Cross-presentation

Cross-presentation is the ability of certain professional antigen-presenting cells (mostly dendritic cells) to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells (cytotoxic T cells). Cross-priming, the result of this process, describes the stimulation of naive cytotoxic CD8⁺ T cells into activated cytotoxic CD8⁺ T cells.^[1] This process is necessary for immunity against most tumors^[2] and against viruses that infect dendritic cells and sabotage their presentation of virus antigens.^[3][4] Cross presentation is also required for the induction of cytotoxic immunity by vaccination with protein antigens, for example, tumour vaccination.^[5]

Cross-presentation is of particular importance, because it permits the presentation of exogenous antigens, which are normally presented by MHC II on the surface of dendritic cells, to also be presented through the MHC I pathway.^[6] The MHC I pathway is normally used to present endogenous antigens that have infected a particular cell. However, cross presenting cells are able to utilize the MHC I pathway to present exogenous antigens (ones not from the cell itself) to trigger an adaptive immune response by activating cytotoxic CD8+ T cells recognizing the exogenous antigens on the MHC class I complexes.