Growing evidence suggests that anaphylatoxins, C3a and C5a, play important roles in innate immunity and may also participate in the pathogenesis of asthma. Previous studies with animal models and immunohistochemistry analysis of lung tissue indicated that anaphylatoxins may regulate airway hyperresponsiveness (AHR) in asthma via the activation of their cell surface G protein-coupled receptors (C3aR and C5aR) in airway smooth muscle (ASM) cells. Using RT-PCR, flow cytometry, and confocal microscopy, we made the surprising observation that while C3aR and C5aR were expressed in human mast cells, they were not present in cultured primary human or murine ASM cells. Furthermore, we could not detect C3aR in smooth muscle-positive cells of human trachea or bronchus. Interestingly, incubation of human mast cells with ASM cells, but not its culture supernatant, caused a significant enhancement of C3a-induced mast cell degranulation. Although stem cell factor (SCF) and its receptor c-kit are constitutively expressed on ASM cells and mast cells, respectively, neutralizing antibodies to SCF and c-kit failed to inhibit ASM cell-mediated enhancement of mast cell degranulation. However, dexamethasone-treated ASM cells were normal for cell surface SCF expression but were significantly less effective in enhancing C3a-induced mast cell degranulation when compared with untreated cells. These findings suggest that cell-cell interaction between ASM cells and mast cells, via a SCF-c-kit-independent but dexamethasone-sensitive mechanism, enhances C3a-induced mast cell degranulation, which likely regulates ASM function, thus contributing to the pathogenesis of asthma.