While healthy CNS tissue is devoid of MHC II protein expression, circulating APC, including myeloid cells and B lymphocytes that have entered the perivascular spaces or parenchyma of the CNS from the peripheral blood constitutively express MHC II. After CNS injury, astrocytes [12,13] and microglial cells [14,15] can also upregulate MHC II in response to inflammatory mediators, including IFN??, the