The efficiency of antigen delivery from macrophage phagosomes into cytoplasm for MHC class I-restricted antigen presentation.

Macrophages can present exogenous antigen in association with MHC class I molecules. Indirect evidence indicates that antigens internalized by phagocytosis can enter cytoplasm before following the conventional MHC class I presentation pathway. However, little is known about how common such entry is, or to what extent it depends on the kind of particle ingested. This study reports quantitative and morphological characterization of antigen delivery from phagosomes into cytoplasm for MHC class I-restricted antigen presentation. Ovalbumin (OVA) was associated with polystyrene particles (PS), biodegradable poly-e-caprolactone particles (PCL), and sheep red blood cells (SRBC), and its delivery into macrophage cytoplasm, via phagocytosis was assessed with a T hybridoma assay for MHC class I-restricted presentation of OVA-derived peptides. Although direct introduction of antigen into cytoplasm by scrape-loading produced the most efficient presentation, comparable signals could be obtained after phagocytosis of PCL or PS. Phagocytosis of OVA-loaded SRBC, and OVA internalized by pinocytosis, did not deliver efficiently. MHC class I-restricted presentation of phagosome-derived OVA required cytoplasmic processing, as it was inhibited by proteasome inhibitors and brefeldin A. Morphological studies showed that biotinylated OVA originating in PCL phagosomes could be delivered into the cytoplasm of 90% of the macrophages. These results indicate that phagocytosis per se is not sufficient to deliver antigen into cytoplasm, but that phagocytosis of solid, synthetic polymeric particles delivers antigen efficiently into cytoplasm for MHC class I processing.