Immunotherapeutic strategies for Alzheimer´s Disease

A primary goal of research on Alzheimer´s disease is to develop therapeutics able to prevent or reverse the cognitive decline of AD patients. As discussed in the previous section the most studied hypothesis emphasizes the neurodegenerative effect exerted by amyloid deposits. Based on the current knowledge provided by these studies, the development of anti-Aß therapeutics appears as a rational approach for treatment.

Present approaches are focused on (i) the modulation of Aß production, (ii) preventing Aß aggregation and (iii) clearance of soluble Aß and amyloid deposits from brain (see Figure 10). The attempt to reduce the production of Aß led to the

development of potent inhibitors that block the activity of ß- and γ-secretase.

Although the inhibiting compounds have been tested in mice and were effective in reducing Aß, testing in humans has been barely attempted due to the concern regarding potential side effects. Preventing the formation of Aß aggregates has been attempted by chelating the metal ions (Zn²⁺, Cu²⁺) reported to enhance Aß deposition [87-89].

The idea of using the ability of the immune system to produce specific antibodies that recognise soluble Aß or amyloid deposits and lead to their clearance from brain has gained increasing interest in recent years. Several immunotherapeutic strategies have been under investigation including active immunization with synthetic Aß, protofibrillar Aß assemblies or Aß peptide fragments conjugated to a carrier protein, and passive immunization with monoclonal Aß-specific antibodies [90, 91]. The studies carried out by B. Solomon and collaborators provided first evidence that antibodies recognizing Aß were effective in blocking the formation of amyloid fibrils in vitro [92], dissolving pre-existing amyloid fibrils [93] and preventing neurotoxicity of Aß fibrils. These observations performed in vitro were followed by the immunization of transgenic mice overexpressing APP with Aß. The first results of immunization

Figure 10: Schematic presentation of the therapeutic strategies aiming to prevent and reverse the pathological events leading to amyloid plaque formation.

transgenic mice, and reduction or even reversal of amyloid deposition if the immunization was performed in older animals [94]. A correlation between plaque reduction and the ability to perform memory tasks was not possible due to the observation that in PDAPP mice cognitive impairment precedes amyloid deposition [95]. However, improvements in cognitive function associated with plaque reduction were described in two parallel studies in which TgCRND8 transgenic mice were immunized with protofibrillar Aß42 [95] and APP/PS1 mice with Aß42 [96]. Both studies did not observe any adverse effect of the vaccine. On the basis of the promising preclinical findings, a first clinical trial was initiated in which 300 patients received active immunisation with AN1792 (Aß42 and adjuvants). Unfortunately, 6%

of the patients developed symptoms of meningoencephalitis and the trial was suspended [97]. At present, the inflammatory response encountered is attributed to the infiltration of the brain with activated T cells [98, 99]. However, the patients with high anti-Aß titers had significantly less deterioration of cognitive performance in the year following the clinical trial, than patients with little or no anti-Aß antibodies [100, 101].

In recent years, the immunotherapeutic studies have been concentrated on the development of passive immunization strategies. The intravenous administration of amyloid specific antibodies might offer potential advantages over the active immunization approach: (i) avoid the variability of the immune response across the individuals receiving the immunogen by providing known amounts of antibody of known epitope specificity; (ii) do not trigger T cell activation; and (iii) can be withdrawn if adverse reaction are encountered [102]. A number of controversies have arisen regarding the antibody specificity for passive immunization studies. While Bard et al. showed that only N-terminal domain antibodies are able to clear amyloid plaques [103], DeMattos et al. argue that an antibody to the Aß mid-domain with little reactivity to brain amyloid might be effective [104]. Alternatively, Morgan et al.

reported that passive administration of antibodies specific to a carboxy terminal domain of Aß was able to reverse cognitive deficits in transgenic mice.

Several mechanisms have been proposed to explain the therapeutic action of anti-amyloid antibodies (see Figure 11). The plaque breakdown hypothesis relies on the ability of a small amount of Aß-antibodies (0.1%) to cross the blood-brain barrier into

the CNS, bind the neuritic plaques and promote Fc-receptor-mediated phagocytosis of the plaques (Figure 11a). According to the results obtained by active immunization, antibodies able to recognize the Aß-plaques target the N-terminal domain of Aß. The epitope recognized by such “plaque-specific” antibodies was elucidated by our laboratory to comprise the Aß(4-10) sequence [16]. The peripheral sink hypothesis is supported by the experiments of DeMattos and coworkers and Morgan and colleagues. Peripheral administration of antibodies that bind monomeric Aß causes the transport of Aß from the CNS to plasma until the equilibrium is reached (see Figure 11b). A third possible mechanism for the action of anti-Aß antibodies suggests a catalytic transformation of Aß peptide into a structure less compatible with amyloid fibril formation (Figure 11c).

a) Plaque breakdown b) Peripheral sink c) Aggregation inhibitor

Brain Plasma

Neuritic plaque

Monomeric Aß Microglia

Anti-Aß antibodies

Brain Plasma

Neuritic plaque

Monomeric Aß Microglia

Anti-Aß antibodies

Figure 11: Schematic representation of proposed immunotherapeutic actions of anti-Aß antibodies (a) entry of anti-Aß antibodies to the brain could result in decoration of amyloid plaques by antibodies and subsequent removal by microglia activated through Fc receptor mechanisms; (b) antibodies to Aß act as a “sink” for Aß as it moves from the CNS to plasma; (c) antibodies to Aß may act to block interactions between monomeric Aß preventing aggregation.

A molecular understanding of the neuroprotective effect of the Aß-specific antibodies can be expected by the elucidation of the antigenic determinant targeted by the active antibodies as well as by the structural characterization of these antibodies. A fast and reliable tool for epitope identification is provided by the mass spectrometric methods in conjunction with affinity chromatography and proteolytic assays.