Age‐ and infection‐related maturation of the nasal immune response in 0–2‐year‐old children

van Benten, I. J.; van Drunen, C. M.; Koopman, L. P.; van Middelkoop, B. C.; Hop, W. C. J.; Osterhaus, A. D. M. E.; Neijens, H. J.; Fokkens, W. J.

Background: The hygiene hypothesis suggests that exposure to micro‐organisms influences development of the immune system in children. Methods: In this study, we examined nasal immune responses in the first 2 years of life in relation to age of children and the number of viral infections they have experienced. Nasal brushes were taken during rhinovirus‐ (n = 20) or respiratory syncytial virus (RSV)‐induced (n = 7) upper respiratory tract infections (URTI), and of controls (n = 40). Results: The number of macrophages were higher during URTI and increased with age. The number of T lymphocytes increased with age in controls and were higher during URTI at all ages. We found an age‐related decrease in the number of interleukin (IL)‐4‐ and IL‐10‐positive cells in controls, while the number of IL‐12‐positive cells remained unchanged. Changes in T lymphocyte and IL‐4 cell number were stronger related to the age of the child than to the number of respiratory infections, while the opposite was true for macrophages. Conclusions: In infants, we found an infection‐ and age‐related increase respectively for nasal macrophages and T lymphocytes during URTI. Furthermore, the number of IL‐4‐ and IL‐10‐positive cells decreased with age. Whether this maturation reflects a natural age‐related maturation, the degree of exposure to respiratory infections, or possibly both, could not be resolved and needs further study.