Microglial Transplants Reverse Age-Related Pathology in Mice

In diseases driven by dysfunctional microglia, could replacing them with healthy versions prevent, or even reverse, pathology? Yes, suggest two papers in the June 18 Neuron. Both describe how mice completely devoid of microglia develop astrogliosis, reactive oligodendrocytes, neurodegeneration, white-matter atrophy, and calcification of the thalamus from middle age. These symptoms mirror a rare neurodegenerative disease in people, which goes by the mouthful adult-onset leukoencephalopathy with axonal spheroids and pigmented glia. ALSP affects people who have mutations in the gene for colony stimulating factor 1 receptor, which is essential for microglial proliferation. By reintroducing microglia to mice devoid of these cells, the scientists were able to prevent, even reverse, ALSP-like pathologies.

  • Without microglia, mice get astrogliosis and neurodegeneration.
  • Their pathology resembles that of leukoencephalopathy in people.
  • Replenishing microglia in young mice prevents this.
  • In adult mice, it reverses it.

In one of the studies, scientists led by first author David Munro and Josef Priller at the University of Edinburgh transplanted mouse microglia. In the other, Hayk Davtyan, Mathew Blurton-Jones, and colleagues at the University of California, Irvine, gave human microglia to a humanized version of the microglia-free mice. Both strategies worked.

“Overall, this is very exciting and certainly something we should continue pursuing in the field,” wrote Anna Martinez-Muriana and Renzo Mancuso, VIB-Center for Molecular Neurology, Antwerp, Belgium. The prospects for basic research impressed Nóra Baligács and Bart de Strooper of KU Leuven, Belgium. “This presents an exciting opportunity to … investigate the role of microglia and their diverse signaling pathways in brain function in unprecedented ways,” they wrote (comments below). Others saw potential for replenishing failing microglia in Alzheimer’s and other neurodegenerative diseases, even though the focus in these two papers was on primary microgliopathies.

The latter arise when too few healthy microglia patrol the brain. In the case of ALSP, people develop memory problems, personality changes, and sensory deficits in their 30s and 40s. They get tremors and walk more slowly. These symptoms are rooted in reactive astrocytes, white-matter atrophy, brain calcification, blood-brain barrier dysfunction, and axonal spheroids, i.e., blebs on axons triggered by inflammation or degeneration (Konno et al., 2017; Robinson et al., 2015; reviewed by Mickeviciute et al., 2021). 

Modeling ALSP in mice was difficult until scientists knocked out the fms-intronic regulatory element in the promoter that drives CSF1R gene expression. At first, the “FIRE” mice, which make no microglia, appeared surprisingly normal, even up to adulthood at 9 months (Rojo et al., 2019). Upon closer inspection, however, scientists found that 6-month-old mice roused a subset of inflammatory oligodendrocytes expressing the serine protease inhibitor Serpina3n and had lost myelin (Oct 2023 conference news). Serpina3n marks disease-associated oligodendrocytes in mouse models of neurodegenerative diseases (Kenigsbuch et al., 2022).

Munro and colleagues reported that as FIRE mice aged, they began to show more signs of ALSP. By 11 months, calcium had deposited in the thalamus, while MRI showed voids appeared in white matter of the thalamus, basal ganglia, and pons. These all worsened with age. At 12 months, some FIRE mice began dragging their hindlimbs and hunching their backs. By 18 months, half of them died. At this point, the thalamus had lost one-quarter of its neurons, with axonal spheroids dotting the hippocampus. Reactive astrocytes patrolled white-matter tracts, hippocampi, and cortices, and Serpina3n-positive oligodendrocytes multiplied throughout the white matter and thalamus (image below).

Reactive Oligos. Serpina3n-positive oligodendrocytes (red) start to gather in the white-matter tracts and thalami in adult FIRE mice (top right) and amass there by old age (bottom right). [Courtesy of Munro et al., Neuron, 2024.]

In Blurton-Jones’ lab, first author Jean Paul Chadarevian saw much the same in FIRE mice he created to support human microglial grafts. Called hFIRE, these also have no microglia. They express the human version of the CSF1R ligand, CSF1, and they lack B, T, and natural killer cells that might mount an immune response to the human microglia the scientists planned to graft.

