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Background: The gut microbiota (GM) plays an important role in the development of immune-related diseases, and the immune response is one of the pathomechanisms of depression (Dep); whether the effect of GM on Dep is mediated by immune cells (ImC) is unclear., Objective: ImC may mediate the effect of GM on Dep. Our aim is to identify and quantify the role of immune characteristics as potential mediators., Methods: Pooled statistics for GM (n = 7738) and ImC (n = 3757) were obtained from publicly available genome-wide association studies (GWAS), and for Dep (n = 47,696) from the Finnish database R10. We used a mediated Mendelian randomization (MR) study to investigate the causal relationship between GM and Dep and the mediating role of ImC between GM and Dep associations., Results: The results showed that the genetically predicted GM was significantly correlated with both ImC as well as Dep. MR analysis identified five microbiomes that had significant causal effects on Dep (Methionine biosynthesis III, PWY-6737-Starch degradation V, Parasutterella excrementihominis, Parasutterella, and Lysine biosynthesis I). In addition, five of the 26 ImC trait significantly associated with GM were most closely associated with Dep (T cell %lymphocyte、CD28-CD127-CD25++CD8br AC、CD28-CD8br AC、CD27 receptor on peripheral blood plasma cells (CD27 on PB/PC) and CD11b receptor on mononuclear myeloid-derived suppressor cells (CD11b on Mo MDSC)). This mediated MR illustrates the causal role of methionine biosynthesis III on Dep (IVW: OR = 1.08, 95%CI [1.04,1.14], P = 0.001). And there was no strong evidence for a causal effect of depression on methionine biosynthesis III. In the B cell group, the proportion of CD27 on PB/PC mediated was 7.88 %(95%CI [-0.04,0.03]) of the total effect. This study further suggests that Dep patients should actively seek immunologic intervention therapy., Conclusion: This MR study found that GM may play a causal role in Dep by mediating ImC. Our findings will help to understand the pathogenic mechanism of GM in Dep and the risk of immune mediation., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)