Amino-Terminal Flanking Residues Determine the Conformation of a Peptide–Class II MHC Complex

Abstract

The peptide spanning residues 48–62 of hen egg white lysozyme presented by I-Ak molecules gives rise to two T cell populations, types A and B, that recognize distinct conformers of the complex generated in late and recycling endosomes. The class II–like accessory molecule H2-DM functions as a conformational editor, eliminating the type B conformer in late endosomes. Here, we show that the conformation of the complex, and its susceptibility to editing by H2-DM, are determined by peptide amino-terminal flanking residues. Elimination of these residues abolished editing, permitting formation of the type B conformer in late endosomes. Substitutions at P(−2) affected the stability of the type B conformer, preventing its formation and/or editing, without hindering peptide binding or formation of the type A conformer of the complex. We conclude that interactions involving amino-terminal flanking residues stabilize peptide-MHC conformers and confer resistance to editing by H2-DM, influencing the nature of the T cell repertoire.

Activation of Type B T Cells after Protein Immunization Reveals Novel Pathways of In Vivo Presentation of Peptides

“…If DM were absent or inactive in these vesicles, or if the rate of transit through the pathway overwhelmed the capacity of DM to function as a conformational editor, then presentation of the type B conformer would be the expected result. Alternatively, phagolysosomal proteases may preferentially generate truncated peptides that lack flanking residues necessary for DM to eliminate the type B conformer; this would result in presentation of the type B conformer, as the complexes that form would be resistant to the editing function of DM (31).…”

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

“…This activity of H2-DM (and DMY) is likely to affect T cell responses to a large array of natural peptides with Nterminal extensions. In contrast, as shown for HEL, some short synthetic peptides devoid of N-terminal extensions will exist in both type A and B conformations independent of the presence of DM (10). We can speculate that in physiological conditions, professional APCs in the tumor environment will process exogenous TAAs and load nominal peptides in the context of DM, thereby eliminating type B complexes.…”

“…DMY reduced the display of type B HEL-I-A k conformers at the cell surface of APCs, causing the poor stimulation of CP1.7 and MLA11.2 type B T cell hybridomas. Unanue and collaborators (10) showed that HEL 48-61 binds MHC II in at least two different conformations depending on the folding of the N-terminal extension, especially at position -2. One of these conformers (type B) is nonoptimal, and thus readily dissociates in the presence of H2-DM.…”

“…Although some T cells (also called type A) can recognize both conformers, other clones (type B) are specific for the type B peptide-MHC II complexes. Interestingly, the type B-specific effector T cells do not recognize the epitope coming from intracellular processing of the native Ag and that are loaded under the control of H2-DM (10). Thus, it is predicted that a substantial fraction of the T cell repertoire activated by peptide-loaded DC vaccines will not recognize tumorresident APCs displaying the target native epitopes, as these are loaded on MHC II in the presence of DM.…”

“…Mouse CD4 + T cells were isolated from splenocytes of 3A9 TCR transgenic BALB-k mice by negative selection (Miltenyi Biotec). 3A9 (type A), CP 1.7 (type B), and MLA 11.2 (type B) T cell hybridomas were provided by Dr. E. Unanue (10). Jurkat HA1.7 T cells were obtained from Dr. R. P. Sékaly (University of Montreal) (18).…”