pet-imaging-neuroinflammation-using-11cdpa-713-mouse-model-ischemic

PET Imaging of Neuroinflammation Using [<sup>11</sup>C]DPA-713 in a Mouse Model of Ischemic Stroke

Neuroinflammation is central to the pathological cascade following ischemic stroke. Non-invasive molecular imaging methods have the potential to provide ...

imaging-cd19-b-cells-an-experimental-autoimmune-encephalomyelitis

Imaging CD19<sup>+</sup> B Cells in an Experimental Autoimmune Encephalomyelitis Mouse Model using Positron Emission Tomography

Multiple sclerosis (MS) is the most common demyelinating central nervous system (CNS) disease affecting young adults, often resulting in neurological ...

Stanford University

Complex regional pain syndrome (CRPS) is a severely disabling disease characterized by pain, temperature changes, motor dysfunction, and edema that most often occurs as an atypical response to a minor surgery or fracture. Inflammation involving activation and recruitment of innate immune cells, including both peripheral and central myeloid cells (ie, macrophages and microglia, respectively), is a key feature of CRPS. However, the exact role and time course of these cellular processes relative to the known acute and chronic phases of the disease are not fully understood. Positron emission tomography (PET) of translocator protein-18 kDa (TSPO) is a method for noninvasively tracking these activated innate immune cells. Here, we reveal the temporal dynamics of peripheral and central inflammatory responses over 20 weeks in a tibial fracture/casting mouse model of CRPS through longitudinal TSPO-PET using [F]GE-180. Positron emission tomography tracer uptake quantification in the tibia revealed increased peripheral inflammation as early as 2 days after fracture and lasting 7 weeks. Centralized inflammation was detected in the spinal cord and brain of fractured mice at 7 and 21 days after injury. Spinal cord tissue immunofluorescent staining revealed TSPO expression in microglia (CD11b+) at 7 days but was restricted mainly to endothelial cells (PECAM1+) at baseline and 7 weeks. Our data suggest early and persistent peripheral myeloid cell activation and transient central microglial activation are limited to the acute phase of CRPS. Moreover, we show that TSPO-PET can be used to noninvasively monitor the spatiotemporal dynamics of myeloid cell activation in CRPS progression with potential to inform disease phase-specific therapeutics.

Longitudinal translocator protein-18 kDa-positron emission tomography imaging of peripheral and central myeloid cells in a mouse model of complex regional pain syndrome.

Acidosis is a key driver for many diseases, including cancer, sepsis, and stroke. The spatiotemporal dynamics of dysregulated pH across disease remain elusive, and current diagnostic strategies do not provide localization of pH alterations. We sought to explore if PET imaging using hydrophobic cyclic peptides that partition into the cellular membrane at low extracellular pH (denoted as pH [low] insertion cycles, or pHLIC) can permit accurate in vivo visualization of acidosis.  Acid-sensitive cyclic peptide c[EWC] pHLIC was conjugated to bifunctional maleimide-NO2A and radiolabeled with Cu (half-life, 12.7 h). C57BL/6J mice were administered lipopolysaccharide (15 mg/kg) or saline (vehicle) and serially imaged with [Cu]Cu-c[EWC] over 24 h. Ex vivo autoradiography was performed on resected brain slices and subsequently stained with cresyl violet to enable high-resolution spatial analysis of tracer accumulation. A non-pH-sensitive cell-penetrating control peptide (c[RWC]) was used to confirm specificity of [Cu]Cu-c[EWC]. CD11b (macrophage/microglia) and TMEM119 (microglia) immunostaining was performed to correlate extent of neuroinflammation with [Cu]Cu-c[EWC] PET signal.  [Cu]Cu-c[EWC] radiochemical yield and purity were more than 95% and more than 99%, respectively, with molar activity of more than 0.925 MBq/nmol. Significantly increased [Cu]Cu-c[EWC] uptake was observed in lipopolysaccharide-treated mice (vs. vehicle) within peripheral tissues, including blood, lungs, liver, and small intestines ( < 0.001-0.05). Additionally, there was significantly increased [Cu]Cu-c[EWC] uptake in the brains of lipopolysaccharide-treated animals. Autoradiography confirmed increased uptake in the cerebellum, cortex, hippocampus, striatum, and hypothalamus of lipopolysaccharide-treated mice (vs. vehicle). Immunohistochemical analysis revealed microglial or macrophage infiltration, suggesting activation in brain regions containing increased tracer uptake. [Cu]Cu-c[RWC] demonstrated significantly reduced uptake in the brain and periphery of lipopolysaccharide mice compared with the acid-mediated [Cu]Cu-c[EWC] tracer.  Here, we demonstrate that a pH-sensitive PET tracer specifically detects acidosis in regions associated with sepsis-driven proinflammatory responses. This study suggests that [Cu]Cu-pHLIC is a valuable tool to noninvasively assess acidosis associated with both central and peripheral innate immune activation.

