Three years ago in a high-profile paper, researchers led by Michal Schwartz at the Weizmann Institute of Science in Rehovot, Israel, deployed an immune-boosting approach pioneered for cancer in a mouse model of Alzheimer’s disease. The results appeared encouraging. The therapeutic antibodies—checkpoint inhibitors—reportedly improved cognition and reduced amyloid plaque in the animals. Now, a follow-up study claims that checkpoint blockade is similarly effective in models of dementia driven by accumulation of pathologic tau. Their results raise the tantalizing prospect that checkpoint inhibitors, several of which are FDA-approved, could be used to treat dementia regardless of its underlying pathology. The study appeared January 28 in Nature Communications.

  • In mouse models, immune checkpoint inhibitors clear amyloid, tau pathology, improve cognition.
  • Researchers are planning to test clinically.
  • Other groups tried to replicate, unsuccessfully.

Groundbreaking, if true. Unfortunately, the earlier work (Jan 2016 news) has not been replicated independently, despite efforts from multiple labs in academia and industry. In one effort, three pharma companies tried, and failed, to recreate the amyloid-clearing effects of antibodies targeting the programmed cell death receptor PD-1, as was first reported by Schwartz in 2016 (Latta-Mahieu et al., 2018). The replication paper’s senior author, Laurent Pradier of the French pharma company Sanofi, told Alzforum that Schwartz’s new results are interesting, but that independent research teams need to confirm and generalize them to other models and antibodies.

Others agree. "This is provocative data, though it comes on the heels of a highly controversial study from this group that has been difficult for others to reproduce," said Todd Golde, University of Florida, Gainesville.

Schwartz is confident in her data. She is working with a startup, ImmunoBrain Checkpoint, Inc., of Ness-Ziona, Israel, to develop checkpoint inhibitors to treat AD. In June 2017, the Danish pharmaceutical company Lundbeck announced it would partner on the project.

Checkpoint inhibitors block the immunosuppressive activity of the programmed cell death (PD-1) receptor and its ligands, PD-L1 and PD-L2. Neutralizing antibodies to PD-1 and PD-L1 stimulate anti-tumor responses; a handful have been approved by the FDA to treat various cancers. In their previous work, Schwartz and colleagues reported that in 5XFAD transgenic mice, injected PD-1 antibodies stimulated the peripheral immune system. As part of this response, interferon-γ produced at the choroid plexus ushered phagocytic monocytes from the blood into the brain. Antibody treatment reportedly reduced amyloid plaque load and improved cognition, even in mice with advanced pathology (Baruch et al., 2016). 

In the new study, first author Neta Rosenzweig tested the same strategy in mice that express the human tau gene with two mutations associated with frontotemporal dementia, K257T and P301S. The animals develop neurofibrillary tangle-like tau pathology and neuroinflammation that worsens with age (Rosenmann et al., 2008). By eight months, their short-term spatial memory has declined, signaled by their apparent inability to distinguish old territory from new in a T maze. One month after eight-month-old mice were injected with either an anti-PD-1 or anti-PD-L1 antibody, but not with isotype-control antibodies, the mice performed almost normally, suggesting an improvement in cognition. Normal mice treated with antibody had no such change in behavior. Treatment reportedly lowered levels of the inflammatory cytokine IL1β and reduced tau hyperphosphorylation and aggregation in the hippocampus and cortex. The treated mice also had more surviving cortical neurons than their untreated counterparts.

Similar to the treated 5XFAD mice, the antibody-treated tau mice had more monocytes cross from their blood to the brain. Single-cell RNA sequencing of these interlopers revealed expression of a collection of scavenger receptors absent from brain-resident microglia. One such receptor, Msr1, also known as Scara1, was previously implicated in clearance of Aβ (Frenkel et al, 2013). Msr1 was required for cognitive improvement: In mice irradiated to destroy bone marrow and deplete blood macrophages, and then reconstituted with bone marrow from wild-type mice, the PD-L1 antibody still improved cognition. But if the bone marrow came from Msr1 knockout mice, the antibody was ineffective.  

In their previous work with AD models, the scientists tested a PD-1 antibody. Here, they report the PD-L1 antibody is just as effective in those animals.

Together, the results of both the amyloid and the tau transgenic models suggest that a dementia treatment need not directly target the pathology in the brain, Schwartz told Alzforum. “This tells us we can target the peripheral immune system, thereby initiating a cascade of immunological events, which, in synergy with the inflammatory signaling from the brain, can lead to restoration of homeostasis and modification of brain pathology, and thus, would be applicable to diverse etiologies in patients,” she said.

As with a car, taking the brakes off the immune system can be dangerous. Checkpoint blockade comes with serious, and in rare cases even fatal, autoimmune side effects (Wang et al. 2018). Schwartz said she plans to use intermittent dosing. With short exposures sufficient to mobilize monocytes to the brain, her group reduced adverse immunological side effects in animal studies, she said.

