Translocator protein is a marker of activated microglia in rodent models but not human neurodegenerative diseases

Regional skull translocator protein elevation in autistic adults detected by PET-MRI

Immune processes have been implicated in the pathophysiology of autism spectrum disorder (ASD). Brain borders, such as the skull, have recently been highlighted as sites where neuro-immune interactions occur with key consequences for brain immunity. Translocator protein (TSPO), a mitochondrial protein involved in immune functions, was measured in the skull using [11C]PBR28 positron emission tomography-magnetic resonance imaging (PET-MRI) in 38 autistic adults (26 males, 12 females) and 29 age-and sex-matched healthy controls (19 males, 10 females). [11C]PBR28 uptake relative to a pseudo-reference region assessed using standardized uptake value ratio (SUVR) revealed elevated TSPO in autistic adults in frontal and temporal skull. We did not observe an association between [11C]PBR28 uptake in total or regional skull areas and autism symptom severity. C-reactive protein levels were positively associated with [11C]PBR28 uptake in the total skull across participants. Lastly, [11C]PBR28 uptake in the total skull was stable across a 4-month period. This work indicates regional TSPO elevations in the skull in autistic adults, which may suggest immune involvement.

[11C]ER176 images brain inflammation across TSPO genotypes and colocalizes with tau

Accurately measuring brain inflammation in Alzheimer's disease (AD) is crucial due to the role of inflammatory processes in neurodegeneration. In a recent study, Appleton, Finn, et al. used [11C]ER176, a novel translocator protein 18 kDa (TSPO)-positron emission tomography (PET) tracer overcoming genotype-related binding issues, to show increased inflammation in early-onset AD, with patterns aligning more closely with tau pathology than amyloid deposition or atrophy.

Radiation dosimetry of the 18 kDa translocator protein ligand [18F]PBR111 in humans

PURPOSE

The 18 kDa translocator protein (TSPO) is a mitochondrial protein that becomes overexpressed during neuroinflammatory conditions, such as in Alzheimer's disease or multiple sclerosis. TSPO is of interest because it serves as a marker for microglial and astrocytic activity, measurable via in vivo positron emission tomography (PET) molecular imaging. [18F]PBR111 is a second-generation TSPO PET radioligand with high signal specificity but a sensitivity to TSPO polymorphism, in comparison with first-generation ligands. This study focused on the biodistribution and dosimetry of [18F]PBR111 in healthy humans.

METHOD

Six volunteers (three males, three females) were administered approximately 200 MBq of [18F]PBR111. Organs such as the lungs and liver showed the highest initial radioactivity level, while the bone marrow and bladder accumulated activity over time, likely reflecting ligand defluorination and elimination.

RESULTS

Dosimetry findings revealed a total effective dose of 16.17 μSv/MBq, equivalent to 3.04 mSv per examination. Compared to animal models, human dosimetry showed lower radiation exposure, highlighting discrepancies in predictive models. Organ-specific dose comparisons with other TSPO ligands ([18F]PBR06, [18F]FEPPA, [18F]FEDAA1106) revealed similar distribution patterns. This study underscores the clinical viability of [18F]PBR111 for TSPO imaging, providing critical data for optimizing its safe use in research and clinical settings.

CONCLUSION

The findings support its potential for studying neuroinflammatory and systemic diseases. The trial registration number is NCT06398392.

Glial activation among individuals with neurological post-acute sequelae of coronavirus disease 2019: A positron emission tomography study of brain fog using [18F]-FEPPA

Background

This study examined the regional distribution of glial activation in essential workers with neurological post-acute sequelae of coronavirus disease 2019 (COVID-19) infections (N-PASC).

Methods

We injected ≤185 MBq of [18F]-FEPPA as an intravenous bolus and positron-emission tomography over 2 h. To measure distribution volume (VT) we recruited 24 essential workers (14 N-PASC, 10 Never-COVID-19 Controls, of whom 22 successfully placed arterial lines). Individuals with low binding affinity were excluded from this study, and VT was adjusted for translocator protein genotype. Analyses that passed the false discovery rate are reported.

Results

Participants at midlife survived mild to moderate COVID-19 without hospitalization but reported onset of post-acute sequelae of COVID-19 (PASC) for, on average, 22 months before undergoing neuroimaging. Hippocampal VT was higher (VT = 1.70, 95% C.I. = [1.30-2.21], p = 0.001) in participants with persistent brain fog after COVID-19, reflecting an increase of 10.58 mL/cm3 in VT (area under the receiver-operating curve, AUC = 0.95 [0.85-1.00]). At a cutoff of 10.6, sensitivity/specificity/accuracy were 0.88/0.93/0.91.

Conclusion

The results from this study imply that neuroimmune response is a distinct and identifiable characteristic of brain fog after COVID-19. Results suggest that [18F]-FEPPA could be used to support N-PASC diagnosis.

Cigarette smoking is associated with reduced neuroinflammation and better cognitive control in people living with HIV

People living with HIV (HIV+) are roughly twice as likely to smoke cigarettes (Smok+) as the general population. With the advent of effective antiretroviral therapies, it is increasingly important to understand the effects of chronic HIV infection and cigarette smoking on brain function and cognition since HIV+ individuals have heightened neuroinflammation and cognitive deficits even with such therapies. Based on prior studies demonstrating that smoking reduces a marker for neuroinflammation in HIV- individuals, we hypothesized that HIV+/Smok+ individuals would have less neuroinflammation and better cognitive control than HIV+/Smok- individuals. Fifty-nine participants (HIV-/Smok- [n = 16], HIV-/Smok+ [n=14], HIV+/Smok- [n = 18], and HIV+/Smok+ [n = 11]) underwent baseline eligibility tests, positron emission tomography (PET) scanning to determine levels of a marker for neuroinflammation, and assessment of cognitive control with the reverse-translated 5-choice continuous performance test (5C-CPT), with smokers having smoked to satiety prior to testing. For the PET data, a significant effect of smoking status on whole brain (WB) standardized uptake value (SUV) was found between HIV+/Smok+ and HIV+/Smok- participants (due to 18.8% lower WB SUV in the HIV+/Smok+ group). HIV+/Smok- participants exhibited a mean 13.5% higher WB SUV than HIV-/Smok- participants. For the 5C-CPT, HIV+/Smok+ participants performed significantly better than HIV+/Smok- participants (d prime), and HIV+/Smok- participants performed worse than HIV-/Smok- participants. Thus, HIV+/Smok+ individuals demonstrated lower levels of the neuroinflammation marker and better cognitive control than HIV+/Smok- individuals. Given that HIV+ individuals whose HIV is well-controlled can still have chronic neurocognitive complications, study results suggest possible paths for future research into nicotine-related treatments to prevent such complications.

Early metabolic changes in the brain of Alzheimer's disease rats are driven by GLAST+ cells

Glucose metabolic dysfunction is a hallmark of Alzheimer's disease (AD) pathology and is used to diagnose the disease or predict imminent cognitive decline. The main method to measure brain metabolism in vivo is positron emission tomography with 2-Deoxy-2-[18F]fluoroglucose ([18F]FDG-PET). The cellular origin of changes in the [18F]FDG-PET signal in AD is controversial. We addressed this by combining [18F]FDG-PET with subsequent cell-sorting and γ-counting of [18F]FDG-accumulation in sorted cell populations. 7-month-old male TgF344-AD rats and wild-type controls (n = 24/group) received sham or ceftriaxone (200 mg/kg) injection prior to [18F]FDG-PET imaging to increase glutamate uptake and glucose utilisation. The same animals were injected again one week later, and radiolabelled brains were dissected, with hippocampi taken for magnetically-activated cell sorting of radioligand-treated tissues (MACS-RTT). Radioactivity in sorted cell populations was measured to quantify cell-specific [18F]FDG uptake. Transcriptional analyses of metabolic enzymes/transporters were also performed. Hypometabolism in the frontal association cortex of TgF344-AD rats was identified using [18F]FDG-PET, whereas hypermetabolism was identified in the hippocampus using MACS-RTT. Hypermetabolism was primarily driven by GLAST+ cells. This was supported by transcriptional analyses which showed alteration to metabolic apparatus, including upregulation of hexokinase 2 and altered expression of glucose/lactate transporters. See Figure 1 for summary.

