Complement-Mediated Events in Alzheimer's Disease: Mechanisms and Potential Therapeutic Targets

This information is current as Andrea J. Tenner of October 1, 2021. J Immunol 2020; 204:306-315; ; doi: 10.4049/jimmunol.1901068 http://www.jimmunol.org/content/204/2/306 Downloaded from References This article cites 143 articles, 31 of which you can access for free at: http://www.jimmunol.org/content/204/2/306.full#ref-list-1

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The Journal of Immunology is published twice each month by The American Association of Immunologists, Inc., 1451 Rockville Pike, Suite 650, Rockville, MD 20852 Copyright © 2020 by The American Association of Immunologists, Inc. All rights reserved. Print ISSN: 0022-1767 Online ISSN: 1550-6606. Complement-Mediated Events in Alzheimer’s Disease: Mechanisms and Potential Therapeutic Targets Andrea J. Tenner An estimated 5.7 million Americans suffer from detrimental damage of nerve cells in (7) and Alzheimer’s disease in the United States, with no stroke (8), and aberrant synapse pruning in neurologic disease-modifying treatments to prevent or treat cog- disorders (reviewed in Refs. 9, 10). In this brief review, the nitive deficits associated with the disease. Genome- systems involved will be introduced, followed by discus- wide association studies suggest that an enhancement sion of data suggesting modulatory roles of complement in of clearance mechanisms and/or promotion of an anti- Alzheimer’s disease (AD). Finally, potential approaches to inflammatory response may slow or prevent disease therapies targeting specific elements of the will be presented. progression. Increasing awareness of distinct roles of Downloaded from complement components in normal brain development Alzheimer’s disease and function and in neurodegenerative disorders align with complement-mediated responses, and thus, thor- AD is the most prevalent progressive neurodegenerative dis- order of the elderly, and the sixth leading cause of death in the ough understanding of these molecular pathways is United States (11). Clinical symptoms of the disease include needed to facilitate successful therapeutic design. Both an unrelenting progressive decline of both memory and ex- http://www.jimmunol.org/ beneficial and detrimental effects of C1q as well as ecutive cognitive function. Unfortunately, there are still no contributions to local inflammation by C5a–C5aR1 treatments that limit the pathologic condition, or, more impor- signaling in brain highlight the need for precision of tantly, slow the progressive cognitive loss characteristic of this therapeutic design. The potential benefit of b-amyloid disorder. Alois Alzheimer first described AD in 1906 as clinically clearance from the circulation via CR1-mediated mech- dementia with brain pathologic condition defined by the presence anisms is also reviewed. Therapies that suppress inflam- of extracellular plaque deposits and neurofibrillary tangles. The mation while preserving protective effects of complement plaques contain aggregated b amyloid peptide (Ab), and the could be tested now to slow the progression of this de- tangles are aggregated hyperphosphorylated t (a microtubule- bilitating disease. The Journal of Immunology, 2020, associated protein) (12). It is now clear that reactive microglia by guest on October 1, 2021 204: 306–315. and astrocytes indicative of a glial neuroinflammatory response surround the fibrillar plaques (plaques containing multimers of Ab in a b-sheet fibrillar conformation [fAb]). This, in he complement system is an evolutionarily ancient conjunction with measured elevated levels of inflammatory system contributing to defense from pathogens and mediators in post mortem AD brains, suggests a role for T injury (1). Although the protective effector functions inflammation in the progression of the disease (Refs. 13, 14 of the complement system in the vasculature were the first to and reviewed in Refs. 15, 16). be described, it is now clear that the components of the sys- Most AD cases (∼95–98%) are diagnosed after the age of tem have a plethora of roles in immune mechanisms of ho- 60 and are classified as sporadic or, perhaps a better term, meostasis, the development and retraction of the adaptive late onset Alzheimer’s disease. Although age is the greatest immune responses, intracellular metabolism, and regenera- risk factor for the disease, large genome-wide association tion of injured tissue (reviewed in Refs. 2–4) and that over- studies have identified .25 genetic risk loci, including two activation or dysregulation contributes to disease. There has complement associated genes, clusterin (CLU) and com- been an explosion of new findings of the role of complement plement 1 (CR1) (17–21). Although most variants in the nervous system, from contributions to migration of confer low risk, many have relatively high prevalence in the cells and synapse elimination during development (5, 6) to population and, thus, can combine to generate significant

Department of Molecular Biology and Biochemistry, University of California Address correspondence and reprint requests to Dr. Andrea J. Tenner, Department of Irvine, Irvine, CA 92697; Department of Neurobiology and Behavior, University Molecular Biology and Biochemistry, University of California Irvine, 3205 McGaugh of California Irvine, Irvine, CA 92697; Department of Pathology and Laboratory Hall, Irvine, CA 92697. E-mail address: [email protected] Medicine, School of Medicine, University of California Irvine, Irvine, CA 92697; Abbreviations used in this article: Ab, b amyloid peptide; AD, Alzheimer’s disease; and Institute for Memory Impairment and Neurological Disorders, University of C4BP, C4 binding protein; CR1, 1; fAb,Ab in a b-sheet fibrillar California Irvine, Irvine, CA 92697 conformation; MAC, membrane attack complex; RNA-Seq, RNA sequencing; SNP, ORCID: 0000-0003-3000-024X (A.J.T.). single-nucleotide polymorphism.