At least five laboratories have tried to corroborate Schwartz’s 2016 paper. In the only published attempt, researchers from Sanofi, Janssen, and Lilly assessed the effect of checkpoint blockade on amyloid plaque load in their respective labs, using three different mouse models and three PD-1 antibodies, including the rat monoclonal employed by the Schwarz lab. Last fall, the FDA approved Sanofi/Regeneron's cemiplimab (Migden et al, 2018). Janssen and Lilly have investigational checkpoint inhibitors targeting PD-1 or PD-L1. The companies could substantially expand their drugs’ market if they were safe and effective in Alzheimer’s disease.

The pharma researchers confirmed checkpoint blockade in the mice’s periphery, and an increase in interferon-γ in one model. Alas they found no infiltration of the brain by myeloid cells, and in no model did they find a reduction in amyloid plaque load. They did not test cognition.

Pradier, who prefers the word “generalize” to “replicate” or “repeat,” told Alzforum he knows of at least one more company lab that also tried and failed to show amyloid clearance with checkpoint blockade, and assumes there are others. An academic investigator, who asked to remain anonymous, told Alzforum his lab, too, has been unable to confirm the Schwartz lab's findings.

Why the discrepancy? Different animals, antibodies, labs, disease stages? Reproducibility disputes frequently pit the original lab, which insists that replication conditions must be exactly identical, against labs trying to replicate, which argue that for a published effect to hold up in clinical development, it must be generalizable to similar models and other variations in experimental conditions. In other words, corroborating results should be obtained across slightly different experimental paradigms. The consensus is that if a tantalizing finding is finicky, it may survive peer review at a high-profile journal, but not the rigors of a drug development program.

Some commentators suggested that even variations in microbiota between one vivarium and another can lead to incongruent results in mouse studies of immune-related treatments. If so, Pradier and other investigators questioned if the PD-1 inhibitor approach would be robust enough to succeed in people, where such variability is the rule.

Schwartz attributed the disagreement in results to a lack of optimization of the treatment in the other labs, including dose calibration. The antibody’s affinity and half-life critically affect the regimen, she said. Her new study expands on the original single-dose work with data on additional models at multiple doses, she said. Schwartz also suggested that cognition be used as the primary readout, not amyloid clearance. “Since the antibodies are not directed at amyloid plaque, the primary dose calibration should be based on disease modification, as measured by cognitive performance,” she wrote to Alzforum (see full comment below).

Pradier said that for purposes of preclinical work, his and many other groups nowadays consider cognition a “soft endpoint” that needs to be backed up with some measurable molecular pathology response to therapy. That’s why the Sanofi, Janssen, and Lilly groups looked at plaques. Use of cognitive behavior outcomes in transgenic AD mice has diminished across pharma and academic labs in the aftermath of years of poor predictive validity from mice to human trials.

“We all wished it had worked,” Pradier told Alzforum. “Many companies have these antibodies. Hopefully, others will be able to generalize the results, and we’ll figure out conditions that could make it work,” he said.

He noted that the Schwartz lab injected rat or human antibodies into mice at high doses. These foreign proteins, in addition to checkpoint blockade, could contribute to peripheral immune responses and mobilization of innate immunity in brain. “It would be important that such potential ‘sensitizing/potentiating’ factors be elucidated in the future,” he and Andreas Ebneth of Janssen wrote to Alzforum (see full comment below).

Eti Yoles of ImmunoBrain Checkpoint, Schwartz’s startup, told Alzforum that the company has replicated the result with three different anti-PDL-1 antibodies. Schwartz said that Lundbeck reproduced the results in an independent study, where anti-PD-L1 reduced cognitive deficits and brain pathology in a different tau mouse. The companies are developing new PD-L1 targeting antibodies that Schwartz said are tailored to AD treatment. She estimated they will be ready to begin testing in people in 2020. Lundbeck declined to comment.

Last October, researchers at the University of Southampton, U.K., reported that genetically deleting PD-1 in a prion model of neurodegeneration did not recruit monocytes to the diseased brain and slightly worsened the behavioral phenotype (Obst et al., 2018). 

Richard Ransohoff, Third Rock Ventures, Boston, expressed strong reservations about immune checkpoint inhibitors for Alzheimer’s or other dementias. More broadly, he questions the value of animal models as predictors of clinical efficacy in neurodegeneration (Ransohoff, 2018). While genetic studies implicate immune responses in Alzheimer’s disease, he sees no evidence for involvement of the PD1/PDL1 pathway. “Success in one animal experiment, with failure to replicate, should mandate caution in moving forward to the clinic. This concern is all the more relevant because of the known risk of autoimmune complications caused by checkpoint inhibitors. With uncertain benefit and known risk, this approach doesn't seem actionable,” Ransohoff said.

Guillaume Dorothee, INSERM, Paris, also worries about the consequences of moving to the clinic too quickly with a strategy that boosts peripheral immune responses indiscriminately. “I think we shouldn’t take for granted that it would be beneficial. Immune responses are complex, and we need to further understand and validate what different immune cells are doing in the context of amyloid and tau pathology, before going ahead with broad-spectrum boosting approaches,” he said.

