Higher plasma proinflammatory cytokines and reduced microglial TREM2 signaling in Late‐life Major Depression

Abstract Background Elevated circulating cytokines leading to microglia activation have been implicated in the increased risk for Alzheimer’s Disease (AD) in depression. However, correlations between peripheral and central inflammatory markers have been inconclusive. Additionally, only some depressives show evidence of increased inflammation. Microglia can assume anti‐inflammatory states via increased signaling of their TREM2 receptor with potential protective effects against AD including increased phagocytosis and NFkB inflammatory pathway inhibition. This process generates a soluble cleavage product, sTREM2, detected in CSF. Importantly in longitudinal human studies, higher CSF sTREM2 is associated with less brain amyloid deposition, less cognitive decline, and fewer degenerative changes. These considerations prompted us to determine if higher levels of plasma proinflammatory cytokines were associated with decreased TREM2 signaling in late‐life major depression (LLMD). Method Fifty‐one cognitively‐intact subjects aged 60 years and older, completed a 3‐year longitudinal study and an optional lumbar puncture (LP). Of these individuals, 49 were included in the analysis (29 LLMD and 20 controls). Independent‐samples Mann‐Whitney‐U Tests were used to compare groups. Non‐parametric correlations were run between CSF sTREM2 and plasma IL‐1B, Il‐6, Il‐8, TNFα, and CSF C3. Result In LLMD, higher plasma IL‐6 (r = ‐0.45, p = 0.02) and TNFα (r = ‐0.41, p = 0.30) were both associated with lower CSF sTREM2, but not with higher CSF C3 (p = 0.19) or other pro‐inflammatory markers consistent with a reduction in TREM2 signaling. These findings were not observed for controls. There were no group differences in Plasma IL‐1B, IL‐6, TNFα and CSF C3. Plasma IL8 was significantly higher in LLMD vs controls (p = 0.03). No correlations between plasma and CSF levels were observed for cytokines within the LLMD group. Conclusion Higher plasma pro‐inflammatory cytokines were associated with lower CSF sTREM2 in LLMD, consistent with a reduction in TREM2 signaling. Future studies should determine if lower CSF sTREM2 levels predict increased AD biomarkers, progressive decline and conversion to AD in LLMD.