Demarcation of Sepsis-Induced Peripheral and Central Acidosis with pH (Low) Insertion Cycle Peptide.

Neuroinflammation is a key pathologic hallmark of numerous neurologic diseases, however, its exact role in vivo is yet to be fully understood. PET imaging enables investigation, quantification, and tracking of different neuroinflammation biomarkers in living subjects longitudinally. One such biomarker that has been imaged extensively using PET is translocator protein 18 kDa (TSPO). Although imaging TSPO has yielded valuable clinical data linking neuroinflammation to various neurodegenerative diseases, considerable limitations of TSPO PET have prompted identification of other more cell-specific and functionally relevant biomarkers. This review analyzes the clinical potential of available and emerging PET biomarkers of innate and adaptive immune responses, with mention of exciting future directions for the field.

Neuroinflammation PET Imaging: Current Opinion and Future Directions.

B cells play a central role in multiple sclerosis (MS) through production of injurious antibodies, secretion of pro-inflammatory cytokines, and antigen presentation. The therapeutic success of monoclonal antibodies (mAbs) targeting B cells in some but not all individuals suffering from MS highlights the need for a method to stratify patients and monitor response to treatments in real-time. Herein, we describe the development of the first CD19 positron emission tomography (PET) tracer, and its evaluation in a rodent model of MS, experimental autoimmune encephalomyelitis (EAE).

Development of a CD19 PET tracer for detecting B cells in a mouse model of multiple sclerosis.

In the brain, REST (Repressor Element-1 Silencing Transcription factor) is a key regulator of neuron cell-specific gene expression. Nuclear translocation of neuronal REST has been shown to be neuroprotective in a healthy ageing context. In contrast, inability to upregulate nuclear REST is thought to leave ageing neurons vulnerable to neurodegenerative stimuli, such as Alzheimer's disease (AD) pathology. Hippocampal and cortical neurons are known to be particularly susceptible to AD-associated neurodegeneration. However, REST expression has not been extensively characterised in the healthy ageing brain. Here, we examined the spatiotemporal immunolocalisation of REST in the brains of healthy ageing wild-type Fischer-344 and transgenic Alzheimer's disease rats (TgF344-AD). Nuclear expression of REST increased from 6 months to 18 months of age in the hippocampus, frontal cortex and subiculum of wild-type rats, but not in TgF344-AD rats. No changes in REST were measured in more posterior cortical regions or in the thalamus. Interestingly, levels of the presynaptic marker synaptophysin, a known gene target of REST, were lower in CA1 hippocampal neurons of 18-month TgF344-AD rats compared to 18-month wild-types, suggesting that elevated nuclear REST may protect against synapse loss in the CA1 of 18-month wild-type rats. High REST expression in ageing wild-type rats did not, however, protect against axonal loss nor against astroglial reactivity in the hippocampus. Taken together, our data confirm that changes in nuclear REST expression are context-, age- and brain region-specific. Moreover, key brain structures involved in learning and memory display elevated REST expression in healthy ageing wild-type rats but not TgF344-AD rats.

Spatiotemporal immunolocalisation of REST in the brain of healthy ageing and Alzheimer's disease rats.

Recent events in America in 2020 have stimulated a worldwide movement to dismantle anti-Black racism in all facets of our lives. Anti-Black racism is, as defined by the Movement for Black Lives, a "term used to specifically describe the unique discrimination, violence, and harm imposed on and impacting Black people specifically." In science, technology, engineering, and mathematics (STEM), we have yet to achieve the goal and responsibility to ensure that the field reflects the diversity of our lived experiences. Members of the Women in Molecular Imaging Network (WIMIN) have come together to take a stand on diversity, equity, and inclusion in the field of molecular imaging. We strongly condemn oppression in all its forms and strive to identify and dismantle barriers that lead to inequities in the molecular imaging community and STEM as a whole. In this series coined "Visions" (Antiracism and Allyship in Action), we identify and discuss specific actionable items for improving diversity and representation in molecular imaging and ensuring inclusion of all members of the community, inclusive of race, disability, ethnicity, religion, or LGBTQ+ identity. Although the issues highlighted here extend to other under-recruited and equity-seeking groups, for this first article, we are focusing on one egregious and persistent form of discrimination: anti-Black racism. In this special article, Black women residing in America present their lived experiences in the molecular imaging field and give candid insights into the challenges, frustrations, and hopes of our Black friends and colleagues. While this special article focuses on the experiences of Black women, we would like the readers to reflect on their anti-Blackness toward men, transgender, nonbinary, and gender non-conforming people. From the vulnerability we have asked of all our participants, these stories are meant to inspire and invoke active antiracist work among the readership. We present strategies for dismantling systemic racism that research centers and universities can implement in the recruitment, retention, mentorship, and development of Black trainees and professionals. We would like to specifically acknowledge the Black women who took the time to be interviewed, write perspectives, and share their lived experiences in hopes that it will inspire genuine and lasting change.

