Peripheral and central dysregulation of the immune system in the Activity-Based Anorexia model

Anorexia nervosa (AN) is a life-threatening psychiatric disorder characterized by self-imposed starvation, distorted body image, and intense fear of gaining weight, often resulting in compulsive exercise. Despite its clear symptomatology, knowledge about its etiology is limited and treatments are ineffective. It is well established that AN leads to multiorgan dysfunction involving also the immune system. Indeed, in the early phase of AN, patients show remarkable resistance to sickness and infections while, in the advanced stages, more frequent and severe infections occur. Our hypothesis is that AN-derived alterations of the peripheral immune system paralleled by central dysfunctions, through the activation of an aberrant microglia response, might contribute to the persistence of the AN phenotype. To test this hypothesis, we took advantage of the activity-based anorexia (ABA) rat model: female rats were exposed 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 hippocampus. Behavioral and molecular data were analyzed by two-way ANOVA followed by Bonferroni or Tuckey post hoc tests. From a behavioral point of view, rats exposed to the combination of food restriction and exercise show increased running wheel activity and severe body weight loss, thus mirroring key features of the anorexic phenotype. Flow cytometry analyses performed on trunk blood and bone marrow highlighted a persistent reduction of circulating CD45+ cells and a shift between granulocytes and B cells in the pool of total immune cells in the bone marrow of ABA rats, resulting in an imbalance of innate and adaptive immunity. Gene expression analyses in ventral hippocampus, a brain subregion well-known for its role in vulnerable phenotypes, revealed that the acute manifestation of AN induced neuroinflammation, as shown by increased TNF- levels. This effect resulted in a change of microglial transcriptional profile after the recovery period including an increase in Trem2, Iba1, Cd11b and Cd68 expression levels. Overall, our results indicate that alterations in the peripheral immune response induced by AN are paralleled by endurable central dysfunctions through the activation of an abnormal late microglial response in the ventral region of the hippocampus. Taken together, these findings may point to a possible mechanism underlying the increased vulnerability to infection and long-term progression to a more severe disease phenotype. Supported by: Cariplo foundation 2023-1003