Two-month-old hFIRE brains looked normal. By 8.5 months, Serpina3n-positive oligodendrocytes predominated in the white matter of the hippocampus, which was packed with lipids. As with the FIRE mice, astrocytosis, calcium deposits, axonal spheroids, and thalamic neurodegeneration hampered the hFIRE mice. Though Serpin3a oligodendrocytes were not detected in the thalamus, microbleeds and deposits of plasma albumin there indicated blood-brain barrier damage.

“It is remarkable that both studies find many overlapping pathological hallmarks of ALSP,” wrote Sabina Tahirovic at DZNE in Munich (comment below). “ALSP provides an ideal condition … to consider a microglia transplantation strategy.”

Transplants to the Rescue?
Munro and colleagues transplanted microglia from wild-type mice into 1-day-old FIRE pups. Nine months later, the cells had flourished throughout the brain. These brains had no calcium deposits, axonal spheroids, Serpina3n-positive oligodendrocytes, and few reactive astrocytes. They boasted 20 percent more neurons in the thalamus than untreated FIRE mice, nearly as many as wild-type.

For their part, Chadarevian and colleagues injected healthy microglia derived from human iPSCs into the hippocampi of 2-month-old hFIRE mice, before pathology. Microglia filled the brain within a month (image below). After 6.5 months, most were homeostatic, as measured by bulk RNA-Seq, and appeared to be highly ramified, as is typical for microglia in a healthy brain.

Microglia on the March. Human microglia (green) spread from the hippocampal injection site (left) throughout the brain (right) within 30 days. [Courtesy of Chadarevian et al., Neuron, 2024.]

Mirroring Munro's findings, transplanted hFIRE mice developed hardly any ALSP pathology. They had no calcium deposits, microbleeds, or axonal spheroids and as many neurons as wild-type mice. Astrogliosis and Serpina3n expression were no different from wild-types.

“Both publications demonstrate the remarkable ability of grafted microglial progenitors to colonize the entire mouse brain, adopt homeostatic signatures and functionally integrate,” wrote Gesine Saher at the Max Planck Institute for Multidisciplinary Sciences, Gottingen, Germany (comment below).

Could introducing microglia reverse established pathology in FIRE mice? Thinking about the possible therapeutic applications, Chadarevian and colleagues created iPSCs from a woman who carried the L786S CSF1R variant that causes ALSP, corrected the mutation to the L786L functional variant to create an isogenic control line, and then differentiated both lines into microglia. Six weeks after injecting the control cells, but not the L786S versions, into the hippocampi of 4-month-old hFIRE mice, axonal spheroids had disappeared, calcium crystals had dissolved, astrocytes were calmed, and oligodendrocyte Serpina3n expression fell by a third (image below). “I was pleasantly surprised how quickly pathology reversed,” Blurton-Jones told Alzforum.

Reverse It. In hFIRE mice, injected human microglia (green) carrying the L786S CSF1R variant are sparse after 6 weeks (left, top). Axonal spheroids (red), astrogliosis (yellow, bottom), and calcium deposits (white, bottom) remain. In contrast, L786L CSF1R microglia flourish (top right), axonal spheroids disappear, astrocytes calm down, and calcium deposits shrink (bottom right). [Courtesy of Chadarevian et al., Neuron, 2024.]

Blurton-Jones thinks this could be a valuable therapeutic strategy. However, these experiments don’t exactly mimic what happens in people because they already have microglia in their brains, even if they are not functioning normally.

To better model how injected microglia might take over from an existing population, Blurton-Jones and colleagues waited six months until the weak CSF1R mutant human microglia populated the hFIRE brain, then added CRISPR-corrected microglia. They are now testing if the latter fill the brain and rescue pathology.

Baligács and de Strooper think this could become a powerful therapeutic approach, particularly for ALSP, where microglia numbers and proliferation are low. “In other conditions, where microglial numbers are not affected, treatments to deplete endogenous microglia before transplantation may be necessary,” they wrote.

Could this work in Alzheimer’s disease, where certain microglial genotypes may be protective? “These suggestions remain futuristic for now. We still lack a clear understanding of the precise role of microglia in Alzheimer’s, and the practical challenges of implementing cell therapies in humans will require significant and persistent research efforts,” they wrote (comment below).

