beneath the basal layer. However macrophages appear to be necessary for the transmission of infection from epithelial cells to basal dendritic cells.

Pathogen recognition receptor crosstalk in respiratory syncytial virus sensing: a host and cell type perspective

Human respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract infection in young children, immunocompromised adults, and the elderly. The innate immune response plays a pivotal role in host defense against RSV, but whether severe outcomes following RSV infection result from excessive or poor innate immune recognition remains unclear. Recent research suggests a situation in which crosstalk between families of pattern recognition receptors (PRRs) occurs in a cell type-dependent manner. The current challenge to empower novel therapeutic approaches and vaccine development is to confirm the role of the individual receptors in RSV pathogenesis in humans.