Therapeutic versus neuroinflammatory effects of passive immunization is dependent on Aβ/amyloid burden in a transgenic mouse model of Alzheimer's disease
© Minami et al; licensee BioMed Central Ltd. 2010
Received: 31 August 2010
Accepted: 28 September 2010
Published: 28 September 2010
Passive immunization with antibodies directed to Aβ decreases brain Aβ/amyloid burden and preserves memory in transgenic mouse models of Alzheimer's disease (AD). This therapeutic strategy is under intense scrutiny in clinical studies, but its application is limited by neuroinflammatory side effects (autoimmune encephalitis and vasogenic edema).
We intravenously administered the monoclonal Aβ protofibril antibody PFA1 to aged (22 month) male and female 3 × tg AD mice with intermediate or advanced AD-like neuropathologies, respectively, and measured brain and serum Aβ and CNS cytokine levels. We also examined 17 month old 3 × tg AD female mice with intermediate pathology to determine the effect of amyloid burden on responses to passive immunization.
The 22 month old male mice immunized with PFA1 had decreased brain Aβ, increased serum Aβ, and no change in CNS cytokine levels. In contrast, 22 month old immunized female mice revealed no change in brain Aβ, decreased serum Aβ, and increased CNS cytokine levels. Identical experiments in younger (17 month old) female 3 × tg AD mice with intermediate AD-like neuropathologies revealed a trend towards decreased brain Aβ and increased serum Aβ accompanied by a decrease in CNS MCP-1.
These data suggest that passive immunization with PFA1 in 3 × tg AD mice with intermediate disease burden, regardless of sex, is effective in mediating potentially therapeutic effects such as lowering brain Aβ. In contrast, passive immunization of mice with a more advanced amyloid burden may result in potentially adverse effects (encephalitis and vasogenic edema) mediated by certain proinflammatory cytokines.
A defining pathological hallmark of Alzheimer's disease (AD) is the accumulation of Aβ/amyloid deposits in brain. The generation and clearance of Aβ, produced from amyloid precursor protein (APP) by β- and γ-secretases, remain key therapeutic targets for transgenic AD mouse models and for clinical studies . Aβ naturally exists as a monomer or as aggregates including oligomers, protofibrils, and fibrils. Of the two most commonly generated isoforms of Aβ, Aβ40 and Aβ42, the latter aggregates more readily to form amyloid fibrils [2, 3]. Oligomers and protofibrils, the intermediate stages of Aβ fibril formation, are neurotoxic in culture and in animal models [4–6] and these aggregates may correlate better with AD severity than neuritic plaque density [7, 8].
Immunotherapy is an effective method of reducing brain Aβ levels and preserving or improving behavioral outcome measures in transgenic mouse models of AD. Initial studies utilized an active immunization approach. Peripheral injections of synthetic human Aβ prevented amyloid deposition, decreased CNS Aβ/amyloid burden, and ameliorated memory deficits in mice [9–11]. However, when this therapy was translated to individuals with AD, the pivotal clinical trial was terminated early due to the development of excessive neuroinflammation (autoimmune encephalitis) in 6% of treated individuals . Passive immunization with antibodies to Aβ may offer a safer, and reversible, alternative by circumventing T-cell responses associated with neuroinflammation in active immunization protocols. Passive immunization studies show similar efficacy in reducing brain Aβ/amyloid and preserving memory in transgenic mouse models of AD [13, 14], but the passive immunization approach is also limited by excessive neuroinflammation and vasogenic edema in a subset of treated individuals with AD.
Passive immunization may reduce brain Aβ by at least two distinct mechanisms that are not mutually exclusive: 1) microglial phagocytosis of Aβ/amyloid with increased cytokine production, which requires peripherally administered antibodies to cross the blood brain barrier , and 2) peripheral sequestration of Aβ by antibody in the blood -- referred to as the sink hypothesis . The Aβ antibody in the periphery may act as a sink by binding Aβ in the blood and promoting clearance of Aβ from the brain to the periphery, resulting in increased plasma Aβ following treatment. Others suggest, however, that peripherally-administered antibody also prolongs the half-life of Aβ in blood, and this explains the resultant increase in plasma levels. To examine these possibilities, it is important to measure CNS cytokine levels in addition to serum and brain Aβ levels following passive immunization. It is unknown whether immunotherapy may prove beneficial with high levels of CNS amyloid burden, or whether treatment may perhaps be limited to a subset of individuals in earlier disease stages. Alternatively, antibody dosage may be tailored to amyloid burden in order to minimize the risk of encephalitis and vasogenic edema. A recent clinical study reported increased vasogenic edema with higher antibody dosage and greater disease burden (as found in ApoE4-positive subjects) . This is the first study to suggest differential responses to passive immunization perhaps due to CNS amyloid burden in a transgenic mouse model of AD.
