To determine the number of Tregs present in our infection model in secondary lymphoid organs and the brain, C57BL/6 mice were infected with rMV-green and analyzed 3, 7, 10, 14 and 28 days post infection. Lymphocytes were isolated from 6 draining cervical lymph nodes (LN), the spleen, and the brain of MVinfected and control (i.c. injected PBS) animals. The fraction of CD4 + CD25 + Foxp3 + T cells of all lymphocytes was determined to be 1.5-3% in the spleen and 4-6% in LN ( Fig. 1 A, B ) with no significant difference between infected and control animals. This corresponds to a total number of Tregs of approximately 1610 6 in the spleen and 2610 5 in the prepared LN at day 28 post infection. Only a small statistically not significant number of Foxp3 + T cells was detected in the brain, irrespective of the infection (data not shown). To obtain statistically significant results and to reduce standard errors caused by the staining procedure, we used DEREG mice, which express GFP in Foxp3 + cells. After infection of these mice with rMV (not expressing GFP), a small but significant number of GFP + cells, approximately 800 per brain, was detected in the brains, whereas almost no GFP + cells were detected in controls ( 