Although the contribution of Notch ligands expressed by antigen-presenting cells (APCs) to the Th1/Th2 differentiation has been suggested in mouse studies, their nature in humans remains obscure. To assess the issue, we examined correlation of Notch ligand mRNA levels of human APCs with Th1/Th2-promoting stimulations, i.e. Th1/Th2 adjuvant activities. The expression patterns of human dendritic cells (DCs) and leukemic cell line models of APCs differed partly from those previously observed in mouse DCs and by cellular types and maturation stages. Most strikingly, Th2 adjuvants enhanced the Delta] expression on human APCs but did not induce Delta4 expression, which was induced by a Th1 adjuvant. The differential expression patterns suggest a distinct role of these Delta members in Th1/Th2 differentiation and their predictive potential for Th1/Th2-promoting activities. (C) 2007 Elsevier Inc. All rights reserved.