C-reactive protein, brain-derived neurotrophic factor, interleukin-2, and stressful life events in drug-naive first-episode and recurrent depression: A cross-sectional study.

AIMS: The aim of the study is to assess and compare serum C-reactive protein (CRP), brain derived neurotrophic factor (BDNF), and interleukin-2 (IL-2) levels in patients with first-episode depression (FED), recurrent depressive disorder (RDD), and healthy controls (HCs) and to determine the relationship between the above-specified inflammatory markers, severity of depression, and stressful life events.
MATERIALS AND METHODS: Consecutive drug-naive patients with FED and RDD (n = 85) and 50 HCs were studied. Serum concentrations of CRP, brain-derived nerve growth factor (BDNF), and IL-2 were measured. All participants were assessed using Mini-International Neuropsychiatric Interview Plus, Beck's depression inventory, and presumptive stressful life events scale.
RESULTS: The FED and RDD groups had statistically significant lower serum concentration of BDNF and higher IL-2 compared to the HC group, whereas no statistically significant difference was observed with regard to CRP level. No statistically significant differences were observed with regard to the severity of depression and serum concentrations of CRP, BDNF, and IL-2 in the FED and RDD groups. No significant correlation was found between severity of depression and serum concentration of CRP, BDNF, and IL-2 in both the groups. Serum CRP concentration was significantly higher in patients with ≥2 stressful life events. No significant difference was observed between number of stressful life events and BDNF and IL-2 in patients with depression.
CONCLUSION: FED and RDD are associated with lower serum concentration of BDNF and higher IL-2 compared to the HCs, whereas there appears no difference with regard to CRP level. Multicentric studies are needed to further elucidate the role of inflammatory markers in depression.

INTRODUCTION

Over the last three-to-four decades, research has focused on the relationship between inflammatory markers and psychiatric disorders, especially with regard to etiology, diagnosis, treatment, and prognosis. There is evidence that in patients with depression, inflammatory markers affect the brain functions through changes in neurotransmitter synthesis and metabolism, neuroendocrine functions, and brain structure.[123456]

Patients with major depressive disorder exhibit all the cardinal features of an inflammatory response, including increased expression of pro-inflammatory cytokines and their receptors and increased levels of acute-phase reactants, chemokines, and soluble adhesion molecules in peripheral blood and cerebrospinal fluid.[47] Interleukin-6 (IL-6) and IL-1 have been most studied in depression. A review of a meta analysis on nine biomarkers of depression reported that IL-6 was most consistently elevated.[8] IL-6 is a pro-inflammatory cytokine that has the ability to cross the blood-brain barrier.[8]

Accumulating evidence suggests low levels of brain-derived neurotrophic factor (BDNF), a candidate molecule involved in the pathophysiology major depression.[91011] IL-2 has received little research attention with regard to its role in depression. Most researchers have measured the serum soluble IL-2 receptor (sIL-2R) in depressed patients but not serum IL-2 levels. Yang et al. reported a significant increase in sIL-2R which primarily acts as a storage reservoir for circulating IL-2 to increase the longevity of IL-2 signaling but appears not to induce signaling cascades of its own.[12]

In a daily diary study, greater interpersonal stress during a 2-week period was related to higher C-reactive protein (CRP).[1314] This may indicate that not only chronic stress but also acute stress is associated with higher level of circulating markers. There are two possible mechanisms of stress-induced activation of immune responses that involve both sympathetic nervous system and hypothalamic pituitary adrenal (HPA) axis pathways.

