Mouse Bone Marrow-derived Microglia-like Cells Secrete Transforming Growth Factor-ß1 and Promote Microglial Aß Phagocytosis and Reduction of Brain Aß.

Accumulation of amyloid-ß (Aß) in brain tissue contributes to the pathophysiology of Alzheimer's disease (AD). We recently reported that intrahippocampal transplantation of mouse bone marrow-derived microglia-like (BMDML) cells suppresses brain amyloid pathology and cognitive impairment in a mouse model of AD. How these transplanted cells interact with resident microglia remains unknown. In the present study, we evaluated the effects of cytokines secreted from mouse BMDML cells on cultured mouse microglia. Conditioned medium from BMDML cells increased microglial Aß phagocytosis. High levels of transforming growth factor-ß1 (TGF-ß1) were present in the conditioned medium, and BMDML cells and microglia expressed Tgf-ß1 mRNA and TGF-ß receptor type 1 (TGF-ßR1) protein, respectively. BMDML conditioned medium also induced microglial Smad2/3 phosphorylation. A TGF-ßR1 inhibitor suppressed Smad2/3 phosphorylation and promotion of microglial Aß phagocytosis induced by conditioned medium. Recombinant mouse TGF-ß1 similarly increased microglial Aß phagocytosis and induced Smad2/3 phosphorylation, which were suppressed by the TGF-ßR1 inhibitor. Brain TGF-ß1 levels and resident microglial TGF-ß1R expression were increased by intrahippocampal injection of BMDML cells in a mouse model of AD. Cotreatment with the TGF-ßR1 inhibitor suppressed the ability of transplanted BMDML cells to increase microglial TGF-ß1R expression and decrease hippocampal Aß levels. Taken together, these findings suggested that transplanted BMDML cells secreted TGF-ß1 to stimulate Aß phagocytosis by resident microglia and decrease brain Aß pathology.

Peripheral Blood-Derived Microglia-Like Cells Decrease Amyloid-ß Burden and Ameliorate Cognitive Impairment in a Mouse Model of Alzheimer's Disease.

Amyloid-ß (Aß) accumulation in the brain triggers the onset of Alzheimer's disease (AD), and its prevention and elimination are high priorities for anti-AD therapeutic strategies. Microglia, the resident immune cells in the brain, promote Aß clearance by phagocytosis. Previously, we demonstrated that injection of primary cultured rat microglia and mouse bone marrow-derived microglia-like cells into the brain decreases the level of Aß and that intrahippocampal injection of these cells ameliorates cognitive impairment in a mouse model of AD. To advance this cell therapeutic strategy to the clinical stage, less invasive ways of preparing autologous microglia-like cells from elderly patients are required. In this study, we demonstrated that hematopoietic stem cells mobilized from the bone marrow to peripheral blood by administering granulocyte colony-stimulating factor and a CXCR4 antagonist to mice differentiated into microglia-like cells upon stimulation with colony-stimulating factor 1 and interleukin-34. The peripheral blood-derived microglia-like (PBDML) cells expressed microglial markers and engaged in Aß phagocytosis. Although PBDML cells were in an anti-inflammatory state under nonstimulated conditions, they expressed mRNAs encoding proinflammatory cytokines following lipopolysaccharide treatment. PBDML cells injected into the hippocampi of a mouse AD model survived for at least 36 days while phagocytosing Aß, contributed to a reduction in brain Aß burden, and ameliorated cognitive impairment in the mice. These results strongly suggest that PBDML cells are a promising source for the development of a novel cell therapy against AD.