M. Bevan showed in the mid-seventies that cytotoxic T lymphocyte (CTL) responses may be initiated by antigenpresenting cells that do not express the antigens themselves (1). He called this process cross-priming. The antigen-presenting cells involved in cross-priming must therefore internalize and present antigens to CD8 T cells in the context of MHC class I molecules. This process is often referred to as “cross-presentation.” Cross-presentation by antigen-presenting cells in vivo results in either cross-priming (initiation of CD8 T cell responses) or in cross-tolerance (induction of CD8 T cell unresponsiveness; reference 2). These results raised the question of the nature of the “crosspresenting cells.” In vitro, dendritic cells cross-present antigens more efficiently than any other antigen-presenting cell (3). They are also the only antigen-presenting cells that activate naive T lymphocytes (4). Dendritic cells, indeed, are sufficient for cross-presentation in vivo (5).