A recent systematic review reported that suicidal behavior in depressive patients is specifically associated with elevations of IL-2 and other cytokines.[15] Even a gender difference has been seen, the association between depression and high-sensitivity CRP was much stronger among men than among women.[16] The severity of depression is associated with increased serum CRP level but not in recurrent depressive disorder (RDD).[17] Emotional stressors are linked to RDD and induce an inflammatory response accompanied by increased production of pro-inflammatory cytokines.[18] Patients with major depression have reduced plasma oxygen radical absorbance capacity in comparison to healthy individuals.[19] Consequently, recurrent depressive episodes can contribute to a pro-inflammatory state. These recent studies indicate that the course of depressive disorder may have different immune responses although the evidence so far is inconclusive. It is likely that there may be a difference in inflammatory markers between first-episode depression (FED) and RDD. Given this background, the present researchers primarily aimed to assess and compare serum CRP, BDNF, and IL-2 levels in patients with FED and RDD and healthy controls (HCs). Secondarily, the authors aim to determine the relationship between the inflammatory markers (CRP, BDNF, and IL-2), severity of depression, and stressful life events.

MATERIALS AND METHODS

A cross-sectional, observational study was conducted in the Department of Psychiatry, Geetanjali Medical College and Hospital, Udaipur, Rajasthan. The department is part of a multispecialty general hospital rendering tertiary level health services. The study was approved by the Institution Ethics Committee, and informed consent was obtained from all participants of the study. The study was conducted from July 2016 to April 2017.

Study participants

A total of 85 consecutive patients with depression constituted the study population. The control group (HC) comprised 50 “healthy” individuals without a history of psychiatric and other medical conditions and biologically unrelated to the patients. All participants were screened, including a lifetime evaluation for psychiatric disorders using Mini-International Neuropsychiatric Interview Plus, and diagnoses of depressive disorders were based on the International Classification of Diseases-10.[2021] Severity of depression was assessed on Beck's depression inventory.[22] Information on stressful life events was obtained using the presumptive stressful life events (PSLE) scale.[23] All patients were categorized on the basis of number of stressful life events: >2 and ≤2 stressful life events in the last 1 year.

Inclusion criteria

Patients aged 18–65 years and those who provided informed written consent with current depressive episode were included in the study. The study participants were drug naive, i.e., no history of exposed to psychotropic drugs, antibiotics, steroids, structured psychotherapy, electroconvulsive therapy, and any other medications in the last 4 weeks, and they were categorized into two groups – FED and RDD.

Exclusion criteria

Individuals with present and past history of substance use disorders, psychotic disorders, bipolar affective disorder, and depression with psychotic symptoms and anxiety disorders; patients with other medical disorders such as atherosclerotic disease, cerebrovascular event, coronary artery disease, dyslipidemia, acute and chronic inflammatory conditions, diabetes mellitus, and other endocrine-metabolic disorders; patients with a positive urine pregnancy test or postpartum period of <1 year; patients with body mass index (BMI) >24.9 kg/m2 were also excluded from the study.

Evaluation for inflammatory markers

All the patients with depression (FED and RDD) and HCs were assessed after overnight fasting and resting state in the morning. A venous blood sample (5 ml) was drawn and collected in serum separator tube. The samples obtained were allowed to clot for 30 min at room temperature before centrifugation for 15 min at 3000/rpm. Serum was removed immediately in aliquot and stored at ≤−20°C. Repeated freeze–thaw cycles were avoided. Before assaying, the frozen samples were brought to room temperature slowly and mixed gently. The concentration of CRP (mg/dl) was measured using the fully automated biochemistry analyzer COBAS C411 based on electrochemiluminescence.[24] The concentration of BDNF (ng/ml) and IL-2 was measured using the Bio-Rad 680 analyzer based on enzyme-linked immunosorbent assay.[2526]

Data analysis

Statistical analyses were done using the Statistical Package for Social Sciences for Windows, version 16 (SPSS Inc., Chicago, Ill., USA). Continuous covariates were expressed as mean with standard deviation and compared between groups using the unpaired Student's t-test. Discrete covariates were expressed as frequencies compared using Chi-square test. To determine the correlation between variables, Pearson's correlation coefficient was used. All statistical analysis was done at 95% confidence interval and P <0.05 was considered statistically significant.