California-based NovoGlia, co-founded by Blurton-Jones, has grants from California’s Stem Cell Agency to work on microglia transplants for ALSP.—Chelsea Weidman Burke

Comments

  1. The two articles published back-to-back propose the exciting therapeutic concept that microglial transplantation can potentially rescue brain defects induced by the congenital lack of microglia. Both papers agree that the lack of microglia in adult mice results in neurodegeneration, brain calcifications, and myelin abnormalities, phenotypes resembling CSF1R-related leukoencephalopathy. Transplantation of wild-type mouse/human microglia rescued these phenotypes and protected against the development of the associated pathologies.

    This is a very interesting strategy for primarily myeloid cells diseases, such as those caused by mutations in colony stimulating factor 1 receptor (CSF1R), which lead to defective maturation, proliferation, and survival of microglia and other tissue-resident macrophages. Although both papers are elegant and show striking effects in the model systems used, there are many things to consider if one wanted to envision microglial transplantations as a therapeutic. For example, in the context of CSF1R mutations, there is a strong contribution of tissue-resident macrophages outside the CNS that should be further explored in follow-up publications.

    In addition, when thinking of translational aspects, correcting mutant stem cells and transplanting them back autologously, as shown by Chadarevian, is very interesting because it would avoid potential graft versus host disease that might happen after an allogeneic transplant. It also has downsides. It would require individual intervention for every single patient, which cannot be generalized. There are, of course, additional obstacles that should be considered, including safety, routes of delivery, etc., before this could be applied clinically.

    Using microglia replacement as a cell therapy is not necessarily limited to CSF1R-related diseases but could also be applied to other microgliopathies/macrophage disorders. For instance, Nasu-Hakola is caused by defects in the myeloid-expressing gene, TREM2/DAP12, resulting in microglial/macrophage dysfunction.

    Overall, we congratulate the authors on these two papers. They reinforce the role of microglia in neurodegeneration and open microglia transplantation strategies as new ways to treat microgliopathies.

    View all comments by Renzo Mancuso View all comments by Anna Martinez-Muriana

  2. These two fascinating publications build on the landmark paper that demonstrated the generation of a mouse completely devoid of microglia by simply removing a specific enhancer in the promoter of the CSFR1 gene (Rojo et al., 2019). The surprise at that time was that the brains of these “FIRE” mice appeared relatively normal, raising numerous questions about the role of microglia in brain development. The current publications focus on the effects of this deficiency in later life. FIRE mice exhibit many features of adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP), a primary microgliopathy caused by mutations in the CSF1R gene. The two papers demonstrate that the transplantation of both primary mouse microglia and human microglia leads to complete microglia engraftment in the brain and protects against the observed pathologies.

    These results provide preclinical evidence that microglia transplantation in the brain could be a powerful therapeutic approach for patients with primary microgliopathies. One reason this approach might be successful in ALSP patients is the reduced microglia numbers and proliferation, creating “empty niches” that can be colonized by the transplanted microglia. In other conditions, where microglia numbers are not affected, treatments to deplete endogenous microglia before transplantation may be necessary. It is also enticing to consider microglia transplantation for other neurological diseases, such as Alzheimer’s disease, which can be exacerbated by severe microglia deficiencies seen with TREM2 mutations. Recent evidence suggests that certain microglia genotypes (e.g., APOE3 Christchurch) (Chen et al., 2024; Arboleda-Velasquez et al., 2019) may be protective in Alzheimer’s. Therefore, one could speculate about microglia replacement in AD using microglia with protective genotypes.

    These suggestions remain futuristic for now. We still lack a clear understanding of the precise role of microglia in Alzheimer’s , and the practical challenges of implementing cell therapies in humans will require significant and persistent research efforts.

    More immediately,  the work described in these two papers will likely benefit basic research. The promising news is that early transplantation of microglia in these FIRE mice can prevent many of the symptoms associated with aging. This presents an exciting opportunity to  investigate the role of microglia and their diverse signaling pathways in brain function in unprecedented ways. We anticipate a surge of publications based on this model in the coming years.