Investigation of patterns and associations of neuroinflammation in cognitive impairment

Neuroinflammation has been identified as an important pathological component of cognitive impairment, and translocator protein imaging has become a valuable tool for assessing its patterns. We aimed to obtain the exact distribution of neuroinflammation in cognitive impairment and its underlying mechanisms with amyloid-beta. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines, two investigators searched literature databases for studies that measured translocator protein binding levels. This measurement was performed between healthy controls and subjects with mild cognitive impairment or Alzheimer's disease via voxel-based positron emission tomography image analysis at the whole-brain level. This meta-analysis was performed with the anisotropic effect-size based algorithm. Neuroinflammation in patients with mild cognitive impairment was mainly concentrated in the left middle temporal gyrus and left amygdala. In Alzheimer's disease patients, the brain regions involved were the left inferior temporal gyrus, left calcarine fissure/surrounding cortex, left parahippocampal gyrus, right lingual gyrus, and right middle temporal gyrus. In addition, neuroinflammation in patients with cognitive impairment was highly correlated with amyloid-beta deposition in the cortex. This study deepens our understanding of the patterns of neuroinflammation in patients with cognitive impairment and its interaction with amyloid-beta, providing potential insights for therapeutic approaches targeting neuroinflammation in Alzheimer's disease.

The Matrix of Mitochondrial Imaging: Exploring Spatial Dimensions

Mitochondria play a crucial role in human biology, affecting cellular processes at the smallest spatial scale as well as those involved in the functionality of the whole system. Imaging is the most important research tool for studying the fundamental role of mitochondria across these diverse spatial scales. A wide array of available imaging techniques have enabled us to visualize mitochondrial structure and behavior, as well as their effect on cells and tissues in a range from micrometers to centimeters. Each of the various imaging techniques that are available offers unique advantages tailored to specific research needs. Selecting an appropriate technique suitable for the scale and application of interest is therefore crucial, but can be challenging due to the large range of possibilities. The aim of this review is two-fold. First, we provide an overview of the available imaging techniques and discuss their strengths and limitations for applications across the sub-mitochondrial, cellular, tissue and organ levels for the imaging of mitochondria. Second, we identify opportunities for novel applications and advancement in the field. We emphasize the importance of integration across scales in mitochondrial imaging studies, particularly to bridge the gap between microscopic and non-invasive techniques. While integrating these diverse scales is challenging, primarily because such multi-scale approaches require expertise that spans different imaging modalities, we argue that integration has the potential to provide groundbreaking insights into mitochondrial biology. By providing a comprehensive overview of imaging techniques, this review paves the way for multi-scale imaging initiatives in mitochondrial research.

Development and evaluation of [11C]DPA-813 and [18F]DPA-814: novel TSPO PET tracers insensitive to human single nucleotide polymorphism rs6971

PURPOSE

The translocator protein 18 kDa (TSPO) is a widely used marker for imaging neuroinflammation via Positron Emission Tomography (PET). However, the vast majority of reported TSPO PET tracers display low binding affinity to a common isoform of human TSPO (rs6971; A147T), making them unsuitable for universal use in the general population. In this study, we have developed and preclinically validated two novel tracers designed to image TSPO in patients of all genotypes.

METHODS

Novel analogues of known TSPO ligands were synthesised, evaluated for TSPO binding affinity in vitro (membranes prepared from transfected HEK-293T cells expressing wild-type (WT) or A147T TSPO) and radiolabelled with carbon-11 or fluorine-18. They were evaluated in situ (autoradiography on genotyped human brain tissue) and in vivo (rat, both WT and clinically relevant experimental autoimmune encephalomyelitis (EAE) neuroinflammation model) as potential polymorphism-insensitive TSPO PET tracers.

RESULTS

Two new TSPO ligands, DPA-813 and DPA-814, displayed equivalent single-digit nanomolar binding affinities in vitro towards both human TSPO isoforms. [11C]DPA-813 and [18F]DPA-814 were synthesised in moderate radiochemical yields, high radiochemical purity, and high molar activity. Autoradiography on human MS tissues showed high specific binding for both tracers, irrespective of the TSPO isoform. The tracers demonstrated high plasma stability after 45 min and no brain metabolism with > 99% intact tracer. Biodistribution in WT animals indicated good brain uptake for both tracers (0.28 and 0.41%ID/g for [18F]DPA-814 and [11C]DPA-813, respectively). PET imaging in the clinically relevant EAE neuroinflammation model in rats showed significantly higher uptake of [11C]DPA-813 and [18F]DPA-814 in the spinal cord of the EAE rats compared to the controls.

CONCLUSION

We have developed two novel PET tracers that display indiscriminately high binding affinity to both common isoforms of human TSPO, show favourable metabolic stability and brain penetration in rats, and significantly higher uptake in the spinal cord of a neuroinflammatory rat model of multiple sclerosis. Going forward, first-in-human clinical validation will mark a critical juncture in the development of these tracers, which could offer substantial improvements over existing imaging tools for detecting neuroinflammation, irrespective of genetic variations.

Extra-axial inflammatory signal and its relationship to peripheral and central immunity in depression

Although both central and peripheral inflammation have been observed consistently in depression, the relationship between the two remains obscure. Extra-axial immune cells may play a role in mediating the connection between central and peripheral immunity. This study investigates the potential roles of calvarial bone marrow and parameningeal spaces in mediating interactions between central and peripheral immunity in depression. PET was used to measure regional TSPO expression in the skull and parameninges as a marker of inflammatory activity. This measure was correlated with brain TSPO expression and peripheral cytokine concentrations in a cohort enriched for heightened peripheral and central immunity comprising 51 individuals with depression and 25 healthy controls. The findings reveal a complex relationship between regional skull TSPO expression and both peripheral and central immunity. Facial and parietal skull bone TSPO expression showed significant associations with both peripheral and central immunity. TSPO expression in the confluence of sinuses was also linked to both central and peripheral immune markers. Group-dependent elevations in TSPO expression within the occipital skull bone marrow were also found to be significantly associated with central inflammation. Significant associations between immune activity within the skull, parameninges, parenchyma and periphery highlight the role of the skull bone marrow and venous sinuses as pivotal sites for peripheral and central immune interactions.

Bridging peripheral and central inflammation with TSPO-PET: insights into depression and beyond

This scientific commentary refers to ‘Extra-axial inflammatory signal and its relationship to peripheral and central immunity in depression’ by Eiff et al. (https://doi.org/10.1093/brain/awae343).

Imaging the translocator protein 18 kDa within cognitive control and declarative memory circuits in virally suppressed people with HIV

OBJECTIVES

Virally suppressed people with HIV (VS-PWH) show heterogeneity in patterns of cognitive dysfunction. To better understand the relationship between the neuroimmune response and cognition, we used PET to image the translocator protein 18 kDa (TSPO). The study examined HIV-serostatus differences in TSPO as well as associations between regional TSPO and select cognitive processes defined using the Research Domain Criteria (RDoC) framework.

DESIGN

Cross-sectional investigation in VS-PWH ( n  = 25) versus HIV-uninfected individuals ( n  = 18) of cognitive control and declarative memory, as well as [ 11 C]DPA-713 PET measures of TSPO within cognitive control and declarative memory regions of interest (ROI).

METHODS

Group differences in [ 11 C]DPA-713 binding ( VT ) in cognitive control or declarative memory regions were examined using linear mixed models. Tests of associations between factor-derived cognitive system measures and PET measures were performed, controlling for TSPO genotype.

RESULTS

There were no group differences in any of the four factor-derived cognitive system measures. VS-PWH had higher log [ 11 C]DPA-713 VT across cognitive control regions [unstandardized beta coefficient reflecting mean difference [ B ] = 0.23, SE = 0.11, 95% confidence interval (CI) 0.01-0.45, P  = 0.04] and declarative memory regions ( B   =  0.24, SE = 0.11, 95% CI 0.02-0.45, P  = 0.03). Higher log [ 11 C]DPA-713 VT in cognitive control regions related to poorer cognitive control in each group, and to worse self-reported cognitive performance in VS-PWH. Log [ 11 C]DPA-713 VT in each declarative memory region did not associate with measured declarative memory.

CONCLUSION

A localized neuroimmune response marked by high TSPO in brain regions that subserve cognitive control may contribute to poorer cognitive control in VS-PWH.

Immune mechanisms and shared immune targets in neurodegenerative diseases

The immune system plays a major part in neurodegenerative diseases. In some, such as multiple sclerosis, it is the primary driver of the disease. In others, such as Alzheimer disease, amyotrophic lateral sclerosis and Parkinson disease, it has an amplifying role. Immunotherapeutic approaches that target the adaptive and innate immune systems are being explored for the treatment of almost all neurological diseases, and the targets and approaches are often common across diseases. Microglia are the primary immune cells in the brain that contribute to disease pathogenesis, and are consequently a common immune target for therapy. Other therapeutic approaches target components of the peripheral immune system, such as regulatory T cells and monocytes, which in turn act within the CNS. This Review considers in detail how microglia, monocytes and T cells contribute to the pathogenesis of multiple sclerosis, Alzheimer disease, amyotrophic lateral sclerosis and Parkinson disease, and their potential as shared therapeutic targets across these diseases. The microbiome is also highlighted as an emerging therapeutic target that indirectly modulates the immune system. Therapeutic approaches being developed to target immune function in neurodegenerative diseases are discussed, highlighting how immune-based approaches developed to treat one disease could be applicable to multiple other neurological diseases.

