Alzheimer’s disease: Metals and the immune systemNEURODEGENERATION, Vol. 4, pp 107–111 (1995)

Hypothesis: is Alzheimer’s Disease a Metal-inducedImmune Disorder?

R.A. Armstrong,1 S.J. Winsper1 and J.A. Blair2

1Vision Sciences and 2Pharmaceutical and Biological Sciences, Aston University,Birmingham B4 7ET

A hypothesis that a metal-induced immune disorder may be involved in the pathogenesis of someforms of Alzheimer’s disease (AD) is presented. The classical complement pathway is activated inAD and T cells and reactive microglia appear in the brain. Studies of metal induced autoimmunityand the use of compounds containing aluminium as vaccine adjuvants suggest that metals canactivate complement and can be taken up by antigen presenting cells. The consequent immuneresponse could contribute to neuronal damage, β-amyloid deposition and cell death. The strengthsand weaknesses of this hypothesis are discussed and tests of some aspects are proposed.

Key words: Alzheimer’s disease, metals, aluminium, immune activation, majorhistocompatability locus antigens

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THERE IS INCREASING evidence that a disorder of theimmune system is involved in the pathogenesis ofAlzheimer’s disease (AD) (Nandy, 1983; Rogers &Luber-Narod, 1988; McGeer et al., 1991). Complementproteins associated with the ‘classical’ pathway(McGeer et al., 1991), brain reactive antibodies (Nandy,1983), possibly immunoglobulins (Ishii & Haga, 1976;Eikelenboom & Stam, 1982), circulating immune com-plexes (CIC) in peripheral tissue and cerebral bloodvessels (Heinonen et al., 1993), helper/inducer andcytotoxic/suppressor T cells (McGeer et al., 1991) andabundant reactive microglia (McGeer et al., 1991) haveall been recorded in AD. Significant alterations in themajor histocompatibility locus (MHC) antigens havealso been demonstrated (Walford & Fortoul, 1983)suggesting either increased resistance or susceptibilityto an antigen in AD patients (Cohen et al., 1982;Henschke et al., 1978). These immune reactions couldbe a response to the pathological processes of AD.However, it is also possible that immune reactions toan environmental antigen could initiate such a patho-logical process. There is evidence that exposure tometals can induce an immune response in peripheral

tissues. First, mercury can induce antibodies againstrenal antigens and inhibit RT61 T-cells (Kosuda et al.,1993). Second, exposure to cobalt, aluminium (A1), tin,zirconium and beryllium has been associated withlung inflammatory disorders (Epstein et al., 1982, Kreisset al., 1993). Lymphocyte transformation, an increasein T cells and the development of a granulomatouspneumonitis are features of these disorders (Kreiss etal., 1993). Third, A1 phosphate and hydroxide are usedas vaccine adjuvants (Gupta et al., 1993), i.e. theyenhance the immune response to an antigen (Edelman,1980). Adjuvants containing A1 can activate comple-ment proteins (Ramanathan et al., 1979), prime helperT cells for IgE production (Kishimoto et al., 1973) andinduce antibody (Reiotella & Orasy, 1969). Since manymetals can be absorbed into the brain (Basun et al.,1991), it is possible that a metal-induced immuneactivation could occur in brain tissue and contribute tothe pathogenesis of AD. The objectives of this presentpaper are: (1) to propose a hypothesis describing howmetals could induce an immune response in AD; (2) toconsider its strengths and weaknesses and (3) to sug-gest studies to test aspects of the hypothesis.

Metals and an immune response

Of the metals that could be involved in an immuneresponse in AD, one possible candidate is A1. It oc-

Correspondence to: Dr R.A. ArmstrongReceived 11 March 1994; revised and accepted for publication 14

