Neuroinflammation and depression: immune-brain interface mechanisms, biomarker stratification, and therapeutic strategies

Abstract

Major depressive disorder is among the most prevalent and disabling conditions in global medicine, yet its biological underpinnings remain incompletely understood, and current pharmacological treatments fail to produce adequate responses in approximately one-third of patients. A rapidly accumulating body of evidence has not simply challenged the long-dominant monoamine deficiency hypothesis but has provided a mechanistic framework that may explain many of the observed monoaminergic alterations in depressive cohorts, including IDO1-driven serotonin depletion, cytokine-mediated AMPA receptor internalization, and HPA-immune feedback dysregulation. Neuroinflammation, particularly glial activation across microglial, astrocytic, and oligodendrocyte lineages, and its downstream consequences for tryptophan metabolism, glutamatergic transmission, synaptic plasticity, and neurotrophic signaling, represents a central pathophysiological mechanism in a substantial subgroup of depressed patients. Peripheral inflammatory markers, including C-reactive protein, interleukin-6, and tumor necrosis factor-alpha, are elevated in a significant proportion of individuals with major depressive disorder, and these elevations predict poor response to conventional antidepressants while identifying patients who may respond preferentially to anti-inflammatory strategies. Post-mortem studies, positron emission tomography imaging of translocator protein density, and transcriptomic analyses of brain tissue have collectively provided consistent, though not yet fully definitive, evidence that microglial activation is a neurobiological feature of depression rather than a consequence of comorbid physical illness. This review synthesizes current mechanistic understanding of the neuroinflammatory hypothesis of depression, examines the evidence base from epidemiological, biomarker, neuroimaging, and interventional studies, including null findings and methodological limitations, evaluates emerging therapeutic strategies targeting the immune-brain interface, and identifies the critical questions that will determine whether immunopsychiatry fulfills its promise as a precision medicine framework for treatment-resistant depression.

Introduction

Major depressive disorder affects approximately 280 million people worldwide and represents the leading cause of years lived with disability globally, yet the biological mechanisms that drive the disease and the principles that should guide its treatment remain subjects of fundamental uncertainty [1, 2]. The monoamine hypothesis, which attributed depression to deficient serotonergic, noradrenergic, or dopaminergic neurotransmission and which underpinned decades of antidepressant pharmacology, has proven inadequate as a comprehensive mechanistic account. The delayed therapeutic onset of selective serotonin reuptake inhibitors relative to their immediate neurochemical effects, the treatment failure rates approaching 40 percent in large pragmatic trials, and the absence of consistent monoamine biomarkers differentiating depressed from non-depressed individuals collectively indicate that monoamine deficiency is at best a partial and downstream description of a more complex and heterogeneous disorder [3, 4].

Against this backdrop, the inflammatory hypothesis of depression has moved from a peripheral speculation to a scientifically mainstream and clinically consequential framework over the past fifteen years. Importantly, this framework does not position neuroinflammatory and monoaminergic biology as mutually exclusive; on the contrary, inflammatory mechanisms may underlie a substantial proportion of the monoaminergic alterations observed in depressed patients, including IDO1-driven tryptophan diversion away from serotonin synthesis, cytokine-mediated internalization of AMPA receptors, and stress-amplified NF-κB programs that suppress monoamine biosynthesis. The immune hypothesis is therefore better understood as providing a mechanistic upstream explanation for downstream neurochemical changes than as an alternative to them. The observation that patients receiving cytokine therapies for hepatitis C or cancer develop major depressive episodes at high rates, that inflammatory medical conditions including rheumatoid arthritis, inflammatory bowel disease, and cardiovascular disease substantially elevate depression prevalence, and that healthy volunteers administered bacterial lipopolysaccharide develop transient depressive and anhedonic symptoms in direct proportion to their inflammatory response established the causal plausibility of immune-to-brain signaling in depression etiology [5, 6]. The subsequent identification of elevated inflammatory biomarkers in a large minority of patients with primary major depressive disorder unconfounded by comorbid inflammatory illness, and the demonstration that these biomarkers predict antidepressant non-response, transformed the inflammatory hypothesis from an observation about medically ill patients to a potentially generalizable biological subtype with direct treatment implications [7, 8].

This review provides a comprehensive synthesis of the neuroinflammatory hypothesis of depression, organized around the key mechanistic pathways by which peripheral and central inflammation disrupts neural circuits governing mood, motivation, and cognition. We examine evidence from post-mortem neuropathology, neuroimaging with TSPO-PET, peripheral biomarker studies, and randomized anti-inflammatory intervention trials, with particular attention to developments since 2018 and explicit consideration of conflicting findings and methodological limitations. We further address the cellular heterogeneity across glial lineages that links systemic inflammation to synaptic dysfunction and identify the precision medicine questions that must be answered to translate this framework into improved outcomes for patients with treatment-resistant depression.

Neuroinflammation-associated depression across disease contexts

Depression is not confined to primary psychiatric illness but emerges as a high-prevalence comorbidity across major neurological, infectious, autoimmune, and systemic diseases in which neuroinflammatory mechanisms are operative, underscoring the transdiagnostic relevance of the immunopsychiatric framework. In HIV infection, sustained microglial activation driven by viral proteins and CCR2-mediated monocyte trafficking, combined with IDO1-induced kynurenine pathway dysregulation and elevated CSF IL-6 and TNF-alpha, produces depressive symptoms in 30–50% of people living with HIV independent of viral load or antiretroviral status; a 2023 scoping review identified IL-6, TNF-alpha, and neopterin as the most consistent neuroimmune biomarkers linking HIV to depression, with chronically activated microglia proposed as the central mechanistic node [9, 10].

