Trans-spliced peptides spanning PDGFRα and Adeno-Associated Virus type 5 trigger autoimmune responses in systemic sclerosis patients

Abstract

Systemic Sclerosis (SSc) is an autoimmune disorder of unknown etiopathogenesis. In SSc stimulatory antibodies targeting the platelet-derived growth factor receptor a (PDGFRa) have been described, cloned and demonstrated to activate human fibroblasts in vitro and ex vivo. Since PDGFRa is the entry receptor for the adeno-associated virus type 5 (AAV5), and is involved in the pathogenesis of SSc, we investigated whether the binding of AAV5 to PDGFRa in specific configurations, might be the target of the immune response to the virus-receptor complex and the loss of tolerance to PDGFRa. Using in silico molecular docking of the receptor interacting with the viral capsid and guided by the conformational epitopes of the receptor recognized by the agonistic antibodies, we predicted three trans-spliced-peptides formed by a covalent link of adjacently exposed segments of PDGFRa and AAV5. Those predicted trans-peptides were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in multiple reaction monitoring scan mode in HLA class I immunopeptidome in PBMCs from 15 SSc patients but not in 8 healthy subjects. Importantly the trans-spliced peptides had high affinity for both HLA-class I and II alleles associated with SSc and displayed immunoreactivity to PDGFRa in SSc patients but not in healthy controls. This is first demonstration of trans-spliced peptides in an autoimmune disorder and their presence in SSc supports the notion that a virus in specific configuration(s) with membrane proteins contributes to break the immunity barrier to specific cellular antigens. Moreover, the results suggest for the first time that the generation of trans-spliced peptides originating from different proteins can be the target of bivalent agonistic antibodies that result in productive autoimmune response.