Immune cells have been shown to play a role in clearing Aβ from the brain, but it's not clear exactly which subset of cells is responsible. Is it the phagocytes already present in the brain—the microglia—or are new cells recruited from the circulatory system? It's hard to tell, because once in the brain, they all look alike. Researchers are trying to pinpoint the key cell group so that they can design targeted therapies to stimulate them to clear Aβ and perhaps ward off Alzheimer's disease. A study in the August 3 Journal of Neuroscience lends new evidence that perivascular macrophages (PVMs)—those phagocytes that line the brain's blood vessels—may be key players.

Previous research suggests that the immune cells responsible for amyloid clearance depend on the chemokine receptor CCR2 to help them find and eat away at amyloid (see ARF related news story on El Khoury et al., 2007). So to figure out which cells depend on CCR2, researchers led by Josef Priller of the Charité Universitätsmedizin Berlin and Marco Prinz of the University of Freiburg, both in Germany, created chimeric animals by irradiating APPswe or APPswe/PS1 transgenic mice and replacing their bone marrow cells with grafted cells from mice that either expressed CCR2 (CCR2+/+) or double mutants that didn't (CCR2-/-). They then looked to see which type became engrafted in the brain.

The team, which also included joint first authors Alexander Mildner and Katrin Kierdorf of the University of Freiberg and Bernhard Schlevogt of Universitätsmedizin Göttingen, also in Germany, showed that circulating CCR2+/+ myeloid cells were the ones that crossed into the parenchyma of APPswe/PS1 mice. Furthermore, they showed that irradiation of the whole body, including the head, was required for them to gain entry. But once there, the engrafted phagocytes didn't seem to chew away at the plaques, as has been reported elsewhere (see ARF related news story on Simard et al., 2006). Instead, the data suggested the bone marrow-derived phagocytes in APPswe mice acted at the perivascular compartment. This falls in line with a previous report suggesting that macrophages in and around the blood vessels may be involved in reducing Aβ in the brain by shuttling it out via the perivascular spaces (see ARF related news story on Hawkes and McLaurin, 2009). "This irradiation paradigm reduces amyloid, not in terms of the plaques, but actually in terms of the overall amyloid in the perivascular compartment, around the vessels," said Priller.

To distinguish between the different macrophage activities that might be responsible for clearing Aβ, the team used different methods of irradiation to control whether circulating cells became engrafted in the brain. Since the researchers found that irradiation of the head was required for circulating myeloid cells to enter the brain, they shielded the skulls of some of the mice from the radiation so that circulating myeloid cells were locked out—they could only go as far as the perivascular space. The method generated chimeric APPswe mice whose perivascular macrophages were either CCR2+/+ or CCR2-/-.

The researchers irradiated the animals at three months old. Seven months later , the APPswe mice with CCR2-/- grafts had significantly more insoluble Aβ in their brains than did CCR2+/+ -grafted mice, which the team took to mean that perivascular macrophages modulate Aβ deposition in the brain's blood vessels in a process that depends on CCR2. Conversely, the team compared aged, unirradiated APPswe CCR2-/- mice with aged APPswe CCR2+/+ and found that clearance of Aβ deposition and the number of immunoreactive cells, as shown by the macrophage/microglia marker Iba-1, was similar in both. The result suggests that brain-resident microglia don't depend on CCR2 for clearance. This finding contrasts with other work suggesting that CCR2-deficient mice have fewer microglia and enhanced parenchymal Aβ plaque deposition (see El Khoury et al., 2007).

Priller said the findings implicate perivascular macrophages, rather than microglia, in Aβ clearance. He said the current paper provides a nice link between results showing that reduction in perivascular macrophages enhances cerebral amyloid angiopathy, or CAA (see Hawkes and McLaurin, 2009), and that CCR2 is needed for Aβ clearance (El Khoury et al., 2007). Researchers led by Joseph El Khoury at Massachusetts General Hospital, Charlestown, contended that CCR2-deficient APPswe mice die early because the microglia no longer function, leading to an increase of Aβ plaques in the brain and early death. But Priller says this new work suggests that, rather than affecting the microglia, the CCR2 deficiency affects the perivascular macrophage clearance. This supports work by Terrence Town at Cedars Sinai Medical Center in Los Angeles, California, who reported that stimulating peripheral macrophages by blocking TGF-β1 signaling cleared Aβ from the brain, including the blood vessels (see ARF related news story).

