Susceptibility to HLA-DM Protein Is Determined by a Dynamic Conformation of Major Histocompatibility Complex Class II Molecule Bound with Peptide

Peptides bind Class II Major Histocompatibility Complex molecules (MHCII) through a thermodynamically non-additive process consequent to the flexibility of the reactants. Currently, how the specific outcome of this binding process effects the ensuing epitope selection needs resolution. Calorimetric assessment of binding thermodynamics for HA306-319 peptide variants to the human MHCII HLA-DR1 (DR1) and a mutant DR1 reveals that peptide/DR1 complexes can be formed with different enthalpic and entropic contributions. Complexes formed with a smaller entropic penalty feature circular dichroism spectra consistent with a non-compact form, and molecular dynamics simulation shows a more flexible structure. The opposite binding mode, compact and less flexible, is associated with greater entropic penalty. These structural variations are associated with rearrangements of residues known to be involved in DM binding, affinity of DM for the complex, and complex susceptibility to DM-mediated peptide exchange. Thus, the thermodynamic mechanism of peptide binding to DR1 correlates with the structural rigidity of the complex, and DM mediates peptide exchange by “sensing” flexible complexes in which the aforementioned residues are rearranged at a higher frequency than in more rigid ones.

Section: Discussionsupporting

confidence: 91%

The Thermodynamic Mechanism of Peptide–MHC Class II Complex Formation Is a Determinant of Susceptibility to HLA-DM

Ferrante

Templeton

Hoffman

et al. 2015

“…Thus we did not observe that DM could directly stabilize MHC II against aggregation. To validate this observation, we loaded DR1 with the weakly binding peptide W1A, which we identified recently as extremely susceptible to DM-mediated peptide release (42), and incubated this complex alone or in the presence of DM or peptide (Fig. 3C).…”

Section: Resultsmentioning

confidence: 76%

“…MHC II-peptide complexes appear to be highly dynamic and adopt various conformations (40, 41). Recently, we and others proposed a model that DM catalyzes peptide release by sensing the dynamic conformation of MHC II-peptide complex constrained by the interactions throughout peptide binding groove (42), and recognizing a conformation involving the rearrangement of MHC II alpha 3 10 helical and extended region near the N-terminal side of bound peptide in the vicinity of P1 pocket (21). …”

Section: Introductionmentioning

confidence: 99%

Evaluating the Role of HLA-DM in MHC Class II–Peptide Association Reactions

Yin

Maben

Becerra

et al. 2015

Self Cite

Antigen presentation by major histocompatibility complex class II molecules (MHC II) to CD4+ T cells plays a key role in the regulation of the adaptive immune response. Loading of antigenic peptides onto MHC II is catalyzed by HLA-DM (DM), a non-classical MHC II molecule. The mechanism of DM-facilitated peptide loading is an outstanding problem in the field of antigen presentation. In this study we systemically explored possible kinetic mechanisms for DM-catalyzed peptide association, by measuring real time peptide association kinetics using fluorescence polarization assays and comparing the experimental data with numerically modeled peptide association reactions. We found that DM does not facilitate peptide association by stabilizing peptide-free MHC II against aggregation. Moreover, DM does not promote transition of an inactive peptide-averse conformation of MHC II to an active peptide-receptive conformation. Instead, DM forms an intermediate with MHC II that binds peptide with faster kinetics than MHC II in the absence of DM. In the absence of peptides, interaction of MHC II with DM leads to inactivation and formation of a peptide-averse form. This study provides novel insights into how DM efficiently catalyzes peptide loading during antigen presentation.

“…Yet peptide binding studies here suggest that cellular c-MYC levels may also alter the conformation or peptide accessibility of class II molecules. Consistent with this, the conformation of class II molecules is known to be flexible, with loss of DM impacting class II structure and peptide loading (86). Inhibition of c-MYC also resulted in improved CD4+ T cell recognition of BL, suggesting that the alterations in peptide binding to class II molecules along with class II components DM/DO may be regulated by elevated c-MYC in tumor cells.…”

Section: Discussionmentioning

confidence: 86%

Elevation of c-MYC Disrupts HLA Class II–Mediated Immune Recognition of Human B Cell Tumors

God

Cameron

Figueroa

et al. 2015

Elevated levels of the transcription factor c-myc are strongly associated with various cancers, and in particular B-cell lymphomas. While many of c-MYC’s functions have been elucidated, its effect on the presentation of antigen (Ag) through the HLA class II pathway has not previously been reported. This is an issue of considerable importance, given the low immunogenicity of many c-MYC-positive tumors. We report here that increased c-MYC expression has a negative effect on the ability of B-cell lymphomas to functionally present Ags/peptides to CD4+ T cells. This defect was associated with alterations in the expression of distinct co-factors as well as interactions of antigenic peptides with class II molecules required for the presentation of class II-peptide complexes and T cell engagement. Using early passage Burkitt’s lymphoma (BL) tumors and transformed cells, we show that compared to B-lymphoblasts, BL cells express decreased levels of the class II editor HLA-DM, lysosomal thiol-reductase GILT, and a 47kDa enolase-like protein. Functional Ag presentation was partially restored in BL cells treated with a c-MYC inhibitor, demonstrating the impact of this oncogene on Ag recognition. This restoration of HLA class II-mediated Ag presentation in early passage BL tumors/cells was linked to enhanced HLA-DM expression and a concurrent decrease in HLA-DO in BL cells. Taken together, these results reveal c-MYC exerts suppressive effects at several critical checkpoints in Ag presentation which contribute to the immunoevasive properties of BL tumors.