Received for publication September 3, 2019. Accepted for publication October 18, Ó 2019. Copyright 2020 by The American Association of Immunologists, Inc. 0022-1767/20/$37.50 This work was supported in part by National Institutes of Health Grants NS 35144, P01 AG 00538, R01 60148, and R21 61746 and the Alzheimer’s Association. www.jimmunol.org/cgi/doi/10.4049/jimmunol.1901068 The Journal of Immunology 307 polygenic risk scores (22, 23). These data strongly suggest bond. C4b binds C2 and the C1s protease, then cleaves C2 that multigenetic, as well as environmental factors, con- into C2a and C2b [in this study, C2b refers to the larger tribute to the disease. Concurrently, large unbiased bio- cleaved fragment of C2 that contains the enzymatic activity informatics analyses reproducibly point to clusters of genetic (36, 37)]. The nascently formed C3 convertase, C4b2b, risk factors in pathways that suggest overlapping processes cleaves C3 into and . C3a influences inflammation contribute to synapse loss, neuronal degeneration, and cog- and cellular responses via C3aR (38–40), and C3b stably nitive dysfunction (21). Importantly, complement, micro- associated with the activator is a potent engaging glia, astrocytes, and immune response pathways involving phagocytic cells, as well as mediating clearance of immune inflammation and/or clearance mechanisms align with the complexes. Some of the cleaved C3b remains associated genetics and pathological features of late onset Alzheimer’s with C4b2b, forming C4b2b3b, the classical pathway C5 disease (13). Gene expression studies in mouse models of convertase. The C5 convertase then cleaves C5 into C5a AD (24–27) support these critical pathways. A somewhat and C5b. C5a can bind to receptors expressed on myeloid, unexpected finding from models that perturb inflammatory mast cells, and other cell types (41, 42), inducing che- pathways via deletion of C5AR1 and TYROBP was that motaxis, increased vascular permeability and stimulating normalization of gene expression pathways and cognitive proinflammatory pathways. C5b initiates the formation of deficits occurred in the absenceofdecreasedamyloidplaque the membranolytic pore-forming C5b-9 complex, capable deposition (25, 26). However, those data are consistent with of lysing pathogens. reports of cognitively normal individuals with substantial The is quite similar to the classical pathway plaque pathologic condition at autopsy (28, 29) and indicate except that the recognition components ficolins, mannan Downloaded from that it is an induced response to plaque that is detrimental in binding lectin (MBL), or other collectins are activated by AD, rather than the plaques alone. Thus, these dysregulated specific repeating carbohydrate structures, and the associated response pathways can provide potential therapeutic targets serine proteases are MASP1/2. The alternative pathway of to alleviate or slow the progression of disease. activation is the result of the direct association of C3b with an Current therapeutics thus far have shown limited efficacy activating surface (43), which is often a pathogen but can also (reviewed in Refs. 30, 31) with recent disappointing results be fibrillar Ab (44). The serine proteases and Factor http://www.jimmunol.org/ from anti-Ab and BACE1 inhibitor therapies. Although B result in the generation of the C3 cleaving enzyme complex steroids, nonsteroidal anti-inflammatories, and antioxidants C3bBb and the alternative C5 cleaving enzyme C3bBb3b. target inflammation, these broad, nonselective drugs have had The alternative pathway can also amplify the classical pathway limited success in AD clinical trials and are accompanied by activation and, thus, could be targeted to decrease excessive substantial side effects (32). These negative clinical results downstream complement activation. may have several possible inadvertent design caveats but Regardless of the activation mechanism, the same major combined with the genome-wide association studies and, effector functions can result: 1) opsonization, via tagging of more recently, gene expression data have led to new concep- activators with cleavage fragments C3b/iC3b (and, to a lesser by guest on October 1, 2021 tual frameworks suggesting that there are many pathways to extent, C4b and C1q), resulting in more efficient receptor- the cognitive loss associated with AD pathologic condition. mediated clearance of pathogens, apoptotic cells, and cellu- Thus, more consideration is now being given to the devel- lar debris by phagocytes; 2) recruitment of immune cells, opment of drugs with highly specific targets that would be including microglia, to the site of injury via production of disease modifying in subsets of AD patients or that would