Selective transport of internalized antigens to the cytosol for MHC class I presentation in dendritic cells

In order for cytotoxic T cells to initiate immune responses, peptides deriv ed from internalized antigens must be presented to the cytotoxic T cells on major histocompatibility complex (MHC) class I molecules. Here we show tha t dendritic cells, the only antigen-presenting cells that initiate immune r esponses efficiently, have developed a unique membrane transport pathway li nking the lumen of endocytic compartments and the cytosol, Endosome-to-cyto sol transport is restricted to dendritic cells, specific to internalized an tigens and selective for the size of the transported molecules. Thus, in de ndritic cells, internalized antigens gain access to the cytosolic antigen-p rocessing machinery and to the conventional MHC class I antigen-presentatio n pathway.