TLR3 and TLR7/8 ligands induce microglia activation in intact rat spinal cord

Objective To study the effects of two stimulators of the innate immune system, polyinosine-polycytidylic acid [Poly (I:C)] and R848, on the activation of microglia in rat spinal cord. Methods Thirty male Lewis rats (6 to 8 weeks old) were divided into Poly(I:C) group (n=12), R848 group (n=15) and control group (n=3). According to the killing time, the Poly(I:C) group and R848 group were divided into 4 and 5 sub-groups, respectively, with 3 rats in each sub-group. The rats in the subgroups received intraperitoneal injection of a single bolus of Poly(I:C) (5 mg/kg) or R848 (1 mg/kg) accordingly, and in the control group, the same volume of phosphate-buffered saline (PBS) was administered. Activated microglia were observed using immunohistochemistry for ED-1, AIF-1, EMAP Ⅱ, OX6, and P2X4R, and BrdU staining was used to identify the proliferating cells. Results Compared with the control group, both Poly (I:C) and R848 groups showed a significant but transient increase of ED-1-positive spinal cord microglia 4 days after the injection, while no significant differences were found in the microglial markers AIF-1, EMAP Ⅱ, OX6, P2X4 receptor (P2X4R), indicating that the microglia were not fully activated. Tracing of the cell proliferation by BrdU revealed that only a small fraction of the proliferating cells were microglia (less than 5%). Conclusion Poly(I:C) and R848 have definite effects on the innate immune system of the spinal cord and modulate the immune activity in the spinal cord, suggesting the value of these agents in modulating local regenerative processes in injured spinal cord. Key words: Polyinosine-polyeytidylic acid; R848; Microglia; Spinal cord