ERAP2 Increases the Abundance of a Peptide Submotif Highly Selective for the Birdshot Uveitis-Associated HLA-A29

Wouter J Venema,Jonas J W Kuiper,Efstratios Stratikos,Siranush Sarkizova,George M C Janssen,Sanne Hiddingh,Peter A Van Veelen,Lars T Van Der Veken,Arend Mulder,Frans H J Claas,Joke H De Boer,Anneke I Den Hollander

doi:10.3389/fimmu.2021.634441

Abstract

Birdshot Uveitis (BU) is a blinding inflammatory eye condition that only affects HLA-A29-positive individuals. Genetic association studies linked ERAP2 with BU, an aminopeptidase which trims peptides before their presentation by HLA class I at the cell surface, which suggests that ERAP2-dependent peptide presentation by HLA-A29 drives the pathogenesis of BU. However, it remains poorly understood whether the effects of ERAP2 on the HLA-A29 peptidome are distinct from its effect on other HLA allotypes. To address this, we focused on the effects of ERAP2 on the immunopeptidome in patient-derived antigen presenting cells. Using complementary HLA-A29-based and pan-class I immunopurifications, isotope-labeled naturally processed and presented HLA-bound peptides were sequenced by mass spectrometry. We show that the effects of ERAP2 on the N-terminus of ligands of HLA-A29 are shared across endogenous HLA allotypes, but discover and replicate that one peptide motif generated in the presence of ERAP2 is specifically bound by HLA-A29. This motif can be found in the amino acid sequence of putative autoantigens. We further show evidence for internal sequence specificity for ERAP2 imprinted in the immunopeptidome. These results reveal that ERAP2 can generate an HLA-A29-specific antigen repertoire, which supports that antigen presentation is a key disease pathway in BU.

Highlights

Birdshot uveitis (BU) is a rare form of uveitis characterized by distinctive inflammatory foci across the retina, hypopigmented choroidal lesions, and cystoid macular edema, which causes visual impairment when undertreated [1, 2]
We showed that ERAP2 shapes the human leukocyte antigen (HLA)-A29 peptidome predominantly by over-trimming peptides carrying susceptible residues at their N-terminus while sparing others carrying a sub-optimal residue at the N-terminal positions
We identified that ERAP2 increases the abundance of peptides with a distinct submotif defined by nonpolar aromatic residues F or Y at P2 that binds to HLA-A29

Summary

Introduction

Birdshot uveitis (BU) is a rare form of uveitis characterized by distinctive inflammatory foci across the retina, hypopigmented choroidal lesions, and cystoid macular edema, which causes visual impairment when undertreated [1, 2]. Because the risk haplotypes of ERAP genes for BU have been shown to result in lower cellular expression and activity of ERAP1 in combination with high cellular expression of functional ERAP2 [10], it is likely that ERAP2 generates a so far unknown, but highly HLA-A29restricted antigen repertoire that dictates T cell- or NK cell responses. This renders antigen processing and presentation a key disease pathway in BU [9]

Methods

Results

Conclusion