Fig. 6
Mechanical strain in stromal cells induces recruitment of classical monocytes into mechano-sensitive regions. a Increased secretion of CXCL1, MMP3, IL6, and CCL2 in stromal cells (synovial- or Achilles tendon fibroblasts) upon in vitro stretch; n = 25 samples of individual mice, Wilcoxon. b Increased migration of classical monocytes and CD11b + F4/80 monocytes upon in vitro stretch of stromal cells. The migration of splenocytes to conditioned supernatants of stretched vs. non-stretched cells were tested by in vitro chemotaxis and measured by flow cytometry, migration index is shown with n = 8 independent samples from five independent experiments, MWU. c Increased accumulation of classical monocytes at mechanostressed tissues by biomechanical loading in arthritic disease. TNF∆ARE mice were divided into three groups (score matched) with different loading regimens during 2 weeks: voluntary running, unloading or control. Control littermates were used as a negative control. Knee synovium, Achilles tendon, and spleen were isolated and prepared for flow cytometry. Increases of classical monocytes in response to loading were seen in synovial tissue, Achilles tendon but not spleen. Synovium and tendon 3 mice/group, spleen individual mice; seven independent experiments. Repeated measures one-way Anova p < 0.01, post-test Tukey’s Multiple comparison (respectively four (synovium) and three (tendon) groups were compared due to missing value in unloaded group). d Representative dot plot of control (top panel) and voluntary running (lower panel) mice show increased frequency of classical monocytes to loading in TNF∆ARE synovium. e Human Achilles tendon of patients with chronic tendinopathy (heightened biomechanical stress) (n = 7) vs. patients unable to walk because of general spasticity (represents unloaded conditions) (n = 3), MWU. f Representative dot plot of isolated mononuclear cells of patients unable to walk because of general spasticity (upper panel) and patients with chronic tendinopathy (lower panel). A marked increase in classical monocytes was observed under conditions of increased mechanical strain. g, h Clinical scoring data of CCL2−/− mice and corresponding C57BL/6 littermates show a significant increase in arthritis severity by voluntary running in the C57BL/6 mice but not in CCL2−/− mice. MMRM day 3–10; n = 10/group. Error bars show the mean ± s.e.m., *p < 0.05, **p < 0.01, ***p < 0.001
