Holding T cells in check--a new role for complement regulators?

Complement and periodontitis

Although the complement system is centrally involved in host defense, its overactivation or deregulation (e.g., due to inherent host genetic defects or due to pathogen subversion) may excessively amplify inflammation and contribute to immunopathology. Periodontitis is an oral infection-driven chronic inflammatory disease which exerts a systemic impact on health. This paper reviews evidence linking complement to periodontal inflammation and pathogenesis. Clinical and histological observations show a correlation between periodontal inflammatory activity and local complement activation. Certain genetic polymorphisms or deficiencies in specific complement components appear to predispose to increased susceptibility to periodontitis. Animal model studies and in vitro experiments indicate that periodontal bacteria can either inhibit or activate distinct components of the complement cascade. Porphyromonas gingivalis, a keystone species in periodontitis, subverts complement receptor 3 and C5a anaphylatoxin receptor signaling in ways that promote its adaptive fitness in the presence of non-productive inflammation. Overall, available evidence suggests that complement activation or subversion contributes to periodontal pathogenesis, although not all complement pathways or functions are necessarily destructive. Effective complement-targeted therapeutic intervention in periodontitis would require determining the precise roles of the various inductive or effector complement pathways. This information is essential as it may reveal which specific pathways need to be blocked to counteract microbial evasion and inflammatory pathology or, conversely, kept intact to promote host immunity.

Implication of complement system and its regulators in Alzheimer's disease

Alzheimer’s disease (AD) is an age-related neurodegenerative disease that affects approximately 24 million people worldwide. A number of different risk factors have been implicated in AD, however, neuritic (amyloid) plaques are considered as one of the defining risk factors and pathological hallmarks of the disease. Complement proteins are integral components of amyloid plaques and cerebral vascular amyloid in Alzheimer brains. They can be found at the earliest stages of amyloid deposition and their activation coincides with the clinical expression of Alzheimer's dementia. This review emphasizes on the dual key roles of complement system and complement regulators (CRegs) in disease pathology and progression. The particular focus of this review is on currently evolving strategies for design of complement inhibitors that might aid therapy by restoring the fine balance between activated components of complement system, thus improving the cognitive performance of patients. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of AD.

Effect of complement and its regulation on myasthenia gravis pathogenesis

Myasthenia gravis (MG) is primarily caused by antibodies directed towards the skeletal muscle acetylcholine receptor, leading to muscle weakness. Although these antibodies may induce compromise of neuromuscular transmission by blocking acetylcholine receptor function or antigenic modulation, the predominant mechanism of injury to the neuromuscular junction is complement-mediated lysis of the postsynaptic membrane. The vast majority of data to support the role of complement derives from experimentally acquired MG (EAMG). In this article, we review studies that demonstrate the central role of complement in EAMG and MG pathogenesis along with the emerging role of complement in T- and B-cell function, as well as the potential for complement inhibitor-based therapy to treat human MG.

Properdin: emerging roles of a pattern-recognition molecule

Complement is an innate immune system that is a first line of defense against pathogens and facilitates elimination of apoptotic and injured cells. During complement activation, the complement convertases are assembled on target surfaces and initiate their proteolytic activities, a process that marks targets for phagocytosis and/or lysis. The complement alternative activation pathway has been implicated in a number of autoimmune conditions including arthritis and age-related macular degeneration. Properdin, a plasma component that is also released by activated neutrophils, is critical in the stabilization of alternative pathway convertases. Recently, it has been shown that properdin is also a pattern-recognition molecule that binds to certain microbial surfaces, apoptotic cells, and necrotic cells. Once bound to a surface, properdin can direct convertase formation and target uptake. New studies are now focusing on a role for properdin in inflammatory and autoimmune diseases. This review examines the new properdin findings and their implications.

Overview of the immune response

The immune system has evolved to protect the host from a universe of pathogenic microbes that are themselves constantly evolving. The immune system also helps the host eliminate toxic or allergenic substances that enter through mucosal surfaces. Central to the immune system's ability to mobilize a response to an invading pathogen, toxin, or allergen is its ability to distinguish self from nonself. The host uses both innate and adaptive mechanisms to detect and eliminate pathogenic microbes, and both of these mechanisms include self-nonself discrimination. This overview identifies key mechanisms used by the immune system to respond to invading microbes and other exogenous threats and identifies settings in which disturbed immune function exacerbates tissue injury.

The increased mucosal mRNA expressions of complement C3 and interleukin-17 in inflammatory bowel disease

Recent studies have demonstrated that the complement system participates in the regulation of T cell functions. To address the local biosynthesis of complement components in inflammatory bowel disease (IBD) mucosa, we investigated C3 and interleukin (IL)-17 mRNA expression in mucosal samples obtained from patients with IBD. The molecular mechanisms underlying C3 induction were investigated in human colonic subepithelial myofibroblasts (SEMFs). IL-17 and C3 mRNA expressions in the IBD mucosa were evaluated by real-time polymerase chain reaction. The C3 levels in the supernatant were determined by enzyme-linked immunosorbent assay. IL-17 and C3 mRNA expressions were elevated significantly in the active lesions from ulcerative colitis (UC) and Crohn's disease (CD) patients. There was a significant positive correlation between IL-17 and C3 mRNA expression in the IBD mucosa. IL-17 stimulated a dose- and time-dependent increase in C3 mRNA expression and C3 secretion in colonic SEMFs. The C3 molecules secreted by colonic SEMFs were a 115-kDa alpha-chain linked to a 70-kDa beta-chain by disulphide bonds, which was identical to serum C3. The IL-17-induced C3 mRNA expression was blocked by p42/44 mitogen-activated protein kinase (MAPK) inhibitors (PD98059 and U0216) and a p38 MAPK inhibitor (SB203580). Furthermore, IL-17-induced C3 mRNA expression was inhibited by an adenovirus containing a stable mutant form of I kappaB alpha. C3 and IL-17 mRNA expressions are enhanced, with a strong correlation, in the inflamed mucosa of IBD patients. Part of these clinical findings was considered to be mediated by the colonic SEMF response to IL-17.

