Other defenses may have ancient origins. Some research shows that cognitive resilience may have evolved from mechanisms of innate immunity that have protected the brain against infection and injury for millennia.
For example, a variant of the gene encoding the CD33 receptor, expressed on innate immune cells such as monocytes, has been linked to Alzheimer pathology and cognitive decline in humans. The variation increases the number of CD33 receptors on the surface of monocytes in young and older individuals (Bradshaw EM et al. Nat Neurosci. 2013:16[7]: 848-850). Using positron emission tomography imaging of older adults and autopsy tissue from older adult brains, the researchers showed that people with the variant have less functional immune cells that cannot clear amyloid well. The result is an accumulation of plaques.
Another variant, in the triggering receptor expressed on myeloid cells 1 (TREM1) gene, is linked with reduced expression of these receptors, which normally help amplify inflammatory immune responses (Repogle JM et al. Ann Neurol. 2015;77[3]:469-477). This variant also was associated with more plaques and tangles and a faster rate of cognitive decline. Together, the results suggest variants that suppress the innate immune system accelerate cognitive decline and Alzheimer-related pathology.
Better understanding how the immune system does double duty in the brain is critical to developing treatments that promote neuroprotection and cognitive resilience.