Dorothee and David Blum, INSERM, Lille, recently reported that depleting peripheral T cells—the opposite of what checkpoint inhibitors do—reduced neuroinflammation and memory loss in tau mice (Laurent et al., 2017). Dorothee and others also reported that selectively amplifying regulatory T cells, which are key controllers of immune responses, was beneficial in several mouse amyloid models (Dansokho et al., 2016; Baek et al., 2016Alves et al., 2017). 

“The Schwartz data are intriguing, but there are contradictory findings that remain to be clarified. We also need supporting data from human studies, to avoid the risk that disappointing clinical results would discourage further exploration of this promising avenue of treatment by immunomodulation,” Dorothee said.—Pat McCaffrey

Comments

  1. We read with great interest the very extensive publication from Dr. Schwartz’ group, which significantly expands on their previous work on checkpoint blockade for treating neurodegenerative diseases. Using both anti-PD-1 and now anti-PD-L1 antibodies, they demonstrate in a large series of animal studies prevention and reversion of cognitive deficits in the well-characterized 5xFAD amyloid transgenic model, and significant protection against cerebral pathology, together with the new finding of increased anti-inflammatory IL10 cytokine. Very interestingly, they extend these findings to tau transgenic animals and further analyze the possible underlying mechanisms.

    We have previously tried to generalize the checkpoint blockade concept to other amyloid transgenic mouse models, with different genetic backgrounds, and in three different animal facilities in pharmaceutical companies. While the checkpoint blockade was confirmed in the periphery in one case, collectively we could not observe a significant impact on brain amyloid pathology in any model (Latta-Mahieu et al., 2018). Experimental conditions cannot be identical (discussed in detail in Latta-Mahieu et al.), and according to Rosenzweig et al. it is indeed of outmost importance to “optimize treatments with the tested antibody in each animal model at each facility.”

    We note that in the two publications from Dr. Schwartz, xenogenic rat anti-PD1 or anti PD-L1 and human anti-PD-L1 antibodies are used and most recently at significant doses, 0.5 and 1.5 mg per mouse. Those, together with the concomitant checkpoint blockade, could participate in enhancing peripheral immune responses and the mobilization of innate immunity in brain. It would be important that such potential “sensitizing/potentiating” factors be elucidated in the future.

    We welcome and congratulate Dr. Schwartz’s group on this new publication and are looking forward to more teams in the world generalizing the checkpoint blockade concept for treating neurodegenerative diseases.

    References:

    . Systemic immune-checkpoint blockade with anti-PD1 antibodies does not alter cerebral amyloid-β burden in several amyloid transgenic mouse models. Glia. 2018 Mar;66(3):492-504. Epub 2017 Nov 14 PubMed.

  2. With regard to Latta-Mahieu et al., 2018, I am confident that the three groups that published this paper would have been able to repeat the positive effect of anti-PD-1 antibody on plaque burden, if they had calibrated the antibody and the treatment protocol in the mouse model that they used, and taken into account the different stages of the disease. The manuscript describes three single experiments, employing animal models that are different from those used in either of our papers. Below, I elaborate on the factors that could explain the failure to detect a decrease in plaque burden, the only read-out that was used to monitor the treatment effect.

    The target of the treatment with anti-PD-1 or anti-PD-L1 is not directed against β-amyloid plaques, but rather, this therapy evokes an immunological pathway that starts in the immune system and facilitates, in synergy with inflammatory cues coming from the brain, the recruitment of immune cells to the brain, together leading to multiple effects that ultimately result in cognitive improvement and neuroprotection. Accordingly, the beneficial effect of the treatment on disease modification is an outcome of the modulation of many processes that contribute to disease escalation; these include, but are not limited to, the clearance of amyloid plaques. In contrast to a treatment that is specifically directed against amyloid pathology, with the primary goal of reducing plaques, and which may or may not affect cognitive performance, in the case of immune checkpoint blockade, the beneficial effect is an outcome of several events that are affected by the therapy, the primary one being immune modulation.  

    Our first paper (Baruch et al., 2016) introduced the concept of adopting immune checkpoint blockade for treating AD, by reporting results of a single dose of anti-PD-1 antibody in 5xFAD mice. In order to generalize the phenomenon, further experiments were needed, including those addressing the mechanism, optimization of the dose/regimen, and testing in additional mouse models, which are all presented in our most recent publication (Rosenzweig et al., 2019). 

    We found that the effect of the antibody is dependent on the dose, and its affinity. Moreover, we found that elevation of IFN-γ, indicating peripheral engagement, is an essential response to the treatment, but is not sufficient. Thus, dosages that were sufficient to induce a peripheral response were insufficient to evoke a central one. In line with that observation, we found that the peripheral immune response can be evoked in wild-type animals as well, though there is no effect on monocyte infiltration and immune activity in their brains, suggesting that IFN-γ is important, but is not a valid marker of efficacy on disease progression of this treatment. In addition, in animal models of dementia, which show no plaque burden, treatment using the correct dose and timing resulted in improved cognition and reduced Tau pathology.

