Processing of MHC class II in dendritic cells

In the past years we performed studies to gain more insight into the processing of major histocompatibility class II (MHC class II) in dendritic cells. We focused on the sorting mechanisms of MHC class II, the degradation of its associated Ii and peptide loading at the endosomal system. In addition, we investigated the regulatory function of these intracellular processes on MHC class II surface expression and antigen presentation. In chapter 2 of this thesis we report that MHC class II can be post-translationally modified by the addition of ubiquitin moieties at a unique lysine residue in the cytosolic domain of the b-chain. Furthermore, it is demonstrated that ubiquitination of MHC class II regulates its cell surface expression by providing a signal for efficient sorting towards degradation in the lysosome. The third chapter describes the effect on MHC class II homeostasis when the formation of ILV is hampered by over-expressing a dominant negative mutant of VPS4. We observed that interference with ILV formation did not affect MHC class II peptide loading but increased MHC class II expression by immature DCs, underlining the role of MVB sorting in MHC class II degradation. The long standing issue on whether the large amount of ILV-localized MHC class II present in immature DCs can be recruited by maturing DCs for antigen presentation is covered in chapter 4. Here we show that upon DC activation, antigens are presented mainly by newly synthesized MHC class II. In chapter 5 we report that, in contrast to claims in existing literature, the degradation of the MHC class II associated invariant chain is a constitutive process and not up-regulated during DC maturation. We also pinpointed a possible artifact that can explain these discrepancies in the literature. In chapter 6 an experimental procedure is presented that improves antigen presentation by immature dendritic cells. We were able to show that an artificially intracellularly accumulated cohort of MHC class II molecules could be synchronously released to enhance cell surface expression of MHC class II and antigen presentation towards T cells. The strategy described in this chapter might be helpful to design immuno-tolerizing vaccines.