Neuroinflammation can both help and harm the Alzheimer’s brain, leaving researchers uncertain of the best way to modulate the immune system therapeutically. Previously, many scientists thought that dialing down inflammation would help, but growing evidence now suggests the opposite. Numerous talks at the annual Society for Neuroscience conference, held November 12-16 in San Diego, reinforced this new view, with converging lines of evidence for the benefits of a pro-inflammatory brain milieu. Specifically, pumping up the appetites of microglia clears amyloid and improves cognition in mouse models, researchers said. They put forward several different ways to stimulate microglia, including cytokines and an antibody that recognizes the microglial receptor TREM2. Other presentations drilled into how aggregated Aβ and TREM2 promote a pro-inflammatory state. Overall, the data suggest an emerging consensus on how inflammation could be harnessed to slow AD.

“Over the last decade, we have been trying to advance the hypothesis that the inflammatory response in AD is ineffective because it is out of balance. Paradoxically, it isn’t too much inflammation that seems to be the culprit in AD; it’s not having the right kind of immune response,” Terrence Town of the University of Southern California, Los Angeles, wrote to Alzforum. In particular, he believes TREM2 expression might be suboptimal in the AD brain, and that stimulating TREM2 signaling might be one way to restore balance.

Too Little TREM2?

In an AD brain section, activated microglia (red) surround an amyloid plaque (green), but express sparse TREM2 (pink). [Courtesy of Brian Leung and Terrence Town.]

Some previous studies had already cast inflammation as a force for good in Alzheimer’s. Todd Golde of University of Florida, Gainesville, reported that the pro-inflammatory cytokine IL-6 stimulates gliosis, enhancing Aβ phagocytosis in transgenic mice (see Chakrabarty et al., 2010). Both Golde and Town separately found that the anti-inflammatory cytokine IL-10 worsens amyloid pathology and cognition in mice (see Feb 2015 conference news). Not all studies agree, however, with other researchers reporting negative effects from different pro-inflammatory cytokines (see Nov 2012 news). 

For his current research, Golde and colleague Yona Levites wondered how the inflammatory state of the brain might affect AD immunotherapy. To address this, Levites injected a vector carrying either IL-6 or IL-10 into the brains of newborn CRND8 mice, which overexpress mutant human APP and typically develop amyloid plaques by three months of age. Beginning two months after birth, the researchers injected 0.5 mg of the in-house anti-Aβ1-16 antibody mAb5 intraperitoneally into half the transfected mice (see Levites et al., 2015). The animals received injections twice weekly for four months.

When the animals were analyzed at six months, mice that expressed exogenous IL-6 had less amyloid than untreated controls. Antibody treatment on top of that lessened the load a little further, displaying a partial additive effect with IL-6. In mice that received both IL-6 and mAb5, insoluble Aβ42 and Aβ40 levels dropped nearly in half, while SDS-soluble Aβ crashed by two-thirds. On the other hand, mice expressing IL-10 fared worse than untreated controls, developing a heavier amyloid load. Moreover, the presence of IL-10 blunted the benefits of antibody treatment, Golde reported.

The data suggest that the brain’s underlying immune status can modulate the outcome of immunotherapy, Golde said. He noted that immune activation follows the “Goldilocks Principle”—it needs to be just right. Recent mouse data point to benefits from turning up the immune system to run a little hotter in the AD brain, Golde said. An audience member wondered if the findings will translate to people, since both IL-6 and IL-10 have been reported to be elevated in AD brain. However, Golde noted that most of those data come from postmortem brains. People with AD often die from infections such as sepsis or pneumonia, which could alter the immune status of the brain. Researchers need to assess the level of innate immune factors in living patients, he suggested. If the data show that immunotherapy does require pro-inflammatory factors to be most effective, researchers might borrow a technique from the cancer field and create “chemobodies” that combine an antibody with another molecule to accomplish both goals at once, Golde suggested.

Others were not so sure. Gary Landreth of Case Western Reserve University, Cleveland, noted that overexpression of IL-6 and IL-10 in these experiments makes it hard to determine how the findings apply to normal physiology, or what this means for phagocytosis. “There is little mechanistic understanding of how a pro- or anti-inflammatory milieu interacts with the phagocytic machinery in the brain,” he wrote to Alzforum.

What else besides cytokines affects microglial activation? Several presentations fingered the microglial receptor TREM2, variations in which triple the risk of AD. Many researchers now believe this is because these variants hamper the ability of microglia to contain or clear plaques (see Nov 2012 news; May 2016 news; Jul 2016 news). 

