Affiliations : The Douglas Institute, McGill, Canada
Journal reference: doi: 10.1016/j.biopsych.2019.11.017
Summary: What if a blood test could help better identify patients with depression? In this summary Dr. Maitra explains how a higher white blood cell count is implicated in depression and the potential avenues for future research.
Context: depression can have different manifestations
Depression is a serious, debilitating mental illness affecting around 300 million people worldwide. Severe depression increases the risk of suicide and is a leading cause of disability globally. Thus, there is an urgent need to better understand the causes of depression and to design effective treatments. In addition to depressed or “low” mood, symptoms of depression can include lack of pleasure in any activity, feelings of hopelessness, thoughts of suicide, and even disruptions in sleep, appetite, and energy levels. However, not every depressed person will show all of these symptoms. Moreover, antidepressant medications – used as the first-line of treatment for depression, are not effective in around one-third of patients. Given the variability in the symptoms of depression and treatment response, recent research has tried to identify subtypes of depressed patients, with the hope of designing more effective specific treatments for each patient group.
Introduction: inflammation in the blood of depressed patients?
In this study, the authors wanted to identify whether there are sub-groups of depressed patients with indications of inflammation in their blood. Inflammation is a defense response of the body which is usually activated during injury or infection and involves an increase in the numbers of white blood cells and the levels of certain proteins called “pro-inflammatory cytokines” in the blood. In some chronic health conditions, such as autoimmune disorders or diabetes, evidence of chronic inflammation may be present even in the absence of infection or injury. In depression, previous studies found increased levels of certain types of white blood cells and increased levels of some pro-inflammatory cytokines, such as C-reactive peptide (CRP) and interleukin-6 (IL-6), in the blood of patients compared to non-depressed subjects.
Material & Method: Counting blood cells in depressed patients
The authors of this study counted the numbers of 14 different types of blood cells in samples from over 200 patients who were currently depressed or had a history of depression and compared these counts to those from around 80 subjects with no history of depression. Among the depressed patients there were more women than men, which is not surprising since depression is more prevalent among women. Most of the depressed subjects were taking antidepressant medication.
Results : Higher number of white blood cells is correlated with higher severity of depression symptoms
The numbers of three specific types of white blood cells were higher on average in the depressed group compared to the control group, similar to findings from previous studies. CRP and IL-6 levels were also higher on average in the depressed subjects, consistent with previous research. Moreover, the counts of certain white blood cell types were positively correlated with depression symptom severity. However, it is important to note that using the blood cells counts alone is not very effective at predicting whether a person is depressed or how depressed they are, because there is still overlap between depressed and non-depressed subjects.
The authors also sub-divided the depressed subjects into groups based on the numbers of blood cells, because they expected variability even among the depressed subjects in terms of their levels of inflammatory markers. They found that the group of depressed patients with higher numbers of white blood cells also had higher severity of depression symptoms as well as higher CRP and IL-6 levels.
Take home message
Overall, the results suggest that depressed subjects may have increased numbers of certain subtypes of white blood cells on average compared to non-depressed subjects. Further, among depressed subjects there is variability in terms of the counts of various white blood cell types and increased counts seem to be associated with more severe symptoms. The findings of this study add to our understanding of the role of inflammation in depression and could help guide decisions about treatment in the future. But, as the authors themselves point out, further work would need to be done with larger numbers of subjects before we get a complete picture of how the numbers of different white blood cell types are altered in sub-groups of depressed patients.