"This study makes me quite comfortable that peripheral macrophages are adept at removing brain vascular Aβ deposits," said Town, who was not involved with the study. But the disagreement with El Khoury’s data still leaves open the question of whether the microglia play a role in plaque breakdown. "I don't think we have closure on the issue of CCR2 dependency of brain parenchymal plaque clearance, because we now have two datasets that aren't completely congruent, " Town added.

Serge Rivest of Laval University, Quebec, Canada, said it's becoming more accepted in the field that neuroprotection doesn't always correlate with changes in plaque load—that subtle changes, such as in cerebrovascular β amyloid clearance, could confer a neuroprotective effect. However, he would have liked to see more behavioral data on the mice, to show that perivascular clearance of Aβ had a positive effect on the brain. "The paper showed that circulating monocytes can be an extremely powerful approach to the disease," said Rivest. "But we need to find out whether these data translate to clearly physiological relevance in these mice using behavioral tests."—Gwyneth Dickey Zakaib

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  1. This paper by Mildner et al. showed that CCR2 localized at subsets of myeloid cells exert differential roles in plaque pathology. CCR2 localized on peripheral macrophages is required for their infiltration into AD mouse brain parenchyma after total-body irradiation and bone marrow transplantation, but does not affect plaque pathology. In contrast, CCR2 is not required for the recruitment of perivascular macrophages (PVM) to vascular Aβ deposits, but exerts strong effects on plaque clearance.

    A few intriguing lessons emerge from this carefully designed and well-controlled study: 1) The difference between protected versus unprotected (conditioned) brains for the engraftment of peripheral macrophages in the brain is profound. Even in AD mice with significant plaque pathology, the engraftment efficiency of peripheral macrophages in the protected brain seems too low to be detected; 2) The role of PVMs in AD is underappreciated, and this study offers an important clue to their distinct effects on plaque clearance. More studies are needed to further explore their properties and functions in AD pathogenesis; and 3) Cytokine receptors could play differential functions depending on their localization. Besides CCR2 and CX3CR1 on residential microglia, those receptors on peripheral macrophages also play differential roles. Several studies support the idea that there is deficient microglial CX3CR1 in the brain, which reduces plaque load and exacerbates toxicity induced by lipopolysaccharide, the mitochondrial toxin MPTP, soluble Aβ, and phosphorylated-tau (Bhaskar et al., 2010; Cardona et al., 2006; Cho et al., 2011). However, deficient CX3CR1 signaling in intraspinal microglia and monocyte-derived macrophages appears to promote recovery after traumatic spinal cord injury in mice (Donnelly et al., 2011). These findings highlight the importance of cell type- and spatial specificity of cytokine receptors.

    References:

    . Regulation of tau pathology by the microglial fractalkine receptor. Neuron. 2010 Oct 6;68(1):19-31. PubMed.

    . Control of microglial neurotoxicity by the fractalkine receptor. Nat Neurosci. 2006 Jul;9(7):917-24. PubMed.

    . CX3CR1 protein signaling modulates microglial activation and protects against plaque-independent cognitive deficits in a mouse model of Alzheimer disease. J Biol Chem. 2011 Sep 16;286(37):32713-22. Epub 2011 Jul 19 PubMed.

    . Deficient CX3CR1 signaling promotes recovery after mouse spinal cord injury by limiting the recruitment and activation of Ly6Clo/iNOS+ macrophages. J Neurosci. 2011 Jul 6;31(27):9910-22. PubMed.