    In Latta-Mathieu et al., 2018, results from three different facilities were presented, which summarize three single experiments. Two of the three experiments reported used Fc chimeras (mIgG1) derived from the RPM1-14 anti-PD-1 antibody; importantly, it was shown that the chimeric Fc region significantly affects the pharmacokinetic profile of such antibodies (Ober et al., 2001), and obviously requires adjustment of the therapeutic dose/regimen. The only experiment which used the same antibody that our team used (Baruch et al., 2016) was carried out by Janssen using the commercially available rat IgG2a anti-PD-1 from BioXcell (clone RMP1-14; Catalog number, BE0146), but was tested on the ThyAPP/PS1 mouse model.

    In light of the above, we suspect that the lack of amyloid-β plaque clearance in all the experiments described in Latta-Mahieu et al., 2018, indicates that the authors, using a single dose, were working under sub-optimal experimental conditions. Based on our understanding of the mechanism, we are currently establishing the nature of the immunological marker of the response induced by the antibody that most closely correlates with treatment efficacy in Alzheimer’s disease modification; such a marker could be used to predict a response leading to disease modification. Until these markers are disseminated, and since the antibodies are not directed at amyloid plaque, the primary dose calibration in any mouse model or facility should be based on disease modification as measured by cognitive performance.

    References:

    . Systemic immune-checkpoint blockade with anti-PD1 antibodies does not alter cerebral amyloid-β burden in several amyloid transgenic mouse models. Glia. 2018 Mar;66(3):492-504. Epub 2017 Nov 14 PubMed.

    . PD-1 immune checkpoint blockade reduces pathology and improves memory in mouse models of Alzheimer's disease. Nat Med. 2016 Feb;22(2):135-7. Epub 2016 Jan 18 PubMed.

    . PD-1/PD-L1 checkpoint blockade harnesses monocyte-derived macrophages to combat cognitive impairment in a tauopathy mouse model. Nat Commun. 2019 Jan 28;10(1):465. PubMed.

    . Differences in promiscuity for antibody-FcRn interactions across species: implications for therapeutic antibodies. Int Immunol. 2001 Dec;13(12):1551-9. PubMed.

  3. This paper is interesting and extends the group's previous findings. The authors show PD-1 or PD-L1 blockade improved murine AD and tauopathy models. Interestingly, they found that macrophages expressing scavenger receptors are involved in this process. However, the relationship between the PD-1/PD-L1 system, which works in T cells, and macrophage/microglial scavenger receptors has not been well-elucidated. To extend this effect observed in mice to the clinic, it is necessary to clarify the detailed mechanism of how PD-1/PD-L1 blockade reduces plaque burden. It must be cautioned that this treatment may promote neuroinflammation, like multiple sclerosis.

  4. Harnessing the peripheral immune system as a neuroprotective therapeutic approach to Alzheimer's disease: Discussion regarding pathways and mechanism

    Immunotherapy targeting the programmed death protein (PD)-1/PD-L1 pathway was shown to be effective in reducing disease manifestations as measured by cognitive performance and pathology in mouse models of AD and tauopathy (Baruch et al., 2016; Rosenzweig et al., 2019). The results, much as they were novel and provocative, were also met with skepticism, and evoked controversy when another group attempted to replicate these results and failed to show reduction in amyloid plaque deposition, as a single readout (Latta-Mahieu et al., 2018). Nevertheless, recently published papers from additional groups support the role of the peripheral immune system in containing AD (He et al., 2020; Yang et al., 2020), and further demonstrate that treatment directed at the peripheral immune system activates a process of repair, leading to disease modification and cognitive improvement in additional mouse models. These and other studies, cited below, are used here as a platform to explain the failure to reduce amyloid plaque burden as a single readout (Latta-Mahieu et al., 2018). 

    In the recent study by He and colleagues, systemic injection of IFN-γ in APP/PSN1 mice resulted in recruitment of monocyte-derived macrophages, enhanced cognitive performance, and reduction of disease pathology (He et al., 2020). Yang and colleagues showed that, in the APP/PSN1 mouse model, elevation of systemic immunosuppressive mechanisms (regulatory T cells) exacerbates disease progression, which could be overcome by influenza vaccination (Yang et al., 2020). Elevated immune suppression, as well as the negative effect of systemic regulatory T cells shown in this study (Yang et al., 2020), were previously documented in 5xFAD mice as well (Baruch et al., 2015). The effects of systemic IFN-γ injection on the brains of AD mice (He et al., 2020) are also consistent with the effect of short-term depletion of regulatory T-cells (Baruch et al., 2015) or of blocking the PD-1/PD-L1 pathway (Baruch et al., 2016; Rosenzweig et al., 2019), shown to be mediated through IFN-γ-producing T cells (Baruch et al., 2016). In AD, the IFN-γ-producing T cells facilitate recruitment of monocyte-derived macrophages to the diseased brain (Baruch et al., 2016; Rosenzweig et al., 2019). 

