Constitutive macropinocytosis allows TAP‐dependent major histocompatibility compex class I presentation of exogenous soluble antigen by bone marrow‐derived dendritic cells

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Abstract
Dendritic cells expanded from mouse bone marrow (BMDC) with granulocyte/macrophage‐colony‐stimulating factor have potent T cell‐stimulatory properties both in vitro and in vivo. This has been well documented for major histocompatibility complex (MHC) class II‐restricted responses, and more recently using peptide‐loaded and protein‐pulsed DC for CD8 responses following adoptive transfer in mice. An unresolved question concerns the capacity of BMDC to present exogenous antigen on MHC class I molecules, an unconventional mode of MHC class I loading for which there is now considerable evidence, particularly in macrophages. Here, we show that BMDC exhibit high levels of macropinocytosis driven by constitutive membrane ruffling activity. Up to one‐third of actively ruffling and macropinocytosing BMDC transferred pinocytosed horseradish peroxidase into the cytosol following a 15‐min pulse, suggesting that they might be capable of presenting exogenous soluble antigen on MHC class I molecules. We show that BMDC presented exogenous ovalbumin to a T cell hybridoma more effectively, more rapidly, and at lower exogenous antigen concentrations than BM macrophages on a cell‐for‐cell basis. Presentation was TAP dependent, brefeldin A sensitive, and blocked by inhibitors of proteasomal processing, demonstrating use of the classical MHC class I pathway. Although effective presentation of exogenous antigen by BMDC occurred in the absence of agents which stimulate macropinocytosis, treatment with phorbol myristate acetate (PMA) enhanced both pinocytosis and MHC class I presentation by BMDC. Finally, PMA‐stimulated BMDC exposed to exogenous ovalbumin in vitro were able to prime an antigen‐specific cytotoxic T lymphocyte response following adoptive transfer in vivo.

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