September 1994© 1995 Academic Press Limited1055-8330/95/010107 + 5 $08.00/0

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curs commonly in food and drinking water (Graves etal., 1990; Gregor, 1992; MAFF, 1993; Bowen et al.,1993), it can enter the brain and individual brain cells(Basun et al., 1991); Candy et al., 1992) and there isepidemiological (Martyn, 1992) and case control data(Graves et al., 1990; Neri and Hewitt, 1991) whichtogether suggest a possible link between A1 and AD.However, this line of evidence is extremely contro-versial and it is possible that other metals could beinvolved in AD. It has been established that metalions may induce: (1) antibodies against neuronal anti-gens (Kosuda et al., 1993), (2) complement factors(Ramanathan et al., 1979) or (3) a cellular immuneresponse (Sartvetnick & Fox, 1990). Sartvetnick andFox (1990) further suggest that the immune systemcan response to antigens expressed by the MHC path-way in non-immune cells. MHC-antigen complexeson the cell surface could then signal a T-cell response.For example, silver taken up into cells via transferrin(Tf) receptors results in the presence of CD41 T cells(McCoy et al., 1993). These are cells which recognizepeptide fragments taken up by macrophages and othercells. In addition, T-cell clones from patients withchronic beryllium disease are MHC class-II restrictedi.e. they only respond to beryllium when it is in asso-ciation with MHC class-II molecules on the surface ofantigen presenting cells (Richeldi et al., 1993). Hence,metals taken up into antigen presenting cells in thebrain could result in immune activation and death ofneurons.

The hypothesis

The essential features of the hypothesis are illustratedin Figure 1. An important aspect of the mechanism isthat metal ions like A1 enter brain cells and, in sus-ceptible individuals, are presented to the immunesystem via the MHC pathway.

First, entry into the cell could occur via Tf receptors(Roskams & Connor, 1990) although other methodsare possible. The density of Tf receptors is greatest inregions of high cortical pyramidal cell density, prob-ably reflecting the high use of iron by such cells (Pullenet al., 1990). However, it is probable that metals wouldalso enter other brain cells including microglia andcapillary endothelial cells. For example, A1 has beendetected inside membrane bound cytoplasmic inclu-sions within macrophages (Leininger et al., 1977), cellswhich share many of the same properties with brainmicroglia (Peress et al., 1993).

Second, we propose that metals are incorporatedinto the membranes of antigen presenting cells via theMHC pathway. This is unlikely to occur in neurons orastrocytes which do not express MHC antigens(McGeer et al., 1991) but could occur in microglia andcapillary endothelial cells which do express these an-tigens (McGeer et al., 1991). In support of this, signifi-cantly increased levels of MHC class II expressionhave been recorded in AD retinae principally due toinduced activity on endothelial cells (Liew et al., 1994).

Third, there is a cellular immune response result-ing in the production of CD41 T cells which mayinfiltrate the neuropil and cluster around antigen pre-senting cells.

Fourth, T cells release lymphokines e.g. interferon-gamma, which activate microglia (Weiss et al., 1993).

Fifth, activated microglia release free radicals in anoxidative attack which damages adjacent neurons.Activated microglia excrete large quantities of theunconjugated pterin neopterin, a marker of the cellu-

Figure 1. Hypothetical scheme of how a metal, e.g. Alu-minium (Al), could initiate the pathology of Alzheimer’sdisease by a metal-induced immune attack. Al-Tf 5 A1bound to transferrin; β/A4 5 β-amyloid peptide;CEC 5 Capillary endothelial cells; CMAC 5 Complementmembrane attack complex; CCC 5 Classical complementcascade; IFN-γ 5 Interferon-gamma; IL-1 5 Interleukin 1;MHC 5 Major histocompatibility complex; M 5 Microglia;A-M 5 Activated Microglia; N 5 Neuronal cell body;Ne 5 Neopterin; SP 5 Senile plaque.

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lar immune response (Weiss et al., 1993; Armstrong etal., 1994). This immune attack could be exacerbatedby the presence of metal ions in pyramidal neuronswhich interfere with enzyme reactions. For example,A1 may interfere with cytoplasmic reactions whichemploy enzymes with Mg21 cofactors (Glick, 1990).Stimulation of the classical complement cascade cul-minating in the production of the ‘membrane attackcomplex’ (MAC) C5b-9 (Ramanathan et al., 1979) couldalso contribute to the immune response. It has re-cently been shown that in AD, neurons are attackedby MAC and attempt to resist the damage by internal-izing MAC fragments (Itagaki et al., 1994).

Sixth, these processes could increase the produc-tion of amyloid precursor protein (APP) and β-amyloid(β/A4) deposit due to loss of synaptic connectionsresulting from neuronal damage (Wallace et al., 1993).