We examined whether passive immunization of 3 × tg AD mice with the monoclonal Aβ protofibril antibody PFA1, which is well-characterized but has not yet been tested as an immunotherapeutic [16, 17], was effective in lowering brain Aβ. The use of 3 × tg AD mice (which express three human mutant transgenes--APP, PS-1, and Tau--found in familial AD or a familial tauopathy) allowed us to investigate CNS Aβ levels as well as downstream effects of PFA1 immunization on tau phosphorylation. The 3 × tg AD mice are also known to have sex-specific differences in CNS amyloid burden, where females develop Aβ pathology at an earlier age and to a much greater extent than males . This allowed us to investigate the potential roles of sex and amyloid burden in response to passive immunotherapy. Passive immunization of intermediate but not more advanced stage 3 × tg AD mice resulted in decreased brain Aβ and increased serum Aβ, supporting the sink hypothesis. In the older female mice with advanced pathology we found no change in brain Aβ and an increase in CNS proinflammatory cytokine levels. Collectively, these data suggest that the potential benefits of passive (and active) immunization may be optimal in individuals with less pathology, and that treatment of individuals with more advanced CNS amyloid burden may engender potentially deleterious neuroinflammation. Passive (and active) immunization therapies may require tailoring to Aβ/amyloid disease burden, and may be considered high risk in subpopulations with advanced disease due to the development of neuroinflammation, encephalitis, and vasogenic edema. The critical mediators of neuroinflammation in response to passive immunization in tg AD mice or in man, however, remain unclear.
We obtained 3 × tg AD mice (breeding pairs) from Dr. Frank LaFerla (University of California, Irvine) to establish our colony. These mice were generated by co-microinjection of mutant APP (K670M/N671L) and tau (P301L) transgenes under the control of the Thy 1.2 promoter into mutant PS-1 (M146V) knock-in mice . We used 3 × tg AD mice at 21.6 ± 0.5 months of age for vehicle (PBS)-treated females (n = 6), PFA1-treated females (n = 7), vehicle-treated males (n = 5), and PFA1-treated males (n = 7). We also used 3 × tg AD mice at 16.8 ± 0.6 months of age for vehicle-treated females (n = 5) and PFA1-treated females (n = 5). Mice were sacrificed by cervical dislocation to eliminate anesthesia-mediated tau phosphorylation , and brains were snap-frozen in dry ice for biochemical analyses. All animal experiments were approved by the Institutional Animal Care and Use Committee at Georgetown University.
Antibodies and chemicals
The protofibril Aβ monoclonal antibody PFA1 was purified as described [16, 17]. The monoclonal antibody 6E10 was purchased from Invitrogen (Carlsbad, CA). Phospho-tau antibodies AT8 (Ser202/Thr205) and AT180 (Thr231) were purchased from Pierce Biotechnology (Rockford, IL), and Tau46 detecting total tau was purchased from Sigma-Aldrich (St. Louis, MO). Synthetic Aβ40 and Aβ42 were purchased from American Peptide (Sunnyvale, CA) and a fraction was dissolved to a concentration of 0.1 μg/μL ddH20 for gel loading for immunoblots.
Mouse treatments and plasma collection
Purified PFA1 (100 μg IgG/mouse) or vehicle control (PBS) was intravenously administered into the tail vein weekly for 4 weeks. Blood was collected from the facial vein 1 day prior to the first infusion and from the heart 1 week following the last injection -- at the time of sacrifice. EDTA-treated plasma was prepared, aliquoted, snap frozen, and stored at -80°C for biochemical assays.
Sample preparation and Aβ ELISAs
Brains were homogenized in a 10-fold volume of 50 mM Tris-HCl buffer, pH 7.6, containing 250 mM sucrose and protease and phosphatase inhibitors. This crude brain homogenate was used for immunoblot analysis.
