Sunill Kumar Saini

• MHC (major histocompatibility complex) class I molecules are membrane proteins that play a central role in the mammalian immune response against pathogens and tumors by presenting the cellular proteome, in the form of peptides of 8-10 amino acids, at the cell surface for inspection by cytotoxic T lymphocytes. The decisive event for this system to function is the selection and stable binding of peptides to MHC class I molecules. In this research work, I investigated the peptide binding properties of MHC class I molecules and used small molecules to modulate the peptide binding process with the aim of improving our understanding of the MHC class I mediated immune defense and utilizing such small molecules for therapeutic applications. Conventionally, in vitro folding of MHC class I requires full-length peptide for fold-ing and stabilization. I found that small molecules such as dipeptides can be used to fold and stabilize MHC class I molecules. The dipeptides keep the class I molecules in a peptide-receptive state that binds peptide faster than the empty state. Exogenously added dipeptides can accumulate and stabilize peptide receptive class I molecules at the cell surface. Intri-guingly, dipeptides also catalyze peptide exchange on human (HLA A*02:01) and murine (H 2Kb) class I molecules, both on recombinant and on cell surface class I molecules. They do this by increasing the dissociation rate of bound peptide, probably interacting with the F pocket area of the class I peptide binding groove, similar to the postulated mechanism for intracellular peptide editing by the chaperone protein tapasin that supports the selection of high affinity peptides for binding to class I. With our collaborators in Tübingen, I also show that dipeptide mediated peptide exchange allows for easy and efficient production of MHC class I tetramers for the detection of antigen specific T lymphocytes. The findings from this project have widespread use in basic research as well as in di-agnostic and therapeutic applications. Dipeptides are efficient tools to generate empty class I molecules for the study of peptide dependent conformational changes, which could be utilized to understand the intracellular peptide selection mechanism. Dipeptide mediated peptide exchange provides an advance in class I multimer preparation that are used to counter viruses and tumors.