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

8 páginas, 5 figuras, 1 figura P1 -- PAGS nros. E415-E422There are currently limited treatment options for AAA when surgery is inapplicable. Therefore, insights into molecular mechanisms underlying AAA pathogenesis may reveal therapeutic targets that could be manipulated pharmacologically or biologically to halt disease progression. Using an elastase-induced AAA mouse model, we previously established that the complement alternative pathway (AP) plays a critical role in the development of AAA. However, the mechanism by which complement AP is initiated remains undefined. The complement protein properdin, traditionally viewed as a positive regulator of the AP, may also initiate complement activation by binding directly to target surfaces. In this study, we sought to determine whether properdin serves as a focal point for the initiation of the AP complement activation in AAA. Using a properdin loss of function mutation in mice and a mutant form of the complement factor B protein that produces a stable, properdin-free AP C3 convertase, we show that properdin is required for the development of elastase-induced AAA in its primary role as a convertase stabilizer. Unexpectedly, we find that, in AAA, natural IgG antibodies direct AP-mediated complement activation. The absence of IgG abrogates C3 deposition in elastase-perfused aortic wall and protects animals from AAA development. We also determine that blockade of properdin activity prevents aneurysm formation. These results indicate that an innate immune response to self-antigens activates the complement system and initiates the inflammatory cascade in AAA. Moreover, the study suggests that properdin-targeting strategies may halt aneurysmal growth.Abdominal aortic aneurysm (AAA) is among the top 20 leading causes of mortality in the United States. It accounts for ∼15,000 deaths/y. AAA is a complex disease associated with male sex, advanced age, hypertension, hypercholesterolemia, coronary artery disease, atherosclerosis, and cigarette smoking (1⇓–3). Presently, open surgical repair of large AAA (greater than 5.5 cm in diameter) is an effective option in preventing death from rupture. However, surgical treatment is associated with high postoperative mortality (up to 6%) (4). Endovascular repair is associated with lower postoperative mortality, but it offers no significant difference in overall long-term survival rate and has a higher incidence of reintervention (5⇓⇓–8). Given the high complication rate and morbidities associated with surgical treatment, medical therapy designed to slow progression of AAA would be desirable. Although the causes of AAA are not completely understood, it is apparent that the immune system is a major contributor to pathogenesis. Therefore, a better understanding of the underlying immune-mediated pathways involved in AAA may identify new targets that could be manipulated pharmacologically or biologically to halt disease progression. The complement system is a branch of immunity that can rapidly respond to foreign intruders or pathogens without prior sensitization or exposure. It plays a key role in the identification and removal of injured tissue and debris (9), modulates the antibody repertoire (10), and participates in the activation of T-cell populations (11). Complement activation is mediated by the classical pathway (CP), which is commonly initiated by antigen–antibody complexes, the lectin pathway (LP), which is initiated by LP-specific carbohydrate recognition complexes, and the alternative pathway (AP), which is constitutively active at low level because of continuous hydrolysis of the complement protein C3. The three pathways converge on a central enzyme, the C3 convertase. This enzyme complex activity generates opsonins that promote target clearance, leads to the assembly of the membrane attack complex, and releases anaphylatoxins (C3a and C5a) that activate and recruit inflammatory cells (9). We have shown that the complement system is a critical mediator of elastase-induced AAA (12). Using mice deficient in factor B (fB; an essential component of the AP) and C4 (a component of both the LP and CP), we established that the AP plays a major role in AAA development, whereas the CP and LP seem dispensable. Unlike the CP and LP, which are initiated by specific target structures, the AP is continuously activated in the fluid phase and on cell surfaces. It is limited to safe steady-state levels through the action of fluid-phase and surface-bound regulatory proteins. Several AP-dependent diseases, including age-related macular degeneration and atypical hemolytic uremic syndrome (aHUS), have been shown to be, in part, the result of inadequate regulation because of genetic variants in the complement regulator genes (13).The AP C3 convertase is formed from three components: fB, a zymogen that carries the convertase catalytic site, C3b, which anchors the convertase to the target surface, and properdin, which stabilizes the complex by 5- to 10-fold (14). More recently, evidence has emerged that properdin may act as an initiator of the AP, binding directly to target cell surfaces and providing a platform for the AP C3 convertase assembly (15, 16). We previously obtained compelling histochemical evidence for the presence of properdin on the luminal surface of human AAA specimens (12), and this finding led us to hypothesize that properdin may serve as a focal point for the initiation of the AP in AAA. Here, we show that properdin plays a critical role in the pathogenesis of elastase-induced AAA, suggesting that properdin-targeting strategies merit additional investigation. We assessed the role of properdin in AAA as a stabilizer vs. initiator of AP convertase by using a mutant form of the fB protein that produces a stable properdin-free AP C3 convertase. Our findings indicate that the primary role of properdin in elastase-induced AAA is as a convertase stabilizer. However, we found that complement activation in AAA is dependent on the presence of natural antibodies, lending support to previous reports suggesting that recognition of self-antigen(s) by antibodies initiates the inflammatory cascade in AAA (17)This work was supported by British Heart Foundation Grant PG/09/053/27836 (to C.M.S.); the Ciber de Enfermedades Raras and the Fundación Renal Iñigo Alvarez de Toledo (T.M.F. and S.R.d.C.); National Institutes of Health Grants AI049261 (To C.T.N.P.), AI049261-08S1 (To C.T.N.P.), AI085596 (to W.-C.S.), HL56701 (to R.W.T.), AI041592 (to J.P.A.), AI051436 (to D.E.H.), and AI049261-08S2 (to C.T.N.P.); and Spanish Ministerio de Ciencia e Innovacion Grant SAF2008-00226 (to S.R.d.C.).Peer reviewe