Keywords:
Major depressive disorder; Interleukin 1 alpha, Interleukin 6; Tumor necrosis factor alpha; psychological distress; Cardiometabolic Disease
Introduction
Major depressive disorder (MDD) is a debilitating mental disorder, and a leading cause of disability globally (GBD, 2017). Individuals with MDD have a higher incidence of inflammatory illnesses such as diabetes, obesity and heart disease (Maes et al. 2011; Buckley and Abbate, 2018) and higher levels of several pro-inflammatory cytokines (Beurel et al. 2020; Osimo et al. 2020; Bhatt et al. 2022) than those without MDD. Accordingly, pro-inflammatory cytokines may contribute to the increased risk for those with MDD of cardiometabolic disease (CMD) (Celano et al. 2011). CMD is characterised by the markers of both cardiovascular disease (CVD) and metabolic syndrome (Castro et al. 2003; Srivastava, 2012), and associated with high mortality rates (WHO, 2017). In non-depressed cohorts, pro-inflammatory cytokines IL-1α, IL-6, and TNF-α have been associated with specific aspects of psychological distress, including stress, anxiety and hostility (Suarez et al. 2004; Postal et al. 2016; Barnard et al. 2019), obesity (Schmidt et al. 2014) and cardiovascular risk factors (Ridker and Rane, 2021; Urschel and Cicha, 2015). However, concurrent assessment of multiple associations between pro-inflammatory cytokines, specific psychological symptoms associated with both MDD and CVD, body mass indices associated with metabolic syndrome, measures of cardiovascular health, including sex differences, among individuals with MDD with comparison to a control cohort is lacking.
The pro-inflammatory cytokines interleukin 1 alpha (IL-1α), interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-α) are produced by mononuclear cells and macrophages and released into the blood stream in response to an immunologic challenge (Dolwati et al. 2010; Beurel et al. 2020). Each coordinates a variety of cell functions that ultimately stimulate and enhance inflammation (Bacchiega et al. 2017; Beurel et al. 2020). However, elevation of these pro-inflammatory cytokines in the absence of infection or tissue injury is considered abnormal (Dolwati et al. 2010). Cytokines can cross the blood brain barrier and affect central neural function, and increasing evidence suggests that abnormal inflammatory responses may play a role in the pathophysiology of MDD (Beurel et al. 2020). A causative role of cytokines in MDD is supported by a recent study that showed that administration of cytokines can cause depression-like behaviors and symptoms (Bhatt et al. 2022) and from evidence that inflammation-based treatments and inflammation illnesses are linked to depressed mood or depressive disorders (Dantzer et al. 2008, Capuron et al. 2009; Bhatt et al. 2022). Further, blockade of cytokine signalling is associated with mood-enhancing effects (Tyring et al. 2006; Hersey et al. 2021) and most cytokines decrease with effective anti-depressant treatment (Dahl et al. 2014; Bhatt et al. 2022). The potential mechanism underlying these effects may be due to the role of pro-inflammatory cytokines in serotonin availability. The cytokines have been shown to mediate serotonin deficiency via tryptophan degradation by indoleamine 2,3-dioxygenase, which results in decreased availability of tryptophan for cerebral serotonin synthesis (Felger and Lotrich, 2013).
Much research has reported that individuals with depression have higher pro-inflammatory cytokines than healthy controls including IL-1α, IL-1β, IL-6 and TNF- α (Simon et al. 2008; Dowlati et al. 2010; Al-Hakeim et al. 2015; Munjiza et al. 2018; Osimo et al., 2020; Bürhan-Çavuşoğlu et al. 2021). However, there are inconsistent results, with some studies showing no difference in plasma IL-6 or TNF-α between individuals with versus without MDD (Brambilla et al. 2004; Cilan et al. 2012; Cassano et al. 2017; Obermanns et al. 2021). Notably, IL-1 exists in two isoforms: IL-1α, the main intracellular isoform, and IL-1β, the main released isoform (Dinarello, 2018); these are equally potent in activating the inflammatory process (Voronov et al. 2013). Most research to-date with respect to MDD has focussed on IL-1β; however, since IL-1α is found constitutively as opposed to IL-1β being the induced form released in an inflammatory disease state (Voronov et al. 2013), it may be more relevant in MDD.