chemotactic peptides, C3a and C5a, and their subsequent reduce excessive detrimental inflammation or promote neu- engagement of specific cell membrane receptors; 3) targeted ronal resilience broadly. death of pathogens because of the sequential interaction of activation cleavage product C5b with C6, C7, C8, and C9, Complement generating the lytic membrane attack complex (MAC); 4) The complement system, composed of over 40 interacting increased vascular permeability; and 5) polarization of cells via proteins in blood and other tissues, is a part of the innate induction of specific gene expression patterns (reviewed in immune response that is critically important for quickly rec- Refs. 33, 45). ognizing and clearing pathogens, dying cells, and misfolded proteins (33). It is also a prominent effector in Ab-mediated Noncanonical roles of complement in the CNS pathogen killing and clearance and contributes to the gener- In 2007, Stevens and colleagues (6) “rocked” the complement ation of the pathogen-specific adaptive immune system re- and neurodevelopment world using high-resolution confocal sponse (34, 35). The complement system is activated by three microscopy to quantify synaptic puncta in mice genetically different recognition pathways (classical, lectin, and alterna- deficient in the complement proteins C1q and C3 to dem- tive), all of which lead to sequential enzyme activation, pro- onstrate a substantial role for the upstream classical comple- tein cleavage, and conformational change to mediate effector ment pathway in eliminating so-called inactive synapses functions (33). The classical complement pathway is activated during the refinement of the developing retinogeniculate by binding of C1q, a component of the , to the Fc system. Microglia ingested C1q- and iC3b/C3b-tagged, pre- domain of Ab in immune complexes or to non-Ig activators sumably “weak,” CD47 low (46), synapses via the comple- such as fibrillar Ab. The interaction-induced conformational ment receptor CR3 (47). At the protein level, C1q was change induces activation of the proenzymes Clr and C1s copurified with synaptosomes containing apoptosis markers within the C1 complex. C1s then cleaves complement com- (48), suggesting that synaptic pruning may involve some of ponent C4, generating and C4b, the latter of which is the same molecular triggers as the complement-mediated covalently deposited on the activator surface by a thioester enhanced clearance of apoptotic cells. Thus, although very 308 BRIEF REVIEWS: COMPLEMENT IN ALZHEIMER’S DISEASE surprising at first, the nervous system appears to have merely (79). During development of the visual system, synthesis of assimilated functions normally provided by complement in C1q is upregulated in neurons (6). In this case, C1q synthesis the tagging of apoptotic cells and cellular material for “silent” by neurons is dependent on TGF-b secreted by astrocytes clearance by phagocytic cells in tissue. However, subsequently, (80). However, in a microglial-specific conditional C1q excessive complement dependent synaptic pruning and/or knockout mouse, blood C1q levels remain unchanged while localized loss of spine density was demonstrated to occur in C1q in the adult brain was absent, implicating microglia as multiple mouse models of neurologic disorders (49–54), often the predominant CNS source of C1q (81). Thus, this com- correlating with deficits in behavioral performance, indicating plement component is present in aging brain even without either aberrant activation or insufficient regulation of the a breakdown of the blood brain barrier. In contrast to the cascade. dominant expression of C1q by microglia, C3 is largely In a distinct mechanism of action, interaction of C1q with produced by reactive astrocytes in brain and C4 appears to myeloid cells, including microglia, in the absence of the en- be synthesized in oligodendrocytes as demonstrated by im- zyme complex C1, suppresses proinflammatory cytokine munohistochemistry (82) and RNA sequencing (RNA-Seq) production and enhances clearance of apoptotic cells and (51, 83, and W.T. Chen, manuscript posted on bioRxiv). C3 neuronal blebs in primary cell cultures (55–58) (Fig. 1). C1q has been used as a marker of A1-activated astrocytes (83). interaction with macrophages or microglia in tissue during Over 35 y ago, it was demonstrated that C1q, the initiating cell turnover, tissue remodeling, or apoptotic cell death, as component of the classical complement cascade, could be may occur in mild sterile injury, is a homeostasis mechanism synthesized in the absence of the C1 serine proteases C1r and to enable clearance without inducing an immune response C1s in peripheral myeloid cells (84). In the brain, upregula- Downloaded from to