Production and functional analysis of rat CD59 and chimeric CD59-Crry as active soluble proteins in Pichia pastoris

On the three-finger protein domain fold and CD59-like proteins in Schistosoma mansoni

Background

It is believed that schistosomes evade complement-mediated killing by expressing regulatory proteins on their surface. Recently, six homologues of human CD59, an important inhibitor of the complement system membrane attack complex, were identified in the schistosome genome. Therefore, it is important to investigate whether these molecules could act as CD59-like complement inhibitors in schistosomes as part of an immune evasion strategy.

Methodology/Principal Findings

Herein, we describe the molecular characterization of seven putative SmCD59-like genes and attempt to address the putative biological function of two isoforms. Superimposition analysis of the 3D structure of hCD59 and schistosome sequences revealed that they contain the three-fingered protein domain (TFPD). However, the conserved amino acid residues involved in complement recognition in mammals could not be identified. Real-time RT-PCR and Western blot analysis determined that most of these genes are up-regulated in the transition from free-living cercaria to adult worm stage. Immunolocalization experiments and tegument preparations confirm that at least some of the SmCD59-like proteins are surface-localized; however, significant expression was also detected in internal tissues of adult worms. Finally, the involvement of two SmCD59 proteins in complement inhibition was evaluated by three different approaches: (i) a hemolytic assay using recombinant soluble forms expressed in Pichia pastoris and E. coli; (ii) complement-resistance of CHO cells expressing the respective membrane-anchored proteins; and (iii) the complement killing of schistosomula after gene suppression by RNAi. Our data indicated that these proteins are not involved in the regulation of complement activation.

Conclusions

Our results suggest that this group of proteins belongs to the TFPD superfamily. Their expression is associated to intra-host stages, present in the tegument surface, and also in intra-parasite tissues. Three distinct approaches using SmCD59 proteins to inhibit complement strongly suggested that these proteins are not complement inhibitors and their function in schistosomes remains to be determined.

Schistosomes are parasites that reside for many years in the blood stream, demanding efficient mechanisms of evading immune response effectors such as complement deposition. A group of genes similar to human CD59, an important complement inhibitor in mammals, were identified in the schistosome genome. Computer predictions of protein structure indicated substantial similarity of the schistosome proteins and the mammalian CD59 family of proteins, which due to their three-finger-shaped spatial conformation are members of the Three-Finger Protein Domain fold superfamily (TFPD). Members of this family of schistosome proteins were also shown to be expressed predominantly during the mammalian stages when worms are exposed to complement and found to be present at the host-interactive surface of schistosomes. Three different methods were employed to test the possible involvement of these proteins in complement inhibition. Our results strongly suggest that these proteins are not involved in the inhibition of complement and that further studies are needed to establish their functional role(s) in schistosomes.

Strategies of therapeutic complement inhibition

The involvement of complement in the pathogenesis of a great number of partly life threatening diseases defines the importance to develop inhibitors which specifically interfere with its deleterious action. Endogenous soluble complement-inhibitors, antibodies or low molecular weight antagonists, either blocking key proteins of the cascade reaction or neutralizing the action of the complement-derived anaphylatoxins have successfully been tested in various animal models over the past years. Promising results consequently led to first clinical trials. This review is focused on different approaches for the development of inhibitors, on their site of action in the cascade, on possible indications for complement inhibition based on experimental animal data, and on potential side effects of such treatment.

Contrasting roles of complement activation and its regulation in membranous nephropathy

The complement system is involved in defense against microorganisms, the processing of immune complexes and apoptotic debris, and the development of an appropriate immune response. Along with these physiologic effects, complement activation has the potential to result in tissue pathology. To limit this, various complement regulatory proteins (CRP) are present on host cells, including the glomerular podocyte. Experimental data from the Heymann nephritis (HN) rat model of human membranous nephropathy (MN) have shown that IgG antibodies in subepithelial immune deposits initiate complement activation and C5b-9-mediated damage of the overlying podocyte. Although IgG can activate the classical pathway, there also is evidence that alternative pathway activation occurs in MN, which could occur because of absent, dysfunctional, or inhibited podocyte CRP. Related to this are experimental data in HN showing the presence of antibodies that bind and inhibit podocyte CRP; although such antibodies have not been documented in human MN, a decrease in CR1 quantity on the podocyte has been observed. A s a result of a relative lack of CRP and the exposure of activating complement proteins to tubular cells, alternative complement pathway activation and C5b-9-mediated tubular injury can occur in MN and other proteinuric diseases. Overall, in a disease such as MN, the balance between complement regulation and activation is tipped toward its being activated. Therefore, a number of therapeutic approaches have been developed to counteract this, including recombinant forms of endogenous CRP and complement-inhibitory monoclonal antibodies. There is good reason to be optimistic that approaches to block complement activation will become viable therapy for human MN in the future.

