Decay-accelerating factor is a component of subendothelial extracellular matrix in vitro, and is augmented by activation of endothelial protein kinase C

Complement C5b-9 and Cancer: Mechanisms of Cell Damage, Cancer Counteractions, and Approaches for Intervention

The interactions of cancer cells with components of the complement system are highly complex, leading to an outcome that is either favorable or detrimental to cancer cells. Currently, we perceive only the "tip of the iceberg" of these interactions. In this review, we focus on the complement terminal C5b-9 complex, known also as the complement membrane attack complex (MAC) and discuss the complexity of its interaction with cancer cells, starting with a discussion of its proposed mode of action in mediating cell death, and continuing with a portrayal of the strategies of evasion exhibited by cancer cells, and closing with a proposal of treatment approaches targeted at evasion strategies. Upon intense complement activation and membrane insertion of sufficient C5b-9 complexes, the afflicted cells undergo regulated necrotic cell death with characteristic damage to intracellular organelles, including mitochondria, and perforation of the plasma membrane. Several pro-lytic factors have been proposed, including elevated intracellular calcium ion concentrations and activated JNK, Bid, RIPK1, RIPK3, and MLKL; however, further research is required to fully characterize the effective cell death signals activated by the C5b-9 complexes. Cancer cells over-express a multitude of protective measures which either block complement activation, thus reducing the number of membrane-inserted C5b-9 complexes, or facilitate the elimination of C5b-9 from the cell surface. Concomitantly, cancer cells activate several protective pathways that counteract the death signals. Blockage of complement activation is mediated by the complement membrane regulatory proteins CD46, CD55, and CD59 and by soluble complement regulators, by proteases that cleave complement proteins and by protein kinases, like CK2, which phosphorylate complement proteins. C5b-9 elimination and inhibition of cell death signals are mediated by caveolin and dynamin, by Hsp70 and Hsp90, by the mitochondrial stress protein mortalin, and by the protein kinases PKC and ERK. It is conceivable that various cancers and cancers at different stages of development will utilize distinct patterns of these and other MAC resistance strategies. In order to enhance the impact of antibody-based therapy on cancer, novel precise reagents that block the most effective protective strategies will have to be designed and applied as adjuvants to the therapeutic antibodies.

CD55 deposited on synovial collagen fibers protects from immune complex-mediated arthritis

INTRODUCTION

CD55, a glycosylphosphatidylinositol-anchored, complement-regulating protein (decay-accelerating factor), is expressed by fibroblast-like synoviocytes (FLS) with high local abundance in the intimal lining layer. We here explored the basis and consequences of this uncommon presence.

METHODS

Synovial tissue, primary FLS cultures, and three-dimensional FLS micromasses were analyzed. CD55 expression was assessed by quantitative polymerase chain reaction (PCR), in situ hybridization, flow cytometry, and immunohistochemistry. Reticular fibers were visualized by Gomori staining and colocalization of CD55 with extracellular matrix (ECM) proteins by confocal microscopy. Membrane-bound CD55 was released from synovial tissue with phospholipase C. Functional consequences of CD55 expression were studied in the K/BxN serum transfer model of arthritis using mice that in addition to CD55 also lack FcγRIIB (CD32), increasing susceptibility for immune complex-mediated pathology.

RESULTS

Abundant CD55 expression seen in FLS of the intimal lining layer was associated with linearly oriented reticular fibers and was resistant to phospholipase C treatment. Expression of CD55 colocalized with collagen type I and III as well as with complement C3. A comparable distribution of CD55 was established in three-dimensional micromasses after ≥3 weeks of culture together with the ECM. CD55 deficiency did not enhance K/BxN serum-induced arthritis, but further exaggerated disease activity in Fcgr2b (-/-) mice.

CONCLUSIONS

CD55 is produced by FLS and deposited on the local collagen fiber meshwork, where it protects the synovial tissue against immune complex-mediated arthritis.

The role of complement in tumor growth

Complement is a central part of the immune system that has developed as a first defense against non-self cells. Neoplastic transformation is accompanied by an increased capacity of the malignant cells to activate complement. In fact, clinical data demonstrate complement activation in cancer patients. On the basis of the use of protective mechanisms by malignant cells, complement activation has traditionally been considered part of the body's immunosurveillance against cancer. Inhibitory mechanisms of complement activation allow cancer cells to escape from complement-mediated elimination and hamper the clinical efficacy of monoclonal antibody-based cancer immunotherapies. To overcome this limitation, many strategies have been developed with the goal of improving complement-mediated effector mechanisms. However, significant work in recent years has identified new and surprising roles for complement activation within the tumor microenvironment. Recent reports suggest that complement elements can promote tumor growth in the context of chronic inflammation. This chapter reviews the data describing the role of complement activation in cancer immunity, which offers insights that may aid the development of more effective therapeutic approaches to control cancer.

CD97 is a multifunctional leukocyte receptor with distinct roles in human cancers (Review)

G-protein coupled receptors (GPCRs) represent the most diverse and biologically ubiquitous protein receptors. The epidermal growth factor seven-span transmembrane (EGF-TM7) family consists of adhesion GPCRs with a diverse functional repertoire. CD97 is the most broadly expressed member with roles in cell adhesion, migration and regulation of intercellular junctions. CD97 is also expressed in a variety of human malignancies including those of the thyroid, stomach, colon and brain. CD97 confers an invasive phenotype and has been shown to correlate with tumor grade, lymph node invasion, metastatic spread and overall prognosis. More recently, CD97 was found to signal through Gα12/13, resulting in increased RHO-GTP levels. Proven roles in tumor invasion and signaling make CD97 an exciting novel therapeutic target. In this review, we will discuss the structure and function of this receptor, with a specific focus on its mechanistic significance in neoplastic diseases.

