Cerebral and Peripheral Amyloid Phagocytes—an Old Liaison with a New Twist

Abstract

In this month’s issue of Nature Medicine, Town et al. suggest that peripheral macrophages invading the brain reduce cerebral amyloidosis and thus may play a key role in the pathogenesis of Alzheimer’s disease (AD). This observation intensifies the longstanding controversy of whether mononuclear cells such as macrophages and/or microglial cells are beneficial or detrimental in AD.

The current discussion is already some twenty years old: It was Henryk Wisniewski who argued that microglia in culture but not in vivo can phagocytose β-amyloid fibrils. His conviction was based on ultrastructural observations in Alzheimer’s disease (AD) brains revealing an intimate relationship of β-amyloid plaques with microglia, which were, however, never found to harbor β-amyloid fibrils within their lysosomal compartments. In contrast, Wisniewski and colleagues described phagocytosed β-amyloid fibrils in macrophages of elderly patients suffering from fatal stroke—a rare complication in AD, which certainly does not represent the usual course of disease (Wisniewski et al., 1991; Frackowiak et al., 1992).

During development, myeloid cells invade the brain and differentiate into microglia. Resident microglia in the adult brain are thought to monitor their local environment and—in contrast to their peripheral counterparts, namely monocytes and extraneural tissue macrophages, which are rapidly and efficiently repopulated (Kennedy and Abkowitz, 1998)—resident microglia appear to have a rather slow turnover (Asheuer et al., 2004; Priller et al., 2001).

In the brains of AD patients as well as the respective transgenic mouse models, microglia become activated and increase in number in response to cerebral β-amyloidosis. The extent to which peripheral macrophages/monocytes contribute to this amyloid-associated microgliosis and its significance for AD remains unclear (Wyss-Coray, 2006).

In the current issue of Nature Medicine, Town and colleagues (2008) crossed CD11c-DNR mice, in which TGF-β-Smad2/3 signaling is blocked in CD11c⁺ cells, to two distinct and widely used amyloid precursor protein (APP) transgenic mouse models of cerebral amyloidosis. Contrary to the authors’ initial expectations, they observed a >50% reduction of both soluble and insoluble amyloid-β peptide (Aβ). Equally surprisingly, an increase in cells morphologically and phenotypically most closely resembling brain-infiltrating macrophages in the absence of detrimental inflammatory side effects was detectable in the brains of aged double-transgenic APP/CD11c-DNR mice over single-transgenic APP littermates (~1.7-fold by morphometric and ~7-fold by FACS analyses). Such an increase was found neither in the brains of single transgenic CD11c-DNR mice nor in younger, though already amyloid-depositing APP/CD11c-DNR mice. Town and colleagues went on to show that inhibition of TGF-β signaling in primary peripheral macrophages increased the uptake of pre-aggregated synthetic Aβ in vitro. These and other observations led the authors to conclude that blockade of TGF-β signaling in peripheral macrophages promotes brain infiltration of these cells with subsequent phagocytosis and attenuation of cerebral β-amyloidosis (Town et al., 2008).

These findings recall results of another recent study: by ablating bone marrow-derived mononuclear cells in an APP transgenic mouse model, Simard and colleagues (2006) concluded that bone marrow-derived macrophages, but not resident microglia, are critical for restricting β-amyloid plaque burden. However, the specificity of targeting exclusively peripheral macrophages was not quantified.

Previous studies using bone marrow chimeric mice revealed that infiltrating peripheral macrophages/monocytes represent only a tiny portion (~1%) of total CD11b⁺/Iba1⁺ cells in the brains of normal mice. In response to cerebral amyloidosis, an increase in this invading cell population has been reported in APP transgenic mice. Yet only a subpopulation (~20%) of amyloid plaques are the target of such infiltrating macrophages, on average with less than one invading macrophage per amyloid plaque (Stalder et al., 2005; Simard et al., 2006). It is a surprising but equally interesting suggestion that a presumably small number of invading phagocytes are accountable for a significant Aβ plaque catabolism, as suggested by Simard et al. (2006) and Town et al. (2008). Alternatively, thorough assessment isneededof whether other mechanisms than phagocytosis of invading macrophages may explain the described changes in amyloid burden.

It is important to note that recent studies challenge data on the role of CNS invasion of peripheral monocyte/macrophages, at least when generated in irradiated bone marrow chimeric mice (Ajami et al., 2007; Mildner et al., 2007). These studies suggest that the increase in microglia in response to a variety of CNS lesions is more a result of local microglial proliferation than of invasion of peripheral mononuclear cells. In contrast, significant CNS invasion of peripheral cells would require a breach of the blood-brain barrier, which in either case may be damaged in the course of cerebral amyloidosis (Winkler et al., 2001).

The potential benefit and clinical implication of therapeutically targeting mononuclear cells to fight AD is further substantiated by an additional recent report by El Khoury et al. (2007). In this study, AD mice deficient in the CC-chemokine receptor-2 (Ccr2) exhibit a significantly reduced recruitment of both peripheral monocytes/macrophages and resident microglia toward β-amyloid plaques and display accelerated amyloid deposition. While this finding does not resolve the issue of the specific mononuclear cell subtype in charge, it clearly supports the notion of the importance of the innate immune system in the pathogenesis of AD.

To further pave the way toward targeting elements of the innate immune axis such as mononuclear cells to fight AD, obvious open questions must be addressed. These include the urgent need to improve our knowledge on the origin of mononuclear cells in the setting of AD, i.e., whether exclusively bone marrow-derived mononuclear cells, resident microglia, dendritic cells, or, finally, all of them hold promise in the treatment of AD. Since CD11c is known to drive protein expression in the majority of dendritic cells and even in subtypes of microglia (Bulloch et al., 2008), it will be interesting to learn whether the increase in cells most closely resembling brain-infiltrating macrophages in the recent study of Town et al. (2008) is also attributable to other mononuclear or myeloid cell types.

Further, proof of β-amyloid phagocytic activity of resident microglia or invading macrophages, i.e., engulfment of amyloid fibrils with the later appearance of fibrils in the lysosomal-phagosomal compartments, is still lacking for the in vivo setting in the above-cited animal studies. Thus, mechanistic insights into how peripheral macrophages, resident microglia, or both, clear Aβ in vivo are required (is it Aβ endocytosis, pinocytosis, or indirect degradation?). This is also of particular interest in the context of Aβ-vaccination where microglia in an ex vivo assay have been described to phagocytose Aβ-antibody decorated amyloid plaques (Bard et al., 2000). Since Aβ-vaccination is a promising anti-AD interventional regime, it will be important to dissect the mechanisms involved, including the contribution of microglia.

Finally, there is the need to carefully assess Aβ biochemistry, amyloid deposition, and its variability with aging in the various mouse models of AD along with its functional implications. Equally important are comparisons among these mouse models. It is important to keep in mind that there may be differences in the efficacy of macrophage/monocyte-recruitment at different stages of AD and in individual disease models. Such analyses may also help to explain the nonlinear increase in plaque formation by Simard and colleagues (2006), which was the basis for assigning an amyloid-reducing capacity to blood-borne macrophages in the latter study.

Henryk Wisniewski, after following our debate, would possibly smile. Despite the modern tools of molecular and transgenic approaches, we are still left with questions he already asked some twenty years ago. However, he would most likely agree with us that the recent data on the role of mononuclear cells is highly encouraging and should promote more research on the role of the innate immune system in AD. Only a thorough elucidation of basic mechanisms will lead to the successful identification of novel therapeutic or prophylactic options and eventual safe implementation of anti-AD regimens that target the immune axis.