Complement system on the attack in

John P. Atkinson

J Clin Invest. 2003;112(11):1639-1641. https://doi.org/10.1172/JCI20309.

Commentary

The antiphospholipid syndrome is characterized clinically by fetal loss and thrombosis and serologically by the presence of autoantibodies to lipid-binding proteins. In a model of this procoagulant condition in which these antibodies are injected into pregnant mice, fetal loss was prevented by blocking of complement activation. Specifically, interaction of complement component 5a (C5a) with its is necessary for thrombosis of placental vasculature (see the related article beginning on page 1644). Inhibition of complement activation may have a therapeutic role in this disease.

Find the latest version: https://jci.me/20309/pdf on the attack tion of the classical pathway is a time- honored concept, antibody can also in autoimmunity serve as a site that is protected from plasma and membrane inhibitors of John P. Atkinson complement activation. In particular, these regulators of complement activa- Washington University School of Medicine, St. Louis, Missouri, USA tion are designed to block amplifica- tion of the alternative pathway (AP) on The antiphospholipid syndrome is characterized clinically by fetal loss self tissue (6). To produce arthritis, anti- and thrombosis and serologically by the presence of autoantibodies to body binding at a site relatively defi- lipid-binding proteins. In a model of this procoagulant condition in cient in complement inhibitors allows which these antibodies are injected into pregnant mice, fetal loss was “firing” of the AP (5). Complement prevented by blocking of complement activation. Specifically, interac- component 5a (C5a), interacting with tion of complement component 5a (C5a) with its receptor is necessary its G-coupled receptor, is absolutely for thrombosis of placental vasculature (see the related article beginning required to recruit and to on page 1644). Inhibition of complement activation may have a thera- activate nearby cells (5). Based on deple- peutic role in this disease. tion studies and knockouts, neu- trophils, mast cells, and Fc receptors are J. Clin. Invest. 112:1639–1641 (2003). doi:10.1172/JCI200320309. also necessary in this model of rheuma- toid arthritis (RA) (5, 7). In the first autoimmune condition cascade by the immune complexes is Antiphospholipid syndrome. In the sec- that was recognized as such over a required for disease development in a ond mouse model of autoimmunity, century ago, antibody and comple- mouse model of APS (2). antiphospholipid antibodies derived ment were shown to lyse human red from patients are injected into preg- blood cells in a rare form of hemolyt- Two informative models of humoral nant mice. Remarkably, these anti- ic anemia known as paroxysmal cold autoimmunity that require bodies react with lipid-binding pro- hemoglobinuria (1). Impressively, complement activation teins on endothelial cells to trigger these same two factors had been to mediate tissue injury complement activation, thrombosis, found to mediate lysis of bacterial Rheumatoid arthritis. In one model, that and fetal death secondary to placen- pathogens just three years earlier in of a spontaneously arising inflamma- tal insufficiency (8–11). These patho- 1892. While lysis was the end point in tory arthritis, autoantibodies reactive to logic antibodies attach to lipid-rich those early studies because it was both the ubiquitously expressed intracellular antigens on the developing placental pathologically meaningful and easy to protein glucose-6-phosphate isomerase vasculature and then bind comple- monitor, we now realize that it is the are synthesized (3–5). These IgG ment (12–15). The complement sys- phlogistic and opsonic activities of autoantibodies bind to this autoanti- tem in turn damages the endothelial complement that account for much of gen in the joint and trigger a destructive cells, leading to a procoagulant state its physiologic and pathologic impor- arthritis. Additionally, an acute tran- and undesirable thrombosis. In this tance. The antiphospholipid syn- sient form of synovitis can be produced issue of the JCI, Girardi et al. (2) con- drome (APS) is an autoimmune dis- by the passive transfer of antibody vincingly demonstrate that both the ease in which autoantibodies are alone. While antibody-mediated activa- classical and alternative pathways are synthesized to lipids and lipid-bind- ing proteins. These antibodies bind to antigens expressed by endothelial cells and produce a clinical syndrome fea- turing thrombosis. Fetal wastage is caused by placental infarction. In this issue of the JCI, Girardi et al. report that activation of the complement

Address correspondence to: John P. Atkinson, Campus Box 8045, 660 South Euclid Avenue, St. Louis, Missouri 63110-1093, USA. Phone: (314) 362-8391; Figure 1 Fax: (314) 362-1366; can be deposited on a target by immune complexes that commonly engage the classi- E-mail: [email protected]. cal pathway, by lectins such as mannose-binding protein that bind to sugars, or through the Conflict of interest: The author has declared that no conflict of interest exists. continuous low-grade turnover of the AP. The deposited C3b serves as a nidus for amplifi- Nonstandard abbreviations used: anti- cation of C3b deposition via the positive feedback loop. Control of this amplification process phospholipid syndrome (APS); alternative is critical to prevent undesirable injury to host cells. The presence of antibodies or lectins on pathway (AP); complement component 5a a membrane at a site relatively lacking in membrane regulators is conducive to excessive com- (C5a); rheumatoid arthritis (RA). plement activation on self tissue.

