Endocytosis of an HIV-derived lipopeptide into human dendritic cells followed by class I-restricted CD8(+) T lymphocyte activation

CD8(+) T lymphocytes, which are major immune effecters, require primary sti mulation by dendritic cells (DC) presenting MHC class I molecule-bound epit opes. Sensitization to exogenous protein epitopes that are not synthesized in DC, such as cross-priming, is obtained through pathways leading to their association with MHC class I. To follow class I-restricted pathways in hum an DC, we have tracked a lipopeptide derived from the conserved HLA-A*0201- restricted HIV-1 reverse transcriptase 476-484 epitope, by N-terminal addit ion of an N epsilon -palmytoyl-lysine. Indeed, lipopeptides elicit cytotoxi c responses from CD8(+) T lymphocytes, whereas peptides without a lipid moi ety do not. The lipopeptide and its parent peptide were labeled unequivocal ly by rhodamine to study their entry into immature monocyte-derived human D C by confocal microscopy. The lipid moiety induced endocytosis of the lipop eptide, assessed by rapid entry into vesicles, colocalization with Dextran- FITC and dependence on energy. Internalization occurred even when actin fil aments were depolymerized by Cytochalasin B. This internalization induced f unctional stimulation of specific CD8(+) T lymphocytes in IFN-gamma ELISPOT assays. The peptide alone was not visualized inside the DC and was present ed through direct surface association to HLA-A*0201. Therefore, lipopeptide s are a unique opportunity to define precisely the pathways that lead exoge nous proteins to associate with MHC class I molecules in DC. The results wi ll also be useful to design lipopeptide vaccines.