Complement proteins and complement inhibitors in Alzheimer's disease

IMMUNOLOGICAL

Complement

FACTORS

IN ALZHEIMER’S

DISEASE

proteins and complement in Alzheimer’s disease

621

inhibitors

P.L. McGeer (*) and E.G. McGeer Kinsmen Laboratory of Neurological Research, University of British Columbia, 2255 Wesbrook Mall, Vancouver, BC V6T IZ3 (Canada)

Following the initial report of Eikelenboom and Stam (1982), numerous groups have reported on the presence of complement proteins associated with Alzheimer’s disease (AD) pathology (Ishii and Haga, 1984; Pouplard-Barthelaix, 1988 ; Eikelenboom et al., 1989; McGeer et al., 1989a,b; Rozemuller et al., 1990). Complement proteins Cl, Clq, C4 and C4d, which are part of the classical pathway, as well as C3, C3b, C3c, C3d, C3d,g, C7, C9 and C5b-9, which are shared by the classical and alternative pathways, have all been identified in AD brain. Since properidin (Eikelenboom and Stam, 1982; McGeer et al., 1989a,b) and fraction Bb of factor B (McGeer et al., 1989a,b) are not present, it is probable that the classical, and not the alternative, pathway is being activated. The identification of C4d and C3d is particularly important since these residual components of C4 and C3 are covalently bound to tissue and therefore represent irreversible chemical linking to pathologically affected AD tissue. The presence of the membrane attack complex CSb-9 (the MAC), indicates that the full classical pathway is activated. Concomitantly, complement receptors CR3 (CD1 lb) and CR4 (CD1 lc) (Myones et al., 1988) are upregulated on reactive microglia. Double immunohistochemical staining experiments show that complement proteins and reactive microglia bearing high levels of complement receptors gather in the same areas of affected AD brain (Akiyama and McGeer, 1990; Rozemuller et al., 1989). The questions that are raised by these experimental findings are the following: 1) What is responsible for activation of the classical complement pathway? Is it in response to the formation of antigen-antibody complexes, or is some alternative form of complement activation occurring? 2) Is complement activity restricted to opsonizing

(*) Corresponding

author.

degenerated material, or is an attack on viable cells also taking place? 3) Are complement proteins coming from the serum or are they locally produced ? 4) If complement proteins are locally produced, which cells are producing them? 5) Will detailed examination of these complement processes lead us to a better understanding of the aetiopathogenesis of AD and new routes of therapy?

Complement

activation

Complement is normally activated by dissociation of Cl following binding of the Clq portion to the Fc chain of antigen-complexed immunoglobulins. There have been reports of immunoglobulins in AD tissue (Ishii et al., 1988; Ishii and Haga, 1976), as well as reports of anti-brain antibodies in AD serum (Singh and Fudenberg, 1986; Gaskin et al., 1987) and CSF (McRae et al., 1991). However, the reports of immunoglobulin presence in AD brain have not been confirmed and specific antigens against which antibodies are raised have not so far been identified. Complement activation can occur by mechanisms other than binding to specific Ig (Colman et al., 1990). Thus, alternative possibilities need to be explored in AD. An intriguing approach is that of Rogers and colleagues (Rogers et al., 1991). They have shown that Clq binds weakly to the amyloid precursor protein (APP) and strongly to its betaamyloid protein (BAP) fragment. These data suggest that extracellular BAP might initiate the complement cascade in the absence of traditional antibodies. Clearly, further work along this promising line is warranted. The important question of what initiates the complement cascade in AD remains open.

