Phenotyping of Tregs from infected/nontreated and infected/667-treated mice and the role of IL-10 produced by Tregs from infected/nontreated mice. (A) Phenotyping of Tregs. Tregs were prepared from 8-week-old noninfected/nontreated, infected/nontreated, or infected/667-treated mice and subjected to TCR stimulation, as described in “Methods,” before flow cytometry analyses of the indicated proteins. Two independent experiments with 3 mice per group were conducted. Data are presented as means ± SEM. The statistical significance between groups was established using the Student t test. *P < .05; **P < .01. (B-D) Role of IL-10 in Tregs from infected/nontreated mice. Tregs were purified from 8-week-old infected/nontreated mice and grafted to infected/667-treated mice of the same age. These mice were virally challenged at the same time, as described in “Methods.” Half of the animals were subjected to IP administration of a neutralizing anti–mouse IL-10 mAb from day 5-10 after viral challenge/Treg grafting and the other half were left untreated. Immune response analyses were carried out on day 10 after grafting as in Figure 4, and the experiments were conducted as described in “Methods.” (B) Assay of GagL-specific CD8⁺ T cells. (C) In vivo assay of anti-FrCas^E–infected cell CTL responses. (D) Assay of anti-FrCasE IgGs. Two independent experiments with 4 mice per group were conducted. Data are presented as means ± SEM. The statistical significance between groups was established using the Student t test. *P < .05; **P < .01.