Adaptive immune responses are induced when dendritic cells (DCs) take up antigens in the peripheral tissue. Upon antigen recognition, DCs mature and migrate into the draining lymph nodes, where they can activate antigen-specific T cells.
After uptake by the DC, internalized antigens are degraded intracellularly. The resulting antigen-derived peptides are then complexed to Major Histocompatibility Complex (MHC) molecules. Peptide loading on MHC II molecules can be recognized by antigen-specific CD4+ T helper cells, whereas presentation of extracellular antigens on MHC I molecules, a process termed cross-presentation, can lead to the activation of cytotoxic CD8+ T cells.
The mechanisms regulating whether internalized antigens are presented on MHC I or MHC II molecules, and therefor what kind of immune response is induced by a specific antigen, were unknown for a long time. In a recent study, we could demonstrate that this decision is determined by the mechanism of antigen uptake. Whilst pinocytosis and scavenger receptor (SR)-mediated endocytosis introduce the model antigen ovalbumin (OVA) into lysosomes for presentation exclusively on MHC II molecules, the mannose receptor (MR) routed the antigen into stable early endosomes. In these endosomes, the antigen was rescued from rapid lysosomal degradation and was processed only for presentation on MHC I, demonstrating that antigens intended for presentation on MHC I or MHC II molecules are internalized by distinct endocytosis mechanisms and are targeted into different cellular organelles.
Furthermore, we are interested in the transport of the members of the MHC I loading machinery, which normally are located in the ER, towards early endosomes. We could demonstrate that such transport of TAP, which is essential for the re-import of proteasome-derived peptides for loading on MHC I, is regulated by the presence of endotoxins. The recognition of LPS by TLR4 induced the transport of TAP towards endosomes containing MR-internalized antigens in a MyD88-dependent but TRIF-independent fashion.
Furthermore, we are investigating the molecular and cell-biological mechanisms of MR-dependent antigen cross-presentation. We could demonstrate that after internalization, MR-internalized antigens are transported out of the endosomes into the cytoplasm for proteasomal degradation. The resulting peptides are then transported back into the same endosomal subset by endosomal TAP. In these endosomes, the antigenic peptides are subsequently loaded on MHC I molecules and transported to the cell membrane for presentation to T cells. Hence, cross-presentation of extracellular antigens is spatially distinct from MHC I-restricted presentation of intracellular antigens, which are loaded on MHC I molecules within the ER.
Additionally, we established a flow cytrometry-based method to monitor protein recruitment towards antigen-containing endosomes. Using this technique, we could demonstrate that p97, another member of the ER-associated degradation (ERAD) machinery that plays an important role in cross-presentation, is recruited towards the endosomal membrane by the MR itself. After ligand recognition, the MR becomes ubiquitinated at its cytosolic tail. Poly-ubiquitinated MR is directly responsible for the recruitment of p97 towards the endosomal membrane, allowing p97 to provide the energy for antigen translocation into the cytosol. Furthermore, we identified TSG101 as a regulator of MR poly-ubiquitination and hence cross-presentation. TSG101 specifically binds mono-ubiquitinated MR and prevents the attachment of further (poly-)ubiquitin residues, thereby inhibiting the recruitment of p97 and antigen translocation into the cytosol.
- a profound characterization of MR-targeted endosomes
- the export of antigens out of the endosomes into the cytoplasm
- the proteasomal degradation of antigens for cross-presentation
- the transport of ER components towards endosomes
- the influence of endotoxins on the molecular mechanisms of cross-presentation
- the induction of T cell tolerance and the regulatory role of endocytic receptors