Mechanisms of DiseaseInterleukin 9 production in the lungs of infants with severe respiratory syncytial virus bronchiolitis
Introduction
Respiratory syncytial virus (RSV) infection is an important cause of morbidity and mortality in children worldwide.1 Although most infants infected with RSV have only mild symptoms such as cough and coryza, many develop bronchiolitis. In the developed world, 25 in every 1000 children are admitted for bronchiolitis and of these, 2% need ventilatory support.2 Clinically, bronchiolitis is characterised by a harsh paroxysmal cough and lower respiratory-tract signs such as wheezes and crackles.2 Thick mucoid secretions are a feature of both upper and lower respiratory tract manifestations of RSV disease. Treatment for RSV bronchiolitis is largely supportive and there is currently no effective vaccine.1 Infants born preterm are at particular risk of severe RSV disease. Although they account for only 6% of all births, preterm infants represent up to 24% of infants admitted for bronchiolitis and up to 63% of infants ventilated for bronchiolitis.3, 4
RSV infection of the human respiratory tract provokes a brisk cellular response, mostly from neutrophils.5 This response is associated with large amounts of proinflammatory cytokines (interleukin 6 and TNF α)6, and chemokines (interleukin 8, monocyte inflammatory protein 1α [MIP 1 α], monocyte inflammatory protein 1β [MIP 1β] and monocyte chemotactic protein 1 [MCP 1]).7, 8 In human RSV infection, high chemokine production is associated with severe disease;8, 9 cytokines and chemokines probably amplify the immune response to RSV infection by increasing cellular recruitment into RSV infected airways. In severe RSV disease, an overexuberant immunological response could be responsible for damage to both infected and uninfected areas of lung.10
The predisposition of infants admitted with RSV bronchiolitis to subsequently develop asthma and recurrent wheeze has caused speculation that an aberrant immune response by type 2 lymphocytes might play a role in RSV infection.11 interleukin 9 is a type 2 cell-derived cytokine with pleiotropic effects on various cell types. Genetic linkage studies have suggested that interleukin 9 might play an important role in the pathogenesis of asthma.12 Increased expression of interleukin 9 and the interleukin 9 receptor has been shown in lung biopsy specimens from adult asthmatics.13 Furthermore, it upregulates the immune response by inducing the production of proinflammatory cytokines and chemokines both in vivo and in vitro.14 There is also increasing evidence that one of the most important actions interleukin 9 might have in the lung is to stimulate mucus production.15, 16, 17 We aimed to investigate whether interleukin 9 was present in the lungs of term and preterm infants with severe RSV disease and if found, to investigate which cells were producing it.
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Participants
We recruited 45 consecutive infants aged younger than 1 year who were ventilated for RSV bronchiolitis at Alder Hey Children's Hospital, Liverpool, UK during the 2000–01 and 2001–02 RSV seasons. 24 participants were born at term (ie, ⩾37 weeks) and 21 were preterm (<37 weeks). 13 of the 21 preterm infants had required ventilation in the neonatal period. Four preterm infants had chronic lung disease of prematurity. We excluded children with other medical conditions such as congenital heart
Results
Interleukin 9 mRNA expression relative to GAPDH was measured on 78 occasions in 35 infants with RSV bronchiolitis (17 term and 18 preterm) and 10 times in 10 controls. A representative autoradiograph of rnase protection assay is shown in figure 1. We noted interleukin 9 mRNA expression in all samples from infants with RSV bronchiolitis and in three samples from controls.
On day 1, interleukin 9 mRNA expression in term and preterm infants with bronchiolitis was significantly greater than in the
Discussion
We have identified interleukin 9 in the airways of children during an acute respiratory infection. Our results suggest that large quantities of interleukin 9 are produced in the lungs of term infants with severe RSV bronchiolitis. Surprisingly, a major source of this type 2 cytokine in the RSV-infected lung seems to be neutrophils.
We assessed changes in the interleukin 9 response between groups over time in two different ways. First, we compared interleukin 9 mRNA and protein production
GLOSSARY
- interleukin 9
- A T-helper-2-cell derived cytokine with pleiotropic effects on a variety of cell lines. It has the ability to stimulate both chemokine and mucus production. The gene for interleukin 9 is located on human chromosome 5q31.1.
- rnase protection assay
- A method for quantifying different mRNA transcripts in a sample relative to “housekeeping” mRNA, expressed in all living cells.
- t-helper-1 and t-helper-2 cells
- CD4+ T lymphocytes distinguished by patterns of cytokine production; T-helper-1
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