Induction of HO-1 inhibits the allogeneic stimulatory capacity of rat and human DCs. (A) Rat BMDCs and (B) human DCs were treated or not with SnPP or CoPP for 2 hours, cultured for 16 hours, and then for a further 24 hours in presence or absence of LPS. Thereafter, DCs were cultured with allogeneic T cells at different ratios for 4 days and proliferating T cells were labeled with ³H-thymidine for 8 hours before being harvested and counted. Results are shown as means ± SD of triplicate values after subtraction of spontaneous ³H-thymidine. Statistical significance compared with LPS-treated DCs is indicated as ^*P < .05. Similar results were obtained in 6 independent experiments for rat DCs and in 2 for human DCs. cpm indicates counts per minute. (C) In parallel, supernatants were collected after 48 hours from rat MLRs and analyzed by ELISA to determine levels of IFN-γ and IL-10. Data presented are mean ± SD of triplicate analysis of one experiment and are representative of 5 independent experiments. Statistical significance compared with LPS-treated DCs is indicated as ^*P < .01.