Background
Lower respiratory tract infection (LRTI) is the leading cause of global child mortality. Respiratory syncytial virus (RSV) is believed to be the most important viral pathogen causing LRTI in young children. Furthermore, the incidence for severe LRTI was highest in infants aged 0–11 months [
1]. RSV is not only the leading pathogen of LRTI including pneumonia and bronchiolitis, but is also closely associated with the development of asthma [
2]. A number of clinical epidemiology studies have revealed that there is a strong association between RSV bronchiolitis in infancy and the development of wheezing or asthma in later childhood. About 40% of children after severe RSV bronchiolitis subsequently develop asthma [
2,
3]. Despite the importance of RSV as a respiratory pathogen, the underlying mechanisms of the development of subsequent wheezing and asthma following primary RSV infection remain unclear.
RSV infection may increase the susceptibility to developing allergic immune responses by breaking immune tolerance to allergens early in life through regulatory T cells [
4]. Moreover, RSV-induced neurogenic inflammation appears to potentiate neural pathways that favor bronchoconstriction and mucus production [
5]. Interestingly, studies have shown that infection with RSV also results in a significant increase in proinflammatory cytokines such as interleukin (IL)-1, IL-6, IL-8, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α, contributing to development of recurrent wheezing or asthma [
3]. Our previous study has shown that low birth weight (LBW) or intrauterine growth retardation significantly increases the risk of childhood asthma. Children with LBW have an approximately 16% higher risk of asthma compared with those with normal birth weight (NBW) [
6]. Another study by Rossi et al. further revealed that LBW was not only associated with the increased risk of asthma or wheezing, but also with RSV infection severity [
7].
Although we recognize the significant role of low birth weight in infantile RSV-induced LRTI or wheezing, the mechanism of this phenomenon remains unknown. In view of the potential role of cytokines, we hypothesized that LBW induces a specific cytokine profile in infantile bronchiolitis or wheezing induced by RSV infection, leading to more significant manifestations related to asthma. The present study aims to evaluate the possible role of cytokines in the pathogenesis of RSV infection in infants with LBW.
Discussion
Respiratory syncytial virus (RSV) is the leading viral cause of acute LRTI worldwide, and the most important viral pathogen in infancy, especially among young infants less than 6 months [
12]. A large number of studies have established a link between infantile RSV infection and wheezing or asthma [
2]. Moreover, the increased evidence indicates that cytokines are strongly correlated with the pathogenic mechanism of RSV-induced wheezing or asthma attacks [
13‐
17].
IL-6 is an important proinflammatory cytokine, promoting Th2 cell differentiation and simultaneously inhibiting Th1 cell polarization. TNF-α is another important proinflammatory cytokine that is involved in both acute and chronic inflammatory responses. IL-6 and TNF-α mRNA and protein levels in bronchoalveolar lavage fluid in term infants with RSV bronchiolitis were greater than the control group. However, in preterm infants with RSV bronchiolitis, IL-6 and TNF-α proteins significantly decreased [
15]. The decrease in IL-6 and TNF-α protein levels in preterm infants may be correlated with the prolonged clinical course seen in those RSV patients [
15]. Another study by Oda et al. demonstrated that serum IL-6 levels from patients with RSV infection were high compared with a control group, but there was no statistical significance [
18]. Unlike local cytokine expression, our study indicated that systemic IL-6 levels in the RSV group were also high (greater than normal reference value), but there was no statistical significance compared with the non-RSV group. However, average serum TNF-α level from the RSV group was significantly lower than from the non-RSV group. It is likely that the decreased local or systemic TNF-α levels contributed to the prolonged clinical course and wheezing seen in those infants with RSV infection. Similar to other studies [
13,
19,
20], no obvious changes in serum IL-2, 4, 10 and IFN-γ levels were observed in the present study.