    View all comments by Li Gan
  2. I recommend the primary papers, and also I want to add some considerations. In APP/PS1 transgenic mice, we can detect a lot of inflammatory mediators (submitted) in the cortex at five and seven months of age compared to wild-type. It is interesting to note that, in transgenic mice, and in AD, macrophages surrounding the brain can access by crossing the blood-brain barrier, and those cell can be beneficial (or perhaps detrimental) to the patient (McGeer et al., 2006; Viña et al., 2007; Vina et al., 2007). On the other hand, we and others have published (Valles et al., 2008 and 2010) that, in the brain, there not only exist microglia such as macrophages playing a role, but also astrocytes that have their own immune function. We should never forget the role of astrocytes inside the CNS, because otherwise, we are only looking at one type of cell. Astrocytes have many functions inside the CNS, such as contributing to inflammation and oxidative stress. The fact that astrocytes surround amyloid-β in AD patients, and the fact that astrocytes change to reactive astrocytes, engulfing Aβ and destroying it (published by us and others), demonstrates a special role for these cells in AD. Furthermore, inside the brain, neurons, astrocytes, microglia, oligodendroglia, and macrophages from outside are in physiological conditions, so we need to be careful when we look for mechanisms producing or clearing Aβ.

    Neurons are cells with delicate lives. They need the help of astrocytes to survive. Neurons can last for a lifetime, but a decrease in astrocyte support will lead to inefficient elimination of Aβ in the elderly. I believe that microglia are helping the brain in young people, but perhaps in older brain the physiologic mechanisms to eliminate Aβ are weak,, and the elimination of Aβ becomes critical, as suggested by Wyss Coray and collaborators (Tesseur et al., 2006), and us (paper submitted). On the other hand nobody is looking for adult stem cells in elderly AD patients (we want to work on that if we can get funding). Our hypothesis is that, in older brain, oxidative stress and inflammation are playing a cooperative job, such as in young people, but problems start when the two mechanisms are affected by age and/or when either (or both) is destroyed or works poorly due to unfavorable conditions (see next publication of the group Garcia-Lucerga et al., submitted).

    References:

    . Oestradiol or genistein rescues neurons from amyloid beta-induced cell death by inhibiting activation of p38. Aging Cell. 2008 Jan;7(1):112-8. Epub 2007 Nov 21 PubMed.

    . Estradiol or genistein prevent Alzheimer's disease-associated inflammation correlating with an increase PPAR gamma expression in cultured astrocytes. Brain Res. 2010 Feb 2;1312:138-44. Epub 2009 Nov 27 PubMed.

    . Mitochondrial oxidant signalling in Alzheimer's disease. J Alzheimers Dis. 2007 May;11(2):175-81. PubMed.

    . Effect of gender on mitochondrial toxicity of Alzheimer's Abeta peptide. Antioxid Redox Signal. 2007 Oct;9(10):1677-90. PubMed.

    . Inflammation, anti-inflammatory agents and Alzheimer disease: the last 12 years. J Alzheimers Dis. 2006;9(3 Suppl):271-6. PubMed.

    . Deficiency in neuronal TGF-beta signaling promotes neurodegeneration and Alzheimer's pathology. J Clin Invest. 2006 Nov;116(11):3060-9. PubMed.

    View all comments by Soraya Valles

References

News Citations

  1. Microglia—Medics or Meddlers in Dementia
  2. Calling for Backup: Microglia from Bone Marrow Fight Plaques in AD Mice
  3. CAA Relief? Specialized Macrophages Help Flush Out Vascular Amyloid
  4. Macrophages Storm Blood-brain Barrier, Clear Plaques—or Do They?

Paper Citations

  1. . Ccr2 deficiency impairs microglial accumulation and accelerates progression of Alzheimer-like disease. Nat Med. 2007 Apr;13(4):432-8. PubMed.
  2. . Bone marrow-derived microglia play a critical role in restricting senile plaque formation in Alzheimer's disease. Neuron. 2006 Feb 16;49(4):489-502. PubMed.
  3. . Selective targeting of perivascular macrophages for clearance of beta-amyloid in cerebral amyloid angiopathy. Proc Natl Acad Sci U S A. 2009 Jan 27;106(4):1261-6. PubMed.

Further Reading

Papers

  1. . Mutation in the alpha-synuclein gene and sporadic Parkinson's disease, Alzheimer's disease, and dementia with lewy bodies. Exp Neurol. 1998 Sep;153(1):164-6. PubMed.

Primary Papers

  1. . Distinct and non-redundant roles of microglia and myeloid subsets in mouse models of Alzheimer's disease. J Neurosci. 2011 Aug 3;31(31):11159-71. PubMed.