Grass pollen immunotherapy: symptomatic improvement correlates with reductions in eosinophils and IL-5 mRNA expression in the nasal mucosa during the pollen season.

Background: Tissue eosinophilia and infiltration by T(H)2-type T cells are characteristic features of allergic rhinitis both after allergen challenge and during natural allergen exposure. Specific immunotherapy inhibits allergen-induced nasal eosinophilia.

Objectives: We sought to assess, in the context of a randomized trial, the relationships between symptomatic improvement after immunotherapy and eosinophil numbers and IL-5 expression in the nasal mucosa during the pollen season.

Methods: Nasal biopsy specimens were taken from 37 adults with severe summer hay fever at baseline (out of season) and at peak season after 2 years of treatment with a depot grass pollen extract or placebo. Biopsy specimens were processed for immunohistochemistry by using mAbs against eosinophils (EG2), T cells (CD3), and IL-2 receptor-positive cells (CD25), as well as for in situ hybridization by using a sulfur 35-labeled antisense riboprobe directed against IL-5.

Results: Immunotherapy significantly reduced symptoms (49%, P =.01) and medication requirements (80%, P =.007) compared with placebo. There was a 400% increase (P =.004) in eosinophils during the pollen season in placebo-treated patients, which was inhibited in the immunotherapy group (20% increase, P =.04 between groups). Seasonal increases were also observed for CD25(+) cells (P =.002), CD3(+) cells (P =.02), and IL-5 mRNA-expressing cells (P =.03) in the placebo group but not in the immunotherapy group. A significant correlation was observed between eosinophils and IL-5 expression (r = 0.5, P <.05). Both eosinophils (r = 0.6, P <.02) and IL-5 (r = 0.6, P <.02) correlated with symptoms after immunotherapy.

Conclusion: Improvement in symptoms after grass pollen immunotherapy may result, at least in part, from inhibition of IL-5-dependent tissue eosinophilia during the pollen season.