Project Summary Maternal infection during pregnancy increases the risk of neuropsychiatric disorders. Although maternal immune activation during pregnancy is a strong risk factor for neurodevelopmental disorders, most cases of maternal infections do not result in neuropsychiatric disorders. However, maternal immune activations appear to act as ‘priming events’, sensitizing the immune system and increasing susceptibility to subsequent perturbations. Early- life adversity, such as maltreatment or neglect, increase the risk for stress-related disorders. Epidemiological studies have found enhanced susceptibility to a second ‘hit’ of adversity stress. Indeed, two-hit models of maternal immune activation and stress induce robust alterations in offspring behavior. Rodent models of maternal immune activation typically induce immune activation through infectious agents such as bacteria or double-stranded RNA. However, some of the prenatal infectious agents that are most strongly linked to the risk for neurodevelopmental disorders are single-stranded viruses like rubella and influenza. The recognition of single-stranded RNA by toll-like receptor 7 (TLR7) initiates a robust immune response. Although the use of single-stranded mimetics that stimulate TLR7 are relatively underutilized in rodent maternal immune activation models, recent data suggest that maternal TLR7 activation during pregnancy induces a robust maternal immune response and impacts offspring neurodevelopment and behavior. Here, we will use a model of maternal TLR7 stimulation by the single-stranded mimetic imiquimod in combination with early postnatal resource deprivation stress through caging in limited bedding and nesting environments. The central hypothesis of this R21 proposal is that a novel two-hit model of maternal immune activation using TLR7 activation by imiquimod during pregnancy in combination with early postnatal resource deprivation stress through LBN results in sex-specific neurodevelopmental susceptibilities. In Aim 1, we will determine whether prenatal TLR7 stimulation induces maternal, placental, and offspring brain inflammatory responses in a sex- biased manner. We will assess whether maternal immune activation induces placental macrophage and fetal brain microglial priming. In Aim 2, we will determine whether microglial morphological alterations and pruning of excitatory neurons in the anterior cingulate cortex, nucleus accumbens, paraventricular nucleus of the hypothalamus, and ventral tegmental areas is induced by prenatal TLR7 stimulation in combination with early postnatal resource deprivation stress by LBN. Determining whether this two-hit model of maternal TLR7 stimulation in combination with early-life resource deprivation stress results in inflammatory alterations within placental macrophages and fetal brain microglia, along with altering microglial phagocytosis of neuronal circuits.