Monocytes are central to innate and adaptive immunity, regulating tissue homeostasis and inflammation. However, data on their steady-state biodistribution in healthy humans are limited. We developed a non-invasive method to track radiolabelled autologous monocytes. Monocytes showed distinct intravascular transit kinetics vs neutrophils, with 45 min recovery of 47% for neutrophils vs 7% for monocytes (p=0.008), indicating predominant distribution in marginated pools. In patients with psoriatic and rheumatoid arthritis but no documented lung disease, we observed preferential early retention of monocytes in the lungs (transit half-life 25.2 min vs 14.2 min; p=0.008) compared with healthy volunteers, with reduced intravascular recovery in patients with rheumatoid arthritis (4.3% vs 12%; p=0.0043). These findings provide the first direct evidence of human monocyte pulmonary transit kinetics, systemic biodistribution and fate in health and inflammatory disease, advancing understanding of in vivo monocyte biology.

INTRODUCTION:Inflammation contributes to Alzheimer's disease (AD), but its stage-specific and amyloid-dependent patterns remain unclear. METHODS:We analyzed 964 participants from the Bio-Hermes cohort (cognitively normal [CN] = 404, mild cognitive impairment [MCI] = 302, mild AD = 258). Plasma levels of 32 cytokines, neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP) were quantified alongside core AD biomarkers. Associations with cognition, amyloid, apolipoprotein E (APOE) ε4, and clinical outcomes were assessed using analysis of covariance, partial correlations, and regression models. RESULTS:Twenty-four cytokines, NfL, and GFAP differed across cognitive groups. Amyloid stratification revealed a core amyloid-independent profile (14 cytokines + NfL) and a broader amyloid-specific profile including GFAP, interleukin (IL)-1β, and IL-18, implicating microglial inflammasome and astrocytic activation. Stage-dependent patterns suggested inflammation may act as early driver, concurrent process, or late amplifier. Paradoxical associations (e.g., eotaxin-2, IL-2R with better memory) and APOE ε4-linked immune differences indicated context-dependent roles. DISCUSSION:This exploratory study reveals biologically plausible, inflammatory heterogeneity in AD and highlights plasma cytokine profiles as candidate biomarkers and therapeutic targets, warranting investigation.

Neuroinflammation is an important contributor to Alzheimer’s disease (AD) pathogenesis. However, it is unclear whether inflammation drives, precedes, or occurs concurrently with AD pathology, and how inflammatory markers relate to established AD biomarkers. This study used the Bio-Hermes cohort to investigate associations between a comprehensive serum inflammatory panel, AD biomarkers, and key clinical/biological factors. Cross-sectional data from 964 participants in the Bio-Hermes study, comprising cognitively normal (CN=404), mild cognitive impairment (MCI=302), and mild AD dementia (AD=258) groups, were analyzed. Serum concentrations of 32 cytokines, NfL, GFAP, and AD biomarkers (Aβ40, Aβ42, Aβ42/Aβ40, total tau, p -tau181, and p -tau217) were included. Age/sex-adjusted ANCOVA compared cytokine, NfL, and GFAP levels across cognitive groups, stratified by amyloid and APOEε4 status (Tukey's HSD, FDR p <0.05). Pearson correlations assessed relationships between significant cytokines and AD plasma biomarkers across cognitive groups. Linear regressions assessed associations with clinical scores (MMSE, FAQ, RAVLT, GDS), adjusted for age, sex, education, and APOEε4. Significant differences across cognitive groups were found for NfL, GFAP, and 24 cytokines. NfL, GFAP, and Eotaxin-2/CCL24 showed stepwise increases (CN<MCI<AD). Seventeen cytokines (including Eotaxin/CCL11, SCF, IL-1β, TNF-RII) were elevated in AD versus both CN and MCI; IL-7 was lower. IL-2R was elevated in MCI and AD vs CN. Four cytokines (IFN-γ, IL-18, MCP-2/CCL8, MIP-1α/CCL3) were elevated in AD vs CN only. Most group differences persisted within amyloid-positive individuals. APOEε4 carriers had lower GFAP but higher IL-15, IL-20, and MCP-2/CCL8. Higher NfL, GFAP, IL-17A, IL-7 associated with lower MMSE in mild AD (IL-18 in MCI); higher Eotaxin-2 and IL-2R associated with better RAVLT scores, and higher IL-17A with lower RAVLT scores ( p <0.05). Serum inflammatory alterations, including 24 cytokines, strongly associate with cognitive status in Bio-Hermes, largely independent of amyloid pathology. Distinct inflammatory markers associate with APOEε4 genotype. Differences in biomarker correlation patterns and clinical associations highlight immune dysregulation across the AD spectrum. These findings reinforce inflammation’s role in AD pathophysiology, suggesting avenues for biomarker development and immunomodulatory therapies, warranting longitudinal investigation.

