Recent developments in the biology of respiratory syncytial virus: are vaccines and new treatments just round the corner?

Cellular Immunity and Lung Injury in Respiratory Virus Infection

Respiratory virus infection may result in considerable lung injury, and host immune responses may be an important contributor to this. Important factors that determine the magnitude of immunopathologic tissue damage include the degree of distal distribution of infection into alveolar cells, the overall viral load, the magnitude of the T-cell responses, the effector mechanisms employed by the T cells, and regulatory mechanisms which may come into play. CD8+ T cells are important contributors to viral clearance, utilizing contact-dependent effector functions (perforin and CD95L) as well as IFN-gamma and TNF-alpha. IFN-gamma and TNF-alpha are primary perpetrators of T-cell-mediated lung injury, with TNF as the major contributor. It is not entirely clear at present the degree to which injury is a necessary consequence of host defense to respiratory virus infection, though there are tantalizing bits of evidence to the contrary. In murine models, TNF plays only a minor role in antiviral activity and clearance of laboratory strains of RSV and influenza. In the event of a pandemic with a highly virulent virus, intervention directed at TNF-alpha should be given consideration, as this may be most likely to provide protection against severe lung injury at the lowest cost to viral clearance.

ERK-1/2 activity is required for efficient RSV infection

Respiratory syncytial virus (RSV) infection up-regulates the expression of genes encoding proinflammatory mediators in bronchial epithelial cells. However, the specific signaling events immediately following RSV exposure are poorly understood. Herein, we report that RSV attachment to A549 cells activates both ERK-1 and ERK-2 pathways within 5 min. Inhibition of ERK pathways significantly decreases RSV infection of these cells compared to controls. These results demonstrate that the activation of the ERK-1/2 is required in RSV-induced early gene expression.

Molecular aspects of allergy

Atopic diseases such as asthma, rhinitis, eczema and food allergies have increased in most industrialised countries of the world during the last 20 years. The reasons for this increase are not known and different hypotheses have been assessed including increased exposure to sensitising allergens or decreased stimulation of the immune system during critical periods of development. In allergic diseases there is a polarisation of the Th2 response and an increase in the production of type 2 cytokines which are involved in the production of immunoglobulin E and the development of mast cells, basophils and eosinophils leading to inflammation and disease. The effector phase of atopy is initiated by interaction with Fc epsilon RI expressed on effector cells such as mast cells and basophils but also found on an ever increasing list of cells. Binding of a polyvalent allergen to the variable part of IgE leads to a cross-link of the receptor that triggers the cell to release histamine and pharmacological mediators of the symptomatic allergic response. Cross-linking of Fc epsilon RI by autoantibodies against the alpha-chain of the Fc epsilon RI, causing subsequent histamine release is thought to be involved in the pathogenesis of other diseases such as chronic idiopathic urticaria (CIU). To date, most therapeutic strategies are aimed at inhibiting and controlling components of the inflammatory response. Recently, new treatment strategies have emerged that focus on the development of preventive and even curative treatments. The most promising therapeutic approaches are aimed at inhibiting the IgE-Fc epsilon RI interaction with the use of non-anaphylactogenic anti-IgE or anti-Fc epsilon RIalpha autoantibodies. Clinical trials in humans using an humanised anti-IgE antibody showed that this antibody was well tolerated and reduced both symptoms and use of medication in asthma and allergic rhinitis. Thus interruption of the atopic cascade at the level of the IgE-Fc epsilon RI interaction with the use of non-anaphylactogenic antibodies is effective and represents an attractive therapy for the treatment of atopic disease.

Severe respiratory syncytial virus infection in early life is associated with increased type 2 cytokine production in Gambian children

BACKGROUND

Severe respiratory syncytial virus (RSV) infection in early childhood has been associated with subsequent wheezing and atopy. The aim of this study was to test if severe RSV infection in early life was associated with an increase in type 2 cytokine production and atopy in Gambian children 5 years later.

METHODS

A cohort of children with severe RSV infection during the first year of life ('cases', n = 66) and without ('controls', n = 122) was followed-up at 5 years of age. Immediate hypersensitivity to common allergens, airway reactivity, serum IgE concentration and the production of IFN-gamma, IL-5 and IL-13 by lymphocytes activated in vitro with RSV F-G or control antigens was determined.

RESULTS

After adjustment for confounders, cases produced significantly higher concentrations of IL-13 in response to RSV F-G and of IL-5 and IL-13 in response to tuberculin. Cases were more likely to have presented with a wheezy lower respiratory tract infection in the first 3 years of life (adjusted odds ratio = 9.9; 95% CI 1.6-61.0), but not thereafter. Cases and controls had similar skin response to allergens, airway reactivity and serum IgE concentrations.

CONCLUSION

Severe RSV infection in early life is associated with a higher production of type 2 cytokines in Gambian children at 5 years of age. However this does not appear to result in increased risk of atopy or clinical allergy at that age.

