Positron emission tomography imaging of peripheral benzodiazepine receptor binding in human immunodeficiency virus-infected subjects with and without cognitive impairment

Differences in neuroinflammation in people who started antiretroviral treatment during primary versus chronic HIV infection: an 18kDa Translocator protein (TSPO) positron emission tomography (PET) study

Persistent inflammation is described in people with HIV (PWH) on antiretroviral treatment (ART). Early ART initiation is associated with reduced inflammation. We aimed to evaluate neuroinflammation, using translocator protein (TSPO) [11C]PBR28 PET neuroimaging in PWH who initiated ART during acute HIV (aPWH) versus chronic HIV infection (cPWH) versus a control population. This was a cross-sectional, observational study. All participants underwent [11C]PBR28 PET-CT neuroimaging. Using a two-tissue compartment model, total volume of distribution (VT) and distribution volume ratios (DVR) using cortical grey matter as a pseudo-reference region at 20 regions of interest (ROIs) were calculated. Differences in VT and DVR were compared between groups using the Kruskall-Wallis test. Seventeen neuro-asymptomatic male PWH on ART (9 aPWH, 8 cPWH) and 8 male control participants (CPs) were included. Median (interquartile range, IQR) age was 40 (30, 46), 44 (41, 47) and 21 (20, 25) years in aPWH, cPWH and CPs, respectively. Median (IQR) CD4 (cells/µL) and CD4:CD8 were 687 (652, 1014) and 1.37 (1.24, 1.42), and 700 (500, 720) and 0.67 (0.64, 0.82) in aPWH and cPWH, respectively. Overall, no significant difference in VT and DVR were observed between the three groups at any ROIs. cPWH demonstrated a trend towards higher mean VT compared with aPWH and CPs at most ROIs. No significant differences in neuroinflammation, using [11C]PBR28 binding as a proxy, were identified between cPWH, aPWH and CPs. A trend towards lower absolute [11C]PBR28 binding was seen amongst aPWH and CPs, suggesting early ART may mitigate neuroinflammation.

The Promise of Molecular Imaging: Focus on Central Nervous System Infections

Central nervous system (CNS) infections can lead to high mortality and severe morbidity. Diagnosis, monitoring, and assessing response to therapy of CNS infections is particularly challenging with traditional tools, such as microbiology, due to the dangers associated with invasive CNS procedures (ie, biopsy or surgical resection) to obtain tissues. Molecular imaging techniques like positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging have long been used to complement anatomic imaging such as computed tomography (CT) and magnetic resonance imaging (MRI), for in vivo evaluation of disease pathophysiology, progression, and treatment response. In this review, we detail the use of molecular imaging to delineate host-pathogen interactions, elucidate antimicrobial pharmacokinetics, and monitor treatment response. We also discuss the utility of pathogen-specific radiotracers to accurately diagnose CNS infections and strategies to develop radiotracers that would cross the blood-brain barrier.

Positron Emission Tomography of Neuroimmune Responses in Humans: Insights and Intricacies

The brain's immune system plays a critical role in responding to immune challenges and maintaining homeostasis. However, dysregulated neuroimmune function contributes to neurodegenerative disease and neuropsychiatric conditions. In vivo positron emission tomography (PET) imaging of the neuroimmune system has facilitated a greater understanding of its physiology and the pathology of some neuropsychiatric conditions. This review presents an in-depth look at PET findings from human neuroimmune function studies, highlighting their importance in current neuropsychiatric research. Although the majority of human PET studies feature radiotracers targeting the translocator protein 18 kDa (TSPO), this review also considers studies with other neuroimmune targets, including monoamine oxidase B, cyclooxygenase-1 and cyclooxygenase-2, nitric oxide synthase, and the purinergic P2X7 receptor. Promising new targets, such as colony-stimulating factor 1, Sphingosine-1-phosphate receptor 1, and the purinergic P2Y12 receptor, are also discussed. The significance of validating neuroimmune targets and understanding their function and expression is emphasized in this review to better identify and interpret PET results.

Have (R)-[11C]PK11195 challengers fulfilled the promise? A scoping review of clinical TSPO PET studies

PURPOSE

The prototypical TSPO radiotracer (R)-[11C]PK11195 has been used in humans for more than thirty years to visualize neuroinflammation in several pathologies. Alternative radiotracers have been developed to improve signal-to-noise ratio and started to be tested clinically in 2008. Here we examined the scientific value of these "(R)-[11C]PK11195 challengers" in clinical research to determine if they could supersede (R)-[11C]PK11195.

METHODS

A systematic MEDLINE (PubMed) search was performed (up to end of year 2020) to extract publications reporting TSPO PET in patients with identified pathologies, excluding studies in healthy subjects and methodological studies.

RESULTS

Of the 288 publications selected, 152 used 13 challengers, and 142 used (R)-[11C]PK11195. Over the last 20 years, the number of (R)-[11C]PK11195 studies remained stable (6 ± 3 per year), but was surpassed by the total number of challenger studies for the last 6 years. In total, 3914 patients underwent a TSPO PET scan, and 47% (1851 patients) received (R)-[11C]PK11195. The 2 main challengers were [11C]PBR28 (24%-938 patients) and [18F]FEPPA (11%-429 patients). Only one-in-ten patients (11%-447) underwent 2 TSPO scans, among whom 40 (1%) were scanned with 2 different TSPO radiotracers.

CONCLUSIONS

Generally, challengers confirmed disease-specific initial (R)-[11C]PK11195 findings. However, while their better signal-to-noise ratio seems particularly useful in diseases with moderate and widespread neuroinflammation, most challengers present an allelic-dependent (Ala147Thr polymorphism) TSPO binding and genetic stratification is hindering their clinical implementation. As new challengers, insensitive to TSPO human polymorphism, are about to enter clinical evaluation, we propose this systematic review to be regularly updated (living review).

Molecular neuroimaging of inflammation in HIV

People with HIV now have near-normal life expectancies due to the success of effective combination antiretroviral therapy (cART). Following cART initiation, immune recovery occurs, and opportunistic diseases become rare. Despite this, high rates of non-infectious comorbidities persist in treated people with HIV, hypothesized to be related to persistent immuno-activation. One such comorbidity is cognitive impairment, which may partly be driven by ongoing neuro-inflammation in otherwise effectively treated people with HIV. In order to develop therapeutic interventions to address neuro-inflammation in effectively treated people with HIV, a deeper understanding of the pathogenic mechanisms driving persistent neuro-inflammatory responses and the ability to better characterize and measure neuro-inflammation in the central nervous system is required. This review highlights recent advances in molecular neuroimaging techniques which have the potential to assess neuro-inflammatory responses within the central nervous system in HIV disease. Proton magnetic resonance spectroscopy (1H-MRS) has been utilized to assess neuro-inflammatory responses since early in the HIV pandemic and shows promise in recent studies assessing different antiretroviral regimens. 1H-MRS is widely available in both resource-rich and some resource-constrained settings and is relatively inexpensive. Brain positron emission tomography (PET) imaging using Translocator Protein (TSPO) radioligands is a rapidly evolving field; newer TSPO-radioligands have lower signal-to-noise ratio and have the potential to localize neuro-inflammation within the brain in people with HIV. As HIV therapeutics evolve, people with HIV continue to age and develop age-related comorbidities including cognitive disorders. The use of novel neuroimaging modalities in the field is likely to advance in order to rapidly assess novel therapeutic interventions and may play a role in future clinical assessments.

