Background/Purpose: The association between HLA-B27 and spondyloarthritis (SpA) remains unexplained more than 40 years after its discovery. To further explore the pathogenic role of HLA-B27, we produced transgenic Drosophila for several human HLA-B alleles, associated (B*2705, B*2704) or not (B*0702) with SpA, and for the b2m invariant chain partner.

Methods: Transgenes were expressed with different drivers in the wing or eye imaginal discs (i.e. the precursor tissues of the adult fly wing or eye). Their expression was examined by PCR and Western-blot. Cell surface or intra-cellular levels of unfolded (HC10 antibody) and well-folded (ME1 and W6/32 antibodies) HLA-B were examined by FACS and by immunofluorescence (IF). Genetic interactions with candidate genes controlling several signaling pathways were addressed.

Results: IF experiments revealed high levels of well-folded HLA-B27 alleles on the cell surface, but less of the HLA-B*0702. FACS analysis confirmed that HLA-B*2705/b2m and HLA-B*2704/b2m better fold than HLA-B0702/ b2m, in Drosophila. Co-expression of HLA-B27 (B2705 or B2704) alleles and b2m in the wing tissue specifically induced the loss of posterior and anterior cross-veins leading to a CVL phenotype. Furthermore, the eye size was reduced when HLA-B27 alleles (but not HLA-B*0702) were expressed in this tissue with b2m. Genetic interaction tests for candidate signaling pathways involved in the eye and wing development showed a selective decrease of the Bone Morphogenetic Protein (BMP) signaling when HLA–B*2705/b2m was expressed with no evidence for ER stress induction. Those results were confirmed by the decreased phosphorylation of the transcription factor MAD. Altogether, our results allow us to hypothesize that HLA-B*2705 could act downstream of the BMP receptor. Moreover, overexpression of the Drosophila SMAD7 homolog Dad, a negative regulator of the BMP pathway, was observed. Interestingly, transcriptomic study showed that SpA-prone HLA-B27/b2m transgenic rat lines also exhibit a specific increase of SMAD7 in their dendritic cells.

Conclusion: HLA-B27 seems to have a better capacity to fold and reach the cell surface than HLA-B*0702, in the presence of b2m in Drosophila, even in the absence of the class-I MHC peptide-loading complex machinery . CVL and small-eye phenotypes resulting from the expression of both SpA-associated HLA-B27 subtypes (i.e. B*2705 and B*2704) but not of HLA-B*0702 was induced by disrupting MAD phosphorylation. Those results highlight the effectors (rSMADs) and inhibitor (SMAD7) of the BMP/TGFb pathway as candidate targets for the pathogenic role of HLA-B27 in SpA.

« Back to 2017 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/hla-b27-human-%ce%b22-microglobulin-%ce%b22m-transgenic-drosophila-expresses-specific-cross-vein-less-cvl-and-small-eye-phenotypes-resulting-from-disruption-of-the-bmptgf%ce%b2-signaling-pathway/