A large number of studies have confirmed a link between infantile RSV infection and the development of wheezing or asthma in later childhood [
2]. Moreover, individuals with LBW were more inclined to develop severe RSV infection, resulting in an increased risk of wheezing or asthma [
6,
7]. The potential mechanism might be explained in part by the fact that serum cytokine concentrations in infants with RSV infection were strongly related to birth weight, not to delivery mode. IL-6 high patients had significantly worse lung function and more frequent asthma exacerbations than IL-6 low patients [
21]. Circulating IL-6 is elevated in asthmatic patients and in bronchoalveolar lavage fluid of patients in whom asthma is clinically active. IL-6 levels probably reflect an activated state of the lung, and may have a role as a biomarker for asthma [
22]. In the present study, increased serum IL-6 levels in infants with LBW might also play an important role in RSV induced wheezing or asthma. Although average serum IL-6 in RSV patients with wheezing was higher than in those without wheezing, there was no significant difference between them. This phenomenon further indicated that only LBW was likely to cause obvious increases in serum IL-6 levels in infants with RSV infection. In general, IL-6 is moderately increased in virus infections, lower than in
Mycoplasma pneumoniae and bacterial infections [
11,
23]. The present study also confirmed this trend. It has been previously documented that the levels of pro-inflammatory cytokines such as IL-6 were consistently higher in RSV patients at hospital admission, discharge, and 1 month after discharge than control levels. There was a trend towards greater IL-6 production in RSV infection, but not IFN-γ production [
13]. These cytokines recruit inflammatory and immunocompetent cells into the sites of airway inflammation. Therefore, it is likely that the increased IL-6 response in RSV patients with wheezing would provide a substratum for the development of subsequent asthma, resulting in more significant changes related to asthma, especially in children with LBW.
Additionally, a previous study by Rusconi et al. showed that the adjusted risk ratio for development of asthma was 1.33 for elective cesarean delivery compared with spontaneous vaginal delivery [
24]. However, Werner et al. found no support for the hypothesis that children delivered by caesarean section have an increased risk of asthma during the first 15–18 years of life [
25]. Furthermore, delivery by caesarean section was not clearly associated with hospitalizations for asthma and other wheezing disorders up to 12 years of age compared to vaginal delivery [
26]. In view of the potential role of delivery mode in the development of asthma or other wheezing diseases, we also evaluated the possible role of delivery mode in cytokine changes during infantile RSV infections. No obvious changes in serum cytokines were observed between the caesarean section and vaginal delivery groups. Therefore, the present study cannot confirm the correlation between delivery mode and cytokine changes in infantile RSV infections.
Nevertheless, our study has some limitations. Besides the six cytokines in this study, other inflammatory cytokines were reported to be related to pathogenesis of RSV infection, such as IL-5, 13, and 17 [
3]. We investigated the above six cytokines only because of the availability of the commercial CBA kit consisting of the six cytokines: IL-2, 4, 6, 10, TNF-α, and IFN-γ. Secondly, because commercial respiratory immunofluorescence kit via NPA in our hospital only detected respiratory syncytial virus (RSV), influenza virus A and B, parainfluenza virus, and adenovirus, not including rhinovirus, we did not perform rhinovirus testing in the present study. It is likely that the RSV-rhinovirus co-infection is the most common co-infection, or Non-RSV infected patients might have rhinovirus infection. Thirdly, although all the samples were collected on admission, we are not certain if they were collected at the same stage of infection. Finally, this study mainly focused on a certain period of hospitalization; long term follow-up study was not performed. Therefore, larger prospective studies are necessary to explore the potential role of serum cytokines in RSV patients with LBW.
Acknowledgements
We thank Dr. Sun for her help. We also thank Dr. Elizabeth L. Kramer from Cincinnati Children’s Hospital Medical Center for reviewing this manuscript. We also thank the Zhejiang Key Laboratory for Diagnosis and Therapy of Neonatal Diseases and the Key Laboratory of Reproductive Genetics Ministry of Education for their assistance.