Expression of decay-accelerating factor is reduced on hyperplastic synovial lining cells in rheumatoid synovitis

Complement and immunity

Our body is in constant interaction with the environment. Some of the interactions involve the recognition and disposal of foreign substances that may harm the delicate balance between health and disease. The foreign elements, or antigens, include infectious organisms and lifeless macromolecules. The ability of the body to recognize what is dangerous and what is inconsequential, and to refrain from damaging what is perceived as self, are the main functions of the immune system. One important component of the innate immune response is the complement system. This article describes the different mechanisms of how complement is activated and the consequence of this activation, followed by a characterization of the complement's role in inflammation and autoimmunity, and the therapeutic considerations emanating from these studies.

Membrane complement regulators protect against the development of type II collagen-induced arthritis in rats

OBJECTIVE

To investigate changes in the distribution patterns of membrane complement regulators (MCRs) during the development of type II collagen-induced arthritis (CIA) and to examine the protective effects of these molecules against the augmentation of CIA in the knee joint.

METHODS

Immunohistochemistry was used to examine the distribution of the MCRs Crry, DAF, and CD59 in the synovium of knee joints before and 2, 4, and 10 weeks after induction of CIA by immunization with type II collagen. In addition, at 2 or 10 weeks after induction of CIA, rats were injected intraarticularly with anti-Crry and/or anti-CD59 as the F(ab')2 fraction of monoclonal antibodies (mAb). Knee joint swelling and histologic changes in the synovium were examined 2 weeks after mAb injection.

RESULTS

Synovial expression of Crry, DAF, and CD59 decreased in parallel with increased inflammation. When Crry and CD59 were functionally blocked at 2 weeks after the induction of CIA, swelling of the knee joints was markedly increased. Blocking of either regulator alone had no effect on swelling. Thickening of the synovial surface and proliferation of subsynovial tissue were all increased after blocking Crry and CD59, whereas blocking of either MCR alone had no effect. When both Crry and CD59 were blocked, deposits of membrane attack complex were found in the synovium.

CONCLUSION

Our findings indicate that in rats with CIA and severely inflamed synovium, local expression of MCR is reduced. The MCRs Crry and CD59 appear to suppress the development of CIA.

Therapeutic efficacy of a novel membrane-targeted complement regulator in antigen-induced arthritis in the rat

OBJECTIVE

Complement system activation is strongly implicated as a factor in the pathogenesis of chronic synovitis in human rheumatoid arthritis. The objective of this study was to explore the therapeutic potential and local retention of a novel membrane-targeting complement regulatory protein, derived from human complement receptor 1, in the experimental setting of rat antigen-induced arthritis.

METHODS

Sensitized animals were treated at the time of arthritis induction with a single intraarticular (IA) dose of the membrane-targeting regulator APT070, a non-membrane-targeting control regulator (APT898), or vehicle control, and disease was assessed clinically and histologically. In addition, immunocytochemical analysis was performed on sections from normal rat knee joints at various time points after IA injection with APT070.

RESULTS

Animals treated with APT070 showed a dose-dependent therapeutic effect, with significantly milder clinical and histologic disease compared with both other treatment groups (P < 0.008 at the higher dose) and minimal evidence of erosive disease at study end in the active treatment group. Immunoperoxidase and immunofluorescence studies demonstrated local retention of APT070 on cell surface membranes within the normal joint up to 48 hours after IA injection.

CONCLUSION

These results show that IA complement inhibition represents an effective therapeutic strategy in experimental arthritis, by demonstrating that the exogenous delivery of a membrane-targeting complement regulator can result in prolonged synovial cell surface binding and significant clinical benefit in vivo. Complement inhibitory strategies of this type should be considered as novel therapies in human inflammatory arthritis.

Comparison of the suppressive effects of soluble CR1 and C5a receptor antagonist in acute arthritis induced in rats by blocking of CD59

We investigated the effects of suppression of complement activation at C3 level and inhibition of C5a on acute synovitis in rats. Acute synovitis was induced in Wistar rats by intra-articular (i.a.) injection into one knee of 0.3 mg of MoAb 6D1 (anti-rat CD59 antibody). In the treatment groups, soluble CR1 (sCR1) or C5a receptor (C5aR) antagonist was administered intra-articularly or intravenously and effects on the course of the acute synovitis were monitored. Synovitis induced by 6D1 was characterized by joint swelling, thickening of synovial tissue, cellular infiltration and deposition of membrane attack complex (MAC) on the synovial surface. Neither inflammatory change nor MAC deposition was found in rats which received an i.a. injection of sCR1 to suppress complement activity in the joint. Intra-articular injection of sCR1 did not reduce plasma complement activity. Intravenous administration of sCR1 suppressed plasma complement activity but had no effect on the course of the arthritis and synovitis with MAC deposition was observed. Neither i.a. nor i.v. injection of C5aR antagonist had any suppressive effects on inflammatory change or MAC deposition in synovium. The data show that inflammatory change induced by 6D1 was mediated by local complement activation and was not accompanied by systemic complement activation. C5a generation was not responsible for the observed inflammation, suggesting that other complement activation products, possibly MAC, mediate the inflammatory change observed in this model of acute synovitis in rats.

