Dendritic cells (DCIs) possess a potent ability to modulate and activate specific T-cell responses to allergens, which play a pivotal role in allergic inflammation by secreting cytokines and other mediators. However, the molecular mechanisms by which allergen-challenged DCs regulate specific T-cell responses are still not well characterized. This study aims at elucidating the molecular mechanisms underlying the DC-T-cell interaction during an allergic immune response to grass pollen, using a genomic and functional approach. Transcriptional analysis was performed on grass allergen Phleum pratense-stimulated DCs and on autologous memory CD4+ T cells co-cultured with allergen-challenged DCs from healthy and allergic donors. DCs from the allergic donors were potent inducers of T-cell proliferation and Th2 polarization, as demonstrated by high IL-4, IL-5 and IL-13, and low IFN-? production. A gradual up-regulation of activation markers on both DCs and T cells was evident during the co-culture period, demonstrating an educational element of the DC-T-cell interaction. The global transcriptional analysis revealed a differential gene regulation in DCs and T cells derived from allergic donors after stimulation with allergen, as compared with the healthy donors. Peripheral memory CD4+ T cells from healthy and allergic donors also responded differently after stimulation with allergen-loaded DCs with respect to cytokine production, proliferation, surface marker expression and gene transcription. We found up-regulated genes involved in Th2 cell biology, such as genes important for homing, adhesion, signaling and transcription, in addition to genes previously not described in the context of allergy. The panel of differentially expressed genes in the allergic group will form the basis for an increased understanding of the molecular mechanisms in allergy. © The Japanese Society for Immunology 2005. All rights reserved.