Class I MHC is stabilized against thermal denaturation by physiological concentrations of NaCl.

Class I MHC molecules are ternary complexes composed of an allotype specific heavy chain, a noncovalently associated protein beta(2)-microglobulin (beta(2)m), and a peptide. The complexes are assembled in the endoplasmic reticulum by a complex series of chaperones and peptide-loading mechanisms. In the absence of beta(2)m or peptide, very little class I heavy chain is transported to the surface of the cell. Complexes that do not contain all three parts of the protein are not made productively in vivo and not at all in vitro. The ability of the complex to withstand thermal denaturation in vitro has been shown to be related to the binding affinity of the peptide. Paradoxically, some low-affinity peptide complexes denature at or below human basal body temperatures in vitro but are effective biological agents in vivo. Here we show that these complexes are stabilized against thermal denaturation by physiological cosolvents and maximally stabilized by 150 mM NaCl. While the degree of stabilization by 150 mM NaCl is greatest for low-affinity peptide/MHC complexes, the mechanism of stabilization is independent of peptide sequence. This effect is hypothesized to occur by multiple mechanisms including increasing the affinity of beta(2)m for the complex and charge screening.

Poor binding of a HER-2/neu epitope (GP2) to HLA-A2.1 is due to a lack of interactions with the center of the peptide.

Class I major histocompatibility complex (MHC) molecules bind short peptides derived from proteins synthesized within the cell. These complexes of peptide and class I MHC (pMHC) are transported from the endoplasmic reticulum to the cell surface. If a clonotypic T cell receptor expressed on a circulating T cell binds to the pMHC complex, the cell presenting the pMHC is killed. In this manner, some tumor cells expressing aberrant proteins are recognized and removed by the immune system. However, not all tumors are recognized efficiently. One reason hypothesized for poor T cell recognition of tumor-associated peptides is poor binding of those peptides to class I MHC molecules. Many peptides, derived from the proto-oncogene HER-2/neu have been shown to be recognized by cytotoxic T cells derived from HLA-A2(+) patients with breast cancer and other adenocarcinomas. Seven of these peptides were found to bind with intermediate to poor affinity. In particular, GP2 (HER-2/neu residues 654-662) binds very poorly even though it is predicted to bind well based upon the presence of the correct HLA-A2.1 peptide-binding motif. Altering the anchor residues to those most favored by HLA-A2.1 did not significantly improve binding affinity. The crystallographic structure shows that unlike other class I-peptide structures, the center of the peptide does not assume one specific conformation and does not make stabilizing contacts with the peptide-binding cleft.

Peptide binding by class I and class II MHC molecules.

Major histocompatibility complex (MHC) antigens bind peptides of diverse sequences with high affinity. They do this in order to generate maximal immunological protection by covering the spectrum of peptides that may be seen by a host over the course of its lifetime. However, in many circumstances the immune system does not recognize a particular peptide that it should for maximum advantage over the pathogen. In other situations, the immune system goes awry and incorrectly recognizes a self-peptide that it should not. This results in disease characterized by recognition and attack of self. Rheumatoid arthritis is an example of just such a disease. In either of these situations, peptide-based modalities for immune therapy would be an advantage. However, peptide-based therapies require a thorough understanding of the forces involved in peptide binding. Great strides have been made in elucidating the mechanisms by which these MHC proteins may bind peptides with diverse sequences and high affinity. This review summarizes the current data obtained from crystallographic analyses of peptide binding for both class I and class II MHC molecules. Unfortunately, as yet these data have not allowed us to predict which peptides will bind with high affinity to a specific MHC molecule.