DIRECT INDUCTION OF MICROGLIA-LIKE CELLS FROM HUMAN MONOCYTES: A NOVEL CELLULAR TOOL FOR TRANSLATIONAL RESEARCH OF NEUROPSYCHIATRIC DISORDERS
Authors: Ohgidani *Masahiro , Kato Takahiro A 
Published: February 2025

Abstract

Background
Microglia, a type of immune cells in our brain, respond rapidly to minute environmental changes and function as a center of the immune response in the brain. It has also been suggested that dysfunction and abnormal activation are involved in a variety of pathological conditions, including pain, Alzheimer's, and neuropsychiatric disorders. Unlike organic diseases, neuropsychiatric disorders such as depression and schizophrenia are unique to humans. Of course, the study of this unique human disease requires experimentation with human cells. But the hurdle is very high. Detailed analysis at the cellular level has been impossible because microglia present in the brain are difficult to biopsy. In this context, we have succeeded in developing a technique to produce microglia-like cells (induced microglia-like cells: iMG cells) from monocytes in peripheral blood.

Aims and Objectives
The purpose of this study is to elucidate the role of human microglia in neuropsychiatric disorders using iMG cells and to investigate their potential as a novel therapeutic target.

Method
In this study, patients with bipolar disorder, microglial disease, brain tumor, and chronic pain were recruited to generate iMG cells. iMG cells were generated by culturing peripheral blood monocytes in RPMI medium containing GM-CSF and IL-34 for two weeks. Using the iMG cells, we evaluated cell functions such as phagocytosis and cytokine production, and gene expression responses to various stimuli. All research related to this presentation was approved by the Ethics Committee of Kyushu University Hospital.

Results
In a series of studies, we have confirmed the utility of iMG cells as a tool for translational research. iMG cells generated from patients with microglial disease, represented by mutations in DAP12 and AIF1, which are expressed in microglia in the central nervous system, retained those gene mutations.
Furthermore, siRNA-based knockdown was also possible, and by artificially knocking down DAP12, the gene responsible for Nasu-Hakola disease, the phenotype was successfully reproduced in patients with Nasu-Hakola disease. In addition, when microglia isolated from actual brain tissue were compared with iMG cells from the same individual, the gene expression patterns were similar. These facts indicate that iMGs are helpful as model cells for human microglia.
Interestingly, in each disease, the phenotype of microglial dysfunction was different. For example, there was a decrease in phagocytosis in microglial disease, accompanied by reduced cytokine production during phagocytosis. In rapid cyclers with bipolar disorder, changes in gene expression were observed between manic and depressed states. Interestingly, the expression of CD206, which is related to the immune system, was altered. In chronic pain, the disease group showed increased inflammatory cytokines (TNF) expression when stimulated with ATP, a known neurotransmitter.

Discussion & Conclusion
We have developed a technology to generate human microglial cells ahead of iPS cells and have reported its usefulness as a translational research tool. We have also accumulated data on individual differences in drug response with a view to companion diagnostics and hope to contribute to the next generation of neuropsychiatric disease treatment.