Fas-associated Death Domain (FADD) is the pivotal adaptor of the apoptotic signal mediated by death receptors. In rheumatoid arthritis (RA), ligands of IL-1R/TLR4 are present in the synovial fluid (SF), and FADD can act as an anti-inflammatory molecule by sequestering MyD88, the adaptor of IL-1R/TLR4 signaling. Thus, absence of FADD in synovial cells can potentially contribute to the establishment of inflammation. Recently, we identified adenosine receptors as the main regulators of a new mechanism of FADD regulation of expression by shedding of microvesicles. As adenosine is abundant in the SF of RA patients, we investigated whether FADD could be detectable in the SF and sera of RA patients compared to non-RA patients.
Method: Detection of intracellular FADD in patients affected with RA and osteoarthritis (OA) was investigated by western blot followed by densitometry in synovial tissues (RA n=11, OA n=14) and cultured synoviocytes (RA n=7, OA n=7). A homemade protocol was used for quantification of FADD expression by ELISA. FADD was assessed in Synovial fluid (SF) from two independent cohorts (n=26 and 67) and in the sera of patients from the ESPOIR cohort (n=551 RA and 67 non-RA patients), [a French multi-centric cohort of patients having early arthritis lasting less than 6 months], and from healthy individuals (n=20). Student’s t tests were conducted to verify statistical significance of our data.
Result: Densitometry analysis (Arbitrary Unit = AU) showed a higher concentration of cytoplasmic FADD in OA tissues and synoviocytes compared to RA tissues and synoviocytes: 0.809±0.06 vs 0.592±0.06 AU and 0.532±0.212 vs 0.317±0.143 AU respectively. Similarly nuclear FADD was higher in OA tissues and synoviocytes: 0.621±0.08 vs 0.320±0.09 and 0.247±0.05 vs 0.138±0.042AU. We detected more FADD in SF of RA patients compared to OA patients in the two cohorts: 249.4 ±72.7 vs 55 ng/ml±17 (P=0.017) and 140.2±28.5 vs 5.8±1.4 ng/ml (P=0.0015) respectively. Also FADD concentration was higher in sera of RA patients than in non-RA and healthy patients: 33.5± 5.2 vs 10.7±1.7 (P=0.00004) and vs 5.6±2.5 ng/ml (P=0.000002). The presence of FADD in RA sera at inclusion correlated with the presence of anti-CCP antibodies (P=0.035), rheumatoid factor (P=0.03) and the DAS28 (P=0.0002). Conversely no link could be made between FADD concentration and joint erosion at inclusion (P=0.2) or at 2 years post-inclusion (p=0.39).
Conclusion: This is the first demonstration that human FADD could be detected specifically during the course of an inflammatory disease. These results raised the hypothesis that low expression of FADD in joint cells could occur through a release of the protein in SF and could contribute to the development of RA
Disclosure: L. Tourneur, None; S. Mistou, None; V. Vilmont, None; N. Cagnard, None; J. E. Gottenberg, None; V. Devauchelle, None; G. Chiocchia, None.