Visions by Women in Molecular Imaging Network: Antiracism and Allyship in Action.

Immunotherapy is a promising approach for many oncological malignancies, including glioblastoma, however, there are currently no available tools or biomarkers to accurately assess whole-body immune responses in patients with glioblastoma treated with immunotherapy. Here, the utility of OX40, a costimulatory molecule mainly expressed on activated effector T cells known to play an important role in eliminating cancer cells, was evaluated as a PET imaging biomarker to quantify and track response to immunotherapy.

Whole-body PET Imaging of T-cell Response to Glioblastoma.

Parkinson's disease (PD) is associated with aberrant innate immune responses, including microglial activation and infiltration of peripheral myeloid cells into the central nervous system (CNS). Methods to investigate innate immune activation in PD are limited and have not yet elucidated key interactions between neuroinflammation and peripheral inflammation. Translocator protein 18 kDa (TSPO) PET is a widely evaluated imaging approach for studying activated microglia and peripheral myeloid lineage cells in vivo but has yet to be fully explored in PD. Here, we investigate the utility of TSPO PET in addition to PET imaging of triggering receptor expressed on myeloid cells 1 (TREM1)-a novel biomarker of proinflammatory innate immune cells-for detecting innate immune responses in the 6-hydroxydopamine mouse model of dopaminergic neuron degeneration.  C57/BL6J and  knockout mice were stereotactically injected with 6-hydroxydopamine in the left striatum; control mice were injected with saline. At day 7 or 14 after surgery, mice were administered F-GE-180, Cu-TREM1 monoclonal antibody (mAb), or Cu-isotype control mAb and imaged by PET/CT. Ex vivo autoradiography was performed to obtain high-resolution images of tracer binding within the brain. Immunohistochemistry was conducted to verify myeloid cell activation and dopaminergic cell death, and quantitative polymerase chain reaction and flow cytometry were completed to assess levels of target in the brain.  PET/CT images of both tracers showed elevated signal within the striatum of 6-hydroxydopamine-injected mice compared with those injected with saline. Autoradiography afforded higher-resolution brain images and revealed significant TSPO and TREM1 tracer binding within the ipsilateral striatum of 6-hydroxydopamine mice compared with saline mice at both 7 and 14 d after toxin. Interestingly, F-GE-180 enabled detection of inflammation in the brain and peripheral tissues (blood and spleen) of 6-hydroxydopamine mice, whereas Cu-TREM1 mAb appeared to be more sensitive and specific for detecting neuroinflammation, in particular infiltrating myeloid cells, in these mice, as demonstrated by flow cytometry findings and higher tracer binding signal-to-background ratios in brain.  TSPO and TREM1 PET tracers are promising tools for investigating different cell types involved in innate immune activation in the context of dopaminergic neurodegeneration, thus warranting further investigation in other PD rodent models and human postmortem tissue to assess their clinical potential.

Tracking Innate Immune Activation in a Mouse Model of Parkinson's Disease Using TREM1 and TSPO PET Tracers.

Chronic neuroinflammation and microglial dysfunction are key features of many neurological diseases, including Alzheimer's Disease and multiple sclerosis. While there is unfortunately a dearth of highly selective molecular imaging biomarkers/probes for studying microglia in vivo, P2Y12R has emerged as an attractive candidate PET biomarker being explored for this purpose. Importantly, P2Y12R is selectively expressed on microglia in the CNS and undergoes dynamic changes in expression according to inflammatory context (e.g., toxic versus beneficial/healing states), thus having the potential to reveal functional information about microglia in living subjects. Herein, we identified a high affinity, small molecule P2Y12R antagonist (AZD1283) to radiolabel and assess as a candidate radiotracer through in vitro assays and in vivo positron emission tomography (PET) imaging of both wild-type and total knockout mice and a non-human primate.

Aisling M. Chaney

Department of Radiology

Stanford University

PET Imaging of Neuroinflammation Using [¹¹C]DPA-713 in a Mouse Model of Ischemic Stroke

Imaging CD19⁺ B Cells in an Experimental Autoimmune Encephalomyelitis Mouse Model using Positron Emission Tomography

Aisling M. Chaney

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Radiology

Stanford University

PET Imaging of Neuroinflammation Using [11C]DPA-713 in a Mouse Model of Ischemic Stroke

Imaging CD19+ B Cells in an Experimental Autoimmune Encephalomyelitis Mouse Model using Positron Emission Tomography

Department of Radiology

PET Imaging of Neuroinflammation Using [¹¹C]DPA-713 in a Mouse Model of Ischemic Stroke

Imaging CD19⁺ B Cells in an Experimental Autoimmune Encephalomyelitis Mouse Model using Positron Emission Tomography