Cerebral Microbleeds and Their Association With Inflammation and Blood-Brain Barrier Leakage in Small Vessel Disease

BACKGROUND

How cerebral microbleeds (CMBs) are formed, and how they cause tissue damage is not fully understood, but it has been suggested they are associated with inflammation, and they could also be related to increased blood-brain barrier (BBB) leakage. We investigated the relationship of CMBs with inflammation and BBB leakage in cerebral small vessel disease, and in particular, whether these 2 processes were increased in the vicinity of CMBs.

METHODS

In 54 patients with sporadic cerebral small vessel disease presenting with lacunar stroke, we simultaneously assessed microglial activation using the positron emission tomography ligand [11C]PK11195 and BBB leakage using dynamic contrast enhanced magnetic resonance imaging, on a positron emission tomography-magnetic resonance imaging system. To assess local inflammation and BBB leakage, 3 one-voxel concentric shells were generated around each CMB on susceptibility-weighted imaging and resampled to positron emission tomography and T1 mapping images, respectively. In these 3 shells, we calculated the mean of PK11195 nondisplaceable binding potential (BPND) as a marker of microglial activation, as well as the mean influx rate as a marker of BBB leakage. In addition, 93 blood biomarkers related to cardiovascular disease, inflammation, and endothelial activation were measured to quantify systemic inflammation.

RESULTS

No significant associations were found between the number of CMBs and the measures for microglial activation (β=2.6×10-5, P=0.050) and BBB leakage (β=-0.0001, P=0.400) in the white matter. There was no difference in measures of microglial activation (P=0.403) or BBB leakage (P=0.423) across the 3 shells surrounding the CMBs. Furthermore, after correcting for multiple comparisons, no associations were observed between systemic inflammation biomarkers and the number of CMBs.

CONCLUSIONS

We found no evidence that CMBs are associated with either microglial activation assessed by [11]CPK11195 positron emission tomography or BBB leakage assessed by dynamic contrast enhanced magnetic resonance imaging, either globally or locally, in sporadic cerebral small vessel disease. There was also no association with markers of systemic inflammation.

[18F]SF51, a novel 18F-labeled PET radioligand for translocator protein 18kDa (TSPO) in brain, works well in monkeys but fails in humans

[18F]SF51 is a novel radioligand for imaging translocator protein 18 kDa (TSPO) that previously displayed excellent imaging properties in nonhuman primates. This study assessed its performance in human brain and its dosimetry. Seven healthy participants underwent brain PET imaging to measure TSPO binding using a two-tissue compartment model (2TCM) to calculate total distribution volume (VT). This cohort included two high-affinity binders (HABs), three mixed-affinity binders (MABs), and two low-affinity binders (LABs). Two other participants received whole-body scans to assess radiation exposure. Peak brain radioactivity reached a standardized uptake value (SUV) of 1.4 at 3 minutes post-injection, diminishing to 30% of peak by 120 minutes. The average VT for all genotype groups was notably low (<1 mL·cm-3), emphasizing the radioligand's poor binding in brain. [18F]SF51 remained sensitive to the TSPO polymorphism in vivo, as shown by a two-fold difference in VT between HABs and LABs. VT stabilization by 80 minutes post-injection suggested minimal radiometabolite accumulation in brain. The average effective dose was 13.8 ± 0.9 µSv/MBq. Contrary to previously published animal data, [18F]SF51 showed low binding to human TSPO, with uptake remaining influenced by the rs6971 polymorphism. These findings highlight the challenges of developing TSPO radioligands and underscore the significant species differences that may influence translational outcomes.ClinicalTrials.gov identifier: NCT05564429; registered 10/03/2022.

Characteristics of TSPO expression in marmoset EAE

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) and is a leading non-traumatic cause of disability in young adults. The 18 kDa Translocator Protein (TSPO) is a mitochondrial protein and positron emission tomography (PET)-imaging target that is highly expressed in MS brain lesions. It is used as an inflammatory biomarker and has been proposed as a therapeutic target. However, its specific pathological significance in humans is not well understood. Experimental autoimmune encephalomyelitis (EAE) in the common marmoset is a well-established primate model of MS. Studying TSPO expression in this model will enhance our understanding of its expression in MS. This study therefore characterizes patterns of TSPO expression in fixed CNS tissues from one non-EAE control marmoset and 8 EAE marmosets using multiplex immunofluorescence. In control CNS tissue, we find that TSPO is expressed in the leptomeninges, ependyma, and over two-thirds of Iba1 + microglia, but not astrocytes or neurons. In Iba1 + cells in both control and acute EAE tissue, we find that TSPO is co-expressed with markers of antigen presentation (CD74), early activation (MRP14), phagocytosis (CD163) and anti-inflammatory phenotype (Arg1); a high level of TSPO expression is not restricted to a particular microglial phenotype. While TSPO is expressed in over 88% of activated Iba1 + cells in acute lesions in marmoset EAE, it also is sometimes observed in subsets of astrocytes and neurons. Additionally, we find the percentage of Iba1 + cells expressing TSPO declines significantly in lesions > 5 months old and may be as low as 13% in chronic lesions. However, we also find increased astrocytic TSPO expression in chronic-appearing lesions with astrogliosis. Finally, we find expression of TSPO in a subset of neurons, most frequently GLS2 + glutamatergic neurons. The shift in TSPO expression from Iba + microglia/macrophages to astrocytes over time is similar to patterns suggested by earlier neuropathology studies in MS. Thus, marmoset EAE appears to be a clinically relevant model for the study of TSPO in immune dysregulation in human disease.

Brain inflammation co-localizes highly with tau in mild cognitive impairment due to early-onset Alzheimer's disease

Brain inflammation, with an increased density of microglia and macrophages, is an important component of Alzheimer's disease and a potential therapeutic target. However, it is incompletely characterized, particularly in patients whose disease begins before the age of 65 years and, thus, have few co-pathologies. Inflammation has been usefully imaged with translocator protein (TSPO) PET, but most inflammation PET tracers cannot image subjects with a low-binder TSPO rs6971 genotype. In an important development, participants with any TSPO genotype can be imaged with a novel tracer, 11C-ER176, that has a high binding potential and a more favourable metabolite profile than other TSPO tracers currently available. We applied 11C-ER176 to detect brain inflammation in mild cognitive impairment (MCI) caused by early-onset Alzheimer's disease. Furthermore, we sought to correlate the brain localization of inflammation, volume loss, elevated amyloid-β (Aβ)and tau. We studied brain inflammation in 25 patients with early-onset amnestic MCI (average age 59 ± 4.5 years, 10 female) and 23 healthy controls (average age 65 ± 6.0 years, 12 female), both groups with a similar proportion of all three TSPO-binding affinities. 11C-ER176 total distribution volume (VT), obtained with an arterial input function, was compared across patients and controls using voxel-wise and region-wise analyses. In addition to inflammation PET, most MCI patients had Aβ (n = 23) and tau PET (n = 21). For Aβ and tau tracers, standard uptake value ratios were calculated using cerebellar grey matter as region of reference. Regional correlations among the three tracers were determined. Data were corrected for partial volume effect. Cognitive performance was studied with standard neuropsychological tools. In MCI caused by early-onset Alzheimer's disease, there was inflammation in the default network, reaching statistical significance in precuneus and lateral temporal and parietal association cortex bilaterally, and in the right amygdala. Topographically, inflammation co-localized most strongly with tau (r = 0.63 ± 0.24). This correlation was higher than the co-localization of Aβ with tau (r = 0.55 ± 0.25) and of inflammation with Aβ (0.43 ± 0.22). Inflammation co-localized least with atrophy (-0.29 ± 0.26). These regional correlations could be detected in participants with any of the three rs6971 TSPO polymorphisms. Inflammation in Alzheimer's disease-related regions correlated with impaired cognitive scores. Our data highlight the importance of inflammation, a potential therapeutic target, in the Alzheimer's disease process. Furthermore, they support the notion that, as shown in experimental tissue and animal models, the propagation of tau in humans is associated with brain inflammation.

