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.     

PK11195 labels activated microglia in Alzheimer's disease and in vivo in a mouse model using PET

Activated microglia may promote neurodegeneration in Alzheimer's disease (AD) and may also help in amyloid clearance in immunization therapies. In vivo imaging of activated microglia using positron emission tomography (PET) could assist in defining the role of activated microglia during AD progression and therapeutics. We hypothesized that PK11195, a ligand that binds activated microglia, could label these cells in postmortem AD tissues and in vivo in an animal model of AD using PET. [(3)H](R)-PK11195 binding was significantly higher in AD frontal cortex compared to controls and correlated mainly with the abundance of immunohistochemically labeled activated microglia. With age, the brains of APP/PS1 transgenic mice showed progressive increase in [(3)H](R)-PK11195 binding and [(11)C](R)-PK11195 retention in vivo assessed using microPET, which correlated with the histopathological abundance of activated microglia. These results suggest that PK11195 binding in AD postmortem tissue and transgenic mice in vivo correlates with the extent of microglial activation and may help define the role of activated microglia in the pathogenesis and treatment of AD.     

Macrophages relate presynaptic and postsynaptic damage in simian immunodeficiency virus encephalitis

Neurodegeneration observed in lentiviral-associated encephalitis has been linked to viral-infected and -activated central nervous system macrophages. We hypothesized that lentivirus, macrophages, or both lentivirus and macrophages within distinct microenvironments mediate synaptic damage. Using the simian immunodeficiency virus (SIV)-infected macaque model, we assessed the relationship between virus, macrophages, and neurological damage in multiple brain regions using laser confocal microscopy. In SIV-infected macaques with SIV encephalitis (SIVE), brain tissue concentrations of SIV RNA were 5 orders of magnitude greater than that observed in nonencephalitic animals. In SIVE, staining for postsynaptic protein microtubule-associated protein-2 was significantly decreased in the caudate, hippocampus, and frontal cortical gray matter compared to nonencephalitic controls, whereas staining for presynaptic protein synaptophysin was decreased in SIV-infected macaques with and without encephalitis. These data suggest that presynaptic damage occurs independent of pathological changes associated with SIVE, whereas postsynaptic damage is more tightly linked to regional presence of both activated and infected macrophages.     

PET imaging with [11C]PBR28 can localize and quantify upregulated peripheral benzodiazepine receptors associated with cerebral ischemia in rat

Peripheral benzodiazepine receptors (PBRs) are upregulated on activated microglia. We recently developed a promising positron emission tomography (PET) ligand, [11C]PBR28, with high affinity and excellent ratio of specific to nonspecific binding. We assessed the ability of [11C]PBR28 PET to localize PBRs in a rat permanent middle cerebral artery occlusion (MCAO) model of neuroinflammation. [11C]PBR28 was intravenously administered to rats at 4 and 7 days after permanent MCAO. In all experiments, arterial blood was sampled for compartmental modeling of regional distribution volumes, and rat brains were sampled after imaging for in vitro [3H]PK 11195 autoradiography and histological evaluation. [11C]PBR28 PET and [3H]PK 11195 autoradiography showed similar areas of increased PBRs, especially in the peri-ischemic core. Results from these in vivo and in vitro methods were strongly correlated. In this first study to demonstrate neuroinflammation in vivo with small animal PET, [11C]PBR28 had adequate sensitivity to localize and quantify the associated increase in PBRs.     

3H]PK 11195 and the localisation of secondary thalamic lesions following focal ischaemia in rat motor cortex.

The peripheral-type benzodiazepine binding site (PTBBS) ligand, PK 11195, is known to be a marker of damage in the central nervous system, the binding being predominantly to macrophages. Using photochemically induced focal cortical ischaemia as a lesion model in the rat, we have investigated the detection of secondary lesions using [3H]PK 11195 and ex vivo autoradiography. Secondary lesions in the thalamus became apparent during the second week post-lesioning, at a time when [3H]PK 11195 binding around the primary lesion was beginning to subside. Using Brain Browser software, the identity of the labelled thalamic nucleus was confirmed, objectively, as the ventrolateral nucleus, known to have reciprocal connections with the lesioned cortical area. As with the primary lesion, high densities of PTBBS correlated with infiltration of macrophages. Three-dimensional reconstruction of [3H]PK 11195 autoradiograph images showed binding along white matter tracts between the primary and secondary lesions. We conclude that radiolabelled PK 11195 given in vivo can be used in the visualisation of secondary lesions and their associated degenerating tracts.     