BACKGROUND:Depression has been increasingly characterised as a disorder of functional brain connectivity. Over the last two decades aberrant functional connectivity of large-scale resting-state brain networks implicated in inhibitory cognitive control, affective regulation, and self-referential thought, has been compellingly linked to depression. Capitalising on network-based accounts of depression, subsequent research endeavours have aimed at identifying functional connectomic signatures of treatment response in depression. However, to date, there has been little research on the connectomic features of psychotherapy response. METHODS:We investigated static and dynamic functional connectivity signatures of response to CBT in forty-six unmedicated patients with depression who underwent resting-state functional magnetic resonance imaging before and two months after completion of an Internet-delivered CBT intervention. RESULTS:At baseline, responders dwelled in a brain state characterised by greater functional connectivity between cognitive control and affective networks. Conversely, functional connectivity between cognitive control and default mode networks was comparatively weaker in the responders group. Notably, baseline functional connectivity significantly classified CBT response at the individual level with an area under the receiver operating characteristic curve of 0.85. CONCLUSION:These results are in accordance with current network-based accounts of CBT neural mechanisms, positing that greater cognitive control over negative emotion processing enables CBT response. This study extends previous findings on the network-based functional connectomic signatures of CBT response in depression.

Rationale: Monocytes play a pivotal role in both the innate and adaptive immune systems, contributing to tissue homeostasis and mediating inflammatory responses, for example in Acute Respiratory Distress Syndrome (ARDS). Despite the critical functions of monocytes, data on their steady-state biodistribution in healthy humans remain limited. To address this, we developed methodology to non-invasively track the biodistribution of radiolabelled autologous monocytes in vivo.Methods: Whole blood was collected from healthy volunteers (n=6), and monocytes isolated using plasma-Percoll gradient centrifugation and clinical-grade CD14 microbeads. Isolated monocytes were labelled with 99mTc-hexamethylpropylene amine oxime (HMPAO). After bolus intravenous injection of autologous 99mTc-labeled monocytes (54.1–150.1 MBq per subject), activity in the chest and upper abdomen was monitored using a double-headed SPECT/CT camera over 6 hours.Results: Monocytes displayed an extended transit time through the lungs, followed by notable uptake in the liver and spleen. The median first pass lung transit time for monocytes was 30.3 seconds, longer than previously reported for neutrophils. Additionally, monocytes exhibit distinct intravascular kinetics compared to neutrophils, with a significantly lower 45-minute recovery (P < 0.01). Conclusion: Our study identifies a substantial marginated pool of monocytes at steady-state homing to the lungs, liver and spleen. Importantly, this work provides the basis for future studies assessing monocyte distribution and intravascular lifespan in inflammatory settings, where there is interest in developing therapies targeting monocytes.