self Ags (autoimmunity) (59, 60) and as reviewed in tion of C1q expression at the mRNA and protein level is an Refs. 61, 62). This is an area of continuing investigation, early response to injury or disease and, in many cases, is ro- as C1q can engage diverse receptors and coreceptors, bustly produced in the absence of the classical pathway serine thereby mediating diverse cellular responses (reviewed in proteases [as reviewed in Ref. 85 and, more recently, (67)], Refs. 63, 64).” supporting C1-independent C1q-mediated functions. C1r, C1q has direct protective effects on primary cultured neu- C1s, and C3 are upregulated later in models of AD when http://www.jimmunol.org/ rons under nutrient stress or amyloid-induced toxicity, again plaques become more abundant (67). Wu and colleagues (51) without the presence or activation of C1 or any of the report differential synthesis of several complement compo- downstream complement components (65–67) (Fig. 2). Early nents in murine astrocytes and microglial as a function of investigations of the intracellular signaling and induced pat- amyloidosis or tauopathy with age. The temporal and cel- terns of gene expression are intriguing (66, 67) and warrant lular regulation of individual component cascade proteins in further investigation in the pursuit of potential therapeutic normal, aging, and diseased brains is clearly important to advantage. Thus, although targeting C1q to avoid damaging define as several components contribute to different disease downstream events due to the activation of the classical processes and tissue repair or resilience. How transcriptional by guest on October 1, 2021 pathway may have benefits, inhibition of these other neuro- and translational control of complement protein availability protective functions may be counterproductive for neurode- contributes to disease progression will become more clear as generative diseases in general. single-cell RNA-Seq and proteomic studies assessing com- plement component genes in regional and temporal di- Synthesis of complement proteins mensions are completed (W.T. Chen, manuscript posted on The liver had long been recognized as the site of complement bioRxiv). protein production (reviewed in Ref. 68, both constitutively and, in some cases (for example, C3 and MBL), as induc- Complement and AD ible acute phase proteins (69, 70). However, the synthesis As early as 1982, complement proteins were found associated of complement proteins is now recognized to be differen- with amyloid plaques in human AD brain (86). It was then tially induced in multiple cell types, including myeloid cells demonstrated that the complement cascade could be directly throughout the body, and, importantly, can be transcrip- activated in vitro by fibrillar Ab and neurofibrillary tangles tionally regulated in CNS resident neurons, astrocytes, oli- (hyperphosphorylated t) (44, 87–89). C1q, C3b, C4b, and godendrocytes, and microglia with injury or aging (reviewed were verified as being readily associated with fi- in Refs. 9, 71). CNS expression of most complement com- brillar Ab plaques in humans and animal models, providing ponents increases with aging and further increases in AD in vivo correlates to the activation of both classical and al- patients and animal models of AD consistent with a role for ternative pathway, as reviewed in Refs. 90,91, clearly impli- complement in the response to injury and progression of cating the complement system as a player in the inflammatory disease (72–75). However, evidence thus far suggests that scenario. complement system regulators such as C1 inhibitor levels in Clearance of Ab plaques is either impaired in AD or not the aging, AD brain are decreased especially relative to the sufficient to overcome an accumulation of Ab (or both). Al- increased activators, such as misfolded proteins, apoptotic though a portion of Ab is transported out of the brain via cells/damaged neurons, or cell debris (76, 77) and reviewed in LRP-1, and microglia have been shown ingesting amyloid, the Ref. 78, leading to a potentially significant imbalance in mechanisms and relative contributions to clearance of Ab control of inflammatory activation. plaques remains to be clarified (92). Although oligomeric Ab More is known about C1q and C3 synthesis in the brain. An directly stresses neurons, the b-sheet fibrillar amyloid plaque increase in C1q mRNA results in the dramatically increased (and likely protofibrils) is the complement activating con- C1q protein in the normal aging of mouse and human brain formation (93). A beneficial result of complement activation The Journal of Immunology 309 Downloaded from http://www.jimmunol.org/ by guest on October 1, 2021

