Enforcing dendritic cell vaccines by manipulating the MHC II antigen presentation pathway
Dendritic cell peptide-based vaccines are the most common immunotherapy approach in cancer therapy. While, in principle, dendritic cells (DCs) could be loaded efficiently by exogenously added tumor peptides, their loading efficacy is severely reduced due to low number of peptide-receptive MHC II on cell surface. Most surface MHC II molecules are either occupied by endogenous peptides or are inactive due to a conformation that is not receptive for free peptides. In MHC II antigen presentation pathway, HLA-DM (DM) in acidic endosomal vesicles removes the self-peptides and grants a peptide receptive conformation to MHC II.  Mutating of an intracellular sorting motif in DM, renders its accumulation on cell surface. We hypothesized that the mutant DM (DMY) is functional on cell surface and can generate peptide receptive MHC II on surface of DCs for exogenous peptide loading. By using an adenoviral vector that expresses DMY, we found that DMY is functional on surface of DCs. DMY supplied peptide receptive MHC II on surface of DCs and improved exogenous peptide loading.  The improvement of peptide loading by DMY is both quantitative and qualitative. DMY improves helper T cell (Th) response in Th1 direction that favors anti-cancer immunity. The qualitative improvement of peptide loading extends to loading of superior conformational isomer (conformer) of peptide-MHC complexes. This superior conformer (type A) is the favourite type for vaccination approaches and DMY successfully edits peptide-MHC conformers on cell surface level by eliminating undesirable one (type B).    Function of DM is regulated by HLA-DO (DO) and it is well accepted that in acidic pH of late endosomes, DO inhibits function of DM by preventing removal of class II associated invariant chain peptide (CLIP) from peptide binding groove of MHC II. My results indicate that DO overexpression, changes binding of peptide-dependent superantigens to MHC II molecules. Superantigens (SAgs) are small microbial proteins that bind out side peptide binding groove of MHC II. DO probably enhances binding of peptide-dependent SAgs by forcing the accumulation of CLIP on the cell surface of antigen presenting cells. DO also neutralizes Th2 polarization by CLIP. Collectively, these results indicate that DMY is a valuable tool for improvement of exogenous peptide loading in DCs vaccines. An unnoticed effect of DO on SAgs binding was also recognized. Further investigations are needed to clarify the mechanisms by which, DMY and DO influence Th polarization. This would provide a better understanding of antigen presentation pathway and its interaction with immune system.
Dendritic cells CD4 T cells MHC class II Cancer vaccine Superantigen SEA SEB HLA-DM HLA-DO CLIP