The prefrontal-hippocampal-entorhinal system is perhaps the most widely-studied circuit in cognitive and systems neuroscience, due to its role in supporting cognitive functions such as working memory and decision-making. Disrupted communication within this circuit is a key feature of disorders such as schizophrenia and dementia. Nucleus reuniens (NRe) is a midline thalamic nucleus that sits at the nexus of this circuit, linking these regions together. As there are no direct projections from prefrontal cortex to hippocampus (HPC), the accepted model is that the NRe mediates prefrontal drive of hippocampal activity, although these connections are poorly defined at the cellular and synaptic level. Using ex vivo optogenetics and electrophysiology in both mice and rats, alongside monosynaptic circuit-tracing, we sought to test the mechanisms through which NRe could drive hippocampal activity. Unexpectedly, we found no evidence that pyramidal cells in CA1 receive input from NRe, with midline thalamic input to HPC proper appearing selective for GABAergic interneurons. In other regions targeted by NRe, we found that pyramidal cells in prosubiculum and subiculum received synaptic inputs from NRe that were at least an order of magnitude weaker than those in prefrontal or entorhinal cortices. We conclude that, contrary to widely-held assumptions in the field, hippocampal pyramidal cells are not a major target of NRe.

Adaptive immune responses are intensely energy-dependent and rely on a local source of fuel-producing molecules which have been proposed to be derived from fat pads in which mammalian lymph nodes are embedded. However, the trigger for their release has not been identified. Here we demonstrate that cutaneous inflammation is directly correlated with rapid atrophy of perinodal fat pads and increase in embedded lymph node size. We further demonstrate that the fat pad atrophy is associated with influx of a CCR2-independent, lipid metabolising, macrophage population. Macrophage depletion ameliorates fat pad atrophy, and lymph node expansion, downstream of inflamed sites. Our data therefore identify peripheral inflammation as an antigen-independent trigger of downstream fat pad and lymph node remodelling and contributes to the release of essential nutrients to drive the energetic requirements of the adaptive immune response. Significance statement To our knowledge, this striking correlation between peripheral inflammation and reciprocal fat pad and lymph node remodelling has not been reported previously. Our report of this correlation and our mechanistic insight, have clear implications for our understanding of the inflammation-driven rapid release of sources of energy from fat pads to drive the immune response. Our data also potentially shed light on additional aspects of the functionality of pro-inflammatory vaccine adjuvants. We believe that our findings are fully novel and will be of interest to immunologists, infectious disease specialists and researchers interested in adipose tissue derived energetics. ### Competing Interest Statement The authors have declared no competing interest. Wellcome Trust, https://ror.org/029chgv08, 217093/Z/19/Z Medical Research Council, https://ror.org/03x94j517, MR/V01972/1

BACKGROUND:Depression is characterized by divergent changes in positive and negative affect. Emerging roles of inflammation in depression portend avenues for novel immunomodulator-based monotherapy, targeting mechanistically distinct symptoms such as anhedonia and pessimism. METHODS:To investigate links between these divergent affective components and inflammation, we used a probabilistic reinforcement-learning fMRI paradigm, testing for evidence of hyposensitivity to reward, and hypersensitivity to punishment in low-inflammation depression cases (loCRP depression; CRP ≤ 3 mg/L; N = 48), high-inflammation depression cases (hiCRP depression; CRP > 3 mg/L; N = 31), and healthy controls (HC; CRP ≤ 3 mg/L; N = 45). We aimed to (i) determine whether depression cases with high and low inflammation showed aberrant neural activation to monetary gains and losses compared to controls, and (ii) examine if these alterations correlated with a continuous measure of C-reactive protein (CRP) in depression, as well as indices of anhedonia and pessimism derived from behavioral instruments in depression. RESULTS:Voxel-wise activation was observed in key brain regions sensitive to monetary reward (ventromedial prefrontal cortex, vmPFC; nucleus accumbens, NAc) and punishment (insula) outcomes across all three groups. However, there was no significant difference in activation between groups. Within depression cases, increasing CRP scaled negatively with activation in the right vmPFC and left NAc but not insula cortex. However, there was no significant association between regional activation and severity of anhedonia or pessimism. CONCLUSIONS:Our results support the previously reported association between CRP and striatal reward reactivity in depression but do not extend this to processing of negatively valenced information.

