FRI0452 Are Gm-Csf-Producing Th17 Cells Important in The Pathogenesis of Psoriatic Arthritis?
Authors: C. Stober, J. Goodall, H. Gaston
Journal: Annals of the Rheumatic Diseases (2016)

<h3>Background</h3> Psoriasis [Ps] is a common chronic inflammatory skin condition affecting about 1–2% of the world population; approximately 20–30% of patients have psoriatic arthritis [PsA]. PsA can affect peripheral and axial joints, eyes, ileum, colon and skin. Genetic studies reveal common candidate genes including <i>IL23R</i>, <i>IL12B</i>, <i>STAT3</i>, and <i>CARD9</i>, all associated with IL-23 signalling. We demonstrated increased frequencies of Th<sub>17</sub> cells in patients with Ps and PsA and the clinical importance of IL-23/Th<sub>17</sub> is revealed by the efficacy of biologics targeting this pathway. Whilst IL-17 was thought to be pathogenic in inflammatory conditions such as experimental autoimmune encephalomyelitis, the redundancy of IL-17 but essential requirement for GM-CSF release by Th<sub>17</sub> cells was shown. GM-CSF<sup>+</sup> T cells were identified in cerebrospinal fluid from patients with multiple sclerosis but a role in the aetio-pathogenesis of PsA has not been established. <h3>Objectives</h3> 1) Are GM-CSF<sup>+</sup> T cells enriched in peripheral blood and synovial fluid obtained from patients with PsA? 2) Is GM-CSF co-produced with IFN-g? 3) Do CD4<sup>+</sup>GM-CSF<sup>+</sup> T cells express surface markers characteristic of Th<sub>17</sub>? 4) Is GM-CSF release augmented by exogenous IL-23 and/or IL-12? <h3>Methods</h3> PBMC or synovial fluid derived from patients with PsA, or healthy donor (HD) PBMC, were stimulated with anti-CD3/anti-CD28 +/− recombinant IL-23 or IL-12. Supernatants were harvested and analysed by ELISA for IL-17, IFN-γ or GM-CSF. Alternatively, cells were activated with phorbol myristate acetate (PMA) and ionomycin, and evaluated for the expression of the Th<sub>17</sub>-associated cell surface markers CCR6, CD161 &amp; IL-23R plus IL-17A, IFN-g and GM-CSF by flow cytometry. <h3>Results</h3> GM-CSF release from PsA PBMC was higher than that of HD, whereas IFN-g levels were higher in HD. This was most significant on comparing the ratio of IFN-g to GM-CSF. On examining the co-expression of GM-CSF and IFN-g in CD4<sup>+</sup> T cells, PsA patients had higher proportions of GM-CSF<sup>+</sup> single positive and fewer GM-CSF<sup>+</sup>IFN-g<sup>+</sup> relative to HD. There was also marked enrichment of CD4<sup>+</sup>GM-CSF<sup>+</sup> cells in synovial fluid. On evaluating the expression of the Th<sub>17</sub>-associated markers CCR6, CD161 and IL-23R, we demonstrated that whereas almost all CD4<sup>+</sup>IL-17<sup>+</sup> cells expressed CCR6, approximately 50% of CD4<sup>+</sup>GM-CSF<sup>+</sup> cells expressed this chemokine receptor. Surprisingly, there were fewer PsA patient CD4<sup>+</sup>GM-CSF<sup>+</sup> peripheral blood T cells co-expressing CCR6, CD161 and IL-23R relative to HD. Finally, IL-23 has been shown to render Th17 cells more pathogenic, hence we examined the effect of this cytokine on GM-CSF release. We showed that whilst IL-17 release was enhanced by exogenous IL-23, GM-CSF release was significantly downregulated with little effect on IFN-g. In contrast, IL-12 increased IFN-g and GM-CSF, and reduced IL-17. <h3>Conclusions</h3> IL-23 and Th<sub>17</sub> cells are therapeutic targets in PsA, and mouse models of inflammation suggest that pathogenic Th<sub>17</sub> cells release GM-CSF. We reveal that GM-CSF release is elevated in patients with PsA, and GM-CSF<sup>+</sup> cells are enriched in diseased joints. However, only a proportion of GM-CSF<sup>+</sup> cells exhibited features of Th<sub>17</sub>. Furthermore, IL-23 downmodulated GM-CSF whilst enhancing IL-17. Given that GM-CSF was mainly produced by cells <i>not</i> co-expressing IFN-g in PsA patients [in contrast to healthy donors], this subset may be pathogenic in PsA and warrants further investigation. <h3>Disclosure of Interest</h3> None declared