C4b-binding protein in Alzheimer's disease: Binding to Aβ1–42 and to dead cells

In the Alzheimer's disease (AD) brain, binding of Clq within the Cl complex, the initiating molecule of the classical complement pathway, to apoptotic cells, DNA and amyloid-β (Aβ), the major constituent of senile plaques, can initiate complement activation. However, the extent of activation is determined by the balance between activation and inhibition.Fluid-phase complement inhibitor C4b-binding protein (C4BP) was immunohistochemically detected in Aβ plaques and on apoptotic cells in AD brain. In vitro, C4BP bound apoptotic and necrotic but not viable brain cells (astrocytes, neurons and oligodendrocytes) and limited complement activation on dead brain cells. C4BP also bound Aβ1–42 peptide directly, via the C4BP α-chain, and limited the extent of complement activation by Aβ. C4BP levels in cerebrospinal fluid (CSF) of dementia patients and controls were low compared to levels in plasma and correlated with CSF levels of other inflammation-related factors.In conclusion, C4BP binds to dead brain cells and Aβ peptide in vitro, is present in CSF and possibly protects against excessive complement activation in AD brains.