FIGURE 1. Complement-mediated functions change with disease progression. Early in disease (Phase 1), C1q expression is upregulated by injury or perceived injury independently of other complement cascade proteins. C1q binds apoptotic neurons and neuronal blebs, thereby enhancing phagocytosis by microglia (pink) while also suppressing inflammatory response in the absence of other pathogen- or damage-associated molecular patterns. Phase 2, the complement cascade is chronically activated as fAb and other activators accumulate. C1r, C1s, C4, and C3 synthesis is induced in response to more local damage, whereas the complement regulators C1-INH and C4BP are not comparatively upregulated. Generated C3b/iC3b covalently links to fAb and may lead to phagocytosis. However, in Phase 3, C5a is also generated, diffuses from the plaque, engages C5aR1 on microglia, and induces chemotactic activity recruiting microglia to the plaques. fAb binds to microglial TLR receptors (or others), inducing a synergistic response including proinflammatory cytokine secretion and reactive oxygen species production. Because large fAb plaques are not efficiently cleared, a chronic inflammatory environment develops, contributing to greater neuronal damage conducive to more fAb production and ultimately neuronal dysfunction and death. The contribution of all these events to AD onset and progression in humans remains to be determined. in the case of AD could be opsonization and clearance of generation of downstream mediators C5a and C5b-9. How- misfolded proteins (94, 95). In addition, glutamate contain- ever, the MAC, C5b-9, has also been found in human AD ing vesicular blebs generated by damaged neurons and apo- brain in areas containing fibrillar plaques and tangles (99). ptotic cells bind C1q and are cleared by microglia (58) Although this could result in MAC-induced damage by in- (Fig. 1). Both membrane-bound and soluble complement sertion into cells (adjacent to the plaques) that may not be regulators, including, for example, CD55, , and C4 well protected by host cell CD59 (76), the contribution to binding protein (C4BP), limit the formation of the down- AD dysfunction by the C5b-9 complex remains to be estab- stream C3 and C5 convertases. Factor H is a cofactor for lished. The presence of the MAC nevertheless demonstrates Factor I–mediated cleavage of C3b, whereas C4BP is a co- that complete activation of the cascade does occur within the factor for cleavage of C4b by Factor I, leading to inactivation AD brain, thereby inferring the generation of the generally of C3b and C4b, respectively, and thus inhibiting further C3 proinflammatory fragment, C5a. A large body of previous cleavage activity. Both Factor H and C4BP are colocalized investigations of C5aR1 signaling (reviewed in Ref. 100) in- with plaques and dead cells in AD brains, demonstrating that dicate that the receptor C5aR1 produces a there is some level of inhibition at the C3 convertase level proinflammatory environment by acting on MAPKs and (78, 96–98) that could permit clearance of apoptotic cells, leading to generation of inflammatory cytokines, many of with concomitant attenuation of inflammation, with limited which are known to be expressed in AD brain. Although 310 BRIEF REVIEWS: COMPLEMENT IN ALZHEIMER’S DISEASE Downloaded from

FIGURE 2. Beneficial complement-mediated functions in the brain are retained in the presence of C5aR1 antagonists. A specific C5aR1 antagonist (inhibitor [INH]) will prevent induction of detrimental proinflammatory responses to chronic fibrillar Ab via blocking C5a-induced microglia responses (and possibly induction of A1 astrocytes), as well as block C5a-induced neuronal apoptosis. The antagonist will not affect the direct interaction of C1q with neurons that http://www.jimmunol.org/ promotes survival and resistance to fAb-induced death (left) or the enhancement of microglial uptake of apoptotic cells and neuronal blebs with suppression of inflammatory cytokine release (right). CR3 will remain functional, mediating phagocytosis by engaging iC3b and C3b bound to Ab, apoptotic cells, or other debris (right). C5aR2 would be unaffected and can act as a scavenger of C5a to contribute to the suppression of proinflammatory responses (left and right). Not pictured: formation of membranolytic C5b-9 on pathogens will not be affected. carboxypeptidase N (CPN) reduces C3a and C5a affinity complicates interpretation of the results of either knocking for their receptors and thus reduces corresponding proin- out or inhibiting the early components of complement, as flammatory actions by rapidly cleaving their C-terminal that can influence the generation of multiple downstream by guest on October 1, 2021 Arg, little is known of the expression and role of CPN in functions, which