Membrane-bound complement regulatory proteins as biomarkers and potential therapeutic targets for SLE

For the last two decades, there had been remarkable advancement in understanding the role of complement regulatory proteins in autoimmune disorders and importance of complement inhibitors as therapeutics. Systemic lupus erythematosus is a prototype of systemic autoimmune disorders. The disease, though rare, is potentially fatal and afflicts women at their reproductive age. It is a complex disease with multiorgan involvement, and each patient presents with a different set of symptoms. The diagnosis is often difficult and is based on the diagnostic criteria set by the American Rheumatology Association. Presence of antinuclear antibodies and more specifically antidouble-stranded DNA indicates SLE. Since the disease is multifactorial and its phenotypes are highly heterogeneous, there is a need to identify multiple noninvasive biomarkers for SLE. Lack of validated biomarkers for SLE disease activity or response to treatment is a barrier to the efficient management of the disease, drug discovery, as well as development of new therapeutics. Recent studies with gene knockout mice have suggested that membrane-bound complement regulatory proteins (CRPs) may critically determine the sensitivity of host tissues to complement injury in autoimmune and inflammatory disorders. Case-controlled and followup studies carried out in our laboratory suggest an intimate relation between the level of DAF, MCP, CR1, and CD59 transcripts and the disease activity in SLE. Based on comparative evaluation of our data on these four membrane-bound complement regulatory proteins, we envisaged CR1 and MCP transcripts as putative noninvasive disease activity markers and the respective proteins as therapeutic targets for SLE. Following is a brief appraisal on membrane-bound complement regulatory proteins DAF, MCP, CR1, and CD59 as biomarkers and therapeutic targets for SLE.

Atypical hemolytic uremic syndrome-associated variants and autoantibodies impair binding of factor h and factor h-related protein 1 to pentraxin 3

Atypical hemolytic uremic syndrome (aHUS) is a renal disease associated with complement alternative pathway dysregulation and is characterized by endothelial injury. Pentraxin 3 (PTX3) is a soluble pattern recognition molecule expressed by endothelial cells and upregulated under inflammatory conditions. PTX3 activates complement, but it also binds the complement inhibitor factor H. In this study, we show that native factor H, factor H-like protein 1, and factor H-related protein 1 (CFHR1) bind to PTX3 and that PTX3-bound factor H and factor H-like protein 1 maintain their complement regulatory activities. PTX3, when bound to extracellular matrix, recruited functionally active factor H. Residues within short consensus repeat 20 of factor H that are relevant for PTX3 binding were identified using a peptide array. aHUS-associated factor H mutations within this binding site caused a reduced factor H binding to PTX3. Similarly, seven of nine analyzed anti-factor H autoantibodies isolated from aHUS patients inhibited the interaction between factor H and PTX3, and five autoantibodies also inhibited PTX3 binding to CFHR1. Moreover, the aHUS-associated CFHR1*B variant showed reduced binding to PTX3 in comparison with CFHR1*A. Thus, the interactions of PTX3 with complement regulators are impaired by certain mutations and autoantibodies affecting factor H and CFHR1, which could result in an enhanced local complement-mediated inflammation, endothelial cell activation, and damage in aHUS.

Human pentraxin 3 binds to the complement regulator c4b-binding protein

The long pentraxin 3 (PTX3) is a soluble recognition molecule with multiple functions including innate immune defense against certain microbes and the clearance of apoptotic cells. PTX3 interacts with recognition molecules of the classical and lectin complement pathways and thus initiates complement activation. In addition, binding of PTX3 to the alternative complement pathway regulator factor H was shown. Here, we show that PTX3 binds to the classical and lectin pathway regulator C4b-binding protein (C4BP). A PTX3-binding site was identified within short consensus repeats 1–3 of the C4BP α-chain. PTX3 did not interfere with the cofactor activity of C4BP in the fluid phase and C4BP maintained its complement regulatory activity when bound to PTX3 on surfaces. While C4BP and factor H did not compete for PTX3 binding, the interaction of C4BP with PTX3 was inhibited by C1q and by L-ficolin. PTX3 bound to human fibroblast- and endothelial cell-derived extracellular matrices and recruited functionally active C4BP to these surfaces. Whereas PTX3 enhanced the activation of the classical/lectin pathway and caused enhanced C3 deposition on extracellular matrix, deposition of terminal pathway components and the generation of the inflammatory mediator C5a were not increased. Furthermore, PTX3 enhanced the binding of C4BP to late apoptotic cells, which resulted in an increased rate of inactivation of cell surface bound C4b and a reduction in the deposition of C5b-9. Thus, in addition to complement activators, PTX3 interacts with complement inhibitors including C4BP. This balanced interaction on extracellular matrix and on apoptotic cells may prevent excessive local complement activation that would otherwise lead to inflammation and host tissue damage.