Microglial activation in healthy aging

Healthy brain aging is characterized by neuronal loss and decline of cognitive function. Neuronal loss is closely associated with microglial activation and postmortem studies have indeed suggested that activated microglia may be present in the aging brain. Microglial activation can be quantified in vivo using (R)-[(11)C]PK11195 and positron emission tomography. The purpose of this study was to measure specific binding of (R)-[(11)C]PK11195 in healthy subjects over a wide age range. Thirty-five healthy subjects (age range 19-79 years) were included. In all subjects 60-minute dynamic (R)-[(11)C]PK11195 scans were acquired. Specific binding of (R)-[(11)C]PK11195 was calculated using receptor parametric mapping in combination with supervised cluster analysis to extract the reference tissue input function. Increased binding of (R)-[(11)C]PK11195 with aging was found in frontal lobe, anterior and posterior cingulate cortex, medial inferior temporal lobe, insula, hippocampus, entorhinal cortex, thalamus, parietal and occipital lobes, and cerebellum. This indicates that activated microglia appear in several cortical and subcortical areas during healthy aging, suggesting widespread neuronal loss.     

Correlation of acute humoral response with brain virus burden and survival time in pig-tailed macaques infected with the neurovirulent simian immunodeficiency virus SIVsmmFGb

Infection of pig-tailed macaques with the simian immunodeficiency virus (SIV) isolate SIVsmmFGb frequently results in SIV encephalitis (SIVE) in addition to immunodeficiency and acquired immune deficiency syndrome. We used in situ hybridization to quantitate the number of SIV-infected cells in brain parenchyma, choroid plexus, and meninges from 17 macaques that developed acquired immune deficiency syndrome after infection with SIVsmmFGb. SIV-infected cells and histopathological lesions of SIVE were identified in 15 of 17 animals (88.2%), including 12 of 12 rapid progressors (RP) and 3 of 5 slow progressors (SP). The parenchymal virus burden was much greater in RP macaques than in the three SP macaques with SIVE (median values of 24.3 versus 0.3 infected cells/mm(2), respectively; P < 0.05). Viral load differences between RP and SP with SIVE were less marked in choroid plexus (29.6 versus 12.8 infected cells/mm(2), respectively) and meninges (133.0 versus 34.2 infected cells/mm(2), respectively). A significant negative correlation was observed between the magnitude of the anti-SIV antibody titer at 1 month after inoculation and brain virus burden at necropsy (r = -0.614; P < 0.01). The close association between immune response and SIVE in this model should prove useful for identifying correlates of immune protection against primate lentiviral encephalitis.     

大鼠肝再生过程中线粒体外周型苯二氮卓类受体的研究

目的：研究肝细胞线粒体通透性转换（PT）的主要调节蛋白外周型苯二氮卓类受体(PBR)在肝再生过程中的表达及其与专一性配体结合动力学变化，探讨与线粒体PT的关系。〖HTH〗方法：健康成年雄性SD大鼠，随机分为3组：肝部分切除（PH）组，切除肝左叶和中叶约全肝的70%；假处理组，同样麻醉和开腹，但不切肝；正常组。手术后3 h、6 h、12 h、 24 h、 48 h、 72 h、 120 h 和168 h 分别以半定量RT-PCR法检测PBR mRNA表达的动态变化。利用PBR专一的配体［3H］PK11195测定肝再生时线粒体膜上PBR的含量以及受体与配体亲和力的变化。〖HTH〗结果：在肝再生过程中PBR基因表达与假处理组无显著差异；［3H］PK11195与PBR最大结合量(Bmax)显著低于对照组（P <0.05），其中PH后3 h 和120 h非常显著（P<0.01），168 h接近正常水平； 平衡解离常数（Kd）在PH后72 h和168 h明显低于假处理照组（P<0.01）。假处理组之间Bmax和Kd无明显差异。〖HTH〗结论：肝再生过程中肝线粒体PBR mRNA水平无明显变化，而PBR与配体结合动力学明显改变提示PBR与线粒体PT变化有关。     

In vivo receptor labeling of peripheral benzodiazepine receptor by ex vivo binding of [3H]PK11195

