. Blocking TGF-beta-Smad2/3 innate immune signaling mitigates Alzheimer-like pathology. Nat Med. 2008 Jun;14(6):681-7. PubMed.

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  1. I wanted to thank Serge Rivest, Mathias Jucker, Tony Wyss-Coray, Joseph El Khoury, and Pritam Das for their helpful and thought-provoking comments, and to address some of their questions. I find it terribly interesting that the recent report by Richard, Rivest, and colleagues showed spontaneously increased TGF-β expression in immune cells near plaques of Tg APP/TLR2-/- mice. I agree that these striking findings are in line with the interpretation that increased TGF-β1 levels in AD patient brains, as shown by Wyss-Coray, Masliah, Mucke, and colleagues, likely serve the maladaptive role of maintaining an “immune privileged” brain milieu in AD patients and in these transgenic mouse models of the disease. We believe that overcoming this non-productive immune state will likely be key in targeting beneficial immune-mediated clearance of cerebral amyloid—and what better immune cell to target than the blood-borne macrophage (Greek etymology—“big eater”)? We also agree with Joseph El Khoury that a key aspect of this therapeutic modality will be promoting the Aβ phagocytosis response while opposing the proinflammatory response, both of which likely exist as a continuum of innate immune cell activation profiles (Town et al., 2005). But, if we can accomplish this, will amyloid-reducing therapies ultimately be successful AD therapeutics? As stated by Dave Morgan and others on this forum, the first test of the amyloid cascade hypothesis of AD in humans will likely be the Aβ vaccine. We anxiously await whether the hypothesis holds up and delivers an efficacious AD therapy. If it does, then the floodgates will open for a whole host of amyloid-targeted AD therapeutics—both immune and non-immune.

    About the issue raised by Mathias Jucker and Tony Wyss-Coray of CD11c as a marker for blood-borne innate immune cells/macrophages versus microglia, I should mention that we initially thought that CD11c would be a microglial marker in the context of AD. However, after examining numerous brain sections from various ages of wild-type versus Tg2576 or mutant APP/PS1 doubly transgenic mice for CD11c expression, we concluded that while microglia in the parenchyma around Aβ deposits were CD11b, CD45, MHC II, F4/80 Ag, and CD68 positive, they were negative for CD11c. However, we did observe a small number of round, non-process bearing CD11c positive cells within the lumen of blood vessels in both Tg2576 and APP/PS1 mice, consistent with Stalder and colleagues’ report of invading hematopoietic cells in brains of aged Tg2576 mice. At the time that we were checking for CD11c expression in AD mice, Alon Monsonego and Harold Weiner published a review in Science where they mentioned (as data not shown) that plaque-associated microglia were CD11c positive. I called Alon and asked him about the methodological details. However, after trying various tissue handling techniques, antibodies, and confocal settings, I was unable to reproduce this despite getting microglia in day 20 MOG-EAE brain sections to light up like a Christmas tree with CD11c. I came away thinking that it is possible to acutely activate microglia with the necessary vigor to promote CD11c expression, for example, in the context of EAE. However, I believe that this form of activation does not occur in AD mice, where the profile more closely resembles a chronic, persistent, low-level inflammation.

    I have recently read the paper by Bulloch and coworkers with great interest, which shows the presence of CD11c/EYFP “dendritic-like” mouse microglia in multiple stages of life. However, because the authors did not quantify their observations, it is unclear how prevalent these cells are in the brain, and/or whether these cells arose from the blood or were long-term CNS residents. Further, the authors had difficulty in co-staining these cells with CD11c antisera in tissue sections, raising a possibility that those who work with transgenics are all too aware of: expression of transgenes is often more promiscuous than expected. In our study, we demonstrated a seven- to eightfold increase in CD45+CD11b+CD11c+CD68+Ly-6C- cells (presumed “anti-inflammatory” macrophages initially immunophenotyped by Littman’s group in Geissmann et al., 2003) in our crossed mice, and immunohistochemical approaches revealed prominent vascular cuffing, where these cells appeared to be entering the brain via cerebrovessels. Regarding the questions from Joseph El Khoury and Pritam Das about the origin of these brain macrophages, we agree that the “acid test” of whether the macrophage-like cells that we see in and around cerebral vessels and β amyloid plaques arise from the periphery or from within the CNS would either be a chimeric approach or parabiosis. We moved away from the chimeric approach following recent reports in Nature Neuroscience (Ajami et al., 2007; Mildner et al., 2007) showing that the act of irradiating the mice leads to brain infiltration of monocytes/macrophages—the very dependent variable that we are interested in testing. However, we believe that 1) parabiosis of AD mice with GFP+CD11c-DNR mice or 2) chemical methods of ablating hematopoietic cells in AD mice followed by reconstitution with GFP+CD11c-DNR bone marrow containing or depleted of macrophages represent possible strategies that we are currently pursuing.

