After enrichment by immobilized Aβ chromatography, Aβ-binding IgG autoantibodies from healthy humans impeded formation of Aβ aggregates and dissolved preformed Aβ fibrils [21
]. Administration of IVIG to AD patients at a cumulative dose of hundreds of grams over several months has provided preliminary evidence of reduced cognitive decline accompanied by transiently increased blood Aβ and decreased cerebrospinal Aβ levels [22
A firm mechanistic basis for the potential benefit of reversibly binding Aβ antibodies present in IVIG is not available. Information gleaned from Aβ-binding humanized monoclonal antibodies is relevant to interpreting the IVIG effects. Studies in Aβ-overexpressing transgenic mice have provided unambiguous evidence for the ability of the monoclonal antibodies to clear brain Aβ deposits and improve cognition [7
]. Like the monoclonal antibodies, small amounts of Aβ-binding IgGs purified from human blood appear to cross the BBB [23
], permitting conception of local antibody-facilitated Aβ clearance. The phase 2 trial of intravenously infused bapineuzumab, a monoclonal IgG to the Aβ N terminus, revealed a modest, reduced risk of cognitive decline in AD patients who were negative for the ApoE4 allele, a genetic trait associated with delayed AD onset [7
]. In another phase 2 trial of an intravenously administered monoclonal IgG directed to an epitope in the middle region of Aβ (Eli Lilly and Co., Indianapolis, IN, USA), increased Aβ levels in cerebrospinal fluid and decreased Aβ levels in peripheral blood of AD patients were noted [7
]. To the extent that the increase of peripheral Aβ is attributable to a release from the brain peptide stores, the antibody may exert a favorable effect.
Safety concerns have been raised about Aβ-binding antibodies [7
]. Mouse studies indicate that clearance of antibody-Aβ immune complexes through the microglial uptake pathway holds the risk of inflammatory mediator release. Immune complex deposition in the vascular walls may cause microbleeds. There was no evidence for an unacceptable inflammatory reaction in the monoclonal IgG human trials, but a dose-limiting incidence of vasogenic edema was evident in magnetic resonance imaging (MRI) tests after bapineuzumab administration, an effect that suggests vascular dysfunction (). Antibodies that do not permeate into the brain should be free of this side effect. The safety profile of IVIG in AD patients remains to be examined by predictive methods such as MRI, but no untoward effects were reported in a recently published trial on eight patients [22
]. From these results, there is cautious support for passive AD immunotherapy using Aβ-binding antibodies. However, there is also concern that their therapeutic efficacy may not be sufficiently robust and that side effects are possible.
Provided that the cleavage of Aβ is sufficiently specific, clearance of Aβ by catalytic antibodies should occur with many of the advantages and none of the disadvantages of Aβ-binding antibodies. Because catalysts that enter the brain will not form long-lived immune complexes, the risk of vascular immune complex deposition and microglial release of inflammatory mediators occurring as a result of Fc-receptor mediated immune complex uptake is minimized (). If the rate of Aβ cleavage is sufficiently rapid, any reduction in Aβ clearance due to loss of the immune complex uptake pathway is inconsequential. Similarly, antibodies bound via the Fc receptor on the brain side of the BBB will cleave Aβ, and the process of antibody-facilitated Aβ efflux from the brain will be rendered redundant (). Catalytic IgM preparations from healthy humans inhibited Aβ aggregation, dissolved preformed Aβ aggregates, and inhibited the toxic effect of Aβ oligomers on cultured neuronal cells [10
]. Other than microbial B cell superantigens [3
], the Aβ-cleaving human IgM preparations did not degrade various self-proteins or foreign proteins. Pooled IgM from human blood, therefore, deserves consideration as a potentially therapeutic catalytic IVIG (CIVIG) formulation. In light of the large mass of IgM molecules (900 kDa), ingress of peripherally injected IgM into the brain is predicted to be more restricted than the smaller IgG molecules (150 kDa). The potential benefit of an IgM CIVIG preparation, therefore, will depend largely on compensatory release of brain Aβ stores following peripheral Aβ clearance by the catalytic IgM ().
IgG purified from the plasma of old humans by an affinity fractionation procedure involving acid treatment also hydrolyzed Aβ detectably (), albeit at levels smaller than the IgM fractions from the same humans. No Aβ hydrolysis was detected by an IVIG preparation (; Carimune, CSL Behring, 2.2 mg/mL; a concentration >100-fold of polyclonal human IgM yielding detectable Aβ hydrolysis in ). Similarly, there was no detectable hydrolysis of 125
I-Aβ by another IVIG preparation (; Intratect, Biotest Pharma GmbH, 0.15 mg/mL). In a previous study [24
], we tested as substrates small peptide microantigens that are recognized by catalytic antibodies largely with minimal contributions from noncovalent epitope recognition. The small peptides were also cleaved poorly by IVIG preparations compared with IgG purified by the acid-affinity purification procedure. Evidently, the IgG catalytic activity does not survive the purification procedures used to prepare IVIG. Recent studies on a recombinant catalytic antibody fragment also suggest the sensitivity of the catalytic site to conformational perturbations [25
]. The antibody fragment displayed profoundly altered levels of Aβ-cleaving activity following structural perturbations remote from the catalytic site [25
]. The catalytic activity of antibodies depends on maintenance of intramolecular activation interactions within precisely positioned amino acid dyads and triads, e.g., the activation of the Ser hydroxyl side chain enabled by hydrogen bonding with properly positioned imidazole nitrogen. Even small, sub-angstrom side-chain movements can weaken the activation reaction and compromise the catalytic activity. In order to purify catalytic antibodies, therefore, procedures that minimize their structural perturbations must be used.
Fig. 3 Failure of IVIG preparations to cleave Aβ40. a IVIG Carimune, 2.2 mg/mL; control monoclonal IgM Yvo, 0.18 mg/mL. Catalytic activity measured as in . b IVIG Intratect and IgM Yvo, 0.18 mg/mL. Catalytic activity measured as in  using (more ...)
The distinction between the catalytic and binding function of antibodies is also illustrated by the recent report that that even mild denaturing steps used for industrial preparation of IVIG expose cryptic autoantibody-binding activities, including an increased ability to bind Aβ [26
]. The activating effect was ascribed to subtle changes in the IgG structure. Removal of physiological autoantibody inhibitors is another possibility. In contrast, the IVIG fractionation technology compromises the catalytic function of antibodies.