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Requirements for complement activation on self tissue in autoimmunity other vascular endothelial cells to cause venous and arterial thrombosis. 1. Autoantibodies What, then, are the implications for 2. Protected site relative to complement inhibitors the treatment of these conditions with Antibodies themselves are a protected site. If they also bind to antigen located in a site that is relatively a complement inhibitor? The trial of deficient in regulators (such as joint cartilage in the RA model or developing placental vasculature in an mAb that inhibits C5 was effica- the APS model), a particularly tissue-damaging situation occurs. cious in RA but not to the same extent as inhibition of TNF (20). For APS, required to generate sufficient C5 model, where the evidence points to treatment has focused primarily on cleavage to cause fetal loss. The antibody binding and complement anticoagulation (21). Complement released C5a binds to its receptor on activation taking place at a site rela- inhibition would seem to be an attrac- neutrophils and endothelial cells. In tively deficient in regulators (5). Fur- tive therapeutic target in some of these this model, neutrophils are again thermore, the autoantibodies them- patients. In particular, C5a and its required but Fcγ receptors are not. selves serve as a protected site for receptor should now become prime Simply put, a procoagulant state is complement activation (6). Genetic targets for inhibition. Girardi and col- set up, clots form, placental infarc- deficiencies of complement inhibitors leagues have substantially advanced tion ensues, and embryos die. reinforce these points. For example, in our understanding of the APS model paroxysmal nocturnal hemoglobin- by showing us factors required to Local requirements for uria, red blood cells are lysed because mediate the associated tissue injury, complement activation of an acquired clonal stem cell inacti- and this, in turn, may have therapeutic The complement-activation scheme is vation of the enzyme required to implications (2). different in the two models. In the APS anchor two regulators of complement 1. Silverstein, A.M. 1989. A history of immunology. model, the classical pathway triggers on the erythrocyte membrane (18). In Academic Press Inc. San Diego, California, USA. the process by depositing a relatively atypical (nondiarrheal) hemolytic 422 pp. 2. Girardi, G., et al. 2003. Complement C5a recep- small amount of C3b on the target, uremic syndrome, heterozygous defi- tors and neutrophils mediate fetal injury in the and then this C3b serves as a nidus for ciency of one of the complement antiphospholipid syndrome. J. Clin. Invest. 112:1644–1654. doi:10.1172/JCI200318817. the AP’s self-amplifying feedback loop inhibitors or CD46 predis- 3. Matsumoto, I., Staub, A., Benoist, C., and Math- (Figure 1). The AP deposits the major- poses to a thrombotic microangiopa- is, D. 1999. Arthritis provoked by linked T and B ity of the C3b and cleaves most of the thy because of excessive complement cell recognition of a glycolytic enzyme. Science. 286:1732–1735. C5. Another particularly intriguing activation (17, 19). The take-home les- 4. Matsumoto, I., et al. 2002. How antibodies to a aspect of this model is the indication sons from these reports are that com- ubiquitous cytoplasmic enzyme may provoke joint-specific autoimmune disease. Nat. Immunol. that neutrophils enhance the comple- plement activation on vessel walls 3:360–365. ment-activation process. In the RA must be rigorously controlled and 5. Ji, H., et al. 2002. Arthritis critically dependent on model, the AP alone can provide the that even heterozygous deficiency of innate immune system players. Immunity. 16:157–168. necessary C3b and subsequent C5 acti- an inhibitor predisposes to thrombo- 6. Ratnoff, W.D., Fearon, D.T., and Austen, K.F. vation (5). As noted, plasma and mem- sis if a vessel wall is under siege. A rea- 1983. The role of antibody in the activation of the brane complement regulators are par- sonable hypothesis, therefore, to alternative complement pathway. Springer Semin. Immunopathol. 6:361–371. ticularly effective at blocking the explain clot formation in APS is that 7. Wipke, B.T., and Allen, P.M. 2001. Essential role feedback loop of the AP (16, 17) but the developing placental vasculature of neutrophils in the initiation and progression of a murine model of rheumatoid arthritis. are less able to control antibody-medi- is relatively deficient in complement- J. Immunol. 167:1601–1608. ated classical-pathway activation. inhibitor function. 8. Branch, D.W., et al. 1990. Thus, this potentially lethal amplifica- fractions from patients with antiphospholipid antibodies cause fetal death in BALB/c mice: a tion process occurs routinely and Implications for therapy model for autoimmune fetal loss. Am. J. Obstet. robustly against foreign materials Are these pathologic situations in the Gynecol. 163:210–216. 9. Blank, M., Cohen, J., Toder, V., and Shoenfeld, Y. (which in most cases lack regulators). mouse similar to the human diseases 1991. Induction of antiphospholipid syndrome This line of reasoning accounts for RA and APS? At the minimum, these in naive mice with mouse lupus monoclonal and why large amounts of autoantibodies studies identify signaling pathways human polyclonal antibodies. Proc. Natl. Acad. Sci. U. S. A. 88:3069–3073. that activate the classical pathway are and mechanisms associated with tis- 10. Piona, A., et al. 1995. Placental thrombosis and so damaging, yet low levels of autoan- sue destruction that should be fur- fetal loss after passive transfer of mouse lupus tibodies or autoreactive lectins are so ther investigated in individuals suf- monoclonal or human polyclonal anti-cardi- olipin antibodies in pregnant naive BALB/c mice. well tolerated. fering from RA or APS. At the Scand. J. Immunol. 41:427–432. Consequently, for excessive and maximum, they represent almost 11. Ikematsu, W., et al. 1998. Human anticardiolipin monoclonal autoantibodies cause placental unwanted complement activation to exactly what takes place in RA and necrosis and fetal loss in BALB/c mice. Arthritis occur on self tissue, something is like- APS. In the former case, one would Rheum. 41:1026–1039. ly to be amiss relative to the host’s argue that we have yet to define the 12. Rand, J.H., et al. 1997. Pregnancy loss in the antiphospholipid-antibody syndrome: a possible complement inhibitors (see Require- autoantibody and autoantigen com- thrombogenic mechanism. N. Engl. J. Med. ments for complement activation on self tis- bination. In the latter case, one would 337:154–160. 13. Pierangeli, S.S., et al. 1999. Antiphospholipid sue in autoimmunity). This point is say that the same pathologic process antibodies from antiphospholipid syndrome clearly illustrated in the arthritis occurs in the human placenta and on patients activate endothelial cells in vitro and in

