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