Concanavalin A, but not glycated albumin, increases subendothelial deposition of von Willebrand factor in vitro

Diabetes is associated with augmentation of prothrombogenic von Willebrand factor (vWF) content in plasma. Earlier, the author and colleagues have shown that high glucose and insulin do not appreciably influence deposition of vWF into the subendothelial extracellular matrix (SECM) produced by cultured human umbilical vein endothelial cells (HUVECs). In the present work, the author used this model to test the effects of nonenzymatically glycated albumin (Glyc-HSA) and two lectins, concanavalin A (ConA) and wheat germ agglutinin (WGA), on vWF deposition into the SECM. First-passage HUVECs were seeded into gelatin-coated 96-well plates and cultured for 6 to 7 days. HSA or Glyc-HSA (at concentrations 25, 50, and 100 microg/mL), and WGA or ConA (4, 8, and 16 microg/mL) were added 3 h after seeding. Cell viability was tested by the MTT method. To determine vWF contents in the SECM, HUVECs were detached by treatment with NH4OH and the residual material was used as a solid phase in an enzyme-linked immunosorbent assay (ELISA)-like assay with primary (anti-vWF) and secondary (peroxidase-conjugated) antibodies. Addition of Glyc-HSA did not essentially influence VWF contents in the SECM (A490 was 0.226 versus 0.268 at 0 and 100 microg/mL, respectively; p > .05, n = 16). Cultivation in the presence of WGA led to the deterioration of cell viability, which was accompanied by a significant decrease of vWF in the SECM (0.248 versus 0.128 at 0 and 16 microg/mL, respectively; p < .001, n = 16). ConA did not influence viability of HUVECs, but this lectin at all concentrations consistently increased the deposition of vWF (up to 164% relative to control, p <.001; n = 16). These data indicate that endothelial carbohydrate determinants and corresponding ligands (namely, mannose-specific lectins) may be involved in the regulation of production and deposition of vWF.

Cross-talk between the complement system and endothelial cells in physiologic conditions and in vascular diseases

The endothelial layer represents a continuous physical barrier that controls coagulation and allows selective passage of soluble molecules and circulating cells across the vessel wall into the tissue. The functional activity of the endothelial cells may be influenced by their interaction with components of the complement system. In this review we shall discuss the complex interplay that can be established between the endothelium and complement proteins or activation products. Endothelial cells may also secrete several complement components which contribute to the circulating pool. This process can be regulated by cytokines and other pro-inflammatory stimuli. In addition, complement activation products stimulate endothelial cells to acquire a pro-inflammatory and pro-coagulant status. Expression of regulatory molecules on the cell surface provides protection against an undesired attack by complement activation products. Unrestricted complement activation under pathological conditions may lead to structural and functional changes of the endothelium resulting in vascular disease.

Complement decay accelerating factor (DAF)/CD55 in cancer

The complement system is a powerful innate mechanism involved in protection of the host against pathogens. It also has a role in the clearance of apoptotic cells and has been implicated in a range of pathologies including autoimmunity and graft rejection. The control of complement is mediated through the complement regulatory proteins (CRPs). These are present on most cells and protect normal cells from complement-mediated attack during innate activation. However, in a range of pathologies and cancer, these molecules are up or down regulated, sometimes secreted and even lost. We will review the expression of CRPs in cancer, focussing on CD55 and highlight other roles of the CRPs and their involvement in leukocyte function. We will also provide some data providing a potential mechanism by which soluble CD55 can inhibit T-cell function and discuss some of the implications of this data.

Decay-Accelerating Factor Prevents Acute Humoral Rejection Induced by Low Levels of Anti-αGal Natural Antibodies

BACKGROUND

Hyperacute and delayed vascular rejection due to natural antibodies (NAb) present major obstacles in pig-to-primate xenotransplantation. Although "supraphysiologic" expression of human complement regulatory proteins (CRPs) can prevent hyperacute rejection in discordant xenogenic recipients, their physiologic role in the homologous setting is undefined. We have evaluated the effect of the absence of decay-accelerating factor (DAF) on cardiac allograft rejection in the presence of different levels of antidonor antibodies (Ab).

METHODS

DAF1-deficient (DAF KO; B6129F2 H-2) mice were used as heart graft donors to alpha1,3-galactosyltransferase deficient (GalT KO; B6, H-2) recipients. Heterotopic heart grafting was performed with or without presensitization. Graft survival, histology, and anti-alphaGal Ab levels were monitored.

RESULTS

DAF knockout (KO) but not wild-type (WT) grafts showed hyperacute or acute humoral rejection in nonsensitized GalT KO mice with low levels of anti-alphaGal IgM NAb. However, humoral rejection of both DAF KO and DAF WT donor grafts occurred in presensitized GalT KO recipients.

CONCLUSIONS

The expression of DAF prevents hyperacute rejection in mice with low titers of anti-alphaGal antibody. These studies demonstrate the physiologic role of DAF in preventing humoral rejection in the presence of low levels of NAb and have implications for transplantation of discordant vascularized xenografts.

