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Differential cytosolic delivery and presentation of antigen by listeriolysin O-liposomes to macrophages and dendritic cells.

Delivery of antigenic protein to the cytosol of antigen-presenting cells (APCs), such as macrophages (MPhi) and dendritic cells (DCs), is required for an efficient CD8 T-cell-mediated immune response. We have previously shown that co-encapsulation of antigenic protein inside pH-sensitive liposomes with listeriolysin O (LLO), a pore-forming protein of Listeria monocytogenes, generates efficient major histocompatibility complex class I (MHC I)-restricted immune responses both in vitro and in vivo. In this study, we sought to analyze the relative efficiency of LLO-mediated cytosolic delivery of liposomal antigen in two important APCs, macrophages and dendritic cells, by examining the sequential steps involved in antigen presentation to T-cells in cultured mouse bone marrow-derived MPhis (BMMPhis) and DCs (BMDCs). BMMPhis overall presented liposomal antigen better than BMDCs at a given concentration of liposomal antigen incubated with cells, and the trend was also observed after the presentation was normalized by the uptake of antigen. When soluble antigen was directly introduced into the cytosol, however, BMDCs presented the antigen more efficiently than BMMPhis. In addition, when the APCs were externally loaded with the antigenic peptide of the protein, BMDCs displayed a higher level of cell surface MHC I-peptide complexes and presented the peptide more efficiently than BMMPhis. These results combined together suggest that LLO-mediated release of liposomal antigen from the endosomal/lysosomal compartment may be more pronounced in BMMPhis than in BMDCs, and further implicates differential activity of LLO and varying efficiency of LLO-mediated endosomal escape in different antigen-presenting cell types.