Properdin: a tightly regulated critical inflammatory modulator

Abstract

The complement alternative pathway is a powerful arm of the innate immune system that enhances diverse inflammatory responses in the human host. Key to the effects of the alternative pathway is properdin, a serum glycoprotein that can both initiate and positively regulate alternative pathway activity. Properdin is produced by many different leukocyte subsets and circulates as cyclic oligomers of monomeric subunits. While the formation of non-physiological aggregates in purified properdin preparations and the presence of potential properdin inhibitors in serum have complicated studies of its function, properdin has, regardless, emerged as a key player in various inflammatory disease models. Here, we review basic properdin biology, emphasizing the major hurdles that have complicated the interpretation of results from properdin-centered studies. In addition, we elaborate on an emerging role for properdin in thromboinflammation and discuss the potential utility of properdin inhibitors as long-term therapeutic options to treat diseases marked by increased formation of platelet/granulocyte aggregates. Finally, we describe the interplay between properdin and the alternative pathway negative regulator, Factor H, and how aiming to understand these interactions can provide scientists with the most effective ways to manipulate alternative pathway activation in complex systems.

Keywords: complement; factor H; inflammation; properdin.

Figure 1

Activation of the alternative pathway. Described from upper right to left: The thioester bond in C3 is spontaneously hydrolyzed by water, leading to the formation of C3(H₂O), which can recruit Factor B (FB). Once bound to C3(H₂O), FB is cleaved by Factor D (FD) to Bb to form the alternative pathway fluid phase C3 convertase, C3(H₂O)Bb. The C3 convertase cleaves C3 to C3a and C3b, which can bind covalently to nearby amino‐ and hydroxyl‐groups via its thioester group. C3b covalently bound to a surface recruits FB, which is subsequently cleaved by FD to form the alternative pathway cell‐surface C3 convertase, C3bBb. While C3bBb has a half‐life of only approximately 90 seconds, properdin (P) stabilizes the convertase to increase its activity 5‐ to 10‐fold

Figure 2

Alternative pathway amplifies all complement activity. C3b originally deposited on a surface by the classical pathway can act as a site for formation of the alternative pathway cell‐surface C3 convertase. The alternative pathway deposits more C3b on the surface, which can act as additional sites for alternative pathway C3 convertase formation. Therefore, even minor complement activity initiated by the classical (or lectin) pathway can be quickly and efficiently amplified by the alternative pathway

Figure 3

Factor H structure and function. (A) Simplified schematic representation of Factor H and disease associations. Factor H is composed of 20 homologous complement control protein (CCP) domains. The N‐terminal 4 domains bind C3b and contain the regulatory functions of Factor H, while the C‐terminal domains 19–20 bind both C3b and polyanions to anchor Factor H to cell surfaces. Dense deposit disease (DDD) occurs when the N‐terminal domains are impaired or absent (due to Factor H deficiency), whereas most mutations in Factor H associated with atypical hemolytic uremic syndrome (aHUS) are found in the C‐terminus, resulting in defective cell‐surface alternative pathway regulation. The Y402H polymorphism in domain 7 is strongly associated with the development of age‐related macular degeneration (AMD). (B) Factor H regulatory functions. Factor H N‐terminal domains 1–4 regulate the alternative pathway via three different mechanisms: (left) competing with Factor B (FB) for binding to C3b; (middle) accelerating the decay of the alternative pathway C3 convertase; and (right) acting as a cofactor for FI‐mediated cleavage of C3b to iC3b, a C3 fragment that cannot bind FB

Figure 4

C‐terminal domains 19–20 mediate Factor H cell‐surface protection. (Left) Factor H domains 19–20 bind simultaneously to C3b and polyanions (blue lines) on cell surfaces. These domains are the primary region of Factor H that anchors the protein to the cell surface to allow efficient alternative pathway regulation by N‐terminal domains 1–4. (Right) AHUS‐related mutations impair the ability of the domains 19–20 to bind to the C3b/polyanion combination on cell surfaces, and thus Factor H is dysfunctional in its ability to regulate the alternative pathway on cell surfaces, but not in the fluid phase

Figure 5

Properdin structure. Properdin monomers are composed of 7 thrombospondin repeat (TSR) domains labeled TSR0‐6. Under physiological conditions, properdin monomers form cyclic dimers, trimers, and tetramers in an approximately 1:2:1 ratio. Molecular modeling suggests that the vertices of properdin oligomers are composed of a total of four domains comprised from two different monomers in a head‐to‐tail organization. The exact organization of domains at the vertices has not been elucidated, but depicted here are theoretical models for oligomers in which the vertices are formed by TSR0‐1 from one monomer and TSR5‐6 from the other. Model proposed by Alorco et al54

Figure 6

Properdin functions. Properdin (P) enhances alternative pathway activity in one of two ways: (1) acting as a positive regulator of pre‐existing alternative pathway activity or (2) initiating alternative pathway activity. Properdin acts as a positive regulator by stabilizing the alternative pathway C3 and C5 convertases, increasing their activity 5‐ to 10‐fold. As a pattern recognition molecule, properdin binds selectively to specific surfaces upon which it recruits C3b or C3(H₂O) to initiate alternative pathway activity

Figure 7

Main physical contacts governing platelet/granulocyte aggregate interactions. P‐selectin initially tethers activated platelets to granulocytes via binding to PSGL‐1. This interaction leads to an intracellular signaling cascade that activates and increases the expression of CR3 on granulocytes. CR3 binds multiple ligands on platelets including GPIIb/IIIa through a fibrinogen bridge, GP1bα directly and via a HMWK bridge, ICAM‐2, and JAM‐3. CD40L and TREM‐1L, expressed on activated platelets, enhance granulocyte functions via binding to their granulocyte counter‐receptors, CD40 and TREM‐1, respectively

Figure 8

Complement activation on platelets. Platelets activate both the alternative and classical pathways on their surface. Expression of the surface receptor, gC1qR, is increased after stimulation with shear stress and binds C1q to initiate classical pathway activation. Chondroitin sulfate released from platelets upon activation also binds C1q to initiate classical pathway activity in the fluid phase. Physiological properdin (P) binds directly to activated platelets and recruits C3(H₂O) to initiate alternative pathway activity. C3(H₂O) also binds directly to activated platelets and leads to alternative pathway activation in the presence of properdin. Properdin‐mediated alternative pathway activation on platelets is controlled by Factor H

Figure 9

Model for the mechanism of the effects of properdin on platelet/granulocyte aggregate (PGA) formation and complement activation on each cell type. (A) Activated platelets initially tether to granulocytes via P‐selectin/PSGL‐1 interactions, where they activate the classical pathway(CP) on their surface and/or secrete chondroitin sulfate, a known activator of the CP. (B) Properdin‐enhanced alternative pathway(AP) activity amplifies CP activity initiated by platelets, leading to the deposition of C3b on the granulocyte surface. The AP can then use deposited C3b to amplify its activity directly on the granulocyte surface. The AP also activates spontaneously on (C) neutrophils and activated platelets, and (D) AP activity is enhanced by high levels of properdin oligomers (P₂, P₃, and especially P₄), secreted from neutrophils. (E) Properdin‐enhanced AP activity ultimately leads to increased levels of C5a that binds to C5aR1 on neutrophils to enhance CR3 expression. (F) Factor H regulates AP/properdin‐mediated PGA formation. Figure originally printed in: Blatt et al156