Salivary lactoferrin as biomarker for Alzheimer's disease: Brain-immunity interactions

Objective: We aim to explain why salivary lactoferrin (Lf) levels are reduced in patients suffering mild cognitive impairment (MCI) and sporadic Alzheimer's disease (sAD). (1)We also will discuss if such Lf decrease could be due to a downregulation of the sAD associated systemic immunity. Background: Several non-neurological alterations have been described in sAD, mainly in skin, blood cell, and immunological capacities. We reviewed briefly the main pathophysiological theories of sAD (amyloid cascade, tau, unfolder protein tau, and amyloid deposits) emphasizing the most brain based hypotheses such as the updated tau-related neuron skeletal hypothesis; we also comment on the systemic theories that emphasize the fetal origin of the complex disorders that include the low inflammatory and immunity theories of sAD. New/updated hypothesis: Lf has important anti-infectious and immunomodulatory roles in health and disease. We present the hypothesis that the reduced levels of saliva Lf could be an effect of immunological disturbances associated to sAD. Under this scenario, two alternative pathways are possible: first, whether sAD could be a systemic disorder (or disorders) related to early immunological and low inflammatory alterations; second, if systemic immunity alterations of sAD manifestations could be downstream of early sAD brain affectations. Major challenges for the hypothesis: The major challenge of the Lf as early sAD biomarker would be its validation in other clinical and population-based studies. It is possible the decreased salivary Lf in early sAD could be related to immunological modulation actions, but other different unknown mechanisms could be the origin of such reduction. Linkage to other major theories: This hypothesis is in agreement with two physiopathological explanations of the sAD as a downstream process determined by the early lesions of the hypothalamus and autonomic vegetative system (neurodegeneration), or as a consequence of low neuroinflammation and dysimmunity since the early life aggravated in the elderly (immunosenescence).