Laboratory Animals of the National Institutes of Health. The protocol was approved by the Committee on the Ethics of Animal Experiments of the University of Würzburg (Permit Number: 55.2-2531.01-67/06). Specific pathogen free C57BL/6 mice were purchased from Harlan-Winkelmann, Germany. DEREG mice [38] were breeded in the animal facilities of the Institute for Virology and Immunobiology and the Centre for Experimental Medicine, Würzburg. Mice were anesthetized using isofluran and infected intracerebrally (i.c.) into the left hemisphere with 20 ml virus suspension containing 1610 3 PFU at an age of two weeks. To expand Tregs, mice were treated intraperitoneally (i.p.) with 100 mg superagonistic anti-CD28 monoclonal antibody (mAb) D665 [36] , and to deplete Tregs, DEREG mice were treated i.p. with 1 mg diphtheria toxin (DT; Merck) [38] .