Background: Epstein-Barr virus-induced gene 3 (EBI3) is a widely expressed interleukin-12-related protein that plays a critical role in the regulation of effector T cells and inflammatory responses. Methods: Allergic rhinitis (AR) was induced in mice by both intraperitoneal injection and intranasal administration of ovalbumin. By using immunohistochemical techniques, we investigated EBI3 expression in mouse spleens. The expressions of EBI3 and forkhead transcription factor 3 (Foxp3) in CD4+CD25+ regulatory T (T-reg) cells isolated from spleens were assessed by flow cytometry. RNA extraction and RT-PCR were used to measure EBI3 and Foxp3 mRNA expression. Results: EBI3 and Foxp3 were expressed mainly in the white pulp of the spleens. Flow cytometry analysis showed that EBI3 and Foxp3 coexpressed in mouse CD4+CD25+ T-reg cells. Furthermore, RT-PCR and flow cytometry analysis showed that expression of both Foxp3 and EBI3 was reduced in CD4+CD25+ T-reg cells in an AR mouse model compared with controls, but there was no difference in the frequency of CD4+CD25+ T-reg cells. Conclusions: The deficiency in CD4+CD25+ T-reg cell suppressive activity in AR is associated with the molecular mediators EBI3 and Foxp3. Copyright (C) 2011 S. Karger AG, Basel