The pro-inflammatory cytokines are also associated with other psychological symptoms associated with depression including stress (Huesten and Deak, 2014; Barnard et al. 2019), hostility (Suarez et al. 2004) and anxiety (Postal et al. 2016; Bhatt et al. 2017). This is significant since these are in turn associated with CMD risk factors (Goldbacher and Matthews, 2007; Nabi et al. 2010, Ojike et al. 2016; Gowey et al. 2019), which may contribute to people with MDD being more vulnerable to developing CMD. Stress and depression are associated with higher waist circumference and poorer metabolic health outcomes generally (Gowey et al. 2019); anxiety is linked to cardiovascular disease and elevated heart rate (Olafiranye et al. 2011; pittig et al. 2013); hostility is associated with increased blood pressure (Spicer and Chamberlain, 1996), lipid dysregulation (Suarez et al. 1998), and the development of metabolic syndrome (Matthews and Salomon, 2003). Further, in relation to increased risk of CMD, pro-inflammatory cytokines are secreted by adipose tissue and higher in obese individuals (Schmidt et al. 2014), and have pro-hypertension effects (Dorrance, 2007; Finnel and Wood, 2016). Given the multiple associations between the pro-inflammatory cytokines IL-1α, IL-6 and TNF-α, and psychopathologies and measures of adiposity and cardiovascular health, elucidating these complex associations in a cohort of MDD, including comparison to healthy controls is needed.
Of interest is that inflammatory responses differ between sexes, with females showing more pronounced inflammatory responses during bacterial and viral infections than males (O’Connor et al. 2007; Klein and Flanagan, 2016; Wegner et al. 2017). A study by Wegner et al. (2017) among health individuals found that when bacterial endotoxin was administered, females had a higher increase in plasma TNF-α and IL-6 concentrations compared with males (Wegner et al. 2017). However, little research has examined sex differences in inflammatory markers in association with depressive symptoms, with inconsistent results. Serum levels of IL-6 and TNF-α were higher in depressed females than depressed males in one study (Birur et al. 2017), but there were no sex differences in IL-6 among depressed participants in another study (Davidson et al. 2009). In addition to females being at a greater risk for depression (Kessler et al. 2005) and showing greater inflammatory reactivity (O’Connor et al. 2007; Klein and Flanagan, 2016; Wegner et al. 2017), there are sex differences in antidepressant treatment response. It was reported that monoamine oxidase inhibitors and selective serotonin reuptake inhibitors are more effective in female patients (Quitkin et al. 2002), while male patients respond more favourably to tricyclic antidepressants (Kornstein et al. 2000). Accordingly, it is important to examine sex differences in pro-inflammatory cytokines and their associations with psychological symptoms, since these may be important to the aetiology of MDD and CMD risk.
Overall, despite much previous research that has investigated pro-inflammatory cytokines in people with depression (Tuglu et al. 2003; Simon et al. 2008; Al-Hakeim et al. 2015), there are limited reports on associations with both psychological symptoms (Suarez et al. 2004; Postal et al. 2016; Barnard et al. 2019) and measures relevant to increased risk of CVD and metabolic syndrome (Ridker et al. 2000; Urschel and Cicha, 2015), particularly in an MDD cohort. The present study aimed to compare the plasma concentration of pro-inflammatory cytokines (IL-1α, IL-6, TNF-α) in participants with MDD versus healthy controls and by sex, and to determine associations between these and measures of adiposity (waist circumference and BMI) and cardiovascular health (blood pressure and heart rate), and psychological symptoms (stress, depression, anxiety, hostility and global severity index of psychopathology), relevant to MDD and CMD risk. It was hypothesized that plasma concentration of these pro-inflammatory cytokines would be: 1) higher in the MDD group than healthy controls, and in females than males; 2) positively associated with waist circumference, BMI, blood pressure and heart rate, and 3) positively associated with psychological symptoms of stress, depression, anxiety and hostility.
Method:
2.1 Participants:
The method and participants of the current study have been previously published (Elgellaie et al. 2021). Participants with MDD and healthy controls were recruited by media and university advertisements. Exclusion criteria across both diagnostic groups included any use of corticosteroids, neurological illness, and substance use disorders. Depressed participants were thoroughly pre-screened to confirm that they met DSM-5 criteria for a current major depressive episode, and had not been receiving any psychological, pharmacological or somatic treatment for MDD within the last two months. All control participants had no significant history of mental health problems or diagnosed mental disorders and they were age and sex matched to MDD participants.