Complement: coming full circle

The complement system has long been known to be a major element of innate immunity. Traditionally, it was regarded as the first line of defense against invading pathogens, leading to opsonization and phagocytosis or the direct lysis of microbes. However, from the second half of the twentieth century on, it became clear that complement is also intimately involved in the induction and "fine tuning" of adaptive B- and T-cell responses as well as lineage commitment. This growing recognition of the complement system's multifunctional role in immunity is consistent with the recent paradigm that complement is also necessary for the successful contraction of an adaptive immune response. This review aims at giving a condensed overview of complement's rise from a simple innate stop-and-go system to an essential and efficient participant in general immune homeostasis and acquired immunity.

Assays on the influence of biomaterials on allogeneic rejection in tissue engineering

In tissue engineering, innate responses to biomaterial scaffolds will affect rejection of allogeneic cells. Biomaterials directly influence innate and adaptive immune cell adhesion, reactive oxygen intermediate production, cytokine secretion, nuclear factor-kappa B nuclear translocation, gene expression, and cell surface markers, all of which are likely to affect allogeneic rejection responses. A major goal in tissue engineering is to induce transplant tolerance, potentially by manipulating the biomaterial component. This review describes methods of measuring responses of macrophages, dendritic cells, and T cells stimulated in vitro and in vivo and addresses key factors in assay development. Such tests include mixed leukocyte reactions, enzyme-linked immunosorbent spot assays, trans-vivo delayed-type hypersensitivity assays, and measurement of dendritic cell subsets and anti-donor antibodies; we propose extending these studies to tissue engineering.

Co-ordinated regulation of plasmacytoid dendritic cell surface receptors upon stimulation with herpes simplex virus type 1

Human plasmacytoid dendritic cells (PDC) are crucial for innate and adaptive immune responses against viral infections, mainly through production of type I interferons. Evidence is accumulating that PDC surface receptors play an important role in this process. To investigate the PDC phenotype in more detail, a chip-based expression analysis of surface receptors was combined with respective flow cytometry data obtained from fresh PDC, PDC exposed to interleukin-3 (IL-3) and/or herpes simplex virus type 1 (HSV-1). CD156b, CD229, CD305 and CD319 were newly identified on the surface of PDC, and CD180 was identified as a new intracellular antigen. After correction for multiple comparisons, a total of 33 receptors were found to be significantly regulated upon exposure to IL-3, HSV-1 or IL-3 and HSV-1. These were receptors involved in chemotaxis, antigen uptake, activation and maturation, migration, apoptosis, cytotoxicity and costimulation. Infectious and ultraviolet-inactivated HSV-1 did not differentially affect surface receptor regulation, consistent with the lack of productive virus infection in PDC, which was confirmed by HSV-1 real-time polymerase chain reaction and experiments involving autofluorescing HSV-1 particles. Viral entry was mediated at least in part by endocytosis. Time-course experiments provided evidence of a co-ordinated regulation of PDC surface markers, which play a specific role in different aspects of PDC function such as attraction to inflamed tissue, antigen recognition and subsequent migration to secondary lymphatic tissue. This knowledge can be used to investigate PDC surface receptor functions in interactions with other cells of the innate and adaptive immune system, particularly natural killer cells and cytotoxic T lymphocytes.

C3 promotes expansion of CD8+ and CD4+ T cells in a Listeria monocytogenes infection

It is known that C3 is required for optimal expansion of T cells during acute viral infections. However, it is not yet determined whether T cell responses to intracellular bacterial infections require C3. Therefore, we have investigated the requirement for C3 to elicit potent T cell responses to Listeria monocytogenes (LM). We show that expansion of Ag-specific CD8 and CD4 T cells during a primary response to LM was markedly reduced in the absence of C3 activity. Further studies indicated that, unlike in an influenza virus infection, the regulation of LM-specific T cell responses by C3 might not involve the downstream effector C5a. Moreover, reduced T cell responses to LM was not linked to defective maturation of dendritic cells or developmental anomalies in the peripheral T cell compartment of C3-deficient mice. Experiments involving adoptive transfer of C3-deficient CD8 T cells into the C3-sufficient environment of wild-type mice showed that these T cells do not have intrinsic proliferative defects, and a paracrine source of C3 will suffice for clonal expansion of CD8 T cells in vivo. However, stimulation of purified C3-deficient CD8 T cells by plastic-immobilized anti-CD3 showed that C3 promotes T cell proliferation directly, independent of its effects on APC. On the basis of these findings, we propose that diminished T cell responses to LM in C3-deficient mice might be at least in part due to lack of direct effects of C3 on T cells. These studies have furthered our understanding of C3-mediated regulation of T cell immunity to intracellular pathogens.

