Controlled Delivery of a Neurotrophic Factor in the Adult Mouse Brain Using Engineered Microglia

Microglia, the resident immune cells of the central nervous system, have been proposed as vehicles for delivering therapeutic biologics. These cells can be genetically engineered in vitro and transplanted into host animals following ablation of endogenous microglia, enabling repopulation of the brain parenchyma. However, current replacement strategies often rely on radiation or transgenic models, limiting their clinical relevance. CSF1R inhibitors offer a more translational approach to microglia ablation, though surviving host cells can compete with transplanted microglia during repopulation. In this study, we successfully ablated endogenous microglia using a CSF1R inhibitor in adult mice and developed a method to transplant engineered microglia expressing Brain-Derived Neurotrophic Factor (BDNF) in a doxycycline-inducible manner. To enhance engraftment, transplanted cells also expressed a constitutively active CSF1R mutant (caCSF1R). BDNF-expressing transplanted microglia spread through large areas of host mice brains, displayed similar morphology and transcriptional profile to repopulating host microglia, and responded to pro-inflammatory stimuli. Treatment with doxycycline resulted in increased BDNF expression and TrkB phosphorylation in the host brain. Expression of caCSF1R provided transplanted cells with a competitive advantage over endogenous repopulating cells, resulting in the accelerated spread of the transplants. Our results demonstrate the functional integration and therapeutic potential of microglia as vehicles for delivering neurotrophic factors to the brain in a controllable manner. Furthermore, we show that caCSF1R expression is able to enhance the spread of transplanted microglia.