ARTHRITIS & RHEUMATOLOGY Vol. 66, No. 9, September 2014, pp 2412–2422 DOI 10.1002/art.38737 © 2014, American College of Rheumatology Increased Production of Interleukin-17 Over Interleukin-10 by Treg Cells Implicates Inducible Costimulator Molecule in Experimental Spondyloarthritis Luiza M. Araujo, 1 Ingrid Fert, 1 Quentin Jouhault, 1 Karine Labroque `re, 2 Muriel Andrieu, 2 Gilles Chiocchia, 1 and Maxime Breban 3 Objective. HLA–B27/human  2 -microglobulin (h 2 m)–transgenic (B27-transgenic) rats develop an inflammatory disorder resembling spondyloarthritis, with accumulation of proinflammatory Th17 cells. Be- cause Treg cells and Th17 cells have opposing effects in inflammatory disorders, we sought to determine whether biased expansion of Th17 cells could result from altered Treg cell frequency and/or function in B27-transgenic rats. Methods. We characterized the phenotype and function of Treg cells from B27-transgenic rats in comparison with those from control rats, by examining their expression of cell surface markers, suppressive activity, cytokine production, and differentiation pat- tern. Results. In B27-transgenic rats, the preferential accumulation of CD4 Teff cells over Treg cells was not associated with a defect in Treg cell differentiation or suppressive activity. The expression of Treg cell mark- ers was similar between B27-transgenic and control rats, with the exception of the inducible costimulator (ICOS) molecule, which was overexpressed in B27- transgenic rats. High levels of ICOS are considered to be a hallmark of Treg cells with heightened suppressive activity and interleukin-10 (IL-10) expression. Paradox- ically, the production of IL-10 by Treg cells was reduced in B27-transgenic rats, whereas the production of IL-17 was enhanced. Moreover, the addition of anti-ICOS monoclonal antibodies during Treg cell differentiation in the presence of dendritic cells from B27-transgenic rats reversed this cytokine profile, restoring the balance between IL-10 and IL-17 in Treg cells from B27- transgenic rats. Conclusion. We observed dysregulated pro- duction of IL-10 and IL-17 by Treg cells from B27- transgenic rats, which may contribute to disease devel- opment. Moreover, our data highlight a key role for ICOS signaling in the generation of imbalanced produc- tion of IL-10 and IL-17 by Treg cells in this experimen- tal model of spondyloarthritis. Spondyloarthritis (SpA) is a type of chronic inflammatory disorder that predominantly affects the spine and sacroiliac joints. These cardinal features fre- quently combine with peripheral joint arthritis, enthesi- tis, and extraarticular manifestations, the most frequent of which are uveitis, psoriasis, and inflammatory bowel disease, including Crohn’s disease and ulcerative colitis (1). SpA is a highly heritable disorder involving multiple genetic factors, among which the HLA–B27 allele plays a prominent role (2). The association between HLA– B27 and SpA was first described 40 years ago, but the mechanism of this association remains largely unex- plained (3). Supported by INSERM and by the French National Agency for Research (DCSPA [Molecular Role of HLA–B27 in Spondyl- arthritis, From Animal Model to Human Disease] grant ANR- Physiopath 2007-2011 and Investments for the Future Programme grant ANR-11-IDEX-0005-02, Sorbonne Paris Cite ´, Laboratoire d’Excellence Inflamex). 1 Luiza M. Araujo, PhD, Ingrid Fert, MS, Quentin Jouhault, MS, Gilles Chiocchia, PhD: INSERM U987 and Universite ´ de Ver- sailles St.-Quentin-en-Yvelines, Montigny-le-Bretonneux, France, and Universite ´ Paris Diderot, Sorbonne Paris Cite ´, and Laboratoire d’Excellence Inflamex, Paris, France; 2 Karine Labroque `re, MS, Muriel Andrieu, PhD: Institut Cochin, INSERM U1016, and CNRS (UMR 8104), Paris, France; 3 Maxime Breban, MD, PhD: INSERM U987 and Universite ´ de Versailles St.-Quentin-en-Yvelines, Montigny-le- Bretonneux, France, Ho ˆpital Ambroise Pare ´, AP-HP, and Universite ´ de Versailles St.-Quentin-en-Yvelines, Boulogne-Billancourt, France, and Universite ´ Paris Diderot, Sorbonne Paris Cite ´, and Laboratoire d’Excellence Inflamex, Paris, France. Address correspondence to Maxime Breban, MD, PhD, Ho ˆpital Ambroise Pare ´, Service de Rhumatologie, 9 Avenue Charles de Gaulle, 92100 Boulogne-Billancourt, France. E-mail: maxime. breban@apr.aphp.fr. Submitted for publication June 30, 2013; accepted in revised form June 3, 2014. 2412