Immunotherapy for mild cognitive impairment patients: a role for omega-3 fatty acids in improving innate immunity against amyloid-beta

Written by Milan Fiala and Matteo Pellegrini, Department of Molecular, Cell and Developmental Biology, University of California, Los Angeles (CA, USA)

Despite over 200 clinical trials, as of yet no drug for Alzheimer’s disease (AD) has been approved, but the search continues for a successful candidate, in particular  for the early stage of AD, mild cognitive impairment (MCI). Recent trials of amyloid-beta (Aβ) antibodies have failed in the search for a blockbuster medication. The antibody approach to enhancing microglial phagocytosis of Aβ (1–42) was developed in mutant amyloid-precursor protein transgenic mouse models. These models were not constructed with the human immune system and therefore did not reproduce the innate immune defects of AD patients. The studies in animal models have produced conflicting results regarding the role of brain microglia and peripheral monocytes. The microglia are poorly phagocytic for Aβ in comparison to peripheral monocytes, and monocytes do not clear Aβ after repopulation of the mouse brain. An example of this lack of translation between mouse models and clinical results is exhibited in the therapeutic use of the antineoplastic agent bexaroten, which was made headlines when it lowered Aβ in animal models [1], yet these findings did not translate into human patients [2].
Our breakthrough discovery of defective phagocytosis of Aβ by monocyte/macrophages of AD patients suggested a new immunotherapeutic approach through enhancement of innate immunity [3]. This approach has been even more compelling because we found that peripheral blood mononuclear cells (PBMCs) of AD patients downregulated N-acetylglucosamine transferase  when exposed to Aβ, probably accounting in part for the defective macrophage phenotype  [4]. Thus, we think that the deregulation of the innate immune system in AD and MCI patients may account for poor brain clearance and needs to be repaired first before treatment with antibody therapies. 

To understand the role of macrophages in Aβ clearance, we have examined AD brain tissues and have observed dense infiltration of plaques and perivascular areas by large macrophages morphologically distinct from microglia and penetrating from microvessels into the neuropil  [5]. In our observations, we found phenotypic and molecular markers of defective function in peripheral blood monocytes of AD patients: defective phagocytosis of Aβ and an extreme M1 inflammatory or extreme M2 non-inflammatory phenotype [6].

Interestingly, macrophages of AD patients are defective specifically in phagocytosis and apoptotic resistance to Aβ (either soluble, oligomeric or fibrillary), but are not defective in phagocytosis of E.coli or Staph.aureus [7]. However, we have not tested clearance of putative infectious agents such as prions, HSV-1 and Chlamydiae by macrophages, and therefore cannot exclude that macrophages could fail to kill these agents.

Our discoveries of defective Aβ phagocytosis by AD macrophages in 2007 [4] and an abnormal inflammatory phenotype of AD macrophages [6] have led to many more studies of macrophage pathology in AD and MCI patients. In more recent studies, we have shown that apoptotic macrophages are present at the blood-brain barrier in the AD brain and that Aβ is released from dead macrophages into congophilic vessels [8]. We perfected a flow cytometric test of faulty Aβ phagocytosis by macrophages that was 94% positive in AD patients and 100% negative in age-matched University professors [7]. We also observed downregulation of the N-acetylglucosamine transferase RNA in peripheral blood mononuclear cells (PBMCs) in 11 out of 14 MCI patients [9]. Thus, we are now investigating the lack of N-acetylglucosamine addition to glycoproteins as a molecular underpinning of immunopathology in AD.

Following on from these basic advances, we are now developing immune therapy targeting these macrophage defects. Nutritional scientists have for a long time recommended and showed the benefits of the dietary components in seafood, omega-3 fatty acids and vitamin D3. Omega-3 fatty acids are precursors of the ‘specialized pro-resolving mediators’ (SPMs), including resolvins, maresins, and protectins, that are anti-inflammatory, pro-phagocytic and pro-resolving, and have made major impact at a basic level in conditions related to acute inflammation with potential in chronic inflammation [10]. To apply these advances to MCI therapy, we are investigating omega-3 supplementation and possible future application of SPMs.

Omega-3 fatty acids have been used with mixed success in clinical trials and observation studies to stabilize cognition in elderly or MCI patients. There are many over-the-counter omega-3 preparations on the market, but 50% have been found to be oxidized [11] and, therefore, inactive. In one controlled clinical trial, a preparation containing the omega-3 fatty acid, docosahexaenoic acid (DHA), prepared from algae unprotected against oxidation failed to change the slope of cognitive decline in mild to moderate AD (MMSE score 14–26) [12]. However, in a clinical trial with a salmon omega-3 preparation, the slope of MCI progression was reduced in very mild MCI (MMSE>27) patients [13].

The preparations used in these previous studies were not protected by antioxidants present in the omega-3 drink called Smartfish with Resveratrol (Smartfish, Oslo, Norway) that we used in our studies, in which we investigated the relationship between omega-3 intake, immune function and MCI symptoms. The drink contains both DHA and another omega-3 fatty acid, eicosapentaenoic acid, at a concentration of 1 gram each in a 200ml dose, plus vitamin D3, antioxidants, and resveratrol. This drink is protected against oxidation by natural anti-oxidants and thus less likely to be oxidized than unprotected preparations. We have shown that Smartfish nutritional supplementation correlates with an increase of Aβ phagocytosis by monocyte/macrophages to a physiological level in AD and MCI patients within 2 to 3 months following the start of supplementation [14]. These and previous [4] results suggest that improving the innate immune system with omega-3 fatty acids from fish may clear the brain of Aβ and possibly repair the pathology at the MCI stage of the disease.

“…these and previous results suggest that improving the innate immune system with omega-3 fatty acids from fish may clear the brain of Aβ and possibly repair the pathology at the MCI stage of the disease.”

In all of our studies, MCI patients have shown strong heterogeneity, a highly inflammatory or a very low inflammatory state, at baseline. We performed these studies in a personalized prospective fashion, measuring the rate of improvement or decline in individual patients over time. Our recent uncontrolled study with the Smartfish drink in MCI and subjective cognitive impairment (SCI) patients showed cognitive improvement on the Minimental state examination scale of 2.2 points per year (P=0.015 compared to zero) in APOE e3/e3 MCI patients (n=5). The results in the e3/e4 MCI group (n=6) were inconsistent: seven patients stabilized but three patients precipitously declined cognitively as well as in the innate immune system [15]. Admittedly, uncontrolled studies, such as our small study, may be unreliable due to the bias by the investigator and the patient. In addition, the composition of the study group strongly influenced these results, as the APOE e3/e3 group included more SCI patients than the APOE e3/e4 group. Although our results uncontestably demonstrate an improvement in phagocytosis (P= 0.016 in the first study [14] and P=0.03 in the second study [15]), cognitive benefits need to be confirmed in controlled studies.

As opposed to MCI patients, omega-3 fatty acids generally do not help patients with established Alzheimer’s dementia [14]. Therefore, early detection and repair of the innate immune deficits at the stage of MCI are exciting possibilities for immune therapy using omega-3 and other approaches. These advances will require nuanced regulation of innate immunity to a homeostatic level, which is under investigation in several laboratories, including our own.

The views expressed in this article are those of the authors and do not necessarily reflect those of Neuro Central or Future Science Group.

References

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