The role of the microbiota-gut-brain axis in schizophrenia: an immunological perspective

The potential role of bile acids (BAs) metabolism dysregulation in SZ is likely mediated through interactions with the gut-brain axis (76). Clinical studies have confirmed significant alterations in both the gut microbiota composition and BAs profiles in SZ patients (77), highlighting a potential correlation between BAs dysregulation and the disorder. A metagenome-wide association study further identified gut bacteria unique to SZ patients, including Alkalibacterium, Enterococcus faecium, and Lactobacillus fermentum (78). Cross-sectional studies found that certain gut microbes such as Collinsella, Corynebacterium, Lactobacillus, and Succinivibrio (67); Lachnospiraceae (79); and Veillonella (75) were positively correlated with the severity of SZ. This association between gut microbiota dysbiosis and SZ severity might be mediated through abnormalities in the BAs decoupling process. Metabolomic analysis of serum BAss in SZ patients revealed significantly altered BAs profiles compared to healthy controls (80). Notably, levels of primary Bas, such as CDCA, were generally elevated in patients, whereas levels of secondary Bas (e.g., DCA and LCA) were reduced (80). These altered BAs profiles may subsequently affect the composition of the gut microbiota, thereby interfering with neural function and contributing to the development and progression of SZ. A recent case report described a 39-year-old Persian male with treatment-resistant SZ who, after receiving 300 mg UDCA daily for 12 weeks, showed significant improvement in both positive and negative symptoms, along with enhanced cognitive abilities. Importantly, UDCA treatment was not only effective but also well-tolerated, with no adverse reactions reported during the treatment period, underscoring the safety and efficacy of UDCA supplementation (81). Animal studies have revealed that secondary bile acids may impair the integrity of the BBB. Among these, DCA has been demonstrated to damage endothelial tight junctions, leading to increased BBB permeability (82). This disruption allows toxic substances and peripheral inflammatory mediators to enter the brain, triggering neuroinflammation.

Polyamines (PAs) are important metabolites produced by gut bacteria. Research has confirmed that the gut microbiota is a significant source of polyamine synthesis, with its production of putrescine, spermidine, and spermine playing a key role in maintaining intestinal polyamine homeostasis (83). Historically, PAs were implicated in the etiology of schizophrenia because certain antipsychotic and antimalarial drugs contain structural components resembling spermidine and were associated with extrapyramidal symptoms and psychosis (84). A multi-omics analysis revealed significant disturbances in the polyamine biosynthesis pathway (85). Currently, elevated blood concentrations of PAs (primarily spermine and/or spermidine) have been observed in cases across various subtypes of schizophrenia (86 –88). However, data on PA concentrations in the brain remain limited. One study on human brain tissue found no difference in polyamine levels in the frontal cortex or hippocampus of schizophrenia patients compared to control subjects (89). Another study reported significantly elevated agmatine concentrations in both the plasma and postmortem frontal cortex tissue of first-episode and chronic schizophrenia patients (90 –93), and found that antipsychotic treatment could reduce blood agmatine levels (94). Elevated spermidine and total PA concentrations were also detected in fibroblasts obtained from SZ patients (95). Furthermore, studies on serum from SZ patients showed elevated levels of polyamine oxidase (the enzyme responsible for degrading PAs) (96, 97), while the activities of three enzymes involved in PA synthesis—ornithine aminotransferase, antizyme inhibitor 1 (AZIN1), and ornithine cyclodeaminase —were found to be reduced in the prefrontal cortex of both treated and untreated patients (98), ultimately potentially leading to disrupted polyamine homeostasis. A translational convergent functional genomics study identified the gene AZIN1, which encodes AZIN1, as a candidate gene for schizophrenia (99), providing genetic support for these findings. The pathophysiological role of PAs in SZ is primarily thought to be mediated through the dopamine pathway and by altering the function of the N-methyl-D-aspartate receptor (100), although the precise mechanisms remain unclear.

