Surfactant protein-A enhances uptake of respiratory syncytial virus by monocytes and U937 macrophages

Effectiveness of drug therapies to treat or prevent respiratory syncytial virus infection-related morbidity

Respiratory syncytial virus (RSV) infection causes a huge burden to the health service, as it results in a large number of in-patient days each year and increases the risk of asthma in childhood. In the acute phase, therapy is supportive as bronchodilators and corticosteroids have resulted, at best, only in short-term benefits; promising treatments for ventilated patients, such as exogenous surfactant, require further testing. Passive immunoprophylaxis reduces hospital admission in high risk groups. In the prevention of chronic respiratory morbidity following RSV infection, however, studies are needed to determine whether immunoprophylaxis will have a useful role and to identify which drug treatment will be most cost-effective.

Binding and entry of respiratory syncytial virus into host cells and initiation of the innate immune response

Respiratory syncytial virus (RSV) is the most common cause of severe lower respiratory tract infection in infants and the elderly. There is currently no effective antiviral treatment for the infection, but advances in our understanding of RSV uptake, especially the role of surfactant proteins, the attachment protein G and the fusion protein F, as well as the post-binding events, have revealed potential targets for new therapies and vaccine development. RSV infection triggers an intense inflammatory response, mediated initially by the infected airway epithelial cells and antigen-presenting cells. Humoral and cell-mediated immune responses are important in controlling the extent of infection and promoting viral clearance. The initial innate immune response may play a critical role by influencing the subsequent adaptive response generated. This review summarizes our current understanding of RSV binding and uptake in mammalian cells and how these initial interactions influence the subsequent innate immune response generated.

Surfactant protein A inhibits lipopolysaccharide-induced in vivo production of interleukin-10 by mononuclear phagocytes during lung inflammation

We previously demonstrated that resident alveolar macrophages from naive mice do not synthesize interleukin (IL)-10, whereas mononuclear phagocytes (MP) recruited during the lung inflammatory process are transiently competent for IL-10 production when exposed to lipopolysaccharide (LPS) in vitro. As surfactant protein A (SP-A), a member of the collectin family, inhibits LPS-induced in vitro IL-10 formation by bone marrow-derived macrophages, we studied its effect on MP under in vivo inflammatory conditions. When mice with LPS-induced inflamed lungs were given a second intranasal LPS administration, IL-10 concentration recovered in the bronchoalveolar lavage fluids varied as a function of the time interval between the two LPS doses. Thus, IL-10 concentration increased with the number of MP up to Day 3, and then decreased to undetectable values within 24 h, despite a continued increase in the number of MP. Analysis of IL-10 mRNA from purified MP indicated that gene expression correlated with the IL-10 level in the bronchoalveolar lavage fluid. In contrast to IL-10 production, SP-A concentrations during LPS-induced inflammation decreased with a nadir at Day 3, and then increased significantly within 24 h. Furthermore, intranasal administration of exogenous SP-A to mice with LPS-induced inflamed lungs led to a repression of the IL-10 production. In summary, this study demonstrates for the first time an in vivo inhibitory role of SP-A on the anti-inflammatory activity of MP, through inhibition of IL-10 production.

Respiratory syncytial virus and pulmonary surfactant

Respiratory syncytial virus (RSV) infections peak in young infants and are associated with significant morbidity. The collectins surfactant protein-A (SP-A) and SP-D are pattern recognition molecules that belong to the innate immune system of the lungs, forming a first line of defense. On the one hand, SP-A and SP-D levels are reduced during RSV infection. This may critically influence the invasion of RSV and also the virus-induced cytokine patterns of the host. Both collectins enhance the in vivo elimination of RSV. Thus, interactions before the virus enters the epithelial cells may determine the course of the infection. On the other hand, during severe RSV infection in infants, the biophysical surfactant function is reduced and exogenous surfactant substitution may be a valid therapeutic option for selected infants. Thus, all components of the pulmonary surfactant system are involved during severe RSV infection. Especially the collectins SP-A and SP-D may play a pivotal role determining the short- and long-term course of RSV infections in early infancy.

Respiratory syncytial virus--viral biology and the host response

Respiratory syncytial virus (RSV) is the most important cause of respiratory tract infection in infants. We have an incomplete understanding of the reasons why some infants are more severely affected by RSV than others. There is no effective antiviral treatment for the infection. Advances in our understanding of the biology of RSV, particularly in relation to the attachment protein G and the fusion protein F, have revealed potential targets for new antiviral therapies and vaccine development. In response to RSV infection an intense inflammatory response is triggered, mediated initially by the infected airway epithelial cells. Cell mediated responses are important in controlling the extent of infection and in viral clearance. Humoral responses are important in protection. There is early evidence that genetic variation of the host response can influence the outcome of RSV-induced bronchiolitis.

The role of surfactant-associated protein A in pulmonary host defense

Resident alveolar macrophages play a key role in the initial defense against inhaled pathogens. Surface molecules bind opsonized as well as nonopsonized microbes and mediate their internalization by the macrophage. The recent discovery that specific C-type lectins can bind to the surface of a wide range of pathogens has led to the hypothesis that these lectins are involved in the initial phases of microbe recognition by the macrophage. Studies in our laboratory focus on the role of the lung-specific lectin surfactant associated protein A (SP-A) in host defense against pulmonary pathogens. SP-A contains a carbohdyrate recognition domain that appears to bind specifically to exposed carbohydrate residues on the surface of microorganisms. This lectin-microorganism interaction leads to entry of specific pathogens into macrophages and activation of intracellular pathways, resulting in the production of antimicrobial mediators such as nitric oxide. Many studies, including those involving SP-A-deficient mice, underscore the importance of this protein in pulmonary innate immunity. However, the intramacrophage mechanisms underlying the effects of SP-A are still unclear. This article describes our current knowledge of SP-A and its interactions with immune cells and pathogens with a focus on recent findings from our laboratory regarding SP-A interactions with mycobacteria.

Surfactant proteins A and D in Eustachian tube epithelium

Surfactant protein (SP) A and SP-D are collectins that have roles in host defense. The Eustachian tube (ET) maintains the patency between the upper airways and the middle ear. Dysfunction of local mucosal immunity in ET may predispose infants to recurrent otitis media. We recently described preliminary evidence of the expression of SP-A and SP-D in the ET. Our present aim was to establish the sites of SP-A and SP-D expression within the epithelium of the ET in vivo. With in situ hybridization, electron microscopy, and immunoelectron microscopy, the cells responsible for SP-A and SP-D expression and storage were identified. SP-A expression was localized within the ET epithelium, and the protein was found in the electron-dense granules of microvillar epithelial cells. Being concentrated in the epithelial lining, only a few cells revealed intracellular SP-D, and it was not associated with granules. The SP-A and SP-D immunoreactivities in ET lavage fluid, as shown by Western blot analyses, were similar to those in bronchoalveolar lavage fluid. We propose that there are specialized cells in the ET epithelium expressing and secreting SP-A and SP-D. SP-A and SP-D may be important for antibody-independent protection of the middle ear against infections.

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.