In Alzheimer’s and Parkinson’s disease, preclinical and human post-mortem studies have shown that microglial and astrocytic activation that drives NLRP3 inflammasome assembly, complement-mediated synaptic loss, and BDNF suppression precedes and accompanies depressive episodes, now recognized as prodromal rather than reactive features [11].

Post-stroke depression, affecting approximately one-third of stroke survivors, is driven by ischemia-induced M1 microglial polarization, NLRP3 inflammasome activation via NF-kappaB signaling, and blood-brain barrier disruption amplifying peripheral cytokine access to peri-infarct tissue [12].

In bipolar disorder, peripheral CRP and IL-6 elevations have been reported across mood phases, including euthymia, in meta-analyses of blood-based biomarkers. However, large-scale CSF studies, which more directly reflect central neuroinflammatory processes, have yielded less consistent findings [13, 14], and the relationship between peripheral and central inflammatory markers in bipolar disorder remains incompletely characterized. In vivo TSPO-PET has suggested hippocampal microglial activation, though this also requires replication, and BBB disruption has been proposed to amplify peripheral cytokine access to the CNS [15, 16]. These findings should be interpreted as preliminary rather than established.

In obsessive-compulsive disorder and multiple sclerosis, overlapping kynurenine pathway dysregulation and HPA-immune feedback loops contribute to depression prevalence exceeding 40 and 50%, respectively [17]. Cancer-related depression, affecting up to 40% of oncology patients, is directly precipitated by tumor-derived cytokines and amplified by interferon-alpha and interleukin-2 therapies, converging on the same IDO1-kynurenine, glutamatergic, and BDNF-suppression pathways operative in primary inflammatory depression [5]. Across these contexts, the shared neuroinflammatory substrate reinforces the case for biomarker-stratified, immune-targeted therapeutic strategies with transdiagnostic scope.

These conditions are not presented as mechanistically identical; rather, they illustrate the broad clinical relevance of inflammatory mechanisms to depressive symptomatology across diverse disease contexts. The underlying pathophysiologies are likely more complex and multifactorial than any single inflammatory hypothesis can fully account for, and this complexity should be borne in mind throughout.

Peripheral inflammation in depression: epidemiological and biomarker evidence

The epidemiological relationship between inflammatory biology and depression is supported by multiple independent lines of evidence that span population cohorts, clinical samples, experimental models, and genetic data. Meta-analyses consistently demonstrate that circulating concentrations of C-reactive protein, interleukin-6, tumor necrosis factor-alpha, and interleukin-1beta are elevated in patients with major depressive disorder compared to healthy controls, with effect sizes in the small-to-medium range that are nevertheless clinically meaningful given the size and consistency of the literature [18, 19]. It should be noted, however, that cytokine measurement is subject to important methodological constraints, including pre-analytical variability, protein instability, particularly for IL-1β and TNF-α, and assay sensitivity differences across platforms. Cytokines with shorter half-lives or lower circulating concentrations are more challenging to quantify reliably than acute-phase reactants such as CRP and IL-6, which may contribute to the greater consistency of the latter in meta-analytic literature. Conclusions regarding specific cytokine mechanisms should therefore be interpreted with appropriate caution [20]. The elevation of these markers is not attributable entirely to confounders such as obesity, smoking, or physical inactivity. Indeed, these factors are increasingly understood not as simple confounders but as integral components of an inflammatory depression phenotype, lifestyle variables that are themselves both downstream of depressive pathophysiology and upstream drivers of inflammatory tone, embedded within a bidirectional system rather than standing outside it [21].

Prospective cohort studies have established that elevated inflammatory markers precede the onset of depression rather than simply accompanying or following it, consistent with a causal rather than consequential role. Kivimäki and colleagues, analyzing data from the Whitehall II cohort, found that elevated interleukin-6 concentrations measured in midlife significantly predicted the onset of depression over the subsequent decade [12, 22]. Wium-Andersen and colleagues similarly demonstrated, in a Danish population study of over 73,000 individuals, that elevated C-reactive protein was prospectively associated with incident depression and antidepressant prescription in a dose-dependent manner [23]. Mendelian randomization analyses have provided further support for a causal relationship, though these findings are not universally consistent across all inflammatory markers or study designs, reflecting the complexity of causal inference in polygenic traits [24, 25].

An important and well-established pathway by which peripheral inflammation produces depressive behavior involves cytokine-mediated activation of hypothalamic circuits, which subsequently engage dopaminergic mesolimbic projections that govern reward anticipation and motivational salience. Peripherally administered cytokines and LPS-induced cytokine release activate brainstem and hypothalamic inflammatory signaling via vagal afferent pathways and via transport through circumventricular organs, producing the constellation of fatigue, psychomotor slowing, social withdrawal, and anhedonia that constitutes sickness behavior, a phylogenetically conserved adaptive response that becomes maladaptive when chronically sustained [26]. Critically, animal studies establish a causal role for peripheral inflammation: peripheral LPS administration induces depressive-like phenotypes through IL-1β-dependent mechanisms [27], and peripheral blockade of inflammatory signaling rescues stress-induced depressive behavior in rodents [28], implicating peripheral inflammatory tone as a direct driver rather than merely a correlate of central depressive pathology. These findings are central to the ongoing debate regarding the relative contributions of peripheral versus central inflammation to depressive symptomatology.

