Acidification of endosomal compartment is needed for cross-presentation of MP by DCs. IDCs were preincubated with increasing doses of NH₄Cl. The NH₄Cl-pretreated DCs were thereafter cocultured with HLA-mismatched apoptotic or necrotic monocytes (1:2 ratio) expressing MP together with the same doses of inhibitor and a maturation stimulus for about 12 hours. All groups of DCs were fixed by glutaraldehyde and cultured with Flu16. Responses were determined in an IFN-γ ELISPOT assay. The left axis shows cross-presentation; the right, peptide presentation. One representative experiment of 4 is shown.