Molecular profiling of CD3-CD4+ T cells from patients with the lymphocytic variant of hypereosinophilic syndrome reveals targeting of growth control pathways

The clonal CD3(-)CD4(+) T-cell population characterizing lymphocytic variant hypereosinophilic syndrome (L-HES) persists for years, with a subgroup of patients ultimately progressing to T lymphoma. The molecular changes associated with the premalignant clone and the emergence of malignant subclones are unknown, precluding the development of targeted therapy for this HES variant. In this study, we used whole genome arrays to examine gene expression in the CD3(-)CD4(+) T cells and found that 850 genes were differentially regulated during chronic disease compared with CD3(+)CD4(+) T cells from healthy donors. Changes in the expression of 349 genes were altered in association with the clinical progression from chronic L-HES to T lymphoma in 1 patient, with 87 of 349 genes representing further changes in genes whose expression was altered in all chronic disease patients (87 of 850). Array analysis after CD2/CD28-mediated activation revealed that the major gene expression changes observed in the CD3(-)CD4(+) T cells do not reflect activation induced alterations but rather pathways involved in T-cell homeostasis, including transforming growth factor-beta signaling, apoptosis, and T-cell maturation, signaling, and migration. Examination of microRNA expression in the CD3(-)CD4(+) T cells from patients with chronic disease identified 23 microRNAs that changed significantly, among which miR-125a further decreased in association with one patient's evolution to T lymphoma.

IL-10 protects monocytes and macrophages from complement-mediated lysis

Phagocytes, such as monocytes and macrophages, are important cells of the innate immunity in the defense against microbes. So far, it is unclear how these cells survive at the site of combat against microbes, where a hostile inflammatory environment prevails with strong complement activity. We hypothesized that IL-10, a key cytokine involved in the resolution of inflammation, induces resistance to complement attack. Here, we demonstrate for the first time such a cell-protective effect of IL-10 on human monocytes and macrophages. IL-10 is indeed able to protect these cell types in an in vitro model of complement lysis triggered by an anti-MHCI antibody or by binding of zymosan. Investigating potential underlying mechanisms, we found that IL-10 up-regulated the expression of complement regulatory membrane protein CD59 and the general cell-protective stress protein HO-1 in human monocytes. However, further functional analysis failed to link these individual IL-10-mediated effects with the increased protection from complement lysis. Blocking the protective effect of CD59 with an antibody increased complement lysis but did not abrogate the IL-10-protective effect. Interestingly, chemical interference with HO-1 activity did abrogate the protective effect of IL-10, but siRNA-mediated knockdown of HO-1 did not confirm this observation. Our results suggest that IL-10 generates pathogen-clearing phagocytes, which are resistant to complement lysis and thereby, enabled to survive longer in a hostile inflammatory environment.

CD59 but not DAF deficiency accelerates atherosclerosis in female ApoE knockout mice

Although the complement system has been implicated in atherosclerosis, the influence of membrane-bound complement regulators in this process has not been well understood. We studied the role of two membrane complement regulators, decay-accelerating factor (DAF) and CD59, in a murine model of atherosclerosis. DAF(-/-) and CD59(-/-) mice were crossed with apolipoprotein E (ApoE)-deficient mice to generate DAF(-/-)ApoE(-/-) and CD59(-/-)ApoE(-/-) mice. Mice were fed a high fat diet (HFD) for 8 or 16 weeks. En face analysis showed that CD59 deficiency led to more extensive lesions in female ApoE(-/-) mice both at 8 weeks (2.07+/-0.27% vs.1.34+/-0.21%, P=0.06) and 16 weeks (17.13+/-1.14% vs. 9.72+/-1.14%, P<0.001). Similarly, lesions measured by aortic root sectioning were larger in female CD59(-/-)ApoE(-/-) mice than in controls at 8 weeks of HFD feeding (20.74+/-1.33% vs. 13.12+/-1.46%, P<0.005). On the other hand, DAF deficiency did not significantly influence atherosclerosis in ApoE(-/-) mice. Immunohistochemistry revealed more abundant membrane attack complex (MAC) deposition and more collagen staining in the aortic roots of CD59(-/-)ApoE(-/-) mice. Unexpectedly, total plasma cholesterol levels in female CD59(-/-)ApoE(-/-) mice were found to be elevated compared with CD59(+/+)ApoE(-/-) mice. We conclude that CD59 but not DAF offered protection in atherosclerosis in the context of ApoE deficiency. The protective role of CD59 was gender-biased and most likely involved prevention of MAC-mediated vascular injury, with possible contribution from an undefined effect on plasma cholesterol homeostasis.

