In order to investigate whether this increase in virus clearance correlates with the presence of higher numbers of virus-specific CD8 + T cells, we first identified T cell receptor recognized peptides presented by MHC class I of C57BL/6 mice (H-2 b ). Two CD8 + T cells were detected in the brain at 7, 10, 14, and 28 dpi (Fig. 5 A) . Interestingly, high percentages of D b MV-H 22-30specific CD8 + T cells are present in the brain during the persistent phase of the infection (at 28 dpi approximately 18% of all CD8 + T lymphocytes). Mock-treated animals (i.c. PBS injection; ctrl) did not contain virus-specific CD8 + T cells. After treatment of DEREG mice with DT, the number of Tregs decreased by approximately 95%, while the number of CD8 + T cells in brains slightly increased (Fig. 5 B) . Interestingly, after depletion of Tregs, the fraction and absolute number of D b MV-H 22-30 -specific CD8 + T cells increased from 2,000 to 8,000 cells per brain, or 5% to 23% of CD8 + T cells (n = 3, P = 0,05; Fig. 5 C) . Thus, depletion of Tregs during the persistent phase of infection led to an increase of virus-specific CD8 + T cells and a significant reduction of the persistent infection in the brain.