Cross priming or direct priming: is that really the question?

Naïve CD8+ T cells differentiate into effector cells after recognizing specific peptides bound to self-MHC class I molecules. Recent work has examined whether endogenously synthesized antigens (direct priming) or antigens from exogenous sources (cross priming) are the primary in vivo source of these peptides, and whether the display of peptide–MHC complexes by specialized professional antigen presenting cells is required for the induction of a primary CD8+ T cell response. Although the results seem contradictory, they support the concept that a multitude of antigen-processing pathways can contribute to the generation of effective immunity by CD8+ T cells.