Microglial Replacement as a Drug Delivery Method for the Brain

Microglia, the central nervous system's (CNS) resident immune cells, show promise as vehicles for treating intractable neurological disorders. Microglia can be transiently ablated using a Colony Stimulating Factor 1 (CSF1r) inhibitor and can quickly repopulate their environment. They can also be genetically engineered in vitro. Given these properties, replacing native microglia with engineered ones could bypass the blood-brain barrier and revolutionize the way drugs are delivered to the CNS. However, most current strategies for microglia replacement use radiation or transgenic models, limiting clinical applicability. Here, without radiation or transgenics, we successfully replaced microglia within a portion of forebrain tissue of an adult immunocompetent mouse. These microglia were engineered to overexpress Brain-Derived Neurotrophic Factor (BDNF), for which we not only measured an increase within the transplanted region of tissue, but also demonstrated increased TrkB phosphorylation. These transplanted microglia transcriptionally mirror endogenous repopulating microglia, retain immune functions, and effectively deliver BDNF to the brain parenchyma, demonstrating significant promise for clinical application. Single Cell RNA Sequencing of Cx3Cr1-Cre Csf1r fx/fx;DTA host animals transplanted with microglia infected with a lentivirus expressing a BDNF construct, or microglia without viral infection. These 4 mice (2 virus 2 no virus) are compared to 4 age matched controls with the same genetic background as the transplanted microglia without transplantation (WT)