Microglial activation in depressed brain: neuroimaging and neuropathological evidence

TSPO-PET imaging of microglial activation

Translocator protein 18 kDa (TSPO) is expressed at low levels in resting microglia and astrocytes and is upregulated in activated glia, making it a radioligand target for in vivo neuroimaging of neuroinflammation. Second-generation TSPO-PET radioligands, including 11C-PBR28, 18F-FEPPA, and 11C-DPA-713, have substantially improved signal-to-noise ratio compared to the first-generation ligand 11C-PK11195 [29]. Multiple independent groups have applied TSPO-PET to patients with major depressive disorder, and the overall pattern is of regionally elevated TSPO binding in corticolimbic circuits, though with notable heterogeneity across studies.

Setiawan and colleagues found significantly elevated 11C-PBR28 binding in the prefrontal cortex, anterior cingulate cortex, and insula of unmedicated depressed patients compared to genotype-matched controls, with binding correlating positively with symptom severity [30]. Subsequent studies by Nettis and colleagues have reported elevated TSPO binding in treatment-resistant depression, and a meta-analysis by Enache and colleagues confirmed a significant overall elevation, particularly in more severe or treatment-resistant presentations [31, 32].

Despite these positive findings, the TSPO-PET literature on depression shows important limitations and heterogeneity. Some studies report no significant difference in TSPO binding between depressed patients and controls, and methodological factors, including the rs6971 TSPO polymorphism, choice of reference region, radioligand kinetic modeling approach, and patient medication status, substantially influence binding estimates and limit cross-study comparability. Critically, TSPO is not exclusive to microglia: it is upregulated in astrocytes and endothelial cells under inflammatory conditions, and TSPO binding reflects mitochondrial density and metabolic activation rather than a specific pro-inflammatory transcriptional program. Elevated TSPO binding should therefore be interpreted as evidence of altered glial metabolic state rather than definitive classical neuroinflammation. These limitations are an important context for interpreting the TSPO-PET evidence base.

Post-mortem neuropathology

Neuropathological studies of post-mortem brain tissue from individuals with major depressive disorder have provided complementary evidence for microglial activation, with the advantage of direct tissue access but the limitation of potential confounds from agonal state, medication exposure, and diagnostic heterogeneity. Steiner and colleagues demonstrated increased microglial density and morphological activation in the dorsal and subgenual anterior cingulate cortex of suicide completers with MDD compared to controls, with the degree of activation correlating with pre-mortem symptom severity [33]. Wohleb and colleagues demonstrated that microglial activation in post-mortem depression tissue is accompanied by transcriptional signatures consistent with an inflammatory state, with enrichment for NF-kappaB target genes, complement pathway components, and chemokine receptors [34].

The regional specificity of microglial activation in post-mortem studies, most consistently the anterior cingulate cortex, subgenual prefrontal cortex, hippocampus, and amygdala, maps onto the regions showing functional hyperactivation and structural volume reductions in neuroimaging studies of depression, suggesting that microglial-driven synaptic pruning may contribute to circuit-level abnormalities [30, 34, 35]. It should be noted, however, that several post-mortem studies of depression do not support prominent microglial activation as a universal feature, and negative findings in this literature should be acknowledged alongside positive ones [36].

Microglial and neurovascular heterogeneity in depression: moving beyond a uniform activation model

Although converging TSPO-PET and post-mortem studies support the presence of neuroimmune alterations in major depressive disorder [30–35], the interpretation of ‘microglial activation’ in depression requires greater nuance than a binary activated-versus-resting framework. Microglia exist across a spectrum of transcriptional and functional states, and recent single-cell transcriptomic analyses challenge the assumption that depression is uniformly characterized by classical pro-inflammatory activation (Fig. 1). Scheepstra and colleagues [35], analyzing cortical tissue from individuals with major depressive disorder, reported a transcriptional profile suggestive of microglia exhibiting inhibited or immune-suppressed activity in gray matter rather than a canonical inflammatory phenotype. These findings may reflect region-, stage-, or phenotype-specific microglial states rather than contradicting TSPO-PET findings.

Fig. 1. Neuroinflammation-driven mechanisms contributing to depressive symptomatology.

Peripheral inflammatory states, characterized by elevated C-reactive protein, interleukin-6, and tumor necrosis factor-alpha, along with chronic stress signals, promote the release of circulating cytokines and immune mediators that communicate with the central nervous system via humoral, vagal afferent, and blood-brain barrier-associated pathways. These signals induce microglial activation within the brain, reflected by increased translocator protein expression and production of pro-inflammatory cytokines, including interleukin-1β and tumor necrosis factor-alpha. Neuroinflammatory activation stimulates indoleamine 2,3-dioxygenase, shifting tryptophan metabolism toward the kynurenine pathway and increasing levels of neuroactive metabolites, including quinolinic acid and kynurenic acid. Quinolinic acid acts as an N-methyl-D-aspartate receptor agonist, contributing to glutamatergic dysregulation, excitotoxic stress, and oxidative damage. Concurrently, diversion of tryptophan away from serotonin synthesis reduces central serotonergic tone. These combined processes impair synaptic plasticity and reduce brain-derived neurotrophic factor expression, ultimately leading to network dysfunction and the emergence of core depressive symptoms, including low mood, anhedonia, and cognitive impairment. This framework reflects the immunopsychiatric paradigm linking systemic inflammation to alterations in neurotransmission and synaptic integrity in major depressive disorder. CRP, C-reactive protein; IL-6, interleukin-6; TNF-α, tumor necrosis factor-alpha; TSPO, translocator protein 18 kDa; IDO, indoleamine 2,3-dioxygenase; NMDA, N-methyl-D-aspartate; BDNF, brain-derived neurotrophic factor.