A critical role for murine complement regulator crry in fetomaternal tolerance

Complement is a component of natural immunity. Its regulation is needed to protect tissues from inflammation, but mice with a disrupted gene for the complement regulator decay accelerating factor were normal. Mice that were deficient in another murine complement regulator, Crry, were generated to investigate its role in vivo. Survival of Crry-/- embryos was compromised because of complement deposition and concomitant placenta inflammation. Complement activation at the fetomaternal interface caused the fetal loss because breeding to C3-/- mice rescued Crry-/- mice from lethality. Thus, the regulation of complement is critical in fetal control of maternal processes that mediate tissue damage.

Complement activation and inhibition in experimental models of arthritis

Complement activation has been implicated as a pathological process in a number of inflammatory and autoimmune disorders including chronic rheumatoid arthritis (RA). Animal models of experimental arthritis have been widely used to investigate the pathogenesis of RA and also in the development of novel therapies. Many of these models are complement-dependent and both incidence and progression of disease can be influenced by complement inhibition. In certain situations, local inhibition is of greater therapeutic benefit than systemic decomplementation. An increasing awareness and availability of a wide range of naturally occurring complement regulatory proteins can now offer a more targeted approach to complement inhibition while the availability of novel engineering strategies has also improved the efficiency of this process. The success of complement inhibition in the experimental models described should offer a novel therapeutic approach to the treatment of human inflammatory arthritis.

CR1, CD35 in synovial fluid from patients with inflammatory joint diseases

OBJECTIVE

To investigate synovial fluid (SF) for the presence of CR1 and to study its relationship to SF leukocytes and to serum levels of soluble CR1 (sCR1) in patients with rheumatic diseases.

METHODS

Synovial fluids were collected from 35 patients with rheumatoid arthritis (RA) and 26 patients with other inflammatory joint diseases. Total CR1 in the SF and serum were measured with a sandwich enzyme-linked immunosorbent assay (ELISA) that recognized both soluble and transmembrane forms of CR1. The characteristics of CR1 in SF were analyzed by ultracentrifugation and by a second ELISA specific for transmembrane CR1.

RESULTS

CR1 was found in all SF samples tested (range 5-281 ng/ml). SF CR1 was higher in patients with RA (mean +/- SD 81 +/- 66 ng/ml) than in those with other inflammatory joint diseases (31.8 +/- 23.8 ng/ml) (P < 0.001). Serum sCR1 was not significantly increased in the patients compared with the normal subjects. There was no correlation between serum sCR1 and SF CR1. In 44% of the patients, the SF CR1 level was higher than the serum sCR1 level. A fraction (30-80%) of SF CR1 was pelleted by ultracentrifugation and, unlike serum sCR1, it reacted in an ELISA specific for transmembrane CR1. Thus, SF contained 2 forms of CR1: a membrane-associated and a soluble form, which was confirmed by sucrose density-gradient ultracentrifugation. SF CR1 levels correlated directly with the number of SF total leukocytes and polymorphonuclear leukocytes (PMN). These 2 forms of CR1 were also found in the supernatant of in vitro-activated PMN from normal subjects. SF CR1 exhibited the capacity to act as a cofactor for the factor I degradation of C3b.

CONCLUSION

CR1 is found in the SF of patients with joint inflammation. The data suggest that SF CR1 originates from the infiltrating leukocytes, which shed both a soluble and a membrane-associated form. Whether SF CR1 participates in the local regulation of complement activation remains to be examined.

The effects of functional suppression of a membrane-bound complement regulatory protein, CD59, in the synovial tissue in rats

OBJECTIVE

To investigate the roles of CD59 in the synovial tissue by functional suppression of CD59.

METHODS

Rats treated with cobra venom factor to deplete complement or untreated rats were injected intraarticularly with 0.3 mg of the F(ab')2 fraction of a monoclonal antibody, 6D1, that inhibits the function of rat CD59. The circumference of knee joints was measured, and histologic changes in the synovium were studied.