In vivo receptor labeling of the peripheral benzodiazepine receptor was investigated using ex vivo binding of [3H]-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carb ox-amide ([3H]PK11195). In autoradiographic studies, high level specific binding of [3H]PK11195 was observed in the olfactory bulb. Intravenous administration of PK11195 dose-dependently (0.03-3 mg/kg) inhibited ex vivo binding of [3H]PK11195 in the olfactory bulb. Likewise, N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (DAA1106), a newly identified peripheral benzodiazepine receptor-specific ligand, dose-dependently (0.1-100 mg/kg) reduced ex vivo binding of [3H]PK11195, when administered intraperitoneally. In contrast, clonazepam, a central benzodiazepine receptor-specific agonist, had negligible effects on ex vivo binding of [3H]PK11195. We propose that the ex vivo receptor binding technique we used will facilitate determination of in vivo receptor occupancy of the peripheral benzodiazepine receptor.     

Localization of diazepam-binding inhibitor and peripheral type benzodiazepine binding sites in the rat ovary

Diazepam-binding inhibitor (DBI) is the precursor of a family of peptides, including an octadecaneuropeptide (ODN), that share with DBI the ability to specifically displace benzodiazepines (BZD) from their receptors. An association of ODN with the peripheral type BZD receptors (PBR) has been reported in the brain and a few peripheral tissues. In order to investigate whether DBI and PBR are present in ovarian tissue, we have localized DBI by means of immunocytochemistry, in situ hybridization and autoradiography of PBR in the rat ovary. Immunocytochemical localization was achieved by means of rabbit antibodies developed against rat ODN. Immunostaining was located in the cytoplasm of the theca interna, corpus luteum and interstitial gland cells, but not in the granulosa cells. Hybridization signal obtained following in situ hybridization with a [³⁵S₁]-labelled single-stranded RNA probe complementary to DBI mRNA was observed in all the steroid-secreting cells, including granulosa cells of developing and mature follicles. Autoradiographic localization of PBR obtained by incubating ovary sections with [³H] PK11195, a ligand selective for PBR, revealed the presence of specific labelling in all the steroid-secreting cells. These results, which demonstrate for the first time that the ovarian steroid-secreting cells contain both PBR and its endogenous ligand, suggest that the BZD receptor might be involved in the regulation of ovarian function.     

PK (‘peripheral benzodiazepine’) – binding sites in the CNS indicate early and discrete brain lesions: microautoradiographic detection of [3H]PK 11195 binding to activated microglia

The isoquinoline PK 11195 has been suggested as a marker of glial pathology in the lesioned brain. The aim of the present study is to clarify the precise cellular location of its binding site in the central nervous system. Here, we report that in the facial nucleus after facial nerve axotomy–a lesion causing a retrograde neuronal reaction without nerve cell death while keeping the blood–brain barrier intact–activated microglia are the predominant source of lesion-induced increases of PK 11195 binding. Likewise, increased PK 11195 binding is seen in the gracile nucleus after anterograde neuronal injury following sciatic nerve transection. The peak of PK 11195 binding, using the single isomer R-PK 11195, was observed 4 days after the peripheral nerve lesion, consistent with the well-known time course of microglial activation. Photoemulsion microautoradiography confirmed the restriction of PK 11195 binding to activated microglia. The increase of PK 11195 binding in the facial nucleus seen after selective cell death of facial motoneurons by retrograde suicide transport of toxic ricin, a lesion that is accompanied by the rapid transformation of microglia into phagocytes, was no higher than that seen following axotomy. This suggests that the full transformation of microglia into parenchymal phagocytes is not necessary to reach maximal levels of PK 11195 binding. PK 11195, therefore, is a well-suited marker to detect microglial activation in areas of subtle brain pathology, where neither a disturbance of the blood–brain barrier function nor the presence of macrophages and inflammatory cells indicate an on-going disease process.     