Major depressive disorder (MDD) is a heterogeneous condition, hardly captured by single biomarkers. Transcriptomic profiles are important indicators for genes and pathways involved in MDD and its phenotypes. We performed whole-transcriptome and pathway analyses on data from 140 individuals (106 with MDD and 34 controls) from the observational, case-control, BIOmarkers in DEPression (BIODEP) study. We further divided MDD participants based on a) inflammation, measured with serum high-sensitivity C-reactive protein (hsCRP, mg/L) (n=39 CRP<1, n=31 CRP1-3, n=35 CRP>3), and b) exposure/response to antidepressants (n=47 non-responders, n=37 responders, n=22 unmedicated). We performed whole-blood RNA- sequencing using Illumina NextSeq 550 and statistical analyses with Deseq2 (RUV-corrected). We observe several transcripts differentially expressed (p<.05, fold-change>|1.2|, ranging from n=393 to n=1107) in all comparisons, both in the CRP-based and treatment-based groupings. Subsequent analyses with Ingenuity Pathway Analysis (p<.05, z-scores>|2|) reveal various immune-related pathways primarily involved in group differences. This involvement reflects the gradient of serum CRP levels within different MDD groups (progressive activation in CRP<1, 1- 3, >3) and is associated with non-responders and unmedicated MDD (both activated vs. responders). Interestingly, immune-related pathways are differentially activated also in all the three MDD treatment-based subgroups, in the MDD CRP 1-3 and >3, and in the whole-MDD (all cases together) vs. controls; while the main pathways involved in the MDD CRP<1 vs. controls are cell cycle-related. Our findings demonstrate different transcriptomic pathways associatedwith MDD and different MDD phenotypes, which could ultimately inform more tailored approaches to clinical management.

Neurological complications, including encephalopathy and stroke, occur in a significant proportion of COVID-19 cases but viral protein is seldom detected in the brain parenchyma. To model this situation, we developed a novel low-inoculum K18-hACE2 mouse model of SARS-CoV-2 infection during which active viral replication was consistently seen in mouse lungs but not in the brain. We found that several mediators previously associated with encephalopathy in clinical samples were upregulated in the lung, including CCL2, and IL-6. In addition, several inflammatory mediations, including CCL4, IFNγ, IL-17A, were upregulated in the brain, associated with microglial reactivity. Parallel in vitro experiments demonstrated that the filtered supernatant from SARS-CoV-2 virion exposed brain endothelial cells induced activation of uninfected microglia. This model successfully recreates SARS-CoV-2 virus-associated para-infectious brain inflammation which can be used to study the pathophysiology of the neurological complications and the identification of potential immune targets for treatment.

Major depressive disorder (MDD) is associated with inflammation, and 25-30% of patients show low-grade inflammation (C-Reactive Protein (CRP)>3 mg/L). Sex hormones act on inflammation and might contribute to sex-specific differences in MDD. We investigated the interplay between sex hormones and inflammation within-sexes. We measured serum high-sensitivity CRP, testosterone, 17β-estradiol (E2), progesterone, sex-hormone binding globulin (SHBG), gonadotropins levels; we calculated testosterone to E2 ratio (T/E2), free androgen and estradiol indexes (FAI, FEI) in 64 controls and 178 MDD patients. We performed One-way ANOVA with Bonferroni's correction, Pearson's correlations, and linear regressions to investigate sex hormones in controls (25 males, 39 females), patients with CRP<3 mg/L (CRP-) (53 males, 72 females), and patients with CRP>3 mg/L (CRP+) (19 males, 34 females). In males, CRP+ patients showed lower testosterone than controls (p=0.001), lower testosterone (p=0.013), T/E2 (p<0.001) and higher FEI (p=0.015) than CRP- patients. In patients only, CRP negatively correlated with testosterone (r=-. 312, p< 0.005), T/E2 (r=-.274, p< 0.05), and progesterone (r=-223, p<0.05), with testosterone (p=0.020) and T/E2 (p=0.042) predicting CRP (R2 =0.214 p=0.001). In females, CRP+ patients showed lower SHBG than controls (p=0.033) and CRP- patients (p=0.034), which became statistical trends when co-variating for ovulatory phases. In patients only, CRP negatively correlated with testosterone (r=-.285 p=0.003) and SHBG (r=-292 p=0.003), with SHBG (p=0.012) predicting CRP (R2 =.115 p=0.002). MDD patients with inflammation show sex hormone imbalance. Future research is needed to understand this association and hormonal therapies' potential.