Transgenic expression of human complement regulators reduces skeletal muscle ischaemia/reperfusion injury in mice

This study aimed to explore the hypothesis that activated complement components contribute significantly to I/R (ischaemia/reperfusion) injury in skeletal muscle. After 50, 70 and 90 min of tourniquet ischaemia and 24 h of reperfusion, viability of the medial gastrocnemius muscle in CBA-C57BL/6 wild-type mice, assessed histochemically by reduction of NBT (Nitro Blue Tetrazolium) dye, was 60, 21 and 8% respectively. Skeletal muscle viability after 70 min of ischaemia and 24 h of reperfusion in transgenic mice expressing a combination of human CD46, CD55 and CD59, all inhibitors of complement activation, was 45% compared with 24% in ischaemic reperfused wild-type mice (P=0.008; n=6 per group). Muscle from sham-treated transgenic mice and wild-type littermates had no significant loss of viability relative to normal contralateral gastrocnemius muscle. A significant reduction in myeloperoxidase activity (a measure of neutrophil infiltration), xanthine oxidase activity (a source of free radicals) and water content (a measure of oedema) was observed in ischaemic reperfused muscle from transgenic mice compared with ischaemic reperfused wild-type muscle (P<0.05). Haematoxylin and eosin-stained histological sections also showed less damage and less apparent leucocyte infiltration in muscles from ischaemic reperfused transgenic mice than those from wild-type animals given the same degree of injury. Muscles from sham-treated transgenic and wild-type controls were almost identical with normal muscle. It is concluded that complement activation contributes to the pathogenesis of I/R injury in murine skeletal muscle, resulting in increased neutrophil infiltration into the injured muscle, increased free radical production and vascular permeability during reperfusion, and a net detrimental effect on muscle viability.

Expression of the largest CD97 and EMR2 isoforms on leukocytes facilitates a specific interaction with chondroitin sulfate on B cells

The EGF-TM7 receptors CD97 and EMR2 are heptahelical molecules predominantly expressed on leukocytes. A characteristic of these receptors is their ability to interact with cellular ligands via the N-terminal epidermal growth factor (EGF)-like domains. The first two EGF domains of CD97 (but not EMR2) bind CD55 (decay-accelerating factor), while the fourth EGF domain of both CD97 and EMR2 interacts with the glycosaminoglycan chondroitin sulfate (CS). Using fluorescent beads coated with soluble recombinant CD97 and EMR2 protein, and isoform-specific monoclonal antibodies, we have determined the cellular and molecular characteristics of the interaction with CS. The fourth EGF domain of CD97 and EMR2 is expressed on activated lymphocytes and myeloid cells, whereas the ligand is specifically found on B cells within the peripheral blood. The interaction between CD97/EMR2 and CS may therefore play a role in the interaction of activated T cells, dendritic cells, and macrophages with B cells.

Increased serum concentrations and surface expression on peripheral white blood cells of decay-accelerating factor (cd55) in patients with active ulcerative colitis

Inflammatory stimuli induce expression and release of decay-accelerating factor (DAF), a complement-regulatory protein present on peripheral-blood cells. Therefore, in ulcerative colitis (UC), an inflammatory colonic disease in which activated leukocytes are involved, DAF may be released from leukocytes into the circulation. In this study we compared serum DAF concentrations and surface DAF expression on peripheral-blood cells in patients with UC with disease activity. Peripheral-blood samples were obtained from 60 patients with UC (30 with active and 30 with inactive disease) and 19 healthy volunteers. Serum DAF concentrations were determined by means of immunoassay, and surface DAF expression on blood cells was examined with the use of flow cytometry. Serum DAF concentrations in patients with active disease (mean 48.6 ng/mL) were significantly higher than those in patients whose disease was in remission (33.3 ng/mL; P =.0003) and those in healthy controls (32.3 ng/mL; P =.0007). Surface DAF expression on neutrophils, CD14+ monocytes, and subsets of lymphocytes in patients with active UC was significantly increased compared with that in patients with UC in remission and in healthy controls. The increased serum DAF concentrations and surface DAF expression on leukocyte fractions in patients with active disease fell to significantly lower levels when the disease had gone into remission after medical therapy. Serum DAF concentrations are increased in UC patients in relation to disease activity. The likely source of increased DAF concentrations is peripheral-blood leukocytes that have been activated as part of the UC disease process.

Complement resistance of human carcinoma cells depends on membrane regulatory proteins, protein kinases and sialic acid

Nucleated cells employ several strategies to evade killing by homologous complement. We studied complement resistance in the human carcinoma cell lines (CA) T47D (mammary), SKOV3 (ovarian), and PC-3 (prostate) with emphasis on the following mechanisms of defense: 1. Expression and shedding of the membrane complement regulatory proteins (mCRP) CD46, CD55 and CD59; 2. Resistance based on protein phosphorylation; 3. Cell surface expression of sialic acid residues; 4. Desensitization to complement upon exposure to sublytic complement doses. Anti-mCRP antibody blocking experiments demonstrated that CD59 is the main mCRP protecting these CA from complement. Soluble CD59 was also found in supernates of PC-3> SKOV3 > T47D cells. Second, inhibitors of PKC, PKA and MEK sensitized the CA to lysis, thus implicating these protein kinases in CA complement resistance. Third, removal of sialic acid residues with neuraminidase also sensitized CA to lysis. Finally, exposure of CA to sublytic doses of complement conferred on them enhanced resistance to lytic complement doses in a PKC-dependent process. Combined treatment of CA with anti-CD59 antibodies, PD98059 (a MEK inhibitor) and neuraminidase produced a large enhancement in CA sensitivity to complement. Our results show that CD59 and sialic acid residues present on the cell surface, and intracellular processes involving protein phosphorylation act additively to secure CA resistance to complement-mediated lysis. Therefore, the effectiveness of antibody- and complement-based cancer immunotherapy will markedly improve by suppression of the various complement resistance mechanisms.

