Transnasal transplantation of human induced pluripotent stem cell‐derived microglia to the brain of immunocompetent mice

Abstract

Microglia are the resident immune cells of the brain, and play essential roles in neuronal development, homeostatic function, and neurodegenerative disease. Human microglia are relatively different from mouse microglia. However, most research on human microglia is performed in vitro, which does not accurately represent microglia characteristics under in vivo conditions. To elucidate the in vivo characteristics of human microglia, methods have been developed to generate and transplant induced pluripotent or embryonic stem cell‐derived human microglia into neonatal or adult mouse brains. However, its widespread use remains limited by the technical difficulties of generating human microglia, as well as the need to use immune‐deficient mice and conduct invasive surgeries. To address these issues, we developed a simplified method to generate induced pluripotent stem cell‐derived human microglia and transplant them into the brain via a transnasal route in immunocompetent mice, in combination with a colony stimulating factor 1 receptor antagonist. We found that human microglia were able to migrate through the cribriform plate to different regions of the brain, proliferate, and become the dominant microglia in a region‐specific manner by occupying the vacant niche when exogenous human cytokine is administered, for at least 60 days.

Strategies for Manipulating Microglia to Determine Their Role in the Healthy and Diseased Brain

“…We were able to show that the iPSMG proliferated and matured morphologically in the mouse brain. Additionally, the iPSMG were found to have a more complex in vivo morphology than endogenous mouse microglia [137,138]. Although it remains unclear how this morphological complexity could affect different brain functions, we speculate that the longer and more complex processes may enable the iPSMG to actively survey and interact with more synapses, thus influencing neurons.…”

“…Furthermore, studies have now succeeded in investigating the in vivo characteristics of iPSMG by injecting the cells into the brains of immunodeficient adult or neonatal mice. For instance, we recently depleted the endogenous microglia in immunocompetent mice using a CSF1R antagonist, and then transplanted human microglia into the mouse brain via a non-invasive, transnasal route [137]. We were able to show that the iPSMG proliferated and matured morphologically in the mouse brain.…”

Microglia and Macrophages in Neuroprotection, Neurogenesis, and Emerging Therapies for Stroke

“…Animals that received iPSCs also performed better at the cylinder test at four and seven weeks after the ischemic event when compared to animals injected with only the vehicle, suggesting the ability of iPSCs to ameliorate functional deficits in a stroke-injured aged brain [ 164 ]. Additionally, it has been shown that IPSC-derived microglia can be transplanted into the brain through a transnasal route, offering a noninvasive method to deliver potential therapies [ 165 ].…”

Replacement of Mouse Microglia With Human Induced Pluripotent Stem Cell (hiPSC)-Derived Microglia in Mouse Organotypic Slice Cultures

“…To study the function of hiPSC-MG in vivo , researchers have developed several methods to transplant hiPSC-MG into a living mouse brain (Abud et al, 2017 ; McQuade et al, 2018 ; Svoboda et al, 2019 ; Xu et al, 2020 ; Parajuli et al, 2021 ). In addition to the transplantation of hiPSC-MG, transplantation of human induced hematopoietic progenitor cells (hiHPCs), which are progenitor cells of microglia differentiated from hiPSCs, has also been developed (Hasselmann et al, 2019 ).…”

“…Above all, experimentally, it is difficult to increase the number of mice transplanted with a stable number of hiPSC-MG. The replacement rate of microglia among brain regions is highly variable, with 80% of microglia replaced by hiPSC-MG in the hippocampus, while almost no hiPSC-MG were present in the cortex 60 days after intranasal administration of hiPSC-MG (Parajuli et al, 2021 ). Furthermore, many in vivo transplantation models use immunodeficient mice, such as the MITRG, Rag2−/−, and NSG-Quad mouse lines, to avoid immune rejection of transplanted hiPSC-MG (Abud et al, 2017 ; McQuade et al, 2018 ; Hasselmann et al, 2019 ; Svoboda et al, 2019 ; Xu et al, 2020 ).…”