We aimed to explore the functional implications of risk HLA alleles in pediatric uveitis. The amino acid position 86 distinguishes the JIA-U associated alleles
HLA-DRB1*11:01 (86-Glycine) in males,
HLA-DRB1*11:04 (86-Valine) in females (
Supplementary Table 1E). This position has been shown to control DR alpha/beta dimer stability, but also peptide binding for the panuveitis-associated
HLA-DRB1*04:03, and is therefore likely to influence antigen presentation.
40,41 Therefore, we asked whether additional allele-defining positions affect the binding of peptides to HLA allotypes encoded by risk HLA alleles. The
HLA-DQB1*05:03 was found to be associated with two distinct forms of uveitis, CAU and PAN, and this allele is functionally understudied compared to other uveitis risk alleles detected in this study. This allele harbors an aspartic acid (D) at position 57 while other nonrisk alleles for
HLA-DQB1*05 have a serine (S) or valine (V) at position 57 (
Fig. 4A). Curiously, the amino acid position 57 of the DQ betachain is located at the edge of the peptide binding groove (
Fig. 4A) and confers risk to several autoimmune conditions suggesting it to be involved in peptide binding of HLA-DQ.
42–46 To assess whether the presence of aspartic acid at position 57 functionally affects binding of peptide cargo to HLA-DQ, we extracted ligands (
n = 157 from the HLA ligand atlas) from immunopeptidomes of
HLA-DQB1*05:01 (57-S),
HLA-DQB1*05:02 (57-V), and
HLA-DQB1*05:03 (57-D) that only differ in the amino acid residue at position 57 (
Supplementary Table 8).
35 Next, we modeled the binding affinity of the naturally occurring ligands to HLA-DQ, by using the full amino acid sequence of
HLA-DQB1*05 in NetMHCIIpan 4.0 and performed amino acid substitutions at position 57.
36 We observed that substitution of position 57 from S or V (nonrisk alleles) to the uveitis-associated amino acid aspartic acid (D) (equivalent to
DQB1*05:03) resulted in a significant increased binding affinity score (S versus D;
P = 1.67 × 10
−27 and V versus D;
P = 1.64 × 10
−27;
Fig. 4B). In more detail, major binding pockets for HLA-DQ are position P4, P6, and P9 in the peptide motif.
47 Therefore, we computed the peptide motif of the 157
HLA-DQB1:05 ligands and the top 50 peptides with the largest difference in binding scores between HLA
-DQB1*05:02 (57-V) and
HLA-DQB1*05:03 (57-D) and compared the core sequence alignments to identify changes in specific amino acid position. This analysis revealed, as expected, a marked change in amino acid residue preference in particular at anchor position P9, which directly interacts with position 57 (
Fig. 4C). Collectively, these data show that position 57 in the HLA-DQ beta chain modulates the peptide binding capacity and indicates risk HLA alleles associated with pediatric uveitis show altered antigen presentation capacity.