Análisis del plegamiento anómalo de HLA-B27 y su papel en la patogenia de la espondilitis anquilosante

Abstract

Ankylosing Spondylitis (AS) is a chronic inflammatory disease which is strongly associated with the human leukocyte antigen B27 (HLA-B27). HLA-B27 subtypes are differentially associated with AS. B*27:05, B*27:04 and B*27:02 are strongly associated with AS, B*27:07 is associated but not always and B*27: 06 and B*27:09 are not associated. There are several hypotheses trying to explain the mechanism that underlies this association. This work is based on the HLA-B27 misfolding hypothesis. This molecule tends to misfold and as a result of misfolding, it accumulates in the endoplasmic reticulum (ER). This generates stress responses that lead to the release of proinflammatory cytokines such as IL-23 that produces enthesitis. The ER associated degradation pathway (ERAD) should be able to eliminate the misfolded B27 heavy chains from the ER. ERAD encompasses recognition and labelling of misfolded proteins, retrotranslocation to the cytosol, its deglycosylation, ubiquitination and transport to the proteasome for degradation. We aimed to analyse each step of the ERAD pathway, in order to know in which step this pathway is blocked in AS-associated subtypes, and to describe the ERAD pathways that are associated to HLA-B27 in the context of US2 and US11 mediated degradation. The human cytomegalovirus products, US2 and US11 hijack the cellular machinery that operates on endogenous misfolded proteins, the ERAD pathway. They induce a rapid dislocation of newly synthesized class I heavy chains. We have demonstrated that all HLA-B27 subtypes analysed, AS-associated or not, misfold to different degrees. B*27:05 and B*27:02 are the ones with the worst folding phenotype and they bind more calnexin in an attempt to correctly fold. The AS-associated subtype B*27:04 folds better and binds less calnexin. Misfolded HLA-B27 molecules are recognised and labelled by the ERAD components BiP and EDEM1. B*27:02 and B*27:05 bind more BiP and EDEM1 than B*27:04, indicating that the latter folds more efficiently. None of the HLA-B27 subtypes analysed bind to OS9, but they do bind to the later components SEL1L and Derlin-1. B*27:02 and B*27:05 bind more SEL1L and Derlin-1 than B*27:04, again indicating that B*27:04 folding is more efficient. OS9 probably does not participate in HLA-B27 degradation. Another unidentified protein could play the role of OS9 and transfer misfolded proteins to SEL1L which, in turn, should transfer them to Hrd1. From Derlin-1, which is a protein involved in retrotranslocation, the interaction of HLA-B27 subtypes with the ERAD machinery is lost...