Cross-Presentation Pathway: Cytosolic vs Vacuolar Antigen Routing (2D)

Dendritic cells can display fragments of antigen they engulfed from outside the cell on MHC class I — a process called cross-presentation that lets them recruit CD8⁺ killer T cells against threats the DC itself was never infected by. This 2D cross-section renders the two documented escape routes an engulfed antigen molecule can take out of the phagosome: the cytosolic route (Sec61 translocon export → proteasome cleavage → TAP-mediated import into the ER → MHC-I loading) and the shorter vacuolar route (direct cathepsin S degradation and MHC-I loading inside the phagosome). Each antigen particle's fate is decided by a real competing-exponential race between the two pathway rate constants, the measured split converges on the textbook k1/(k1+k2) prediction as more antigen is processed, and toggling a pharmacological block on either route lets you verify — live, by watching total surface MHC-I throughput drop — that removing an available route genuinely reduces cross-presentation efficiency rather than being silently compensated by the other.