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vivo. Circulation. 99:1997–2002. J. Immunol. 168:6298–6304. complement regulator, membrane cofactor pro- 14. Simantov, R., et al. 1995. Activation of cultured 17. Manuelian, T., et al. 2003. Mutations in factor H tein (CD46), predispose to development of vascular endothelium by antiphospholipid anti- reduce binding affinity to C3b and heparin and familial hemolytic uremic syndrome. Proc. Natl. bodies. J. Clin. Invest. 96:2211–2219. surface attachment to endothelial cells in Acad. Sci. U. S. A. 100:12966–12971. 15. Laudes, I.J., et al. 2002. Expression and function hemolytic uremic syndrome. J. Clin. Invest. 20. Tesser, J., et al. 2001. Safety and efficacy of the of in mouse microvascular 111:1181–1190. doi:10.1172/JCI200316651. humanized anti-C5 antibody h5G1.1 in patients endothelial cells. J. Immunol. 169:5962–5970. 18. Atkinson, J.P., and Bessler, M. 2000. Paroxysmal with rheumatoid arthritis. Arthritis Rheum. 16. Barilla-LaBarca, M.L., Liszewski, M.K., Lambris, nocturnal hemoglobinuria. In The molecular basis 44(Suppl.):S274. (Abstr.) J.D., Hourcade, D., and Atkinson, J.P. 2002. Role of blood disease. W.B. Saunders Co. Philadelphia, 21. Petri, M. 2003. Evidence-based management of of membrane cofactor protein (CD46) in regu- Pennsylvania, USA. 564–577. thrombosis in the antiphospholipid antibody lation of C4b and C3b deposited on cells. 19. Richards, A., et al. 2003. Mutations in human syndrome. Curr. Rheumatol. Rep. 5:370–373.

