ROLE OF STRESS EARLY IN LIFE AND OF INFLAMMATION IN THE RISK TO DEVELOP DEPRESSION LATER IN LIFE

Major depressive disorder (MDD) is a severe mental illness affecting approximately 280 million people worldwide (Vos et al., 2020). It is characterized by physical symptoms such as fatigue, weight and appetite loss, along with core features like anhedonia, diminished motivation, sleep and cognitive disturbances, and emotional symptoms including excessive guilt (Cui et al., 2024). Increasing evidence links depression to both systemic and central immune alterations. Indeed, one of the most consistent biological findings in individuals with MDD is the presence of a chronic, low-grade inflammatory state, reflected by elevated circulating levels of inflammatory biomarkers such as interleukin-6 (IL-6), tumor necrosis factor α (TNFα), and C-reactive protein (CRP) (Osimo et al., 2018). Interestingly, elevated levels of peripheral and central pro-inflammatory cytokines are consistently reported not only in individuals with depression but also in those exposed to early adversities, supporting the hypothesis that immune dysregulation may serve as a key mediator between early environmental insults and later psychopathology. However, although the role of early-life adversity in shaping mental health outcomes is well recognized, the biological mechanisms underlying this association remain incompletely understood. This thesis aims to elucidate how prenatal stress (PNS) impacts maternal health and programs vulnerability to psychiatric disorders in the offspring. Specifically, I focused on maternal inflammation, placental immune signaling, and long-term neuroimmune outcomes. Using a multi-level approach that integrates behavioral, molecular, immunological, and transcriptomic analyses, I investigated the long-term effects of early stress exposure in a well-established PNS model routinely used in my laboratory (Creutzberg et al., 2023; Marchisella et al., 2021). My work was organized into four interconnected aims. First, I assessed how gestational stress affects maternal responses during pregnancy and the early postpartum period. Behavioral analyses revealed that PNS impairs maternal care, with significant alterations in pup-directed behaviors. Concurrently, I observed neuroendocrine dysregulation in the hypothalamic-pituitary-adrenal (HPA) axis and oxytocin/vasopressin systems, accompanied by elevated circulating pro-inflammatory cytokines such as TNFα and mitochondrial dysfunction both during pregnancy and in the postpartum. These findings suggest that maternal physiology and behavior are both disrupted, potentially influencing fetal development via biological and caregiving pathways. Secondly, given the placenta’s central role in fetal programming, I investigated whether maternal immune activation alters the intrauterine environment. Placental analyses revealed increased expression of inflammatory mediators and mitochondrial dysfunction pointing to a dysregulated immune-metabolic environment. These changes likely mediate the transfer of maternal stress signals to the fetus, shaping brain development and immune trajectories. To examine long-term outcomes, I assessed behavioral and immune phenotypes in adult offspring. Animals exposed to PNS displayed a range of stress-related behaviors, including anhedonia, cognitive deficits, and impaired social interactions. Based on their social behavior, I stratified animals into resilient and vulnerable subgroups, allowing me to dissect the biological correlates of individual susceptibility in the stress response. Vulnerable offspring showed elevated peripheral cytokines, increased corticosterone levels, and mitochondrial dysfunction in blood cells, linking behavioral impairments to systemic inflammation and increased stress response. Finally, I explored neuroimmune alterations in the ventral and dorsal hippocampus and prefrontal cortex of adult offspring exposed to prenatal stress, focusing on microglial activity and mitochondrial integrity. Vulnerable animals exhibited increased microglial activation, alongside mitochondrial dysfunction in the hippocampus. Transcriptomic profiling in the ventral hippocampus, a key region for emotional regulation, revealed altered expression of genes related to immune signaling, synaptic function, glucocorticoid responses, and energy metabolism. Importantly, changes identified in peripheral blood and in the brain reflect the dysregulation of the same biological mechanisms, indicating that circulating biomarkers may mirror central neurobiological states and serve as accessible indicators of risk. TNFα emerged as a central molecular node: its expression was consistently elevated in maternal plasma and both brain and blood of vulnerable offspring. Moreover, TNFα levels correlated with social impairments and stress sensitivity, highlighting its relevance as both a biomarker and a potential mechanistic driver of neurodevelopmental vulnerability. These findings align with emerging preclinical and clinical research supporting TNFα inhibition as a therapeutic approach for inflammation-linked depression (Bortolato et al., 2015; Brymer et al., 2019). In conclusion, this work presents a comprehensive, multi-level analysis of how prenatal stress shapes maternal and offspring biology. By integrating behavioral, endocrine, immune, and transcriptomic data across maternal, placental, and offspring levels, I identified signatures of vulnerability and resilience and highlighted TNFα as a potentially promising target for early intervention. These findings advance the understanding of psychopathology and offer insights for translational strategies aimed at mitigating the intergenerational impact of prenatal adversity.