MHC class I - Peptide binding and complex stability

The major histocompatibility class I (MHC-I) genes are highly polymorphic and the proteins that they encode play a crucial role in both the innate and the adaptive immune response. A MHC-I molecule consists of three parts, one polymorphic heavy chain, one invariant light chain, β2-microglobulin and a peptide of usually between 8-11 amino acids in length. The maturation and quality control of MHC-I takes place in the endoplasmic reticulum and involves several different proteins including the MHC-I dedicated protein tapasin.

In this thesis we have studied different parameters important for MHC-I formation and stability in humans and birds. We have used various approaches including in silico prediction methods, biochemical assays and cellular assays to elucidate the MHC-I maturation. We show that the functional relationships between MHC-I molecules in passerine birds of different species are based on the MHC-I characteristics such as peptide-binding specificity rather than species characteristics. In addition, passerine MHC-I molecules similar to human MHC-I molecules, have a complex dissociation. This suggests that just as in humans, passerine MHC-I molecules go through different maturation stages that most likely include interaction with quality control proteins such as tapasin.

The cell surface expression of stable MHC-I molecules is crucial for the function of the adaptive immune response and for this reason MHC-I and its related proteins are often a target for viral and tumour evasion strategies. In human cells we show that tapasin promotes the formation of stable cell surface expressed MHC-I molecules and that the dependency on tapasin for a stable cell surface expression varies between different allomorphs (allele specific protein products). The dysregulation of tapasin results in alterations in the peptide repertoire that is presented by MHC-I at the cell surface and most often this induces a decreased stability of the expressed molecules. We here show that by adding certain peptides exogenously to cells deficient in tapasin we were able to increase MHC-I cell surface stability significantly suggesting that exogenous modulations of tapasin deficient cells might be a possible approach in immunotherapy. The formation of aberrant conformations of HLA-B*27:05 has been suggested to play a role in the pathogenesis of ankylosing spondylitis and here we showed that tapasin has a preventive effect on the formation and presentation of aberrant conformations of HLA-B*27:05 at the cell surface.

In conclusion we show that the complex kinetics of MHC-I maturation and stability is a trait shared between birds and humans and we suggest that by studying MHC-I in other species than human we can gain valuable insight into the complex world of MHC-I.