Morning Stiffness and Neutrophil Circadian Disarming

To the Editor,

We read with interest the article by Orange et al. (1) assessing whether histologic parameters of synovium were related to symptoms of rheumatoid arthritis (RA). The authors analyzed synovial tissue samples of 176 RA patients who underwent hip or knee arthroplasty. The detection of neutrophils and fibrin in synovial tissue was associated with patient-reported morning stiffness of >1 hour. They suggested that ongoing neutrophil recruitment in synovial tissue may be associated with prolonged morning stiffness.

This interesting observation may provide insights into understanding the mechanism of one of the most bothersome symptoms of RA. We want to highlight the role of neutrophil circadian disarming as another plausible contributor to synovial neutrophil infiltration and subsequent morning stiffness. Experimental studies and retrospective data reveal that many inflammatory processes manifest circadian periodicity (2). Recent studies reported the release of neutrophils from the bone marrow during the night, with a peak in the early morning and gradual degranulation during the day (2,3). Orange et al. showed the presence of neutrophils as the only histologic feature associated with morning stiffness. However, if there is a steady migration of neutrophils to the synovium throughout the day and neutrophils only live for a short duration after infiltrating the synovium, why is there joint stiffness only in the morning? We hypothesize that neutrophil circadian disarming may be a plausible explanation for the joint stiffness to predominantly exhibit during early morning hours. The neutrophils especially the ones with higher protein content in their granules are released in early morning, recruit to the synovial tissue, and contribute to local inflammation causing joint stiffness especially in the morning.

Orange et al. (1) also reported that fibrin deposits along the synovial membrane with enmeshed neutrophils and necrotic neutrophil DNA were less amenable to fibrinolysis. The study suggested that this increased stability and rigidity of neutrophil-trapped fibrin deposits in the synovium was associated with morning stiffness. There is increased NETosis in synovial fluid and netting neutrophil infiltration in synovial tissue in RA (5). The increased presence of NETs promote immune dysregulation and contribute to tissue damage in many inflammatory diseases including RA (4). Furthermore, NETosis leads to fibrin deposition, formation of fibrin network and promotes resistance to tPA-mediated fibrinolysis (6). Hence, it is possible that synovial membrane necrotic neutrophilic DNA detected may be related to synovial NETosis causing fibrin deposition and its resistance to degradation. The neutrophils follow a circadian rhythm in their ability to form NETs. In healthy volunteers, the granule-loading and the capacity of NET formation of neutrophils are at its highest level at around 8 a.m. and gradually decrease later in the day. In summary, the circadian rhythm of neutrophils, NET formation, and their relation to fibrin maybe an additional explanation of the relationship between synovial neutrophil infiltration and morning stiffness.

The presence of NETs was shown to be associated with loss of tolerance to citrullinated antigens and the development of anti-citrullinated protein/peptide antibody (ACPA) (7). In addition, rheumatoid factor (RF) and ACPA may induce neutrophils to form NETs (5). It would be interesting to learn if there are any differences in neutrophil infiltration between ACPA and/or RF-positive and RF-negative groups in the synovium of patients with RA.