Mapping neuroinflammation in frontotemporal dementia with molecular PET imaging

Compared with healthy subjects and AD patients, an increased prevalence of related autoimmune diseases has been reported in FTD patients with semantic variant primary progressive aphasia (svPPA) who were GRN mutation carriers [13,16]. It has also been found that a proapoptotic protein (regulated partially by vasoactive neuropeptides) in astrocytes called Bax showed immunoreactivity in FTD, which suggested autoimmunity in the pathology of FTD [17]. In tau-negative FTD, neuroinflammation may play a more important role in the pathogenesis of FTD because mutations in the progranulin (GRN) genes lead to tau-negative FTD [10]. Progranulin acts as a mediator of the inflammatory response [18], and deficiency in progranulin may lead to greater microglial activation and a dysregulated inflammatory response in microglia that could cause neuron death and disease progression in FTD [10]. In addition, head trauma that triggers neuroinflammation has been associated with behavioral variant FTD [19]. Furthermore, some biomarkers of inflammation, such as elevated cytokines (for example, tumor necrosis factor (TNF)-α) in the cerebrospinal fluid (CSF), have been observed in patients with FTD [20]. These findings support the hypothesis that neuroinflammation has a detrimental role in FTD [10]. However, the pathogenesis of FTD, in particular the role of neuroinflammation, is still poorly understood.

Microglia are the resident immune cells in the central nervous system (CNS), representing the first line of defense against pathogens: they sense subtle pathological changes and become activated before obvious functional or anatomical abnormalities occur. Normal protective microglia mediate clearance of abnormal protein (such as Aβ or tau) aggregates, remove cell debris, and promote neuroregeneration. However, activated microglia secrete inflammatory mediators (for example, interleukin (IL)-1β), coactivate astrocytes, and induce neuronal death, which further increases brain tissue damage with amplified microglial activation [14]. The neuroinflammatory reaction involves dramatic upregulation of a mitochondrial transmembrane protein, TSPO, which is a marker for microglial activation and a target for imaging neuroinflammation with PET ligands designed to bind TPSO. It has been reported that PET imaging with the widely used first-generation TSPO ligand [¹¹C]-PK11195 detected neuroinflammation in patients with mild cognitive impairment (MCI) [21]. This and other evidence [22-25] suggests that neuroinflammation is an early and continuous process in neurodegenerative dementias [26]. It is now known that microglia detect pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) through pattern recognition receptors (PRRs), and in neurodegenerative disorders, microglia cannot discriminate between invading pathogens and aberrant molecules (or abnormal proteins) of the host, which leads to DAMP-triggered neuroinflammation through sustained and excessive release of pro-inflammatory cytokines such as IL-1β [10]. In addition, age-related microglia priming is crucial in exaggerated neuroinflammation [27], and microglial activation may be related to dementia progression [28].

Genes related to microglial activation (for example, a variant of the triggering receptor expressed on myeloid cells 2 (TREM2)) have now been associated with FTD [29,30]. Abnormal protein aggregates such as amyloid or tau deposition could cause microglial activation. New hypothesis has suggested that microglia could be functionally impaired by abnormal protein aggregates, leading to reduced microglial motility and phagocytic activity in vivo [31]. There is evidence that sustained exposure to bacterial lipopolysaccharide (LPS) or other pro-inflammatory mediators restricts microglial phagocytosis of protein aggregation and suppresses axonal transportation [10]. Several animal studies and postmortem studies have revealed that neuroinflammation stimulated neuronal degeneration [32,33]. On the other hand, microglia-driven neuroinflammation could lead to the formation of tau aggregation [34]. In FTD, the neuronal and axonal degeneration are sufficient to induce microglial activation [19].

Few studies to date have examined neuroinflammation in vivo in FTD patients. Molecular PET imaging with [¹¹C]-PK11195 has found that compared with controls, the mean [¹¹C]-PK11195 binding was significantly increased in patients with FTD (n = 5, including four patients with progressive non-fluent aphasia and one patient with behavioral variant FTD) in regions such as the left dorsolateral prefrontal cortex and the right hippocampus and parahippocampus [35]. The pattern of microglial activation partially overlapped with the pattern of brain atrophy, but there was also increased [¹¹C](R)-PK11195 binding in regions contralateral to predominant lobar atrophy suggesting that microglial activation was present at early stages of FTD prior to anatomical changes [19]. Using postmortem brain tissues, Vennetic et al. found that [¹¹C]-PK11195 binded specifically to activated microglia in FTD, and the binding was correlated with microglial activation identified by immunohistochemistry in situ [36]. Postmortem immunohistochemistry study further demonstrated that compared with controls, higher level of microglial activation detected by [¹¹C]-PK11195 was found in the frontal and temporal cortex in patients with FTD (n = 78), and greater microglial activation was found in the temporal subcortical white matter in FTD-MART than in other FTD genetic types [37].

