Defective suppression of Th2 cytokines by CD4⁺CD25⁺ regulatory T cells in birch allergics during birch pollen season

Summary Background CD4 + CD25 + regulatory T cells suppress proliferation and cytokine production by human T cells both to self‐antigens and exogenous antigens. Absence of these cells in human newborns leads to multiple autoimmune and inflammatory disorders together with elevated IgE levels. However, their role in human allergic disease is still unclear. Objective This study aimed to evaluate the capacity of CD4 + CD25 + regulatory T cells to suppress proliferation and cytokine production outside and during birch‐pollen season in birch‐allergic patients relative to non‐allergic controls. Methods CD4 + cells were obtained from blood of 13 birch‐allergic patients and six non‐allergic controls outside pollen season and from 10 birch‐allergic patients and 10 non‐allergic controls during birch‐pollen season. CD25 + and CD25 − fractions were purified with magnetic beads and cell fractions, alone or together in various ratios, were cultured with antigen‐presenting cells and birch‐pollen extract or anti‐CD3 antibody. Proliferation and levels of IFN‐γ, IL‐13, IL‐5 and IL‐10 were measured by thymidin incorporation and ELISA, respectively. Numbers of CD25 + cells were analysed by flow cytometry. Results CD4 + CD25 + regulatory T cells from both allergics and non‐allergics potently suppressed T cell proliferation to birch allergen both outside and during birch‐pollen season. However, during season CD4 + CD25 + regulatory T cells from allergic patients but not from non‐allergic controls were defective in down‐regulating birch pollen induced IL‐13 and IL‐5 production, while their capacity to suppress IFN‐γ production was retained. In contrast, outside pollen season the regulatory cells of both allergics and non‐allergic controls were able to inhibit T‐helper 2 cytokine production. Conclusion This is the first study to show differential suppression of Th1 and Th2 cytokines, with CD4 + CD25 + regulatory T cells from birch‐pollen‐allergic patients being unable to down‐regulate Th2, but not Th1 responses during birch‐pollen season.