Inflammation with eosinophils and Th2 T cells is a major pathological feature of the target organ mucosa in asthma and atopic dermatitis. While Th2 T cells are well known to regulate eosinophil function through Th2 cytokines such as IL-5, relatively little attention has been paid to the reverse process, that is, the possibility that eosinophil products can regulate Th2 cell function. Strong expression of local tissue lymphopoietins such as TSLP at epithelial surfaces can activate dendritic cells to induce a Th2 permissive microenvironment and expression of new genes on committed CRTh2+CD4+ memory T cells. One such gene is the receptor for the cytokine IL-25, one of IL-17 family of cytokines. Systemic over-expression of IL-25 in animal results in Th2-skewed immune responses and pathological changes in the respiratory and gut mucosa reminiscent of allergic inflammation.

Our preliminary data suggest that this cytokine may play an important role in the mechanistic “cross talk” between Th2 T cells and eosinophils. We have found that eosinophils are the major source of IL-25. Conversely, IL-25R expression is confined principally to memory Th2 T cells. These observations suggest the hypotheses that (1) IL-25 plays an amplifying role in allergic inflammation; (2) CRTh2+CD4+ T cells are an important target for IL-25 in allergic inflammation; (3) eosinophils are a significant source of IL-25. To explore these hypotheses, we will use the well characterized late-phase cutaneous reaction to allergen challenge in the skin of atopic subjects. In this model, skin biopsies can be obtained from individual subjects at intervals of up to 72 hours following cutaneous allergen challenge, so that the time courses of cellular infiltration and cytokine expression can be charted. In this project we will track infiltration of CRTh2+CD4+ T memory cells, as well as IL-25 and IL-25 receptor immunoreactive cells. We will also identify the phenotypes of the latter cells. This will be accomplished using techniques including double, sequential immunohistochemistry (IHC) and IHC/in situ hybridization. We will also examine bronchial mucosal biopsies from asthmatics and controls in a similar fashion. We see these ex vivo, dynamic experiments as complementing and amplifying static experiments addressing the functional effects of IL-25 on CRTh2+CD4+ T cells and the production of IL-25 by eosinophils. We can also make comparisons with disease severity and examine expression of IL-25 in relation to other asthma-relevant cytokines.

The data obtained from these experiments will allow us to establish new roles for CRTh2+CD4+ T cells and eosinophils in allergic inflammation and will open new avenues for therapeutic intervention.