SCF-activated mast cells release adenosine to suppress the immune response. (A) Assay of adenosine released by BMMCs after the stimulation with SCF. BMMCs were cultured in the absence or presence of SCF for 48 hours. The adenosine in the supernatant was assayed as described in “Assay of adenosine.” (B) Assay of adenosine in tumor tissues. The mice bearing WT H22 tumor received the intravenous injection of mast cells and anti-c-Kit antibody as indicated. The mice bearing SCF-knockdown H22 tumor received the intratumor injection of mast cells. Seventy-two hours later, the adenosine in tumor tissues was assayed as described in “Assay of adenosine.” *P < .05, compared with the 0 ng/mL SCF group or the WT tumor group. (C,D) Mast cell–produced adenosine inhibits T cells and NK cells. Splenic T cells and NK cells were cultured with the culture supernatant of SCF-stimulated mast cells or control SCF medium in the presence or absence of adenosine receptor A_2A antagonist SCH-58261 (C). The T cells and NK cells from tumor were isolated from the mice bearing WT H22 tumor 72 hours after the intratumor injection of mast cells or control bone marrow cells with or without SCH-58261 (D). The proliferation of T cells (left) and the production of IFN-γ by NK cells (right) were determined as described in “Assay of soluble SCF and IFN-γ by enzyme-linked immunosorbent assay.” *P < .05, compared with the 0 ng/mL SCF, control, or WT tumor groups; #P < .05, compared with the no-SCH-58261 group. Error bars represent SD.