Interleukin-25 initiates Th2 differentiation of human CD4(+) T cells and influences expression of its own receptor.

Human CRTh2(+) Th2 cells express IL-25 receptor (IL-25R) and IL-25 has been shown to potentiate production of Th2 cytokines. However, regulation of IL-25R and whether it participates in Th2 differentiation of human cells have not been examined. We sought to characterize IL-25R expression on CD4(+) T cells and determine whether IL-25 plays a role in Th2 differentiation. Naïve human CD4(+) T cells were activated in the presence of IL-25, IL-4 (Th2 conditions) or both cytokines to assess their relative influence on Th2 differentiation. For experiments with differentiated Th2 cells, CRTh2-expressing cells were isolated from differentiating cultures. IL-25R, GATA3, CRTh2 and Th2 cytokine expression were assessed by flow cytometry, qRT-PCR and ELISA. Expression of surface IL-25R was induced early during Th2 differentiation (2 days). Addition of IL-25 to naïve CD4(+) T cells revealed that it induces expression of its own receptor, more strongly than IL-4. IL-25 also increased the proportions of IL-4-, GATA3- and CRTh2-expressing cells and expression of IL-5 and IL-13. Activation of differentiated CRTh2(+) Th2 cells through the TCR or by CRTh2 agonist increased surface expression of IL-25R, though re-expression of CRTh2 following TCR downregulation was impeded by IL-25. These data suggest that IL-25 may play various roles in Th2 mediated immunity. We establish here it regulates expression of its own receptor and can initiate Th2 differentiation, though not as strongly as IL-4.