Research

Mannan-Abeta₂₈ conjugate prevents Abeta-plaque deposition, but increases microhemorrhages in the brains of vaccinated Tg2576 (APPsw) mice

Irina Petrushina^1* , Anahit Ghochikyan^2* , Mikayel Mkrtichyan² , Grigor Mamikonyan² , Nina Movsesyan^1,2 , Rodmehr Ajdari¹ , Vitaly Vasilevko¹ , Adrine Karapetyan¹ , Andrew Lees³ , Michael G Agadjanyan^1,2* and David H Cribbs^1,4*

¹  The Institute for Brain Aging and Dementia, University of California Irvine, Irvine, CA 92697-4540, USA

²  The Institute for Molecular Medicine, Department of Immunology, Huntington Beach, CA 92649, USA

³  Biosynexus Incorporated, Gaithersburg, MD 20877, USA

⁴  Professor of Neurology, Department of Neurology, The Institute for Brain Aging and Dementia, 1207 Gillespie NRF, University of California Irvine, Irvine, CA 92697-4540, USA

author email corresponding author email* Contributed equally

Journal of Neuroinflammation 2008, 5:42doi:10.1186/1742-2094-5-42

Published:	29 September 2008

Abstract

Background

New pre-clinical trials in AD mouse models may help to develop novel immunogen-adjuvant configurations with the potential to avoid the adverse responses that occurred during the clinical trials with AN-1792 vaccine formulation. Recently, we have pursued an alternative immunization strategy that replaces QS21 the Th1 type adjuvant used in the AN-1792 clinical trial with a molecular adjuvant, mannan that can promote a Th2-polarized immune response through interactions with mannose-binding and CD35/CD21 receptors of the innate immune system. Previously we established that immunization of wild-type mice with mannan-Aβ₂₈ conjugate promoted Th2-mediated humoral and cellular immune responses. In the current study, we tested the efficacy of this vaccine configuration in amyloid precursor protein (APP) transgenic mice (Tg2576).

Methods

Mannan was purified, activated and chemically conjugated to Aβ₂₈ peptide. Humoral immune responses induced by the immunization of mice with mannan-Aβ₂₈ conjugate were analyzed using a standard ELISA. Aβ₄₂ and Aβ₄₀ amyloid burden, cerebral amyloid angiopathy (CAA), astrocytosis, and microgliosis in the brain of immunized and control mice were detected using immunohistochemistry. Additionally, cored plaques and cerebral vascular microhemorrhages in the brains of vaccinated mice were detected by standard histochemistry.

Results

Immunizations with low doses of mannan-Aβ₂₈ induced potent and long-lasting anti-Aβ humoral responses in Tg2576 mice. Even 11 months after the last injection, the immunized mice were still producing low levels of anti-Aβ antibodies, predominantly of the IgG1 isotype, indicative of a Th2 immune response. Vaccination with mannan-Aβ₂₈ prevented Aβ plaque deposition, but unexpectedly increased the level of microhemorrhages in the brains of aged immunized mice compared to two groups of control animals of the same age either injected with molecular adjuvant fused with an irrelevant antigen, BSA (mannan-BSA) or non-immunized mice. Of note, mice immunized with mannan-Aβ₂₈ showed a trend toward elevated levels of CAA in the neocortex and in the leptomeninges compared to that in mice of both control groups.

Conclusion

Mannan conjugated to Aβ₂₈ provided sufficient adjuvant activity to induce potent anti-Aβ antibodies in APP transgenic mice, which have been shown to be hyporesponsive to immunization with Aβ self-antigen. However, in old Tg2576 mice there were increased levels of cerebral microhemorrhages in mannan-Aβ₂₈ immunized mice. This effect was likely unrelated to the anti-mannan antibodies induced by the immunoconjugate, because control mice immunized with mannan-BSA also induced antibodies specific to mannan, but did not have increased levels of cerebral microhemorrhages compared with non-immunized mice. Whether these anti-mannan antibodies increased the permeability of the blood brain barrier thus allowing elevated levels of anti-Aβ antibodies entry into cerebral perivascular or brain parenchymal spaces and contributed to the increased incidence of microhemorrhages remains to be investigated in the future studies.