The role of CD55 in protecting the tumour environment from complement attack

CD55 is a complement regulatory protein expressed by cells to protect them from bystander killing by complement. CD55 is over-expressed 2-100-fold on tumour cells and is deposited in large amounts within tumour matrix. Vascular endothelial growth factor (VEGF) produced by tumours to stimulate angiogenesis, also up-regulates endothelial cell surface expression of CD55 and stimulates the release of matrix degrading metalloproteinases. This study investigated the effects of VEGF on CD55 deposition into matrix and the release of CD55 by metalloproteinases. In contrast to inflammatory cytokines, CD55 was up-regulated by VEGF at the cell surface and within the extracellular matrix (ECM). Interestingly, human umbilical vein endothelial cells (HUVEC) exposed to VEGF released similar amounts of CD55 into the ECM as a tumour cell line expressing 50-fold higher level of CD55 on its cell surface. Furthermore, in contrast to earlier studies, both tumour and HUVEC-derived CD55 was functionally active. However, in contrast to papain that degrades CD55, and collagenase that fails to release CD55, MMP-7 released intact CD55 from ECM. This suggests that it may have a further role to play in protecting cells during inflammation and invasion.

CD97-decay-accelerating factor interaction is not involved in leukocyte adhesion to endothelial cells

BACKGROUND

Effective improvement in xenograft survival is achieved using transplants from transgenic pigs expressing human complement (C) regulatory proteins, including decay-accelerating factor (DAF), CD59, and CD46 on endothelial cells (ECs). The aim of this study was to investigate whether human DAF expression in porcine ECs, as well as regulating C activation, can modify intercellular events through its interaction with its receptor, CD97, on human leukocytes.

METHODS

Cellular interactions between human leukocytes and porcine ECs were investigated in vitro using ECs from either wild-type or DAF-transgenic pigs. Static leukocyte adhesion and T cell activation assays were performed using porcine ECs as target or effector cells, respectively. The role of the DAF-CD97 interaction was investigated using specific blocking monoclonal antibodies (mAbs) against human DAF and its receptor, CD97, in adhesion assays.

RESULTS

Adhesion of U937 or Jurkat T cells, both expressing human DAF and CD97, was quantitatively similar for wild-type and transgenic-DAF-expressing pig ECs. Furthermore, blocking the CD97-DAF interaction did not inhibit xenogeneic leukocyte-endothelium adhesion, whereas blocking the very late antigen 4-vascular cell adhesion molecule-1 pathway reduced this adhesion by 50-80%. Furthermore, DAF and CD97 expression was not up-regulated during tumor necrosis factor-alpha- or lipopolysaccharide-mediated EC activation, unlike the adhesion molecules E-selectin, vascular cell adhesion molecule-1, and intracellular adhesion molecule (ICAM)-1.

CONCLUSION

We found that high levels of human DAF expressed on ECs abrogates C-mediated cell damage but did not affect the in vitro adhesive properties or antigen-presenting cell function of genetically modified porcine ECs.

K562 erythroleukemic cells are equipped with multiple mechanisms of resistance to lysis by complement

Resistance of tumor cells to lysis by complement is generally attributed to several protective mechanisms. The relative impact of these mechanisms in the same tumor cell, however, has not been assessed yet. We have analyzed the interaction of the human erythroleukemia tumor cell line K562 with human complement. K562 cells express the membrane complement regulatory proteins CD59, CD55 and CD46. As shown here for the first time, K562 also spontaneously release the soluble regulators C1 inhibitor, factor H, and soluble CD59. Complement resistance of K562 cells is augmented upon treatment with PMA, TNF or even with sublytic complement. Unlike TNF and sublytic complement, PMA enhanced the expression of membrane-bound CD55 and CD59 and led to increased secretion of soluble CD59. In addition, we show that complement-resistant K562 cells express a membrane-associated proteolytic activity, higher than the complement-sensitive K562/S cells. Treatment of complement-resistant K562 cells with serine protease inhibitors enhance their sensitivity to complement-mediated lysis. Inhibitors of protein kinase C (PKC) also sensitize K562 cells to complement lysis, implicating PKC-mediated signaling in cell resistance to complement. Neutralization of the CD55 and CD59 but not of CD46 regulatory activity with specific antibodies significantly increases complement-mediated K562 cell lysis. Treatment of K562 cells with a mixture of inhibitory reagents results in a significant additive enhancing effect on complement-mediated lysis of K562. In conclusion, K562 cells resist a complement attack by concomitantly using multiple molecular evasion strategies. Future attempts in antibody-based tumor therapy should include strategies to interfere with those resistance mechanisms.

Upregulation of DAF (CD55) on orbital fibroblasts by cytokines. Differential effects of TNF-beta and TNF-alpha

PURPOSE

Decay accelerating factor (DAF) and membrane cofactor protein (MCP) are membrane complement regulators that protect self cells from deposition of autologous C3b on their surfaces. CD59, a third downstream regulator of the cascade, prevents the assembly on self cells of autologous membrane-attack complexes. All three proteins are highly expressed on corneal and conjunctival epithelia, and are present in lower levels on multiple intraocular and adnexal cell types. The purpose of this study was to determine whether, and if so, how DAF, MCP and CD59 expression by ocular and adnexyl cells is modulated by cytokines.