Single-cell RNA sequencing studies in neuropsychiatric and neurodegenerative disorders demonstrate multiple microglial phenotypes, including interferon-responsive states, synapse-remodeling states, and disease-associated microglia characterized by altered lipid metabolism and complement signaling. While these phenotypes were first characterized in Alzheimer’s disease [37], analogous state diversification is increasingly observed in stress-based depression models. Chronic stress paradigms induce microglial priming characterized not only by cytokine upregulation but also by enhanced complement-mediated synaptic tagging and elimination, suggesting that depression-relevant pathology may involve excessive or mistimed synaptic remodeling rather than purely inflammatory cytotoxicity [38].

Oligodendrocytes are a relatively underemphasized cellular contributor to inflammatory depression. Accumulating evidence from post-mortem and animal studies indicates that pro-inflammatory cytokines impair oligodendrocyte precursor cell differentiation and myelin maintenance, with consequences for white matter integrity in prefrontal-limbic circuits. Reduced myelin basic protein expression and oligodendrocyte density have been reported in prefrontal regions in MDD, and inflammatory challenge in rodents produces myelin disruption accompanied by depressive-like behavior [39]. These findings suggest that the white matter abnormalities consistently documented in diffusion tensor imaging studies of depression may have a partially neuroimmune basis.

Astrocytes constitute a second critical cellular node in inflammatory depression. Post-mortem studies consistently report reduced astrocytic density and decreased GFAP expression in the prefrontal cortex of depressed individuals. Astrocyte reactivity itself is not monolithic; reactive astrocytes exhibit distinct molecular states, broadly characterized as neuroprotective (A2) or neurotoxic (A1), with context-dependent functional consequences [40]. Astrocytes regulate glutamate clearance via EAAT2/GLT-1, maintain extracellular potassium homeostasis, and provide metabolic lactate support to neurons. Pro-inflammatory cytokines downregulate EAAT2 expression, impair glutamate reuptake, and amplify excitotoxic vulnerability initiated by quinolinic acid-mediated NMDA receptor activation, thus synergizing with microglial activation of the kynurenine pathway to destabilize excitatory-inhibitory balance in corticolimbic circuits.

The neurovascular interface further complicates the inflammatory model. Systemic inflammation alters BBB permeability through cytokine-mediated disruption of tight junction proteins: IL-6 and TNF-alpha disrupt claudin-5 and occludin, thereby increasing permeability and permitting peripheral immune mediators to access the perivascular spaces [41]. Depression has been associated with subtle BBB dysfunction and altered endothelial signaling, suggesting that vascular-immune coupling may represent an upstream event in susceptible individuals [42, 43]. Rodent studies have directly linked BBB dysfunction to depressive behavior: endothelial barrier compromise in the prefrontal cortex and hippocampus, induced by chronic stress or inflammatory challenge, promotes monocyte extravasation, microglial activation, and depressive-like behavioral phenotypes [42, 43], providing mechanistic support for the clinical associations.

Crucially, the relationship between inflammation and depression is bidirectional. While peripheral inflammatory states can precipitate depressive symptoms [5, 6], depressive behaviors, including sleep disturbance, physical inactivity, and psychosocial stress, sustain inflammatory signaling through sympathetic nervous system-mediated NF-kappaB activation [44]. Chronic stress induces microglial priming, rendering the brain more responsive to subsequent immune challenges and creating a feed-forward loop in which psychological stress and inflammatory signaling mutually reinforce one another, shifting the system toward a maladaptive steady state characterized by glucocorticoid resistance [45, 46], kynurenine pathway bias [47], glutamatergic dysregulation [35, 48], and impaired neurotrophic signaling [49]. Within this framework, inflammatory depression is best conceptualized not as static neuroinflammation but as a dynamic failure of immune-neural homeostasis.

Mechanisms linking inflammation to depression

Kynurenine pathway and tryptophan metabolism

The kynurenine pathway represents one of the most mechanistically developed links between peripheral inflammation and depressive symptomatology. Tryptophan, the precursor to serotonin, is metabolized predominantly through the kynurenine pathway under inflammatory conditions, driven by indoleamine 2,3-dioxygenase 1 (IDO1), which is potently induced by interferon-gamma, TNF-alpha, and LPS in macrophages, microglia, and astrocytes. IDO1 induction diverts tryptophan flux from serotonin synthesis toward kynurenine and its downstream metabolites, resulting in central serotonin deficiency while generating neuroactive metabolites that directly affect neuronal function [50].

