Ligand Exchange of Major Histocompatibility Complex Class II Proteins Is Triggered by H-bond Donor Groups of Small Molecules

Abstract

Hydrogen bonds (H-bonds) are crucial for the stability of the peptide-major histocompatibility complex (MHC) complex. In particular, the H-bonds formed between the peptide ligand and the MHC class II binding site appear to have a great influence on the half-life of the complex. Here we show that functional groups with the capacity to disrupt hydrogen bonds (e.g. -OH) can efficiently catalyze ligand exchange reactions on HLA-DR molecules. In conjunction with simple carrier molecules (such as propyl or benzyl residues), they trigger the release of low affinity ligands, which permits the rapid binding of peptides with higher affinity. Similar to HLA-DM, these compounds are able to influence the MHC class II ligand repertoire. In contrast to HLA-DM, however, these simple small molecules are still active at neutral pH. Under physiological conditions, they increase the number of "peptide-receptive" MHC class II molecules and facilitate exogenous peptide loading of dendritic cells. The drastic acceleration of the ligand exchange on these antigen presenting cells suggests that, in general, availability of H-bond donors in the extracellular milieu controls the rate of MHC class II ligand exchange reactions on the cell surface. These molecules may therefore be extremely useful for the loading of antigens onto dendritic cells for therapeutic purposes.Peptide ligands bind to the peptide-binding groove of MHC 1 class II molecules by an array of intermolecular hydrogen bonds (H-bonds). These hydrogen bonds are mostly formed between the backbone of the peptide and conserved residues of the MHC class II molecule. Some of these H-bonds are particularly crucial for the stability of the ligand complex (1). It has been shown for a murine MHC class II molecule that the elimination of H-bonds between the ligand and residues His-81 or Asp-82 of the I-A d ␤-chain results in a rapid loss of the bound peptide (2). Detailed kinetic studies with these mutated MHC molecules revealed peptide dissociation rates that were increased up to 200-fold (3). This increase was in the same range observed after addition of HLA-DM to the peptide complex of the nonmutated I-A d molecule. It was therefore proposed that HLA-DM-mediated ligand release (4, 5) is also accomplished by the disruption of H-bonds (6), a hypothesis also introduced when the crystal structure of HLA-DM was published (7).Because H-bonds appear to be fundamental in maintaining the stability of the MHC class II peptide complexes, we started to investigate small molecules capable of disrupting H-bonds with the goal of achieving an HLA-DM-like catalytic effect on the kinetics of peptide binding. H-bonds require a hydrogen donor and an acceptor group, which provides a free electron pair. Some of the functional groups that can fulfill this function are hydroxyl or amino groups. They are present in a variety of natural and synthetic molecules, such as lipids, metabolites, amino acids, and pharmaceutical drugs. One example is ethanol, where the well known physiological effects appear to resul...

Small Molecules That Enhance the Catalytic Efficiency of HLA-DM

“…This analog had partial activity (ϳ30% relative to M19), indicating that substantial changes to this part of the molecule can be made without complete loss of activity. These small molecules are thus structurally and functionally distinct from previously reported compounds (phenolic compounds with a single ϪOH group) that accelerate release of low-affinity peptides from DR in the absence of DM, presumably through disruption of hydrogen bonds between the peptide and the MHC molecule (35,36).…”

Cutting Edge: TCR Contacts as Anchors: Effects on Affinity and HLA-DM Stability

“…One possibility was that substitutions containing -OH groups might interfere with the H-bond network (Ser at P2, Asp at P3), or allow increased solvent access to disrupt the H-bond network (Gly at P10). To test this idea, we took advantage of the ability of small molecules containing -OH groups (e.g., 1-propanol) to disrupt H-bonds and/or partially destabilize MHCII (24). In the presence of 1-propanol the P2, 3, and 10 substitutions significantly decreased complex stability (Fig.…”

Forced Expression of HLA-DM at the Surface of Dendritic Cells Increases Loading of Synthetic Peptides on MHC Class II Molecules and Modulates T Cell Responses

“…More recently, DM mimics capable of catalyzing peptide exchange have been described. These simple organic chemicals or peptidic molecules operate at neutral pH and enhance T cell activation (26,27,(43)(44)(45). Interestingly, a better loading was obtained by combining n-propanol and DMY, suggesting that the former acts in a DM-independent and -dependent fashion.…”

“…We tested the possibility of combining DMY with small molecules capable of forming H-bonds and that act as chemical analogs of DM (26). Such MHC-loading enhancers, like n-propanol, modulate both off and on rates of peptides on soluble and cellbound DR (27).…”