Retina-arrestin specific CD8+ T cells are not implicated in HLA-A29-positive birdshot chorioretinitis

BACKGROUND

HLA-A29-positive birdshot chorioretinitis (BCR) is an inflammatory eye disorder that is generally assumed to be caused by an autoimmune response to HLA-A29-presented peptides from retinal arrestin (SAG), yet the epitopes recognized by CD8+ T cells from patients remain to be identified.

OBJECTIVES

The identification of natural ligands of SAG presented by HLA-A29. To quantify CD8+ T cells reactive to antigenic SAG peptides presented by HLA-A29 in patients and controls.

METHODS

We performed mass-spectrometry based immunopeptidomics of HLA-A29 of antigen-presenting cell lines from patients engineered to express SAG. MHC-I Dextramer technology was utilised to determine expansion of antigen-specific CD8+ T cells reactive to SAG peptides in complex with HLA-A29 in a cohort of BCR patients, HLA-A29-positive controls, and HLA-A29-negative controls.

RESULTS

We report on the naturally presented antigenic SAG peptides identified by sequencing the HLA-A29 immunopeptidome of antigen-presenting cells of patients. We show that the N-terminally extended SAG peptide precursors can be trimmed in vitro by the antigen-processing aminopeptidases ERAP1 and ERAP2. Unexpectedly, no enhanced antigen engagement by CD8+ T cells upon stimulation with SAG peptides was observed in patients or HLA-A29-positive controls. Multiplexed HLA-A29-peptide dextramer profiling of a case-control cohort revealed that CD8+ T cells specific for these SAG peptides were neither detectable in peripheral blood nor in eye biopsies of patients.

CONCLUSIONS

Collectively, these findings demonstrate that SAG is not a CD8+ T cell autoantigen and sharply contrast the paradigm in the pathogenesis of BCR. Therefore, the mechanism by which HLA-A29 is associated with BCR does not involve SAG.

[Pseudoxanthoma elasticum: A disorder with different manifestations]

BACKGROUND

Pseudoxanthoma elasticum (PXE) is a rare, autosomal recessive inheritable disorder characterized by progressive elastic fibre calcification.

CASE DESCRIPTION

Here we describe two patients with different presentations of PXE. Patient A, an 11-year-old girl, visited the dermatologist because of yellow papules (pseudoxanthomas) on the side of her neck. With the aid of a skin biopsy, the dermatologist diagnosed PXE. Some years later, patient A developed symptoms of intermittent claudication due to arterial calcifications. Supervised exercise training diminished these symptoms. Patient B, a 55-year-old man, visited the ophthalmologist due to recent onset of metamorphopsia. The ophthalmologist discovered a subretinal haemorrhage and observed changes in the retina consistent with PXE. Severe loss of vision was prevented by intraocular anti-VEGF injections. Upon further investigation, pseudoxanthomas and arterial calcifications were found.

CONCLUSION

PXE is a rare monogenetic disorder with dermatological, ocular and vascular manifestations. With these two case reports we have illustrated how the initial clinical presentation and symptomatology may vary widely.