Vaccination with recombinant alphavirus or immune-stimulating complex antigen against respiratory syncytial virus

Respiratory syncytial virus (RSV) causes severe respiratory diseases in infants and young children. Inappropriate immunity to the virus can lead to disease enhancement upon subsequent infection. In this study, we have characterized the antiviral immunity elicited by the recombinant Semliki Forest virus (SFV) encoding the RSV fusion (F) and attachment (G) protein, and compared with that induced by the immune-stimulating complex (ISCOM)-incorporated FG proteins. Antiviral immunity against RSV elicited nasally or parentally by either of the immunogen having divergent profiles could reduce lung RSV titers upon challenge. However, resistance to RSV without disease enhancement was only observed in those vaccinated with SFV recombinants via nasal route. Presence of postvaccination pulmonary IFN-gamma response to the H-2K(d)-restricted T cell epitope (F(85-93); KYKNAVTEL) was found to be associated with absence of enhanced pulmonary disease and goblet cell hyperplasia as well as reduced Th2-cytokine expression. This result demonstrates that the SFV recombinants can result in enhanced clearance of RSV without enhancing the RSV-associated disease, and underlines the importance in priming pulmonary MHC class I-restricted T cells when RSV FG-based vaccines are used.

The pathogenesis of respiratory syncytial virus disease in childhood

Respiratory syncytial virus (RSV) is a leading cause of severe respiratory infection in infants and children. RSV is an RNA virus whose genome encodes 10 proteins. The G protein is responsible for viral attachment to cells whilst the F protein promotes syncytia formation. These proteins are also important in the immune response to RSV. Both the innate and adaptive arms of the cellular immune system are involved in the immunological response to RSV. The cytopathic effects of the virus explain many of the pathological findings in RSV disease. However, there is compelling evidence to suggest that the host cell immune response also has a prominent role in disease pathogenesis. Non-immunological factors may also be important.

Immunopathology in RSV infection is mediated by a discrete oligoclonal subset of antigen-specific CD4(+) T cells

Vaccination with the respiratory syncytial virus (RSV) attachment (G) protein results in immune-mediated lung injury after natural RSV infection with pathogenic features characteristic of an exaggerated Th2 response. Here we demonstrate that approximately half of the CD4(+) T cells infiltrating the lungs of G-primed mice utilize a single V beta gene (V beta 14) with remarkably limited CDR3 diversity. Furthermore, elimination of these V beta 14-bearing CD4(+) T cells in vivo abolishes the type 2-like pulmonary injury. These results suggest that a novel subset of CD4(+) T cells may be crucial in the development of pathology during human RSV infection and that genetic or environmental factors prior to or at the time of G antigen exposure may affect the commitment of this discrete antigen-specific T cell subset to Th2 differentiation.

Inhibition of tumor necrosis factor reduces the severity of virus-specific lung immunopathology

TNF antagonists are effective treatments for rheumatoid arthritis and Crohn's disease, and have been tried with variable success in other diseases caused by immune damage. To test the hypothesis that viral lung diseases caused by respiratory syncytial virus or influenza virus are partly due to overproduction of TNF, we used anti-TNF antibody to treat mice with lung disease caused by these viruses. TNF depletion reduced pulmonary recruitment of inflammatory cells, cytokine production by T cells and the severity of illness without preventing virus clearance. These broad beneficial effects suggest that TNF antagonists might be tested as treatments of human viral lung diseases.

Implication of the proprotein convertases furin, PC5 and PC7 in the cleavage of surface glycoproteins of Hong Kong, Ebola and respiratory syncytial viruses: a comparative analysis with fluorogenic peptides

Fluorogenic peptides encompassing the processing sites of envelope glycoproteins of the infectious influenza A Hong Kong virus (HKV), Ebola virus (EBOV) and respiratory syncytial virus (RSV) were tested for cleavage by soluble recombinants of the proprotein convertases furin, PC5 and PC7. Kinetic studies with these intramolecularly quenched fluorogenic peptides revealed selective cleavages at the physiological dibasic sites. The HKV peptide is cleaved by both furin and PC5 with similar efficacy; in comparison, PC7 cleaves this substrate poorly. In contrast with the basic tetrapeptide insertion within the haemagglutinin sequence of HKV, two other dipeptide insertions revealed a poorer cleavage with a similar rank order of potency. These results demonstrate that the N-terminal RERR insertion to the wild-type avian RKKR downward arrow sequence is functionally significant, and suggest that the approx. 5-fold increase in cleavage efficacy contributes to the high infectivity of the H5N1 virus subtype. With regard to RSV peptide processing, PC7 is twice as effective as PC5 and furin. The EBOV peptide was processed with similar efficiency by the three enzymes. Our observations that all of these cleavages can be effectively inhibited by a plant andrographolide derivative at 250 microM or less might aid in the design of potent convertase inhibitors as alternative antiviral therapies.