CD4 and infection: This is mainly orchestrated by T cell activation following widespread infection of the CNS parenchyma as lack of MHC I and II in β2-Macroglobulin–/– and Aβ–/– MHV-J2.2-v1 mice or deficiency of CD4 in CD4–/– mice resulted in reduced viral clearance with limited demyelination (Houtman, 1996; Lane et al., 2000).