INCREASED EXPRESSION OF TOLL-LIKE RECEPTOR (TLR)-3 IN ALZHEIMER’S DISEASE BRAINS: A PROTECTIVE OR PATHOGENIC RESPONSE?

Abstract

Toll-like receptors (TLR) are pattern recognition receptors involved in immune responses to microbial organisms. Studies have also shown their involvement in inflammatory responses to amyloid beta (Aβ) and aggregated α-synuclein. Activation of these receptors can amplify inflammation, but also promote phagocytosis of Aβ. Activation of TLR-9 resulted in significant reduction of Aβ in animal AD models. Activation of TLR-3 could have therapeutic potential due to its induction of beta-interferon responses. The premise of this study was observation of increased numbers of TLR-3 immunoreactive cells in AD affected brains. This study had two components; firstly staged series of human brain samples from low plaque (LP) and high plaque (HP) non-demented (ND) cases and AD cases were used to identify TLR-3 cellular localization using immunohistochemistry, and to quantify changes of expression of TLR-3 mRNA with disease by quantitative polymerase chain reaction (PCR). The second component involved in vitro studies of TLR-3 expression and activation using human postmortem brain-derived microglia and vascular endothelial cells. Quantification of TLR-3 immunoreactivity in middle temporal gyrus using Image J software to measure area occupied showed increase of 101.8% in AD cases compared to LPND (p=0.0036), and 16.5% increase between LPND and HPND (NS). TLR-3 immunoreactivity showed significant correlation with scores for brain plaque loads (Spearman r=0.464, p<0.01) and tangle loads (r=0.561, p<0.001). TLR-3 immunoreactivity was primarily in microglia, but also visible in subsets of astrocytes and vascular endothelial cells. Increased TLR-3 immunoreactivity was noticeable in microglia associated with Aβ plaques. Quantitative PCR showed significant increase in TLR-3 mRNA between AD and ND cases (p=0.043), interferon-β (p=0.036) and interferon regulatory factor-3 (p=0.013). Significant increase in TLR-9 mRNA expression in AD cases was also detected in these samples (p=0.0019). In vitro experiments showed that treatment with TLR-3 ligand poly I:C did not induce significant increased expression of TLR-3 mRNA in microglia, but a tenfold increase in brain-derived endothelial cells.