. Alzheimer's Abeta vaccination of rhesus monkeys (Macaca mulatta). Mech Ageing Dev. 2004 Feb;125(2):149-51. PubMed.

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  1. This paper shows that immunization of aged monkeys against the Aβ peptide produces measurable antibody titers and sizeable increases in circulating Aβ levels. These data are consistent with the argument that anti-Aβ immunotherapy may reduce brain amyloid by sequestering Aβ in the plasma. Somewhat surprisingly, the results with protein G imply that even though much of the increased circulating Aβ found after immunization is associated with antibody, some of the increase in Aβ remains even after removal of antibodies.

    These results differ from those reported by Hock et al., where humans vaccinated against Aβ did not reveal detectable increases in circulating Aβ, suggesting that the antibodies generated in humans did not create a peripheral sink for Aβ. However, it is important to recognize that measurement of serum Aβ and anti-Aβ antibodies may be complicated when both agents are present in the sample to be evaluated. Certainly, if an antibody against Aβ is bound to circulating Aβ peptide before placing the serum into an ELISA assay, the antibody cannot bind to additional Aβ tethered to the ELISA plate. For high-affinity antibody-antigen interactions, the off rate may be too slow for dissociation to occur during the period of incubation on the ELISA plate, and the antibody concentration will be underestimated. We have evidence that this sort of antibody masking does occur in transgenic mice when antibody titers are not in excess of circulating Aβ (Li et al., in review).

    Conversely, measurement of Aβ may also be modified in sandwich ELISA assays by the presence of anti-Aβ antibodies derived from the serum. First, if the circulating anti-Aβ antibody and the capture antibody have overlapping epitopes, they may compete and prevent the Aβ from being captured and thus detected by the ELISA. However, if the two epitopes do not overlap, permitting capture of Aβ still bound to the circulating host antibody, and the detection antibody can also bind the Aβ, there is an opportunity for magnification of the signal. Assuming a secondary antibody binding the detection antibody can cross-react with the circulating host antibody, the apparent signal may be doubled, relative to a standard curve made from Aβ without attached antibody.

    These complications make direct comparisons between papers difficult. Often, manuscripts do not provide the detailed steps used for the ELISAs measuring Aβ and anti-Aβ antibodies, as these are viewed as standard techniques within the respective laboratories. However, the antibodies used and their extent of cross-reactivity and epitope overlap may be important to the overall results obtained. Even the time that sera are in a diluted state may influence the results, depending upon antibody-Aβ dissociation rates To avoid these problems, we have recently started dissociating serum antibody-bound Aβ with a mild acid denaturation step (pH 2.5) followed by centrifugation through a size sieving filter to separate Aβ and antibody prior to ELISA. Obviously, other techniques may be used that accomplish the same result.

    Thus, the question regarding a peripheral sink for Aβ remains with regard to humans vaccinated against Aβ. Our view of the literature, coupled with our own data, finds support for at least three mechanisms by which immunotherapy lowers Aβ in transgenic mouse models of amyloid deposition (Wilcock et al., 2003; Wilcock et al., 2004) It would be surprising if all three were not also at work in humans vaccinated against Aβ. The work from Gandy et al. would suggest that more detailed and controlled analyses will be needed to reach a final conclusion.

    References:

    . Antibodies against beta-amyloid slow cognitive decline in Alzheimer's disease. Neuron. 2003 May 22;38(4):547-54. PubMed.

    . Intracranially administered anti-Abeta antibodies reduce beta-amyloid deposition by mechanisms both independent of and associated with microglial activation. J Neurosci. 2003 May 1;23(9):3745-51. PubMed.

    . Microglial activation facilitates Abeta plaque removal following intracranial anti-Abeta antibody administration. Neurobiol Dis. 2004 Feb;15(1):11-20. PubMed.

    View all comments by Dave Morgan
  2. This paper deals with immunization of healthy old monkeys with fibrillar Aβ42. These animals showed age-related cerebral amyloidosis but no Alzheimer's disease pathology (1) like plaques and gliosis. I wonder if vaccination of healthy old monkeys could be a good model for treatment of AD, as apart from aging they showed no sign of the disease (or cognitive impairment?).

    The changes in treated monkeys of plasma levels of Aβ, similar to those found in young AD transgenic mice before plaque appearance, may support the peripheral sink theory (2). Treatment with intravenous immunoglobulin (IVIG), containing natural anti-Aβ antibodies, of elderly people suffering from neurological diseases other than AD (such as multiple sclerosis, peripheral neuropathy, LEMNS, dermatomyositis) showed a similar pattern of reduction of CSF Aβ and Aβ42 and an increase of CSF anti-Aβ antibodies as compared to the baseline. Total serum Aβ and anti-Aβ antibodies both increased, with a nonsignificant trend toward increased serum Aβ42 after treatment, suggesting the possibility of increased antibody-mediated clearance of Aβ from CSF to serum (3) unrelated to Alzheimer's disease.

    In the absence of AD brain pathology, antibodies bind to soluble Aβ and may interfere with the equilibrium between the brain and peripheral Aβ peptide, which supports the sink theory. However, immunotherapy of AD patients who show plaque pathology did not support this theory (4). Therefore, it seems that this research, done on only four monkeys exhibiting no signs of AD, cannot support the sink theory, as appealing as it is.

    See also:

    Walker LC, Cork LC. The neurobiology of aging in nonhuman primates. In: Terry RD, et al., eds. Alzheimer's Disease. Philadelphia: Lippincott Williams and Wilkins, 1999: 233-243.

    References:

    . Peripheral anti-A beta antibody alters CNS and plasma A beta clearance and decreases brain A beta burden in a mouse model of Alzheimer's disease. Proc Natl Acad Sci U S A. 2001 Jul 17;98(15):8850-5. PubMed.

    . Human antibodies against amyloid beta peptide: a potential treatment for Alzheimer's disease. Ann Neurol. 2002 Aug;52(2):253-6. PubMed.

    . Antibodies against beta-amyloid slow cognitive decline in Alzheimer's disease. Neuron. 2003 May 22;38(4):547-54. PubMed.

    View all comments by Beka Solomon

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