T-cell infiltration after chronic constriction injury of mouse sciatic nerve is associated with interleukin-17 expression and secondary macrophage recruitment

Aims: Cytokine networks are involved in adaptive responses after peripheral nervous system injury. While it is well established that macrophages play a decisive role in nerve degeneration after chronic constriction injury (CCI) the role of T-cells is not yet well defined. Interleukin (IL)-17 is a novel proinflammatory cytokine produced by activated T-cells, but not macrophages.

Objectives: In the present study, we investigated the role of T-cells and the T-cell derived cytokine IL-17 in CCI, an animal model for neuropathic pain.

Materials and Methods: Wildytpe (WT) and RAG-1-/- (KO) mice, which lack functional T-cells, were subjected to CCI of the right sciatic nerve. The temporal gene expression profiles of IL-17A, macrophage chemoattractant protein (MCP)-1 and the macrophage antigen F4/80 were assessed in the distal nerve segments of WT and KO mice by real-time RT-PCR. IL-17A protein expression and T-cell infiltration were further investigated by immunohistochemistry. Paw withdrawal latencies to heat were measured in both groups to study the role of T-cells during development of thermal hyperalgesia.

Results: IL-17 mRNA was virtually absent in control specimens from healthy WT mice and at early stages of Wallerian degeneration at day (d) 1. One week after CCI IL-17 transcripts showed a 20fold induction (p<0.0001) and rapidly declined thereafter. In RAG-1 KO mice IL-17 mRNA was not detectable at any time point after CCI. IL-17 protein was found in sciatic nerves from WT mice 7d after CCI and was located predominantly within the endoneurium. CD3/IL-17 double staining confirmed the co-localization of IL-17 and T-cells. Interestingly, the maximum of F4/80 and MCP-1 mRNA expression in WT animals paralleled the IL-17 peak and was reached one week after CCI (F4/80: ˜25fold induction; MCP-1: ˜35fold induction, p<0.0001). Supporting a role of T-cells in secondary macrophage recruitment, RAG-1 KO mice showed lower F4/80 and MCP-1 mRNA levels (F4/80: ˜10fold induction; MCP-1: ˜15fold induction). Moreover, RAG-1 KO mice developed significantly less thermal hyperalgesia compared to WT animals indicating a functional role of T-cells in pain induction. The difference was already evident at d3 post CCI and even more pronounced at d7 (p<0.0001).

Conclusions: Our data suggest that T-cells are critically involved in secondary recruitment of macrophages and modulation of pain-related behaviour after CCI.