must be considered as potential mecha- the CNS (101). C5aR2, an alternate receptor for C5a, is nisms for the observed results. thought to have C5a scavenger and neuroprotective func- TLRs are pattern-recognition receptors that are important tions (102). C5aR2 can also heterodimerize with C5aR1, sensors for innate immune system that can synergize with or resulting in modulation of that receptor expression/activity antagonize the complement system to initiate and enhance (103), again demonstrating multiple potential levels of the response to pathogens and misfolded proteins (reviewed regulation. in Ref. 107). In the periphery, C5a–C5aR1 signaling was To explore functions and consequences of complement found to synergize with TLR2 and TLR4 stimulation and activation in vivo, mouse models of AD were crossed to C3 enhance proinflammatory cytokine responses (TNF-a and knockout mice or to a transgenic mouse overexpressing the IL-1b) in mouse models (108, 109), mouse macrophages, C3 convertase inhibitor, Crry. Greater amyloid accu- and human monocytes, all of which can be beneficial in the mulation and greater cognitive deficits were observed in resolution of infections (reviewed in Ref. 107). Although it the C3-deficient or Crry-overexpressing mice (94, 104), remains to be determined if similar synergy occurs in CNS, suggesting a beneficial role for activated C3. In contrast, in Ab binding to microglia has been found to involve TLR4 the Tg2576-transgenic mouse, genetic ablation of C1q and TLR2 (110). The early in vitro findings that C5a in- resulted in reduced plaque and glial activation, less loss of creased the release of IL-1b and IL-6 in Ab-primed human synaptophysin in stratum lucidum of CA3 (where mossy monocytes (111) and induced in microglia fibers synapse CA3 pyramidal dendrites), less loss of MAP2 (112) was consistent with a scenario in which fibrillar am- in CA3 pyramidal neurons dendrites, and less cognitive yloid plaque complement activation generates C5a, which decline relative to the C1q sufficient Tg2576 (105). In more then recruits microglia to the plaque. Interaction of fibrillar recent studies, deletion of C1q or C3 in other AD models plaques with TLR (113) on the recruited microglia would was found to be beneficial in cognitive studies (53, 106). synergistically initiate an inflammatory response leading to a This protection was attributed to suppression of excessive neurotoxic environment (Fig. 1). An unbiased integrated synaptic pruning. Similar evidence of the involvement of systems approach identified immune functions and micro- early complement components in detrimental excessive glial activation products including complement, TLR, and synapse elimination has been reported in models of frontal cytokine networks as key nodes correlating with attributes temporal dementia and West Nile virus infection (49, 54). of human late-onset AD (13), and thus as in the periphery, However, the complex role of complement in the brain C5a may play a synergistic role with other damage-associated The Journal of Immunology 311 molecular patterns in the response to perceived danger in patients (29). A potential caveat of all mouse models of AD the brain. studied thus far is that they are more closely aligned with the early onset AD that is the result of mutations in the amyloid Role of C5a and inflammation in AD precursor protein (APP) or the presenilins that cleave APP to b Evidence from multiple systems suggests that inflammation form A , overexpress these proteins under a variety of caused by the complement system is triggered by the promoters that lack endogenous regulation and lack genetic activation-induced cleavage product C5a. C5a is chemotactic diversity seen in the human population (123, 124). New for phagocytes (including microglia) and leads to an alteration candidate models of late-onset AD are currently being gen- of their functional states (reviewed in Ref. 114). In AD, the erated, with a few currently distributed commercially. Thus, complement system may be continually activated by both whereas accumulation of misfolded proteins, complement fibrillar Ab (88, 115) and extracellular tangles formed by activation, and inflammation are hallmarks of AD dementia hyperphosphorylated t (87). This could contribute to a pathologic condition, use of these models will certainly ac- chronic inflammatory state mediated substantially by the celerate the path to effective therapeutic intervention. complement activation product C5a upon binding to its receptor C5aR1 on microglia (Figs. 1, 2). C5aR1 as a therapeutic target Evidence for the concept of microglial priming has long been PMX53 and related PMX205 are cyclic hexapeptide analogs of proposed with proponents advocating peripheral or