Background During synovial inflammation, platelets and their associated microparticles escape from the synovial microvasculature and provide pro-inflammatory factors leading to the activation of synovial fibroblasts (SF) that actively contribute to joint damage.¹

In patients with rheumatoid arthritis (RA), SF up-express the surface protein Podoplanin (PDPN), a ligand for CLEC-2.^2,3 Although the function of PDPN is still poorly understood, recent data suggest that the PDPN/CLEC-2 axis can modulate cellular responses. Within the RA synovium, platelets are considered the sole source of CLEC-2.³ Despite these observations, clear experimental approaches that explore the role of PDPN/CLEC-2 interactions in RA pathogenesis are lacking.

Materials and methods PDPN expression by freshly isolated mouse synoviocytes was measured by flow cytometry (FACS) during joint inflammation and after resolution. PDPN expressing cells were characterised by FACS and quantitative PCR. Arthritis severity in the absence of CLEC-2 was assessed in the global tamoxifen-inducible Clec1b deleting mice (ie, Rosa26-Ert2Cre x Clec1b^Flox/Flox) as well as in the platelet-specific Clec1b deleting mice (ie, Pf4-Cre x Clec1b^Flox/Flox). Wild-type mice with auto-immune arthritis were injected with anti-PDPN antibody. The disease was monitored by body weight, clinical scores, ankle and paw thicknesses. The synovium cellular contents from arthritic animals were analysed by FACS and cartilage damage quantified by histology while bone erosion and remodelling were monitored by MicroCT.

Results Joint inflammation triggered PDPN up-expression on a pro-inflammatory SF subset with concurrent accumulation of PDPN+ anti-inflammatory macrophages. These populations disappeared with the resolution of inflammation. In the absence of CLEC-2, arthritis was more severe and led to more pronounced bone erosion and remodelling. Mice treated with an agonist anti-PDPN antibody were partially protected from induced auto-immune arthritis as demonstrated by their clinical features and reduced leucocyte infiltrations into the joints.

Conclusions We provide the first in vivo evidence that the PDPN/CLEC-2 interaction restrains arthritis as ablation of platelet-derived CLEC-2 leads to worse arthritis, bone erosion and remodelling. Accordingly, mimicking PDPN/CLEC-2 interactions with an agonist anti-PDPN restrains auto-immune arthritis in mice. These observations suggest that platelets, known for promoting joint inflammation, also contribute to the suppression of arthritis in a CLEC-2 dependent manner. The mechanisms underlying this anti-inflammatory process are currently under investigation.

References 1. Boilard E. et al. Nature Review Rheumatol. 2012;8:534–42.

2. Ekwall A.K. et al. Arthritis Res Ther. 2011;13:R40.

3. Del Rey M.J. et al. PLoS One.2014;9:e0099607.