Microbiome and immuno-metabolic dysregulation in patients with major depressive disorder with atypical clinical presentation

Highlights

• Emerging evidence suggests that metabolic dysregulation and chronic low-grade inflammation are more consistently associated with a symptom profile often referred to as “atypical” depression.

• A highly interconnected complex of adverse physiological adaptations are most likely common underlying biological substrates that dominate clinical presentations depending on individual predisposition and additional factors.

• Accumulating evidence has identified human-associated microbes as a major player in the interplay between immune and energy dysregulation, linking atypical depressive symptoms to comorbid metabolic disorders.

• Dysfunctional interplay among microbiome composition, immunity, and metabolism that predisposes to atypical depression and cardiometabolic disorders may occur early in development.

• Dysbiosis of the oral microbiome facilitates the passage of pathogenic microbes and their metabolites across the blood-brain barrier and may affect neuronal function.

Abstract

Depression is highly prevalent (6% 1-year prevalence) and is the second leading cause of disability worldwide. Available treatment options for depression are far from optimal, with response rates only around 50%. This is most likely related to a heterogeneous clinical presentation of major depression disorder (MDD), suggesting different manifestations of underlying pathophysiological mechanisms.

Poorer treatment outcomes to first-line antidepressants were reported in MDD patients endorsing an “atypical” symptom profile that is characterized by preserved reactivity in mood, increased appetite, hypersomnia, a heavy sensation in the limbs, and interpersonal rejection sensitivity. In recent years, evidence has emerged that immunometabolic biological dysregulation is an important underlying pathophysiological mechanism in depression, which maps more consistently to atypical features.

In the last few years human microbial residents have emerged as a key influencing variable associated with immunometabolic dysregulations in depression. The microbiome plays a critical role in the training and development of key components of the host's innate and adaptive immune systems, while the immune system orchestrates the maintenance of key features of the host-microbe symbiosis. Moreover, by being a metabolically active ecosystem commensal microbes may have a huge impact on signaling pathways, involved in underlying mechanisms leading to atypical depressive symptoms. In this review, we discuss the interplay between the microbiome and immunometabolic imbalance in the context of atypical depressive symptoms. Although research in this field is in its infancy, targeting biological determinants in more homogeneous clinical presentations of MDD may offer new avenues for the development of novel therapeutic strategies for treatment-resistant depression.

This article is part of the Special Issue on “Microbiome & the Brain: Mechanisms & Maladies”.