Upregulated expression of complement inhibitory proteins on bladder cancer cells and anti-MUC1 antibody immune selection

Membrane complement inhibitors (CD46, CD55 and CD59) are upregulated in some human cancers indicating that they play a role in immune evasion. We investigated complement inhibitor expression in bladder cancer and examined the hypothesis that selective pressure of an antibody response (anti-MUC1) results in the upregulated expression of complement inhibitors on tumor cells. Paired samples of tumor and normal tissue from 22 bladder cancer patients were analyzed for expression of MUC1, CD46, CD55 and CD59, and matched serum samples analyzed for anti-MUC1 IgM and IgG levels. Relationships between anti-MUC1 antibody levels and complement inhibitor expression were investigated. MUC1 mRNA was upregulated in 86% of tumor samples. CD46 was upregulated in 77%, CD55 in 55% and CD59 in 59% of tumors. Low titer anti-MUC1 IgM was detected in normal human sera, but was elevated in 41% of the bladder cancer patients. Anti-MUC1 IgG was virtually absent from normal sera, but present in 32% of the cancer patients. There was a direct relationship between anti-MUC1 antibody titer and expression level of complement inhibitors. Analysis of the correlation of each antibody with the expression of each complement inhibitor by Spearman's rank test revealed a strong correlation between both anti-MUC1 IgM and IgG levels and increased expression of CD46 and CD55, and combined anti-MUC1 IgM/IgG levels correlated with increased expression of all 3 complement inhibitors. In conclusion, the data demonstrate upregulated complement inhibitor expression and the presence of an anti-MUC1 antibody response in bladder cancer patients and support the hypothesis of antibody-mediated immune selection.

Modulation of protective T cell immunity by complement inhibitor expression on tumor cells

Complement-inhibitory proteins expressed on cancer cells can provide protection from antitumor antibodies and may potentially modulate the induction of an immune response to tumor-associated antigens. In the current study, we investigated the consequences of complement inhibitor down-regulation on the effector and inductive phases of an immune response. Stable small interfering RNA-mediated down-regulation of the complement inhibitor Crry on MB49 murine bladder cancer cells increased their susceptibility to monoclonal antibody and complement in vitro. In a syngeneic model of metastatic cancer, the down-regulation of Crry on i.v.-injected MB49 cells was associated with a significant decrease in tumor burden and an increase in the survival of challenged mice. However, monoclonal antibody therapy had no additional benefit. There was an antitumor IgG response, but the response was not effected by Crry down-regulation on inoculated tumor cells. Down-regulation of Crry on MB49 cells resulted in an enhanced antitumor T-cell response in challenged mice (measured by lymphocyte IFN-gamma secretion), and CD8+ T cell depletion of mice prior to injection of MB49 cells completely abrogated the effect of Crry down-regulation on tumor burden and survival. Deficiency of C3 also abrogated the effect of Crry down-regulation on the survival of MB49-challenged mice, indicating a complement-dependent mechanism. These data indicate that complement inhibitors expressed on a tumor cell can suppress a T cell response and that enhancing complement activation on a tumor cell surface can promote protective T cell immunity.

The classical complement pathway in transplantation: unanticipated protective effects of C1q and role in inductive antibody therapy

Though complement (C) deposition within the transplant is associated with allograft rejection, the pathways employed have not been established. In addition, evidence suggests that C-mediated cytolysis may be necessary for the tolerance-inducing activities of mAb therapies. Hence, we assessed the role of the classical C pathway in acute allograft rejection and its requirement for experimental mAb therapies. C1q-deficient (C1q-/-) recipients rejected allografts at a faster rate than wild-type (WT) recipients. This rejection was associated with exacerbated graft pathology but not with enhanced T-cell responses in C1q-/- recipients. However, the humoral response to donor alloantigens was accelerated in C1q-/- mice, as an early IgG response and IgG deposition within the graft were observed. Furthermore, deposition of C3d, but not C4d was observed in grafts isolated from C1q-/- recipients. To assess the role of the classical C pathway in inductive mAb therapies, C1q-/- recipients were treated with anti-CD4 or anti-CD40L mAb. The protective effects of anti-CD4 mAb were reduced in C1q-/- recipients, however, this effect did not correlate with ineffective depletion of CD4+ cells. In contrast, the protective effects of anti-CD40L mAb were less compromised in C1q-/- recipients. Hence, this study reveals unanticipated roles for C1q in the rejection process.

Patching of ganglioside(M1) in human erythrocytes - distribution of CD47 and CD59 in patched and curved membrane

Membrane rafts may act as platforms for membrane protein signalling. Rafts have also been implicated in the sorting of membrane components during membrane budding. We have studied by fluorescence microscopy cross-linking of ganglioside GM1 in the human erythrocyte membrane, and how membrane proteins CD47 and CD59 distribute in GM1 patched discoid cells and calcium-induced echinocytic cells. Patching of ganglioside(M1) (GM1) by cholera toxin subunit B (CTB) plus anti-CTB resulted in the formation of usually 40-60 GM1 patches distributed over the membrane in discoid erythrocytes. Pre-treatment of erythrocytes with methyl-beta-cyclodextrin abolished GM1 patching. GM1 patching was insensitive to pre-fixation (paraformaldehyde) of cells. Patching of GM1 did not affect the discoid shape of erythrocytes. Membrane proteins CD47 and CD59 did not accumulate into GM1 patches. No capping of patches occurred. GM1 accumulated in calcium-induced echinocytic spiculae. Also CD59, but not CD47, accumulated in spiculae. However, CD59 showed a low degree of co-localization with GM1 and frequently accumulated in different spiculae than GM1. In conclusion, our study describes a novel method for examining properties and composition of rafts. The study characterizes raft patching in the human erythrocyte membrane and emphasizes the mobility and 'echinophilicity' of GM1. Glycosyl phosphatidylinositol-anchored CD59 was identified as a mobile 'echinophilic' but 'raftophobic(GM1)' protein. Largely immobile CD47 showed no segregation.