Under pathological conditions such as cerebral ischemia and trauma, stress states can induce significant disruption in polyamine metabolism, particularly the abnormal accumulation of putrescine. This alteration is closely associated with vasogenic edema and the disruption of the BBB (101). Research has confirmed that inhibiting polyamine biosynthesis with the ornithine decarboxylase inhibitor α-difluoromethylornithine significantly mitigates increased BBB permeability, Evans blue extravasation, and brain tissue water content in models of cerebral ischemia (102). Notably, this protective effect can be reversed by the administration of exogenous putrescine. This indicates that upregulation of endogenous polyamines, especially putrescine, serves as a critical mediator in BBB injury.

The structural integrity of the BBB is maintained by tightly joined brain endothelial cells, astrocytic end-feet, pericytes, and various transport proteins, which stringently regulate the penetration of substances from the blood into the brain tissue (103). SCFAs—such as acetate, propionate, and butyrate—produced by gut microbial fermentation, can enter the systemic circulation, reach the brain, and are present in considerable concentrations in the cerebrospinal fluid (104).

Recent research has shown that SCFAs play a crucial role in modulating the structure and function of the BBB. On the one hand, SCFAs can enhance BBB integrity by regulating the expression of tight junction proteins. For instance, studies in germ-free or antibiotic-treated mouse and rhesus monkey models have demonstrated reduced levels of BBB tight junction proteins (e.g., claudin-5, occludin) and increased BBB permeability; colonization with SCFA-producing strains (such as Clostridium tyrobutyricum and Bacteroides) reversed these effects (105 –108). On the other hand, SCFAs can protect the BBB directly or indirectly through anti-inflammatory mechanisms. SCFAs can interact with free fatty acid receptor 3 present on brain endothelial cells (109), suppress inflammatory responses, and alleviate oxidative stress, thereby promoting BBB stability. For example, propionate exerts anti-inflammatory and antioxidant effects by reducing cell surface CD14 expression and influencing the translocation of Nuclear Factor Erythroid 2-Related Factor 2 (110). Peripherally, certain SCFAs (e.g., acetate, propionate, and butyrate) have also been identified to possess anti-inflammatory properties (111 –113).

Current studies have identified elevated transcript levels of complement components (C1qA, C3, C4, C5) in patients with schizophrenia (114) (see Table 2), suggesting, albeit not entirely consistently, that increased activation of the classical complement pathway may be associated with SZ.

Genetic susceptibility to schizophrenia is significantly associated with polymorphisms in genes related to the complement system, largely attributable to alleles of the complement C4 gene located within the major histocompatibility complex region on chromosome 6 (132). The C4 allele demonstrates the strongest association with SZ risk (132)and is closely linked to increased synaptic phagocytosis and elimination by microglia (8). Furthermore, recent research has found that C4 gene overexpression triggers impaired GluR1 trafficking through an intracellular mechanism involving the endosomal protein SNX27, leading to pathological synaptic loss.

Pathogen exposure has long been recognized as a risk factor for the development of schizophrenia (142). Immune-related environmental variables, such as pathogen infection and gut microbiota dysbiosis, may interact with complement system dysfunction (143). Particularly noteworthy is the finding that genetic polymorphisms of complement components C4A and C4B are significantly associated with various microbial environmental variables in schizophrenia patients, including a history of pathogen exposure and gut ecological imbalance (143). Among these, a negative correlation was observed between plasma lipopolysaccharide-binding protein (LBP) levels and C4A gene copy number was observed in patients with schizophrenia, but not in healthy controls (143). This discovery is significant because LBP, a marker of bacterial translocation, reflects the host's response to gut microbial translocation and circulating bacterial LPS (144). Together, these results suggest that the complement system, particularly C4A, may play a key role in the gene-environment interactions of schizophrenia by modulating the gut microbiota and systemic immune activation status.

