The Interaction Between CD4+ T Cells and the Blood–Brain Barrier and Its Implications for Depression

Depression is a widespread psychiatric disorder characterized by sadness, sleepSleep disturbances, fatigueFatigue, and concentration difficulties, influenced by genetic, biological, environmentalEnvironmental, and psychological factors, and exacerbated by life events and chronic diseases. Depression is treatable through psychotherapyPsychotherapy, antidepressant medications, lifestyle changes, and electroconvulsive therapyElectroconvulsive therapy (ECT) for severe cases. Research on depression aims to understand its mechanisms, develop effective treatmentsTreatment, and explore neurobiological approaches, genetic markers, and substances like ketamineKetamine and psychedelicsPsychedelics for treatmentTreatment-resistant depression. Research on neuroinflammationNeuroinflammation in depression suggests it may contribute to depression. Recent studies reveal peripheral immune cells infiltrate the brain, causing symptoms like astrogliosis and demyelination. CD4+ T cells, originating from bone marrow precursors, mature in the thymus, recognizing foreign antigens and assisting other immune cells. They differentiate into various subsets for coordinated immune responses. The TH1/TH2 axis regulates the immune system, involving the differentiation and balance of TH1 and TH2 subsets. TH1 cells promote cell-mediated immunity against intracellular pathogens, while TH2 cells promote humoral immunity. Maintaining balance is crucial for immune homeostasisHomeostasis, as imbalances can lead to diseases and allergiesAllergy. TH17 and Treg cells, both part of the CD4+ T cell family, have distinct functions. TH17 cells can cause pathogenic effects, while Treg cells regulate their functions. The TH17/Treg axis is highly reprogrammable, with Treg cells transforming into TH17-like cells. In depression, immune cells cross the BBB, a crucial brain defense mechanism, through diapedesis, aiming to migrate to the disease's location. They challenge the neurovascular unit, endothelial cells, basal membrane, pericytes, and astrocytes, ensuring optimal brain function and minimizing toxins exposure. This chapter covers in detail the interaction between various CD4+ T cells and the BBB.