Background/Purpose: Synovial inflammation is a common feature of osteoarthritis (OA) and rheumatoid arthritis (RA) patients. Comparative studies indicated that synovitis, measured by histology or imaging, is more severe in RA than in OA, which is also reflected in more infiltrating immune cells in RA than in OA. Intriguingly, mast cell numbers were found in some studies to be equal or lower in RA than OA. Here, we wished to expand these findings using immunofluorescence  and to assess the possible relevance of mast cells in OA by studying  their association with clinical parameters, such as radiographic damage and pain.    

Methods: Synovial tissues of 56 symptomatic OA and 49 RA patients were obtained from arthroscopy (early disease) (22 OA, 23 RA) or arthroplasty (late disease) (34 OA, 26 RA). Two to three paraffin slides were used to quantify inflammation using haematoxylin and eosin staining (synovitis score 0-9), and to quantify and determine degranulation state of mast cells (per 10 high-power fields) using double immunofluorescence for CD117 and tryptase. Average scores per patient were used for analysis. Knee radiographs of OA patients were scored according to the Kellgren and Lawrence (KL) system and self-reported pain was determined in OA patients by visual analogue scale at baseline.

Results:

Median (range) of mast cells was significantly higher in OA samples 45 (1-168) compared to RA samples 4 (1-47) (p-value < 0.001), despite a lower median (range) synovitis score in OA (2.5 (0-6.0)) compared to 4.6 (0-8.0) in RA samples. This held true when comparing the subgroups of early and late samples between diagnoses. Similar percentages of mast cells were degranulated in both diseases at all stages, indicating that the abundance of these cells rather than degranulation state could be relevant for the disease process. The synovitis score correlated with the number of mast cells both in OA (Spearman’s rho = 0.3, p =0.023) and RA (Spearman’s rho = 0.5,p < 0.001), indicating that mast cells could contribute to synovial inflammation in both diseases, although their contribution is probably moderate. Interestingly, the number of mast cells was associated with an increased KL-grade (p-value 0.05) in OA patients, independently of synovitis. No associations were found with self-reported pain. 

Conclusion:

Prevalence of mast cells in OA synovial tissue is higher than in RA. Interestingly, it appears to be associated with synovial inflammation, as well as structural damage in OA patients, suggesting a role of mast cells in this disease.

« Back to 2015 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/characterization-of-synovial-mast-cells-in-knee-osteoarthritis-association-with-clinical-parameters/