Comparison of [11C]-PBR28 Binding Between Persons Living With HIV and HIV-Uninfected Individuals

OBJECTIVE

Despite combined antiretroviral therapy, neuroinflammation may persist in persons living with HIV (PLWH) and contribute to cognitive impairment in this population. Positron emission tomography (PET) imaging targeting 18 kDa translocator protein (TSPO) has been used to localize neuroinflammation. We aimed to use TSPO-PET imaging to evaluate neuroinflammation in PLWH.

DESIGN

Twenty-four virologically suppressed PLWH on combined antiretroviral therapy and 13 HIV-negative (HIV-) controls completed TSPO-PET imaging using the radiotracer [C]PBR28. Because of tracer complexity and differing procedures used in previous studies, we employed an expansive methodological approach, using binding potential (BP) and standard uptake value ratio and multiple different reference regions to estimate [C]PBR28 binding.

METHODS

[C]PBR28 binding was measured in 30 cortical and subcortical regions and compared between PLWH and HIV- controls. Pearson correlation evaluated the association between [C]PBR28 binding and cognition and clinical measures of HIV.

RESULTS

Analyses conducted using multiple reference regions and measures of tracer uptake revealed no significant differences between [C]PBR28 binding in PLWH compared with HIV- controls. In addition, [C]PBR28 binding in PLWH was not significantly associated with clinical measures of HIV or plasma biomarkers of inflammation. [C]PBR28 binding was not significantly elevated in cognitively impaired PLWH compared with unimpaired PLWH, but there were inverse relationships between cognitive performance (executive and global function) and [C]PBR28 binding in PLWH.

CONCLUSIONS

Our results suggest that neuroinflammation may play a role in cognitive deficits, but overall neuroinflammatory levels as measured by TSPO-PET imaging in PLWH are not significantly different from those seen in HIV- controls.

Neuroimaging the Neuropathogenesis of HIV

PURPOSE OF REVIEW

This review highlights neuroimaging studies of HIV conducted over the last 2 years and discusses how relevant findings further our knowledge of the neuropathology of HIV. Three major avenues of neuroimaging research are covered with a particular emphasis on inflammation, aging, and substance use in persons living with HIV (PLWH).

RECENT FINDINGS

Neuroimaging has been a critical tool for understanding the neuropathological underpinnings observed in HIV. Recent studies comparing levels of neuroinflammation in PLWH and HIV-negative controls show inconsistent results but report an association between elevated neuroinflammation and poorer cognition in PLWH. Other recent neuroimaging studies suggest that older PLWH are at increased risk for brain and cognitive compromise compared to their younger counterparts. Finally, recent findings also suggest that the effects of HIV may be exacerbated by alcohol and drug abuse. These neuroimaging studies provide insight into the structural, functional, and molecular changes occurring in the brain due to HIV. HIV triggers a strong neuroimmune response and may lead to a cascade of events including increased chronic inflammation and cognitive decline. These outcomes are further exacerbated by age and age-related comorbidities, as well as lifestyle factors such as drug use/abuse.

PET imaging of neuroinflammation in neurological disorders

A growing need exists for reliable in-vivo measurement of neuroinflammation to better characterise the inflammatory processes underlying various diseases and to inform the development of novel therapeutics that target deleterious glial activity. PET is well suited to quantify neuroinflammation and has the potential to discriminate components of the neuroimmune response. However, there are several obstacles to the reliable quantification of neuroinflammation by PET imaging. Despite these challenges, PET studies have consistently identified associations between neuroimmune responses and pathophysiology in brain disorders such as Alzheimer's disease. Tissue studies have also begun to clarify the meaning of changes in PET signal in some diseases. Furthermore, although PET imaging of neuroinflammation does not have an established clinical application, novel targets are under investigation and a small but growing number of studies have suggested that this imaging modality could have a role in drug development. Future studies are needed to further improve our knowledge of the cellular mechanisms that underlie changes in PET signal, how immune response contributes to neurological disease, and how it might be therapeutically modified.

Brain PET Imaging: Value for Understanding the Pathophysiology of HIV-associated Neurocognitive Disorder (HAND)

PURPOSE OF REVIEW

The purpose of this review is to summarize recent developments in PET imaging of neuropathologies underlying HIV-associated neurocognitive dysfunction (HAND). We concentrate on the recent post antiretroviral era (ART), highlighting clinical and preclinical brain PET imaging studies.

RECENT FINDINGS

In the post ART era, PET imaging has been used to better understand perturbations of glucose metabolism, neuroinflammation, the function of neurotransmitter systems, and amyloid/tau protein deposition in the brains of HIV-infected patients and HIV animal models. Preclinical and translational findings from those studies shed a new light on the complex pathophysiology underlying HAND. The molecular imaging capabilities of PET in neuro-HIV are great complements for structural imaging modalities. Recent and future PET imaging studies can improve our understanding of neuro-HIV and provide biomarkers of disease progress that could be used as surrogate endpoints in the evaluation of the effectiveness of potential neuroprotective therapies.

Bidirectional Associations among Nicotine and Tobacco Smoke, NeuroHIV, and Antiretroviral Therapy

People living with HIV (PLWH) in the antiretroviral therapy (ART) era may lose more life-years to tobacco use than to HIV. Yet, smoking rates are more than twice as high among PLWH than the general population, contributing not just to mortality but to other adverse health outcomes, including neurocognitive deficits (neuroHIV). There is growing evidence that synergy with chronic inflammation and immune dysregulation that persists despite ART may be one mechanism by which tobacco smoking contributes to neuroHIV. This review will summarize the differential effects of nicotine vs tobacco smoking on inflammation in addition to the effects of tobacco smoke components on HIV disease progression. We will also discuss biomarkers of inflammation via neuroimaging as well as biomarkers of nicotine dependence (e.g., nicotine metabolite ratio). Tobacco smoking and nicotine may impact ART drug metabolism and conversely, certain ARTs may impact nicotine metabolism. Thus, we will review these bidirectional relationships and how they may contribute to neuroHIV and other adverse outcomes. We will also discuss the effects of tobacco use on the interaction between peripheral organs (lungs, heart, kidney) and subsequent CNS function in the context of HIV. Lastly, given the dramatic rise in the use of electronic nicotine delivery systems, we will discuss the implications of vaping on these processes. Despite the growing recognition of the importance of addressing tobacco use among PLWH, more research is necessary at both the preclinical and clinical level to disentangle the potentially synergistic effects of tobacco use, nicotine, HIV, cognition and immune dysregulation, as well as identify optimal approaches to reduce tobacco use. Graphical Abstract Proposed model of the relationships among HIV, ART, smoking, inflammation, and neurocognition. Solid lines represent relationships supported by evidence. Dashed lines represent relationships for which there is not enough evidence to make a conclusion. (a) HIV infection produces elevated levels of inflammation even among virally suppressed individuals. (b) HIV is associated with deficits in cognition function. (c) Smoking rates are higher among PLWH, compared to the general population. (d) The nicotine metabolite ratio (NMR) is associated with smoking behavior. (e) HIV and tobacco use are both associated with higher rates of psychiatric comorbidities, such as depression, and elevated levels of chronic stress. These factors may represent other mechanisms linking HIV and tobacco use. (f) The relationship between nicotine, tobacco smoking, and inflammation is complex, but it is well-established that smoking induces inflammation; the evidence for nicotine as anti-inflammatory is supported in some studies, but not others. (g) The relationship between tobacco use and neurocognition may differ for the effects of nicotine (acute nicotine use may have beneficial effects) vs. tobacco smoking (chronic use may impair cognition). (h) Elevated levels of inflammation may be associated with deficits in cognition. (i) PLWH may metabolize nicotine faster than those without HIV; the mechanism is not yet known and the finding needs validation in larger samples. We also hypothesize that if HIV-infection increases nicotine metabolism, then we should observe an attenuation effect once ART is initiated. (j) It is possible that the increase in NMR is due to ART effects on CYP2A6. (k) We hypothesize that faster nicotine metabolism may result in higher levels of inflammation since nicotine has anti-inflammatory properties.

