<p>Tumor-infiltrating nerves play critical roles in promoting tumor growth and progression; however, the mechanisms that drive tumor innervation remain unclear. Upon transformation, tumors recruit surrounding peripheral nerves into the tumor microenvironment (TME) to obtain their own innervation, a process called axonogenesis. While in vitro studies suggest tumor cell-derived neurotrophins, such as brain-derived neurotrophic factor (BDNF), drive axonogenesis, this has yet to be demonstrated in vivo. During wound healing, macrophages are the primary source of neurotrophins. Given the critical role of macrophages in breast cancer growth, we investigated whether these immune cells drive tumor axonogenesis in breast tumors in vivo. Syngeneic Py230 mouse triple-negative breast cancer (TNBC) cells were transplanted into intact mice and mice lacking immune-derived BDNF. Bone marrow-derived macrophages from either wild-type or immune-BDNF-deficient mice were transplanted into tumor-bearing recipients to determine if macrophage-derived BDNF was sufficient to restore tumor growth and innervation. We found that transplanted TNBC cannot grow in the absence of immune-derived BDNF, and that depletion of macrophages from the TME compromises tumor innervation. Remarkably, the introduction of wild-type macrophages restores tumor growth and innervation in mice lacking immune-derived BDNF, demonstrating that macrophages are both necessary and sufficient for tumor axonogenesis in vivo. In the absence of sensory tumor innervation, tumor growth was significantly reduced. Moreover, targeting BDNF signaling diminished TNBC growth and innervation. Our findings identify macrophages as the critical source of BDNF driving axonogenesis in breast cancer and suggest that selectively targeting BDNF signaling could provide a novel therapeutic strategy for treating TNBC through compromising tumor innervation.</p>