Peptides translocated back into the phagosomal lumen are loaded onto MHC class I molecules. Guermonprez et al. (2003) detected the components of the peptide loading complex (TAP, tapasin, calreticulin and ERp57) together with MHC class I in purified early phagosomes. They observed peptide loading in the presence of ATP in purified phagosomes expressing HLA-A2, incubated with iodinated S-9-L peptide. From these experiments they concluded that TAP-imported peptides can be loaded on MHC class I molecules in the lumen of phagosomes. Houde et al. (2003) also observed OVA peptide SIINFEKL:MHC-I complex in the phagosome lumen.