Specific immunotherapy to birch allergen does not enhance suppression of Th2 cells by CD4⁺CD25⁺ regulatory T cells during pollen season

Introduction: The aim of this study was to investigate the suppressive capacity of CD25⁺ regulatory T cells on birch allergen-induced T-cell responses during the first birch pollen season after initiation of specific immunotherapy (SIT). Methods: CD25^pos and CD25^neg T cells were purified from blood of birch-allergic SIT patients and birch-allergic controls, stimulated with birch pollen extract, and analyzed for T-cell proliferation and production of interferon gamma (IFN-γ), interleukin (IL)-5 and IL-10. Results: We show that allergen-induced proliferation and IFN-γ production were suppressed equally well by CD25^pos T cells from SIT patients and controls, while the IL-5 production was not suppressed by either of the groups. IL-10 levels were higher in SIT patients relative to controls only when CD25^neg and CD25^pos were cultured together. Furthermore, neither FOXP3 levels nor proportions of CD25^high T cells were enhanced in SIT patients compared to allergic controls. Discussion: These results suggest that the Th2-suppressive capacity of allergen-stimulated CD25^pos Treg in vitro is not improved by SIT in spite of increased IL-10 production from T cells.