T-cell regulation by CD46 and its relevance in multiple sclerosis

CD46 is a complement regulatory molecule expressed on every cell type, except for erythrocytes. While initially described as a regulator of complement activity, it later became a 'magnet for pathogens', binding to several viruses and bacteria. More recently, an alternative role for such complement molecules has emerged: they do regulate T-cell immunity, affecting T-cell proliferation and differentiation. In particular, CD46 stimulation induces Tr1 cells, regulatory T cells characterized by massive production of interleukin-10 (IL-10), a potent anti-inflammatory cytokine. Hence, CD46 is likely to control inflammation. Indeed, data from CD46 transgenic mice highlight a role for CD46 in inflammation, with antagonist roles depending on the cytoplasmic tail being expressed. Furthermore, recent data have shown that CD46 is defective in multiple sclerosis, IL-10 production being severely impaired in these patients. This lack of IL-10 production probably participates in the inflammation observed in patients with multiple sclerosis. This review will summarize the data on CD46 and T cells, and how CD46 is likely involved in multiple sclerosis.

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.

Dysferlin deficiency enhances monocyte phagocytosis: a model for the inflammatory onset of limb-girdle muscular dystrophy 2B

Dysferlin deficiency causes limb-girdle muscular dystrophy type 2B (LGMD2B; proximal weakness) and Miyoshi myopathy (distal weakness). Muscle inflammation is often present in dysferlin deficiency, and patients are frequently misdiagnosed as having polymyositis. Because monocytes normally express dysferlin, we hypothesized that monocyte/macrophage dysfunction in dysferlin-deficient patients might contribute to disease onset and progression. We therefore examined phagocytic activity, in the presence and absence of cytokines, in freshly isolated peripheral blood monocytes from LGMD2B patients and in the SJL dysferlin-deficient mouse model. Dysferlin-deficient monocytes showed increased phagocytic activity compared with control cells. siRNA-mediated inhibition of dysferlin expression in the J774 macrophage cell line resulted in significantly enhanced phagocytosis, both at baseline and in response to tumor necrosis factor-alpha. Immunohistochemical analysis revealed positive staining for several mononuclear cell activation markers in LGMD2B human muscle and SJL mouse muscle. SJL muscle showed strong up-regulation of endocytic proteins CIMPR, clathrin, and adaptin-alpha, and LGMD2B muscle exhibited decreased expression of decay accelerating factor, which was not dysferlin-specific. We further showed that expression levels of small Rho family GTPases RhoA, Rac1, and Cdc 42 were increased in dysferlin-deficient murine immune cells compared with control cells. Therefore, we hypothesize that mild myofiber damage in dysferlin-deficient muscle stimulates an inflammatory cascade that may initiate, exacerbate, and possibly perpetuate the underlying myofiber-specific dystrophic process.

Effects of biomaterial-induced inflammation on fibrosis and rejection

Evidence is emerging that biomaterials cause inflammation by ligating innate immune receptors on antigen presenting cells. Although inflammation is usually viewed as detrimental, it has unexpected and potentially beneficial effects on fibrosis and transplant rejection. For example, the magnitude of inflammation due to a biomaterial is not predictive of the extent of fibrosis. Similarly, biomaterials do not always show adjuvancy. Some biomaterials suppressed T cell rejection responses in vivo and in vitro, while others non-specifically stimulated T cell proliferation. Understanding these complex inter-relationships is the key to designing a biomaterial that stimulates regeneration and induces tolerance in tissue engineering applications.

Complement-targeted therapeutics

The complement system is a central component of innate immunity and bridges the innate to the adaptive immune response. However, it can also turn its destructive capabilities against host cells and is involved in numerous diseases and pathological conditions. Modulation of the complement system has been recognized as a promising strategy in drug discovery, and a large number of therapeutic modalities have been developed. However, successful marketing of complement-targeted drugs has proved to be more difficult than initially expected, and many strategies have been discontinued. The US Food and Drug Administration's approval of the first complement-specific drug, an antibody against complement component C5 (eculizumab; Soliris), in March 2007, was a long-awaited breakthrough in the field. Approval of eculizumab validates the complement system as therapeutic target and might facilitate clinical development of other promising drug candidates.

New concepts of complement in allorecognition and graft rejection

In transplantation, activation of complement has largely been equated to antibody-mediated rejection, but complement is also important in recognition of apoptotic and necrotic cells as well as in modifying antigen presentation to T cells and B cells. As a part of the innate immune system, complement is one of the first responses to injury, and it can determine the direction and magnitude of the subsequent responses. Consequently, the effects of complement in allorecognition and graft rejection are increased when organs are procured from cadaver donors because these organs sustain a series of stresses from brain death, prolonged life support, ischemia and finally reperfusion that initiate proinflammatory processes and tissue injury. In addition, these organs are transplanted to patients, who frequently have been sensitized to histocompatibility antigens as the result of transfusions, pregnancies or transplants. Complement activation generates a series of biologically active effector molecules that can modulate graft rejection by directly binding to the graft or by modifying the response of macrophages, T and B cells of the recipient. However, complement is regulated and the process of regulation produces split products that can decrease as well as increase immune responses. Small animal models have been developed to test these variables. The guide for evaluating results from these models remains clinical findings because there are significant differences between the rodent and human complement systems.

