Immature human DCs efficiently translocate endocytosed antigens into the cytosol for proteasomal processing

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HIGHLIGHTSFor cross-presentation by dendritic cells, small external antigens are imported as intact proteins from endolysosomes into the cytosol.In immature dendritic cells but not in mature dendritic cells, the imported antigens are immediate processed by proteasomes.Other cross-presenting antigen-presenting cells like B cells and monocytes do not shuttle internalized antigen into the cytosol.Large antigens need proteolytical pre-processing in the endolysosomes before transfer into the cytosol.Cross-presentation of endocytosed antigen is essential for induction of CD8 effector T cell responses and a hallmark of dendritic cells (DCs). The mode of antigen processing in this context is controversial and some models imply translocation of the antigen from the endosomes into the cytosol. To test this hypothesis we made use of the pro-apoptotic properties of cytochrome c when in the cytosol, and confirmed that it indeed triggered apoptosis of human immature DCs but only at high concentrations. Proteasome inhibitors reduced the required concentration of cytochrome c thousand-fold, indicating that protein translocated into the cytosol is rapidly degraded by proteasomes. Mature DCs were also susceptible to cytochrome c-triggered apoptosis at high concentrations but proteasome inhibitors did not increase their sensitivity. Other cross-presenting cells such as B cells and monocytes were not sensitive to cytochrome c at all, indicating that they do not shuttle internalized antigen into the cytosol. Thus, processing of internalized antigens seems to follow different pathways depending on cell type and, in case of DCs, maturation state. Immature DCs appear to have a unique capacity to shuttle external antigen into the cytosol for proteasomal processing, which could explain their efficiency in antigen cross-presentation.

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