The kynurenine branch-point is of critical mechanistic importance. In activated microglia, kynurenine is preferentially converted by kynurenine monooxygenase (KMO) to 3-hydroxykynurenine, which is subsequently converted to quinolinic acid, a potent NMDA receptor agonist that is neurotoxic at elevated concentrations, thereby driving oxidative stress, excitotoxic neuronal damage, and impaired synaptic plasticity. In astrocytes, kynurenine is preferentially converted by kynurenine aminotransferase (KAT) to kynurenic acid, a broad glutamate receptor antagonist with neuroprotective properties. The ratio of quinolinic acid to kynurenic acid, therefore, reflects the microglial-to-astrocytic metabolic balance, and its elevation signals a shift toward the neurotoxic arm of the pathway. Elevated quinolinic acid concentrations in CSF and plasma have been reported across multiple independent MDD cohorts and correlate with depression severity and suicidality [47, 51]. Kynurenic acid deficiency in this context removes a key brake on NMDA excitotoxicity, amplifying the glutamatergic dysregulation described below. The IDO1 inhibitor 1-methyl-tryptophan reverses inflammation-induced depressive behavior in rodents, and IDO1 genetic variants modify the relationship between inflammatory exposure and depression risk in human genetic studies, establishing IDO1 as a causal rather than merely correlative node in the inflammatory depression pathway.

Glutamatergic dysregulation

The convergence of the kynurenine pathway, through quinolinic acid-mediated NMDA receptor overactivation, with the broader evidence for glutamatergic dysregulation in depression provides a mechanistic bridge between neuroinflammation and synaptic pathology. Elevated quinolinic acid preferentially targets NMDA receptors on GABAergic interneurons in the prefrontal cortex and hippocampus, disrupting excitatory-inhibitory balance and reducing GABAergic inhibition that normally maintains prefrontal cognitive control and suppresses amygdalar hyperreactivity [35, 48]. This GABA deficit has been consistently documented in magnetic resonance spectroscopy studies of depression.

Microglial release of pro-inflammatory cytokines, including IL-1beta and TNF-alpha, further dysregulates glutamatergic transmission through cytokine-mediated internalization of AMPA receptors from postsynaptic membranes, reducing synaptic AMPA receptor density in the prefrontal cortex and hippocampus and impairing the synaptic strengthening that underlies adaptive mood regulation and stress resilience [52, 53]. The reduction in AMPA receptor-mediated synaptic transmission driven by neuroinflammation mirrors the synaptic deficit produced by chronic stress and is precisely the deficit that ketamine reverses through mTOR-dependent AMPA receptor insertion, providing a biologically plausible explanation for why patients with elevated inflammatory markers may show differential response to ketamine, though direct clinical evidence for this interaction remains preliminary and requires replication in larger biomarker-stratified trials [54].

HPA axis dysregulation and glucocorticoid resistance

Proinflammatory cytokines, including IL-1beta, IL-6, and TNF-alpha, activate the HPA axis through direct effects on the hypothalamus and pituitary, driving cortisol hypersecretion. Simultaneously, chronic cortisol elevation drives glucocorticoid receptor resistance in immune cells, impairing the normal anti-inflammatory feedback that glucocorticoids provide and allowing inflammatory signaling to persist despite elevated cortisol, creating a state of simultaneous HPA overdrive and glucocorticoid resistance uniquely characteristic of inflammatory depression [45, 46]. Glucocorticoid receptor resistance in peripheral immune cells is associated with increased NF-κB activation and enhanced pro-inflammatory cytokine production, creating a constitutive inflammatory tone that is further amplified by psychosocial stress through sympathetic nervous system-mediated upregulation of inflammatory gene transcription. This mechanistic circuit provides a unifying explanation for the well-documented association between adverse childhood experiences, chronic psychosocial stress, and inflammatory depression in adulthood [46, 55].

Neurotrophic signaling impairment, neurogenesis, and plasticity

Brain-derived neurotrophic factor (BDNF) expression is suppressed by pro-inflammatory cytokines, including IL-1beta and TNF-alpha, through NF-kappaB-dependent transcriptional repression of BDNF promoters in cortical and hippocampal neurons, establishing a direct mechanistic link between neuroinflammation and the neurotrophic deficits consistently documented in major depressive disorder [35, 49]. Reduced hippocampal BDNF concentrations are among the most replicated biological findings in post-mortem depression studies, and peripheral BDNF levels are reduced in meta-analyses of depressed patients compared to controls, with normalization following effective antidepressant treatment.

Beyond the suppression of BDNF transcription, neuroinflammation directly impairs hippocampal neurogenesis. Pro-inflammatory cytokines, including IL-1β and TNF-α, reduce proliferation and survival of newborn neurons in the dentate gyrus in rodent models, while IDO1-derived quinolinic acid suppresses neural progenitor cell differentiation through NMDA receptor-mediated excitotoxicity [56]. Anti-inflammatory interventions that normalize the quinolinic acid: kynurenic acid ratio restore hippocampal neurogenesis in chronic stress models [57]. Astrogliogenesis is also disrupted under inflammatory conditions: cytokine-driven conversion of reactive astrocytes toward a neurotoxic A1 phenotype reduces trophic support for newly generated neurons and impairs dendritic spine maintenance in corticolimbic circuits [40]. These findings position neuroinflammation as a mechanism that undermines structural plasticity at multiple simultaneous levels, consistent with the volumetric and connectivity abnormalities documented by structural and functional MRI in depression.

