The origin, function, and sometimes even the existence of macrophages in the brain have been vigorously debated over the past century (see, for example, the historical overview by Peters, Palay, and Webster in The Fine Structure of the Nervous System, Oxford, 1991). Many issues have been resolved in recent years, but the cells remain surprisingly refractory to scientific interrogation. In Alzheimer disease, reactive microglia are a prominent cellular component of senile plaques, and hence, they have attracted the attention of researchers who wish to establish whether they are harmful or beneficial. The microglia themselves furnish evidence to support both views: As macrophages, they are equipped to rid the brain of unwanted material, yet this capability also gives them the potential to do collateral damage in the process.

This intriguing paper by Simard, Rivest, and colleagues provides evidence for a beneficial role of microglia in removing excess β-amyloid in vivo. Their data indicate, in plaque-producing transgenic mice (including a model that also expresses thymidine...  Read more

The origin, function, and sometimes even the existence of macrophages in the brain have been vigorously debated over the past century (see, for example, the historical overview by Peters, Palay, and Webster in The Fine Structure of the Nervous System, Oxford, 1991). Many issues have been resolved in recent years, but the cells remain surprisingly refractory to scientific interrogation. In Alzheimer disease, reactive microglia are a prominent cellular component of senile plaques, and hence, they have attracted the attention of researchers who wish to establish whether they are harmful or beneficial. The microglia themselves furnish evidence to support both views: As macrophages, they are equipped to rid the brain of unwanted material, yet this capability also gives them the potential to do collateral damage in the process.

This intriguing paper by Simard, Rivest, and colleagues provides evidence for a beneficial role of microglia in removing excess β-amyloid in vivo. Their data indicate, in plaque-producing transgenic mice (including a model that also expresses thymidine kinase), that many macrophages associated with relatively mature senile plaques originate from the periphery, and that it is these cells, rather than the resident microglia, that phagocytose Aβ and thereby lower plaque load. The bone marrow-derived macrophages are summoned to dispose only of senile plaques that have evolved to the point where, presumably, they present a distinct threat to the integrity of the brain. This finding supports the view that activated microglia probably do not mediate the early deposition of Aβ in APP-transgenic mice (Stalder et al., 1999) or in aged humans (Vogelgesang et al., 2002). It also suggests a tentative resolution to the question of what microglia are doing in plaques: It depends in part on the origin of the cells. On balance, these interesting data in mice imply that promoting the phagocytosis of brain amyloid by bone marrow-derived microglia could reduce plaque load in AD.

That said, provoking a cellular attack on Aβ is a potentially high-reward but also risky strategy, as has been made clear by the Aβ immunization trials for AD. With this in mind, there are some additional issues that might be considered before calling on peripheral macrophages to join the fray. First, the relevance of Aβ clearance in transgenic mice to the situation in human AD remains uncertain, as there is considerable evidence for critical differences between murine and human senile plaques/Aβ deposits, and possibly in the way that the two species respond immunologically to Aβ. Second, it is important to repeat this study in larger numbers of mice, and preferably in male and female mice, which can differ significantly in their tendency to deposit Aβ. Studies in biologically intermediate species might help to bridge this gap. In addition, it would be beneficial to know if, for example, oligomeric Aβ, tau, and neuronal integrity are affected by blood-borne microglia in animal models. Ultimately, behavioral improvement will be the arbiter of the value of any potential therapy.

It would be ideal to impede the Aβ-cascade before the toxic effects of Aβ become manifest in the brain. If, however, recruitment of peripheral macrophages can be demonstrated to be selective, safe, and effective in humans, this approach could indicate a path to treating AD at a point in the pathogenesis of the disorder when other types of therapy might be too late.

View all comments by Lary Walker

This is an interesting manuscript. It confirms an earlier study by Wisniewski et al., 1991, which reported that invading macrophages after brain injury phagocytose amyloid while resident microglia appear not to do so. However, the question of whether a CNS lesion, that is, disruption of the blood-brain barrier (BBB), is necessary for a “phagocytotic” activity of invading macrophages still remains unanswered in this new study. That is because Simard and collaborators inserted a catheter into the ventricle, which obviously affected the integrity of the BBB.

The authors also suggest amyloid phagocytosis of the invading macrophages based on co-staining of a lysosomal marker with Aβ. Unfortunately this co-staining was not done for resident microglia. Colocalization of Aβ/amyloid at the level of confocal microscopy does not unequivocally prove amyloid phagocystosis (see, e.g., Wisniewski et al. above; Stalder et al., 2001). Because the role of resident microglia was not studied, further...  Read more

This is an interesting manuscript. It confirms an earlier study by Wisniewski et al., 1991, which reported that invading macrophages after brain injury phagocytose amyloid while resident microglia appear not to do so. However, the question of whether a CNS lesion, that is, disruption of the blood-brain barrier (BBB), is necessary for a “phagocytotic” activity of invading macrophages still remains unanswered in this new study. That is because Simard and collaborators inserted a catheter into the ventricle, which obviously affected the integrity of the BBB.

