CD200 expression on apoptotic DCs results in increased T-cell proliferation and diminished secretion of proinflammatory cytokines in caspase-dependent autologous MLR cultures. (A) WT CD11c⁺ DCs were cultured in complete DMEM with or without 200 μM Z-VAD-FMK for 24 hours, washed thoroughly, and then used as stimulators in autologous MLR cultures with purified CD4⁺ T cells from C57BL/6 mice as responders. ^*P < .05 versus medium control. (B) Cell surface protein expression on wild-type (WT) and CD200-deficient (CD200^-/-) DCs. CD11c⁺ DCs were cultured in complete DMEM for 24 hours to induce apoptosis and stained with specific antibodies (shaded histograms) to detect cell surface protein expression by flow cytometry. Cells were more than 97% CD11c⁺, and approximately 70% were apoptotic by Annexin V staining. ΔG_mean represents the change in MFI from isotype-matched controls (open histograms). (C) CD11c⁺ DCs were isolated from wild-type C57BL/6 (H-2^b; CD200^+/+) and CD200-deficient (H-2^b; CD200^-/-) mice and cultured in complete DMEM for 24 hours to induce apoptosis. Stimulators were more than 97% CD11c⁺, and approximately 70% were apoptotic by Annexin V staining at the time responders were added. Proliferation was measured at 72 hours by [³H]-thymidine incorporation. Results are representative of 4 replicate experiments. ^*P < .05 versus WT. (D) Cytokine levels were measured from supernatants at 72 hours by cytometric bead array. Results are the average of 4 replicate experiments. ^*Statistically significant from CD200^-/- (P < .05) by Student t test. Error bars indicate ±1 SD (A, C-D).