    Finally, Pritam Das raises the interesting questions of the long-term consequences of inhibiting TGF-β signaling on peripheral macrophages and the effects on T cells. We did not observe increased peripheral numbers of innate immune cells (including macrophages and dendritic cells), CD4+ or CD8+ T cells, or B cells in CD11c-DNR mice alone or in Tg2576xCD11c-DNR crossed mice, suggesting that an autoimmune state was not generated and that the increased abundance of macrophages in the brains of our crossed mice was β amyloid-directed. We also quantified T cells in brains of our crossed mice versus singly transgenic animals, and detected that about 4-5 percent of brain hematopoietic cells were TcRαβ positive (presumed T cells), and they were divided about equally between CD4+ and CD8+ subsets—however, these numbers were similar amongst wild-type, CD11c-DNR, APP/PS1, and APP/PS1xCD11c-DNR mice, suggesting that neither the CD11c-DNR nor the APP/PS1 transgenes were able to modify brain entry of T cells. Finally, regarding the issue of assessing neurodegeneration, we are currently pursuing this line of investigation by quantitative synaptophysin immunohistochemistry and hope to answer this question in the near future.

    References:

    . Local self-renewal can sustain CNS microglia maintenance and function throughout adult life. Nat Neurosci. 2007 Dec;10(12):1538-43. PubMed.

    . CD11c/EYFP transgene illuminates a discrete network of dendritic cells within the embryonic, neonatal, adult, and injured mouse brain. J Comp Neurol. 2008 Jun 10;508(5):687-710. PubMed.

    . Blood monocytes consist of two principal subsets with distinct migratory properties. Immunity. 2003 Jul;19(1):71-82. PubMed.

    . Microglia in the adult brain arise from Ly-6ChiCCR2+ monocytes only under defined host conditions. Nat Neurosci. 2007 Dec;10(12):1544-53. PubMed.

    . Immunotherapeutic approaches to Alzheimer's disease. Science. 2003 Oct 31;302(5646):834-8. PubMed.

    . Toll-like receptor 2 acts as a natural innate immune receptor to clear amyloid beta 1-42 and delay the cognitive decline in a mouse model of Alzheimer's disease. J Neurosci. 2008 May 28;28(22):5784-93. PubMed.

    . Invasion of hematopoietic cells into the brain of amyloid precursor protein transgenic mice. J Neurosci. 2005 Nov 30;25(48):11125-32. PubMed.

    . The microglial "activation" continuum: from innate to adaptive responses. J Neuroinflammation. 2005 Oct 31;2:24. PubMed.

    . Amyloidogenic role of cytokine TGF-beta1 in transgenic mice and in Alzheimer's disease. Nature. 1997 Oct 9;389(6651):603-6. PubMed.

    View all comments by Terrence Town
  2. In this study, the authors have found that TGF-β signaling inhibits the natural properties of macrophages to clear Aβ and infiltrate the CNS of APP mice. Knocking out such signaling events was found to improve both the brain infiltration of bone marrow-derived macrophages/microglia and their clearance of Aβ, which prevented the cognitive decline in mouse models of AD.

    These data fit very well with the novel concept that systemic innate immune cells have the capacity to fight against toxic proteins but do not do it in an efficient manner. That's probably because of anti-inflammatory signals (e.g., TGF-β), as elegantly demonstrated by Town and colleagues.

