WDFY4-dependent cross-presentation proceeds via a vacuolar antigen-processing route

The intracellular processing route used by type I conventional dendritic cells (cDC1) for cross-presentation of viral- or tumor-associated antigens remains controversial. One model proposes that captured antigens exit from damaged phagolysosomes and enter the cytosol, where they are processed for presentation by MHC class I molecules (MHC-I). This model relies on proteasomal degradation and TAP-dependent peptide transport into the endoplasmic reticulum (ER) for loading by the peptide loading complex (PLC). An alternative model proposes a vesicular route in which captured antigens are retained and processed within an endocytic compartment. A compelling argument favoring the cytosolic model is the dramatic loss of cross-presentation by TAP-deficient cDC1, which presumes that peptides derived from phagocytosed cells require TAP for their transport into the ER to reach the PLC. However, here we show that cross-presentation by cDC1 is TAP-dependent because TAP is required for the normal trafficking of MHC-I molecules. Our work demonstrates that MHC-I must reach a post-Golgi compartment for loading in cross-presentation and that this process may occur independently of TAP.