['Cysteinyl-leukotrienes (cysLTs) are lipid mediators recognized for their role in asthma and allergic inflammation. CysLT1, one of the receptors for cysLTs, is expressed, among others, in bronchial smooth muscle cells, phagocytes, and B lymphocytes. The potential role of CysLT1 in B lymphocytes remains unexplored.', 'We examined the modulation of expression and function of CysLT1 in human B lymphocytes.', 'Human B lymphocytes were purified from peripheral blood and analyzed by means of RT-PCR and flow cytometry, after culture with IL-4 and anti-CD40 antibody or CD154-transfected fibroblasts. Functional responses to leukotriene (LT) D4 in terms of cytosolic calcium flux, proliferation, and immunoglobulin production were also examined.', 'B lymphocytes expressed CysLT1 at both the mRNA and protein levels. Two-fold to 3-fold enhancement of CysLT1 expression was observed after B-cell exposure to a combination of activating anti-CD40 antibody and IL-4. The expression of CysLT1 was also enhanced when B lymphocytes were cocultured with CD154-transfected fibroblasts in the presence of IL-4. Moreover, IL-4 and CD40-activated B lymphocytes showed an increased responsiveness to LTD4 in terms of cytosolic calcium flux, which was totally prevented by the selective CysLT1 antagonist montelukast. Stimulation of IL-4 and CD40-activated B lymphocytes with picomolar concentrations of LTD4 induced mature epsilon transcripts and upregulated IgE and IgG production 2-fold to 3-fold.'] Does leukotriene D4 enhance immunoglobulin production in CD40-activated human B lymphocytes?