Human brain parenchymal microglia express CD14 and CD45 and are productively infected by HIV-1 in HIV-1 encephalitis

Microglia are endogenous brain macrophages that show distinct phenotypes such as expression of myeloid antigens, ramified morphology, and presence within the neural parenchyma. They play significant roles in a number of human CNS diseases including AIDS dementia. Together with monocyte-derived (perivascular) macrophages, microglia represent a major target of HIV-1 infection. However, a recent report challenged this notion based on findings in SIV encephalitis. This study concluded that perivascular macrophages can be distinguished from parenchymal microglial cells by their expression of CD14 and CD45, and that macrophages, but not microglia, are productively infected in SIV and HIV encephalitis. To address whether parenchymal microglia are productively infected in HIV encephalitis, we analyzed expression of CD14, CD45 and HIV-1 p24 in human brain. Microglia were identified based on their characteristic ramified morphology and location in the neural parenchyma. We found that parenchymal microglia are CD14+ (activated), CD45+ (resting and activated), and constitute approximately two thirds of the p24+ cells in HIV encephalitis cases. These results demonstrate that microglia are major targets of infection by HIV-1, and delineate possible differences between HIVE and SIVE. Because productively infected tissue macrophages serve as the major viral reservoir, these findings have important implications for AIDS.     

Minocycline treatment attenuates microglia activation and non-angiotensin II [125I] CGP42112 binding in brainstem following nodose ganglionectomy

We have previously shown that following unilateral nodose ganglionectomy, [125I] CGP42112 binds to a non-angiotensin II (Ang II) related binding site in rat dorsal motor nucleus of the vagus nerve, ambiguus nucleus and nucleus of the solitary tract. Furthermore, this up-regulated binding site localizes with activated microglia. Given that some tetracyclines may inhibit microglia activation in brain, we examined the effect of minocycline treatment on the binding of [125I] CGP42112 and [3H] PK11195 (an established radioligand for microglia), as well as OX-42 immunoreactivity (an immunomarker for activated microglia), following nodose ganglionectomy. Male Wistar Kyoto rats underwent unilateral nodose ganglionectomy or sham operation and were treated with saline or minocycline (50 mg/kg i.p.) 12 h before surgery and twice daily after surgery (each 50mg/kg i.p.) for 3 days. Subsequent to nodose ganglionectomy, [125I] CGP42112 binding (insensitive to PD123319 or Ang II) was increased approximately two-fold in the ipsilateral nucleus of the solitary tract and was also induced in the ipsilateral dorsal motor nucleus of the vagus nerve and ambiguus nucleus of saline-treated rats. Treatment with minocycline reduced this non-angiotensin II [125I] CGP42112 binding (40-50% reduction) in the nucleus of the solitary tract, dorsal motor nucleus of the vagus nerve and ambiguus nucleus. Analogous experiments using [3H] PK11195 also revealed up-regulated binding in the ipsilateral nucleus of the solitary tract ( approximately 205%), dorsal motor nucleus of the vagus nerve (approximately 80%) and ambiguus nucleus (approximately 210%) of saline-treated rats following nodose ganglionectomy, which was reduced by 40-100% with minocycline treatment. Immunoreactivity to OX-42 confirmed an increase in microglia activation and accumulation of macrophages in these brain stem nuclei following nodose ganglionectomy, which was also attenuated following treatment with minocycline. These data demonstrate that non-Ang II [125I] CGP42112 binding following nodose ganglionectomy is attenuated by minocycline treatment. This minocycline-induced effect was associated with reduced activation of microglia and an apparent reduction in the number of macrophages in the abovementioned nuclei. This evidence suggests that a non-Ang II [125I] CGP42112 binding site is located on, or associated with, activated microglia and macrophages, providing a useful tool with which to quantitate the neuroprotective effects of centrally acting anti-inflammatory compounds.     

Perivascular macrophages in the neonatal macaque brain undergo massive necroptosis after simian immunodeficiency virus infection

We previously showed that rhesus macaques neonatally infected with simian immunodeficiency virus (SIV) do not develop SIV encephalitis (SIVE) and maintain low brain viral loads despite having similar plasma viral loads compared to SIV‐infected adults. We hypothesize that differences in myeloid cell populations that are the known target of SIV and HIV in the brain contribute to the lack of neonatal susceptibility to lentivirus‐induced encephalitis. Using immunohistochemistry and immunofluorescence microscopy, we examined the frontal cortices from uninfected and SIV‐infected infant and adult macaques (n = 8/ea) as well as adults with SIVE (n = 4) to determine differences in myeloid cell populations. The number of CD206+ brain perivascular macrophages (PVMs) was significantly greater in uninfected infants than in uninfected adults and was markedly lower in SIV‐infected infants while microglia numbers were unchanged across groups. CD206+ PVMs, which proliferate after infection in SIV‐infected adults, did not undergo proliferation in infants. While virtually all CD206+ cells in adults are also CD163+, infants have a distinct CD206 single‐positive population in addition to the double‐positive population commonly seen in adults. Notably, we found that more than 60% of these unique CD206+CD163? PVMs in SIV‐infected infants were positive for cleaved caspase‐3, an indicator of apoptosis, and that nearly 100% of this subset were concomitantly positive for the necroptosis marker receptor‐interacting protein kinase‐3 (RIP3). These findings show that distinct subpopulations of PVMs found in infants undergo programmed cell death instead of proliferation following SIV infection, which may lead to the absence of PVM‐dependent SIVE and the limited size of the virus reservoir in the infant brain.     