    References:

    Rojo R, Raper A, Ozdemir DD, Lefevre L, Grabert K, Wollscheid-Lengeling E, Bradford B, Caruso M, Gazova I, Sánchez A, Lisowski ZM, Alves J, Molina-Gonzalez I, Davtyan H, Lodge RJ, Glover JD, Wallace R, Munro DA, David E, Amit I, Miron VE, Priller J, Jenkins SJ, Hardingham GE, Blurton-Jones M, Mabbott NA, Summers KM, Hohenstein P, Hume DA, Pridans C. Deletion of a Csf1r enhancer selectively impacts CSF1R expression and development of tissue macrophage populations. Nat Commun. 2019 Jul 19;10(1):3215. PubMed.

    Chen Y, Song S, Parhizkar S, Lord J, Zhu Y, Strickland MR, Wang C, Park J, Tabor GT, Jiang H, Li K, Davis AA, Yuede CM, Colonna M, Ulrich JD, Holtzman DM. APOE3ch alters microglial response and suppresses Aβ-induced tau seeding and spread. Cell. 2024 Jan 18;187(2):428-445.e20. Epub 2023 Dec 11 PubMed.

    Arboleda-Velasquez JF, Lopera F, O'Hare M, Delgado-Tirado S, Marino C, Chmielewska N, Saez-Torres KL, Amarnani D, Schultz AP, Sperling RA, Leyton-Cifuentes D, Chen K, Baena A, Aguillon D, Rios-Romenets S, Giraldo M, Guzmán-Vélez E, Norton DJ, Pardilla-Delgado E, Artola A, Sanchez JS, Acosta-Uribe J, Lalli M, Kosik KS, Huentelman MJ, Zetterberg H, Blennow K, Reiman RA, Luo J, Chen Y, Thiyyagura P, Su Y, Jun GR, Naymik M, Gai X, Bootwalla M, Ji J, Shen L, Miller JB, Kim LA, Tariot PN, Johnson KA, Reiman EM, Quiroz YT. Resistance to autosomal dominant Alzheimer's disease in an APOE3 Christchurch homozygote: a case report. Nat Med. 2019 Nov;25(11):1680-1683. Epub 2019 Nov 4 PubMed.

    View all comments by Nóra Baligács View all comments by Bart De Strooper

  3. These two excellent studies explore FIRE mice to model rare autosomal-dominant leukodystrophy (ALSP). FIRE mice carry a homozygous loss of the key microglial-regulating gene CSF1R, and thus lack microglia. Of note, most ALSP patients carry a heterozygous CSFR1 mutation. However, modelling the heterozygous state in mice seems not to be sufficient to trigger ALSP-like neuropathology, motivating the authors to focus on the complete loss of CSF1R signalling in microglia.

    Chadarevian et al. generated and characterized xenotolerant mice (hFIRE) to enable transplantation of human microglia, while Munro et al. used a previously published non-humanized FIRE mouse (Rojo et al., 2019) or their study. Both report ALSP-like neuropathological hallmarks, such as axonal spheroids, white-matter abnormalities, astrogliosis, and brain calcifications that develop during aging.

    Surprisingly, microglial depletion did not provoke major differences in transcriptomic signatures of other brain cells at the young age of 6-7 weeks (Munro et al.). However, upon aging, transcriptional alterations were captured in oligodendrocytes and astrocytes, but not in neurons. It remains open whether this is a technical limitation reflecting the low neuronal coverage of the scRNA-sequencing datasets (Munro et al.) or whether the neuronal transcriptional network is indeed less sensitive to the lifelong absence of microglia.

    Beyond the relevance of this work for ALSP, both studies support a concept that microglial loss of function is sufficient to trigger pathology in other brain cells over time, supporting a key role of microglia in maintaining brain homeostasis during aging. Notably, both report a subtle and regional neuronal loss (thalamus), supporting the idea that lack of microglial function alone is insufficient to provoke major and widespread neurodegeneration.

    The question of cell- and region-specific sensitivity is an important aspect reflected in both studies. Thalamus and white matter appear as hot-spot pathological regions in FIRE mice. As the CSF1R depletion is present in all microglia, and microglial loss is not region-selective, differences are likely attributable to intercellular cross talk and regional vulnerabilities of other brain cells. In contrast to neuronal loss, axonal spheroids were observed throughout the brain (Chadarevian et al.), supporting the relevance of microglia for axonal health. Thus, a pathological cross talk of microglia to oligodendrocytes, astrocytes, and neurons—that is well supported by both studies—is an exciting feature that should be further mechanistically dissected.