METHODS

Primary cultures of orbital fibroblasts and corneal epithelial cells were incubated with TNF-alpha, TNF-beta, TGF-beta1, IFN-gamma, MIF or blocking anti-MIF mABs and extracts of the cells quantitated for DAF, MCP and CD59 by two-site immunoradiometric assays. Where inductions occurred, the kinetics of the increases, the effect of combining cytokines, and the effect of protein kinase-C inhibition were studied.

RESULTS

DAF expression on orbital fibroblasts was upregulated 6.3-, 3.7- and 4.2-fold by TGF-beta1, TNF-beta and IFN-gamma, respectively, but that its expression on corneal epithelial cells was minimally affected. These same (or other) cytokines did not significantly upregulate MCP or CD59. The cytokine-induced upregulation of DAF expression on orbital fibroblasts requires 24 hr for IFN-gamma or 48 hr for TGF-beta1 or TNF-beta, is dependent on new protein synthesis, and does not involve protein kinase-C activation.

CONCLUSIONS

TGF-beta1-, TNF-beta- and IFN-gamma-mediated upregulation of DAF should serve to prevent complement-mediated injury to orbital fibroblasts in the course of ocular inflammation. The induction by TNF-beta rather than TNF-alpha contrasts with that on all other cell types studied.

Induction of endothelial cell decay-accelerating factor by vascular endothelial growth factor: a mechanism for cytoprotection against complement-mediated injury during inflammatory angiogenesis

OBJECTIVE

Decay-accelerating factor (DAF) is a widely expressed, multifunctional cell surface protein involved in complement regulation and cell signaling. Previous studies have demonstrated that endothelial cell (EC) DAF is up-regulated by tumor necrosis factor alpha and inhibits complement binding. Because vascular endothelial growth factor (VEGF) is cytoprotective to endothelium and is expressed at sites of chronic inflammation, we hypothesized that VEGF may induce DAF expression during inflammatory angiogenesis.

METHODS

Human umbilical vein and dermal microvascular EC were isolated using routine procedures, and the regulation and function of DAF, as well as other complement-regulatory proteins (membrane cofactor protein and CD59), were analyzed following stimulation with VEGF.

RESULTS

Incubation of large- or small-vessel EC with VEGF led to increased expression of DAF, with maximal expression after 48-72 hours of stimulation. This effect depended on the activation of protein kinase C (PKC) and required increased steady-state messenger RNA levels and de novo protein synthesis. Although VEGF-induced EC proliferation was inhibited by both p38 and p42/44 mitogen-activated protein kinase (MAPK) antagonists, DAF up-regulation in response to VEGF was only sensitive to inhibition of p38 MAPK. VEGF-stimulated EC showed a 60% reduction in C3 deposition following complement activation, and this resulted in a marked reduction in complement-mediated EC lysis. These protective effects were abolished by anti-DAF monoclonal antibody 1H4.

CONCLUSION

This study confirms the importance of PKC for the regulation of DAF expression by EC and reveals VEGF to be a physiologic agonist for this pathway. The up-regulation of DAF expression by VEGF may represent an important mechanism for the protection of EC from complement-mediated injury during angiogenesis in inflammatory rheumatic diseases.

CD55 is over-expressed in the tumour environment

CD55 is a protein that protects cells from complement-mediated attack. 791Tgp72 is an antigen which has been used successfully as a target for both tumour imaging and cancer vaccines. 791Tgp72 has recently been identified as CD55. Quantitative expression of CD55 in the tumour environment was therefore studied. Tumour cells showed a 4-100-fold increase in CD55 cell surface expression when compared to normal cells. Immunohistochemical staining of colorectal tumours also revealed high expression of CD55 in the stroma. To examine the source of this stromal CD55 the ability of both epithelial cells and endothelial cells to produce extracellular CD55 was measured. Tumour cell lines deposit CD55 into their extracellular matrix (ECM) in direct proportion to their cell surface expression. In contrast the ECM from HUVEC cells contained large amounts of CD55 despite expressing low levels of CD55 on their cell surface. Furthermore expression of CD55 on HUVEC cells was increased by exposure to VEGF. Although it remains unclear why CD55 is upregulated in the tumour environment its high level of expression on tumour cells and associated endothelium may explain why it is a good target for both imaging and immunotherapy.

