Background/Purpose:  Elevated levels of the pro-inflammatory cytokine IL-6 have been found in the affected joint of patients with osteoarthritis (OA), although the prevalence of OA patients demonstrating IL-6 pathway activation is not well-characterized.   Our goal was to estimate the proportion of OA patients with increased IL-6 signaling in the diseased joint, and to correlate IL-6 pathway activation with Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores.

Methods:   Synovial membranes were obtained from OA patients (N= 101) undergoing knee replacement surgery. The WOMAC pain questionnaire was used to assess levels of pain on a visual analog scale of 100mm.  Scores within each WOMAC subfunction (pain, stiffness and physical function) were averaged to derive a single composite subfunction score, and the WOMAC total score was calculated as the average of all subfunction scores.   Synovial fluid and serum collected at the time of surgery was assayed for IL-6 and CCL8 proteins.    Gene expression levels were measured using Affymetrix HG-U133Plus2 microarrays.  Transcripts with >2-fold over-expression following treatment with IL-6 and IL6R in synovial fibroblasts were identified. Hierarchical clustering using expression levels of these inducible transcripts in synovial tissue was used to identify OA patients  showing high levels of IL-6 activity. A signature score was calculated as the median expression intensities of IL-6 –inducible genes for each subject. 

Results:     CCL8 and SOCS3 were identified as IL-6 inducible genes in synovial fibroblasts and clustering of synovial membrane specimens from OA subjects revealed a cluster (19/101 subjects, 19%) with a 3.5-fold increase in the median expression of these two genes compared to the remaining subjects (388.4±270.1 vs 111.4 ± 38.7, p=3.0×10^-4) suggesting significantly elevated IL-6 pathway activation in this subgroup. These “IL-6 signature high subjects” also had significantly higher mean levels of IL-6 (155.2± 4.6 pg/mL vs 58.6 ± 4.1 pg/mL p=7.4×1-^-3) and CCL8 (30.5 ± 2.5 pg/mL vs 14.4 ± 2.1 pg/mL  p=7.2×10^-5) proteins in synovial fluid.  The levels of CCL8 and IL-6 proteins in synovial fluid were significantly correlated (rho=0.52, p=1.6×10^-7).  Compared to subjects with low levels of IL-6 inducible genes in the synovium, with those in the IL-6 signature high cluster were more likely to be women (Odds ratio 4.2 95% C.I. [1.1, 15.4] p=0.03), and had higher mean WOMAC scores for pain (65.6 ± 16.4 vs 55.6 ±18.3, p= 4×10^-2), stiffness (76.9 ± 17.0 vs 60.5 ± 24.8, p=3.4×10^-3), physical activity (67.2 ±15.8 vs 56.3 ± 18.2, p=4.4×10^-2) and total score (69.9 ± 14.6 vs 57.5 ± 18.1, p=8.9×10^-3).

Conclusion:   An IL-6 inducible gene signature consisting of CCL8 and SOCS3 was used to identify a subgroup of OA subjects with elevated IL-6 pathway activation in the synovial membrane.  OA subjects in this IL-6-signature high group were more likely to be women and had higher mean WOMAC pain scores.   CCL8 is a potential biomarker of IL-6 pathway activation in the affected joint.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-subset-of-osteoarthritis-individuals-has-elevated-il-6-pathway-activation-associated-with-worse-symptoms/