Tapasin - The star of the show in HLA-I maturation
Human leukocyte antigen class I (HLA-I) molecules are present on the cell surface of all nucleated cells. They bind intracellular peptides and present them to the immune system thus providing a readout to cytotoxic T-cells about the status of a cell. Maturation of the HLA-I molecule includes peptide binding in the endoplasmic reticulum with the assistance of several other proteins collectively called the antigen processing machinery (APM). One of these proteins, tapasin, is of particular interest as it is dedicated to HLA-I and plays a key role in the selection and binding of peptides that induce stable HLA-I molecules. In this thesis I have studied HLA-I maturation, and tapasin in particular, both biochemically and in tumor material. The first paper is a study of HLA-I folding in the presence and absence of a part of recombinant tapasin (tpn1-87). Here we studied how different HLA-I allomorphs depend on tapasin and the influence of peptide length. This is the first study where a large number of HLA-I allomorphs have been simultaneously analyzed for tapasin facilitation. We found that the influence of peptide length for the different allomorphs increased with their tapasin dependence. In paper II and III we studied HLA-I, tapasin and other APM proteins in tumor sections and cell lines of glioblastoma multiforme (GBM). In tumors APM proteins are commonly downregulated as a strategy to evade the immune system. We also found the APM proteins in our tumor material to be highly dysregulated with strongly linked HLAI and tapasin expression. In tumor tissue sections HLA-I and tapasin expression also correlated with survival of GBM patients. High resolution HLA-I typing allowed us to study the HLA-I allomorphs expressed in GBM patients and also each allomorph’s tapasin dependence. We found that GBM patients display HLA-I allomorph profiles with mixed dependency of tapasin, similar as in a healthy cohort. In addition we show that tapasin deficient cells present suboptimally loaded HLA-I molecules on the cell surface. By exogenous addition of high affinity peptides we were able to increase the stability of presented HLA-A*02:01 molecules. The tapasin dependency of each allomorph as well as composition and proportions of HLA-I allomorphs presented on the cell surface is of importance not only for mechanistic understanding but also for immunotherapy settings in different diseases. GBM is an aggressive brain tumor with poor prognosis and there are high demands for new and more effective treatments. We propose individualized immunotherapy protocols where tapasin expression and tapasin dependency of allomorphs expressed in each patient are taken into consideration to improve the selection of peptide:HLA-I combinations for peptide vaccines.
Pharmacology and Toxicology, Tapasin, HLA-I, tumor, antigen presentation
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