Induction of decay-accelerating factor by thrombin through a protease-activated receptor 1 and protein kinase C–dependent pathway protects vascular endothelial cells from complement-mediated injury

There is increasing evidence for functional crosstalk between inflammatory and thrombotic pathways in inflammatory vascular diseases such as atherosclerosis and vasculitis. Thus, complement activation on the endothelial cell (EC) surface during inflammation may generate thrombin via the synthesis of tissue factor. We explored the hypothesis that thrombin induces EC expression of the complement-regulatory proteins decay-accelerating factor (DAF), membrane cofactor protein (MCP), and CD59 and that this maintains vascular integrity during coagulation associated with complement activation. Thrombin increased DAF expression on the surface of ECs by 4-fold in a dose- and time-dependent manner as measured by flow cytometry. DAF up-regulation was first detectable at 6 hours and maximal 24 hours poststimulation, whereas no up-regulation of CD59 or MCP was seen. Thrombin-induced expression required increased DAF messenger RNA and de novo protein synthesis. The response depended on activation of protease-activated receptor 1 (PAR1) and was inhibited by pharmacologic antagonists of protein kinase C (PKC), p38 and p42/44 mitogen-activated protein kinase, and nuclear factor-κB. The increased DAF expression was functionally relevant because it significantly reduced C3 deposition and complement-mediated EC lysis. Thus, thrombin—generated at inflammatory sites in response to complement activation—is a physiologic agonist for the PKC-dependent pathway of DAF regulation, thereby providing a negative feedback loop protecting against thrombosis in inflammation.

Induction of decay-accelerating factor by thrombin through a protease-activated receptor 1 and protein kinase C–dependent pathway protects vascular endothelial cells from complement-mediated injury

There is increasing evidence for functional crosstalk between inflammatory and thrombotic pathways in inflammatory vascular diseases such as atherosclerosis and vasculitis. Thus, complement activation on the endothelial cell (EC) surface during inflammation may generate thrombin via the synthesis of tissue factor. We explored the hypothesis that thrombin induces EC expression of the complement-regulatory proteins decay-accelerating factor (DAF), membrane cofactor protein (MCP), and CD59 and that this maintains vascular integrity during coagulation associated with complement activation. Thrombin increased DAF expression on the surface of ECs by 4-fold in a dose- and time-dependent manner as measured by flow cytometry. DAF up-regulation was first detectable at 6 hours and maximal 24 hours poststimulation, whereas no up-regulation of CD59 or MCP was seen. Thrombin-induced expression required increased DAF messenger RNA and de novo protein synthesis. The response depended on activation of protease-activated receptor 1 (PAR1) and was inhibited by pharmacologic antagonists of protein kinase C (PKC), p38 and p42/44 mitogen-activated protein kinase, and nuclear factor-κB. The increased DAF expression was functionally relevant because it significantly reduced C3 deposition and complement-mediated EC lysis. Thus, thrombin—generated at inflammatory sites in response to complement activation—is a physiologic agonist for the PKC-dependent pathway of DAF regulation, thereby providing a negative feedback loop protecting against thrombosis in inflammation.

Role and regulation of pig CD59 and membrane cofactor protein/CD46 expressed on pig aortic endothelial cells

BACKGROUND

Hyperacute rejection in xenotransplantation is caused by activation of complement (C) on endothelium. We have previously shown that purified C-regulators of the pig (CD59 and membrane cofactor protein [MCP]) are efficient regulators of human C (HuC). The aim of this study was to clarify the role of endogenously expressed C-regulatory molecules on pig endothelium in the protection against hyperacute rejection.

METHODS

Porcine aortic endothelial cells (PAEC) were harvested and cultured for various passages. PAEC were examined for the expression of endogenous pig CD59 and MCP by flow cytometry. PAEC were assessed for their susceptibility to lysis by HuC. The effect of phorbol 12-myristate 13-acetate and various cytokines on the expression of MCP and CD59 and C-susceptibility was assessed.

RESULTS

Primary PAEC showed an initial high level of expression of pig CD59, however, upon culturing, CD59 levels decreased dramatically to about 20% after five passages. In contrast, levels of MCP doubled upon culturing of PAEC to confluency and remained stable during at least five passages. Primary cells and cells in the early passages were more resistant to HuC than cells that were cultured for longer. Blocking the function of CD59 but not of MCP using monoclonal antibody increased the susceptibility to HuC. Purified human CD59 incorporated to a level of expression similar to that of pig CD59 reversed the increased C-susceptibility, suggesting that pig and human CD59 are similarly protective against HuC. Increase of C-resistance and of expression of pig MCP, but not of CD59, was achieved upon incubation with phorbol 12-myristate 13-acetate. Tumor necrosis factor-alpha, interleukin-1beta, interleukin-4, or interferon-gamma had no effect on C-regulator expression or C-susceptibility.

CONCLUSIONS

These data demonstrate the importance of using primary PAEC or cells in the first passages of culturing in in vitro models of xenotransplantation and show that pig MCP and, in particular, pig CD59 play an important role in protection of PAEC from HuC.

Structural characterization of mouse CD97 and study of its specific interaction with the murine decay-accelerating factor (DAF, CD55)