The inflammatory depression subtype: clinical features and biomarker stratification

The inflammatory hypothesis does not propose that all depression is inflammatory in origin, but rather that a biologically meaningful subgroup, estimated at between 25 and 40 percent of clinical samples at CRP thresholds above 3 mg/L, is characterized by elevated inflammatory tone that drives depressive symptoms through the mechanistic pathways described above [7, 46]. This inflammatory subtype has a recognizable clinical phenotype that may guide biomarker-based patient selection and therapeutic decision-making.

Patients with inflammatory depression show higher rates of atypical depressive features, including hypersomnia, increased appetite, fatigue disproportionate to mood severity, and pronounced psychomotor slowing, in contrast to the classical melancholic features that characterize HPA-driven depression [25, 58] (Fig. 1). They also demonstrate greater anhedonia severity relative to sadness, consistent with the preferential effects of inflammatory mediators on dopaminergic mesolimbic circuits that govern reward anticipation and motivational salience. Haroon and colleagues demonstrated, using functional neuroimaging, that patients with elevated CRP levels showed reduced ventral striatal activation during reward anticipation, which correlated inversely with both CRP levels and anhedonia severity [53].

The clinical utility of CRP as a stratification biomarker has been examined in multiple studies. However, the evidence must be interpreted carefully: the finding that patients with CRP above 3 mg/L derive significantly less benefit from monoaminergic antidepressants is not universally replicated across trials, and the evidence base should be considered preliminary rather than definitive [59]. There is also emerging evidence for differential pharmacological responses based on inflammatory status; some data suggest that elevated inflammatory markers may predict a better response to dopaminergic versus serotonergic agents, consistent with the preferential effects of inflammatory mediators on mesolimbic dopamine circuits [60]. This differential pharmacological sensitivity, if confirmed, would have direct implications for treatment selection within the inflammatory subtype. The case for CRP as a clinically actionable stratification biomarker is compelling but awaits confirmation from prospective, pre-registered, biomarker-stratified trials currently underway in the United Kingdom and the United States.

The association between elevated inflammatory markers and treatment resistance is one of the most clinically consequential findings in the neuroinflammatory literature. Patients with TRD show significantly higher CRP, IL-6, and TNF-α concentrations than treatment-responsive patients, and the magnitude of inflammatory elevation predicts the degree of antidepressant non-response in several independent cohorts [61]. The mechanistic basis for this association includes cytokine-mediated impairment of serotonin transporter activity, suppression of monoamine synthesis via IDO1, and AMPA receptor internalization that directly counteracts the synaptic potentiation mechanism of conventional antidepressants. These findings position inflammatory biomarker screening as directly relevant to the clinical management of TRD and suggest that anti-inflammatory augmentation strategies may be most valuable precisely in the patients who have failed conventional treatment.

Therapeutic strategies targeting neuroinflammation in depression (Table 1)

Table 1.

Key Randomized and Interventional Trials of Anti-Inflammatory Agents in Major Depressive Disorder.

Agent	Design	N	Biomarker selection	Primary outcome	Key finding
Infliximab (anti-TNF)	RCT	60	None (post-hoc CRP subgroup)	HAMD-17	No overall benefit; significant benefit in CRP > 5 mg/L subgroup [62]
Tocilizumab (anti-IL-6R)	RCT (preliminary)	TBD	Elevated IL-6	MADRS	Phase II open-label trial in TRD ongoing (NCT02660528); preclinical data show antidepressant-like effects of IL-6 receptor blockade in rodent models [69]
Minocycline	RCT	70	Elevated CRP	QIDS	Significant symptom reduction vs. placebo; effect size increased with baseline CRP tertile [66]
Anti-inflammatory agent	RCT	TBD	Unselected/minimal enrichment	Depression scale	Null result - no significant antidepressant benefit; negative trial underscoring need for biomarker selection [63, 64]
P2X7 antagonist	Preclinical / early phase	—	Elevated P2X7/IL-1β	Inflammatory / behavioral	Antidepressant-like effects in preclinical models; preliminary clinical data [68]

Completed and key ongoing trials of anti-inflammatory agents for MDD are summarized and organized by therapeutic class. Studies were selected to represent the full range of trial designs and outcomes, including null results. Biomarker enrichment status is noted for each trial, as this variable has emerged as the primary determinant of treatment response. Null findings in unselected populations are included to underscore that anti-inflammatory efficacy in depression is conditional on biomarker-stratified patient selection rather than a general property of the treatment class.

TBD to be determined from primary publications, RCT randomized controlled trial, HAMD-17 hamilton depression rating scale 17-item, MADRS montgomery-åsberg depression rating scale, QIDS quick inventory of depressive symptomatology, CRP C-reactive protein, IL-6 interleukin-6, TNF-α tumor necrosis factor-alpha, MDD major depressive disorder, TRD treatment-resistant depression.

Anti-cytokine and anti-TNF therapies

The most mechanistically specific test of the inflammatory hypothesis has come from trials of biologic agents that neutralize specific cytokines in patients selected for elevated inflammatory markers. The landmark randomized controlled trial by Raison and colleagues examined intravenous infliximab in 60 patients with treatment-resistant MDD and found that, while infliximab did not produce antidepressant benefit overall, it produced significant improvement in the subgroup with baseline CRP above 5 mg/L, with a treatment effect size comparable to that of conventional antidepressants in unselected populations [62]. This biomarker-by-treatment interaction provided proof of concept for the inflammatory subtype hypothesis. However, as of 2025, the completed RCT evidence base for biologic agents specifically in MDD remains narrow. Beyond the Raison et al. infliximab trial, IL-6 receptor antagonism with tocilizumab has been evaluated in a phase II open-label trial for treatment-refractory MDD (NCT02660528); however, published results are not yet available, and trials of satralizumab in biomarker-enriched populations are ongoing. The scarcity of completed trials reflects practical and regulatory challenges in developing psychiatric indications for biologic agents, not an absence of scientific rationale.

