Autism spectrum disorder (ASD) is a neurodevelopmental disorder with hallmark symptoms including social deficits, communication deficits and repetitive behaviors. Accumulating evidence suggests a potential role of the immune system in the pathophysiology of ASD. The complement system represents one of the major effector mechanisms of the innate immune system, and regulates inflammation, and orchestrates defense against pathogens. However, the role of CNS complement system in ASD is not well understood. In the present study, we found a significant increase in C2, C5, and MASP1, but a decrease in C1q, C3, and C4 mRNA levels in the middle frontal gyrus of ASD subjects compared to controls. Significant decreases in the mRNA levels of 2 key proinflammatory cytokines, IL-17 and IL-23 were observed in ASD subjects. Our study further demonstrated a strong association of complement genes with IL-17 and IL-23, suggesting a possible role of the complement system in immune dysregulation in ASD. We observed significant associations between complement components and abnormality of development scores in subjects with ASD. In rodents, C3 knockdown in the prefrontal cortex induced social interaction deficits and repetitive behavior in mice. Together, these studies suggest a potential role of C3 in the pathophysiology of ASD.

Keywords:
Complement system, Behavior, Rodents, Autism spectrum disorder, Immune system
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