The ins-and-outs of endosomal antigens for cross-presentation.

The efficiency of antigen cross-presentation, which is the presentation of extracellular antigens on MHC I molecules, critically depends on the stability of the internalized antigens. Since rapid degradation within the lysosomal compartment impairs cross-presentation, potent cross-presenting cells display several mechanisms to prevent activation of lysosomal proteases. Additionally, distinct endocytic receptors can target internalized antigens towards non-degradative early endosomes, from where efficient cross-presentation can occur. From these endosomes, antigens need to be processed for loading on MHC I molecules, which can occur by endo/lysosomal proteases or after translocation into the cytosol by the proteasome. Although the underlying mechanisms require further investigations, increasing evidence points out a decisive role of the ER-associated degradation machinery in such antigen translocation.