18 kDa TSPO targeting drives polarized human microglia towards a protective and restorative neurosteroidome profile

An aberrant pro-inflammatory microglia response has been associated with most neurodegenerative disorders. Identifying microglia druggable checkpoints to restore their physiological functions is an emerging challenge. Recent data have shown that microglia produce de novo neurosteroids, endogenous molecules exerting potent anti-inflammatory activity. Here, the role of neurosteroidogenesis in the modulation of microgliosis was explored in human microglia cells. In particular, CYP11A1 inhibition or TSPO pharmacological stimulation, crucial proteins involved in the rate limiting step of the neurosteroidogenic cascade, were employed. CYP11A1 inhibition led microglia to acquire a dysfunctional and hyperreactive phenotype, while selective TSPO ligands promoted the establishment of an anti-inflammatory one. Analysis of specific neurosteroid levels (neurosteroidome) identified allopregnanolone/pregnanolone as crucial metabolites allowing controlled activation of microglia. Importantly, the neurosteroid shift towards a greater androgenic/estrogenic profile supported the transition from pro-inflammatory to neuroprotective microglia, suggesting the therapeutic potential of de novo microglial neurosteroidogenesis stimulation for neuroinflammatory-related disorders.

[11C]PS13 Demonstrates Pharmacologically Selective and Substantial Binding to Cyclooxygenase-1 in the Human Brain

Our laboratory recently developed [11C]PS13 as a PET radioligand to selectively measure cyclooxygenase-1 (COX-1). The cyclooxygenase enzyme family converts arachidonic acid into prostaglandins and thromboxanes, which mediate inflammation. The total brain uptake of [11C]PS13, which is composed of both specific binding and background uptake, can be accurately quantified with gold standard methods of compartmental modeling. This study sought to quantify the specific binding of [11C]PS13 to COX-1 in healthy human brain using scans performed with arterial input function at baseline and after blockade by the COX-1-selective inhibitor ketoprofen. Methods: Eight healthy volunteers underwent two 90-min [11C]PS13 PET scans with radiometabolite-corrected arterial input function, at baseline and about 2 h after oral administration of ketoprofen (75 mg). Results: Two-tissue compartment modeling effectively identified the total uptake of radioactivity in the brain (as distribution volume), showing the highest densities in the hippocampus, the occipital cortex, and the banks of the central sulcus. All brain regions exhibited displaceable and specific binding, and thus none could be used as a reference region. Ketoprofen blocked approximately 84% of the binding sites on COX-1 in the whole brain. After full occupancy was extrapolated, the average whole-brain values of [11C]PS13 were 1.6 ± 0.8 mL·cm-3 for specific uptake, 1.7 ± 0.6 mL·cm-3 for background uptake, and 1.1 ± 0.5 for the specific-to-background ratio. The hippocampus had the highest specific-to-background ratio value of 2.7 ± 0.9. Conclusion: [11C]PS13 exhibited high specific binding to COX-1 in the human brain, but its quantification requires arterial blood sampling.

Neuroinflammation, Stress-Related Suicidal Ideation, and Negative Mood in Depression

Importance

Brain translocator protein 18k Da (TSPO) binding, a putative marker of neuroinflammatory processes (eg, gliosis), is associated with stress and elevated in depressed and suicidal populations. However, it is unclear whether neuroinflammation moderates the impact of daily life stress on suicidal ideation and negative affect, thereby increasing risk for suicidal behavior.

Objective

To examine the association of TSPO binding in participants with depression with real-world daily experiences of acute stress-related suicidal ideation and negative affect, as well as history of suicidal behavior and clinician-rated suicidal ideation.

Design, Setting, and Participants

Data for this cross-sectional study were collected from June 2019 through July 2023. Procedures were conducted at a hospital-based research center in New York, New York. Participants were recruited via clinical referrals, the Columbia University research subject web portal, and from responses to internet advertisements. Of 148 participants who signed informed consent for study protocols, 53 adults aged 18 to 60 years who met DSM-5 diagnostic criteria for current major depressive disorder completed procedures with approved data and were enrolled. Participants were free of schizophrenia spectrum disorders, active physical illness, cognitive impairment, and substance intoxication or withdrawal at the time of scan.

Exposures

All participants underwent positron emission tomography imaging of TSPO binding with 11C-ER176 and concurrent arterial blood sampling.

Main Outcome and Measures

A weighted average of 11C-ER176 total distribution volume (VT) was computed across 11 a priori brain regions and made up the primary outcome measure. Clinician-rated suicidal ideation was measured via the Beck Scale for Suicidal Ideation (BSS). A subset of participants (n = 21) completed 7 days of ecological momentary assessment (EMA), reporting daily on suicidal ideation, negative affect, and stressors.

Results

In the overall sample of 53 participants (mean [SD] age, 29.5 [9.8] years; 37 [69.8%] female and 16 [30.2%] male), 11C-ER176 VT was associated at trend levels with clinician-rated suicidal ideation severity (β, 0.19; 95% CI, -0.03 to 0.39; P = .09) and did not differ by suicide attempt history (n = 15; β, 0.18; 95% CI, -0.04 to 0.37; P = .11). Exploratory analyses indicated that presence of suicidal ideation (on BSS or EMA) was associated with higher 11C-ER176 VT (β, 0.21; 95% CI, 0.01 to 0.98; P = .045). In 21 participants who completed EMA, 11C-ER176 VT was associated with greater suicidal ideation and negative affect during EMA periods with stressors compared with nonstress periods (β, 0.12; SE, 0.06; 95% CI, 0.01 to 0.23; P = .03 and β, 0.19; SE, 0.06; 95% CI, 0.08 to 0.30; P < .001, respectively).

Conclusion and Relevance

TSPO binding in individuals with depression may be a marker of vulnerability to acute stress-related increases in suicidal ideation and negative affect. Continued study is needed to determine the causal direction of TSPO binding and stress-related suicidal ideation or negative affect and whether targeting neuroinflammation may improve resilience to life stress in patients with depression.

Neuroglia in neurodegeneration: Amyotrophic lateral sclerosis and frontotemporal dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are devastating neurodegenerative diseases sharing significant pathologic and genetic overlap, leading to consider these diseases as a continuum in the spectrum of their pathologic features. Although FTD compromises only specific brain districts, while ALS involves both the nervous system and the skeletal muscles, several neurocentric mechanisms are in common between ALS and FTD. Also, recent research has revealed the significant involvement of nonneuronal cells, particularly glial cells such as astrocytes, oligodendrocytes, microglia, and peripheral immune cells, in disease pathology. This chapter aims to provide an extensive overview of the current understanding of the role of glia in the onset and advancement of ALS and FTD, highlighting the recent implications in terms of prognosis and future treatment options.

Imaging neuroglia

Imaging can help us understand the role neuroglia plays in health and during the course of neurologic disorders. In vivo microscopy has had a great impact on our understanding of how neuroglia behaves during health and disease. While initially the technique was hindered by the limited penetration depth in brain tissue, recent advancements lead to increasing possibilities for imaging of deeper brain structures, even at super-resolution. Unfortunately, in vivo microscopy cannot be applied in a clinical setting and thus cannot be used to study neuroglia in patient populations. However, noninvasive imaging techniques like positron emission tomography (PET) and magnetic resonance imaging (MRI) can. PET has provided valuable information on the involvement of neuroglia in neurologic disorders. To more specifically image microglia and astrocytes, many new PET biomarkers have been defined for which PET tracers are continuously developed, evaluated, and improved. A cell-type specific PET tracer with favorable imaging characteristics can have a huge impact on neuroglia research. While being less sensitive than PET, MRI is a more accessible imaging technique. Initially, only general neuroinflammation processes could be imaged with MRI, but newly developed methods and sequences allow for increasing cell-type specificity. Overall, while each imaging method comes with limitations, improvements are continuously made, all with the aim to truly understand the role that neuroglia play in health and disease.

Neurosteroids and Translocator Protein (TSPO) in neuroinflammation

Neurosteroids have a crucial role in physiological intrinsic regulations of the Central Nervous System functions. They are derived from peripheral steroidogenic sources and from the de novo neurosteroidogenic capacity of brain cells. Significant alterations of neurosteroid levels have been frequently observed in neuroinflammation and neurodegenerative diseases. Such level fluctuations may be useful for both diagnosis and treatment of these pathological conditions. Beyond steroid administration, enhancing the endogenous production by Translocator Protein (TSPO) targeting has been proposed to restore these altered pathological levels. However, the neurosteroid quantification and the prediction of their final effects are often troublesome, sometimes controversial and context dependent, due to the complexity of neurosteroid biosynthetic pathway and to the low produced amounts. The aim of this review is to report recent advances, and technical limitations, in neurosteroid-related strategies against neuroinflammation.