CNS the C-terminal region of C5a and function as antagonists of origin of the priming and/or secondary inflammatory stimuli C5aR1. PMX205 has a more lipophilic nature than PMX53 Downloaded from (116, 117). Many groups have recognized the dichoto- and, thus, more easily gains access to the brain than PMX53 mous roles of microglia, both detrimental and beneficial, (125, 126). Treatment with PMX205 led to less fibrillar forsometime(118).RNA-Seqand immunohistochemical amyloid accumulation in two mouse models of AD, less ac- data provide evidence that microglia near fAb plaques tivation of microglia and astrocytes, and reduction of cogni- adopt different functional states that enable them to pro- tive loss (127) (Fig. 2). In the 3xTg model that acquires duce proinflammatory cytokinesaswellasreactiveoxygen tangles, PMX205 treatment resulted in a 70% decrease in http://www.jimmunol.org/ species, whereas other microglial subsets enhance phago- hyperphosphorylated t. PMX205 and PMX53 have been cytosis and generate neuroprotective growth factors (24, protective in other models of neurodegeneration (reviewed in 26, 27, 119–121). The functional state of microglia is Ref. 9) and prevented neuronal death in vitro (128, 129). critical because the location and “balance” of different Such a therapy would be less immunocompromising than microglia populations may have significantly different ef- complete inhibition of C1q, C1, or C3, as it leaves opsoni- fects on astrocytes and neurotoxicity versus neurons and zation and the ability to form the MAC on pathogens intact resiliency during CNS diseases (83, 122). If microglia can in the brain and periphery. Complement activation by fAb be inhibited in their differentiation to the disease-activated leading to C5a generation also produces C5b and, in the microglial state or repolarized to perform a more beneficial presence of C6, C7, C8, and C9, would lead to the formation by guest on October 1, 2021 role in neurodegenerative and/or neuroinflammatory dis- of MAC. Given that the plaque would not be damaged by eases such as AD (27, 120), it would certainly be a valuable MAC and that MAC would be susceptible to neutralization therapeutic treatment target. by clusterin or vitronectin complexes in solution (20, 130– Recently, in an additional AD mouse model (Arctic), genetic 132), only insertion into membranes of bystander cells could ablation of C5aR1 prevented spatial specific memory deficits be detrimental. Although in the described mouse models of (26) independent of changes in amyloid load. Importantly, AD C5aR1 receptor antagonist treatment was protective, Sholl analysis of neurons in the CA1 region of the hippo- suggesting a limited consequence of generated MAC, it, of campus revealed a profound decrease in neuronal complexity course, remains to be seen if the protection would also be temporally aligning with cognitive deficits in the C5aR1- effective in humans and at all stages of disease. sufficient AD model mice, whereas no such loss of neuronal PMX53 and another orally available selective antagonist of branching was seen in the AD mice lacking C5aR1. Gene C5aR1, Avacopan, have been tested in human clinical trials for expression data from adult microglia isolated from brain at safety. PMX53 was found to be safe in a human Phase 1 clinical four different ages were examined to investigate the effect of trial for autoimmune diseases (reviewed in Ref. 114). Thus, complement C5a on microglial gene expression. Indeed, a PMX53 and PMX205 could have an accelerated path to lack of C5aR1 prevents the polarization of microglia to a human clinical trials. It is important to note that PMX53 more inflammatory state seen with age and amyloid accu- binds to human C5aR1 with much higher affinity than mouse mulation, whereas expression of genes involved in phagocy- C5aR1 (133), suggesting that beneficial results in murine tosis and lysosomal degradative enzymes were enhanced (26), models may underestimate that in humans. Avacopan (134) is consistent with induction of clearance and repair functions. a small molecule specific antagonist for C5aR1 that was ef- Because C5aR1 knockout Arctic mice maintained neuronal fective in mouse models of antineutrophil cytoplasmic Ab integrity and had no behavior deficit as seen in the C5aR1- (ANCA)–associated vasculitis (135) and is currently in Phase 3 sufficient Arctic mice, treatment with a C5aR1 antagonist, clinical trial for ANCA-associated vasculitis. Brain permeability without blocking C1q or C3, may be an effective treatment has not yet been reported. As mentioned above, whereas a C5a for slowing the progression of cognitive loss in AD and AD- receptor antagonist would block the proinflammatory effects related dementias. The data