    Yang’s results are also consistent with the observed effect of immunization with Bacillus Calmette-Guérin (BCG) in APP/PS1 mice, which reduced the levels of regulatory cells, resulting in a 10-fold increase in the serum levels of IL-2, and a fivefold increase in IFN-γ (Zuo et al., 2017). Also in line with these observations, bladder cancer patients treated with BCG were significantly less likely to develop AD at any age compared with patients who were not treated with this modality (Gofrit et al., 2019). 

    In PD-1 or PD-L1 blockade (Baruch et al., 2016Rosenzweig et al., 2019), similarly to the new studies, described above (He et al., 2020; Yang et al., 2020), the effect of activation of the immune system was shown to be mediated, at least in part, through monocyte-derived macrophages. Monocyte-derived macrophages were repeatedly shown in mouse models of AD and dementia to be beneficial in coping with the disease, by reducing pathological manifestations and enhancing cognitive performance (Baruch et al., 2015; El Khoury et al., 2007; Naert and Rivest, 2011; Simard et al., 2006; Butovsky et al., 2007; Butovsky  et al., 2006; Koronyo et al., 2015; Koronyo-Hamaoui et al., 2009; Schwartz and Shechter, 2010; Greenhalgh et al., 2018; Dionisio-Santos et al., 2019; Li et al., 2020). Notably, such monocyte-derived macrophages, which were often referred to as “monocyte-derived microglia” (Simard et al., 2006; Mildner et al., 2007) or “dendritic-like cells” (Butovsky  et al., 2006), display activities distinct from those of the activated resident microglia (Yamasaki et al., 2014; Thériault  et al., 2015; Ajami et al., 2018). Thus, for example, the beneficial effect observed following blocking of PD-L1/PD-1 pathway was shown to be dependent upon enhanced recruitment to the brain of monocyte-derived macrophages expressing the scavenger receptor, MSR1 (Baruch et al., 2016; Rosenzweig et al., 2019). Myeloid cells expressing MSR1 were shown to be able to remove soluble Aβ (Frenkel et al., 2013), which includes the oligomers, the neurotoxic form of misfolded proteins (Shankar et al., 2007). Removal of these forms was shown to have a direct impact on cognitive performance and synaptic activity (Li et al., 2020), arguing in favor of boosting recruitment of immune cells to the brain (Baruch et al., 2016; Rosenzweig et al., 2019). 

    Recent studies using Trem2-/-5xFAD mice clearly show that cognitive impairment is correlated with the level of soluble Aβ, rather than with levels of insoluble amyloid deposits and plaques, and that Trem2-expressing cells mostly affect plaque pathology, but not soluble oligomers nor cognition, in mice over 6 months of age (Griciuc et al., 2019). 

    Taken, together, the reported apparent failure of anti PD-L1 treatment to show an effect on plaque burden, as a single, acute readout (Latta-Mahieu et al., 2018), can be explained based on the mechanism described above. According to our results, we propose that the effect of anti-PD-L1 or anti-PD-L1 treatment is mediated via monocyte-derived macrophages, which contribute to disease modification by removal of soluble Aβ that contain the neurotoxic oligomers, with effect on plaques or insoluble Aβ representing a secondary outcome, as reduction of Aβ oligomers should result in a fewer or smaller plaques over time.

    Accordingly, measuring plaque burden as a single readout for a treatment that is not directed at plaques is an unreliable marker of treatment effects (Latta-Mahieu et al., 2018). In all other studies in which the immune system was harnessed to treat AD, multiple parameters were measured, as summarized above. In contrast to the measurement of plaques, reduction of soluble toxic oligomers measured by biochemical assays, as well as improvement in cognitive performance, could be expected to be observed following a treatment that targets the PD-1/PD-L1 pathway (Baruch et al., 2016; Rosenzweig et al., 2019) or any similar treatment (He et al., 2020; Yang et al., 2020). Moreover, measuring plaques as a sole readout following PD-1 blockade (Latta-Mahieu et al., 2018) or of any treatment that does not directly target amyloid plaques, provides a limited measure of the efficacy, while soluble Aβ is a valid measure of disease modification, with likely relevance to the clinic.

    To date, treatment with anti-PD-L1 antibody has been repeated in five different mouse models of AD and tauopathy, demonstrating improvement of cognitive performance, reduction via monocyte-derived macrophages of the soluble forms of misfolded proteins, measured biochemically in the brain and in the CSF, and skewing the brain toward an anti-inflammatory state, with an increase in the presence of FoxP3+ regulatory T cells (unpublished data; manuscripts in preparation).