Neuroglia in mood disorders

Multiple lines of evidence indicate that mood disorders, such as major depressive and bipolar disorder, are associated with abnormalities in neuroglial cells. This chapter discusses the existing literature investigating the potential role of astrocytes, oligodendrocytes, and microglia in mood pathology. We will describe evidence from in vivo imaging, postmortem, animal models based on (stress) paradigms that mimic depressive-like behavior, and biomarker studies in blood and cerebrospinal fluid in patients with mood disorders. The effect of medication used in the treatment of mood disorders, such as antidepressants and lithium, on glial function is discussed. Lastly, we highlight the most relevant findings about potential deficiencies in glia-glia crosstalk in mood disorders. Overall, decreased astrocyte and oligodendrocyte density and expression and microglial changes in homeostatic functions have frequently been put forward in MDD pathology. Studies of BD report similar findings to some extent; however, the evidence is less well established. Together, these findings are suggestive of reduced glial cell function leading to potential white matter abnormalities, glutamate dysregulation, disrupted neuronal functioning, and neurotransmission. However, more research is required to better understand the exact mechanisms underlying glial cell contributions to mood disorder development.

Myeloid cell-specific loss of NPC1 in mice recapitulates microgliosis and neurodegeneration in patients with Niemann-Pick type C disease

Niemann-Pick type C (NPC) disease is an inherited lysosomal storage disorder mainly driven by mutations in the NPC1 gene, causing lipid accumulation within late endosomes/lysosomes and resulting in progressive neurodegeneration. Although microglial activation precedes neuronal loss, it remains elusive whether loss of the membrane protein NPC1 in microglia actively contributes to NPC pathology. In a mouse model with depletion of NPC1 in myeloid cells, we report severe alterations in microglial lipidomic profiles, including the enrichment of bis(monoacylglycero)phosphate, increased cholesterol, and a decrease in cholesteryl esters. Lipid dyshomeostasis was associated with microglial hyperactivity, marked by an increase in translocator protein 18 kDa (TSPO). These hyperactive microglia initiated a pathological cascade resembling NPC-like phenotypes, including a shortened life span, motor impairments, astrogliosis, neuroaxonal pathology, and increased neurofilament light chain (NF-L), a neuronal injury biomarker. As observed in the mouse model, patients with NPC showed increased NF-L in the blood and microglial hyperactivity, as visualized by TSPO-PET imaging. Reduced TSPO expression in blood-derived macrophages of patients with NPC was measured after N-acetyl-l-leucine treatment, which has been recently shown to have beneficial effects in patients with NPC, suggesting that TSPO is a potential marker to monitor therapeutic interventions for NPC. Conclusively, these results demonstrate that myeloid dysfunction, driven by the loss of NPC1, contributes to NPC disease and should be further investigated for therapeutic targeting and disease monitoring.

Neural and immune interactions linking early life stress and anhedonia

Early experiences of stress and adversity are associated with blunted reward sensitivity and altered reward learning. Meanwhile, anhedonia is characterized by impairments in reward processing, including motivation, effort, and pleasure. Early life stress (ELS) and anhedonia share psychological, behavioral, and neurobiological correlates, and the system-level interactions that give rise to anhedonia have yet to be fully appreciated. The proposed framework uses a multilevel, multisystem approach to aid in understanding neural-immune interactions that link ELS and anhedonia. The interactions linking anhedonia and ELS presented here include reduced reward sensitivity, alterations in hypothalamic-pituitary-adrenal (HPA) axis response, elevated inflammatory cytokines or physiological markers of stress, and blunted reward circuitry functioning along the mesocorticolimbic pathway. The clinical implications and areas for future research are also discussed. Ultimately, this research may inform the development of more specific and individualized treatments for anhedonia.

Phenotyping in vivo chronic inflammation in multiple sclerosis by combined 11C-PBR28 MR-PET and 7T susceptibility-weighted imaging

BACKGROUND

11C-PBR28 positron emission tomography (PET), targeting the translocator protein, and paramagnetic rim lesions (PRL) have emerged as promising imaging markers of MS chronic inflammation. No consensus on which is the optimal marker exists.

OBJECTIVES

To investigate the ability of 11C-PBR28 PET and PRL assessment to identify chronic inflammation in white matter (WM) MS lesions and their relation to neurological impairment.

METHODS

Based on 11C-PBR28 uptake, brain WM lesions from 30 MS patients were classified as PET active or inactive. The PRL presence was assessed on 7T phase reconstructions, T1/T2 ratio was calculated to measure WM microstructural integrity.

RESULTS

Less than half (44%) of non-PRL WM lesions were active on 11C-PBR28 imaging either throughout the lesion (whole active) or at its periphery. PET peripherally active lesions and PRL did not differ in T1/T2 ratio and 11C-PBR28 uptake. A positive correlation was observed between PRL and active PET lesion count. Whole active PET lesion volume was the strongest predictor (β = 0.97, p < 0.001) of increased Expanded Disability Status Scale scores.

CONCLUSION

11C-PBR28 imaging reveals more active WM lesions than 7T PRL assessment. Although PRL and PET active lesion counts are related, neurological disability is better explained by PET whole active lesion volume.

18F-DPA714 PET/MRI as a potential imaging tool for detecting possible antibody-negative autoimmune encephalitis: a prospective study

BACKGROUND AND OBJECTIVES

Conventional magnetic resonance imaging (MRI) used for detecting possible antibody-negative autoimmune encephalitis (AIE) often fails to meet the diagnostic requirements of this disease. Positron emission tomography (PET) with a translocator protein radioligand can help visualize microglia distribution density in inflammation-related diseases, thereby offering potentially incremental value to conventional MRI for the in vivo assessment of possible antibody-negative AIE.

METHODS

In this prospective study, 15 participants diagnosed with possible antibody-negative AIE and 10 healthy controls were enrolled (ClinicalTrials.gov: NCT05293405, dated March 15, 2022). All participants underwent hybrid 18F-DPA714 PET/MRI and evaluation for modified Rankin scale (mRS) score, clinical assessment scale for AIE (CASE), and appropriate antibodies. A positive finding was defined as the intensity of 18F-DPA714 uptake that was above a threshold of mean standardized uptake value ratio (SUVR) + two standard deviations of SUVR within the corresponding brain regions of healthy controls.

RESULTS

The positive detection rate of 18F-DPA714 PET for possible antibody-negative AIE was significantly higher than that of brain MRI (10/15 [67%] vs. 3/15 [20%]; P = 0.039). In addition, both the intensity and extent of 18F-DPA714 uptake were significantly associated with the CASE score (P = 0.002 and 0.001). Meanwhile, SUVR levels in the cerebellar region were significantly higher in patients with ataxia than in those without ataxia (P = 0.006). Furthermore, 18F-DPA714 uptake decreased in 5/10 [50%] patients who underwent follow-up PET/MRI, which mirrored their symptom relief.

CONCLUSION

18F-DPA714 PET demonstrated its potentially incremental value to conventional MRI for detecting possible antibody-negative AIE.

A Comprehensive Functional Investigation of the Human Translocator Protein 18 kDa (TSPO) in a Novel Human Neuronal Cell Knockout Model

The translocator protein 18 kDa (TSPO) is a multifunctional outer mitochondrial membrane protein associated with various aspects of mitochondrial physiology and multiple roles in health and disease. Here, we aimed to analyse the role of TSPO in the regulation of mitochondrial and cellular functions in a human neuronal cell model. We used the CRISPR/Cas9 technology and generated TSPO knockout (KO) and control (CTRL) variants of human-induced pluripotent stem cells (hiPSCs). In a multimodal phenotyping approach, we investigated cellular and mitochondrial functions in neural progenitor cells (NPCs), astrocytes, and neurons differentiated from hiPSC CTRL and TSPO KO cell lines. Our analysis revealed reduced mitochondrial respiration and glycolysis, altered Ca2+ levels in the cytosol and mitochondrial matrix, a depolarised MMP, and increased levels of reactive oxygen species, as well as a reduced cell size. Notably, TSPO deficiency was accompanied by reduced expression of the voltage-dependent anion channel (VDAC). We also observed a reduced TSPO and VDAC expression in cells derived from patients suffering from major depressive disorder (MDD). Considering the modulatory function of TSPO and the similar functional phenotype of cells derived from patients with depression, we discuss a role of TSPO in the etiology or pathology of MDD. In summary, our findings indicate a general impairment of mitochondrial function in TSPO knockout (KO) cells. This deepens our insight into the intricate role of TSPO in a range of physiological and pathological processes.