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622 Bystander

IN IMMUNOLOGY

lysis

Complement assists the process of phagocytosis by opsonizing tissue. It is probably present whenever there is a requirement for removal of debris. However, activation of complement, whether it be by the classical or alternative pathway, involves the potential for generating significant amounts of MAC C5b-9. C5b-9 has the potential for attachment to the lipid bilayer membrane of viable cells and, through multiple molecules of C9 polymerizing in circular array, to punch holes in the cell membrane. This sequence of events, intended for the purpose of lysing foreign cells, is potentially disastrous for host cells. The phenomenon is called bystander lysis. AD dystrophic neurites, neuropile threads and some neurofibrillary tangles are decorated by antibodies to CSb-9 (McGeer et al., 1989a,b). Such deposits are not found in association with either diffuse or consolidated BAP deposits. This is understandable since BAP deposits do not have a lipid membrane for MAC insertion. The presence of the MAC on neuronal cell membranes may represent a dividing line between passive phagocytosis and active attack on still living cells. Many inhibitors of complement exist to protect against unwanted attack on host tissue, including bystander lysis. Some of the proteins which have been particularly studied in this regard are vitronectin, also known as S-protein (Falk et al., 1987; Akiyama et al., 1991), and clusterin (Cheng et al., 1988; McGeer et al., 1992), also known as SP40,40 (Murphy et al., 1988) or sulphated glycoprotein-2 (May et al., 1990). These are both secreted proteins which occur normally in serum. Soluble C5b-9 is tightly bound to both these proteins in a tri-molecular complex. They have each been shown to inhibit the insertion of MAC. They have also been shown by immunohistochemistry to be upregulated in AD. Antibodies to them decorate senile plaques, neurofibrillary tangles, dystrophic neurites and neuropile threads. The pattern is remarkably similar to that observed for the MAC, except that these solub!e proteins are also found to decorate senile plaques themselves (Akiyama et al., 1991; McGeer et al., 1991, 1992). The mRNA for clusterin has also been shown to be upregulated in AD, establishing that this protein is locally produced in brain (Duguid et al., 1989; May et al., 1990). Other complement inhibitors that have been well studied are the homologous restriction factors. These are phosphoinositol-proteoglycan-tailed (PIG-tailed) proteins that are loosely attached by a diacylglycer01 linkage to the outer surface of the lipid bilayer membrane. It has been hypothesized that these proteins can serve a patrolling function because of their loose attachment to the outer cell surface membrane (Rosse, 1990). Protectin (membrane inhibitor of reactive lysis)

is one member of this family that has been investigated in AD (McGeer et al., 1990). This protein specifically inhibits the insertion of MAC. It too shows an upregulation in AD and a pattern of distribution remarkably similar to MAC. Upregulation in AD of such complementdefensive proteins as vitronectin, clusterin and protectin are further evidence that neurons in AD are responding to complement activation, and are trying to defend themselves against bystander lysis. Inhibiting proteins are not the only source of defence against complement and insertion of MAC does not automatically result in the demise of the cells. Host cells are known to recover from bystander lysis, principly by internalizing the complement proteins in order to restore a normal surface membrane (Morgan, 1989). It is possible, therefore, that the localization to neurofibrillary tangles of proteins such as C3d, C4d and C5b-9, that would normally be attached to the outer lipid layer of the cell membrane, might be the result of such an internalization process. Sources of complement

Complement proteins are thought to be produced mainly by the liver, with delivery to the site of inflammation by exudation of serum. This would not account for their presence in brain, because the blood/brain barrier normally excludes serum proteins from the tissue matrix. Studies by positron emission tomography and by postmortem examination of tissues for serum exudate have suggested that the blood/brain barrier is not seriously compromised in AD. Thus, the possibility must be considered that complement proteins are locally produced. Since monocytes and macrophages have been shown peripherally to generate complement proteins, and since reactive microglia have been demonstrated to be phenotypically related to the monocyte cell line, activated microglia would be logical candidates to generate complement proteins locally in brain. The mRNA for complement proteins Cl, C3 and C4 have been shown to be present in brain RNA extracts by Northern blot and polymerase chain reaction analysis, with C3 and C4 being elevated several-fold in AD (Walker and McGeer, 1991). By in situ hybridization, Lampert-Etchells et al. (1991) detected grains over neuron-like cells, indicating the intriguing possibility that even neurons might be able to generate complement proteins. This unusual finding will need to be confirmed. These data indicate that complement proteins can indeed be generated by brain and may constantly be produced at low levels. Microglial cells would be the logical ones to undertake this function, but further work is obviously required to establish this point.

