Autism spectrum disorder (ASD) is a complex neurodevelopmental condition in which affected individuals experience difficulties in social communication and exhibit restricted, repetitive patterns of behavior or interests. A growing body of research suggests that neurobiological changes, particularly abnormalities in dendritic spines, tiny protrusions on nerve cells where synapses form, may be a hallmark of ASD. In particular, studies have found an unusually high number of these spines in individuals with autism. This overabundance of synaptic connections could disrupt normal communication pathways in the brain, potentially contributing to the behavioral and cognitive features seen in ASD.
Under normal circumstances, the brain undergoes synaptic pruning, a process involving the removal of unnecessary or weak synaptic connections to make way for more efficient neural networks. This pruning is crucial during early development and adolescence. Microglia, the resident immune cells in the brain, play a critical role in this process. In addition to defending the brain against infection or injury, they act like gardeners by trimming away excess synapses to help shape healthy neural circuits. However, studying how microglia function in the human brain, especially in individuals with autism, has proven difficult. Unlike in animal models, directly observing and measuring microglial activity in living humans poses technical and ethical challenges, leaving many questions about their precise role in ASD unanswered.
To study immune-related mechanisms in ASD, researchers used macrophages—immune cells derived from blood monocytes—as stand-ins for brain microglia. Then, they differentiated the macrophages into two subtypes using specific colony-stimulating factors (CSFs): granulocyte-macrophage CSF (GM-CSF) induced a pro-inflammatory "M1-like" phenotype, while macrophage CSF (M-CSF) induced an "M2-like" phenotype associated with tissue repair and immune regulation. To assess the ability of macrophages to clear synaptic material, the researchers introduced synaptosomes—fragments of neuronal connections—generated from human induced pluripotent stem cells (hiPSCs). The study entailing the breakthrough results was published online in the journal Molecular Psychiatry on April 4, 2025.
The study found that M-CSF-induced macrophages (M-CSF MΦ) from typically developed individuals were more efficient at phagocytosis—engulfing and clearing synaptosomes—compared to GM-CSF-induced macrophages (GM-CSF MΦ). However, when derived from individuals with ASD, the M-CSF MΦ exhibited a significantly reduced ability to perform phagocytosis. This impairment in synaptic phagocytosis was associated with lower expression of the CD209 gene, which may play a critical role in the ability of macrophages to phagocytose synaptic proteins. These findings suggest that dysfunctional phagocytosis could contribute to the synaptic pruning deficits seen in ASD, with the CD209 gene potentially serving as a molecular mediator.
"This study is the first to reveal lower phagocytosis capacity of synaptosomes in ASD-M-CSF macrophages compared to typically developed-M-CSF macrophages, with a correlation to CD209 gene expression," said Dr. Michihiro Toritsuka, Senior Assistant Professor at the Division of Transformative Psychiatry and Synergistic Research, International Center for Brain Science, Fujita Health University School of Medicine, Japan, and lead author of the study.
This pioneering research adds to a growing body of evidence implicating immune dysfunction in the neurodevelopmental alterations of ASD. While previous postmortem and imaging studies have reported increased dendritic spine density and hyperconnectivity in the brains of individuals with ASD, this study provides the first direct evidence of impaired synaptic pruning activity in human immune cells outside the brain.
Given these findings support the idea that dysfunctions in synapse elimination may extend beyond microglia to peripheral immune cells such as macrophages, Dr. Manabu Makinodan, who is the Professor at the same institution and co-corresponding author of the study, noted, "If the decreased phagocytosis capacity of synaptosomes and decreased CD209 expression are similarly identified in the microglia of individuals with ASD in future studies, it may lead to more effective drug discovery targeting core symptoms of ASD."
By identifying a measurable impairment in macrophage function associated with ASD, this research opens a new avenue for understanding the immune-synapse interface in autism, which can find potential applications in therapies aimed at restoring proper phagocytic function, which could be a promising future direction.