    We recently reported that Toll-like receptor 2 gene deletion is also associated with Aβ42 accumulation and cognitive impairment, while TLR2 gene expression in bone marrow-derived cells rescued such a memory deficit (Richard et al., 2008). Of great interest here is that APPtg/TLR2 knockout mice had a spontaneous increase in TGF-β gene expression in immune cells adjacent to the senile plaques. We can therefore propose that macrophages are not properly activated and do not efficiently infiltrate the CNS of APP mice. This natural innate immune mechanism against endogenously produced toxic elements may prevent chronic diseases, such as AD (see Soulet and Rivest, 2008). Improving both the infiltration and immune properties of these cells will hopefully soon be an effective new therapy to cure AD. The debate about the physiological relevance of these cells in the CNS will be over once patients are cured.

    References:

    . Toll-like receptor 2 acts as a natural innate immune receptor to clear amyloid beta 1-42 and delay the cognitive decline in a mouse model of Alzheimer's disease. J Neurosci. 2008 May 28;28(22):5784-93. PubMed.

    . Bone-marrow-derived microglia: myth or reality?. Curr Opin Pharmacol. 2008 Aug;8(4):508-18. PubMed.

  3. I am glad that the researchers studying transgenic models are finally confirming our results published in 2002 (Fiala et al., 2002), which showed transmigration of macrophages across the brain vessel wall and clearance of plaques by these large macrophages.

    The migrating macrophages broke through ZO-1 tight junction barrier and aggregated around brain vessels similarly as in HIV encephalitis. This has been followed by a recent publication in PNAS (Fiala et al., 2007). The animal studies cannot resolve the crucial question: are macrophages of patients with AD different from those of control subjects? The answers for interested readers are available in our PNAS article and more current work presented at ICAD. Not only macrophages penetrate across the blood-brain barrier but also clear oligomeric amyloid-β from neurons.

    References:

    . Cyclooxygenase-2-positive macrophages infiltrate the Alzheimer's disease brain and damage the blood-brain barrier. Eur J Clin Invest. 2002 May;32(5):360-71. PubMed.

    . Innate immunity and transcription of MGAT-III and Toll-like receptors in Alzheimer's disease patients are improved by bisdemethoxycurcumin. Proc Natl Acad Sci U S A. 2007 Jul 31;104(31):12849-54. PubMed.

    View all comments by Milan Fiala
  4. In this study, Town et al. present some fascinating findings with regard to the role of peripheral macrophages and Aβ amyloid clearance from the brains of Tg2576 mice. The authors genetically interrupted TGF-β signaling specifically in peripheral macrophages of Tg2576 mice and then evaluated Aβ pathology during aging of these mice. To the authors’ surprise, Aβ deposits were significantly attenuated in both brain parenchymal and cerebral blood vessels in these mice. Based on their data (both in vivo and in vitro), the authors suggest that the mechanism for this reduction in Aβ deposition may be due to increased infiltration of these altered peripheral macrophages into the brain and around cerebral blood vessels, resulting in increased Aβ phagocytosis. Although there are much recent data for the role of resident microglial cells in enhancing microglial-mediated phagocytosis of Aβ plaques, this is the first report to directly indicate peripheral macrophages in Aβ phagocytosis and clearance mechanisms.

    Undoubtedly, these interesting results will facilitate future investigations in this area; however, several questions from this report need further clarifications. Certainly, the gold standard for such experiments will be studies in chimeric mice to evaluate the effects of such altered peripheral macrophages on Aβ pathology. However, caution is warranted with regard to consequences of long-term peripheral inhibition of TGF-β signaling on macrophages. What is the effect on peripheral T cells in this scenario? Do peripheral T cells infiltrate into the CNS, and what is the phenotype of these cells? Although the authors evaluated extensively Aβ pathology in these studies, they missed an opportunity to evaluate any effects on neurodegeneration. One of the concerns of having long-term peripheral infiltration of macrophages, regardless of their phenotype, is unwanted reactions on other cell types/tissues in the brain.

    In any event, these new findings do provide a unique peripheral approach for attenuating Aβ deposition in the brain and certainly warrant further investigations.