Neuroinflammation & pre-mature aging in the context of chronic HIV infection and drug abuse: Role of dysregulated autophagy

In the era of combined antiretroviral therapy (cART), HIV-1 infection has transformed from adeath sentenceto a manageable, chronic disease. Although the lifeexpectancy of HIV+ individuals is comparable to that of the uninfectedsubjects paradoxically, there is increased prevalence ofage-associatedcomorbidities such asatherosclerosis, diabetes, osteoporosis & neurological deficits in the context of HIV infection. Drug abuse is a commoncomorbidityofHIV infection andis often associated withincreased neurological complications. Chronic neuroinflammation (abnormal microglial and astrocyte activation) and neuronal synaptodendritic injury are the features of CNS pathology observed inHIV (+) individualsthat are takingcART & that abuse drugs. Neuroinflammation is thedrivingforceunderlying prematureaging associated with HIV (+) infection, cART and drugs of abuse. Autophagy is a highly conserved process critical for maintaining cellular homeostasis. Dysregulated autophagyhas been shown to be linked with abnormal immune responses & aging. Recent emerging evidence implicatesthe role ofHIV/HIV proteins, cART, & abused drugsin disrupting theautophagy process in brain cells such as microglia, astrocytes, and neurons. It can thus be envisioned that co-exposure of CNS cells to HIV proteins, cART and/or abused drugs couldhavesynergistic effects on theautophagy process, thereby leading to exaggerated microglial/astrocyte activation, ultimately, promotingthe aging process. Restoration of autophagic functioncould thusprovide an alternative therapeuticstrategy formitigating neuroinflammation & ameliorating the premature aging process. The current review aims to unravel the role of dysregulated autophagy in the context of single or co-exposure of microglia, astrocytes, and neurons to HIV/HIV proteins, drugs of abuse &/or cART and will also discuss the pathways involved in dysregulated autophagy-mediated neuroinflammation.

PET Evaluation of Microglial Activation in Non-neurodegenerative Brain Diseases

PURPOSE OF THE REVIEW

Microglial cell activation is an important component of neuroinflammation, and it is generally well accepted that chronic microglial activation is indicative of accumulating tissue damage in neurodegenerative conditions, particularly in the earlier stages of disease. Until recently, there has been less focus on the role of neuroinflammation in other forms of neurological and neuropsychiatric conditions. Through this review, we hope to demonstrate the important role TSPO PET imaging has played in illuminating the pivotal role of neuroinflammation and microglial activation underpinning these conditions.

RECENT FINDINGS

TSPO is an 18 kDa protein found on the outer membrane of mitochondria and can act as a marker of microglial activation using nuclear imaging. Through the development of radiopharmaceuticals targeting TSPO, researchers have been able to better characterise the spatial-temporal evolution of chronic neurological conditions, ranging from the focal autoimmune reactions seen in multiple sclerosis to the Wallerian degeneration at remote parts of the brain months following acute cerebral infarction. Development of novel techniques to investigate neuroinflammation within the central nervous system, for the purposes of diagnosis and therapeutics, has flourished over the past few decades. TSPO has proven itself a robust and sensitive biomarker of microglial activation and neuroimaging affords a minimally invasive technique to characterise neuroinflammatory processes in vivo.

Molecular Imaging of Neuroinflammation in HIV

The development of combined antiretroviral therapy (cART) has increased the lifespan of persons living with HIV (PLWH), with most PLWH having a normal life expectancy. While significant progress has occurred, PLWH continue to have multiple health complications, including HIV associated neurocognitive disorders (HAND). While the exact cause of HAND is not known, persistent neuroinflammation is hypothesized to be an important potential contributor. Molecular imaging using positron emission tomography (PET) can non-invasively evaluate neuroinflammation. PET radiotracers specific for increased expression of the translocator protein18kDa (TSPO) on activated microglia can detect the presence of neuroinflammation in PLWH. However, results from these studies have been inconsistent and inconclusive. Future studies are needed to address key limitations that continue to persist with these techniques before accurate conclusions can be drawn regarding the role of persistent neuroinflammation in PLWH.

CSF concentrations of soluble TREM2 as a marker of microglial activation in HIV-1 infection

Objective

To explore changes in CSF sTREM2 concentrations in the evolving course of HIV-1 infection.

Methods

In this retrospective cross-sectional study, we measured concentrations of the macrophage/microglial activation marker sTREM2 in CSF samples from 121 HIV-1-infected adults and 11 HIV-negative controls and examined their correlations with other CSF and blood biomarkers of infection, inflammation, and neuronal injury.

Results

CSF sTREM2 increased with systemic and CNS HIV-1 disease severity, with the highest levels found in patients with HIV-associated dementia (HAD). In untreated HIV-1-infected patients without an HAD diagnosis, levels of CSF sTREM2 increased with decreasing CD4⁺ T-cell counts. CSF concentrations of both sTREM2 and the neuronal injury marker neurofilament light protein (NFL) were significantly associated with age. CSF sTREM2 levels were also independently correlated with CSF NFL. Notably, this association was also observed in HIV-negative controls with normal CSF NFL. HIV-infected patients on suppressive antiretroviral treatment had CSF sTREM2 levels comparable to healthy controls.

Conclusions

Elevations in CSF sTREM2 levels, an indicator of macrophage/microglial activation, are a common feature of untreated HIV-1 infection that increases with CD4⁺ T-cell loss and reaches highest levels in HAD. The strong and independent association between CSF sTREM2 and CSF NFL suggests a linkage between microglial activation and neuronal injury in HIV-1 infection. CSF sTREM2 has the potential of being a useful biomarker of innate CNS immune activation in different stages of untreated and treated HIV-1 infection.

Imaging studies of the HIV-infected brain

Human immunodeficiency virus (HIV) enters the brain early after infecting humans and may remain in the central nervous system despite successful antiretroviral treatment. Many neuroimaging techniques were used to study HIV+ patients with or without opportunistic infections. These techniques assessed abnormalities in brain structures (using computed tomography, structural magnetic resonance imaging (MRI), diffusion MRI) and function (using functional MRI at rest or during a task, and perfusion MRI with or without a contrast agent). In addition, single-photon emission computed tomography with various tracers (e.g., thallium-201, Tc99-HMPAO) and positron emission tomography with various agents (e.g., [18F]-dexoyglucose, [11C]-PiB, and [11C]-TSPO tracers), were applied to study opportunistic infections or HIV-associated neurocognitive disorders. Neuroimaging provides diagnoses and biomarkers to quantitate the severity of brain injury or to monitor treatment effects, and may yield insights into the pathophysiology of HIV infection. As the majority of antiretroviral-stable HIV+ patients are living longer, age-related comorbid disorders (e.g., additional neuroinflammation, cerebrovascular disorders, or other dementias) will need to be considered. Other highly prevalent conditions, such as substance use disorders, psychiatric illnesses, and the long-term effects of combined antiretroviral therapy, all may lead to additional brain injury. Neuroimaging studies could provide knowledge regarding how these comorbid conditions impact the HIV-infected brain. Lastly, specific molecular imaging agents may be needed to assess the central nervous system viral reservoir.