Furthermore, the negative correlation between C4A and C4B copy numbers in SZ patients may reflect a non-random association between these loci. Further analysis revealed that C4A and C4B haplotypes are significantly associated not only with the diagnosis of schizophrenia and environmental factors but also with psychiatric symptoms and cognitive function (143). While multiple previous studies have confirmed the association between the C4A gene and an increased risk of schizophrenia (132), the intrinsic mechanism underlying the negative correlation between C4B and schizophrenia risk remains unclear. It is currently unknown whether a lower C4B copy number has a protective effect against the disease or merely reflects an associated pathological protein deficiency. The negative correlations between C4A and C4B, as well as between C4L and C4S copy numbers, more likely indicate that these loci reside at opposing ends of a risk (C4A and C4L) and protective (C4B and C4S) spectrum, existing in a state of linkage disequilibrium (143). Biochemically, C4A exhibits higher affinity for amino groups, whereas C4B binds more readily to hydroxyl groups, suggesting that C4A may be more involved in binding immune complexes and protein antigens, while C4B plays a more important role in binding carbohydrate-rich microbial antigens (145). Consequently, a deficiency in C4B protein in patients may impair its ability to bind microbial antigens. Conversely, another possibility is that a lower C4B copy number might protect the stability of the host microbiome to some extent by reducing the excessive clearance of beneficial microbes mediated by C4B. A study on pediatric inflammatory bowel disease indicated that individuals with low C4B copy numbers had milder inflammation and higher gut microbial diversity compared to those with high C4B copy numbers (146).

As research on the gut microbiome in schizophrenia expands, integrating C4 genotyping with microbiome analysis is of great importance. Furthermore, these haplotype associations require further analysis to exclude nearby HLA gene variants—which may influence the observed immune phenotype due to linkage disequilibrium with specific C4 alleles (123).

Given the dysfunction of the BBB in patients with schizophrenia, changes in cytokine levels in the periphery (peripheral blood) and the center (cerebrospinal fluid, CSF) are valuable for assessing the CNS inflammatory state. Numerous studies have identified abnormal alterations in various cytokines in the CSF and peripheral blood of SZ patients (see Table 3), most of which are pro-inflammatory cytokines (IL-1β, IL-2, IL-8, TNF-α, and IFN-γ) (188 –191), with a smaller proportion being anti-inflammatory cytokines (including TGF-β1, IL-4, and IL-10) (192). The prevailing research view is that a dynamic imbalance between pro-inflammatory and anti-inflammatory cytokines may contribute to the pathogenesis of schizophrenia and subsequent psychopathological symptoms (193 –196).

According to Table 3, a characteristic pro-inflammatory state can be observed in SZ patients, manifested by persistently elevated levels of pro-inflammatory cytokines such as IL-6, IL-1β, and TNF-α in both peripheral blood and CSF, accompanied by a relative deficiency of anti-inflammatory cytokines like IL-10 and TGF-β. This cytokine imbalance is closely related to symptom severity, cognitive deficits, and treatment response. This aligns with current mainstream research; for example, the TLR4/NF-κB/IL-1β signaling pathway is activated in chronic SZ patients (197), and clozapine reduces the expression of pro-inflammatory genes such as IL-1β and IL-6 by inhibiting the TLR4/NF-κB pathway (198). Additionally, IL-8 mediates the migration and survival of neural stem cells and oligodendrocyte progenitor cells early in life, and elevated CSF cytokines IL-1β and IL-6 suggest the possibility of microglial activation (199 –201).

Aggregating data from Table 3 reveals consistent cross-barrier alterations in cytokine levels in both CSF and peripheral blood across different clinical stages (first-episode/FES, acute/ASZ, and chronic/CSZ), which hold clinical translational significance: 1). Significantly elevated levels of IL-1β, IL-6, and IL-8 in FES and ASZ SZ patients suggest they may be state markers for the onset or acute exacerbation of SZ; 2). The dose-dependent decrease in pro-inflammatory cytokine levels (e.g., IL-1β, IFN-γ, IL-6, TNF-α) following antipsychotic treatment (risperidone and clozapine) indicates value for monitoring treatment response; meanwhile, a significant increase in anti-inflammatory cytokine levels (e.g., sIL-2R) was observed in some patients (181, 184), a finding suggesting that antipsychotic drugs may exert anti-inflammatory-like effects.