In neurodegenerative disorders such as FTD, there is a common pattern in the mechanisms of sensing abnormal protein aggregates, activating microglia, transducing to the release of cytokines, and amplifying the neurotoxic effects in a chronic inflammatory process [14]. A better understanding of the immune response in the brain is critical for possible modulation of microglial activity to slow down or reverse the course of neurodegeneration [14]. Further development of PET imaging with TSPO ligands represents a potential in vivo tool for tracking the progression of neuroinflammation in neurodegenerative disorders such as FTD.

The value of TSPO PET imaging is in detecting microglia activation in the diseased brain, visualizing neuroinflammation and its progression, and monitoring treatment effect, which is highly needed in the diagnosis and treatment of FTD. TSPO is an interesting target for molecular PET imaging because it is involved in a number of neurodegenerative disorders (such as AD, FTD, and Parkinson’s disease (PD)) and neuroinflammatory disorders (such as ischemic stroke and multiple sclerosis). However, there are several limitations in the first-generation TSPO ligand [¹¹C]-PK11195, mainly high non-specific binding, low brain penetration, and high plasma protein binding, which may explain the negative findings in several studies using [¹¹C]-PK11195 [38,39]. In recent years, novel tracers such as [¹¹C]-DAA1106, [¹¹C]vinpocetine, [¹¹C]-DPA-713, [¹¹C]-PBR28, [¹⁸F]-FEDAA1106, [¹⁸F]-PBR06, [¹⁸F]-PBR111, [¹⁸F]-DPA-714, and [¹⁸F]-FEPPA have been developed as the second-generation TSPO radioligands. Comparative studies showed that novel TSPO ligands such as [¹⁸F]-DPA-714 have higher specific binding and lower non-specific binding than [¹¹C]-PK11195 in rodent models [40,41]. Further, Venneti et al. reported that [³H]-DAA1106 showed a higher binding affinity than [¹¹C]-PK11195 in postmortem brain tissues of patients with neurodegenerative disorders such as FTD [36]. Similarly, Vas et al. found higher binding of [¹¹C]vinpocetine than [¹¹C]-PK11195 in patients with multiple sclerosis [42]. These findings suggest that the second-generation TSPO ligands are better than the first-generation TSPO ligand [¹¹C]-PK11195 in imaging activated microglia in vivo in neurodegenerative disorders due to improved pharmacokinetic properties [36].

In recent years, PET imaging with novel TSPO radioligands has been applied to visualizing neuroinflammation in neurodegenerative disorders, although there is very limited data on in vivo PET imaging with the second-generation TSPO tracers in FTD. Using PET imaging with [¹¹C]-DAA1106, Miyoshi et al. examined patients with FTD (n = 3) who were presymptomatic MART gene carriers with parkinsonism linked to chromosome 17 (FTDP-17) and found increased microglial activation in regions such as frontal cortex in patients compared with controls, although such increase was not overt throughout the diseased brain in FTD [43]. In addition, regional increased [¹¹C]-DAA1106 binding has been found in patients with MCI (n = 7) compared to healthy controls [22], and patients with AD (n = 19) had greater regional [¹¹C]-PBR28 binding than controls, which was correlated with severity of the disease [44]. These findings suggest that neuroinflammation is an intrinsic process in tau pathology, which exists even at a presymptomatic stage.

However, research on imaging neuroinflammation with the second-generation TSPO ligands is still in its infancy, and the sample sizes of such research are usually small. In vivo and in vitro studies with the second-generation TSPO ligands have shown significant inter-subject variability because of differences in binding affinity in individual subjects [44-47]. Three affinity patterns of binding variations have been reported: high-affinity binders (HABs), low-affinity binders (LABs), and mixed-affinity binders (MABs) [47,48]. A single polymorphism (rs6971) located in the exon 4 of the TSPO gene determines the binding affinity of the second-generation ligands and causes large inter-subject variation [44,49,50], while for the first-generation TSPO ligand [¹¹C]-PK11195, the inter-subject variation in binding affinity is little.

Compared with carbon-11-labeled TSPO ligands (for example, [¹¹C]-PK11195), tracers labeled with fluorine 18 have a longer half-life ([¹⁸F] vs. [¹¹C]: approximately 110 min vs. approximately 20 min), which is suitable for long-distance dissemination and larger clinical studies. Among the fluorine-18-labeled second-generation TSPO ligands, [¹⁸F]-FEDAA1106 has proved ineffective [51,52] and [¹⁸F]-PBR06 produces a metabolite that confounds quantification of TSPO binding [53], but [¹⁸F]-FEPPA (FEPPA) and [¹⁸F]-DPA-714 (DPA714) may be promising [54-56]. FEPPA PET imaging has been applied to animals [54,57-62] and humans [63-70] and has demonstrated that increased neuroinflammation was not associated with normal aging, but regional increased FEPPA uptake was associated with AD or PD [67-70]. Similarly, there are a number of PET imaging studies with [¹⁸F]-DPA-714 [40,56,71-79], and focal increase in DPA714 uptake has been found in patients with neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS) [78].