Imaging microglial activation and glucose consumption in a mouse model of Alzheimer's disease

In Alzheimer's disease (AD), persistent microglial activation as sign of chronic neuroinflammation contributes to disease progression. Our study aimed to in vivo visualize and quantify microglial activation in 13- to 15-month-old AD mice using [¹¹C]-(R)-PK11195 and positron emission tomography (PET). We attempted to modulate neuroinflammation by subjecting the animals to an anti-inflammatory treatment with pioglitazone (5-weeks' treatment, 5-week wash-out period). [¹¹C]-(R)-PK11195 distribution volume values in AD mice were significantly higher compared with control mice after the wash-out period at 15 months, which was supported by immunohistochemistry data. However, [¹¹C]-(R)-PK11195 μPET could not demonstrate genotype- or treatment-dependent differences in the 13- to 14-month-old animals, suggesting that microglial activation in AD mice at this age and disease stage is too mild to be detected by this imaging method.     

YKL-40: A Candidate Biomarker for Simian Immunodeficiency Virus and Human Immunodeficiency Virus Encephalitis?

This Commentary explores the possible role for YKL-40, a secreted mammalian chitinase-like protein, as a predictive biomarker for SIV encephalitis (SIVE) and HIV encephalitis (HIVE).Human immunodeficiency virus type 1 (HIV-1) affects more that 32 million people worldwide¹ and induces central nervous system-associated neurological dysfunction in at least 30% of infected individuals.² Cognitive disorders range from mild impairment of executive functions to frank dementia (HIV-associated dementia; HAD), and these are collectively termed HIV-associated neurocognitive disorders (HAND).³ Although the incidence of HAD has decreased with the use of highly active anti-retroviral therapy (HAART), it is only partially neuroprotective. The persistence and increasing prevalence of less severe HAND syndromes remains a serious concern, and the assessment of the risk of developing progressive disability in treated individuals is a clear priority. In underdeveloped countries where HAART availability is limited, increasing life expectancy remains the primary goal of HIV treatment, but our experience in the United States forewarns of the likelihood of increasing cognitive disability in these surviving populations.⁴ In this issue of The American Journal of Pathology, Bonneh-Barkay et al⁵ use one of several pigtailed macaque models to investigate a possible role for YKL-40, a secreted mammalian chitinase-like protein, as a predictive biomarker for simian immunodeficiency virus (SIV) encephalitis (SIVE), the primate homologue of HIV encephalitis (HIVE). Elevations in cerebrospinal fluid (CSF) YKL-40 preceded death due to SIVE by up to 8 weeks and correlated with CSF SIV viral RNA (viral load), indicating that YKL-40 could potentially serve as a biomarker for SIVE.     

血小板膜外周型苯二氮卓受体在初发脑梗死后抑郁中的作用

 目的：观察卒中后抑郁（PSD）患者血小板膜外周型苯二氮卓受体（PBRs）的变化,探讨PBRs在PSD中的作用。方法：卒中后抑郁组为初发脑梗死后PSD患者43例、脑梗死组为初发脑梗死患者59例、对照组为健康献血者46名。采用Hamilton抑郁量表（HAMD）评定患者抑郁程度。提取外周血血小板膜,应用放射性配基［3H］PK11195结合实验测定PBRs特异结合活性。结果：［3H］PK11195结合活性3组之间有显著差异（P<0.01）。与对照组［（298.2±25.1) pmol/（g protein）］比较,脑梗死组［3H］PK11195结合活性［（1 410.8±41.4） pmol/(g protein)］显著升高（P<0.01）。与脑梗死组比较,PSD组［3H］PK11195结合活性［（361.7±30.6） pmol/g protein］显著降低（P<0.01）。PSD组男性、女性患者之间比较,［3H］PK11195结合活性差异不显著。PSD组［3H］PK11195结合活性与患者病程无显著相关性（r=0.27,P＞0.05）,与HAMD评分呈显著负相关（r=-0.44,P＜0.01）。结论:PSD患者血小板膜PBRs结合活性下降, PBRs影响抑郁程度。     