At SfN, Taylor Jay and Margaret Broihier at Case Western further elucidated how TREM2 affects microglia. Working with Landreth and Bruce Lamb at Indiana University, Indianapolis, Broihier compared APPPS1 mice with normal levels of TREM2 to mice that lack the protein. Microglia from the TREM2 knockouts expressed lower levels of several pro-inflammatory markers, such as IL-1β, IL-6, iNOS, TLR4, and TNFβ, and higher levels of anti-inflammatory markers such as Fizz1, Arg1, and TGFβ, supporting the idea that TREM2 promotes inflammation. TREM2 knockout microglia proliferated poorly and tended to die more readily than those with the receptor. That said, the presence or absence of TREM2 did not affect the cells’ ability to gobble up fluorescent beads, contradicting some previous studies, Broihier reported (see Jul 2014 webinar). 

If TREM2 helps trigger beneficial inflammation, could it be targeted therapeutically? Jennifer Gooch at the University of Kentucky, Lexington, working with Donna Wilcock there, investigated this question. She used a commercial anti-TREM2 antibody made by the San Francisco-based biotech company Alector that activated signaling through TREM2 and its co-receptor, DAP12. The antibody did not bind to the brains of TREM2 knockout mice, confirming its specificity. Gooch injected the antibody into the bilateral frontal cortices and hippocampi of APPPS1 mice. After 72 hours, she sacrificed the mice and examined their microglia. Antibody treatment had boosted the amount of pro-inflammatory, healing, and repair markers expressed by microglia, while roughly doubling immunoreactivity of CD11b, a marker for microglia. Amyloid deposits in the vicinity of the injection, as seen by immunohistochemistry, had fallen by one-third to one-half compared with mice that received a control antibody. This is comparable to the reduction in total Aβ seen after injecting an anti-Aβ antibody in the same regions, Wilcock noted (see Wilcock et al., 2003). Activating TREM2 with an antibody appears to stimulate the phagocytosis of Aβ, and might make a promising therapeutic, Gooch suggested. The researchers next plan to dose mice systemically with the anti-TREM2 antibody once weekly for several weeks, and examine the effects on behavior. “Our data indicate that TREM2 activation promotes amyloid clearance, but whether this results in a functional benefit remains to be determined,” Wilcock wrote to Alzforum.

Other speakers proposed different ways to stimulate microglia. Alex Vesling, in Town’s group at the University of Southern California, Los Angeles, turned to the toll-like receptor (TLR) signaling pathway. Aggregated Aβ binds TLRs, activating signaling through IL-1 receptor-associated kinases (IRAKs) and TNF-receptor associated factor 6 (TRAF6). This pathway switches on NF-κB and cranks up inflammation (see Sep 2009 news). The response can be inhibited by activation of IRAK-M, a protein found only in microglia and macrophages. Thus, IRAK-M dampens inflammation in microglia. Notably, IRAK-M has been reported to be dysregulated in aging and AD (see Cribbs et al., 2012). 

Vesling examined hippocampal lysates from postmortem AD brains, and found an excess of cleaved, non-functional IRAK-M, as well as an abundance of TRAF6, suggestive of increased inflammation. Vesling then transfected human microglial cultures with IRAK-MΔDD, a mutant form that mimics the cleaved product, before treating the cells for two or six hours with aggregated Aβ42. TRAF6 preferentially binds IRAK-MΔDD over the wild-type IRAK-M, suggesting the mutant should have a dominant-negative effect, Vesling noted. Indeed, in cells that expressed IRAK-MΔDD, TRAF6 expression rose, along with Aβ uptake and expression of pro-inflammatory cytokines including IL-6 and TNFα. Expression of anti-inflammatory cytokines such as TGFβ dropped. Inhibiting IRAK-M pumps up phagocytosis, Vesling concluded. He speculated that cleavage of IRAK-M in AD brain is a compensatory response to try to clear amyloid, but perhaps it occurs too late to help. Activation of TRAF6 may represent a pharmacologic target, he suggested.—Madolyn Bowman Rogers

Comments

Make a Comment

Comments on this content

  1. I would strongly agree that striking the right balance with stimulation of immunity to effect a beneficial response is critical.

