Neuronal complement cascade drives bone cancer pain via C3R mediated microglial activation.

Activation of spinal cord microglia is crucial for the development of bone cancer pain (BCP). The essential signal between neuronal excitability and microglial activation is not fully understood. In the present study, carcinoma implantation into tibia was used to induce BCP and RNAi-lentivirus was injected into spinal cord to knock down C1, C2 or C3 of complement cascade. We showed that C1, C2 and C3 co-localized in the same neurons and increased in cancer-bearing rats along with microglial activation. Knocked down of C1, C2 or C3 inhibited microglial activation and prevented the development of cancer-induced bone pain. Intrathecal administration of either minocycline (an inhibitor of microglial activity) to inhibit the activation of microglia or compstatin (a C3-targeted complement inhibitor) to block the complement cascade reversed cancer induced bone pain. Further study indicated that neuronal complement promoted the activation of microglia via complement 3 receptor (C3R). In the in vitro experiments, the proliferation of microglia was enhanced by the activation product of C3 (iC3b), but was inhibited by compstatin. These results indicated that neuronal complement pathway promoted the activation of microglia via C3R and contributed to the development of BCP.