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A1.10 The GM-CSF/CCL17 axis in the rheumatoid synovial environment
  1. SV Kidger1,
  2. CD Ellson2,
  3. IB McInnes1,
  4. MA Sleeman2,
  5. MJ Robinson2,
  6. CS Goodyear1
  1. 1Institute of Infection, Immunity and Inflammation, College of Medical Veterinary and Life Sciences, University of Glasgow, UK
  2. 2MedImmune, Cambridge, UK

Abstract

Background and objectives GM-CSF is a pivotal growth factor/cytokine that can drive aspects of RAimmunopathogenesis. In particular, GM-CSF can activate monocytes and induce their differentiation towards alternative phenotypes. The synovial environment also contains additional factors that can influence monocyte activation such as damage associated molecular patterns (DAMPs); many of which signal via TLR4. However, the impact of GM-CSF on monocytes in a synovial setting is not well understood and therefore we aimed to investigate the response of monocytes to GM-CSF in the context of synovial fluid (SF) co-stimulation.

Materials and methods Monocytes isolated from human blood or murine bone marrow were cultured in the presence or absence of GM-CSF (1ng/ml) and co-stimulated with SF (2.5%/5%) from RA or OA patients. Supernatants were harvested after 24 h and chemokine and cytokine secretion was analysed by ELISA and/or MSD. Immune complexes were made by incubating human IgG and Staphylococcal Protein A at a ratio of 4:2 respectively.

Results Monocytes from healthy individuals and RA patients, secreted substantial amounts of CCL17 upon stimulation with GM-CSF. In healthy monocytes, GM-CSF driven induction of CCL17 was significantly inhibited (P < 0.01) upon co-stimulation with RA-SF but not OA-SF, whilstthe expression of other chemokines and cytokines was un-affected. GM-CSF induced phosphorylation of STAT5 was not affected by RA-SF exposure, suggesting the inhibition was downstream of receptor and proximal signalling. Furthermore, inhibition of CCL17-induction via RA-SF was TLR-independent, as RA-SF could inhibit CCL17 induction in MyD88/TRIF double knockout murine monocytes. In addition to DAMPs, SF also contains immune complexesand cytokines. We tested if TNF, IL1a or IL-6 could reproduce the effect of SF, but none of these cytokines impacted GM-CSF induction of CCL17. To investigate if the inhibitory capacity of SF may be due to immune complexes, small immune complexes were made, and these did inhibit GM-CSF-mediated production of CCL17 in monocytes.

Conclusions GM-CSF induced production of CCL17 in monocytes is inhibited by RA-SF. The underlying mechanism is not known, however, our work shows that it is TLR-independent and downstream of STAT5 phosphorylation. Immune complexes also inhibit GM-CSF induction of CCL17 suggesting these may contribute to the phenomenon.

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