It is now known that antigen-presenting cells (APCs) can present extracellular antigen to CD4+ T cells on MHC class II molecules and to CD8+ T cells on MHC class I molecules (a process known as cross-presentation), but what determines which presentation pathway the extracellular antigen is routed to? Burgdorf et al. now show that APCs use distinct methods of endocytosis to simultaneously direct extracellular antigen into specific intracellular compartments, and that these compartments then dictate whether the antigen is presented to CD4+ or CD8+ T cells.

Endocytosis of ovalbumin (OVA) mediated by the mouse mannose receptor expressed by dendritic cells (DCs) targeted the antigen for cross-presentation to OVA-specific CD8+ T (OT-I) cells but not to OVA-specific CD4+ T (OT-II) cells. By contrast, treatment of DCs with dimethylamiloride (DMA; an inhibitor of pinocytosis) during OVA uptake blocked the activation of OT-II cells but not of OT-I cells, indicating that pinocytosed antigens only activated CD4+ T cells. Pinocytosis occurred in DCs with or without the expression of the mannose receptor but mannose-receptor-sufficient cells endocytosed large amounts of antigen compared with cells lacking this receptor. So, all DCs internalize small amounts of antigen through pinocytosis that is then presented to CD4+ T cells, whereas mannose-receptor-expressing DCs, such as CD8α+ DCs, simultaneously take up large amounts of antigen that is then targeted for cross-presentation to CD8+ T cells.

But how do these different methods of endocytosis direct antigen to these different pathways of antigen presentation? The authors showed that mannose-receptor-endocytosed antigen was localized to early endosomes, as determined by their expression of early endosomal antigen 1 (EEA1) and RAB5, and was excluded from late endosomes and lysosomes. By contrast, pinocytosed antigen was routed exclusively towards lysosomes. Further investigation showed that lucifer yellow (a marker of pinocytosis), when taken up by DCs, specifically co-localized with MHC class II molecules in lysosomes, and mannose-receptor-endocytosed OVA exclusively co-localized with MHC class I molecules in early endosomes.

Together, these data indicate that the uptake of antigen mediated by the mannose receptor results in routing of the antigen to the early endosomes for subsequent cross-presentation to CD8+ T cells. Simultaneously, antigens that are internalized by pinocytosis are directed to lysosomes for subsequent MHC-class-II-restricted presentation to CD4+ T cells. So, targeting antigens for uptake by the mannose receptor could be used to enhance the efficacy of vaccines that are designed to induce CD8+ T-cell-mediated immunity.