IL-4 production by allergen-stimulated primary cultures: identification of basophils as the major IL-4-producing cell type.

As a potent inducing agent for IgE production and differentiation factor for allergen-specific Th2 cells, IL-4 is a key regulatory cytokine both in the pathogenesis of allergic disease and in the ongoing allergic response. The assay of in vitro IL-4 production has often been used to compare the allergen responses of T cells isolated from atopic and non-atopic subjects. Because peripheral blood basophils also have the capacity to respond to specific allergen by producing IL-4, we investigated the relative contribution of these two cell types to IL-4 production in allergen-stimulated primary cultures. Among unfractionated peripheral blood mononuclear cells (PBMC), the major producers of detectable IL-4 in primary in vitro cultures were found to be basophils based on: (i) an allergen dose-response corresponding closely to that required for basophil histamine release and lower than that required for T cell activation; (ii) a rapid time course for IL-4 production (detectable at 3 h), inconsistent with the typical activation requirements of fresh T cells; (iii) the production of comparable levels of IL-4 in cultures stimulated with allergen or anti-IgE; and (iv) the complete loss of detectable IL-4 production following specific depletion of basophils from PBMC. The T cells in these cultures were functionally able to produce IL-4, as demonstrated by mitogen activation of basophil-depleted PBMC. These findings demonstrate that although IL-4 production in primary in vitro cultures can be used as a sensitive indicator of allergen responsiveness, the accurate interpretation of this result requires identification of the responding cell type. Furthermore, these findings raise the possibility that basophil production of IL-4 early in the course of allergen stimulation may shape subsequent T cell responses both in vivo and in vitro.