A recent review of studies has highlighted the growing significance of inflammatory biomarkers in understanding the complexities of major depressive disorder (MDD). Increasing evidence points to inflammation as a central element in the development and severity of depression, with specific biomarkers linked to symptoms and potential treatments. The findings were published in the journal BJPsychOpen.
The review, which analysed 44 studies published between 2020 and 2024, found strong associations between inflammatory markers and the presence of depressive symptoms. These signs, like cytokines such as interleukin-6 (IL-6), tumour necrosis factor-alpha (TNF-α), and C-reactive protein (CRP), not only show that someone is depressed, but they also help guess how bad their symptoms will be. Elevated levels of IL-6 and CRP were particularly associated with an increased likelihood of depressive symptoms such as anhedonia and a lack of motivation.
Walter Paganin, PhD, a researcher at the University of Rome, said: “The primary motivation for this study was to explore in greater depth the connection between inflammatory biomarkers and depression. Despite growing evidence that inflammation plays a crucial role in major depressive disorder, much remains to be understood about the underlying mechanisms and how biomarkers such as IL-6, CRP, TNF-α, IL-1β, and others influence both the severity of symptoms and the response to treatment, especially in patients with difficult-to-treat depression.”
The role of these biomarkers extends beyond diagnosis. The review also suggested that inflammatory markers could potentially serve as a tool to guide treatment strategies for MDD. For example, patients exhibiting elevated levels of certain inflammatory markers, such as IL-6, were found to have a poorer response to traditional antidepressant medications. This implies that inflammation may play a role in treatment-resistant depression, pointing towards the need for more tailored therapeutic interventions. Some studies have indicated that anti-inflammatory treatments, including the use of non-steroidal anti-inflammatory drugs (NSAIDs), might benefit patients who do not respond well to standard antidepressants.
Paganin further noted the implications of these findings: “The key findings confirmed that elevated levels of inflammatory biomarkers are closely associated with increased severity of depressive symptoms. However, an important discovery was the discrepancy between these biomarkers’ levels in peripheral blood and those detected in the central nervous system. This highlights the need for more precise measurement techniques to better understand central versus peripheral inflammation. The implications for personalised medicine are significant, as correctly identifying these biomarkers could pave the way for targeted anti-inflammatory therapies for patients who do not respond to conventional treatments.”
The review highlighted how inflammation can affect neurotransmitter systems that are implicated in depression. High levels of pro-inflammatory cytokines such as IL-1β, IL-6, and TNF-α were shown to reduce the availability of serotonin, a key neurotransmitter linked to mood regulation. Enzymes like indoleamine 2,3-dioxygenase help with this by sending tryptophan, which is a building block for serotonin, down a different path. The resulting biochemical changes lead to increased production of kynurenine, which has been associated with exacerbated depressive symptoms.
In addition to altering serotonin levels, inflammatory processes can also lead to changes in dopamine and norepinephrine systems, contributing to mood disturbances and cognitive symptoms often observed in depression. Studies within the review noted that inflammation may impair neuroplasticity, affecting the brain’s ability to adapt and form new neural connections, especially in areas of the brain responsible for mood regulation.
Paganin outlined the direction for future research: “Future projects will focus on deepening the study of central inflammatory biomarkers, utilising more advanced techniques to directly measure inflammation levels in the central nervous system. The goal is to determine whether the differences between peripheral and central biomarkers can be narrowed and how this data can be used to develop more effective treatments. Additionally, another research line will explore the combination of anti-inflammatory therapies with traditional antidepressants to determine if therapeutic efficacy can be enhanced in patients with treatment-resistant depression.”
One of the significant insights from the review was how demographic factors like age and gender might influence the inflammatory response in depression. Women and individuals over the age of 50 were found to exhibit higher levels of inflammatory markers, which may correlate with a greater risk for developing or worsening depressive symptoms. This phenomenon, known as “inflammaging”, reflects the increased inflammatory activity that comes with ageing and chronic stress exposure.