    Another way of accomplishing this goal is simulation of microglia/macrophages via Toll-like receptors (TLRs, as also suggested by Dr. Vesling). We have focused on reconciling the age-related defects in immune cell function and tackling AD via Toll-like receptor 9 (TLR9). TLR9 recognizes the unmethylated CpG motifs present at high frequency in bacterial and viral DNA and at low frequency in human DNA. Oligodeoxynucleotides (ODNs) containing these unmethylated CpG motifs trigger cells that express TLR9 (including monocytes, plasmacytoid dendritic cells, and B cells) to mount an innate immune response. Several CpG ODNs have shown excellent safety profiles in humans and have been explored in numerous human clinical trials as anti-tumor, antimicroglial agents, and vaccine adjuvants.

    We have previously shown in Tg2576 and 3xTg-AD mouse models that stimulation of innate immunity via TLR9 ligand class B CpG ODN has the advantage of concurrently ameliorating Aβ and tau pathologies, in association with behavioral improvements. Very recently, in an Early Release article that appeared in the Journal of Neuroscience on December 15, 2016, we have shown that TLR9 is also highly effective at reduction of vascular amyloid pathology in TgSwDI mice, in association with cognitive benefits. Hence the elegant work discussed above and our own findings are supportive of the concept that simulating the immune system "just right" is effective at reducing all aspects of AD pathology.

    References:

    . Induction of toll-like receptor 9 signaling as a method for ameliorating Alzheimer's disease-related pathology. J Neurosci. 2009 Feb 11;29(6):1846-54. PubMed.

    . Amyloid β and Tau Alzheimer's disease related pathology is reduced by Toll-like receptor 9 stimulation. Acta Neuropathol Commun. 2014 Sep 2;2:101. PubMed.

    . Innate Immunity Stimulation via Toll-like Receptor 9 Ameliorates Vascular Amyloid Pathology in Tg-SwDI mice with Associated Cognitive Benefits. J Neurosci. 2016 Dec 15; PubMed.

  2. The above conversation neglects to consider the impact of microglial activation on other pathologies, particularly tau deposition. Over 15 years ago, lipopolysaccharide injected intracranially was demonstrated to rapidly clear some forms of amyloid (DiCarlo et al., 2001). However, the same manipulation in a tau mouse exacerbates the pathology (Lee et al., 2010). It is conceivable that the microglial activation provoked by amyloid in an aged brain may launch the tau aggregation response and lead to neurodegeneration. Amyloid alone appears relatively benign in human brain and murine models. The absence of tau pathology in amyloid models would miss this potentially critical response to these activating stimuli. Clinical trials with agents designed to manipulate microglia need to be carefully vetted preclinically, and application needs to judiciously consider disease stage before administration to trial participants.

    References:

    . Intrahippocampal LPS injections reduce Abeta load in APP+PS1 transgenic mice. Neurobiol Aging. 2001 Nov-Dec;22(6):1007-12. PubMed.

    . LPS- induced inflammation exacerbates phospho-tau pathology in rTg4510 mice. J Neuroinflammation. 2010;7:56. PubMed.

Make a Comment

To make a comment you must login or register.

References

News Citations

  1. Cytokine Takes Aβ Off the Menu for Microglia
  2. Soothing Neuroinflammation Quells Plaques in Mice
  3. Enter the New Alzheimer’s Gene: TREM2 Variant Triples Risk
  4. Barrier Function: TREM2 Helps Microglia to Compact Amyloid Plaques
  5. TREM2 Helps Phagocytes Gobble Up Aβ Coated in Antibodies
  6. New Pathways With Promise in AD—An Inflammatory Statement?

Research Models Citations

  1. TgCRND8
  2. APPPS1

Webinar Citations

  1. Mutations Impair TREM2 Maturation, Processing, and Microglial Phagocytosis

Paper Citations

  1. . Massive gliosis induced by interleukin-6 suppresses Abeta deposition in vivo: evidence against inflammation as a driving force for amyloid deposition. FASEB J. 2010 Feb;24(2):548-59. PubMed.
  2. . A human monoclonal IgG that binds aβ assemblies and diverse amyloids exhibits anti-amyloid activities in vitro and in vivo. J Neurosci. 2015 Apr 22;35(16):6265-76. PubMed.
  3. . Intracranially administered anti-Abeta antibodies reduce beta-amyloid deposition by mechanisms both independent of and associated with microglial activation. J Neurosci. 2003 May 1;23(9):3745-51. PubMed.
  4. . Extensive innate immune gene activation accompanies brain aging, increasing vulnerability to cognitive decline and neurodegeneration: a microarray study. J Neuroinflammation. 2012;9:179. PubMed.

Further Reading