T lymphocytes respond to peptide fragments of protein antigens that are displayed by MHC molecules on antigen-presenting cells (APCs). In general, extracellular antigens are presented via MHC class II molecules to CD4 + T cells while intracellular antigens are presented via MHC class I molecules to CD8 + T cells [8, 9] . However, a number of reports have demonstrated that a significant level of crossover, so-called 'cross-presentation', occurs in APCs [10] [11] [12] [13] [14] . Using this phenomenon, novel vaccine preparation inducing antigen-specific CTLs that effectively eliminate virus-infected cells is expected. The mechanisms of cross-presentation have been studied intensively [15] [16] [17] while the details have been left unclear. Part of the antigens taken via phagocytosis by APCs are known to be translocated into the cytosol and degraded by local proteases [18, 19] . In another pathway, some antigens internalized into endocytic compartments are loaded onto MHC class I molecules [20] .