PET brain imaging in HIV-associated neurocognitive disorders (HAND) in the era of combination antiretroviral therapy

Effective combination antiretroviral therapy (cART) has lead to a significant reduction in the prevalence and incidence of central nervous system (CNS) HIV-associated brain disease, particularly CNS opportunistic infections and HIV encephalitis. Despite this, cognitive deficits in people living with HIV, also known as HIV-associated neurocognitive disorders (HAND) have become more prevalent in recent years. The pathogenesis of HAND is likely to be multifactorial, however recent evidence suggests that brain microglial activation is the most likely pathogenic mechanism. Recent developments in positron emission tomography (PET) brain neuroimaging using novel brain radioligands targeting a variety of physiological changes in the brains of HIV-positive individuals have improved our understanding of the mechanisms associated with the development of HAND. This review will highlight recent PET brain neuroimaging studies in the cART era, focusing on physiological and neurochemical changes associated with HAND in people living with HIV.

Chronic distress and the vulnerable host: a new target for HIV treatment and prevention?

Pathologic stress (distress) disturbs immune, cardiovascular, metabolic, and behavioral homeostasis. Individuals living with HIV and those at risk are vulnerable to stress disorders. Corticotropin-releasing hormone (CRH) is critical in neuroendocrine immune regulation. CRH, a neuropeptide, is distributed in the central and peripheral nervous systems and acts principally on CRH receptor type 1 (CRHR1). CRH in the brain modulates neuropsychiatric disorders. CRH and stress modulation of immunity is two-pronged; there is a direct action on hypothalamic-pituitary-adrenal secretion of glucocorticoids and through immune organ sympathetic innervation. CRH is a central and systemic proinflammatory cytokine. Glucocorticoids and their receptors have gene regulatory actions on viral replication and cause central and systemic immune suppression. CRH and stress activation contributes to central nervous system (CNS) viral entry important in HIV-associated neurocognitive disorders and HIV-associated dementia. CNS CRH overproduction short-circuits reward, executive, and emotional control, leading to addiction, cognitive impairment, and psychiatric comorbidity. CRHR1 is an important therapeutic target for medication development. CRHR1 antagonist clinical trials have focused on psychiatric disorders with little attention paid to neuroendocrine immune disorders. Studies of those with HIV and those at risk show that concurrent stress-related disorders contribute to higher morbidity and mortality; stress-related conditions, addiction, immune dysfunction, and comorbid psychiatric illness all increase HIV transmission. Neuropsychiatric disease, chronic inflammation, and substance abuse are endemic, and chronic distress is a pathologic factor. It is being understood that stress and CRH are fundamental to neuroendocrine immunity; therapeutic interventions with existing and novel agents hold promise for restoring homeostasis, reducing morbidity and mortality for those with HIV and possibly reducing future disease transmission.

Volume, metabolites and neuroinflammation of the hippocampus in bipolar disorder - A combined magnetic resonance imaging and positron emission tomography study

BACKGROUND

The hippocampus is one of the brain regions that is involved in several pathophysiological theories about bipolar disorder (BD), such as the neuroinflammation theory and the corticolimbic metabolic dysregulation theory. We compared hippocampal volume and hippocampal metabolites in bipolar I disorder (BD-I) patients versus healthy controls (HCs) with magnetic resonance imaging (MRI) and spectroscopy (MRS). We post hoc investigated whether hippocampal volume and hippocampal metabolites were associated with microglial activation and explored if potential illness modifying factors affected these hippocampal measurements and whether these were associated with experienced mood and functioning.

MATERIALS AND METHODS

Twenty-two BD-I patients and twenty-four HCs were included in the analyses. All subjects underwent psychiatric interviews as well as an MRI scan, including a T1 scan and PRESS magnetic resonance spectroscopy (MRS). Volumetric analysis was performed with Freesurfer. MRS quantification was performed with LC Model. A subgroup of 14 patients and 11 HCs also underwent a successful [(11)C]-(R)-PK11195 neuroinflammation positron emission tomography scan.

RESULTS

In contrast to our hypothesis, hippocampal volumes were not decreased in patients compared to HC after correcting for individual whole-brain volume variations. We demonstrated decreased N-acetylaspartate (NAA)+N-acetyl-aspartyl-glutamate (NAAG) and creatine (Cr)+phosphocreatine (PCr) concentrations in the left hippocampus. In the explorative analyses in the left hippocampus we identified positive associations between microglial activation and the NAA+NAAG concentration, between alcohol use and NAA+NAAG concentration, between microglial activation and the depression score and a negative relation between Cr+PCr concentration and experienced occupational disability. Duration of illness associated positively with volume bilaterally.

CONCLUSION

Compared to HCs, the decreased NAA+NAAG concentration in the left hippocampus of BD-I patients suggests a decreased neuronal integrity in this region. In addition we found a positive relation between microglial activation and neuronal integrity in vivo, corresponding to a differentiated microglial function where some microglia induce apoptosis while others stimulate neurogenesis.

An Update of the Review of Neuropsychological Consequences of HIV and Substance Abuse: A Literature Review and Implications for Treatment and Future Research

Neuropyschological dysfunction, ranging from mild cerebral indicators to dementia has been a consistent part of the medical picture of HIV/AIDS. However, advances in medical supervision, particularly as a result of antiretroviral (ARV) treatment, have resulted in some mitigation of the neuropsychological effects of HIV and necessitate re-evaluation of the pattern and nature of HIV-related cognitive or mental deficits. The associated enhancements in morbidity and mortality that have occurred as a result of ARV medication have led to a need for interventions and programs that maintain behaviors that are healthy and stop the resurgence of the risk of HIV transmission. Risk factors such as mental illness and substance use that may have contributed to the initial infection with HIV still need consideration. These risk factors may also increase neuropsychological dysfunction and impact observance of prevention for treatment and recommendations. Explicitly, a better comprehension of the role of substance use on the progression of HIV-related mental decline can enlighten management and evaluation of persons living with HIV with concurrent disorders of substance use. This review provides a summary of the neurophyschology of substance use and HIV and the existing research that has looked at the effects of both substance use and HIV disease on neurophyscological function and suggestions for future research and treatment.

Neuroinflammation in treated HIV-positive individuals: A TSPO PET study

OBJECTIVE

To explore the effects of microglial activation on brain function and structure, and its relationship with peripheral inflammatory markers, in treated, HIV-positive individuals, using in vivo [(11)C]PBR28 PET (to measure the 18 kDa translocator protein [TSPO]).

METHODS

Cognitively healthy HIV-positive individuals on suppressive antiretroviral therapy and HIV-negative individuals (controls) underwent brain [(11)C]PBR28 PET and MRI. HIV-positive patients completed neuropsychological testing and CSF testing for chemokines. The concentration of bacterial ribosomal 16sDNA in plasma was measured as a marker of microbial translocation.

RESULTS

HIV-positive individuals showed global increases in TSPO expression compared to controls (corrected p < 0.01), with significant regional increases in the parietal (p = 0.001) and occipital (p = 0.046) lobes and in the globus pallidus (p = 0.035). TSPO binding in the hippocampus, amygdala, and thalamus were associated with poorer global cognitive performance in tasks assessing verbal and visual memory (p < 0.05). Increased TSPO binding was associated with increased brain white matter diffusion MRI mean diffusivity in HIV-positive individuals, a lower CD4/CD8 ratio, and both high pretreatment HIV RNA and plasma concentration ribosomal 16s DNA (p < 0.05).

CONCLUSIONS

Cognitively healthy HIV-positive individuals show evidence for a chronically activated brain innate immune response and elevated blood markers of microbial translocation despite effective control of plasma viremia. Increased brain inflammation is associated with poorer cognitive performance and white matter microstructural pathology, suggesting a possible role in cognitive impairments found in some HIV-positive patients despite effective treatment.

