Fig 2: Spinal neutralization of TREM2 reduces fracture-associated chronic pain and CCL21-induced acute pain. (A,B) A neutralizing antibody against TREM2 (anti-TREM2, i.t., 2 μg, indicated by a red arrow) was injected on day 14 after tibial fracture. Behavioral test showed the attenuation of the established fracture-associated mechanical allodynia and cold allodynia by anti-TREM2. (C,D) Anti-TREM2 (i.t., 2 μg, indicated by a red arrow) was injected 60 min prior to recombinant CCL21 (i.t., 0.1 μg). Exogenous CCL21-induced acute inflammatory pain was alleviated by the pre-application of anti-TREM2. All results are mean ± SEM (n = 8) and analyzed by two-way ANOVA with Bonferroni post hoc comparisons. *p < 0.05, **p < 0.01, ***p < 0.001.

Fig 3: Intrathecal pre-administration of artesunate reduces fracture-associated spinal over-expressions of CCL21, TREM2, and DAP12 and microglia activation. Intrathecal artesunate (100 μg) was administered daily for three consecutive days on days 4, 5, and 6 after tibial fracture with orthopedic surgery. All biochemical data were collected on day 7 after sham and fracture surgeries. (A–C) ELISA identified that pretreatment with artesunate downregulated the increased levels of spinal CCL21, TREM2, and DAP12 proteins after tibial fracture. (D,E) Immunohistochemistry staining showed representative photomicrographs of the marker of microglia activation (Iba1) in the spinal dorsal horn after fracture intervention and artesunate treatment (scale bar, 50 μm). All biochemical results are expressed as mean ± SEM (n = 3–5) and analyzed by one-way ANOVA with Bonferroni post hoc comparisons. # p < 0.05 vs. group sham + DMSO, *p < 0.05 vs. group fracture + DMSO.

Fig 4: Spinal neutralization of CCL21 reduces fracture-associated postoperative pain. (A,B) A neutralizing antibody against CCL21 (anti-CCL21) was intrathecally injected (0.1, 1 and 10 μg) on days 4, 5, and 6 (indicated by red arrows) after fracture surgery. Mechanical allodynia and cold allodynia were measured by the von Frey test and acetone test, respectively. (C,D) Intrathecal injection of anti-CCL21 (10 μg) on day 14 (indicated by a red arrow) after fracture surgery significantly inhibited the established mechanical and cold allodynia. All behavioral results are mean ± SEM (n = 8) and analyzed by two-way ANOVA with Bonferroni post hoc comparisons. # p < 0.05 vs. group sham + DMSO, *p < 0.05 vs. group fracture + DMSO.

Fig 5: Fracture-associated behavioral allodynia and spinal over-expressions of CCL21, TREM2, and DAP12 after orthopedic surgery in mice. The development of mechanical allodynia was assessed by paw withdrawal mechanical threshold (A) and paw withdrawal mechanical frequency to 0.16 g filament (B) in the von Frey test after sham surgery and tibial fracture (n = 8). The development of cold allodynia was assessed by cold response scoring (C) in acetone test after sham surgery and tibial fracture (n = 8). The spinal dorsal horn L4-5 segments were collected for biochemical experiments. (D–F) ELISA identified the increased levels of spinal CCL21, TREM2, and DAP12 proteins after tibial fracture (n = 5). All the data are expressed as mean ± SEM and analyzed by two-way ANOVA with Bonferroni post hoc comparisons. *p < 0.05, **p < 0.01, ***p < 0.001 vs. group sham surgery.