Importantly, trials in unselected or minimally enriched populations have reported no significant antidepressant benefit from anti-inflammatory interventions [63, 64], reinforcing the need for biomarker-stratified patient selection to demonstrate efficacy. These null findings should not be interpreted as evidence against the hypothesis but as evidence that it applies to a biologically defined subgroup rather than to depression broadly.

More recent work has examined IL-6 receptor antagonism in depression, motivated by Mendelian randomization evidence specifically implicating IL-6 signaling in depression causation. Preliminary data in treatment-resistant depression with elevated inflammatory markers suggest antidepressant signals in biomarker-enriched populations, with effects most pronounced on anhedonia and fatigue [32].

Minocycline and microglial-targeted strategies

Minocycline, a second-generation tetracycline antibiotic with well-characterized anti-inflammatory and microglial-inhibitory properties, inhibits microglial activation, reduces pro-inflammatory cytokine production, suppresses IDO1 activity, and limits quinolinic acid synthesis, addressing multiple nodes of the neuroinflammatory-kynurenine-glutamate pathway simultaneously [48]. Nettis and colleagues conducted a four-week randomized controlled trial of minocycline added to ongoing antidepressant treatment in 70 patients with treatment-resistant depression and elevated CRP, finding significant reductions in depressive symptoms compared to placebo, with effect size increasing across tertiles of baseline inflammation [65]. A 2025 systematic review of minocycline in depression and related neuroinflammatory conditions [66] further characterized the evidence base and identified biomarker enrichment as the key moderator of treatment response.

The broader landscape of microglial-targeted strategies includes CSF1R inhibitors, which deplete microglia and allow their repopulation with a deactivated phenotype, and NLRP3 inflammasome inhibitors, which block assembly of the multiprotein complex responsible for processing and releasing mature IL-1beta and IL-18. Preclinical studies in chronic stress depression models demonstrate that both CSF1R inhibition and NLRP3 blockade produce robust antidepressant-like effects, reduce synaptic AMPA receptor deficits, and restore BDNF expression in the hippocampus [38, 67].

P2X7 purinergic receptors, which are highly expressed on microglia and mediate ATP-driven NLRP3 inflammasome activation and IL-1β release, represent an additional target of emerging interest. P2X7 antagonism has demonstrated antidepressant-like effects in preclinical models, and preliminary clinical data suggest P2X7 blockade reduces inflammatory signaling in neuropsychiatric contexts [68]. Clinical development of P2X7 antagonists for depression is at an early stage but provides a mechanistically distinct complement to existing anti-cytokine and microglial-inhibitory strategies.

Conventional antidepressants with anti-inflammatory properties

Several conventional antidepressants exert anti-inflammatory effects that may contribute to their therapeutic activity in the inflammatory depression subtype. Fluoxetine inhibits microglial activation, reduces NF-kappaB signaling, and suppresses pro-inflammatory cytokine production in both in vitro and in vivo models, with evidence that these anti-inflammatory effects are independent of serotonin reuptake inhibition [35]. Bupropion, which acts on norepinephrine and dopamine reuptake, also shows anti-inflammatory properties and may be particularly effective in the inflammatory atypical subtype characterized by fatigue, hypersomnia, and dopaminergic anhedonia. The recognition that anti-inflammatory effects are embedded within the pharmacological profiles of some conventional antidepressants suggests that the success or failure of monoaminergic agents in individual patients may partly reflect the degree to which these agents happen to address the specific biological mechanisms operative in that patient’s depression.

Sex differences, stress, and the social immune interface

The neuroinflammatory hypothesis of depression offers mechanistic explanations for clinical epidemiological patterns that the monoamine hypothesis struggled to account for. The two-fold higher prevalence of major depressive disorder in women than men is partially explicable through sex-specific differences in inflammatory reactivity, with women showing greater and more sustained cytokine responses to psychosocial stressors and heightened sensitivity of the kynurenine pathway to estrogen-driven IDO1 induction [44, 49]. Reproductive transitions, including the peripartum period and perimenopause, are associated with elevated depression risk and characterized by transient increases in microglial activation and inflammatory cytokine production as part of normal hormonal transition; pathological amplification of these inflammatory signals may underlie the vulnerability of these periods to depression onset.

The role of social and environmental stress as a driver of neuroinflammation provides the mechanistic link between psychosocial adversity and biological depression risk. Slavich and Irwin systematically documented that social-evaluative threats, social rejection, social isolation, and interpersonal loss activate inflammatory transcription programs through sympathetic nervous system-mediated upregulation of NF-kappaB activity in peripheral immune cells [44]. Early-life adversity programs a lasting proinflammatory bias in both peripheral immune function and central microglial reactivity through epigenetic mechanisms that persist into adulthood, substantially explaining the epidemiological relationship between adverse childhood experiences and adult depression risk and treatment resistance [55]. This social-immune-neural pathway positions neuroinflammation as the biological mechanism through which social determinants of health become embedded in the nervous system as depressive vulnerability.