    Both have convincingly shown that transplantation of healthy human (Chadarevian et al.) or rodent (Munro et al.) microglia prevents the development of age-associated brain abnormalities. Furthermore, the work of Chadarevian and colleagues provides evidence that transplantation of ALSP patient-corrected microglia can also reverse some of the disease phenotypes. These studies pave the road to clinical translation because the primary microgliopathy ALSP provides an ideal setting to test the potential of microglia transplantation strategies. The findings clearly suggest the preventive potential of microglial transplantation and prove that microglial loss of function is the disease-triggering factor in ALSP.

    What remains challenging is to evaluate microglial repair potential when disease pathology is fully established, as it is in ALSP patients. Furthermore, more work is needed to characterize possible detrimental effects of the remaining “ill” microglia in the brain and their interaction with the transplanted healthy microglia. Because some of the pathological hallmarks of ALSP, including cognitive and motor disturbances, were not recapitulated in FIRE mice, it raises the question whether some “ill” microglia that remain in the brains of ALSP patients may, in addition to the microglial loss of function, provoke further neuropathology.

    Considering the important contribution of microglia to brain diseases, microglia transplantation is an extremely dynamic and exciting research area with a translational potential beyond ALSP.

    References:

    Rojo R, Raper A, Ozdemir DD, Lefevre L, Grabert K, Wollscheid-Lengeling E, Bradford B, Caruso M, Gazova I, Sánchez A, Lisowski ZM, Alves J, Molina-Gonzalez I, Davtyan H, Lodge RJ, Glover JD, Wallace R, Munro DA, David E, Amit I, Miron VE, Priller J, Jenkins SJ, Hardingham GE, Blurton-Jones M, Mabbott NA, Summers KM, Hohenstein P, Hume DA, Pridans C. Deletion of a Csf1r enhancer selectively impacts CSF1R expression and development of tissue macrophage populations. Nat Commun. 2019 Jul 19;10(1):3215. PubMed.

    View all comments by Sabina Tahirovic
    • User Profile Image Gesine Saher
      Max Planck Institute for Multidisciplinary Sciences
    • Posted:

    These two exciting papers by Munro et al. and Chadarevian et al. significantly advance our understanding of microglial function in maintaining brain integrity, demonstrating that these remarkable cells play a crucial role in preventing more pathological alterations than previously suspected.

    Microglia, the brain’s resident immune cells, maintain tissue homeostasis by constantly surveilling the brain parenchyma. During development and in disease states, microglia adopt specific signatures tailored to meet these diverse challenges. Further understanding of such cellular responses and their associated repair processes is essential, as efficient and timely restoration of brain function is vital for delaying the progression of neurodegenerative diseases. However, in certain disease conditions often linked to aging, microglial states transition to profiles that exacerbate disease, promoting neurodegeneration and pathology. Rejuvenating aged microglia has emerged as a therapeutic strategy in experimental models of neurological disease.

    Colony-stimulating factor 1 (CSF1) receptor signaling is crucial for microglial survival, as demonstrated by various genetic and pharmacological experiments. These findings in rodents are paralleled by CSF1 receptor-related human leukoencephalopathies. The two studies employ an elegant genetic tool that specifically targets the Csf1r FIRE enhancer leading to the absence of CNS microglia and a subset of peripheral macrophages.

    Munro et al. investigated the impact of the permanent absence of murine microglia on long-term brain integrity. Chadarevian et al. used immune-compromised microglial mutants to explore the relationship between pathogenesis and grafted human iPSC microglia. They observed that brain aging is significantly accelerated in microglial mutants and focused on brain calcification as a newly recognized age-related pathological condition. Brain calcifications predominantly develop in thalamic nuclei and are associated with the neurovascular unit, especially dysfunctional pericytes. In the long run, calcium deposits can lead to microhemorrhages and microinfarcts. Previous studies suggested that defective calcium and phosphate metabolism, in addition to an osteoclast-like/pro-mineralizing state, could be causal for this histopathology. Surprisingly, Munro et al. found only very few mural and myeloid cell clusters that expressed the respective gene set, leaving the origin of calcium phosphate deposits unresolved.