The microglial translocator protein (TSPO) in Alzheimer's disease reflects a phagocytic phenotype

Translocator protein (TSPO) is a mitochondrial protein expressed by microglia, ligands for which are used as a marker of neuroinflammation in PET studies of Alzheimer's disease (AD). We previously showed increasing TSPO load in the cerebral cortex with AD progression, consistent with TSPO PET scan findings. Here, we aim to characterise the microglial phenotype associated with TSPO expression to aid interpretation of the signal generated by TSPO ligands in patients. Human post-mortem sections of temporal lobe (TL) and cerebellum (Cb) from cases classified by Braak group (0-II, III-IV, V-VI; each n = 10) were fluorescently double labelled for TSPO and microglial markers: Iba1, HLA-DR, CD68, MSR-A and CD64. Quantification was performed on scanned images using QuPath software to assess the microglial phenotype of TSPO. Qualitative analysis was also performed for TSPO with GFAP (astrocytes), CD31 (endothelial cells) and CD163 (perivascular macrophages) to characterise the cellular profile of TSPO. The percentage of CD68+TSPO+ double-labelled cells was significantly higher than for other microglial markers in both brain regions and in all Braak stages, followed by MSR-A+TSPO+ microglia. Iba1+TSPO+ cells were more numerous in the cerebellum than the temporal lobe, while CD64+TSPO+ cells were more numerous in the temporal lobe. No differences were observed for the other microglial markers. TSPO expression was also detected in endothelial cells, but not detected in astrocytes nor in perivascular macrophages. Our data suggest that TSPO is mainly related to a phagocytic profile of microglia (CD68+) in human AD, potentially highlighting the ongoing neurodegeneration.

Exploring neuroinflammation: A key driver in neuropathic pain disorders

Inflammation is a fundamental part of the body's natural defense mechanism, involving immune cells and inflammatory mediators to promote healing and protect against harm. In the event of a lesion or disease of the somatosensory nervous system, inflammation, however, triggers a cascade of changes in both the peripheral and central nervous systems, ultimately contributing to chronic neuropathic pain. Substantial evidence links neuroinflammation to various conditions associated with neuropathic pain. This chapter will explore the role of neuroinflammation in the initiation, maintenance, and resolution of peripheral and central neuropathic pain. Additionally, biomarkers of neuroinflammation in humans will be examined, emphasizing their relevance in different neuropathic pain disorders.

Diagnostic utility of plasma translocator protein 18 kDa (TSPO) in sepsis: A case-control study

Translocator protein 18 kDa (TSPO) is a mitochondrial membrane protein that is involved in inflammation, oxidative stress, and steroidogenesis. TSPO may be a marker of inflammatory responses in the brain and other organs, but there have been few studies of the potential clinical significance of measuring the circulating TSPO concentration, especially in patients with sepsis. In this study, we compared the circulating TSPO concentrations of patients with sepsis and healthy controls to investigate the utility of plasma TSPO for the diagnosis of sepsis. Patients with sepsis admitted to the intensive care unit of Hiroshima University Hospital between January 2020 and April 2024 were enrolled. Plasma samples were collected from patients within 24 hours of admission and also from healthy volunteers, and their plasma TSPO concentrations were compared. Receiver operating characteristic analysis was used to evaluate the usefulness of plasma TSPO concentration for the diagnosis of sepsis. We also investigated the relationships of TSPO concentration with the severity of sepsis, complications, and prognosis of the patients. Eighty subjects (52 patients and 28 controls) were included in this study. The plasma TSPO concentrations of the patients with sepsis were significantly lower than those of the healthy controls (0.094 vs 0.25 ng/mL, P < .001), and receiver operating characteristic analysis generated an area under the curve of 0.81 (95% confidence interval: 0.72-0.91). In patients with sepsis, the TSPO concentration was not associated with the severity of sepsis, complications, or prognosis. Plasma TSPO may be a useful biomarker for the diagnosis of sepsis.

Emerging TSPO-PET Radiotracers for Imaging Neuroinflammation: A Critical Analysis

The translocator protein (TSPO) is a biomarker for imaging neuroinflammation via Positron Emission Tomography (PET) across a broad range of CNS conditions. Most clinically used PET ligands targeting TSPO have limitations, including high lipophilicity and off-target binding or poor binding to a mutated TSPO isoform present in up to 30% of the population. Research efforts over the past decade have focused on development of improved TSPO PET radiotracers that overcome these limitations. This review provides a critical analysis of the development and validation of these so-called "third-generation" radiotracers in clinical and preclinical settings. We also offer our perspective on the future directions of TSPO PET imaging, including recommendations for overcoming current challenges and capitalizing on emerging opportunities in molecular imaging for neuroinflammatory diseases.

"Neuroinflammation": does it have a role in chronic pain? Evidence from human imaging

ABSTRACT

Despite hundreds of studies demonstrating the involvement of neuron-glia-immune interactions in the establishment and/or maintenance of persistent pain behaviors in animals, the role (or even occurrence) of so-called "neuroinflammation" in human pain has been an object of contention for decades. Here, I present the results of multiple positron emission tomography (PET) studies measuring the levels of the 18 kDa translocator protein (TSPO), a putative neuroimmune marker, in individuals with various pain conditions. Overall, these studies suggest that brain TSPO PET signal: (1) is elevated, compared to healthy volunteers, in individuals with chronic low back pain (with additional elevations in spinal cord and neuroforamina), fibromyalgia, migraine and other conditions characterized by persistent pain; (2) has a spatial distribution exhibiting a degree of disorder specificity; (3) is parametrically linked to pain characteristics or comorbid symptoms (eg, nociplastic pain, fatigue, depression), as well as measures of brain function (ie, functional connectivity), in a regionally-specific manner. In this narrative, I also discuss important caveats to consider in the interpretation of this work (eg, regarding the cellular source of the signal and the complexities inherent in its acquisition and analysis). While the biological and clinical significance of these findings awaits further work, this emerging preclinical literature supports a role of neuron-glia-immune interactions as possible pathophysiological underpinnings of human chronic pain. Gaining a deeper understanding of the role of neuroimmune function in human pain would likely have important practical implications, possibly paving the way for novel interventions.

More than microglia: myeloid cells and biomarkers in neurodegeneration

The role of myeloid cells (granulocytes and monocytes) in neurodegeneration and neurodegenerative disorders (NDD) is indisputable. Here we discuss the roles of myeloid cells in neurodegenerative diseases, and the recent advances in biofluid and imaging myeloid biomarker research with a focus on methods that can be used in the clinic. For this review, evidence from three neurodegenerative diseases will be included, Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). We discuss the potential for these biomarkers to be used in humans with suspected NDD as prognostic, diagnostic, or monitoring tools, identify knowledge gaps in literature, and propose potential approaches to further elucidate the role of myeloid cells in neurodegeneration and better utilize myeloid biomarkers in the understanding and treatment of NDD.

Remote Neuroinflammation in Newly Diagnosed Glioblastoma Correlates with Unfavorable Clinical Outcome

PURPOSE

Current therapy strategies still provide only limited success in the treatment of glioblastoma, the most frequent primary brain tumor in adults. In addition to the characterization of the tumor microenvironment, global changes in the brain of patients with glioblastoma have been described. However, the impact and molecular signature of neuroinflammation distant of the primary tumor site have not yet been thoroughly elucidated.

EXPERIMENTAL DESIGN

We performed translocator protein (TSPO)-PET in patients with newly diagnosed glioblastoma (n = 41), astrocytoma WHO grade 2 (n = 7), and healthy controls (n = 20) and compared TSPO-PET signals of the non-lesion (i.e., contralateral) hemisphere. Back-translation into syngeneic SB28 glioblastoma mice was used to characterize Pet alterations on a cellular level. Ultimately, multiplex gene expression analyses served to profile immune cells in remote brain.

RESULTS

Our study revealed elevated TSPO-PET signals in contralateral hemispheres of patients with newly diagnosed glioblastoma compared to healthy controls. Contralateral TSPO was associated with persisting epileptic seizures and shorter overall survival independent of the tumor phenotype. Back-translation into syngeneic glioblastoma mice pinpointed myeloid cells as the predominant source of contralateral TSPO-PET signal increases and identified a complex immune signature characterized by myeloid cell activation and immunosuppression in distant brain regions.

CONCLUSIONS

Neuroinflammation within the contralateral hemisphere can be detected with TSPO-PET imaging and associates with poor outcome in patients with newly diagnosed glioblastoma. The molecular signature of remote neuroinflammation promotes the evaluation of immunomodulatory strategies in patients with detrimental whole brain inflammation as reflected by high TSPO expression.

Neuroinflammatory activation in sensory and motor regions of the cortex is related to sensorimotor function in individuals with low back pain maintained by nociplastic mechanisms: A preliminary proof-of-concept study

BACKGROUND

Chronic pain involves communication between neural and immune systems. Recent data suggest localization of glial (brain immune cells) activation to the sensorimotor regions of the brain cortex (S1/M1) in chronic low back pain (LBP). As glia perform diverse functions that impact neural function, activation might contribute to sensorimotor changes, particularly in LBP maintained by increased nervous system sensitivity (i.e., nociplastic pain). This preliminary proof-of-concept study aimed to: (i) compare evidence of neuroinflammatory activation in S1/M1 between individuals with and without LBP (and between nociceptive and nociplastic LBP phenotypes), and (ii) evaluate relationships between neuroinflammatory activation and sensorimotor function.