Strengths and weaknesses of the hypothesis

The strengths of the present hypothesis are that itcould account of the following observations in AD:(1) the presence of immune activation in AD patients(McGeer et al., 1991; Heinonen et al., 1993; Armstronget al., 1994); (2) variations in MHC antigens in ADpatients (Walford & Fortoul, 1983); (3) the epidemio-logical evidence (Graves et al., 1990; Neri & Hewitt,1991; Martyn, 1992); (4) the production of excess APPand β/A4 deposits and (5) the association between b/A4 production and capillaries in the brain (Armstronget al., 1993). However, the hypothesis has a number ofweaknesses.

First, although there is evidence that metals enterbrain cells, the mechanism of entry into such cells hasnot been conclusively demonstrated. Although entryvia Tf receptors is possible (Roskams & Connor, 1990),other methods may also occur e.g. entry into glialcells could be via the haemopexin transport system.

Second, although a significant proportion of the A1which enters cells is found in the cytoplasm (Shi &Huang 1989; Dobson et al., 1993) it has not been dem-onstrated that A1 or any other metal can react withMHC proteins in human brain cells.

Third, there is no established link between varia-tions in MHC antigens in AD patients and suscepti-bility to metal-induced immunity.

Fourth, the hypothesis purposes that the immunereaction is a cause of the pathological ‘cascade’ in AD,i.e. it results in the production of β/A4. Against this,no association has been found between the presenceof A1 and β/A4 in chronic renal dialysis cases al-

though changes in tau protein did occur (Harringtonet al., 1994). However, we propose that such a linkwould only be seen in particular individuals whoseimmune system was susceptible to a particular metaland this may not be the case in such dialysis patients.In addition, the most obvious immune response ob-served in histological preparations is the presence ofmicroglia associated with mature β/A4 deposits inthe brain (Ohgami et al., 1991; Mann et al., 1992).Hence, an immune reaction could be a response to theproduction of APP or APP fragments in the brain.However, the later appearance of activated microgliadoes not preclude the possibility of an earlier im-mune response in which small numbers of T cells andmicroglia were present more locally in relation toglial or capillary endothelial cells. Diffuse β/A4 de-posits may not necessarily be associated with microgliaif the β/A4 is formed at synaptic terminals as a re-sponse to an immune attack on neuronal cells bodies.In addition, the earliest immune response could in-volve complement activation rather than a cellularimmune response.

Further studies

The most important question in relation to immunemechanisms in AD is whether an immune response isa cause or an effect of the pathological ‘cascade’ inAD. This could be studied in Down’s syndrome (DS)patients in whom the chronology of the pathologicalchanges in the brain has been well established (Mannet al., 1992) and in whom an immune response hasalso been detected (Hodgkins et al., 1993; Armstronget al., 1994). If a cellular immune response precedes β/A4 deposition in such patients then one would expectto see T cells infiltrating the neuropil close to bloodvessels and some activated microglia adjacent to neu-rons and capillary endothelial cells before the appear-ance of β/A4 deposits. In addition, markers of immuneactivation which include neopterin (Wachter, 1992;Weiss et al., 1993; Armstrong et al., 1994), tumournecrosis factor, complement factors, lymphokines andCIC (Heinonen et al., 1993) could be measured in theplasma of DS patients to determine at what age im-mune activation occurs relative to the appearance ofdiffuse β/A4 deposits. If immune markers like thesesuggest immune activation occurs after the appear-ance of β/A4, this would also argue against the hy-pothesis that a primary immune response initiates the‘pathological cascade’ in AD. The hypothesis wouldalso predict a relationship between immune markers

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and the concentration of metals in the body. Hence,the correlation between levels of immune markersand levels of metals bound to Tf or to low molecularweight species in the plasma (Hodgkins et al., 1993)could be studied. Studies of metal uptake intomicroglia and possible interactions between metals inthe cytoplasm and the MHC system should also bestudied. Epidemiological studies have not foundstrong correlations between A1 and AD and it is un-likely that further studies of this type would be usefulin testing the present hypothesis without taking intoaccount variations in MHC antigens in the popula-tion.

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