The authors also suggest amyloid phagocytosis of the invading macrophages based on co-staining of a lysosomal marker with Aβ. Unfortunately this co-staining was not done for resident microglia. Colocalization of Aβ/amyloid at the level of confocal microscopy does not unequivocally prove amyloid phagocystosis (see, e.g., Wisniewski et al. above; Stalder et al., 2001). Because the role of resident microglia was not studied, further work is needed to elucidate the function and impact of resident microglia versus invading macrophages.

View all comments by Mathias Jucker

I would like to thank Erene Mina and Drs. Walker and Jucker. They provide insightful comments regarding specific aspects of the study. I'd like to address a few points here.

The first one regards irradiation and its effects on the blood-brain barrier (BBB). There is not very strong evidence that irradiation alters the BBB, and brain infiltration of bone marrow-derived cells has been reported with other techniques as well. Messengale and colleagues have validated this concept using both lethal irradiation and parabiosis techniques in mice (Massengale et al., 2005). Although most (if not all) GFP cells found in the brains of chimeric mice have a microglial phenotype, the overall contributions of such cells to the brain-resident microglial populations of normal mice remain quite low (e.g., 0.5-11.5 percent of resident microglia). This is what we generally observe in our mice (Simard and Rivest, 2004). In APP mice, however, there is a robust microglial recruitment toward the plaques, and those that derive from the bone marrow are attracted at a specific time of the disease....  Read more

I would like to thank Erene Mina and Drs. Walker and Jucker. They provide insightful comments regarding specific aspects of the study. I'd like to address a few points here.

The first one regards irradiation and its effects on the blood-brain barrier (BBB). There is not very strong evidence that irradiation alters the BBB, and brain infiltration of bone marrow-derived cells has been reported with other techniques as well. Messengale and colleagues have validated this concept using both lethal irradiation and parabiosis techniques in mice (Massengale et al., 2005). Although most (if not all) GFP cells found in the brains of chimeric mice have a microglial phenotype, the overall contributions of such cells to the brain-resident microglial populations of normal mice remain quite low (e.g., 0.5-11.5 percent of resident microglia). This is what we generally observe in our mice (Simard and Rivest, 2004). In APP mice, however, there is a robust microglial recruitment toward the plaques, and those that derive from the bone marrow are attracted at a specific time of the disease. Other groups have observed a similar pattern in irradiated APP mice (Malm et al., 2005; Stalder et al., 2005), and one can appreciate the robust microglia infiltration in the plaques of non-irradiated mice (Fig. 1 and supp. movie 1). Therefore, I do not think that infiltration is caused by alteration of the BBB in irradiated mice, but is a natural process that is especially dynamic while the plaques progress. The mechanisms explaining why bone marrow-derived microglia are no longer recruited toward the plaques at a specific time point in the disease have yet to be unraveled with future experiments.

Another point raised is that we did not look at the colocalization of Aβ in the lysosomes of the resident microglia. We actually did a meticulous analysis of such processes in the chimeric APP, and while GFP cells were almost always associated with lysosomal Aβ, the resident cells were not. This is the reason that we did not show these results, but we have discussed them.

We observed phagocytosis by bone marrow-derived microglia during a very specific time. This takes place around 6 months of age in the APP/PS1 mice, and we no longer see these cells at 9 months. Therefore, cell recruitment (of bone marrow origin) and phagocytosis are dynamic and transient phenomena, which may explain why other groups have not detected it. This also explains why inhibition of cell recruitment (APP/TK mice) from 5 to 6 months has such profound consequences on plaque growth. We are now working on new genetic strategies to enhance and improve the recruitment of these cells for a longer period of time to see if we can prevent the amyloid cascade and cognitive deficit.

Finally, multiple staining and 3D reconstructions using confocal laser-scanning microscopy are powerful tools to determine cellular compartmentalization, such as Aβ within the lysosomal GFP cells.

References:
Malm TM, Koistinaho M, Parepalo M, Vatanen T, Ooka A, Karlsson S, Koistinaho J. Bone-marrow-derived cells contribute to the recruitment of microglial cells in response to β-amyloid deposition in APP/PS1 double transgenic Alzheimer mice. Neurobiol Dis. 2005 Feb;18(1):134-42. Abstract

Massengale M, Wagers AJ, Vogel H, Weissman IL. Hematopoietic cells maintain hematopoietic fates upon entering the brain. J Exp Med. 2005 May 16;201(10):1579-89. Abstract

Simard AR, Rivest S. Bone marrow stem cells have the ability to populate the entire central nervous system into fully differentiated parenchymal microglia. FASEB J. 2004 Jun;18(9):998-1000. Epub 2004 Apr 14. Abstract

Simard AR, Soulet D, Gowing G, Julien JP, Rivest S. 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. Abstract

Stalder AK, Ermini F, Bondolfi L, Krenger W, Burbach GJ, Deller T, Coomaraswamy J, Staufenbiel M, Landmann R, Jucker M. Invasion of hematopoietic cells into the brain of amyloid precursor protein transgenic mice. J Neurosci. 2005 Nov 30;25(48):11125-32. Abstract

View all comments by Serge Rivest