    Based on the data emerging from the novel concept of targeting the PD-1/PD-L1 pathway as an immune-therapy for AD and dementia, a biopharmaceutical company, ImmunoBrain Checkpoint, Inc. (IBC), was founded in 2015. IBC has engineered a proprietary anti-PD-L1 antibody, tailored to the specific therapy mechanism of activity in AD (which is distinct from that in cancer), with a high efficacy and superior safety profile in terms of risk of triggering an adverse autoimmune response (unpublished data). The antibody is currently in production scale-up in order to be ready for first in human clinical trial in 2021. Beyond testing of safety, markers have been established to determine antibody activity mechanism of action in the patient’s blood and CSF.

    References:

    . PD-1 immune checkpoint blockade reduces pathology and improves memory in mouse models of Alzheimer's disease. Nat Med. 2016 Feb;22(2):135-7. Epub 2016 Jan 18 PubMed.

    . PD-1/PD-L1 checkpoint blockade harnesses monocyte-derived macrophages to combat cognitive impairment in a tauopathy mouse model. Nat Commun. 2019 Jan 28;10(1):465. PubMed.

    . Systemic immune-checkpoint blockade with anti-PD1 antibodies does not alter cerebral amyloid-β burden in several amyloid transgenic mouse models. Glia. 2018 Mar;66(3):492-504. Epub 2017 Nov 14 PubMed.

    . Intraperitoneal injection of IFN-γ restores microglial autophagy, promotes amyloid-β clearance and improves cognition in APP/PS1 mice. Cell Death Dis. 2020 Jun 8;11(6):440. PubMed.

    . Influenza vaccination in early Alzheimer's disease rescues amyloidosis and ameliorates cognitive deficits in APP/PS1 mice by inhibiting regulatory T cells. J Neuroinflammation. 2020 Feb 19;17(1):65. PubMed.

    . Breaking immune tolerance by targeting Foxp3(+) regulatory T cells mitigates Alzheimer's disease pathology. Nat Commun. 2015 Aug 18;6:7967. PubMed.

    . Immunization with Bacillus Calmette-Guérin (BCG) alleviates neuroinflammation and cognitive deficits in APP/PS1 mice via the recruitment of inflammation-resolving monocytes to the brain. Neurobiol Dis. 2017 May;101:27-39. Epub 2017 Feb 9 PubMed.

    . Bacillus Calmette-Guérin (BCG) therapy lowers the incidence of Alzheimer's disease in bladder cancer patients. PLoS One. 2019;14(11):e0224433. Epub 2019 Nov 7 PubMed.

    . Ccr2 deficiency impairs microglial accumulation and accelerates progression of Alzheimer-like disease. Nat Med. 2007 Apr;13(4):432-8. PubMed.

    . Hematopoietic CC-chemokine receptor 2-(CCR2) competent cells are protective for the cognitive impairments and amyloid pathology in a transgenic mouse model of Alzheimer's disease. Mol Med. 2011 Nov 29; PubMed.

    . 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.

    . Selective ablation of bone marrow-derived dendritic cells increases amyloid plaques in a mouse Alzheimer's disease model. Eur J Neurosci. 2007 Jul;26(2):413-6. PubMed.

    . Glatiramer acetate fights against Alzheimer's disease by inducing dendritic-like microglia expressing insulin-like growth factor 1. Proc Natl Acad Sci U S A. 2006 Aug 1;103(31):11784-9. PubMed.

    . Therapeutic effects of glatiramer acetate and grafted CD115⁺ monocytes in a mouse model of Alzheimer's disease. Brain. 2015 Aug;138(Pt 8):2399-422. Epub 2015 Jun 6 PubMed.

    . Attenuation of AD-like neuropathology by harnessing peripheral immune cells: local elevation of IL-10 and MMP-9. J Neurochem. 2009 Dec;111(6):1409-24. PubMed.

    . Systemic inflammatory cells fight off neurodegenerative disease. Nat Rev Neurol. 2010 Jul;6(7):405-10. PubMed.

    . Peripherally derived macrophages modulate microglial function to reduce inflammation after CNS injury. PLoS Biol. 2018 Oct;16(10):e2005264. Epub 2018 Oct 17 PubMed.

    . Exploiting microglial and peripheral immune cell crosstalk to treat Alzheimer's disease. J Neuroinflammation. 2019 Apr 5;16(1):74. PubMed.

    . Activated Bone Marrow-Derived Macrophages Eradicate Alzheimer's-Related Aβ42 Oligomers and Protect Synapses. Front Immunol. 2020;11:49. Epub 2020 Jan 31 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.

    . Differential roles of microglia and monocytes in the inflamed central nervous system. J Exp Med. 2014 Jul 28;211(8):1533-49. Epub 2014 Jul 7 PubMed.

    . The dynamics of monocytes and microglia in Alzheimer's disease. Alzheimers Res Ther. 2015;7(1):41. Epub 2015 Apr 15 PubMed.

    . Single-cell mass cytometry reveals distinct populations of brain myeloid cells in mouse neuroinflammation and neurodegeneration models. Nat Neurosci. 2018 Apr;21(4):541-551. Epub 2018 Mar 5 PubMed.

    . Scara1 deficiency impairs clearance of soluble amyloid-β by mononuclear phagocytes and accelerates Alzheimer's-like disease progression. Nat Commun. 2013;4:2030. PubMed.

    . Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway. J Neurosci. 2007 Mar 14;27(11):2866-75. PubMed.

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  5. Reproduction, Replication, and Responsibility

    I have followed with interest the correspondence that began with this report, which reviewed the work of Michal Schwartz and colleagues leveraging the PD-1/PD-L1 pathway, an important regulator of immune function, to provide neuroprotection in rodent models of amyloidopathy and tauopathy, and the subsequent commentary.

    In their Alzforum comments posted on 5 Feb 2019, Laurent Pradier and Andreas Ebneth, representatives of competing pharmaceutical companies, reported an apparent failure to replicate the originally reported rodent efficacy results in three different animal models of cerebral amyloidosis (Latta-Mahieu et al., 2018). Pradier and Ebneth admit that “experimental conditions cannot be identical,” and this was indeed the case. In the Latta-Mahieu paper, three different mouse strains were used, but interestingly, none of these groups used the 5xFAD strain used by Baruch et al. (2016) and by Rosenzweig et al. (2019) as an independent control. Thus, comparability of disease severity could not be established between their experiments and those of Rosenzweig et al. or Baruch et al. The antibodies used were not identical, and the doses used were around 60 percent of the maximally effective dose in the Baruch paper (Figure 2).

    Pradier et al. comment on the need for replication across multiple animal models to support validity of a given therapeutic antibody approach. In general, most amyloid antibodies have been tested in a single animal model; i.e. that which is favored in the laboratories of the company sponsoring their development. When tested outside the originating company, data is not always confirmatory, as in the case of gantenerumab, which was originally studied in APP751(Swedish)xPS2(N141I) transgenic mice (Bohrmann et al., 2012), and crenezumab (Adolfsson et al., 2012). Both these antibodies failed to clear plaque from the brains of Tg2576 mice in one study (Fuller et al., 2015). Interestingly, the study of Fuller et al. was also supported at least in part by a competing pharmaceutical company. On the other hand, Xing et al. (2020) recently published a study that replicated key findings from the Baruch et al., 2016, paper, this time using an APP-PS1 model and the same commercially available anti-PD1 tool antibody used by the Schwartz group.

    At the recent AAIC meeting, Baruch et al., now working at the biopharmaceutical company Immunobrain Checkpoint, presented results describing further experiments with a different anti-PD-L1 antibody in several rodent models (Baruch et al., 2020). Of note, the antibody used had been modified to ablate Fc effector function and carried a second modification that shortened its pharmacokinetic half-life.

    Baruch et al. confirmed efficacy on cognitive performance in 5xFAD mice, as well as two transgenic tauopathy models. Among other findings reported on the poster were (1) dose-dependent pharmacodynamic effects on peripheral immune markers, and (2) prolonged efficacy in a longitudinal study in DM-hTAU (K257T/P301S) transgenic mice. Treatment efficacy in this intermittent administration regimen was found to be effective for at least five months from treatment initiation, and persisted for at least eight weeks after the last injection. These results are consistent with the hypothesis that immune checkpoint inhibition activates a process of repair that is common to several experimental models independently of the underlying proteinopathy. Since most neurodegenerative disorders are believed to represent “multi-proteinopathies” (Robinson et al., 2018), this observation could augur well in predicting success going forward to the clinic.  

    Importantly, the authors demonstrated that the modifications made to the antibody’s backbone significantly reduced its propensity to precipitate early onset of autoimmune diabetes in the well-known NOD mouse model susceptible to autoimmune diabetes. This finding could prove important for future clinical trials in that it may indicate that similar modifications incorporated into the anti-human PD-L1 antibody (IBC-Ab002), currently being readied for clinical trials, would translate to an advantageous safety profile in terms of immune-related adverse effects compared to anti-PD-L1 antibodies currently in the clinic.

    In the original Alzforum commentary, reference was made to the paper by Richard Ransohoff titled “All (animal) models (of neurodegeneration) are wrong. Are they also useful?” (Ransohoff et al., 2018). In his insightful review, Ransohoff points out that our current animal models have been woefully inadequate in predicting the outcome of human clinical trials. However, he concludes by stating that: “… animal models remain extraordinarily valuable for investigating biological processes. In the drug development process, these models can also serve as the preferred testing platform for prospective pharmacodynamic biomarkers.”  

    We as a field need to come to grips with when “perfect is the enemy of good enough” and, recognizing the limits of animal model testing, agree that internally consistent datasets that (1) yield testable hypotheses and (2) biomarker findings reflecting mechanism of action in the animals that can be translated into the clinic, and (3) suggest acceptable risk-benefit in patients, can and should be tested responsibly in human clinical trials.

    Disclosure: I am a paid advisor to, and shareholder in, Immunobrain Checkpoint, and am acting as the study director for the trial mentioned here, which received a 2020 Alzheimer’s Association Part the Clouds Gates Partnership grant. 

