Major depressive disorder (MDD) is a chronic mental illness, with 60% lifetime risk of recurrence after the first MDD episode. Despite available treatment options for MDD, only about half to two-thirds of patients respond to first-line antidepressant treatment, and only 30% to 45% of patients achieve remission. Scholars assume that this low remission rate and high rate of treatment resistance are due to the polyetiological nature of the disease, the heterogeneity of the clinical picture of depression, and the lack of biomarkers to stratify MDD subtypes. The aetiology of MDD, although researched extensively, remains unclear. None of the known mechanisms alone explains the pathogenesis of depression, meaning that the interplay of several factors contributes to the development of MDD. Accumulated scientific evidence has supported the importance of the immune system in the etiopathogenesis of MDD. Until now, the cause of the low-grade inflammation observed in this subgroup of MDD patients has been unclear. In the proposed study, the investigators will test a new hypothesis of the immune theory of the development of MDD: the endotoxin hypothesis of neurodegeneration. This hypothesis states that endotoxin, causes or contributes to neurodegeneration. Blood plasma levels of LPS are normally low but are elevated during infections, gut inflammation, gum disease, and neurodegenerative diseases. Dysbiosis may promote increased intestinal permeability ("leaky gut"), which leads to bacterial translocation across the intestinal barrier and into the circulation, thus forming of LPS and LPS-binding protein complex, which triggers the secretion of cytokines. Data suggest that LPS-induced peripheral inflammation can activate neuroinflammation. However, it is not known whether a low-level persistent presence of LPS in the circulatory system can cause low-grade chronic neuroinflammation leading to neurodegeneration and/or symptoms of MDD. Based on existing preclinical and clinical research data, the investigators hypothesise that an increase in blood plasma endotoxin and peripheral cytokines induce BBB dysfunction, neuroinflammation and neurodegenerative processes in specific etiologically relevant structures of the brain and cause clinical manifestation of depressive symptoms and cognitive damage. In this study the investigators are also going to investigate the effects of single nucleotide polymorphisms of four genes in relation to blood plasma endotoxin and peripheral cytokines concentrations and clinical manifestation of MDD.

Inclusion Criteria:

• ≥18 years old
• signed informed consent
• patients diagnosed with MDD (according to the WHO's [2019] International Classification of Diseases and related health problems categorization of mental disorders) for MDD group.

Exclusion Criteria:

• diagnosis of other mental disorders during the past one-year period (for MDD group).
• diagnosis of any mental disorders within the past one-year period, previous suicide attempt, or current suicide risk identified in the study (for control group).
• diagnosis of somatic diseases that may affect changes in inflammatory factors in the body (for both groups)

The study is being conducted at the hospital of the Lithuanian University of Health Sciences Kaunas Clinics Psychiatry Clinic (inpatient department), and the Nervous System Diseases Outpatient Department.