Lack of neuroinflammation in the HIV-1 transgenic rat: an [(18)F]-DPA714 PET imaging study

Background

HIV-associated neuroinflammation is believed to be a major contributing factor in the development of HIV-associated neurocognitive disorders (HAND). In this study, we used micropositron emission tomography (PET) imaging to quantify neuroinflammation in HIV-1 transgenic rat (Tg), a small animal model of HIV, known to develop neurological and behavioral problems.

Methods

Dynamic [¹⁸F]DPA-714 PET imaging was performed in Tg and age-matched wild-type (WT) rats in three age groups: 3-, 9-, and 16-month-old animals. As a positive control for neuroinflammation, we performed unilateral intrastriatal injection of quinolinic acid (QA) in a separate group of WT rats. To confirm our findings, we performed multiplex immunofluorescent staining for Iba1 and we measured cytokine/chemokine levels in brain lysates of Tg and WT rats at different ages.

Results

[¹⁸F]DPA-714 uptake in HIV-1 Tg rat brains was generally higher than in age-matched WT rats but this was not statistically significant in any age group. [¹⁸F]DPA-714 uptake in the QA-lesioned rats was significantly higher ipsilateral to the lesion compared to contralateral side indicating neuroinflammatory changes. Iba1 immunofluorescence showed no significant differences in microglial activation between the Tg and WT rats, while the QA-lesioned rats showed significant activation. Finally, cytokine/chemokine levels in brain lysates of the Tg rats and WT rats were not significantly different.

Conclusion

Microglial activation might not be the primary mechanism for neuropathology in the HIV-1 Tg rats. Although [¹⁸F]DPA-714 is a good biomarker of neuroinflammation, it cannot be reliably used as an in vivo biomarker of neurodegeneration in the HIV-1 Tg rat.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0390-9) contains supplementary material, which is available to authorized users.

Improved Automated Radiosynthesis of [(11)C]PBR28

Microglial activation is commonly identified by elevated levels of the 18 kDa translocator protein (TSPO) in response to several inflammatory processes. [(11)C]PBR28 is one of the most promising PET tracers to image TSPO in both human and non-human primates. In this study, we optimized the radiolabeling procedure of [(11)C]PBR28 for higher radiochemical yield, radiochemical purity, and specific activity, which can be easily translated to any automated module for clinical trials. Time-activity curves (TACs) derived from the dynamic PET imaging of male rhesus monkey brains demonstrated that [(11)C]PBR28 had suitable kinetics with radiotracer accumulation observed in the caudate, putamen, cerebellum, and frontal cortex region.

Neuroimaging of HIV-associated neurocognitive disorders (HAND)

PURPOSE OF REVIEW

HIV enters the brain after initial infection, and with time can lead to HIV-associated neurocognitive disorders (HAND). Although the introduction of combination antiretroviral therapy has reduced the more severe forms of HAND, milder forms are still highly prevalent. The 'gold standard' for HAND diagnosis remains detailed neuropsychological performance testing but additional biomarkers (including neuroimaging) may assist in early detection of HAND.

RECENT FINDINGS

We review the application of recently developed noninvasive MRI and PET techniques in HIV+ individuals. In particular, magnetic resonance spectroscopy may be more sensitive than conventional MRI alone in detecting HIV associated changes. Diffusion tensor imaging has become increasingly popular for assessing changes in white matter structural integrity due to HIV. Both functional MRI and PET have been limitedly performed but could provide keys for characterizing neuropathophysiologic changes due to HIV.

SUMMARY

It is hoped that continued progress will allow novel neuroimaging methods to be included in future HAND management guidelines.

Neuroinflammation in bipolar disorder - A [(11)C]-(R)-PK11195 positron emission tomography study

BACKGROUND

The "monocyte-T-cell theory of mood disorders" regards neuroinflammation, i.e. marked activation of microglia, as a driving force in bipolar disorder. Microglia activation can be visualized in vivo using [(11)C]-(R)-PK11195 PET. Indirect evidence suggests the hippocampus as a potential focus of neuroinflammation in bipolar disorder. We aim to determine if there is increased [(11)C]-(R)-PK11195 binding to activated microglia in the hippocampus of patients with bipolar I disorder when compared to healthy controls.

MATERIAL AND METHODS

Fourteen patients with bipolar I disorder and eleven healthy controls were included in the analyses. Dynamic 60-min PET scans were acquired after the injection of [(11)C]-(R)-PK11195. All subjects underwent psychiatric interviews as well as an MRI scan, which was used for anatomic co-registration in the data analysis. The data from the PET scans was analyzed with a two-tissue-compartment model to calculate the binding potential, using the metabolite-corrected plasma and blood curve as input.

RESULTS

A significantly increased [(11)C]-(R)-PK11195 binding potential, which is indicative of neuroinflammation, was found in the right hippocampus of the patients when compared to the healthy controls (1.66 (CI 1.45-1.91) versus 1.33 (CI 1.16-1.53); p=0.033, respectively). Although the same trend was observed in the left hippocampus, this difference was not statistically significant.

CONCLUSION

This study is the first to demonstrate the presence of focal neuroinflammation in the right hippocampus in bipolar I disorder.

Regional brain distribution of translocator protein using [(11)C]DPA-713 PET in individuals infected with HIV

Imaging the brain distribution of translocator protein (TSPO), a putative biomarker for glial cell activation and neuroinflammation, may inform management of individuals infected with HIV by uncovering regional abnormalities related to neurocognitive deficits and enable non-invasive therapeutic monitoring. Using the second-generation TSPO-targeted radiotracer, [(11)C]DPA-713, we conducted a positron emission tomography (PET) study to compare the brains of 12 healthy human subjects to those of 23 individuals with HIV who were effectively treated with combination antiretroviral therapy (cART). Compared to PET data from age-matched healthy control subjects, [(11)C]DPA-713 PET of individuals infected with HIV demonstrated significantly higher volume-of-distribution (VT) ratios in white matter, cingulate cortex, and supramarginal gyrus, relative to overall gray matter VT, suggesting localized glial cell activation in susceptible regions. Regional TSPO abnormalities were evident within a sub-cohort of neuro-asymptomatic HIV subjects, and an increase in the VT ratio within frontal cortex was specifically linked to individuals affected with HIV-associated dementia. These findings were enabled by employing a gray matter normalization approach for PET data quantification, which improved test-retest reproducibility, intra-class correlation within the healthy control cohort, and sensitivity of uncovering abnormal regional findings.

HIV-associated neurocognitive disorder

Neurological involvement in HIV is often associated with cognitive impairment. Although severe and progressive neurocognitive impairment has become rare in HIV clinics in the era of potent antiretroviral therapy, most patients with HIV worldwide have poor outcomes on formal neurocognitive tests. In this Review, we describe the manifestations of HIV-associated neurocognitive disorder in the era of effective HIV therapy, outline diagnosis and treatment recommendations, and explore the research questions that remain. Although comorbid disorders, such as hepatitis C infection or epilepsy, might cause some impairment, their prevalence is insufficient to explain the frequency with which it is encountered. HIV disease markers, such as viral load and CD4 cell counts, are not strongly associated with ongoing impairment on treatment, whereas cardiovascular disease markers and inflammatory markers are. New cerebrospinal fluid and neuroimaging biomarkers are needed to detect and follow impairment. Ongoing research efforts to optimise HIV therapy within the CNS, and potentially to intervene in downstream mechanisms of neurotoxicity, remain important avenues for future investigation. Ultimately, the full control of virus in the brain is a necessary step in the goal of HIV eradication.

