Antibody directs properdin-dependent activation of the complement alternative pathway in a mouse model of abdominal aortic aneurysm

Hui-fang Zhoua, Huimin Yana, Cordula M. Stoverb, Tamara Montes Fernandezc, Santiago Rodriguez de Cordobac, Wen-Chao Songd, Xiaobo Wua,RobertW.Thompsone, Wilhelm J. Schwaebleb, John P. Atkinsona, Dennis E. Hourcadea,1,andChristineT.N.Phama,1 aDivision of Rheumatology, Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110; bDepartment of Infection, Immunity and Inflammation, University of Leicester, Leicester LE1 9HN, United Kingdom; cDepartment of Cellular and Molecular Medicine, Centro de Investigaciones Biológicas (CSIC) and Ciber de Enfermedades Raras (CIBERER), 28040 Madrid, Spain; dInstitute for Translational Medicine and Therapeutics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104; and eSection of Vascular Surgery, Department of Surgery, Washington University School of Medicine, St. Louis, MO 63110 AUTHOR SUMMARY

Abdominal aortic aneurysm response, in general, and pro- (AAA) is a complex, inflam- perdin, in particular, merit matory vascular disease, and it is additional exploration. among the top 20 leading causes Properdin is found systemi- of death in the United States. cally in the circulation but is also Surgical repair is currently the stored and rapidly released from only option for management. the granule components of However, mortality and morbid- a type of immune cells called ity are significant even after neutrophils during cell stimula- elective surgery, especially in the tion. We showed that, although older patients who are commonly addition of an outside source of affected. A better understanding neutrophils to properdin-de- Fig. P1. Proposed mechanism of AP complement activation and fi of the underlying immune-medi- order of events in elastase-induced AAA disease. (1) Elastase cient mice could deliver the ated inflammatory pathways in- perfusion unmasks hidden neoepitopes (newly exposed self-antigens) properdin needed to cause AAA volved in AAA may lead to that are recognized by IgG antibody, providing a platform for to some extent, adding pro- treatments that could halt dis- properdin-stabilized AP C3 convertase (C3bBbP) assembly. (2) perdin in the form of mouse se- ease progression. Here, we found Complement activation leads to the release of anaphylatoxins (C3a rum is fully sufficient to promote roles for a particular protein, and C5a. (3) These molecules attract immune cells called neutrophils. aneurysm formation in our fi (4) Activated neutrophils release chemicals that attract other mouse model. To determine properdin, and speci c natural inflammatory immune cells, including monocytes and macrophages. antibodies in the genesis (5) Macrophages release proteases and other inflammatory mediators whether properdin initiates the of AAA. that cause structural damage (weakening of the aortic wall) and AP, we induced a substitution The complement system is eventual aneurysmal changes characteristic of AAA. mutation, D279A, of human a part of the immune system that factor B (fB), an essential com- can rapidly respond to foreign ponent of the AP that is a pro- intruders without prior exposure, causing inflammation and activator of C3. This substitution, which has been described in other biological responses. Complement is activated through previous studies (4), produces a stable C3 convertase in the three pathways, including the alternative pathway (AP), which absence of properdin. We reasoned that, if the essential role of operates continuously because of hydrolysis of the protein C3. properdin in AAA development is to stabilize the AP convertase, We previously established that the AP plays a major role in AAA then the introduction of such a stabilizing fB mutant protein development (1); however, the molecular mechanisms behind into properdin-deficient animals would compensate for the this role remained undefined. properdin deficiency, restoring AAA pathology. For these Properdin is a component of the AP C3 convertase, a critical experiments, a D279G-equivalent mouse fB mutant protein, enzyme in the AP activation pathway. Properdin has long been D276G, was generated. Introduction of D276G protein into known to stabilize this otherwise short-lived molecular complex properdin-deficient mice restored susceptibility to aneurysm (2). Recently, evidence has accumulated that properdin can development, suggesting that the critical function of properdin in also bind certain cellular and microbial surfaces and provide this case is stabilization of the AP convertase and not initiation. a platform for AP C3 convertase assembly and function (3). We previously found compelling evidence for an association of properdin with human AAA (1). In the present study, we sought Author contributions: H.-f.Z., H.Y., D.E.H., and C.T.N.P. designed research; H.-f.Z., H.Y., to test the hypothesis that properdin plays a central role in T.M.F., and C.T.N.P. performed research; C.M.S., T.M.F., S.R.d.C., W.-C.S., X.W., and W.J.S. AAA development with a model of AAA induced by transient contributed new reagents/analytic tools; H.-f.Z., H.Y., T.M.F., D.E.H., and C.T.N.P. analyzed perfusion of elastase (an enzyme that breaks down proteins). data; and C.M.S., S.R.d.C., R.W.T., W.J.S., J.P.A., D.E.H., and C.T.N.P. wrote the paper. Using properdin-deficient mice, we first confirmed that pro- The authors declare no conflict of interest. perdin is required for AP complement-mediated acute in- This article is a PNAS Direct Submission. flammatory response in elastase-induced AAA. Mice deficient 1To whom correspondence may be addressed. E-mail: [email protected] or in properdin were protected against aneurysm formation. [email protected]. Blockade of properdin activity also protected against AAA de- See full research article on page E415 of www.pnas.org. velopment, suggesting that strategies that target the complement Cite this Author Summary as: PNAS 10.1073/pnas.1119000109.

