Major depression is the principal comorbidity of GAD. After the first reports by Maes et al. about immune dysfunction in the early 1990s, the role of chronic low-grade inflammationand immune dysregulation in depression is well characterized. Little is known, however, about the possible relation between anxiety and inflammation.
In the case control study, published in Brain, Behavior, and Immunity, Ruihua Hou and colleagues, from the Faculty of Medicine, University of Southampton, UK analyzed the pro- and anti- inflammatory cytokine profiles, including serum levels of IFN-γ and TNF-α, and IL-10 in patients with a primary diagnosis of GAD.
IFN-γ is crucial for the activation of the immune system against intracellular pathogens and for antitumor immunity, whereas TNF-α is a major pro-inflammatory cytokine, involved in the acute phase reaction. The type 1 or T helper (Th)1-related cytokines such as IFN-γ and TNF-α also stimulate the synthesis of nitric oxide and other inflammatory mediators that drive chronic delayed type inflammatory responses.
On the other hand, IL-10 is a potent anti-inflammatory cytokine that down-regulates Th1 responses but potentiates humural immunity and Th2 responses, directed against extracellular pathogens and involved in allergic conditions.
Hou R et al. found that patients with GAD, after controlling for age, gender, and BMI, had high levels of IFN-γ and TNF-α, but low IL-10 serum levels when compared to healthy control subjects. These differences remained statistically significant after adjustment for co-morbid depression. Thus, the authors speculate that the inflammatory response in patients with GAD is specific for this pathology.
The study demonstrates a distinct cytokine anxiety phenotype independent of the presence and degree of depression as well as an altered cytokine balance in patients with generalized anxiety disorder. The authors suggest that the pro-inflammatory state is related to altered activity of an enzyme involved in the metabolism of tryptophan, leading to degradation of serotonin in patients with GAD, with a mechanism similar to the one observed in major depression.
It appears that this is the first case-controlled study identifying altered pro- and anti-inflammatory cytokine profiles in patients with GAD. Further studies may be useful to clarify some unresolved issues, such as what is the cellular source of these cytokines, and if there are other cytokines, which may have important roles in this disease.
Additionally, better understanding of the mechanisms by which cytokines act as intermediaries between the central nervous and the immune systems, could contribute to the discovery of new biomarkers and perhaps providing the basis for the development of new therapeutic approaches for anxiety.
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