HLA class I maturation - in the presence and absence of tapasin

Human leukocyte antigen class I (HLA-I) molecules are present on all nucleated cells and present the cell content to cytotoxic T lymphocytes in the form of peptides. Maturation of HLA-I occurs in the endoplasmic reticulum and results in stable peptide-HLA-I complexes, in the presence of proper quality control. For proper peptide binding most HLA-I molecules interact with the peptide-loading complex (PLC), which is a multi-protein complex consisting of the transporter associated with antigen presentation, calreticulin, ERp57 and tapasin. Tapasin integrates HLA-I into the PLC and mediates quality control of the HLA-I maturation. When an optimal peptide is bound, tapasin releases the peptide-loaded HLA-I molecule that is next transported to the cell surface. The mechanisms for tapasin quality control of HLA-I maturation and the criteria defining optimal peptides are not completely known.

Here, a recombinant part of tapasin, the first 87 N-terminal amino acids (Tpn1-87), was produced and shown to facilitate folding of different HLA-I molecules, i.e. allomorphs, to different degree in a peptide dependent manner. Folding of HLA-A*02:01 molecules with natural ligands, i.e. with peptides purified mainly from HLA-I expressed on the cell surface, was not facilitated by Tpn1-87, while folding of non-natural ligands, i.e. not presented at the cell surface, was facilitated. The folding facilitation exerted by Tpn1-87, tapasin-facilitation, inversely correlated with the stability of the peptide-HLA-I complex to some extent. The inverse correlation of these two parameters, tapasin-facilitation and stability, was studied in detail in the third paper. An increased stability was shown to not necessarily be associated with a decreased tapasin-facilitation. In the last paper of this thesis the tapasin-facilitation was studied in a large set of allomorphs folded with peptides of 7 to 13 amino acids in length. The influence of peptide-length for the different allomorphs increased with their tapasin dependence.

In conclusion, Tpn1-87 facilitates folding of HLA-I in a peptide- and allomorph-dependent manner. Based on the above studies and data showing tapasin retention of immature HLA-I molecules, and studies of a suggested peptide-editing role of tapasin, we propose that tapasin keeps HLA-I molecules in a peptide-receptive conformation, which reduces the risk degradation of HLA-I molecules and increases the possibility for binding of optimal peptides.