Letter to the EditorCytokines in schizophrenia: Methodological Issues
Introduction
Although the etiology of schizophrenia remains unclear, there is evidence that increase in pro-inflammatory and decrease in anti-inflammatory cytokine signaling and receptors are factors in the pathophysiology of schizophrenia (Potvin et al., 2008). A meta-analysis of 62 studies with 2298 people with schizophrenia and 1858 controls verified the presence of cytokine abnormalities in schizophrenia (Potvin et al., 2008). In a meta-analysis of 40 studies, 19 of which were also in the Potvin et al. study (2008), similar effect sizes in patients compared to controls were seen in patients with acute relapse of schizophrenia and in first episode psychosis (Miller et al., 2011). These two meta-analyses are summarized in Table 1. The purpose of this letter is to shed light on methodological issues that need to be addressed in order to advance the study of cytokines in schizophrenia.
Section snippets
Are cytokine abnormalities independent of body mass index and smoking?
The metabolic syndrome is widespread in schizophrenia. Cytokines, such as IL-1, IL-1RA, IL-6, IL-8, IL-10, TNF-α, and C-reactive protein (CRP) are elevated in people with the metabolic syndrome and obesity (Balistreri et al., 2010). Hence, the metabolic syndrome and/or obesity could be a confounding factor in cytokine studies. Similarly, cigarette smoking is widely prevalent in people with schizophrenia, and smoking may increase concentrations of CRP, IL-1β, IL-6, IL-8, and decrease
Cytokines in blood versus cerebrospinal fluid
Another methodological issue relates to the tissue used to measure cytokine concentrations. Blood may not be the most informative source for studying cytokines in schizophrenia, since the association between peripheral and central cytokine concentrations is poorly understood. However, obtaining cerebrospinal fluid (CSF) is very difficult in this population. Only seven out of 40 studies included in the meta-analysis (Miller et al., 2011) used CSF as the tissue source; all 62 studies included in
Conclusions and future directions
In sum, there is considerable evidence for a role of inflammation in the pathophysiology of schizophrenia. However, cytokine findings would be strengthened by the inclusion of the potential confounds of waist circumference, obesity and smoking in future studies. Understanding the association of cytokines and symptoms in schizophrenia independent of obesity and smoking is critical to understanding the role of inflammation in this illness. A better understanding of the association between
Contributors
Koola wrote the first draft of the manuscript with significant intellectual contributions from Duncan. We thank Drs. Robert Buchanan and Robert McMahon for editing the manuscript.
Conflict of interest
Dr. Duncan is a full time attending psychiatrist in the Mental Health Service Line of the Atlanta Veterans Affairs Medical Center, Decatur, Georgia, and receives grant support from Posit Science Corporation and Auspex Pharmaceuticals, Inc. Dr. Koola has no conflict of interest.
Acknowledgments
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