In general, extracellular antigens are presented via MHC class II molecules to CD4 + T cells, whereas intracellular antigens are presented via MHC class I molecules to CD8 + T cells. Consequently, most APCs do not present exogenous antigens via MHC class I since exogenous antigens do not gain access to the cytosolic compartment. Therefore, exogenous antigens usually do not prime CTL responses in vivo. This segregation of exogenous antigens from the class I pathway is important to prevent CTL from killing healthy cells that have been exposed to foreign antigens but are not infected [26] . However, there are several exceptions to this rule, reflecting the ability of the exogenous antigens to be delivered into the cytosolic compartments [13] [14] [15] [16] [17] .