HLA-B27–mediated activation of TNAP phosphatase promotes pathogenic syndesmophyte formation in ankylosing spondylitis
Chin-Hsiu Liu, … , Shih-Chieh Hung, Kuo-I Lin
Chin-Hsiu Liu, … , Shih-Chieh Hung, Kuo-I Lin
Published December 2, 2019; First published November 4, 2019
Citation Information: J Clin Invest. 2019;129(12):5357-5373. https://doi.org/10.1172/JCI125212.
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Research Article Autoimmunity Bone biology

HLA-B27–mediated activation of TNAP phosphatase promotes pathogenic syndesmophyte formation in ankylosing spondylitis

Abstract

Ankylosing spondylitis (AS) is a type of axial inflammation. Over time, some patients develop spinal ankylosis and permanent disability; however, current treatment strategies cannot arrest syndesmophyte formation completely. Here, we used mesenchymal stem cells (MSCs) from AS patients (AS MSCs) within the enthesis involved in spinal ankylosis to delineate that the HLA-B27–mediated spliced X-box–binding protein 1 (sXBP1)/retinoic acid receptor-β (RARB)/tissue-nonspecific alkaline phosphatase (TNAP) axis accelerated the mineralization of AS MSCs, which was independent of Runt-related transcription factor 2 (Runx2). An animal model mimicking AS pathological bony appositions was established by implantation of AS MSCs into the lumbar spine of NOD-SCID mice. We found that TNAP inhibitors, including levamisole and pamidronate, inhibited AS MSC mineralization in vitro and blocked bony appositions in vivo. Furthermore, we demonstrated that the serum bone-specific TNAP (BAP) level was a potential prognostic biomarker to predict AS patients with a high risk for radiographic progression. Our study highlights the importance of the HLA-B27–mediated activation of the sXBP1/RARB/TNAP axis in AS syndesmophyte pathogenesis and provides a new strategy for the diagnosis and prevention of radiographic progression of AS.

Authors

Chin-Hsiu Liu, Sengupta Raj, Chun-Hsiung Chen, Kuo-Hsuan Hung, Chung-Tei Chou, Ing-Ho Chen, Jui-Teng Chien, I-Ying Lin, Shii-Yi Yang, Takashi Angata, Wen-Chan Tsai, James Cheng-Chung Wei, I-Shiang Tzeng, Shih-Chieh Hung, Kuo-I Lin

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Figure 7

The HLA-B27–mediated activation of the p-IRE1/sXBP1 pathway promotes the upregulation of the RARB/TNAP axis in AS MSCs.

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The HLA-B27–mediated activation of the p-IRE1/sXBP1 pathway promotes the...
(A) Unfolded or misfolded HCs were immunoprecipitated (IP) from lysates of AS MSCs and control MSCs with HC10 antibody, followed by immunoblotting with HLA-B27 and anti-BiP antibodies. In AS MSCs, monomers of HLA-B27 HCs and dimers of disulfide-linked HLA-B27 HCs are indicated by an arrow and an arrowhead, respectively. (B and C) Immunoblot showing the expressions of p-IRE1 (B) and sXBP1 (C) protein in AS and control MSCs at day 7 under osteogenic induction. (D) Immunoblot showing the expressions of p-IRE1 and sXBP1 in AS MSCs transduced with shHLA-B or shCtrl. (E) Immunoblot showing the expressions of p-IRE1 and sXBP1 protein in control MSCs transduced with pLAS2w-HLA-B27 or pLAS2w. (F) ChIP assay showing sXBP1 binding at fragments (Frags) 1, 2, 3, 6, and 7 within 1400 bp upstream of the RARB transcriptional start site in AS MSCs. RARB 3′-untranslated region (3′UTR) was used as the negative control locus. (G) Immunoblot showing the expressions of RARB and TNAP in AS MSCs expressing shXBP1 or shCtrl. (H) ARS staining of mineralization in AS MSCs transduced with shXBP1 or shCtrl under osteogenic induction with quantification (I). All experiments done in the AS patient group and control group are from AS MSCs (A1, A2, and A3) and control MSCs (C1, C2, and C3), respectively, with at least 2–3 experimental repeats. Data are the mean ± SEM. **P < 0.05; ***P < 0.001; ****P < 0.0001 by 2-tailed Student’s t test (2 groups) or ****P < 0.0001 by 1-way ANOVA, followed by Tukey’s HSD test. Representative immunoblots from AS (A1) MSCs are shown in G.