Key knowledge gaps and future directions

Despite the conceptual and empirical progress of the neuroinflammatory hypothesis, several fundamental questions must be resolved before it can fully deliver on its clinical promise. The heterogeneity of microglial states documented in recent single-cell transcriptomic studies raises the critical question of which specific microglial transcriptional program mediates depression-relevant pathology. Not all forms of microglial activation are equivalent; the disease-associated microglial state, the interferon-responsive state, the NLRP3-driven inflammatory state, and the synaptic pruning-enhanced state each produce distinct downstream consequences for neuronal function, and the depression-associated microglial state has not yet been precisely characterized at the single-cell transcriptomic level in human tissue [35, 37].

Biomarker development for patient stratification represents perhaps the most immediately actionable research priority. C-reactive protein, while accessible and clinically established, is a nonspecific acute-phase reactant that imperfectly reflects central microglial activation. More specific candidate biomarkers, including soluble TREM2 in cerebrospinal fluid and the quinolinic acid-to-kynurenic acid ratio, have shown promise in small studies but require validation in large, prospectively designed biomarker-stratified treatment trials [35, 44]. The development of blood-based tests that accurately predict central neuroinflammation and therapeutic response would transform the clinical applicability of the immunopsychiatric framework.

The brain-body inflammatory relationship also requires more precise mechanistic dissection. Current models propose that peripheral inflammatory signals reach the brain via vagal afferents, direct cytokine transport across circumventricular organs, and indirect microglial priming mechanisms. The relative contributions of these routes to human depression, and the identification of which are most amenable to therapeutic intervention, represent important gaps that require experimental medicine designs, advanced neuroimaging, and peripheral immune phenotyping [5, 48]. The field also requires larger, prospectively registered, biomarker-stratified trials with pre-specified inflammatory subgroup analyses to move beyond the proof-of-concept stage and establish the evidence base needed for clinical implementation.

Conclusions

The neuroinflammatory hypothesis of depression has matured from a provocative observation into a mechanistically rich and clinically actionable framework that is reshaping the conceptual foundations of psychiatric research. The convergence of epidemiological evidence for inflammatory causation, in vivo neuroimaging confirmation of glial activation in the depressed brain, mechanistic elucidation of the kynurenine pathway, glutamatergic, neurotrophic, and HPA-immune feedback mechanisms, and randomized trial proof-of-concept for anti-inflammatory antidepressant strategies in biomarker-selected patients collectively constitute one of the most compelling paradigm shifts in modern psychiatry. Importantly, this framework does not replace the monoaminergic model but rather provides an upstream mechanistic layer that explains many of the neurochemical observations previously attributed solely to monoamine deficiency.

The evidence base, while compelling, must be interpreted with appropriate nuance. Important null findings exist in both the neuroimaging and clinical trial literature, methodological limitations affect several lines of evidence, and the precise characterization of depression-relevant glial states in human tissue remains incomplete. Progress requires not only confirmation of positive findings but rigorous engagement with negative results and a commitment to pre-registered, biomarker-stratified trial designs.

The central implication for clinical practice is the necessity of biological subtyping in depression. Treating all depression as a homogeneous disorder with a common monoaminergic mechanism guarantees that anti-inflammatory treatments will appear ineffective when tested in unselected populations. The precision medicine imperative, to measure the inflammatory biology of individual patients and direct them toward treatments most likely to address their specific pathophysiology, is not a future aspiration but a scientifically supported framework awaiting the infrastructure and clinical tools for routine implementation. The next decade of neuroinflammatory depression research will be defined by the success or failure of that translation.

Methods and ai disclosure

This review was conducted as a narrative synthesis of mechanistic, epidemiological, neuroimaging, and clinical trial evidence. Literature was identified through searches of PubMed and Scopus using the following terms: neuroinflammation, depression, microglia, astrocytes, oligodendrocytes, TSPO, kynurenine pathway, IDO1, CRP, IL-6, TNF-alpha, glutamate, BDNF, anti-inflammatory antidepressants, and treatment-resistant depression, with emphasis on studies published between 2015 and 2025. Reference lists of identified reviews were hand-searched for primary experimental studies. Studies were selected based on mechanistic, biomarker, or clinical relevance to the neuroinflammatory hypothesis of depression. This approach does not constitute a formal systematic review, and the synthesis may not exhaustively capture all available evidence; in particular, the evidence base contains important negative and null findings that are incorporated and discussed throughout.

Author contributions

MS, NS, and BES all edited the manuscript.

Competing interests

The authors declare no competing interests.

AI disclosure

Preparation of this manuscript involved the use of an AI-assisted writing tool (Grammarly) for drafting and editing support. The figure was created with AI assistance (ChatGPT). In accordance with Springer Nature policy and the journal’s authorship guidelines, a statement has been added to the Acknowledgments section. All scientific content, data interpretation, literature selection, and conclusions are the sole responsibility of the authors. The AI tool does not meet authorship criteria and is not listed as an author.

AI assistance

Preparation of this manuscript involved using an AI-assisted writing tool (Grammarly) for language correction and editing support and ChatGPT to generate the Figure. All scientific content, data interpretation, literature selection, and conclusions are the sole responsibility of the authors. The AI tools do not meet authorship criteria and are not listed as authors.