Extracellular DNA from the blood plasma of patients with schizophrenia stimulates the TLR9-NF-kB signaling pathway in cultured human lymphocytes

Schizophrenia is a mental illness of complex etiology. Recently, there has been increased interest in the role of the immune system in the pathophysiology of mental disorders. The concept of neuroinflammation in neurodevelopmental disorders is gaining widespread interest, including the role of Toll-like receptors.

Schizophrenia is associated with an increase in the concentration of cfDNA in human blood, and the composition of cfDNA fragments changes significantly compared to cellular DNA: GC-rich fragments of the ribosomal repeat accumulate and base oxidation occurs. Similar changes, but less pronounced, also occur for cfDNA from healthy donors.

To confirm the hypothesis about the possible participation of cfDNA in the inflammation induction, we studied the effect of cfDNA samples on cultured mononuclear cells.

Unlike cellular DNA, cfDNA(SZ) and cfDNA(K) stimulate transcription of the TLR9 gene in mononuclear cells. After 1 hour the amount of TLR9 RNA increases by 2.9 and 3.3 times compared to the control. After 24 hours, the TLR9 RNA level decreases slightly, but is still 2-3 times higher than the control level. After 1 hour, TLR9 protein increases by 1.5 and 1.7 times, respectively, and further increased after 24h of culture.

An increase TLR9 protein expression correlates with an increase of the transcription factor NF-kB in lymphocytes and is accompanied by an increase in proinflammatory cytokine IL8 RNA, the transcription of IL8 is controlled by the NF-kB factor.

Thus, cfDNA(SZ) and cfDNA(K) stimulate the TLR9-NF-kB-proinflammatory cytokine signaling pathway in lymphocytes. The effect of cfDNA also depends on the concentration of these fragments in the extracellular environment. Since the concentrations of cfDNA in the blood of patients with schizophrenia are significantly increased compared to healthy donors, we should expect a much higher level of activation of the TLR9-NF-kB signaling pathway in the body cells of sick people.

Samples of cfDNA from patients with schizophrenia have a pronounced biological effect on cells of the immune system, stimulating the synthesis of pro-inflammatory cytokines by activating the TLR9-NF-kB-proinflammatory cytokines signaling pathway. High levels of cfDNA in blood plasma may be one of the reasons for the induction and maintenance of low-level inflammation in schizophrenia.

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