Free
energy perturbation (FEP) calculations can predict relative
binding affinities of an antigen and its point mutants to the same
human leukocyte antigen (HLA) with high accuracy (e.g., within 1.0
kcal/mol to experiment); however, a more challenging task is to compare
binding affinities of wholly different antigens binding to completely different HLAs using FEP. Researchers have used
a variety of different FEP schemes to compute and compare absolute
binding affinities, with varied success. Here, we propose and assess
a unifying scheme to compute the relative binding affinities of different
antigens binding to completely different HLAs using absolute binding
affinity FEP calculations. We apply our affinity calculation technique
to HLA–antigen–T-cell receptor (TCR) systems relevant
to celiac disease (CeD) by investigating binding affinity differences
between HLA–DQ2.5 (enhanced CeD risk) and HLA–DQ7.5
(CeD protective) in the binary (HLA–gliadin) and ternary (HLA–gliadin–TCR)
binding complexes for three gliadin derived epitopes: glia-α1,
glia-α2, and glia-ω1. Based on FEP calculations with our
carefully designed thermodynamic cycles, we demonstrate that HLA–DQ2.5
has higher binding affinity than HLA–DQ7.5 for gliadin and
enhanced binding affinity with a common TCR, agreeing with known results
that the HLA–DQ2.5 serotype exhibits increased risk for CeD.
Our findings reveal that our proposed absolute binding affinity FEP
method is appropriate for predicting HLA binding for disparate antigens
with different genotypes. We also discuss atomic-level details of
HLA genotypes interacting with gluten peptides and TCRs in regard
to the pathogenesis of CeD.