SCFAs (such as acetate, propionate, and butyrate) activate receptors like FFAR2/3 and Gpr109a and inhibit histone deacetylase (HDAC), collectively suppressing the activation of the NF-κB signaling pathway, thereby downregulating the expression of pro-inflammatory cytokines (202 –204). It is well-known that the NF-κB signaling pathway is typically activated in inflammatory and autoimmune diseases (205 –207). Interestingly, all three cytokines elevated in SZ patients in Table 1 (IL-1β, IL-6, and IL-8) are regulated via the NF-κB pathway. Since cytokines can also regulate the activity of tryptophan catabolism in astrocytes and microglia, this finding corresponds to the alterations in the kynurenine pathway observed in the brains of individuals with schizophrenia (186).

SCFAs play an important regulatory role on various immune cells, particularly those central to maintaining immune homeostasis and anti-inflammatory responses, by inhibiting HDAC. For instance, SCFAs can inhibit NF-κB activation and the secretion of inflammatory cytokines like TNF-α in peripheral blood monocytes, neutrophils, and macrophages, thereby mitigating excessive immune responses. In dendritic cells, butyrate and propionate can impede their normal differentiation and induce an immune tolerant phenotype. Furthermore, SCFAs upregulate the expression of Foxp3, a key transcription factor for regulatory T cells, through HDAC inhibition (208, 209), promoting Treg differentiation and suppressing the production of pro-inflammatory cytokines (210). In SZ patients, the abundance of SCFA-producing bacteria is often altered, leading to reduced SCFA levels, which weakens the capacity to suppress inflammation; the abundance of SCFA-producing bacteria also changes post-treatment (211, 212).

Specific neurotransmitters, such as GABA, dopamine, glutamate, and serotonin (5-HT), are derived from precursors tyrosine and tryptophan, which are transported across the BBB into the CNS and subsequently converted into neurotransmitters (213). Tryptophan is primarily absorbed in the gut and metabolized by the gut microbiota through three downstream pathways: the 5-HT pathway, the kynurenine pathway, and the indole pathway (214). The kynurenine pathway of tryptophan metabolism represents a critical link through which the MGB axis participates in the immunopathology of schizophrenia. In schizophrenia, immune activation upregulates pro-inflammatory cytokines (e.g., IFN-γ, IL-1, TNF-α), which activate indoleamine 2,3-dioxygenase (IDO) via both IFN-γ receptor (IFN-γR)-dependent and independent pathways (e.g., synergistic action of TLR4, IL-1R, TNF-αR) (215). Enhanced activity of IDO enzymes (IDO-1 and IDO-2) shifts tryptophan metabolism towards the kynurenine pathway, leading to increased concentrations of kynurenic acid (KYNA) within the CNS (216 –220). Kynurenine (KYN) is transported across the BBB and metabolized by glial cells into KYNA and quinolinic acid (221, 222) ultimately resulting in excessive KYNA production. Acting as an endogenous antagonist, KYNA inhibits NMDA receptor and α7 nicotinic receptor function (223), suppresses glutamate and acetylcholine neurotransmission (223, 224), and leads to an imbalance in the glutamate, dopamine, and acetylcholine systems, thereby affecting neurotransmission, synaptic organization, and brain connectivity (225, 226).

On the other hand, activation of the KP results in substantial tryptophan consumption, reducing the substrate available for 5-HT synthesis (see Figure 2). Over 90% of the body's 5-HT is synthesized in the gut by enterochromaffin cells, which absorb tryptophan from dietary proteins as a substrate for 5-HT synthesis; this process is regulated by SCFAs and the kynurenine synthesis pathway (227, 228). Studies in animal models with gut microbiota depletion have shown increased plasma tryptophan, elevated brain serotonin concentrations, and reduced kynurenine pathway activity, all of which normalize following microbiota restoration (229 –231). Gut microbiota dysbiosis and immune activation can further enhance IDO activity, exacerbating the conversion of tryptophan to KYNA. This leads to decreased peripheral and central 5-HT levels, impaired receptor function, reduced concentrations of tryptophan and 5-HT (232), worsened symptoms of affective disorders, elevated levels of tryptophan catabolites (233, 234), and increased concentrations of toxic metabolites in the CNS (235).