Dominant-negative diabetes insipidus and is characterized by an autosomal dom- inant (AD) mode of inheritance (Figure other endocrinopathies 1b). For example, familial neurohy- pophyseal diabetes insipidus (FNDI) John A. Phillips III occurs in subjects heterozygous for a variety of AVP gene mutations. Division of Medical Genetics, Vanderbilt University School of Medicine, Nashville, When a mutation in one allele pre- Tennessee, USA vents both it and the normal allele from producing sufficient gene prod- Familial neurohypophyseal diabetes insipidus (FNDI) in humans is an uct to prevent clinical symptoms, the autosomal dominant disorder caused by a variety of mutations in the disorder is also characterized by an AD arginine vasopressin (AVP) precursor. A new report demonstrates how mode of inheritance (Figure 1c). Note heterozygosity for an AVP mutation causes FNDI (see the related article that the sum of the gene-expression beginning on page 1697). Using an AVP knock-in mutation in mice, the activities of the mutant and normal study shows that FNDI is caused by retention of AVP precursors and pro- alleles is less than that of a single nor- gressive loss of AVP-producing neurons. mal allele, because of the effect of the dominant-negative (DN) mutation on J. Clin. Invest. 112:1641–1643 (2003). doi:10.1172/JCI200320441. the normal allele (Figure 1c). The DN mutation decreases expression of the Autosomal recessive, autosomal severity of disease. For example, the normal gene in the same cell so that dominant, and dominant-negative sums of the amounts of gene products the sum of their gene expression does mutations synthesized from both alleles at an not reach the threshold needed to pre- Genes are transcribed into mRNA, autosomal locus, representing the net vent disease. which is translated into a protein prod- activity of gene expression, are shown uct. In the case of arginine vasopressin in Figure 1. When one of two paired FNDI (AVP), the protein product is synthe- alleles produces sufficient protein to Both the AVP and the oxytocin (OT) sized as a much larger prohormone, overcome the presence of a mutation prohormones are synthesized in the which includes AVP, its carrier protein in the second allele, homeostasis is supraoptic nucleus (SON) and par- called neurophysin, and a glycoprotein. maintained and clinical manifesta- aventricular nucleus (PVN) of the The activity of different versions of tions of protein deficiency do not hypothalamus (2). AVP and OT are genes (called alleles) and the quantita- occur. In such cases, the related disor- produced by separate populations of tive and qualitative characteristics of der is characterized by an autosomal magnocellular neurons in both nuclei, their respective gene products can be recessive (AR) mode of inheritance, packaged into neurosecretory vesicles correlated with the incidence and and the heterozygous carrier does not with the neurophysins, and trans- manifest clinical symptoms of disease ported to the neurohypophysis, where Address correspondence to: John A. Phillips (Figure 1a, left). However, when the they are either stored or secreted into III, Division of Medical Genetics, Department net activity of gene expression of two the circulation. of Pediatrics, Vanderbilt University School of mutant alleles is not sufficient to pre- In three members in three genera- Medicine, DD-2205 Medical Center North, Nashville, Tennessee 37232-2578, USA. vent disease, such as in the case of tions of a Japanese family with FNDI, Phone: (615) 322-7601; Fax: (615) 343-9951; Brattleboro rats homozygous for a sin- Nagasaki et al. (3) found a TGC-to- E-mail: [email protected]. gle-base deletion in exon 2 of both AVP TGA transversion at nucleotide posi- Conflict of interest: The author has declared genes, diabetes insipidus with an AR tion 1891 that encodes a Cys67Ter that no conflict of interest exists. mode of inheritance occurs (ref. 1; Fig- change. The prematurely terminated Nonstandard abbreviations used: arginine vasopressin (AVP); autosomal recessive (AR); ure 1a, right). AVP product was predicted to lack autosomal dominant (AD); familial In contrast, when a single normal part of the neurophysin II and glyco- neurohypophyseal diabetes insipidus (FNDI); allele does not have sufficient gene- protein moieties. While the function dominant-negative (DN); oxytocin (OT); expression activity to prevent clinical of AVP is well characterized, and neu- supraoptic nucleus (SON); paraventricular nucleus (PVN); growth hormone (GH); symptoms of disease, the heterozygous rophysin II is known to act as a carrier isolated GH deficiency type II (IGHD II). individual is affected and the disorder protein, the function of the glycopro-

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