Acknowledgements
We wish to acknowledge the excellent assistance received from Rune Fledelius Jensen and Julia Sid Hansen at the Department of Infectious Disease Immunology at Statens Serum Institut. As well as the experimental animal facilities at Statens Serum Institut for animal handling, particularly Jan Villumsen, for his excellent help with the activity monitoring system. The study was supported by the Danish 3R-Center (to G.K.P) and Independent Research Fund Denmark (grant no. 9041-00160B to J.Z.).