Painful peripheral neuropathy constitutes a major dose limiting side effect of chemotherapeutics that remains poorly controlled by available analgesics. Elucidation of mechanisms underlying chemotherapy-induced pain will identify targets for prophylactic and therapeutic analgesia. We recently showed that monocyte/macrophage infiltration in sciatic nerve orchestrates the development of vincristine (VCR)-induced allodynia in mice. Specifically, VCR administration induced expression of endothelial adhesion molecules and CX3CR1+ monocyte infiltration into the sciatic nerve. CX3CR1 receptor activation with endothelial fractalkine (FKN) induced formation of reactive oxygen species (ROS) by macrophages. ROS activated TRPA1 channels on sensory axons which evoked a pain response. VCR-allodynia was prevented by blocking monocyte trafficking into the sciatic nerve. However, mouse (and human) monocytes include a heterogeneous population of cells. Here we plan to identify the specific monocyte subsets that mediate initiation and maintenance of VCR- and paclitaxel-induced allodynia. Our hypothesis is that chemotherapeutic administration results in alteration of sciatic nerve microvasculature so that patrolling monocytes (CX3CR1+CCR2-) are retained by the endothelium as an early response. They trigger the generation of a local inflammatory environment and promote inflammatory monocyte (CX3CR1+CCR2+) to infiltrate the nerve. By combining behavior and immunohistochemistry with intravital microscopy in transgenic mice (KCL), we will define location and function of monocyte subsets in the sciatic nerve during chemotherapy-allodynia. Furthermore, we will establish whether manipulation of monocyte trafficking in the sciatic nerve provides prophylactic and therapeutic analgesia by testing CCR2 receptor antagonists and peripherally restricted Medivir cathepsin S inhibitors (by inhibiting enzymatic release of endothelial FKN, CX3CR1-monocyte activation is blocked).