Expression of complement regulatory proteins on human natural killer cell subsets

The cell surface complement regulatory (CReg) proteins CD46, CD55 and CD59 are widely distributed on human leucocytes and protect against complement-mediated damage. To investigate heterogeneity in CReg protein expression by human natural killer (NK) cells, levels were assessed on resting and activated NK cell subsets identified phenotypically on the basis of expression of CD56 and CD158 markers. Levels of all three CReg proteins on CD56+ cells were lower than on T cells (p<0.05). Freshly isolated CD56(bright) cells expressed higher levels of CD55 than CD56dim cells (p<0.05). CD158a+ cells expressed significantly lower levels of both CD46 and CD59, and CD158e+ cells expressed significantly lower levels of CD46, than CD158a(-) CD158e(-) cells, respectively (both p<0.05). Stimulation with PHA did not significantly alter NK cell surface CReg protein levels whereas, following culture with IL-2, CD46 and CD59 were decreased on both CD56bright and CD56dim subsets (p<0.05). In the case of CD59, this was independent of T cells. Only CD46 was significantly downregulated on CD158b+ (GL183+) and CD158e (NKB1+) subsets (p<0.05). However, culture in IL-15 significantly increased levels of all three CReg proteins. These observations that CReg proteins are downregulated on certain NK cell subsets following activation with IL-2 are opposite to previous findings for other leucocyte subpopulations. Activated NK cells may instead use other strategies for protection against complement-mediated damage in a local inflammatory response.

Enhancement of Antibody-Dependent Mechanisms of Tumor Cell Lysis by a Targeted Activator of Complement

Complement inhibitors expressed on tumor cells provide a hindrance to the therapeutic efficacy of some monoclonal antibodies (mAb). We investigated a novel strategy to overwhelm complement inhibitor activity and amplify complement activation on tumor cells. The C3-binding domain of human complement receptor 2 (CR2; CD21) was linked to the complement-activating Fc region of human IgG1 (CR2-Fc), and the ability of the construct to target and amplify complement deposition on tumor cells was investigated. CR2 binds C3 activation fragments, and CR2-Fc targeted tumor cells by binding to C3 initially deposited by a tumor-specific antibody. Complement deposition on Du145 cells (human prostate cancer cell line) and anti-MUC1 mAb-mediated complement-dependent lysis of Du145 cells were significantly enhanced by CR2-Fc. Anti-MUC1 antibody-dependent cell-mediated cytotoxicity of Du145 by human peripheral blood mononuclear cells was also significantly enhanced by CR2-Fc in both the presence and the absence of complement. Radiolabeled CR2-Fc targeted to s.c. Du145 tumors in nude mice treated with anti-MUC1 mAb, validating the targeting strategy in vivo. A metastatic model was used to investigate the effect of CR2-Fc in a therapeutic paradigm. Administration of CR2-Fc together with mAb therapy significantly improved long-term survival of nude mice challenged with an i.v. injection of EL4 cells. The data show that CR2-Fc enhances the therapeutic efficacy of antibody therapy, and the construct may provide particular benefits under conditions of limiting antibody concentration or low tumor antigen density.

The relative importance of local and systemic complement production in ischaemia, transplantation and other pathologies

Besides a critical role in innate host defence, complement activation contributes to inflammatory and immunological responses in a number of pathological conditions. Many tissues outside the liver (the primary source of complement) synthesise a variety of complement proteins, either constitutively or response to noxious stimuli. The significance of this local synthesis of complement has become clearer as a result of functional studies. It revealed that local production not only contributes to the systemic pool of complement but also influences local tissue injury and provides a link with the antigen-specific immune response. Extravascular production of complement seems particularly important at locations with poor access to circulating components and at sites of tissue stress responses, notably portals of entry of invasive microbes, such as interstitial spaces and renal tubular epithelial surfaces. Understanding the relative importance of local and systemic complement production at such locations could help to explain the differential involvement of complement in organ-specific pathology and inform the design of complement-based therapy. Here, we will describe the lessons we have learned over the last decade about the local synthesis of complement and its association with inflammatory and immunological diseases, placing emphasis on the role of local synthesis of complement in organ transplantation.

Reduced expression of decay-accelerating factor 1 on CD4+ T cells in murine systemic autoimmune disease

OBJECTIVE

Deficiency of decay-accelerating factor 1 (termed Daf1 in mice) has been shown to exacerbate autoimmunity, and recent studies have suggested that this may be explained by Daf1 acting as a regulator of T cell immunity. The aim of this study was to determine whether Daf1 expression on T cells is modulated during development of autoimmunity in mice.

METHODS

To test this hypothesis, we examined Daf1 levels in NZB, DBA/2, and B10.S mice before and after induction of murine mercury-induced autoimmunity (mHgIA). Daf1 was measured by real-time polymerase chain reaction and flow cytometry, and levels of Daf1 were correlated with markers of lymphocyte activation and cytokine production.

RESULTS

Autoimmune-prone NZB mice had low endogenous levels of Daf1 irrespective of the induction of mHgIA. Induction of autoimmunity reduced Daf1 expression in mHgIA-sensitive B10.S mice, particularly on activated/memory (CD44(high)) CD4+ T cells that accumulate as a result of exposure to mercury. Murine mercury-induced autoimmunity-resistant DBA/2 mice, which fail to accumulate CD44(high) T cells, showed no change in Daf1 expression. Modulation of Daf1 expression was found to require CD4+ T cell costimulation, since B10.S mice deficient in CD28 were unable to down-regulate Daf1 or accumulate activated/memory CD4+ T cells. In B10.S mice exposed to mercury, the production of interleukin-4 (IL-4), but not that of IL-2 or interferon-gamma, in the spleen was associated with CD44(high),Daf1(low),CD4+ T cells.

