Cytotoxic T lymphocyte activation by cross-priming.

Cross-priming represents the induction of cytotoxic T lymphocyte responses to exogenous, usually cell associated, antigens that are captured and re-presented on bone-marrow-derived antigen-presenting cells. For effective cross-priming, cytotoxic T lymphocytes require help from CD4(+) T cells, which mediate this help indirectly via modification of the antigen-presenting cell. Recent advances made in research into the cellular and molecular interactions required for cross-priming have greatly improved our understanding of the antigenic requirements for effective priming and have revealed a role for CD40 and its ligand in the provision of T cell help.