The production of TH₂‐type cytokines [interleukin‐4 (IL‐4) and IL‐5] and tissue eosinophilia are characteristic features of allergic diseases. It was previously reported that at 24 h after allergen provocation, CD3⁺ T‐lymphocytes were the principal cell source of IL‐4 and IL‐5 mRNA transcripts in both atopic asthma and rhinitis.

To investigate whether IL‐4 and IL‐5 mRNA are expressed earlier during late nasal responses and if so, which cell(s) are responsible.

Nasal biopsies were obtained at 6 h after nasal allergen challenge and following a control challenge with the allergen diluent. Sections were immunostained for T‐lymphocytes (CD3⁺, CD4⁺) and eosinophils (EG2⁺). In situ hybridization was used to detect the number of cells expressing messenger RNA (mRNA) for IL‐4 and IL‐5.

In patients with allergic rhinitis, eosinophils (EG2⁺ cells P = 0.006) but not T‐ cells (CD3⁺ cells) increased in the nasal mucosa at 6 h after allergen challenge. The number of cells expressing IL‐4 mRNA (P = 0.01) and IL‐5 mRNA (P = 0.05) also increased at 6 h. Co‐localization studies showed that 76% of IL‐4 mRNA⁺ cells and 77% of IL‐5 mRNA⁺ cells were eosinophils, whereas at this time point, T‐cells and mast cells accounted for ≤5% of mRNA expression; the identity of the remaining 20% of IL‐4 and IL‐5 mRNA⁺ cells was not determined. By use of immunohistology, cytokine protein expression at 6 h was confirmed for IL‐4 but not for IL‐5. No increases in T‐cells, eosinophils or cytokine expression were detected in non‐atopic subjects.

Eosinophils represent an early source of IL‐4 which may contribute to TH₂‐type responses during late nasal responses and ongoing allergic rhinitis.