RESULTS

Joint swelling, thickening of the synovial tissues, infiltration of inflammatory cells into the synovium, and deposition of membrane attack complex (MAC) on the synovial surface were observed after intraarticular injection of 6D1. The inflammatory reaction reached its peak at 24 hours after injection, and finally subsided to normal within 3 days. It was suggested that functional suppression of CD59 in the synovium induced MAC formation followed by synovitis. Serum complement depletion did not completely suppress this reaction. This indicates that complement existing in the joint space is important for the formation of MAC on the synovial surface and for induction of synovitis.

CONCLUSION

The membrane-bound complement regulatory protein, CD59, plays a key role in the protection of joints against MAC-mediated synovial injury and in maintaining the normal integrity of the joint.

Glycosylphosphatidylinositol (GPI)-anchored membrane proteins are constitutively down-regulated in psoriatic skin

Hyperproliferation of keratinocytes (KCs) in psoriasis has been found to be associated with excessive activation of a phospholipase C (PLC)/protein kinase C (PKC) signal transduction system. The molecular species of PLCs which are activated in psoriasis have not been thoroughly investigated. It was envisaged that if glycosylphosphatidylinositol (GPI)-specific PLC was activated in the membrane of psoriatic epidermal cells, it would render these cells devoid of those proteins which are anchored to the cell membrane through their GPI moiety. In order to test this possibility, four GPI proteins (CD16, CD55, CD58, and CD59) were determined immunohistochemically in normal and psoriatic skin. In normal skin, CD55 and CD59 were strongly expressed on epithelium and vascular structures, whereas CD16 and CD58 were strongly expressed only on epithelium. The expression of all four GPI proteins was decreased in non-lesional psoriatic skin and virtually abolished in lesional psoriatic skin. A control transmembrane protein, CD46, was strongly expressed in normal and non-lesional psoriatic skin, and its expression was not significantly decreased in psoriatic lesions. The absence or reduction of GPI proteins was not seen in the lesions of several other inflammatory and proliferative diseases studied.

Upregulation of complement regulators MCP (CD46), DAF (CD55) and protectin (CD59) in arthritic joint disease

CD46, CD55 and CD59 are cell surface glycoproteins which are widely distributed on normal tissue, where they function in the prevention of complement-mediated damage. In this study we have investigated the altered expression of these molecules under inflammatory conditions both in vitro and in vivo. By using immunocytochemical techniques we demonstrated marked but disparate upregulation of these molecules in IL1-treated cartilage and in diseased cartilage from arthritic joints compared to normal cartilage in both humans and pigs. Expression of these proteins was restricted to the chondrocyte surface, and was also demonstrated on isolated chondrocytes grown in monolayer culture and stimulated with IL1. It is suggested that the elevated levels of these regulatory proteins may be necessary to ameliorate the multiple damaging effects of the inflammatory processes associated with destructive joint diseases.

Expression of decay-accelerating factor on synovial lining cells in inflammatory and degenerative arthritides

The decay-accelerating factor (DAF) is a complement regulatory cell surface protein that protects cells from complement-mediated lysis. We analysed synovial tissue biopsies from patients with chronic arthritides for the presence of DAF using immunohistochemistry. DAF was expressed in the synovial lining cell layer both in rheumatoid arthritis (RA) and in osteoarthritis (OA). DAF was also on vascular endothelial cells of synovial tissue. A significant correlation was found between the expression of DAF and of HLA-DR in the lining layer, suggesting that DAF may be induced during a local inflammatory response. In addition, C5b-9 terminal complement complexes were found in several DAF-positive cases, suggesting that complement activation might, in itself, induce DAF expression. We propose that the occurrence of DAF may represent a physiological mechanism for local complement regulation in synovial tissue.

Immunolocalization of a glycosylphosphatidylinositol-specific phospholipase D in mast cells found in normal tissue and neurofibromatosis lesions

A large number of eukaryotic proteins have been shown to be anchored to the cell membrane by glycosylphosphatidylinositol (GPI). This glycolipid anchor can serve as a substrate for anchor-specific phospholipases that convert the GPI-anchored membrane proteins into soluble forms. Soluble forms of many GPI anchored proteins have been identified in vivo in connective tissue, plasma, and urine. The authors have discovered that mammalian plasma contains a GPI-specific phospholipase D (GPI-PLD). Because it recognizes a portion of the conserved glycan core structure, all GPI-anchored proteins are potential substrates. The authors report the development of a murine monoclonal antibody specific for one form of the human GPI-PLD and the immunohistochemical localization of this enzyme to mast cells.