Proteins for immunoblot analysis of tau were separated on a 4-15% gradient Tris-HCl SDS-PAGE gel and transferred to a PVDF membrane. Aβ samples were separated on a 10-20% gradient Tris-Tricine gel (BioRad, Hercules, CA). The membrane was blocked with 5% non-fat dry milk and probed with the appropriate primary antibody and HRP-coupled anti-mouse IgG secondary antibody (Jackson ImmunoResearch, West Grove, PA). Bands were visualized using a chemiluminescence kit (Pierce, Rockford, IL) and densitometrically quantified (Quantity One, BioRad).
Soluble Aβ was extracted in 0.4% diethylamine (DEA) as previously described . Briefly, crude 10% homogenate was mixed with an equal volume of 0.4% DEA, sonicated, and ultracentrifuged for 1 hour at 100,000 × g. The supernatant was collected and neutralized with 10% 0.5 M Tris base, pH 6.8. The resulting DEA fraction was used for soluble Aβ analyses. For analysis of 22 month old mice, total Aβ was analyzed from 10% crude brain homogenate mixed with 70% FA, which was then sonicated and ultracentrifuged for 1 hour at 100,000 × g. The insoluble fraction was calculated as the difference between total and soluble Aβ. For analysis of 17 month old mice, we undertook a more direct approach for measurement of insoluble Aβ by resuspending the remaining pellet following DEA fractionation in 70% FA, sonicating, and ultracentrifuging to yield the insoluble Aβ fraction. Total Aβ was calculated as the sum of measured soluble and insoluble Aβ. Sensitive and specific ELISAs to human Aβ1-40 and Aβ1-42 were purchased from IBL Transatlantic (Toronto, Canada) and conducted per the manufacturer's protocol.
Levels of inflammatory cytokines IL1β, TGFβ, TNFα, MCP-1, and IL4 were measured using Multiplex ELISA assays from Search Light Sample Testing Service (Pierce). Levels of SDF1 and RANTES were measured using ELISA kits from R & D Systems (Minneapolis, MN).
Immunoblots for PFA1 characterization were performed a minimum of three times. Immunoblots for phospho-tau and total tau quantification were performed a minimum of two times for all samples collected. All statistics were calculated using GraphPad Prism 5 software, using student's t-test with significance determined as p < 0.05. Descriptive statistics are displayed as mean ± S.E.M. Correlational analyses were performed on pairs of variables and the correlation is indicated as an R2 value within a 95% confidence interval, with p < 0.05.
PFA1 detects Aβ monomers and oligomers
Total brain Aβ is decreased in immunized 22 month old male mice
Plasma Aβ is increased in immunized 22 month old male mice
CNS TNFα and MCP-1 levels are increased in immunized 22 month old female mice, and unchanged in 22 month old male mice
TGFβ and SDF1 are negatively correlated with brain Aβ, while TNFα and MCP-1 are positively correlated
Immunized males show a decreased trend in phospho-tau
Immunized 17 month old female mice reveal a decreasing trend in brain Aβ
Plasma Aβ is increased in immunized 17 month old female mice
MCP-1 is decreased, and is positively correlated with brain Aβ, in immunized 17 month old female mice
Finally, we determined whether specific CNS cytokines were consistently correlated with brain Aβ levels, and found that MCP-1 decreased as soluble, insoluble, and total Aβ40 in female mice decreased (R2 = 0.45, p < 0.05; R2 = 0.615, p < 0.01, R2 = 0.764, p < 0.01, respectively) (Fig. 9B-D). This data is consistent with results found with the 22 month old mice.
Here, we examined the efficacy of a passive immunization paradigm using the monoclonal antibody PFA1 in reducing brain Aβ in a triple transgenic model of AD. We first demonstrate that the antibody binds to both Aβ40 and Aβ42 species, and binds to both oligomeric and monomeric forms (Fig. 1). Chronic passive immunization of 22 month-old 3 × tg AD mice resulted in a significant decrease in brain Aβ accompanied by an increase in serum Aβ in male mice with intermediate (less advanced) Aβ/amyloid pathology (Fig. 2, 3). However, immunization of 22 month-old female 3 × tg AD mice with advanced Aβ pathology had no effect on reducing brain Aβ while increasing levels of inflammatory cytokines TNFα and MCP-1 (Fig. 2, 3, 4). These effects were presumably pathology burden- and not sex-dependent, as immunization of 17 month-old female mice with intermediate pathology resulted in trends towards decreased brain Aβ, increased clearance of Aβ to the periphery, and a decrease in the inflammatory cytokine MCP-1 (Figs. 7, 8, 9).