Increased microglia activation in neurologically asymptomatic HIV-infected patients receiving effective ART

BACKGROUND

Neuroinflammation plays an important role in HIV-associated neurological disorders; however, its role prior to the onset of symptomatic disease is unclear. We imaged microglial activation, the hallmark of neuroinflammation, in asymptomatic HIV-infected patients on effective combination ART.

METHODS

Seven neurologically and cognitively asymptomatic adults with chronic HIV-infection and nine healthy volunteers were investigated with [11C]-PK11195 PET, a marker of translocator protein (TSPO) expressed by activated microglia. In the HIV-infected patients, cognitive speed, accuracy and executive function were also assessed. Between-group differences in [11C]-PK11195 binding potential were localized throughout the brain with statistical parametric mapping (SPM) and associations between levels of [11C]-PK11195 binding and cognitive performance were interrogated using linear regression modelling.

RESULTS

In HIV-infected patients, Statistical parametric mapping detected clusters of significantly increased [11C]-PK11195 binding in corpus callosum (P = 0.001), anterior cingulate (P = 0.001), posterior cingulate (P = 0.008) and temporal (P = 0.026) and frontal (P = 0.038) areas. Cognitive functions were intact in the HIV group, however, a significant association between greater [11C]-PK11195 binding and poorer executive function performance was observed in the anterior cingulate (P = 0.031), corpus callosum and posterior cingulate (P = 0.001).

CONCLUSION

Despite effective control of HIV infection, neuroinflammation, as evidenced by the presence of focal cortical areas of activated microglia, occurs in asymptomatic HIV-infected patients and levels correlate with poorer executive performance. Further studies are needed to establish whether detection of activated microglia in HIV-infected patients represents a marker of future neurocognitive decline.

Feasibility and potential role of ferumoxytol-enhanced neuroimaging in HIV-associated neurocognitive disorder

We assessed ferumoxytol-enhanced brain MRI to identify monocyte/macrophage accumulation in HIV-associated neurocognitive disorder (HAND). Four HIV-infected subjects with undetectable HIV RNA levels on antiretroviral therapy, HIV DNA level in CD14+ cells ≥10 copies/10(6) cells, and cognitive impairment underwent ferumoxytol-enhanced brain MRI. On post-ferumoxytol susceptibility-weighted images, all HIV-infected subjects demonstrated a diffuse "tram track" appearance in the perivascular regions of cortical and deep white matter vessels suggesting ferumoxytol uptake in monocytes/macrophages. This finding was not present in an HIV-seronegative control. While ferumoxytol may have potential as an imaging biomarker for monocyte/macrophage accumulation in patients with HAND, future study is needed.

Acute HCV/HIV coinfection is associated with cognitive dysfunction and cerebral metabolite disturbance, but not increased microglial cell activation

BACKGROUND

Microglial cell activation and cerebral function impairment are described in both chronic hepatitis C viral (HCV) and Human-Immune-Deficiency viral (HIV) infections. The aim of this study was to investigate the effect of acute HCV infection upon cerebral function and microglial cell activation in HIV-infected individuals.

METHODS

A case-control study was conducted. Subjects with acute HCV and chronic HIV coinfection (aHCV) were compared to matched controls with chronic HIV monoinfection (HIVmono). aHCV was defined as a new positive plasma HCV RNA within 12 months of a negative RNA test. Subjects underwent neuro-cognitive testing (NCT), cerebral proton magnetic resonance spectroscopy ((1)H-MRS) and positron emission tomography (PET) using a (11)C-radiolabeled ligand (PK11195), which is highly specific for translocator protein 18 kDA receptors on activated microglial cells. Differences between cases and controls were assessed using linear regression modelling.

RESULTS

Twenty-four aHCV cases completed NCT and (1)H-MRS, 8 underwent PET. Of 57 HIVmono controls completing NCT, 12 underwent (1)H-MRS and 8 PET. Subjects with aHCV demonstrated on NCT, significantly poorer executive function (mean (SD) error rate 26.50(17.87) versus 19.09(8.12), p = 0.001) and on (1)H-MRS increased myo-inositol/creatine ratios (mI/Cr, a marker of cerebral inflammation) in the basal ganglia (ratio of 0.71(0.22) versus 0.55(0.23), p = 0.03), compared to subjects with HIVmono. On PET imaging, no difference in (11)C-PK11195 binding potential (BP) was observed between study groups (p>0.10 all cerebral locations), however lower BPs were associated with combination antiretroviral therapy (cART) use in the parietal (p = 0.01) and frontal (p = 0.03) cerebral locations.

DISCUSSION

Poorer cognitive performance and disturbance of cerebral metabolites are observed in subjects with aHC,V compared to subjects with HIVmono. Higher (11)C-PK11195 BP was not observed in subjects with aHCV, but was observed in subjects not on cART.

Noninvasive molecular imaging of neuroinflammation

Inflammation is a highly dynamic and complex adaptive process to preserve and restore tissue homeostasis. Originally viewed as an immune-privileged organ, the central nervous system (CNS) is now recognized to have a constant interplay with the innate and the adaptive immune systems, where resident microglia and infiltrating immune cells from the periphery have important roles. Common diseases of the CNS, such as stroke, multiple sclerosis (MS), and neurodegeneration, elicit a neuroinflammatory response with the goal to limit the extent of the disease and to support repair and regeneration. However, various disease mechanisms lead to neuroinflammation (NI) contributing to the disease process itself. Molecular imaging is the method of choice to try to decipher key aspects of the dynamic interplay of various inducers, sensors, transducers, and effectors of the orchestrated inflammatory response in vivo in animal models and patients. Here, we review the basic principles of NI with emphasis on microglia and common neurologic disease mechanisms, the molecular targets which are being used and explored for imaging, and molecular imaging of NI in frequent neurologic diseases, such as stroke, MS, neurodegeneration, epilepsy, encephalitis, and gliomas.

Cerebral microglial activation in patients with hepatitis C: in vivo evidence of neuroinflammation

Patients with chronic hepatitis C infection may exhibit neuropsychological symptoms and cognitive impairment. Post-mortem studies of hepatitis C virus HCV quasispecies and replicative intermediates indicate that the brain might act as a separate compartment for viral replication and microglia may be the locus for infection and subsequent neuroinflammatory activity. We sought to use two independent in vivo imaging techniques to determine evidence of neuroinflammation in patients with histologically mild chronic hepatitis C. Using positron emission tomography (PET) with a ligand for microglial/brain macrophage activation, (11)C-(R)-PK11195 (PK11195) and cerebral proton magnetic resonance spectroscopy, we determined whether there was evidence of neuroinflammation in a pilot study of 11 patients with biopsy-proven mild chronic hepatitis C, compared to healthy volunteers. Patients were characterized by cognitive testing and the fatigue impact scale to assess for CNS impairment. PK11195 binding potential was significantly increased in the caudate nucleus of patients, compared to normal controls (P = 0.03). The caudate and thalamic binding potential were more significantly increased in six patients with genotype 1 infection (P = 0.007) and positively correlated with viraemia (r = 0.77, P = 0.005). Basal ganglia myo-inositol/creatine and choline/creatine ratios were also significantly elevated in patients with chronic hepatitis C compared to normal controls (P = 0.0004 and P = 0.01, respectively). Using PET, we demonstrated evidence of microglial activation, which positively correlated with HCV viraemia and altered cerebral metabolism in the brains of patients with mild hepatitis C. This provides further in vivo evidence for a neurotropic role for HCV.

