The relationships between neuroinflammation, beta-amyloid and tau deposition in Alzheimer's disease: a longitudinal PET study

Background The aim of this longitudinal study was to assess with positron emission tomography (PET) the relationship between levels of inflammation and the loads of aggregated beta-amyloid and tau at baseline and again after 2 years in prodromal Alzheimer's disease. Methods Forty-three subjects with mild cognitive impairment (MCI) had serial C-11-PK11195 PET over 2 years to measure inflammation changes, and C-11-PiB PET to determine beta-amyloid fibril load; 22 also had serial F-18-Flortaucipir PET to determine tau tangle load. Cortical surface statistical mapping was used to localise areas showing significant changes in tracer binding over time and to interrogate correlations between tracer binding of the tracers at baseline and after 2 years. Results Those MCI subjects with high C-11-PiB uptake at baseline (classified as prodromal Alzheimer's disease) had raised inflammation levels which significantly declined across cortical regions over 2 years although their beta-amyloid levels continued to rise. Those MCI cases who had low/normal C-11-PiB uptake at baseline but their levels then rose over 2 years were classified as prodromal AD with low Thal phase 1-2 amyloid deposition at baseline. They showed levels of cortical inflammation which correlated with their rising beta-amyloid load. Those MCI cases with baseline low C-11-PiB uptake that remained stable were classified as non-AD, and they showed no correlated inflammation levels. Finally, MCI cases which showed both high C-11-PiB and F-18-Flortaucipir uptake at baseline (MCI due to AD) showed a further rise in their tau tangle load over 2 years with a correlated rise in levels of inflammation. Conclusions Our baseline and 2-year imaging findings are compatible with a biphasic trajectory of inflammation in Alzheimer's disease: MCI cases with low baseline but subsequently rising beta-amyloid load show correlated levels of microglial activation which then later decline when the beta-amyloid load approaches AD levels. Later, as tau tangles form in beta-amyloid positive MCI cases with prodromal AD, the rising tau load is associated with higher levels of inflammation.