    Interestingly, transplantation of intact microglia, either of murine or human origin, largely prevented neurodegeneration and calcification. Thus, the authors conclude that microglia are instrumental in clearing calcium phosphate deposits. Both publications demonstrate the remarkable ability of grafted microglial progenitors to colonize the entire mouse brain, adopt homeostatic signatures, and functionally integrate. The authors propose that microglia transplantation could provide a therapeutic avenue to combat neuropathological conditions. Future studies will determine whether transplantation can serve as a therapeutic means to replace dysfunctional or neurotoxic microglia.

    Together, these important studies from the Priller and Burton-Jones labs link microglial function to the prevention of accelerated neurodegeneration and suggest that long-term pharmacological depletion of microglia in neurological conditions should be approached with caution.

    View all comments by Gesine Saher

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References

News Citations

  1. Does the Brain Use Microglia to Maintain Its Myelin?

Paper Citations

  1. Konno T, Yoshida K, Mizuno T, Kawarai T, Tada M, Nozaki H, Ikeda SI, Nishizawa M, Onodera O, Wszolek ZK, Ikeuchi T. Clinical and genetic characterization of adult-onset leukoencephalopathy with axonal spheroids and pigmented glia associated with CSF1R mutation. Eur J Neurol. 2017 Jan;24(1):37-45. Epub 2016 Sep 29 PubMed.
  2. Robinson JL, Suh E, Wood EM, Lee EB, Coslett HB, Raible K, Lee VM, Trojanowski JQ, Van Deerlin VM. Common neuropathological features underlie distinct clinical presentations in three siblings with hereditary diffuse leukoencephalopathy with spheroids caused by CSF1R p.Arg782His. Acta Neuropathol Commun. 2015 Jul 4;3:42. PubMed.
  3. Mickeviciute GC, Valiuskyte M, Plattén M, Wszolek ZK, Andersen O, Danylaité Karrenbauer V, Ineichen BV, Granberg T. Neuroimaging phenotypes of CSF1R-related leukoencephalopathy: Systematic review, meta-analysis, and imaging recommendations. J Intern Med. 2022 Mar;291(3):269-282. Epub 2021 Dec 22 PubMed.
  4. Rojo R, Raper A, Ozdemir DD, Lefevre L, Grabert K, Wollscheid-Lengeling E, Bradford B, Caruso M, Gazova I, Sánchez A, Lisowski ZM, Alves J, Molina-Gonzalez I, Davtyan H, Lodge RJ, Glover JD, Wallace R, Munro DA, David E, Amit I, Miron VE, Priller J, Jenkins SJ, Hardingham GE, Blurton-Jones M, Mabbott NA, Summers KM, Hohenstein P, Hume DA, Pridans C. Deletion of a Csf1r enhancer selectively impacts CSF1R expression and development of tissue macrophage populations. Nat Commun. 2019 Jul 19;10(1):3215. PubMed.
  5. Kenigsbuch M, Bost P, Halevi S, Chang Y, Chen S, Ma Q, Hajbi R, Schwikowski B, Bodenmiller B, Fu H, Schwartz M, Amit I. A shared disease-associated oligodendrocyte signature among multiple CNS pathologies. Nat Neurosci. 2022 Jul;25(7):876-886. Epub 2022 Jun 27 PubMed.

Further Reading

No Available Further Reading

Primary Papers

  1. Munro DA, Bestard-Cuche N, McQuaid C, Chagnot A, Shabestari SK, Chadarevian JP, Maheshwari U, Szymkowiak S, Morris K, Mohammad M, Corsinotti A, Bradford B, Mabbott N, Lennen RJ, Jansen MA, Pridans C, McColl BW, Keller A, Blurton-Jones M, Montagne A, Williams A, Priller J. Microglia protect against age-associated brain pathologies. Neuron. 2024 Jun 17; PubMed.
  2. Chadarevian JP, Hasselmann J, Lahian A, Capocchi JK, Escobar A, Lim TE, Le L, Tu C, Nguyen J, Kiani Shabestari S, Carlen-Jones W, Gandhi S, Bu G, Hume DA, Pridans C, Wszolek ZK, Spitale RC, Davtyan H, Blurton-Jones M. Therapeutic potential of human microglia transplantation in a chimeric model of CSF1R-related leukoencephalopathy. Neuron. 2024 Jun 12; PubMed.

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