Presence of severe neuroinflammation does not intensify neurofibrillary degeneration in human brain

This study investigated the allegedly causal relationship between microglial activation and neurofibrillary degeneration (NFD) typical of Alzheimer's disease (AD) by determining if presence of extreme microglial activation coincides with intensified NFD. We performed comparative histopathological analyses of NFD and microglial reactivity in 18 primary subjects ranging from 4 to 51 years of age. Ten of these subjects (median age 34) died from infectious disease (HIV, sepsis) and CNS trauma, while eight subjects (median age 32.5) died from non-infectious conditions (controls). Second, we also examined two 52-year-old subjects with Down syndrome where one had comorbid sepsis and the other one did not. We found that all 10 subjects with infectious/traumatic diagnoses showed severe neuroinflammation, while the 8 control subjects completely lacked neuroinflammatory changes. However, all 18 primary subjects were found to show the same early-stage, pretangle neuropathology of Braak stage 1a and 1b, that is, they exhibited primarily subcortical NFD in the locus coeruleus and sporadic lesions in the transentorhinal cortex. Similarly, the two subjects with Down syndrome showed the same high levels of NFD (Braak stage VI) irrespective of the comorbid sepsis-related neuroinflammation present in one of these individuals. Collectively, our findings show that despite rampant microglial activation in all subjects with neuroinflammatory conditions the extent of NFD is at the same level as seen in non-inflamed controls. These findings demonstrate that microglial activation does not initiate or exacerbate NFD, and we conclude that CNS inflammation is unlikely to be causally involved in the development of NFD characteristic of AD dementia.