Generation of a recombinant, membrane-targeted form of the complement regulator CD59: activity in vitro and in vivo

Inappropriate activation of complement contributes to pathology in diverse inflammatory diseases. Soluble recombinant forms of the natural cell membrane regulators of complement are effective in animal models and some human diseases. However, their use is limited for reasons related to cost, short half lives, and propensity to cause unwanted systemic effects. Some of these limitations may be overcome by use of bacterial expression systems, specific targeting moieties, and judicious choice of regulator. Here we describe the application of these strategies to the generation of a membrane-targeted form of CD59. A recombinant soluble form of rat CD59, comprising the first 71 residues of the mature protein and missing the membrane-anchoring signal, was expressed in bacteria, purified, and refolded in a fully active form. The protein was coupled through its carboxyl terminus to a short, synthetic address tag that confers membrane binding activity. Attachment of the membrane address tag markedly increased complement-inhibitory activity assessed in vitro in hemolysis assays. Intra-articular administration of the tagged agent markedly suppressed disease in a model of rheumatoid arthritis in Lewis rats. This novel type of agent, termed sCD59-APT542, offers for the first time the prospect of efficient and specific inhibition of membrane attack complex activity in vivo.

Rat glomerular epithelial cells produce and bear factor H on their surface that is up-regulated under complement attack

BACKGROUND

Factor H is a potent complement inhibitory molecule that is primarily produced by the liver and appears in plasma as a soluble protein. Yet there is evidence that other cells, including those in the kidney, can produce factor H, and that it can be cell-associated as well as present as a plasma protein. Here we studied factor H in rat glomerular epithelial cells (GEC).

METHODS

A polyclonal antibody to factor H was used to identify factor H protein. A polymerase chain reaction (PCR)-based strategy was utilized to clone the full-length cDNA of GEC factor H. The relative quantity of factor H mRNA was measured by quantitative reverse transcription (RT)-PCR in cultured GEC exposed to complement activation and in the passive Heymann nephritis (PHN) model of membranous nephropathy.

RESULTS

By immunofluorescence microscopy, factor H protein was present on the plasma membranes of cultured GEC. Based upon Western blot studies, this appeared to be the full-length 150 kD factor H protein. Factor H cDNA cloned from GEC was identical to the newly deposited sequence for rat liver factor H cDNA. In cultured GEC in which complement was activated, factor H mRNA increased over time. Similarly, in the PHN model in which complement was activated on GEC in vivo, factor H mRNA and protein also increased over time.

CONCLUSION

Cultured GEC and glomeruli express factor H mRNA and protein. As modeled both in vitro and in vivo in the rat, factor H is up-regulated in membranous nephropathy. This is likely to be a direct response of GEC to complement attack and may represent a protective response of this cell.

Complement receptor 2-mediated targeting of complement inhibitors to sites of complement activation

In a strategy to specifically target complement inhibitors to sites of complement activation and disease, recombinant fusion proteins consisting of a complement inhibitor linked to a C3 binding region of complement receptor (CR) 2 were prepared and characterized. Natural ligands for CR2 are C3 breakdown products deposited at sites of complement activation. Fusion proteins were prepared consisting of a human CR2 fragment linked to either the N terminus or C terminus of soluble forms of the membrane complement inhibitors decay accelerating factor (DAF) or CD59. The targeted complement inhibitors bound to C3-opsonized cells, and all were significantly more effective (up to 20-fold) than corresponding untargeted inhibitors at protecting target cells from complement. CR2 fusion proteins also inhibited CR3-dependent adhesion of U937 cells to C3 opsonized erythrocytes, indicating a second potential anti-inflammatory mechanism of CR2 fusion proteins, since CR3 is involved in endothelial adhesion and diapedesis of leukocytes at inflammatory sites. Finally, the in vivo validity of the targeting strategy was confirmed by the demonstration that CR2-DAF, but not soluble DAF, targets to the kidney in mouse models of lupus nephritis that are associated with renal complement deposition.