CONCLUSION

These findings demonstrate that reduction of Daf1 expression is closely associated with CD4+ T cell activation and the accumulation of CD44(high)(activated/memory),CD4+ T cells in both spontaneous and induced systemic autoimmune disease.

Transcription factor Pax5 activates the chromatin of key genes involved in B cell signaling, adhesion, migration, and immune function

The transcription factor Pax5 represses B lineage-inappropriate genes and activates B cell-specific genes in B lymphocytes. Here we have identified 170 Pax5-activated genes. Conditional mutagenesis demonstrated that the Pax5-regulated genes require continuous Pax5 activity for normal expression in pro-B and mature B cells. Expression of half of the Pax5-activated genes is either absent or substantially reduced upon Pax5 loss in plasma cells. Direct Pax5 target genes were identified based on their protein synthesis-independent activation by a Pax5-estrogen receptor fusion protein. Chromatin immunoprecipitation (ChIP) of Pax5 together with chromatin profiling by ChIP-on-chip analysis demonstrated that Pax5 directly activates the chromatin at promoters or putative enhancers of Pax5 target genes. The Pax5-activated genes code for key regulatory and structural proteins involved in B cell signaling, adhesion, migration, antigen presentation, and germinal-center B cell formation, thus revealing a complex regulatory network that is activated by Pax5 to control B cell development and function.

CD59a deficiency exacerbates influenza-induced lung inflammation through complement-dependent and -independent mechanisms

Influenza-specific immune activity not only promotes virus clearance but also causes immunopathology, thereby underlining the importance of mounting a measured anti-viral immune response. Since complement bridges both the innate and adaptive immune systems and has been implicated in defence against influenza, the role of the complement regulator CD59a in modulating the response to influenza was explored. For this purpose, immune responses to influenza virus, strain E61-13-H17, in mice deficient in the complement regulator protein CD59a (Cd59a^–/– mice) were compared to those in wild-type mice. The severity of lung inflammation was significantly enhanced in the lungs of Cd59a^–/– mice with increased numbers of infiltrating neutrophils and CD4⁺ T cells. When complement was inhibited using soluble complement receptor1, the frequency of lung-infiltrating neutrophils in influenza-infected Cd59a^–/– mice was much reduced whilst numbers of CD4⁺ T cells remained unchanged. These results demonstrate that CD59a, previously defined as a complement regulator, modulates both the innate and adaptive immune response to influenza virus by both complement-dependent and-independent mechanisms.

Complement activation in Lyme neuroborreliosis — Increased levels of C1q and C3a in cerebrospinal fluid indicate complement activation in the CNS

A strong initial inflammatory response is important in neuroborreliosis. Since complement is a main player in early inflammation, we monitored the concentration and activation of complement in plasma and cerebrospinal fluid from 298 patients, of whom 23 were diagnosed with neuroborreliosis. Using sandwich ELISAs, we found significantly elevated levels of C1q, C4, C3, and C3a in cerebrospinal fluid, but not in plasma, in patients with neuroborreliosis. This finding indicates that complement plays a role in the human immune response in neuroborreliosis, that the immunologic process is compartmentalized to the CNS, and that complement activation may occur via the classical pathway.

T-cell regulation: with complements from innate immunity

The complement system was traditionally known as an effector arm of humoral immunity. Today we also recognize it as a main element of the innate immune system. In blood and other body fluids complement is a first line of defence against pathogens, because it becomes fully active within seconds. Active complement fragments attach to the invading pathogen to promote opsonization and lysis, triggering a local inflammatory response. This Review focuses on the evolving role of the complement system in the regulation of T-cell responses, from directing the initiation phase, through driving lineage commitment, to regulating the contraction phase.

Ligation of the cell surface receptor, CD46, alters T cell polarity and response to antigen presentation

Lymphocyte function in vivo is dictated by multiple external cues, but the integration of different signals is not well understood. Here, we show that competition for the axis of polarization dictates functional outcomes. We investigated the effect of ligation of the immunoregulatory cell surface receptor, CD46, on lymphocyte polarity during antigen presentation and cytotoxic effector function. Ligation of CD46 on human T cells prevented recruitment of the microtubule organizing center, CD3, and perforin to the interface with the antigen-presenting cell and caused a reduction in IFN-gamma production. In human NK cells, similar changes in polarity induced by CD46 ligation inhibited the recruitment of the microtubule organizing center and perforin to the interface with target cells and correlated with reduced killing. These data indicate that external signals can alter lymphocyte polarization toward antigen-presenting cells or target cells, inhibiting lymphocyte function.

