Abstract
Designing vaccines against viruses or tumor cells that elicit CD8 T cell responses depends on understanding how viral and tumor antigens initiate activation of na�ve CD8 T cells. In his Perspective, Ploegh explores several studies published here ( Norbury et al., Wolkers et al.) and elsewhere that reveal the pathway from viral or tumor antigens to long-lived peptides that are capable of priming CD8 T cells.