    References:

    . Systemic immune-checkpoint blockade with anti-PD1 antibodies does not alter cerebral amyloid-β burden in several amyloid transgenic mouse models. Glia. 2018 Mar;66(3):492-504. Epub 2017 Nov 14 PubMed.

    . PD-1 immune checkpoint blockade reduces pathology and improves memory in mouse models of Alzheimer's disease. Nat Med. 2016 Feb;22(2):135-7. Epub 2016 Jan 18 PubMed.

    . PD-1/PD-L1 checkpoint blockade harnesses monocyte-derived macrophages to combat cognitive impairment in a tauopathy mouse model. Nat Commun. 2019 Jan 28;10(1):465. PubMed.

    . Gantenerumab: A Novel Human Anti-Aβ Antibody Demonstrates Sustained Cerebral Amyloid-β Binding and Elicits Cell-Mediated Removal of Human Amyloid-β. J Alzheimers Dis. 2012;28(1):49-69. PubMed.

    . An effector-reduced anti-β-amyloid (Aβ) antibody with unique aβ binding properties promotes neuroprotection and glial engulfment of Aβ. J Neurosci. 2012 Jul 11;32(28):9677-89. PubMed.

    . Comparing the efficacy and neuroinflammatory potential of three anti-abeta antibodies. Acta Neuropathol. 2015 Nov;130(5):699-711. Epub 2015 Oct 3 PubMed.

    . Influenza vaccine combined with moderate-dose PD1 blockade reduces amyloid-β accumulation and improves cognition in APP/PS1 mice. Brain Behav Immun. 2020 Sep 19; PubMed.

    . Neurodegenerative disease concomitant proteinopathies are prevalent, age-related and APOE4-associated. Brain. 2018 Jul 1;141(7):2181-2193. PubMed.

    . All (animal) models (of neurodegeneration) are wrong. Are they also useful?. J Exp Med. 2018 Dec 3;215(12):2955-2958. Epub 2018 Nov 20 PubMed.

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References

News Citations

  1. Cancer Drug Clears Plaque, Improves Mouse Memory

Research Models Citations

  1. 5xFAD (B6SJL)

Mutations Citations

  1. MAPT K257T

Paper Citations

  1. . Systemic immune-checkpoint blockade with anti-PD1 antibodies does not alter cerebral amyloid-β burden in several amyloid transgenic mouse models. Glia. 2018 Mar;66(3):492-504. Epub 2017 Nov 14 PubMed.
  2. . PD-1 immune checkpoint blockade reduces pathology and improves memory in mouse models of Alzheimer's disease. Nat Med. 2016 Feb;22(2):135-7. Epub 2016 Jan 18 PubMed.
  3. . A novel transgenic mouse expressing double mutant tau driven by its natural promoter exhibits tauopathy characteristics. Exp Neurol. 2008 Jul;212(1):71-84. PubMed.
  4. . Scara1 deficiency impairs clearance of soluble amyloid-β by mononuclear phagocytes and accelerates Alzheimer's-like disease progression. Nat Commun. 2013;4:2030. PubMed.
  5. . Fatal Toxic Effects Associated With Immune Checkpoint Inhibitors: A Systematic Review and Meta-analysis. JAMA Oncol. 2018 Dec 1;4(12):1721-1728. PubMed.
  6. . PD-1 Blockade with Cemiplimab in Advanced Cutaneous Squamous-Cell Carcinoma. N Engl J Med. 2018 Jul 26;379(4):341-351. Epub 2018 Jun 4 PubMed.
  7. . PD-1 deficiency is not sufficient to induce myeloid mobilization to the brain or alter the inflammatory profile during chronic neurodegeneration. Brain Behav Immun. 2018 Oct;73:708-716. Epub 2018 Aug 4 PubMed.
  8. . All (animal) models (of neurodegeneration) are wrong. Are they also useful?. J Exp Med. 2018 Dec 3;215(12):2955-2958. Epub 2018 Nov 20 PubMed.
  9. . Hippocampal T cell infiltration promotes neuroinflammation and cognitive decline in a mouse model of tauopathy. Brain. 2017 Jan;140(Pt 1):184-200. Epub 2016 Nov 5 PubMed.
  10. . Regulatory T cells delay disease progression in Alzheimer-like pathology. Brain. 2016 Apr;139(Pt 4):1237-51. Epub 2016 Feb 1 PubMed.
  11. . Neuroprotective effects of CD4+CD25+Foxp3+ regulatory T cells in a 3xTg-AD Alzheimer's disease model. Oncotarget. 2016 Oct 25;7(43):69347-69357. PubMed.
  12. . Interleukin-2 improves amyloid pathology, synaptic failure and memory in Alzheimer's disease mice. Brain. 2017 Mar 1;140(3):826-842. PubMed.

External Citations

  1. FDA approved

Further Reading

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Primary Papers

  1. . PD-1/PD-L1 checkpoint blockade harnesses monocyte-derived macrophages to combat cognitive impairment in a tauopathy mouse model. Nat Commun. 2019 Jan 28;10(1):465. PubMed.