Isolation and characterization of a novel rat factor H-related protein that is up-regulated in glomeruli under complement attack

The factor H family in humans is composed of seven distinct proteins, including factor H-related proteins (FHR) 1-5. All members contain tandemly arranged short consensus repeats (SCR) typical of the regulators of complement activation gene family. FHR-5 is unusual for this group of proteins, as it was initially identified as a component of immune deposits in glomerular diseases. During our cloning of the cDNA for rat factor H from glomerular epithelial cells (GEC), we identified an alternative 2729-bp cDNA transcript. The translated sequence encoded a protein containing 11 SCRs, most similar to SCRs 7-15 and 19-20 in native rat factor H, which is the same basic structure of human FHR-5. As such, this rat protein was termed FHR. Recombinant rat FHR produced in a eukaryotic expression system had a molecular mass of 78 kDa. In functional studies, recombinant FHR bound C3b and inhibited the complement alternative pathway in a dose-dependent fashion. Given the prominent expression of FHR-5 in human membranous nephropathy, a disease in which complement activation occurs in the vicinity of GEC, the expression of FHR in a rat model of this disease was evaluated. In both in vitro and in vivo models of complement activation on the GEC, FHR mRNA was up-regulated by a factor of 3-6-fold compared with controls in which complement could not be activated. Thus, we have identified a novel factor H family member in rats. This FHR protein is analogous to human FHR-5, both in structure and in potential involvement in glomerular immune complex diseases.

Bacterial expression and membrane targeting of the rat complement regulator Crry: a new model anticomplement therapeutic

Inappropriate or unregulated activation of complement can contribute to pathology in inflammatory diseases. Previous studies have shown that soluble recombinant regulators of complement are effective in animal models and some human diseases. However, limitations include cost, rapid clearance, and unwanted systemic effects. To avoid some of these problems, bacterial expression of regulators has been optimized and methods for the addition of a membrane-targeting moiety to the complement regulator developed. When administered directly to sites of inflammation, membrane-targeted human regulators are retained and inhibit complement-activation locally. To test the efficacy of membrane-targeted complement regulators in vivo, we have undertaken the expression and membrane targeting of the rat-complement regulator Crry. A soluble recombinant form of Crry, containing only the first four short consensus repeats, was expressed in a mammalian expression system and shown to be functional as a fluid phase regulator. To generate the quantities required for testing in vivo, Crry was expressed in bacteria and refolded successfully. Refolded protein had full-complement regulatory activity in vitro. Attachment of a membrane address tag conferred membrane-binding capacity and greatly increased complement regulatory function in vitro. This novel anticomplement agent can now be applied to rat models of arthritis and other inflammatory diseases.

Use of complement inhibitors in tissue injury

A great deal of information has accumulated implicating the complement system in several human disease processes. Although some of this information is circumstantial, protein inhibitors of the complement system have been developed and applied successfully to experimental disease models in animals. Two inhibitors, soluble complement receptor 1 (sCR1) and anti-C5 monoclonal antibody, are now being investigated in a variety of clinical conditions such as systemic lupus erythematosus and rheumatoid arthritis (RA), diseases for which current therapy has changed little and remains unsatisfactory. Preliminary successes in Phase II clinical trials of RA have provided optimism that complement inhibition might prove useful in these diseases and become part of standard medical therapy.

Coupling complement regulators to immunoglobulin domains generates effective anti-complement reagents with extended half-life in vivo

Complement activation and subsequent generation of inflammatory molecules and membrane attack complex contributes to the pathology of a number of inflammatory and degenerative diseases, including arthritis, glomerulonephritis and demyelination. Agents that specifically inhibit complement activation might prove beneficial in the treatment of these diseases. Soluble recombinant forms of the naturally occurring membrane complement regulatory proteins (CRP) have been exploited for this purpose. We have undertaken to design better therapeutics based on CRP. Here we describe the generation of soluble, recombinant CRP comprising rat decay accelerating factor (DAF) or rat CD59 expressed as Fc fusion proteins, antibody-like molecules comprising two CRP moieties in place of the antibody Fab arms (CRP-Ig). Reagents bearing DAF on each arm (DAF-Ig), CD59 on each arm (CD59-Ig) and a hybrid reagent containing both DAF and CD59 were generated. All three reagents inhibited C activation in vitro. Compared with soluble CRP lacking Fc domains, activity was reduced, but was fully restored by enzymatic release of the regulator from the Ig moiety, implicating steric constraints in reducing functional activity. In vivo studies showed that DAF-Ig, when compared to soluble DAF, had a much extended half-life in the circulation in rats and concomitantly caused a sustained reduction in plasma complement activity. When given intra-articularly to rats in a model of arthritis, DAF-Ig significantly reduced severity of disease. The data demonstrate the potential of CRP-Ig as reagents for sustained therapy of inflammatory disorders, including arthritis, but emphasize the need for careful design of fusion proteins to retain function.

