The late-phase of allergic asthma is characterized by infiltration of the airway with eosinophils within 6 h of mast cell activation. Pro-eosinophilic/pro-allergic (TH2) cytokines, originally described as T-lymphocyte products, have recently been ascribed to mast cells as well. To date, however, it is unknown if TH2 cytokine gene expression by the human mast cells is subject to receptor-mediated regulation analogous to that of T-cells, and if messenger RNA (mRNA) expression results in protein secretion occurring in a temporal context consistent with the late-phase response. We examined interleukin-4 (IL-4), IL-5, and IL-6 mRNA expression induced by anti-IgE activation of human lung explants as assessed using reverse transcription/polymerase chain reaction (RT-PCR). Anti-IgE stimulation resulted in rapid and sustained upregulation of IL-5 message, but did not have analogous effects on IL-4 or IL-6. Using quantitative-competitive PCR, we demonstrated that 100 ng of total cellular RNA from human lung contained 1 fg of IL-5 mRNA; this increased to 100 fg 4 h after anti-IgE activation. The source of the anti-IgE-enhanced IL-5 mRNA is likely the mast cell itself, as anti-CD3 activation of lung led to a dissimilar array of cytokine expression. In addition, human lung mast cells purified to near homogeneity expressed IL-5 mRNA after activation, as shown by both RT-PCR and in situ hybridization. In both lung fragments and purified human lung mast cells, the modulation of IL-5 mRNA expression preceded the secretion of IL-5 protein, detected as early as 4 h after activation. Neither isolated purified mast cells nor purified peripheral blood T cells could be induced to secrete detectable amounts of IL-5 protein when activated only with antibodies against IgE or CD3-T cell receptor complex, respectively. However, mast cells (n = 4) and T cells (n = 6) cultured at comparable concentrations (4 x 10(6)/ml) activated through their respective antigen receptors in the presence of phorbol ester yielded comparable IL-5 production (253 +/- 126 pg/ml versus 183 +/- 75 pg/ml, mean +/- SE). We conclude that mast cells are analogous to T cells in the requirement of co-stimuli for the production of IL-5 protein. Moreover, the rapid kinetics of IgE-mediated IL-5 transcription and protein elaboration are consistent with a primary role for mast cell activation directly leading to late-phase airway eosinophilia.