Childhood trauma (CT) is associated with increased inflammation, independently, and comorbidly with, depression. There is also evidence for differential inflammatory gene expression in depression treatment response. Previous analysis identified a trend for the role of CT on NR3C1 expression, however, differences were attributable to depression treatment response. The present work explores the role of CT in inflammation and depression treatment response. The BIOmarkers in DEPression study recruited 170 participants as controls (n=40), responders (n=36), treatment-resistant depressed (TRD) (n=58), or untreated-depressed (UD)(n=36). The Hamilton Depression Rating Scale measured depression. CT was categorised as none-low (n=90), or moderate-severe trauma (n=80) according to the Childhood Trauma Questionnaire. mRNA quantitative polymerase chain reaction (qPCR) measured 7 candidate inflammatory gene expression: A2M, FKBP5, IL6, MIF, SGK1, STAT1, and TNF. Univariate analyses explored inflammatory gene expression differences across CT and treatment response groups. CT did not play a significant role in the differential expression; depression treatment response groups were significantly associated with increased A2M (p<.001), FKBP5 (p<.001), IL6 (p<.001), MIF (p<.001), SGK1 (p<.001) and TNF (p<.001). Responders showed highest mean A2M (.821, SE=.015) and IL-6 expression (.820, SE=.012); TRD showed highest FKBP5 (.819, SE=.010) and TNF (.840, SE=.008); UD participants showed the highest MIF (.832, SE=.013), SGK1 (.801, SE=.008) and STAT1 (.800, SE=.014). Despite a previous trend for CT in NR3C1 expression, increases in further inflammatory gene expression were driven by differences in depression treatment response, with no effect of CT.

The last two decades have seen a flourishing of research into the immunobiology of psychiatric phenotypes, in particular major depressive disorder. Both preclinical and clinical data have highlighted pathways and possible mechanisms that might link changes in immunobiology, most especially inflammation, to clinically relevant behaviour. From a therapeutics perspective, a major impetus has been the action of Biologics, often monoclonal antibodies, that target specific cytokines acting as "molecular scalpels" helping to uncover the actions of those proteins. These interventions have been associated with improvements in mood and related symptoms. There are now enough studies and participants to permit meta-analytic analyses of the actions of these and other anti-inflammatory agents.In this chapter, the focus is on the evidence for the role of inflammation biology in depression and the meta-analytic data from trials. The putative mechanisms that might underpin the antidepressant effect of anti-inflammatory drugs are also explored. Lastly, I describe the more stubborn difficulties around heterogeneity, deep phenotyping and stratification as well as improved animal models and greater understanding of the biology that might be addressed by future studies.

Sex hormones have biological effects on inflammation, and these might contribute to the sex -specific features of depression. C -reactive protein (CRP) is the most widely used inflammatory biomarker and consistent evidence shows a significant proportion (20 -30 %) of patients with major depressive disorder (MDD) have CRP levels above 3 mg/L, a threshold indicating at least low-grade inflammation. Here, we investigate the interplay between sex hormones and CRP in the cross-sectional, observational Biomarkers in Depression Study. We measured serum high -sensitivity (hs-)CRP, in 64 healthy controls and 178 MDD patients, subdivided into those with hs-CRP below 3 mg/L (low -CRP; 53 males, 72 females) and with hs-CRP above 3 mg/L (high -CRP; 19 males, 34 females). We also measured interleukin-6, testosterone, 17-8-estradiol (E2), progesterone, sex -hormone binding globulin (SHBG), follicle -stimulating and luteinising hormones, and calculated testosterone -to -E2 ratio (T/E2), free androgen and estradiol indexes (FAI, FEI), and testosterone secretion index. In males, high -CRP patients had lower testosterone than controls ( p = 0.001), and lower testosterone ( p = 0.013), T/E2 ( p < 0.001), and higher FEI ( p = 0.015) than low -CRP patients. In females, high -CRP patients showed lower SHGB levels than controls ( p = 0.033) and low -CRP patients ( p = 0.034). The differences in testosterone, T/E2 ratio, and FEI levels in males survived the Benjamini-Hochberg FDR correction. In linear regression analyses, testosterone (8 = -1.069 p = 0.033) predicted CRP concentrations (R 2 = 0.252 p = 0.002) in male patients, and SHBG predicted CRP levels (8 = -0.628 p = 0.009, R 2 = 0.172 p = 0.003) in female patients. These findings may guide future research investigating interactions between gonadal and immune systems in depression, and the potential of hormonal therapies in MDD with inflammation.