RESULTS

The mean age of patients in the FED group and RDD group was 29.11 ± 8.28 and 38.51 ± 9.75 years, respectively [Table 1]. The RDD group comprised more number of patients between the ages of 50–65 years in comparison to the other groups (Χ2 = 15.354, 4 d.f., P = 0.004).

Table 1

The distribution of individuals according to the age group and gender

The levels of the inflammatory markers were normally distributed. Patients in the FED and RDD groups had statistically significant lower serum concentration of BDNF and higher IL-2 compared to the HC group, whereas no statistically significant difference was observed with regard to CRP level [Table 2]. No statistically significant differences were observed with regard to the severity of depression and serum concentrations of CRP, BDNF, and IL-2 in patients of the FED and RDD groups [Table 2].

Table 2

Comparisons of severity of depression and concentrations of C-reactive protein, brain-derived nerve growth factor, and interleukin-2 among patients with first-episode depression, recurrent depressive episodes, and healthy controls

No significant correlation was found between severity of depressive episode and serum concentration of CRP, BDNF, and IL-2 in the FED and RDD groups [Table 3].

Table 3

Correlation between severity of depression and serum concentrations of C-reactive protein, brain-derived nerve growth factor, and interleukin-2 in patients with first-episode depression and recurrent depressive disorder

Serum CRP concentration was significantly higher in patients with two or more stressful life events. No significant difference was observed between number of stressful life events and BDNF and IL-2 in patients with depression [Table 4].

Table 4

Relationship between number of stressful life events and serum concentrations of C-reactive protein, brain-derived nerve growth factor, and interleukin-2 in patients with depression

DISCUSSION

The present study aims to understand the role of biological markers in depression with focus on serum CRP, BDNF, and IL-2 levels in FED, RDD, and HC groups. The pathogenesis of depression has not been fully elucidated, and studies suggest that low-grade systemic inflammation contributes to the development of depression.

According to a systematic review and meta-analysis, the relations of CRP to depression were all cross-sectional and inconsistent.[27] Many researchers have found elevated level of CRP in depressive patients.[2829] Matthews et al. found that higher CRP levels led to higher subsequent depressive symptoms, albeit the effect was small.[30] Confounding factors are also play an important role in measuring elevated CRP.[31] However, after adjusting for BMI, there was no significant relationship between CRP and depression.[31] Similarly, in the HUNT population study,[32] CRP levels were raised in those with myocardial infarction (MI) and comorbid MI and depression; the positive association with depression could be explained by confounding factors such as smoking, chronic physical, BMI ≥30 kg/m2, and high coffee consumption.[30] Interaction tests indicated a lower effect of old age and smoking on elevated CRP levels in women compared with men.[32] Previous investigations[33] indicate that obesity is also associated with raised inflammatory markers; hence, individuals with BMI >24.9 kg/m2 were excluded from the study. In contrast to earlier research,[17] we found that serum CRP level among FED and HC groups was positively correlated although weakly (P = 0.697). In the present investigation, RDD and HC groups had positive correlation, but it was statistically nonsignificant (P = 0.060).

BDNF, one of the major neurotrophic factors, plays an important role in the maintenance and survival of neurons, synaptic integrity, and synaptic plasticity. We found significantly lower level of serum BDNF in FED and RDD drug-naive patients compared to HCs. Karege et al.[34] were the first to compare serum BDNF levels in depressed subjects with HCs and they found that the BDNF level was significantly lower compared with that in HCs. This decrease was negatively correlated with the severity of depression. Similarly, many researchers reported decreases in serum BDNF level in depressed patients.[35363738] BDNF levels have been reported to be related to both recurrence and severity of depression.[39] Karege et al. also reported negative correlation in severity of depression and serum BDNF levels.[34] The present study revealed a negative correlation between RDD and serum BDNF. Contrary to earlier reports, we did not observe a statistically significant correlation with severity of depression with serum BDNF level in the FED and RDD groups. BDNF could play an important role in the modulation of neuronal networks and such neuronal plasticity may have an influence on recurrence or recovery of depressive episodes.