A complement-dependent model of thrombotic thrombocytopenic purpura induced by antibodies reactive with endothelial cells

Thrombotic thrombocytopenic purpura (TTP) is an immunologically mediated disease characterized by thrombocytopenia, hemolytic anemia, and pathologic changes in various organs, including the kidney, which are secondary to widespread thromboses. Central to TTP is platelet activation, which may occur from a variety of mechanisms, including endothelial cell activation or injury. In this study, injection of K6/1, a monoclonal antibody with widespread reactivity toward endothelia, led to dose-dependent thrombocytopenia in rats. This was magnified if animals were preimmunized with mouse IgG, thereby resulting in an accelerated autologous phase of injury. In this setting, significant anemia also resulted. Rats injected with K6/1 developed renal injury, consisting of tubular damage and glomerular thrombi. Thrombocytopenia and renal morphological abnormalities were eliminated if animals were complement depleted with cobra venom factor prior to K6/1 injection and worsened when the activity of the ubiquitous complement regulator Crry was inhibited with function-neutralizing antibodies. Therefore, we have developed a complement-dependent model of TTP in rats by injecting monoclonal antibodies reactive with endothelial cells. Antibody-directed complement activation leads to stimulation of platelets, through direct interactions with complement fragments and/or indirectly through endothelial cell activation or injury, with the subsequent development of TTP.

Glomerular complement regulation is overwhelmed in passive Heymann nephritis

BACKGROUND

An injection of anti-Fx1A antibodies in rats leads to passive Heymann nephritis (PHN), a model of membranous nephropathy. Fx1A is a crude extract of renal cortex that contains megalin as a principal component. However, when rats are given anti-megalin antibodies, abnormal proteinuria does not occur. Because of the established complement dependence of PHN, we hypothesized that antibodies neutralizing complement regulatory proteins in the rat glomerulus also were required to induce PHN. Two likely targets are Crry and CD59, proteins abundant on the rat podocyte and contained within Fx1A that inhibit the C3 convertase and C5b-9 assembly, respectively.

METHODS

Rats were injected with anti-megalin monoclonal antibodies, followed by anti-Crry and/or anti-CD59 F(ab')(2) antibodies five days later. In a second group of experiments, rats were injected with anti-Fx1A or anti-Fx1A immunodepleted of reactivity against Crry and/or CD59.

RESULTS

In the setting of podocyte-associated anti-megalin monoclonal antibodies, simultaneous neutralization of Crry and CD59 function led to the development of significant proteinuria (11.0 +/- 2.1 mg/day, P < 0.001 vs. all other groups). In contrast, animals that had neither or only one of these complement regulators inhibited had normal urinary protein excretion (< or =6 mg/day). In animals given anti-Fx1A depleted of anti-Crry and/or anti-CD59, all groups developed typical PHN, characterized by heavy proteinuria and extensive glomerular deposition of C3 and C5b-9.

CONCLUSION

Crry and CD59 play an important role in restraining complement-mediated injury following subepithelial immune complex deposition; however, in PHN, their regulatory capacity is overwhelmed.

A protein with characteristics of factor H is present on rodent platelets and functions as the immune adherence receptor

Complement-coated particles interact with specific immune adherence receptors (IAR). In primates, this function is served by complement receptor 1 (CR1) on erythrocytes. In contrast, rodent platelets bear IAR distinct from CR1, the identity of which was studied here. A 150-kDa C3b-binding protein was isolated from rat platelets, which had immunochemical and biochemical identity to plasma factor H. Immunofluorescence microscopy and flow cytometry demonstrated that factor H was present on the surface of rat and mouse platelets, which could be removed by treatment with neuraminidase. Sheep erythrocytes bearing C3b underwent immune adherence with rat and mouse platelets, which was blocked with anti-factor H F(ab')(2) antibodies, but not with antibodies binding to the complement regulator, Crry, on the platelet surface. By reverse transcription-polymerase chain reaction using rat platelet RNA and primers designed from mouse factor H, a 472-base pair product was generated that was identical in sequence to that produced from rat liver RNA. The translated protein product was 85% similar to mouse liver factor H. The 3'-nucleotide sequence from platelets predicted a soluble factor H protein. By Northern analysis, liver and platelets had identically sized factor H mRNA. Thus, rat and mouse platelets have a membrane protein with characteristics of factor H that is linked via sialic acid residues and functions as the IAR. Whether platelet factor H is acquired by passive adsorption from sera and/or is produced by platelets remains to be determined.