METHODS

Simultaneous PET-fMRI measured neuroinflammatory activation in functionally defined S1/M1 in pain-free individuals (n = 8) and individuals with chronic LBP (n = 9; nociceptive: n = 4, nociplastic: n = 5). Regions of S1/M1 related to the back were identified using fMRI during motor tasks and thermal stimuli. Sensorimotor measures included single and paired-pulse transcranial magnetic stimulation (TMS) and quantitative sensory testing (QST). Sleep, depression, disability and pain questionnaires were administered.

RESULTS

Neuroinflammatory activation was greater in the lower back cortical representation of S1/M1 of the nociplastic LBP group than both nociceptive LBP and pain-free groups. Neuroinflammatory activation in S1/M1 was positively correlated with sensitivity to hot (r = 0.52) and cold (r = 0.55) pain stimuli, poor sleep, depression, disability and BMI, and negatively correlated with intracortical facilitation (r = -0.41).

CONCLUSION

This preliminary proof-of-concept study suggests that neuroinflammation in back regions of S1/M1 in individuals with nociplastic LBP could plausibly explain some characteristic features of this LBP phenotype.

SIGNIFICANCE STATEMENT

Neuroinflammatory activation localized to sensorimotor areas of the brain in individuals with nociplastic pain might contribute to changes in sensory and motor function and aspects of central sensitization. If cause-effect relationships are established in longitudinal studies, this may direct development of therapies that target neuroinflammatory activation.

Understanding Proton Magnetic Resonance Spectroscopy Neurochemical Changes Using Alzheimer's Disease Biofluid, PET, Postmortem Pathology Biomarkers, and APOE Genotype

In vivo proton (1H) magnetic resonance spectroscopy (MRS) is a powerful non-invasive method that can measure Alzheimer's disease (AD)-related neuropathological alterations at the molecular level. AD biomarkers include amyloid-beta (Aβ) plaques and hyperphosphorylated tau neurofibrillary tangles. These biomarkers can be detected via postmortem analysis but also in living individuals through positron emission tomography (PET) or biofluid biomarkers of Aβ and tau. This review offers an overview of biochemical abnormalities detected by 1H MRS within the biologically defined AD spectrum. It includes a summary of earlier studies that explored the association of 1H MRS metabolites with biofluid, PET, and postmortem AD biomarkers and examined how apolipoprotein e4 allele carrier status influences brain biochemistry. Studying these associations is crucial for understanding how AD pathology affects brain homeostasis throughout the AD continuum and may eventually facilitate the development of potential novel therapeutic approaches.

The mitochondrial translocator protein (TSPO) in Alzheimer's disease: Therapeutic and immunomodulatory functions

The translocator protein (TSPO) has been widely investigated as a PET-imaging biomarker of neuroinflammation and, more recently, as a therapeutic target for the treatment of neurodegenerative disease. TSPO ligands have been shown to exert neuroprotective effects in vivo and in vitro models of Alzheimer's disease (AD), by reducing toxic beta amyloid peptides, and attenuating brain atrophy. Recent transcriptomic and proteomic analyses, and the generation of TSPO-KO mice, have enabled new insights into the mechanistic function of TSPO in AD. Using a multi-omics approach in both TSPO-KO- and TSPO ligand-treated mice, we have demonstrated a key role for TSPO in microglial respiratory metabolism and phagocytosis in AD. In this review, we discuss emerging evidence for therapeutic and immunomodulatory functions of TSPO in AD, and new tools for studying TSPO in the brain.

Microglia-mediated neuroimmune suppression in PTSD is associated with anhedonia

Dynamic brain immune function in individuals with posttraumatic stress disorder is rarely studied, despite evidence of peripheral immune dysfunction. Positron emission tomography brain imaging using the radiotracer [11C]PBR28 was used to measure the 18-kDa translocator protein (TSPO), a microglial marker, at baseline and 3 h after administration of lipopolysaccharide (LPS), a potent immune activator. Data were acquired in 15 individuals with PTSD and 15 age-matched controls. The PTSD group exhibited a significantly lower magnitude LPS-induced increase in TSPO availability in an a priori prefrontal-limbic circuit compared to controls. Greater anhedonic symptoms in the PTSD group were associated with a more suppressed neuroimmune response. In addition, while a reduced granulocyte-macrophage colony-stimulating factor response to LPS was observed in the PTSD group, other measured cytokine responses and self-reported sickness symptoms did not differ between groups; these findings highlight group differences in central-peripheral immune system relationships. The results of this study provide evidence of a suppressed microglia-mediated neuroimmune response to a direct immune system insult in individuals with PTSD that is associated with the severity of symptoms. They also provide further support to an emerging literature challenging traditional concepts of microglial and immune function in psychiatric disease.

Pilot Feasibility Study: 18 F-DPA-714 PET/CT Macrophage Imaging in Triple-Negative Breast Cancers (EITHICS)

ABSTRACT

Tumor-associated macrophages are targets of interest in triple-negative breast cancer (TNBC). The translocator protein 18 kDa (TSPO) is a sensitive marker for macrophages and holds potential relevance in TNBC stratification. This pilot prospective study (EITHICS, NCT04320030) aimed to assess the potential of TSPO PET/CT imaging using 18 F-DPA-714 in primary TNBC, compared with immunohistochemistry, autoradiography, and TSPO polymorphism.

PATIENTS AND METHODS

Thirteen TNBC patients were included. They underwent TSPO genotyping (HAB, MAB, LAB), 18 F-FDG PET/CT, and breast MRI. Semiquantitative PET parameters were computed. VOIs were defined on the tumor lesion, healthy breast tissue, and pectoral muscle to obtain SUV, tumor-to-background ratio (TBR), and time-activity curves (TACs). Additionally, immunohistochemistry, 3 H-DPA-714, and 3 H-PK-11195 autoradiography were conducted.

RESULTS

The majority of TNBC tumors (11/13, 84%) had a preponderance of M2-polarized macrophages with a median proportion of 82% (range, 44%-94%). 18 F-DPA-714 PET/CT clearly identified TNBC tumors with an excellent TBR. Three distinct patterns of 18 F-DPA-714 TACs were identified, categorized as "above muscular," "equal to muscular," and "below muscular" with reference to the muscular background. For the "above muscular" group (2 HAB and 2 MAB), "equal muscular" group (3 HAB, 3 MAB, and 1 LAB), and "below muscular" group (1 LAB and 1 MAB), tumor TACs showed a 18 F-DPA-714 accumulation slope of 1.35, 0.62, and 0.22, respectively, and a median SUV mean of 4.02 (2.09-5.31), 1.66 (0.93-3.07), and 0.61 (0.43-1.02).

CONCLUSIONS

This study successfully demonstrated TNBC tumor targeting by 18 F-DPA-714 with an excellent TBR, allowing to stratify 3 patterns of uptake potentially influenced by the TSPO polymorphism status. Further studies in larger populations should be performed to evaluate the prognostic value of this new biomarker.

New Imaging Markers in Multiple Sclerosis and Related Disorders: Smoldering Inflammation and the Central Vein Sign

Concepts of multiple sclerosis (MS) biology continue to evolve, with observations such as "progression independent of disease activity" challenging traditional phenotypic categorization. Iron-sensitive, susceptibility-based imaging techniques are emerging as highly translatable MR imaging sequences that allow for visualization of at least 2 clinically useful biomarkers: the central vein sign and the paramagnetic rim lesion (PRL). Both biomarkers demonstrate high specificity in the discrimination of MS from other mimics and can be seen at 1.5 T and 3 T field strengths. Additionally, PRLs represent a subset of chronic active lesions engaged in "smoldering" compartmentalized inflammation behind an intact blood-brain barrier.

A blood-free modeling approach for the quantification of the blood-to-brain tracer exchange in TSPO PET imaging

Introduction

Recent evidence suggests the blood-to-brain influx rate (K1 ) in TSPO PET imaging as a promising biomarker of blood-brain barrier (BBB) permeability alterations commonly associated with peripheral inflammation and heightened immune activity in the brain. However, standard compartmental modeling quantification is limited by the requirement of invasive and laborious procedures for extracting an arterial blood input function. In this study, we validate a simplified blood-free methodologic framework for K1 estimation by fitting the early phase tracer dynamics using a single irreversible compartment model and an image-derived input function (1T1K-IDIF).

Methods

The method is tested on a multi-site dataset containing 177 PET studies from two TSPO tracers ([11C]PBR28 and [18F]DPA714). Firstly, 1T1K-IDIF K1 estimates were compared in terms of both bias and correlation with standard kinetic methodology. Then, the method was tested on an independent sample of [11C]PBR28 scans before and after inflammatory interferon-α challenge, and on test-retest dataset of [18F]DPA714 scans.

