CD8A and infection: However, MHC I in professional APCs, particularly DCs, can also present peptides from internalized exogenous proteins via a process known as cross-presentation, which is crucial for CD8 T cell responses against infections and tumorigenesis.11,12 In this process, the exogenous proteins are either transported from the endosomal vesicles into the cytosol for proteasomal processing or directly processed within the endosomal compartments.