CD97 is a newly identified, activation-associated human leucocyte antigen with seven putative transmembrane domains. It has an extended extracellular segment containing several adhesion molecule structure motifs, and has been shown to interact with the human complement regulator, decay-accelerating factor (DAF, CD55). To understand further the interaction between CD97 and DAF, as well as the structure and function of CD97 in general, we have cloned the mouse CD97 cDNA and studied the encoded protein for its membrane association property and ability to interact specifically with the murine decay-accelerating factor. The full-length mouse CD97 cDNA that we have cloned and characterized encodes a protein that is 60% identical to the three epidermal growth factor (EGF) domain-containing form of human CD97 but does not contain the Arg-Gly-Asp (RGD) motif which is present in human CD97. Two other alternatively spliced forms of mouse CD97 were also identified. These forms differ by the number of EGF-like sequence repeats present in the N-terminal region. Northern blot analysis revealed that CD97 is expressed widely in mouse tissues and in resting as well as activated cultured mouse splenocytes. Transient transfection of human embryonic kidney (HEK) 293 cells with the mouse CD97 cDNA in a green-fluorescence protein vector (pEGFP-N1) showed plasma membrane targeting of the expressed protein. Western blot analysis confirmed its membrane association and identified the existence of a processed C-terminal fragment, supporting the notion that CD97 on the cell membrane is composed of post-translationally generated subunits. Adhesion studies demonstrated that normal, but not DAF knockout mouse erythrocytes and splenocytes adhered to mouse CD97-transfected HEK cells. The interaction of CD97 and DAF was found to be species-restrictive in that human erythrocytes were unable to bind to mouse CD97-transfected HEK cells. These results indicate that the general structure, membrane association property and DAF-binding ability of CD97 are conserved and that the adhesive interaction between CD97 and DAF is independent of the RGD motif. The finding that CD97 is distributed widely among various mouse tissues suggests that CD97 may have other roles beyond lymphocyte activation.

Induction of Decay-Accelerating Factor by Cytokines or the Membrane-Attack Complex Protects Vascular Endothelial Cells Against Complement Deposition

Vascular endothelium is continuously exposed to complement-mediated challenge, and this is enhanced during inflammation. Although the complement-regulatory proteins decay-accelerating factor (DAF), CD59, and membrane cofactor protein (MCP) protect endothelial cells (ECs) against complement-mediated injury, the control of their expression and relative contributions to vascular protection is unclear. We explored the hypothesis that mechanisms exist which induce upregulation of complement-regulatory proteins on ECs to maintain vascular function in inflammation. Tumor necrosis factor alpha (TNF) and interferon gamma (IFNγ) each increased DAF expression but not CD59 or MCP expression, and a combination of these cytokines was more potent than either alone. Cytokine-induced expression depended on increased DAF mRNA and de novo protein synthesis and was maximal by 72 hours. In addition, assembly of the membrane-attack complex (MAC) on ECs induced a 3-fold increase in DAF expression, and this was enhanced by cytokines. DAF upregulation was not inhibited by protein kinase C (PKC) antagonists. The increase in DAF was functionally relevant since it reduced complement 3 (C3) deposition by 40%, and this was inhibited by an anti-DAF monoclonal antibody. These observations indicate that upregulation of DAF expression by cytokines or MAC may represent an important feedback mechanism to maintain the integrity of the microvasculature during subacute and chronic inflammatory processes involving complement activation.

Induction of Decay-Accelerating Factor by Cytokines or the Membrane-Attack Complex Protects Vascular Endothelial Cells Against Complement Deposition

Vascular endothelium is continuously exposed to complement-mediated challenge, and this is enhanced during inflammation. Although the complement-regulatory proteins decay-accelerating factor (DAF), CD59, and membrane cofactor protein (MCP) protect endothelial cells (ECs) against complement-mediated injury, the control of their expression and relative contributions to vascular protection is unclear. We explored the hypothesis that mechanisms exist which induce upregulation of complement-regulatory proteins on ECs to maintain vascular function in inflammation. Tumor necrosis factor alpha (TNF) and interferon gamma (IFNγ) each increased DAF expression but not CD59 or MCP expression, and a combination of these cytokines was more potent than either alone. Cytokine-induced expression depended on increased DAF mRNA and de novo protein synthesis and was maximal by 72 hours. In addition, assembly of the membrane-attack complex (MAC) on ECs induced a 3-fold increase in DAF expression, and this was enhanced by cytokines. DAF upregulation was not inhibited by protein kinase C (PKC) antagonists. The increase in DAF was functionally relevant since it reduced complement 3 (C3) deposition by 40%, and this was inhibited by an anti-DAF monoclonal antibody. These observations indicate that upregulation of DAF expression by cytokines or MAC may represent an important feedback mechanism to maintain the integrity of the microvasculature during subacute and chronic inflammatory processes involving complement activation.

Complement Activation Occurs on Subendothelial Extracellular Matrix In Vitro and Is Initiated by Retraction or Removal of Overlying Endothelial Cells

Vascular endothelium is continuously exposed to plasma complement, which could generate a potent proinflammatory signal if activated on the vascular wall. Normal endothelium, however, expresses an anti-inflammatory phenotype, which includes resistance to complement fixation. As activated endothelium converts to a proinflammatory phenotype, we investigated the effect of cytokines on endothelial susceptibility to complement fixation. Cytokine-treated HUVEC were exposed to human serum as a source of complement, and C3 deposition was quantified. IL-1β and TNF-α in combination with IFN-γ markedly increased endothelial C3 deposition; however, immunofluorescence microscopy revealed that the endothelial cells had retracted, and that bound C3 was concentrated not on cells but in areas of exposed subendothelial extracellular matrix (ECM). Studies with cell-free ECM indicated that complement activation required only ECM exposure and was independent of cellular activation. C3 deposition on ECM was reproduced by reconstituting the alternative pathway, which generated a stable C3 convertase on ECM, but not on endothelial cells. C3b and iC3b were identified on ECM exposed to purified alternative pathway components and serum, respectively. In conditions associated with endothelial disruption, exposure of subendothelial ECM could induce complement fixation and contribute to inflammation and vascular damage.