SIV Encephalitis Lesions Are Composed of CD163+ Macrophages Present in the Central Nervous System during Early SIV Infection and SIV-Positive Macrophages Recruited Terminally with AIDS

Macrophage recruitment to the central nervous system (CNS) during AIDS pathogenesis is poorly understood. We measured the accumulation of brain perivascular (CD163⁺) and inflammatory (MAC387⁺) macrophages in SIV-infected monkeys. Monocyte progenitors were 5-bromo-2′-deoxyuridine (BrdU) labeled in bone marrow, and CNS macrophages were labeled serially with fluorescent dextrans injected into the cisterna magna. MAC387⁺ macrophages accumulated in the meninges and choroid plexus in early inflammation and in the perivascular space and SIV encephalitis (SIVE) lesions late. CD163⁺ macrophages accumulated in the perivascular space and SIVE lesions with late inflammation. Most of the BrdU⁺ cells were MAC387⁺; however, CD163⁺BrdU⁺ macrophages were present in the meninges and choroid plexus with AIDS. Most (81.6% ± 1.8%) of macrophages in SIVE lesions were present in the CNS before SIVE lesion formation. There was a 2.9-fold increase in SIVp28⁺ macrophages entering the CNS late compared with those entering early (P < 0.05). The rate of CD163⁺ macrophage recruitment to the CNS inversely correlated with time to death (P < 0.03) and increased with SIVE. In SIVE animals, soluble CD163 correlated with CD163⁺ macrophage recruitment (P = 0.02). Most perivascular macrophages that comprise SIVE lesions and multinucleated giant cells are present in the CNS early, before SIVE lesions are formed. Most SIV-infected macrophages traffic to the CNS terminally with AIDS.HIV-associated neurological disorders are associated with central nervous system (CNS) pathologies and include motor, behavioral, and cognitive impairment.^1–3 Proposed explanations for the high prevalence of HIV-associated neurological disorders (approximately 50%), despite effective antiretroviral therapy (ART), include incomplete CNS drug penetrance, continued viral replication in the brain, persistent and chronic macrophage activation, CNS toxicity associated with ART, and the normal effects of aging.^2–7 In SIV-infected monkeys, HIV-infected humans pre-ART, and some HIV-infected humans after ART, infection of the CNS may be associated with encephalitic lesions composed of a focal accumulation of macrophages and microglia, and productive viral infection. Macrophages and microglia that drive CNS pathology are targets of productive viral infection.^3,8–16The timing of monocyte and macrophage entry and accumulation in the CNS as well as the entry of HIV and SIV quasispecies that are associated with pathogenesis are not well understood because of the technical limitations of such studies in humans. An increased rate of monocyte egress from the bone marrow, increased numbers and percentages of CD16⁺ monocytes in the blood, and the accumulation of macrophages in the CNS support the idea that a basal rate of monocyte and macrophage recruitment to the CNS is augmented with HIV infection.^16–22 In a macaque model of SIV infection, carboxyfluorescein succinimidyl ester–labeled autologous monocytes were shown to traffic to the cerebral perivascular space and choroid plexus during acute SIV infection [12 to 14 days postinfection (dpi)] at an accelerated rate compared with uninfected controls.²³ Analyses of macrophage recruitment to and turnover within the CNS from early infection up to the development of AIDS and SIV encephalitis (SIVE) have not been performed.CNS macrophages are heterogeneous and can be classified on the basis of tissue localization and/or immune phenotype.^9,24,25 Parenchymal microglia, the resident macrophages of the CNS, are yolk sac–derived, myeloid lineage cells that engraft the CNS during embryonic development and are then maintained as a stable population.^26,27 Perivascular, meningeal, and choroid plexus macrophages are of bone marrow origin and are thought to be replenished from circulating monocytes in rodents, nonhuman primates, and humans, likely at different rates.^17,28–32 In the normal CNS, microglia express the pan-macrophage marker CD68 and have low to undetectable hemoglobin-haptoglobin scavenger receptor CD163 (CD68⁺CD163⁻).²⁵ Perivascular macrophages express both CD163 and CD68 (CD163⁺CD68⁺).