OBJECTIVE:Chronic fatigue is a major clinical unmet need among patients with rheumatoid arthritis (RA). Current therapies are limited to nonpharmacological interventions, such as personalized exercise programs (PEPs) and cognitive-behavioral approaches (CBAs); however, most patients still continue to report severe fatigue. To inform more effective therapies, we conducted a magnetic resonance imaging (MRI) brain study of PEPs and CBAs, nested within a randomized controlled trial (RCT), to identify their neurobiological mechanisms of fatigue reduction in RA. METHODS:A subgroup of patients with RA (n = 90), participating in an RCT of PEPs and CBAs for fatigue, undertook a multimodal MRI brain scan following randomization to either usual care (UC) alone or in addition to PEPs and CBAs and again after the intervention (six months). Brain regional volumetric, functional, and structural connectivity indices were curated and then computed employing a causal analysis framework. The primary outcome was fatigue improvement (Chalder fatigue scale). RESULTS:Several structural and functional connections were identified as mediators of fatigue improvement in both PEPs and CBAs compared to UC. PEPs had a more pronounced effect on functional connectivity than CBAs; however, structural connectivity between the left isthmus cingulate cortex (L-ICC) and left paracentral lobule (L-PCL) was shared, and the size of mediation effect ranked highly for both PEPs and CBAs (ßAverage = -0.46, SD 0.61; ßAverage = -0.32, SD 0.47, respectively). CONCLUSION:The structural connection between the L-ICC and L-PCL appears to be a dominant mechanism for how both PEPs and CBAs reduce fatigue among patients with RA. This supports its potential as a substrate of fatigue neurobiology and a putative candidate for future targeting.

BackgroundAltered neural haemodynamic activity during decision making and learning has been linked to the effects of inflammation on mood and motivated behaviours. So far, it has been reported that blunted mesolimbic dopamine reward signals are associated with inflammation-induced anhedonia and apathy. Nonetheless, it is still unclear whether inflammation impacts neural activity underpinning decision dynamics. The process of decision making involves integration of noisy evidence from the environment until a critical threshold of evidence is reached. There is growing empirical evidence that such process, which is usually referred to as bounded accumulation of decision evidence, is affected in the context of mental illness.MethodsIn a randomised, placebo-controlled, crossover study, 19 healthy male participants were allocated to placebo and typhoid vaccination. Three to four hours post-injection, participants performed a probabilistic reversal-learning task during functional magnetic resonance imaging. To capture the hidden neurocognitive operations underpinning decision-making, we devised a hybrid sequential sampling and reinforcement learning computational model. We conducted whole brain analyses informed by the modelling results to investigate the effects of inflammation on the efficiency of decision dynamics and reward learning.ResultsWe found that during the decision phase of the task, typhoid vaccination attenuated neural signatures of bounded evidence accumulation in the dorsomedial prefrontal cortex, only for decisions requiring short integration time. Consistent with prior work, we showed that, in the outcome phase, mild acute inflammation blunted the reward prediction error in the bilateral ventral striatum and amygdala.ConclusionsOur study extends current insights into the effects of inflammation on the neural mechanisms of decision making and shows that exogenous inflammation alters neural activity indexing efficiency of evidence integration, as a function of choice discriminability. Moreover, we replicate previous findings that inflammation blunts striatal reward prediction error signals.