2.2 Measures and procedure:
The design of this study was cross-sectional. All participants attended the university clinical trials research unit. Participants were given information about the study and provided written informed consent prior to participating. Participants in the MDD group were interviewed using the Mini Neuropsychiatric Interview, version 7.0.2 for Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5) (MINI; Sheehan, 2015). All participants had their height, weight, waist circumference measured and their BMI (kg/m2) was calculated, and their blood pressure and heart rate recorded. A phlebotomist then collected a non-fasted morning (between 9:00 – 11:00 a.m.) sample of blood (10 mL) into an EDTA tube. Within 5 minutes of blood collection, 200 μL of aprotinin was added to each blood sample, which was then immediately centrifuged at 4°C and 3000 rpm for 10 minutes. Plasma aliquots were stored at −80 °C and thawed prior to hormone analysis and measurements. Plasma concentrations of inflammatory cytokines (IL-1α, IL-6, TNF-α) were measured using standard ELISA methods.
For measures of psychopathology, all participants completed the Brief Symptom Inventory (BSI) questionnaire, which consists of 53 items covering nine symptom dimensions. For the current study, the scores for the symptom dimensions of depression (e.g. feeling no interest in things), anxiety (e.g. feeling tense or keyed up), and hostility (e.g. having urges to break or smash things) were used, along with the Global Severity Index (GSI) as a measure of overall symptomatology during the past seven days (Derogatis, 1975). Good internal consistency reliability is reported for all dimensions of the BSI (Derogatis, 1975). Participants also completed the depression, anxiety and Stress Scale (DASS), which a 21 item self-report questionnaire assessing depression, anxiety and stress severity over the past week (Lovibond and Lovibond, 1995), and with high internal consistency (Brown et al. 1997). Subscale scores were summed and then doubled for comparison with the DASS 42. For the current study, each participant’s score for stress was included in analyses. Severity descriptors were calculated for both groups according to the established cut off scores for each DASS subscale (Lovibond and Lovibond, 1995).
2.3 Data statistical analysis:
Statistical analyses were conducted using ‘Statistical Package for the Social Sciences’ (SPSS, Version 26). Shapiro-Wilk Test of Normality was used to examine the normality distribution of the data. Logarithmic transformations were used if the data were not normally distributed, and outliers as identified on boxplots were removed. Two-way factorial analyses of variance (ANOVAs) were used to compare each of the plasma inflammatory cytokines levels (IL-1α, IL-6, TNF-α), and measures of adiposity, cardiac health and psychopathology symptoms between the MDD versus control groups, by sex. Spearman’s correlations were performed to determine associations between each of the plasma inflammatory cytokines (IL-1α, IL-6, TNF-α) and the measures of adiposity, cardiac health and psychopathology symptoms across both groups. Due to previously reported differences in inflammatory cytokines level by MDD diagnosis and sex, correlations were also conducted separately by diagnostic group and sex. Multiple regression analyses were conducted to determine the unique contribution of each of the pro-inflammatory cytokines and sex to the psychopathology symptoms. For all statistical tests α < .05 was considered statistically significant.
Results:
3.1 Participants’ characteristics
A total of 60 MDD participants and 60 healthy control participants aged between 18 and 54 years (M = 25.05, SD = 6.61 years) were included in data analyses. There were 34 females and 26 males in each group. Participant characteristics including anthropometric and mental health symptoms are reported in Table 1, as reported previously in the context of prolactin (Elgellaie et al. 2021). There were no significant differences between diagnostic groups for age or any of the measures of adiposity or cardiac health. Weight, systolic blood pressure and heart rate were higher in males than females (Elgellaie et al. 2021). There were no further sex differences or group by sex interactions for any of the adiposity or cardiac health measures. The psychometric measures (depression, anxiety, hostility, stress, GSI) were all significantly higher in the MDD than the control group, and females scored significantly higher than males on GSI and marginally higher in anxiety (p = 0.051).
Table 1 : Biometric and psychometric data (mean ± standard deviation) overall, by group (MDD and control) and sex