2202–2203 | PNAS | February 14, 2012 | vol. 109 | no. 7 www.pnas.org/cgi/doi/10.1073/pnas.1119000109 Downloaded by guest on October 2, 2021 Several studies have previously described the presence of a molecule recognized by an antibody) that bind natural IgG PNAS PLUS antibodies or Igs in human AAA tissues, but it has proven dif- antibodies, and the resulting surface-bound Igs provide pro- ficult to establish a causative role for these antibodies in AAA. tected sites that promote the assembly and activation of pro- In general, antibodies are immune system molecules that rec- perdin-stabilized AP C3 convertase. The ensuing generation of ognize and bind foreign invaders. IgM antibodies are usually anaphylatoxins (C3a and C5a) by the convertases results in produced immediately after exposure to foreign invaders, activation and recruitment of immune cells to the site of injury, whereas IgG antibodies evolve later in the immune response. where they initiate inflammation that causes the formation of Natural antibodies (IgM and IgG), however, are Igs present in an AAA (Fig. P1). individual before foreign invader exposure. Natural antibodies Collectively, our results suggest that properdin plays a critical can bind to self-antigens (i.e., act as autoantibodies in recog- role in AAA development. It, therefore, represents a potentially nizing constituents of the body’s own tissues). Because properdin attractive therapeutic target given that it is unique to the AP did not seem to initiate complement activation, we next in- and present at relatively low concentrations. Moreover, we re- vestigated whether the AP might be activated through an anti- veal an unexpected role for autoantibodies in the pathogenesis of body-dependent mechanism (5). First, we examined aortas AAA. These findings support additional efforts aimed at eluci- obtained from mice perfused with elastase. We found abundant dating the identity of the self-antigens recognized by these IgG, but not IgM, deposition in the aortic wall. autoantibodies, because this identification may yield additional Next, we induced disease in a cohort of μMT mice that are therapeutic targets for this common disorder. deficient in B cells (immune system cells that produce Igs) and hence, deficient in all Igs. The absence of Igs abrogated C3 de- 1. Pagano MB, et al. (2009) Complement-dependent neutrophil recruitment is critical for position in elastase-perfused aortic wall and protected μMT the development of elastase-induced abdominal aortic aneurysm. Circulation 119: mice from AAA development. Moreover, reconstitution with 1805–1813. fi 2. Fearon DT, Austen KF (1975) Properdin: Binding to C3b and stabilization of the C3b- natural mouse IgG antibodies puri ed from normal, unmutated dependent C3 convertase. J Exp Med 142:856–863. mice restored susceptibility to AAA in μMT mice. These results 3. Kemper C, Hourcade DE (2008) Properdin: New roles in pattern recognition and target suggest that natural IgG antibodies recognize an exposed clearance. Mol Immunol 45:4048–4056. self-antigen in the elastase-perfused aortic wall, and their pres- 4. Hourcade DE, Mitchell LM, Oglesby TJ (1999) Mutations of the type A domain of fi ence supports AP activation and aneurysm formation. complement factor B that promote high-af nity C3b-binding. J Immunol 162: 2906–2911. In summary, we propose that, in the AAA mouse model, 5. Ratnoff WD, Fearon DT, Austen KF (1983) The role of antibody in the activation of the elastase treatment unmasks hidden epitopes (i.e., the parts of alternative complement pathway. Springer Semin Immunopathol 6:361–371. IMMUNOLOGY

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