Anorexia-induced pheripheral immune dysregulations activate aberrant microglial responses in the hippocampus of adolescent female rats

Anorexia nervosa (AN) is a life-threatening psychiatric disorder that mainly affects young females. Despite the clear symptomatology, the knowledge about its aetiology and risk factors is limited, treatments are ineffective and only symptomatic. AN induces severe multiorgan dysfunctions and it is known to trigger immune system dysregulation of both systemic and central origin, leading to increased risk, recurrence, and severity of infections in anorexic patients. In this scenario, our hypothesis is that the combination of food restriction and hyperactivity, i.e. AN core symptoms, induces neuro-immune dysfunctions potentially critical in worsening the course of the disease and in increasing the patients’ vulnerability to relapse. To test this hypothesis, we took advantage of the activity-based anorexia (ABA) rat model which consists in exposing female rats to the combination of food restriction and wheel access during adolescence (from postnatal day (P) 38 to P42). Animals were sacrificed at P42, at the acute phase, or at P49, after a 7-day recovery period. Trunk blood, bone marrow, and hippocampus were collected. Flow cytometry analyses were performed in blood and bone marrow cell suspensions, whereas gene expression analyses in ventral and dorsal hippocampus. Behavioural and molecular data were analysed by two-way ANOVA followed by Bonferroni or Tuckey post hoc tests. At the acute phase ABA rats showed an overall reduction of circulating CD45+ cells and a shift between B cells and granulocytes in the pool of total immune cells in the bone marrow, a condition that persisted even after body weight recovery. At central level, despite the acute manifestation of AN induced hippocampal neuroinflammation, as shown by increased Tnf-a levels in both ventral and dorsal subregions, ABA rats did not show signs of early microglial activation, since Cd11b and Iba1 mRNA levels were unaltered. Interestingly, after recovery, microglial cells shifted toward an active state, both pro-inflammatory and pro-regenerative, increasing Cd11b, Iba1, Cd68 and Trem2 levels, only in the ventral hippocampus. Conversely, in the dorsal subregion, after 7 days of recovery ABA rats showed increased levels of Cd68 gene expression, an effect that suggest a persistent inflammatory and phagocytic activity of microglial cells. Overall, our results suggest that AN-induced alterations in the peripheral immune response might cause endurable central dysfunctions throughout the activation of an aberrant late hippocampal microglial response, which mainly involve the ventral hippocampus, indeed known to be critically implicated in vulnerable phenotypes. Together, these data may represent a potential mechanism underpinning the increased risk of susceptibility to infections and of the transition toward a severe phenotype in the long range. Sponsored by: Cariplo Foundation (2023-1003)