Animal experiments have demonstrated that acute tryptophan depletion reduces brain tryptophan concentration by 70%, leading to decreased serotonin levels, diminished 5-HT receptor binding (236, 237), and effects on compulsive behavior (238). Similar results have been observed in human cerebrospinal fluid (CSF) studies (239). This suggests that immune dysregulation-induced gut microbiota dysbiosis and activation of the kynurenine pathway cause acute tryptophan depletion in the gut. This, in turn, leads to reduced brain tryptophan concentration, decreased 5-HT levels, and diminished 5-HT receptor binding, which may underlie the 5-HT system dysfunction, impaired neurotransmission, and negative cognitive effects observed in SZ.

This study has several limitations. First, despite conducting an extensive literature search, we cannot entirely exclude the possibility that some published studies may have been overlooked. Second, the multiple meta-analyses and systematic reviews cited in this article may carry a risk of accumulated type I errors (240). Furthermore, although we endeavored to include studies that performed stratified analyses of patients with schizophrenia, the interpretability and generalizability of the results may still be constrained by limited sample sizes and considerable population heterogeneity—such as confounding factors including age, gender, ethnicity, smoking history, and BMI (125).

By integrating the evidence discussed throughout this review, we propose a comprehensive model illustrating the MGB-immune interactions in schizophrenia (See Figure 3). At the peripheral level, gut microbiota dysbiosis and a reduction in microbial metabolites—particularly SCFAs—promote increased antigen exposure and activation of innate immune responses. This leads to the activation of immune cells (e.g., T cells, B cells, NK cells, monocytes/macrophages) and dysfunction of glial cells, culminating in the release of pro-inflammatory cytokines and complement components such as C4. Accompanied by impaired BBB integrity, these peripheral immune factors gain access to the CNS, where they mediate neuroinflammatory responses, resulting in synaptic damage and neuronal dysfunction.

In the CNS, dysbiosis directly or indirectly modulates the function of microglia and astrocytes via metabolites including SCFAs. Aberrantly activated glial cells exacerbate neuroinflammation and influence synaptic pruning processes, with complement C4-mediated synaptic phagocytosis playing a critical role. Concurrently, pro-inflammatory cytokines regulate the activity of key enzymes in tryptophan metabolism—indoleamine IDO and TDO—promoting the production of kynurenine pathway metabolites such as KYNA. This disrupts multiple neurotransmitter systems, including dopamine, GABA, glutamate, and serotonin, further exacerbating neurotransmission dysfunction. Moreover, compromised intestinal barrier integrity and gut-derived microbial components such as LPS activate TLR/NF-κB/NLRP3 pathways, which not only drive systemic inflammation but also modulate gut-derived tryptophan metabolism and serotonin synthesis, thereby coupling immune regulation with neuromodulatory functions.

The gut microbiota, as a critical "microbial organ," has garnered extensive attention for its role in neuropsychiatric disorders such as SZ. However, the causal relationships among microbial dysbiosis, immune dysregulation, and disease onset remain elusive. It is still unclear whether dysbiosis acts as a driver, a consequence, or both. Elucidating this issue is complicated by multiple confounding factors, including population heterogeneity, medication use, dietary and lifestyle variations, disease staging, and inconsistencies in research methodologies. Although small-scale studies and disparities in technical procedures—such as sample processing, sequencing, and bioinformatic analyses—limit the reliability and generalizability of the findings. Therefore, further large-scale and better-standardized clinical studies with stratified populations are urgently needed to provide more robust data on the association between altered gut microbial features and schizophrenia.