Microglia: key innate immune cells of the brain

Microglia are the histiocytes of the central nervous system. These long-lived cells undergo very little turnover in normal physiological states; however, in pathological conditions, increased egress from the bone marrow and chemoattractive signals in the brain can substantially modulate the indigenous population. Although they were initially conceived of as "resting" cells, recent data suggest that they would be more aptly described as "surveillance" cells. Microglia are specifically adapted to sense various types of danger and differentially react with a classical or alternative reparative response. Our understanding of macrophage function has shifted away from focusing on cell lineage to a more systems-based biology of gene networks accomplishing the detoxification and immune functions. With our greater appreciation of microglial involvement in the innate immune response, we have entered a new era in which the modulation of microglia can be proposed as a means of modulating neurological disease.

Positron emission tomography imaging in human immunodeficiency virus-1-associated neurocognitive disorders: an interesting case and a review of the positron emission tomography literature

Human immunodeficiency virus (HIV)-1-associated neurocognitive disorder can manifest with a variety of neurologic, cognitive, and behavioral impairments. We report a case of a 49-year-old non-HIV risk woman with an occult HIV infection who posed a diagnostic challenge as she suffered from a HIV-1-associated neurocognitive disorder with predominant motor symptoms mimicking upper motor neuron disease. Functional imaging using F-18 fluorodeoxyglucose positron emission tomography provided evidence of involvement of several cerebral regions which exhibited a distinct pattern of relative cerebral hypermetabolism (subcortical, brainstem, and cerebellar regions) and hypometabolism (sensorimotor cortex, mesiofrontal, and mesiotemporal areas) and functionally corresponded to the clinical symptoms. The results of the positron emission tomography scan are discussed in comparison with the current positron emission tomography literature and future perspectives are illustrated.

Macrophage delivery of nanoformulated antiretroviral drug to the brain in a murine model of neuroAIDS

Antiretroviral therapy (ART) shows variable blood-brain barrier penetration. This may affect the development of neurological complications of HIV infection. In attempts to attenuate viral growth for the nervous system, cell-based nanoformulations were developed with the focus on improving drug pharmacokinetics. We reasoned that ART carriage could be facilitated within blood-borne macrophages traveling across the blood-brain barrier. To test this idea, an HIV-1 encephalitis (HIVE) rodent model was used where HIV-1-infected human monocyte-derived macrophages were stereotactically injected into the subcortex of severe combined immunodeficient mice. ART was prepared using indinavir (IDV) nanoparticles (NP, nanoART) loaded into murine bone marrow macrophages (BMM, IDV-NP-BMM) after ex vivo cultivation. IDV-NP-BMM was administered i.v. to mice resulting in continuous IDV release for 14 days. Rhodamine-labeled IDV-NP was readily observed in areas of HIVE and specifically in brain subregions with active astrogliosis, microgliosis, and neuronal loss. IDV-NP-BMM treatment led to robust IDV levels and reduced HIV-1 replication in HIVE brain regions. We conclude that nanoART targeting to diseased brain through macrophage carriage is possible and can be considered in developmental therapeutics for HIV-associated neurological disease.

Imaging microglial activation during neuroinflammation and Alzheimer's disease

Microglial activation is an important pathogenic component of neurodegenerative disease processes. This state of increased inflammation is associated not only with neurotoxic consequences but also neuroprotective effects, e.g., phagocytosis and clearance of amyloid in Alzheimer's disease. In addition, activation of microglia appears to be one of the major mechanisms of amyloid clearance following active or passive immunotherapy. Imaging techniques may provide a minimally invasive tool to elucidate the complexities and dynamics of microglial function and dysfunction in aging and neurodegenerative diseases. Imaging microglia in vivo in live subjects by confocal or two/multiphoton microscopy offers the advantage of studying these cells over time in their native environment. Imaging microglia in human subjects by positron emission tomography scanning with translocator protein-18 kDa ligands can offer a measure of the inflammatory process and a means of detecting progression of disease and efficacy of therapeutics over time.

Neuropsychological consequences of HIV and substance abuse: a literature review and implications for treatment and future research

Neuropsychological dysfunction, ranging from mild cognitive symptoms to dementia has been a consistent part of the clinical picture of HIV/AIDS. However, advances in clinical management, particularly antiretroviral (ARV) treatment, have mitigated the neuropsychological effects of HIV and revised the pattern and nature of cognitive deficits, which are observed in HIV-infected individuals. The attendant improvements in mortality and morbidity have led to a need for programs and interventions that sustain healthy behavior and prevent a resurgence of HIV transmission risk. Psychiatric risk factors, particularly substance use, which often contribute to initial acquisition of HIV, still require attention. These risk factors may also exacerbate neuropsychological dysfunction and compromise adherence to prevention recommendations and treatment. Specifically, a more complete understanding of the effects of substance abuse on the progression of HIV related cognitive decline can inform evaluation and management of HIV seropositives with concurrent substance use disorders. This review provides an overview of the neuropsychology of HIV and substance abuse and the extant research that has examined the effects of both HIV disease and substance use on neuropsychological functioning and implications for treatment and future research.

Expression of the translocator protein of 18 kDa by microglia, macrophages and astrocytes based on immunohistochemical localization in abnormal human brain

AIMS

Microglia are involved in neurodegeneration, are prime targets for anti-inflammatory therapy and are potential biomarkers of disease progression. For example, positron emission tomography imaging employing radioligands for the mitochondrial translocator protein of 18 kDa (TSPO, formerly known as the peripheral benzodiazepine receptor) is being scrutinized to detect neuroinflammation in various diseases. TSPO is presumably present in activated microglia, but may be present in other neural cells.

METHODS

We sought to elucidate the protein expression in normal human central nervous system, several neurological diseases (HIV encephalitis, Alzheimer's disease, multiple sclerosis and stroke) and simian immunodeficiency virus encephalitis by performing immunohistochemistry with two anti-TSPO antibodies.

RESULTS

Although the overall parenchymal staining was minimal in normal brain, endothelial and smooth muscle cells, subpial glia, intravascular monocytes and ependymal cells were TSPO-positive. In disease states, elevated TSPO was present in parenchymal microglia, macrophages and some hypertrophic astrocytes, but the distribution of TSPO varied depending on the disease, disease stage and proximity to the lesion or relation to infection. Staining with the two antibodies correlated well in white matter, but one antibody also stained cortical neurones. Quantitative analysis demonstrated a significant increase in TSPO in the white matter of HIV encephalitis compared with brains without encephalitis. TSPO expression was also increased in simian immunodeficiency virus encephalitis.

CONCLUSIONS

This report provides the first comprehensive immunohistochemical analysis of the expression of TSPO. The results are useful for informing the usage of positron emission tomography as an imaging modality and have an impact on the potential use of TSPO as an anti-inflammatory pharmacological target.

The positron emission tomography ligand DAA1106 binds with high affinity to activated microglia in human neurological disorders

Chronic microglial activation is an important component of many neurological disorders, and imaging activated microglia in vivo will enable the detection and improved treatment of neuroinflammation. 1-(2-chlorphenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carbox-amide (PK11195), a peripheral benzodiazepine receptor ligand, has been used to image neuroinflammation, but the extent to which PK11195 binding distinguishes activated microglia and reactive astrocytes is unclear. Moreover, PK11195 may lack sufficient sensitivity for detecting mild neuroinflammation. We hypothesized that N-(2,5-dimethoxybenzyl)-N-(4-fluoro-2-phenoxyphenyl) acetamide (DAA1106), a new ligand that binds specifically to peripheral benzodiazepine receptor, binds to activated microglia in human neurological diseases with higher affinity than does PK11195. We therefore compared the pharmacological binding properties of [3H](R)-PK11195 and [3H]DAA1106 in postmortem tissues from patients with cerebral infarcts, amyotrophic lateral sclerosis, Alzheimer disease, frontotemporal dementia, and multiple sclerosis (n=10 each). In all diseases, [3H]DAA1106 showed a higher binding affinity as reflected by lower dissociation constant (KD) values than that of [3H](R)-PK11195. Moreover, specific binding of both ligands correlated with the presence of activated microglia identified by immunohistochemistry in situ. We conclude that 1) ligands that bind peripheral benzodiazepine receptor mainly label activated microglia in human neurological disorders and that 2) DAA1106 may possess binding characteristics superior to those of PK11195, which may be beneficial for in vivo positron emission tomography imaging.