also confirm that amyloid plaques of C5a (and perhaps enhance phagocytosis and clearance may be necessary (by definition), but not sufficient, to cause pathways), the complement activation fragment, C5b, would the cognitive decline seen in this AD mouse model (26) still be intact and able to participate in the MAC formation and in high-amyloid pathologic condition cognitively intact during a bacterial infection. Thus, this protective function of 312 BRIEF REVIEWS: COMPLEMENT IN ALZHEIMER’S DISEASE complement as well as opsonization, normal synaptic prun- CR1 knock-in mouse has been generated by the Model ing, and neuroprotective effects will not be compromised Organism Development and Evaluation for Late-Onset during systemic treatment with the antagonist. Finally, it Alzheimer’s Disease (MODEL-AD) consortium. Both mice should be noted that 16 y of clinical experience with eculi- are available through The Jackson Laboratory and may enable zumab (an anti-C5 therapeutic Ab that blocks generation of preclinical testing of this novel therapeutic approach. C5a) have indicated a lack of toxicity/adverse effects resulting from the absence of C5a–C5aR1 signaling. In summary, the Conclusions mouse and human data together predict that C5aR1 inhibi- Prevalence of AD in the United States is projected to reach 13 tion is safe and may suppress inflammation and enhance million by 2050, creating a great financial, as well as emotional, homeostatic processes in AD. burden. With no cure or disease-modifying therapeutics available, there is currently a significant unmet medical need to CR1, a multifunctional protein develop therapies for AD. The relatively recent awareness of the CR1 is a host cell–associated regulatory protein that binds role of the early complement components in synapse pruning C3b and, more weakly, C4b and C1q (136). A major CR1 in multiple neurologic diseases adds another level of com- function is to promote the dissociation of C3b-containing C3 plexity and/or opportunity for selective modulation of this convertases and to act as cofactor for Factor I cleavage of C3b process. In addition, colocalization of complement compo- to iC3b (which can no longer participate in C3 or C5 con- nents with both plaques and tangles and the detection of C5b-9 vertase activity), thereby preventing the accumulation of C3b in brain provide clear evidence of activation of the entire Downloaded from and formation of cell lytic C5b-9 on host cells (137). In complement cascade by a variety of activators within the AD humans, erythrocyte CR1 plays a major role in the clearance brain. In animal models, direct and specific inhibition of the of C3b-opsonized immune complexes from the circulation via function of C5aR1 had beneficial effects on both pathologic a mechanism called . C3b linked to im- condition and cognitive behavior in several murine models of mune complexes via its thioester bond, binds to erythrocyte AD. This strategy would leave the beneficial functions of other CR1, which then transports the immune complexes to the complement components, such as C1q and C3b, and C5b-9 http://www.jimmunol.org/ liver and spleen for ingestion, degradation, and thus clearance intact. How suppressing C5aR1 would affect synaptic pruning (138). Polymorphisms in CR1 have been associated with AD is yet unknown. Quantitative studies on the balance between risk, although most single-nucleotide polymorphisms (SNPs) activators and inhibitors of complement in injured brain and are located in noncoding regions of the CR1 gene. Keenan the influence of the complotype (147) on progression of AD and colleagues (18) identified an SNP that is within the could be useful in designing personalized therapies in the coding region of CR1,resulting in an amino acid change future. Although much remains to be clarified, targeting (S1610T) within the protein domain that has been attributed specific effector pathways of complement is justified now as to C1q binding (136). However, interestingly, these SNPs a potential therapeutic strategy for this debilitating neuro- often cosegregate with another genetic polymorphism that is degenerative disease. by guest on October 1, 2021 associated with decreased expression of CR1 on erythrocytes (139–141). As a result, it has been hypothesized that this Acknowledgments lower density of CR1 on red cells lowers peripheral clearance The author thanks Karntipa Pisalyaput and Tracy Cole for the design of Figs. 1 and of Ab and, thus, may contribute to the phenotype of CR1 risk 2, and Nicole Schartz and Gema Olivarria for critically reading the manuscript. variants, although a contribution of CR1 as an extrinsic reg- ulator of complement is not excluded. 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