Initial evidence indicating the functional utility of the catalytic function is available as follows. Aβ40 is neurotoxic.3
We studied the viability of SH-SY5Y neuronal cells following treatment with Aβ40 alone, Aβ40 pretreated with IgM Yvo and IgM Yvo alone. The toxic effect of Aβ40 was reduced significantly in the presence of IgM Yvo (P<0.0001).27
The IgM alone did not influence the level of cell viability. Sierks and coworkers have reported similar results using a catalytic Ig light chain with Aβ hydrolyzing activity.7
Oligomeric and fibrillar Aβ structures are thought to be responsible for the neurotoxicity. Using atomic force microscopy, we observed reduced formation of Aβ oligomers (spherical particles with diameter 4–20 nm), protofibrils and short fibrils following Aβ40 treatment with a model catalytic IgM compared to the control reaction mixture of the peptide and a noncatalytic IgM (by 72–83%).27
Aβ40 was present at 13-fold excess over the IgM in this experiment. No Aβ40 binding by the IgM was detected by ELISA. Therefore, the observed effects of IgM Yvo can not be attributed Aβ40 binding.
Peripherally injected monoclonal IgGs (150 kD mass) can cross the BBB at low levels in the APP-Tg mouse model.30
A recent mouse study has raised the possibility of selective transport of a monoclonal IgM into the brain.31
However, IgMs are very large molecules (900 vs
150 kDa). IgM concentrations in the cerebrospinal fluid (CSF) of non-demented humans and AD patients are no different, and the CSF IgM concentrations are very low (<0.001 mg/ml, compared to blood IgM levels of ~2 mg/ml).32
If CSF IgM expresses catalytic activity equivalent to blood-borne IgM, only 0.01% of Aβ40 and Aβ42 present in CSF will undergo IgM-catalyzed hydrolysis in 5 days (CSF Aβ40 and Aβ42 concentrations are ~2.7 and ~0.28 ng/ml, respectively4
). It is debatable, therefore, whether catalytic IgMs are present in the brain at concentrations sufficient to degrade Aβ appreciably. In contrast, clearance of large amounts of Aβ found in peripheral blood by the IgMs can be anticipated based on the observed rates of hydrolysis. Peripheral blood Aβ concentrations are ~0.25 ng/ml, respectively.4
The IgM catalytic rates are sufficient to hydrolyze 93 % of blood-borne Aβ in 5 days, corresponding to the half-life of IgM in humans.33
Under similar conditions, noncatalytic IgMs with Kd
(equilibrium dissociation constant) equivalent to the observed Michaelis constant (Km
) will bind only 7.7 % of the Aβ in blood at equilibrium.
Peripheral and brain Aβ exist in a state of equilibrium. Other groups have observed that peripheral Aβ binding reagents induce the release of Aβ from the brain Aβ,11
leading to suggestions that peripheral administration of Aβ binding IgGs can be applied to clear Aβ from the brain. In a preliminary study, a preparation of catalytic human IgM from pooled human serum was administered intravenously on day 0 and day 8 to 6 month old APP-Tg mice that overexpress human Aβ (APPSwe
mouse strain). A sustained increase of intact Aβ concentrations in peripheral blood determined was evident (). As the injected human IgM did not bind Aβ detectably, the evident increase of pepripheral Aβ is not due to peptide stabilization by formation of immune complexes. This suggests the feasibility of depleting brain Aβ as a consequence of peripheral IgM catalyzed Aβ hydrolysis. Receptors for the Fc region of IgG expressed on the abluminal side of the BBB have recently been implicated in enhancing IgG-dependent Aβ efflux from the brain.17
Fcμ/α receptors expressed on the luminal side of the BBB could fulfill a similar function in enhancing catalytic IgM-dependent efflux of the peptide (). These receptors are abundantly distributed on various cells.34,35
Local IgM-catalyzed Aβ hydrolysis at BBB can be predicted to strengthen the trans-BBB Aβ concentration gradient, resulting in enhanced peptide efflux be in the microenvironment, and explaining the sustained increase of peripheral Aβ concentrations noted after peripheral catalytic IgM administration.
An important consideration is whether the catalytic Igs can cause pathogenic effects. If Aβ fulfils a vital physiological function, its removal may be deleterious. The neurotrophic effects of very low Aβ concentrations in tissue culture have been reported.36
However, there appears to be no physiological purpose for accumulation of Aβ in the brains of AD patients. Therefore, it is assumed that removal of excess Aβ will be without negative effects. Active vaccination of humans with Aβ42 resulted in the development of encephalitis in some AD subjects, a finding attributed to undesirable T lymphocyte responses.37
However, there is no formal proof for this, and the potential negative role of Aβ binding IgGs remains an open issue. This is highlighted by observations of undesirable inflammatory and vascular effects of the IgGs in tissue culture. Aβ-IgG immune complexes bind Fcγ receptors expressed by microglial cells and induce the release of inflammatory mediators.16
This could exacerbate the already inflamed state of AD brain. In mouse AD models, clearance of amyloid plaques from the brain parenchyma induced by Aβ-binding IgGs can be accompanied by Aβ deposition in the blood vessels and microhemorrhages.38,39
In human trials of the Wyeth-Elan humanized monoclonal IgG, abnormal magnetic resonance images suggestive of angiogenic edema have been observed.40
In theory, catalytic Igs can be predicted to exert lesser side effects compared to Aβ binding IgGs. If the catalytic rate constant is sufficiently rapid, stable immune complexes will not be formed, and Fc receptor mediated inflammatory release from inflammatory cells should be precluded. Concerning microhemorrhages, provided the Aβ fragments generated by the catalysts are less aggregogenic than intact Aβ, permanent clearance of Aβ from the brain should occur, and no catalytic Ig-induced Aβ deposition in the blood vessel walls in anticipated.