Early-life IL-4 administration induces long-term changes in microglia in the cerebellum and prefrontal cortex

Abstract

Microglia are crucial for brain development and their function can be impacted by postnatal insults, such as early-life allergies. These are characterized by an upregulation of interleukin (IL)-4 levels. Allergies share a strong comorbidity with Autism Spectrum Disorders (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD). We previously showed that early-life allergic asthma induces hyperactive and impulsive behaviors in mice. This phenotype was reproduced in animals administered with IL-4 in the second postnatal week. Mechanistically, elevated IL-4 levels prevented microglia-mediated engulfment of neurons in the cerebellum, resulting in a surplus of granule cells and consequent dysfunction in cerebellar connectivity. Here, we aimed to further understand the impact of early IL-4 administration in microglia of the cerebellum and the prefrontal cortex (PFC), two brain regions with protracted developmental programs and susceptible to immune system malfunction after birth. While IL-4 administration induced differential short-term effects on microglia in the cerebellum and PFC, both regions presented similar microglial features in adult mice. Although Sholl analysis did not reveal significant alterations in overall microglia morphology at postnatal day (P)10, the density of microglia was decreased in the cerebellum at this age, especially in the granular layer (GL), but remained unaltered in the PFC. Interestingly, the presence of microglia with phagocytic cups, morphological features important for whole-cell engulfment, was decreased in both regions. When assessing the long-term consequences of IL-4 administration, cerebellar and PFC microglia were hypo-ramified and exhibited increased overall density. Importantly, microglia alterations were exclusive to the GL of the cerebellum and the infralimbic region of the PFC. Our results show that postnatal elevated levels of IL-4 impair the percentage of microglia engaged in cell clearing in two brain regions with protracted developmental programs. Interestingly, IL-4-exposed microglia adapt a similar phenotype in the adult cerebellum and PFC. Our data suggest that this early-life increase in IL-4 levels is sufficient to elicit long-lasting alterations in microglia, potentially increasing cell susceptibility to later insults.