PATHOPHYSIOLOGICAL ANALYSIS OF AUTISM SPECTRUM DISORDER FOCUSING ON PERIPHERAL BLOOD MACROPHAGES

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder characterized by impaired social interaction, poor communication skills, and restricted or repetitive patterns of behavior and interests. Although its etiology and pathogenesis remain unclear, recent studies suggest that inflammation and immune system abnormalities affect brain development. Microglia, the main immune cells in the brain, are known to influence brain development through the secretion of cytokines, trophic factors, and synaptic phagocytosis. However, it is difficult to directly collect and analyze microglia from living humans. Although the origins of microglia and macrophages differ, microglia are identified as tissue-resident macrophages with similar functions. Therefore, we hypothesized that examining macrophages in ASD could provide new insights into microglial function and ASD pathology. We performed our analysis using macrophages polarized from peripheral blood mononuclear cells by granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF). We previously reported that the expression of tumor necrosis factor-α (TNF-α) in differentiated GM-CSF induced macrophages (GM-CSF MΦ) was significantly higher in individuals with ASD than in typically developing (TD) individuals (Yamauchi et al., Autism Research 2021). However, since it remained unclear how this high expression of inflammatory cytokines affects the nervous system, we established a co-culture system using human induced pluripotent stem cell (hiPSC)-derived neurons and macrophages. The results showed that co-culture of GMCSF MΦ induced dendritic atrophy through the secretion of pro-inflammatory cytokines, interleukin-1α(IL-1α) and TNF-α. Macrophages derived from individuals with ASD exerted more severe effects than those derived from TD individuals. Furthermore, we demonstrated that co-administering neutralizing antibodies of TNF-α and IL-1α rescued the dendritic atrophy (Takada et al., Molecular Autism 2024). Next, we focused on synaptic phagocytosis capacity. We established an experimental system to evaluate macrophage synaptic phagocytosis by adding synaptosomes purified from hiPSC-derived neurons to polarized macrophages. We revealed that M-CSF MΦ exhibited higher phagocytosis capacity compared to GM-CSF MΦ, whereas ASD-M-CSF MΦ showed a marked impairment in phagocytosis. Additionally, we found a positive correlation between phagocytosis capacity and the expression of CD209, a C-type lectin receptor (Nishi et al., Molecular Psychiatry 2025). These results support the hypothesis of microglial abnormalities in ASD and provide new insights into the pathology of ASD.