We demonstrated a significant reduction in total Aβ42 in immunized 22 month-old male 3 × tg AD mice. Although effects on total Aβ40 or soluble Aβ40 or Aβ42 were not statistically different, all species of Aβ showed a consistent trend towards decreased Aβ following passive immunization. Similarly, 17 month-old female 3 × tg AD mice showed consistent, but non-significant, trends toward decreased Aβ of all forms. These results were in contrast to those observed in the 22 month-old females, where there was no evidence of decreased Aβ of any species, with a possible increase in soluble Aβ42. While our results suggest that there may be an effect of PFA1 passive immunization on reducing all forms of brain Aβ at intermediate pathological stages, it will be necessary to expand the size and scope of this study due to address this issue.
The peripheral sink hypothesis of Aβ clearance suggests that increasing Aβ-binding agents in the periphery enhances Aβ efflux from the brain [14, 22]. In our studies, we observed a concurrent increase in serum Aβ with a decrease in brain Aβ in 22 month-old male mice, and a concurrent increase in serum Aβ with a trend towards decreased brain Aβ in 17 month-old female mice. Our findings support the possibility that both passive and active immunization involve antibody-mediated clearance of Aβ from the brain to the periphery. This effect can occur independently of an antibody effect in the brain which may incite microglial phagocytosis. Aβ-binding agents, not limited to antibodies, such as heparin  and the Nogo receptor , are also effective in reducing brain Aβ. In addition, active immunization of an AD mouse model lacking the FcR, which is necessary for immune activation, still reduces brain Aβ , and deglycosylated antibodies, which do not bind the FcR, increase Aβ clearance to the periphery . These findings suggest that peripheral clearance of Aβ is a viable mechanism by which passive or active immunization may decrease brain Aβ levels.
Although passive immunization does not directly involve T-cell activation, it can still induce microglial activation and upregulation of inflammatory cytokines. Indeed, we observed that the inflammatory cytokine TNFα and chemokine MCP-1 were increased following immunization in 22 month-old female mice in the absence of any benefit on lowering brain Aβ. TNFα is increased in human AD brains  and is increased following antibody treatment in cultured cells . MCP-1 is a chemokine which is also upregulated in serum of AD patients and may play a role in early inflammatory events . The effects of microglial activation may be pathology-dependent, as TNFα induction in younger PS1xAPP mice results in microglial phagocytosis while activation at older ages results in cytotoxic effects . Thus, immune modulation at advanced pathology may increase potentially adverse inflammatory effects of microglial activation compared to a neuroprotective phagocytic Aβ/amyloid clearance effect found with lower disease burden.
In addition to the increased cytokine levels following passive immunization in mice with advanced pathology, we also observed significant positive correlations between increasing Aβ levels and increasing TNFα and MCP-1 levels, further supporting a pro-inflammatory role for these cytokines. In contrast, we observed an inverse correlation with other cytokines -- as Aβ levels increased, TGFβ and SDF1 decreased, implicating a beneficial role for these cytokines in mediating anti-Aβ effects. This hypothesis is supported by evidence that overexpressing TGFβ in hAPP mice results in decreased numbers of plaques  and reducing TGFβ signaling in mice results in increased Aβ accumulation and neurodegeneration . Expression of the chemokine SDF1 is decreased in early AD brains , suggesting that disregulation of its chemotactic signaling properties may either be an early symptom of AD or may contribute to later AD pathology.
In conclusion, our study is the first to establish pathology burden-dependent effects of passive immunization in an animal model of AD. Treatment at an intermediate pathological stage is more effective at reducing brain Aβ, while treatment at a more advanced stage induces neuroinflammatory effects. In addition, our findings support previous studies that passive immunization, as well as active immunization, results in the clearance of Aβ from the brain to the periphery. High levels of Aβ pathology may contribute to neurotoxic effects mediated by microglial inflammation, while immunomodulation at an intermediate stage may allow the larger pool of soluble Aβ to be sequestered into the periphery. These findings highlight the importance of considering disease burden in determining optimal passive immune timing and dosage in order to maximize therapeutic effects and minimize adverse effects when considering a human intervention.
amyloid precursor protein
transforming growth factor β
tumor necrosis factor α
monocyte chemotactic protein 1
stromal cell-derived factor 1
chemokine ligand 5, RANTES
We thank Dr. Paul Patterson, Jan Ko, and Susan Ou at Caltech for production of the PFA1. This work was supported by NIH grants NS059178 (SSM), R01 AG026478 (RST), and the Intramural Research Program of the NIH, National Institute on Aging (FB).
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