IMMUNOLOGICAL Therapeutic

FACTORS

considerations

The complement system is one of the many sophisticated families of proteins designed to distinguish friend from foe in the body. There is nothing specific about the involvement of complement in a disease process and therefore this must be a consequence, and not a cause, of AD. Nevertheless, inappropriate functioning of the complement system might have a very great deal to do with the pathogenesis of AD. The critical role of the complement system in immune defences is illustrated by the genetic disorders leukocyte adhesion deficiency (Anderson and Springer, 1987) and paroxysmal nocturnal haemoglobinuria (Rosse, 1990). In leukocyte adhesion deficiency there is a failure to produce the beta-chain of the beta-2 integrin family. Functioning complement receptors cannot be produced, and patients generally sucumb to intercurrent infections at an early age. An opposite genetic disease is paroxysmal nocturnal haemoglobinuria (Rosse, 1990). Here, the deficiency is a failure to synthesize the tail of PIG-tailed proteins. The result is failure to defend against selfattack by complement, typically affecting erythrocytes and platelets, so that patients usually succumb to repeated attacks of haemolysis and thrombosis. Intervention in the complement system might present a sensitive way of altering the course of AD. The most obvious focus is that of complement initiation. Blocking activation could theoretically diminish the levels of the MAC. This would tip the balance in favour of the defensive proteins that are generated, and thus possibly limit autodestruction by bystander lysis. Developing blockers for complement receptors on microglia, creating synthetic inhibitors for insertion of MAC, or blocking intracellular production of complement might also be theoretical points for investigation. In summary, further study of the complement system offers both important opportunities for understanding the phenomenon of AD, as well as developing new routes for therapy.

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Complement activation and P-amyloid-med’ la t ed neurotoxicity in Alzheimer’s

disease

J. Rogers (l), J. Schultz (l), L. Brachova (I), L.-F. Lue (I), S. Webster (I), B. Bradt (‘1, N.R. Cooper t2) and D.E. Moss t3) ‘I) Institute for Biogerontology Research, 13220 North 105th Avenue, Sun City, AZ 85351 (USA.i. “) Scripps Clinic and Research Foundation, 10666 North Torrey Pines Road, La Jolla, CA 92037 (KM j* ;/m’ “) University of Texas at El Paso, El Paso, TX 79968 (USA)

Introduction

may be seen in the non-demented

Alzheimer’s disease (AD) is an age-related, progressive neurological disorder leading to dementia. Its approximate age of onset is 65 years old, and the average time between clinical diagnosis and death is approximately 8.5 years. Pathologically, AD is characterized by loss of neurons and their processes and by neuritic plaques and neurofibrillary tangles. Neuritic plaques are approximately

50-100~pm diame-

tered argentophylic structures that include dystrophic neurites. Neurofibrillary tangles occur intracellularly in neurons or as the remnants of neurons (“tombstone” or “ghost” tangles). Both plaques and tangles

elderly (ND) at 3ti-

topsy, but in general they are so much more profuse in AD that differences are evident even with the most cursory examination. In both ND and AD patiettt:;, plaques and tangles have their highest densities in a:;sociation cortex and limbic structures (Rogers ;md Morrison, 1985). Plaques and tangles, pa~ticuh:riy the former, are also typically associated with excessive deposition of a 40-42-amino-acid pep5de ca!led the P-amyloid peptide (P-AP), derived from a larger amyloid precursor protein (APP) (Cllenner ;ml Wong, 1984; Kang et al., 1987; Selkoe, 1991,. Although APP is highly conserved throughout phylogeny and cell types, suggesting an important but

Correspondence to: Dr. Joseph Rogers, IBR, PO Box 1278,Sun City, AZ 85372(USA).