Longitudinal in vivo positron emission tomography imaging of infected and activated brain macrophages in a macaque model of human immunodeficiency virus encephalitis correlates with central and peripheral markers of encephalitis and areas of synaptic degeneration

Human immunodeficiency virus encephalitis is characterized by infiltration of the brain with infected and activated macrophages; however, it is not known why disease occurs after variable lengths of infection in 25% of immunosuppressed acquired immune deficiency syndrome patients. We determined in vivo correlates (in peripheral blood and the central nervous system) for the development and progression of lentiviral encephalitis by longitudinally following infected and activated macrophages in the brain using positron emission tomography (PET). Using human postmortem brain tissues from both lentivirus-infected encephalitic patients and cell culture systems, we showed that the PET ligand [(3)H](R)-PK11195 bound specifically to virus-infected and activated macrophages. We longitudinally imaged infected and activated brain macrophages in a cohort of macaques infected with simian immunodeficiency virus using [(11)C](R)-PK11195. [(11)C](R)-PK11195 retention in vivo in the brain correlated with viral burden in the brain and cerebrospinal fluid, and with regions of both presynaptic and postsynaptic damage. Finally, longitudinal changes in [(11)C](R)-PK11195 retention in the brain in vivo correlated with changes in circulating monocytes as well as in both natural killer and memory CD4(+) T cells in the periphery. Our results suggest that development and progression of simian immunodeficiency virus encephalitis in vivo correlates with changes in specific cell subtypes in the periphery. A combination of PET imaging and the assessment of these peripheral immune parameters may facilitate longitudinal assessment of lentiviral encephalitis in living patients as well as evaluation of therapeutic efficacies.

The high affinity peripheral benzodiazepine receptor ligand DAA1106 binds to activated and infected brain macrophages in areas of synaptic degeneration: implications for PET imaging of neuroinflammation in lentiviral encephalitis

HIV encephalitis (HIVE) is characterized by neurodegeneration mediated by toxins derived from infected and activated brain macrophages. Since the peripheral benzodiazepine receptor (PBR) is abundant on brain macrophages, we hypothesized that [(3)H]DAA1106, a new PBR ligand, can label infected and activated brain macrophages in HIVE. Using cell culture and postmortem brain tissues from HIVE and a macaque model of HIVE, we show that [(3)H]DAA1106 binds with high affinity to activated and infected macrophages in regions of synaptic damage. Further, binding affinity reflected by lower K(D) (dissociation constant) values and the B(max) (total number of binding sites) to K(D) ratios reflective of ligand-binding potential was significantly higher with [(3)H]DAA1106 compared to the extensively characterized PBR ligand [(3)H](R)-PK11195. These data suggest that DAA1106 binds with high affinity to activated and infected brain macrophages and possesses binding characteristics beneficial for in vivo use in the detection and clinical monitoring of HIVE using positron emission tomography.

The high affinity peripheral benzodiazepine receptor ligand DAA1106 binds specifically to microglia in a rat model of traumatic brain injury: implications for PET imaging

Traumatic brain injury (TBI) is a significant cause of mortality, morbidity, and disability. Microglial activation is commonly observed in response to neuronal injury which, when prolonged, is thought to be detrimental to neuronal survival. Activated microglia can be labeled using PK11195, a ligand that binds the peripheral benzodiazepine receptor (PBR), receptors which are increased in activated microglia and sparse in the resting brain. We compared the binding properties of two PBR ligands PK11195 and DAA1106 in rats using the controlled cortical impact (CCI) model of experimental TBI. While both ligands showed relative increases with specific binding in the cortex ipsilateral to injury compared to the contralateral side, [(3)H]DAA1106 showed higher binding affinity compared with [(3)H](R)-PK11195. Combined immunohistochemistry and autoradiography in brain tissues near the injury site showed that [(3)H]DAA1106 binding co-registered with activated microglia more than astrocytes. Further, increased [(3)H]DAA1106-specific binding positively correlated with the degree of microglial activation, and to a lesser degree with reactive astrocytosis. Finally, in vivo administration of each ligand in rats with TBI showed greater retention of [(11)C]DAA1106 compared to [(11)C](R)-PK11195 at the site of the contusion as assessed by ex vivo autoradiography. These results in a rat model of TBI indicate that [(11)C]DAA1106 binds with higher affinity to microglia when compared with PK11195, suggesting that [(11)C]DAA1106 may represent a better ligand than [(11)C](R)-PK11195 for in vivo PET imaging of activated microglia in TBI.

A comparison of the high-affinity peripheral benzodiazepine receptor ligands DAA1106 and (R)-PK11195 in rat models of neuroinflammation: implications for PET imaging of microglial activation

Activated microglia are an important feature of many neurological diseases and can be imaged in vivo using 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195), a ligand that binds the peripheral benzodiazepine receptor (PBR). N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl) acetamide (DAA1106) is a new PBR-specific ligand that has been reported to bind to PBR with higher affinity compared with PK11195. We hypothesized that this high-affinity binding of DAA1106 to PBR will enable better delineation of microglia in vivo using positron emission tomography. [(3)H]DAA1106 showed higher binding affinity compared with [(3)H](R)-PK11195 in brain tissue derived from normal rats and the rats injected intrastriatally with 6-hydroxydopamine or lipopolysaccharide at the site of the lesion. Immunohistochemistry combined with autoradiography in brain tissues as well as correlation analyses showed that increased [(3)H]DAA1106 binding corresponded mainly to activated microglia. Finally, ex vivo autoradiography and positron emission tomography imaging in vivo showed greater retention of [(11)C]DAA1106 compared with [(11)C](R)-PK11195 in animals injected with either lipopolysaccaride or 6-hydroxydopamine at the site of lesion. These results indicate that DAA1106 binds with higher affinity to microglia in rat models of neuroinflammation when compared with PK11195, suggesting that [(11)C]DAA1106 may represent a significant improvement over [(11)C](R)-PK11195 for in vivo imaging of activated microglia in human neuroinflammatory disorders.

The peripheral benzodiazepine receptor (Translocator protein 18kDa) in microglia: from pathology to imaging

Microglia constitute the primary resident immune surveillance cell in the brain and are thought to play a significant role in the pathogenesis of several neurodegenerative disorders, such as Alzheimer's disease, multiple sclerosis, Parkinson's disease and HIV-associated dementia. Measuring microglial activation in vivo in patients suffering from these diseases may help chart progression of neuroinflammation as well as assess efficacy of therapies designed to modulate neuroinflammation. Recent studies suggest that activated microglia in the CNS may be detected in vivo using positron emission tomography (PET) utilizing pharmacological ligands of the mitochondrial peripheral benzodiazepine receptor (PBR (recently renamed as Translocator protein (18kDa)). Beginning with the molecular characterization of PBR and regulation in activated microglia, we examine the rationale behind using PBR ligands to image microglia with PET. Current evidence suggests these findings might be applied to the development of clinical assessments of microglial activation in neurological disorders.