²⁵ A third phenotypic population of macrophages is labeled by the antibody MAC387, which recognizes migration inhibitory factor–related protein (MRP) 14 or the MRP8/MRP14 heterodimer, but does not have detectable CD68 or CD163 (MAC387⁺CD68⁻CD163⁻).¹⁴ MAC387⁺ macrophages are not present in the uninfected or noninflamed CNS but are recruited with inflammation.^14,33–36 Parenchymal microglia and CD163⁺ perivascular macrophages are considered primary targets of HIV and SIV infection in the CNS.^5,7 MAC387⁺ macrophages are rarely found to be productively infected.^{9,13,14,25,37} The timing of monocyte and macrophage entry into the CNS and the role of macrophage subsets mediating the progression or resolution of CNS inflammation due to HIV and SIV infection are not well defined.Current research suggests that virus enters the CNS via trafficking of infected monocytes and macrophages, although other mechanisms, including infection of or transcytosis through endothelial cells and direct transmission of free virus from the blood to the cerebrospinal fluid, have been suggested.^9,20,38 Productive infection of the CNS can be detected within days to weeks of initial infection, primarily within perivascular macrophages, but then resolves until the development of AIDS (pre-ART) or chronic infection with HIVE lesion formation (post-ART).^{23,32,38–42} Productive infection of the CNS at end-stage disease may represent recrudescence of virus seeded in the CNS with primary/acute infection, reintroduction of virus into the CNS terminally with AIDS, or both. Regardless, productive infection of macrophages in the perivascular space, encephalitis lesions, and meninges and choroid plexus likely results in production of toxic factors, including cytokines and viral proteins, which contribute metabolic encephalopathy with resultant neuronal and glial cell aberrations.^8,43We studied macrophage recruitment to the CNS in a rapid simian model of neuro-AIDS by 5-bromo-2′-deoxyuridine (BrdU) labeling of myeloid progenitors in the bone marrow, which traffic to the CNS, and serial labeling perivascular macrophages, within the CNS, by intracisternal injection of dextran conjugates in the cerebrospinal fluid. We compared macrophage recruitment rates between early/acute and terminal disease, or between animals with SIVE and animals with SIV but no encephalitis (SIVnoE). Early CNS inflammation was characterized by an influx of MAC387⁺ macrophages in acute infection. Later, recruitment of both MAC387⁺ and CD163⁺ macrophages was ongoing and was greater terminally in animals that developed AIDS and SIVE. BrdU⁺ macrophages present in CNS tissues were primarily MAC387⁺, but CD163⁺BrdU⁺ macrophages were present in the meninges and choroid plexus terminally with AIDS. Overall, few BrdU⁺ macrophages were present in the perivascular space and SIVE lesions. The ratio of CD163⁺/MAC387⁺ macrophages in the CNS was greater in animals with SIVE compared with SIV-infected animals without SIVE. Recruitment of CD163⁺ macrophages in the CNS correlated with plasma soluble CD163 (sCD163) in animals with SIVE. In all animals, a greater rate of CD163⁺ macrophage recruitment correlated with shorter time to death. Terminally with AIDS, CD163⁺ macrophages accumulated in the perivascular space and SIVE lesions, and not in the meninges or choroid plexus. Interestingly, SIVE lesions were composed primarily of CD163⁺ macrophages that were present in the CNS early in infection, by 20 dpi, before SIVE lesions are typically present. In SIVE lesions, the percentage of productively infected CD163⁺ macrophages was 2.9 times higher in macrophages that entered the CNS terminally with AIDS compared with macrophages present in the CNS at 20 dpi. These data indicate a role for resident perivascular macrophages that line CNS blood vessels and migrate to form SIVE lesions and suggest that virus is reintroduced to the CNS terminally with AIDS.     

Microglial activation in Alzheimer's disease: an (R)-[C]PK11195 positron emission tomography study

Inflammatory mechanisms, like microglial activation, could be involved in the pathogenesis of Alzheimer's disease (AD). (R)-[¹¹C]PK11195 (1-(2-chlorophenyl)-N-methyl-N-1(1-methylpropyl)-3-isoquinolinecarboxamide), a positron emission tomography (PET) ligand, can be used to quantify microglial activation in vivo. The purpose of this study was to assess whether increased (R)-[¹¹C]PK11195 binding is present in AD and mild cognitive impairment (MCI), currently also known as “prodromal AD.”