Levels of expression of complement regulatory proteins CD46, CD55 and CD59 on resting and activated human peripheral blood leucocytes

The cell surface complement regulatory (CReg) proteins CD46, CD55 and CD59 are widely expressed on human lymphoid and non-lymphoid cells. This study aimed to compare systematically levels of CReg expression by different leucocyte subsets and to determine whether levels were increased following activation in vitro. Levels of each CReg protein were similar on freshly isolated monocytes and all major lymphocyte subsets, except that CD4(+) cells expressed significantly less CD46 than CD8(+) cells (P < 0.05) while the reverse was observed for CD55 (P < 0.02). CD56(+) cells, predominantly natural killer cells, expressed significantly lower levels of CD59 than T cells (P < 0.02). CD45RO(+) cells had higher levels of surface CD46 and CD59, but lower levels of CD55, than CD45RO(-) cells (P < 0.02); CD25(+) cells also expressed significantly less CD55 than CD25(-) cells (P < 0.002). Neutrophils expressed higher levels of CD59, but lower levels of CD55, than monocytes. Following activation with phytohaemagglutinin, CD46 was up-regulated on all leucocyte subsets with the exception of CD56(+) cells. Both CD55 and CD59 were also markedly up-regulated on monocytes, and CD55 expression was greater on CD8(+) than CD4(+) cells following activation (P < 0.02). Lipopolysaccharide treatment did not significantly alter B-cell expression of CReg proteins whereas CD55 and CD59, but not CD46, were significantly up-regulated on monocytes (P < 0.02). These observations that CReg proteins are up-regulated on certain activated leucocyte subsets indicate that levels would be increased following immune responses in vivo. This could enhance both protection against local complement activation at inflammatory sites and also the immunoregulatory properties of these leucocytes.

Alternative roles for CD59

CD59 was first identified as a regulator of the terminal pathway of complement, which acts by binding to the C8/C9 components of the assembling membrane attack complex (MAC), to inhibit formation of the lytic pore. Structurally, CD59 is a small, highly glycosylated, GPI-linked protein, with a wide expression profile. Functionally, the role of CD59 in complement regulation is well-defined but studies have also shown clear evidence for signalling properties, which are linked to its glycophosphatidyl inositol (GPI) anchor and its location within lipid rafts. Cross-linking of CD59 using specific monoclonal antibodies drives both calcium release and activation of lipid-raft associated signalling molecules such as tyrosine kinases. These observations clearly show that CD59 exhibits roles independent of its function as a complement inhibitor. In this review, we examine the progression of research in this area and explore the alternative functions of CD59 that have recently been defined.

Complement receptor 2, natural antibodies and innate immunity: Inter-relationships in B cell selection and activation

Complement receptor type 2 (CR2) is a receptor that serves as an important interface between the complement system and adaptive immunity. Recent studies have shown that CR2 is also centrally involved in innate immunity, and one key area is the development of potentially pathogenic natural antibodies that target neo-epitopes revealed in ischemic tissue undergoing reperfusion. Mice lacking either total immunoglobulins or CR2 alone are protected from the development of ischemia-reperfusion injury, and this effect can be reversed by introducing CR2-sufficient B-1 cells or by transferring polyclonal natural IgM antibody from wild type mice as well as monoclonal antibodies that recognize phospholipids, DNA or non-muscle myosin. We will report at the XXI ICW an additional membrane-associated protein to which pathogenic IgM antibodies are directed. Whether B cells producing these natural antibodies are differentially selected in CR2-deficient mice is as yet not well understood, and the complement-related mechanism(s) whereby this differential repertoire selection process could occur have yet to be explored in any detail. In addition to this important role in innate immunity, CR2 can also act as a receptor for other components or activators of innate immunity. One such component is interferon-alpha, an anti-viral cytokine that binds CR2 and induces a component of its mRNA signature in B cells through this receptor. Other potential CR2 ligands are DNA and DNA-containing complexes such as chromatin. The biologic role of these CR2 interactions with interferon-alpha and DNA-containing complexes is not well understood, but may be important in the development of the autoimmune disease systemic lupus erythematosus that is characterized by enhanced interferon-alpha levels and loss of self tolerance to DNA-containing self antigens.

Role of dendritic cell synthesis of complement in the allospecific T cell response

Although extrahepatic synthesis of complement and particularly C3 has been widely studied in most cells and tissues, new information is emerging on dendritic cells (DCs). This research has shown that mouse bone marrow (BM) derived DCs are able to synthesise C3 and this synthesis has a substantial impact on DC activation, affecting the diverse range of DC functions relevant to the allospecific T cell response. Thus, local production of C3 appears to regulate the capacity of DCs to trigger the primary T cell response against donor alloantigen. Understanding of the key mechanisms by which complement activation modulates DC maturation could lead to the development of therapeutic strategies to down regulate DC activation thus reduce allograft rejection.

Costimulation via CD55 on Human CD4+T Cells Mediated by CD97

Decay-accelerating factor (CD55) is a complement regulatory protein, which is expressed by most cells to protect them from complement-mediated attack. CD55 also binds CD97, an EGF-TM7 receptor constitutively expressed on granulocytes and monocytes and rapidly up-regulated on T and B cells upon activation. Early results suggested that CD55 could further enhance T cell proliferation induced by phorbol ester treatment. The present study demonstrates that coengagement of CD55, using either cross-linking mAbs or its natural ligand CD97, and CD3 results in enhanced proliferation of human peripheral blood CD4(+) T cells, expression of the activation markers CD69 and CD25, and secretion of IL-10 and GM-CSF. Recently, an increase in T cell responsiveness in CD55(-/-) mice was shown to be mediated by a lack of complement regulation. In this study, we show that direct stimulation of CD55 on CD4(+) T cells with CD97 can modulate T cell activation but does not interfere with CD55-mediated complement regulation. Our results support a multifaceted role for CD55 in human T cell activation, constituting a further link between innate and adaptive immunity.