Interleukin-10 is a pleiotropic cytokine known to have inhibitory effects on the accessory functions of dendritic cells. In vitro, interleukin-10 converts immature dendritic cells into tolerizing antigen- presenting cells. To assess whether interleukin-10-treated dendritic cells exert tolerizing effects in vivo, CD4+ T cells from DO11.10 ovalbumin-T cell receptor transgenic mice were transferred to syngeneic BALB/c recipients. Recipient animals were treated with ovalbumin-pulsed/unpulsed, interleukin-10-treated/untreated CD11c+ dendritic cells thereafter and ovalbumin-specific proliferation of lymph node cells was assessed by restimulation with the peptide in vitro. In prophylactic experiments, recipients received naive CD4+ DO11.10 T cells and were immunized with ovalbumin323-339 peptide in incomplete Freund's adjuvant after treatment with various subtypes of dendritic cells. Strong ovalbumin-specific proliferation was observed in animals immunized with control ovalbumin-dendritic cells. Minimal proliferation was found in mice treated with ovalbumin-pulsed, interleukin-10-treated dendritic cells. In therapeutic experiments, preactivated CD4+ DO11.10 T cells were transferred, and recipients were treated with dendritic cells as described. Ovalbumin-specific proliferation was strong in recipients treated with ovalbumin-dendritic cells. CD4+ T cell proliferation from ovalbumin-interleukin-10-dendritic cell treated animals was below background. When delayed type hypersensitivity reactions in the footpads of prophylactically or therapeutically vaccinated animals were tested, mice treated with ovalbumin-interleukin-10-dendritic cells showed no footpad swelling compared with controls. Rechallenge with the antigen in vitro and in vivo did not alter the inhibitory effect of interleukin-10-treated dendritic cells. Thus, interleukin-10-treated dendritic cells inhibit ovalbumin-specific immune responses in naive and sensitized mice.