There is increasing evidence that major depression is accompanied by an immune response including activation of T-lymphocytes and cells of the monocyte/macrophage lineage.[40] One of the indices of T-cell activation is the increase in sIL-2Rs in the blood of depressed patients and depressed patients who have attempted suicide.[4142] The circulating form of the IL-2R p55 subunit, i.e., the sIL-2R, is released from activated T-cells into the blood, and sIL-2R concentrations appear to correlate with T-cell activation and IL-2 secretion in various pathological conditions.[43] The observation that sIL-2R circulating levels are significantly higher in unipolar depressed patients than controls[4244] is consistent with the findings of the present study.

Maes et al. observed that plasma sIL-2R concentrations were elevated in major depression and suggested a coordinated and upregulated production sIL-2R, and other cytokines may constitute a trait marker of major depression.[45] The present investigators also found significantly higher level of serum IL-2 in FED and RDD patients.[46] No relationship was found between severity of depression and serum level of IL-2 in FED and RDD patients in our study. Whereas, Brietzke et al.[47] found positive correlation of mood symptoms with IL-2 and IL-6. The present study estimated serum IL-2 levels in depressed patients unlike previous investigations that have focused on sIL-2R. Whether IL-2 abnormalities are at the level of T-lymphocyte function or IL-2 receptor requires further exploration.

Steptoe et al. suggested a modest increase in circulating inflammatory markers following laboratory-induced psychological stress.[48] Research has demonstrated that psychological distress (and greater frequency of daily interpersonal stress) is associated with higher level of CRP with or without depression.[144950] In the present study, most of the patients had a history of stressful life events in the last 1 year as assessed on PSLE, and those with two or more stressful life events have higher serum CRP concentration. It may be reasoned that frequent stressful life events or chronic stress associated with HPA overactivity and elevated serum cortisol level may result in the elevation of circulatory inflammatory markers, especially CRP.

In the present study, there were more patients in the age of 50–65 years in the RDD group which may have a bearing on the results. It has been suggested that aging leads to modified/modulated responses of the immune system, influencing the ability to cope with stressors, and not just to an eventually terminal deterioration of the immune system.[51] Testosterone is generally immunosuppressive while estrogen tends to be immunoenhancing.[52] This needs to be borne in mind as there were more males than females in our study. Gender difference in level of inflammatory markers in depression has been reported previously.[16]

The presence of a comparison group, estimation of serum IL-2 levels, inclusion of drug-naive depressed patients, appropriate tools, and exclusion criteria are relative merits of the study. The study is encumbered by certain limitations such as cross-sectional design with small sample size which limits the generalization of the findings. Although the severity and number of depressive episodes were noted, details pertaining to the phenomenological aspects of depression and its relationship with the inflammatory markers were not assessed. There might be a possibility of carrying minor infections by some participants that may have influenced the markers studied and it cannot be ruled out entirely despite our best efforts to exclude those with obvious ailments on the basis of history. No laboratory investigation was carried out to exclude other medical diseases. Future research may include a group with chronic medical illness to allow for stronger comparisons. The categorization of patients with <2 and >2 stressful life events was arbitrary. Clinical sample was drawn from a single tertiary care center, and hence, the findings need replication in other settings for confirmation.

CONCLUSION

It is still very early to say that depression is associated with global inflammatory markers. FED and recurrent depression are associated with lower serum concentration of BDNF and higher IL-2 compared to the HCs, whereas there appears no difference with regard to CRP level. There was no significant correlation between the severity of depressive episode and serum concentration of CRP, BDNF, and IL-2. In patients with depression, serum CRP concentration was higher in those with two or more stressful life events. Multicentric studies are needed to further elucidate the role of inflammatory markers in depression.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.