It is well established that the gut microbiota regulates brain function and behavior. The immune-mediated dysregulation of the MGB axis detailed in this review is observed not only in schizophrenia but also across various neurological disorders (241), including neurodevelopmental disorders (242), epilepsy (243), and depression (244), among others. Major depressive disorder is characterized by an elevated Bacteroidetes/Firmicutes ratio (5), accompanied by enrichment of Bacteroides and depletion of Blautia, Faecalibacterium, and Coprococcus, alongside increased abundance of Eggerthella and elevated levels of pro-inflammatory genera such as Escherichia (245). In contrast, anxiety disorders demonstrate an increased Firmicutes/Bacteroidetes ratio (246), reduced abundance of SCFA-producing genera including Bifidobacterium and Lactobacillus, while Akkermansia abundance shows a negative correlation with anxiety severity (247, 248). Bipolar disorder similarly exhibits disruption of the Firmicutes/Bacteroidetes ratio and reduced α-diversity, featuring increased Streptococcaceae and Bacteroidaceae abundance contrasting with depletion of anti-inflammatory commensals such as Faecalibacterium (249).Among neurodegenerative disorders, Parkinson's disease shows increased abundance of Lactobacillus and Bifidobacterium with concurrent reduction in Faecalibacterium, Coprococcus, and Blautia (250, 251), where decreased Blautia abundance correlates with clinical severity and reduced fecal butyrate levels (252). Alzheimer's disease manifests through reduced beneficial bacteria including Bifidobacterium and elevated opportunistic pathogens such as Escherichia and Clostridium (253, 254). Autism spectrum disorder(ASD) presents with reduced microbial diversity yet increased biomass in pediatric populations (255, 256), featuring a shift from beneficial microorganisms toward spore-forming, antibiotic-resistant, and/or neurotoxin-producing species (257). Specific alterations include reductions in Prevotella, Coprococcus, and Veillonellaceae, alongside overgrowth of Desulfovibrio (positively correlated with Autism spectrum disorder severity) (258), Sutterella, Ruminococcus, Clostridium, Megamonas, and Candida (259, 260), accompanied by an elevated Firmicutes/Bacteroidetes ratio (255). Notably, Candida overgrowth and associated toxin production may exacerbate neurobehavioral symptoms (37). Epilepsy research similarly reveals substantial gut microbiota dysbiosis, with most studies demonstrating reduced α-diversity in treatment-resistant epilepsy (261 –264). Phylum-level analyses indicate predominant Firmicutes with relative Bacteroidetes reduction in some studies (264, 265), while others report increased Actinobacteria, Verrucomicrobia, or pro-inflammatory Proteobacteria, alongside potential reduction of beneficial Bacteroidetes and Actinobacteria (262, 266).

In summary, these disorders share fundamental microbial alterations: 1) Structural dysbiosis manifested through disrupted Bacteroidetes/Firmicutes ratios and reduced α-diversity; 2) Functional impairment characterized by universal depletion of anti-inflammatory, SCFA-producing genera including Faecalibacterium, Blautia, and Bifidobacterium; 3) Common pathophysiological mechanisms involving impaired SCFA production, immune-inflammatory activation, and dysregulated neuroactive metabolite metabolism. These alterations promote systemic low-grade inflammation through an imbalance between elevated pro-inflammatory cytokines (e.g., IL-6, TNF-α) and anti-inflammatory mediators (e.g., IL-10) (267 –269). This systemic inflammation, akin to the processes described in schizophrenia, can traverse the blood-brain barrier or transmit via vagal afferents, subsequently activating microglia and exacerbating neuroinflammatory processes while impairing prefrontal cortex-mediated executive functions including decision-making and emotional regulation (270). Pro-inflammatory cytokines disrupt neurotransmitter metabolism through mechanisms such as inhibited tryptophan conversion to serotonin while driving microglial activation, thereby amplifying neuroinflammation (271). Activated microglia release chemokines, cytokines, and reactive oxygen species, crucially contributing to neuroinflammatory cascades while disrupting neurotransmitter balance and synaptic plasticity through pro-inflammatory phenotypic transformation. Notwithstanding these commonalities, disorder specific patterns emerge. Major depressive disorder demonstrates Bacteroidetes/Firmicutes ratio elevation contrasting with anxiety disorders, while ASD presents the distinct profile of reduced diversity with increased biomass and concurrent expansion of multiple opportunistic pathogens. These shared and distinct features collectively illustrate the multifaceted involvement of gut microbiota dysbiosis in neuropsychiatric disorders. Beyond inflammation, psychological stress represents another prevalent pathophysiological feature in microbiota-associated diseases. Stress contributes to depression (272), schizophrenia (273), autism spectrum disorder (274), epilepsy (275), and migraine (276). Significant comorbidity exists among these conditions, exemplified by the frequent co-occurrence of depression and ASD with epilepsy (277), the common comorbidity of depression with migraine (277), and the elevated prevalence of inflammatory bowel disease or irritable bowel syndrome among migraine patients (278).