Results

Comparison with standard kinetic methodology showed good-to-excellent intra-subject correlation for regional 1T1K-IDIF-K1 (ρintra  = 0.93 ± 0.08), although the bias was variable depending on IDIF ability to approximate blood input functions (0.03-0.39 mL/cm3/min). 1T1K-IDIF-K1 unveiled a significant reduction of BBB permeability after inflammatory interferon-α challenge, replicating results from standard quantification. High intra-subject correlation (ρ = 0.97 ± 0.01) was reported between K1 estimates of test and retest scans.

Discussion

This evidence supports 1T1K-IDIF as blood-free alternative to assess TSPO tracers' unidirectional blood brain clearance. K1 investigation could complement more traditional measures in TSPO studies, and even allow further mechanistic insight in the interpretation of TSPO signal.

Neuroimaging and fluid biomarkers in Parkinson's disease in an era of targeted interventions

A major challenge in Parkinson's disease is the variability in symptoms and rates of progression, underpinned by heterogeneity of pathological processes. Biomarkers are urgently needed for accurate diagnosis, patient stratification, monitoring disease progression and precise treatment. These were previously lacking, but recently, novel imaging and fluid biomarkers have been developed. Here, we consider new imaging approaches showing sensitivity to brain tissue composition, and examine novel fluid biomarkers showing specificity for pathological processes, including seed amplification assays and extracellular vesicles. We reflect on these biomarkers in the context of new biological staging systems, and on emerging techniques currently in development.

Are central and systemic inflammation associated with fatigue in cerebral small vessel disease?

BACKGROUND

Fatigue is a common symptom in cerebral small vessel disease (SVD), but its pathogenesis is poorly understood. It has been suggested that inflammation may play a role. We determined whether central (neuro) inflammation and peripheral inflammation were associated with fatigue in SVD.

METHODS

Notably, 36 patients with moderate-to-severe SVD underwent neuropsychometric testing, combined positron emission tomography and magnetic resonance imaging (PET-MRI) scan, and blood draw for the analysis of inflammatory blood biomarkers. Microglial signal was taken as a proxy for neuroinflammation, assessed with radioligand 11C-PK11195. Of these, 30 subjects had full PET datasets for analysis. We assessed global 11C-PK11195 binding and hotspots of 11C-PK11195 binding in the normal-appearing white matter, lesioned tissue, and combined total white matter. Peripheral inflammation was assessed with serum C-reactive protein (CRP) and using the Olink cardiovascular III proteomic panel comprising 92 biomarkers of cardiovascular inflammation and endothelial activation. Fatigue was assessed using the fatigue severity scale (FSS), the visual analog fatigue scale, and a subscale of the Geriatric Depression Scale.

RESULTS

Mean (SD) age was 68.7 (11.2) years, and 63.9% were male. Of these, 55.6% showed fatigue on the FSS. Fatigued participants had higher disability scores (p = 0.02), higher total GDS scores (p = 0.02), and more commonly reported a history of depression (p = 0.04). 11C-PK11195 ligand binding in the white matter was not associated with any measure of fatigue. Serum CRP was significantly associated with average fatigue score on FSS (ρ = 0.48, p = 0.004); this association persisted when controlling for age, sex, disability score, and depression (β = 0.49, 95% CI (0.17, 2.26), p = 0.03). Blood biomarkers from the Olink panel showed no association with fatigue.

CONCLUSION

In symptomatic SVD patients, neuroinflammation, assessed with microglial marker 11C-PK11195, was not associated with fatigue. We found some evidence for a role of systematic inflammation, evidenced by an association between fatigue severity and raised CRP, but further studies are required to understand this relationship and inform whether it could be therapeutically modified to reduce fatigue severity.

DATA ACCESS STATEMENT

Data for this study are available from the corresponding author upon reasonable request.

Neuroinflammation in post-acute sequelae of COVID-19 (PASC) as assessed by [11C]PBR28 PET correlates with vascular disease measures

The COVID-19 pandemic caused by SARS-CoV-2 has triggered a consequential public health crisis of post-acute sequelae of COVID-19 (PASC), sometimes referred to as long COVID. The mechanisms of the heterogeneous persistent symptoms and signs that comprise PASC are under investigation, and several studies have pointed to the central nervous and vascular systems as being potential sites of dysfunction. In the current study, we recruited individuals with PASC with diverse symptoms, and examined the relationship between neuroinflammation and circulating markers of vascular dysfunction. We used [11C]PBR28 PET neuroimaging, a marker of neuroinflammation, to compare 12 PASC individuals versus 43 normative healthy controls. We found significantly increased neuroinflammation in PASC versus controls across a wide swath of brain regions including midcingulate and anterior cingulate cortex, corpus callosum, thalamus, basal ganglia, and at the boundaries of ventricles. We also collected and analyzed peripheral blood plasma from the PASC individuals and found significant positive correlations between neuroinflammation and several circulating analytes related to vascular dysfunction. These results suggest that an interaction between neuroinflammation and vascular health may contribute to common symptoms of PASC.

PET Study of Microglial Activation in Kleine-Levin Syndrome

OBJECTIVES

Kleine-Levin syndrome (KLS) is a rare recurrent hypersomnolence disorder associated with cognitive and behavioral disturbances, of unknown origin, but inflammatory mechanisms could be involved. We aimed to explore in vivo microglia activation using [18F]DPA-714 PET imaging in patients with KLS compared with controls, and during symptomatic vs asymptomatic periods.

METHODS

Patients with KLS and controls underwent a standardized clinical evaluation and PET imaging, using a radiolabeled ligand specific to the 18 kDa translocator protein. Images were processed on the PMOD (peripheral module) interface using a standard uptake value (SUV). Five regions of interest (ROIs) were analyzed: hypothalamus, thalamus, frontal area, cerebellum, and whole brain. SUV ratios (SUVr) were calculated by normalizing SUV with cerebellum uptake.

RESULTS

Images of 17 consecutive patients with KLS (7 during episodes, 10 out of episodes) and 14 controls were analyzed. We found no SUV/SUVr difference between KLS and controls, between patients in and out episodes in all ROIs, and no correlation between SUVr and episode duration at the time of PET scan. No association was found between SUVr and sex, disease duration, or orexin levels.

DISCUSSION

Our findings do not support the presence of neuroinflammation in KLS. Further research is needed to identify relevant biomarkers in KLS.

Tau follows principal axes of functional and structural brain organization in Alzheimer's disease

Alzheimer's disease (AD) is a brain network disorder where pathological proteins accumulate through networks and drive cognitive decline. Yet, the role of network connectivity in facilitating this accumulation remains unclear. Using in-vivo multimodal imaging, we show that the distribution of tau and reactive microglia in humans follows spatial patterns of connectivity variation, the so-called gradients of brain organization. Notably, less distinct connectivity patterns ("gradient contraction") are associated with cognitive decline in regions with greater tau, suggesting an interaction between reduced network differentiation and tau on cognition. Furthermore, by modeling tau in subject-specific gradient space, we demonstrate that tau accumulation in the frontoparietal and temporo-occipital cortices is associated with greater baseline tau within their functionally and structurally connected hubs, respectively. Our work unveils a role for both functional and structural brain organization in pathology accumulation in AD, and supports subject-specific gradient space as a promising tool to map disease progression.

In vivo translocator protein in females with autism spectrum disorder: a pilot study

Sex-based differences in the prevalence of autism spectrum disorder (ASD) are well-documented, with a male-to-female ratio of approximately 4:1. The clinical presentation of the core symptoms of ASD can also vary between sexes. Previously, positron emission tomography (PET) studies have identified alterations in the in vivo levels of translocator protein (TSPO)-a mitochondrial protein-in primarily or only male adults with ASD, with our group reporting lower TSPO relative to whole brain mean in males with ASD. However, whether in vivo TSPO levels are altered in females with ASD, specifically, is unknown. This is the first pilot study to measure in vivo TSPO in the brain in adult females with ASD using [11C]PBR28 PET-magnetic resonance imaging (MRI). Twelve adult females with ASD and 10 age- and TSPO genotype-matched controls (CON) completed one or two [11C]PBR28 PET-MRI scans. Females with ASD exhibited elevated [11C]PBR28 standardized uptake value ratio (SUVR) in the midcingulate cortex and splenium of the corpus callosum compared to CON. No brain area showed lower [11C]PBR28 SUVR in females with ASD compared to CON. Test-retest over several months showed stable [11C]PBR28 SUVR across time in both groups. Elevated regional [11C]PBR28 SUVR in females with ASD stand in stark contrast to our previous findings of lower regional [11C]PBR28 SUVR in males with ASD. Preliminary evidence of regionally elevated mitochondrial protein TSPO relative to whole brain mean in ASD females may reflect neuroimmuno-metabolic alterations specific to females with ASD.