Linking innate and acquired immunity: divergent role of CD46 cytoplasmic domains in T cell induced inflammation

Cytokines in Atherosclerosis: Pathogenic and Regulatory Pathways

Atherosclerosis is a chronic disease of the arterial wall where both innate and adaptive immunoinflammatory mechanisms are involved. Inflammation is central at all stages of atherosclerosis. It is implicated in the formation of early fatty streaks, when the endothelium is activated and expresses chemokines and adhesion molecules leading to monocyte/lymphocyte recruitment and infiltration into the subendothelium. It also acts at the onset of adverse clinical vascular events, when activated cells within the plaque secrete matrix proteases that degrade extracellular matrix proteins and weaken the fibrous cap, leading to rupture and thrombus formation. Cells involved in the atherosclerotic process secrete and are activated by soluble factors, known as cytokines. Important recent advances in the comprehension of the mechanisms of atherosclerosis provided evidence that the immunoinflammatory response in atherosclerosis is modulated by regulatory pathways, in which the two anti-inflammatory cytokines interleukin-10 and transforming growth factor-β play a critical role. The purpose of this review is to bring together the current information concerning the role of cytokines in the development, progression, and complications of atherosclerosis. Specific emphasis is placed on the contribution of pro- and anti-inflammatory cytokines to pathogenic (innate and adaptive) and regulatory immunity in the context of atherosclerosis. Based on our current knowledge of the role of cytokines in atherosclerosis, we propose some novel therapeutic strategies to combat this disease. In addition, we discuss the potential of circulating cytokine levels as biomarkers of coronary artery disease.

Immunosuppression caused by measles virus: role of viral proteins

Measles virus (MV) causes transient but profound immunosuppression resulting in increased susceptibility to secondary bacterial and viral infections. Due to the development of these opportunistic infections, measles remains the leading vaccine-preventable cause of child death worldwide. Different immune abnormalities have been associated with measles, including disappearance of delayed-type hypersensitivity reactions, impaired lymphocyte and antigen-presenting cell functions, down-regulation of pro-inflammatory interleukin 12 production and altered interferon alpha/beta signalling pathways. Several MV proteins have been suggested to hinder immune functions: hemagglutinin, fusion protein, nucleoprotein and the non-structural V and C proteins. This review will focus on the novel functions attributed to MV proteins in the immunosuppression associated with measles. Here, we highlight new advances in the field, emphasising the interaction between MV proteins and their cellular targets, in particular the cell membrane receptors, CD46, CD150, TLR2 and FcgammaRII in the induction of immunological abnormalities associated with measles.

Adenovirus types 11p and 35 attach to and infect primary lymphocytes and monocytes, but hexon expression in T-cells requires prior activation

Hematopoietic cells are attractive targets for gene therapy, but the conventional adenovirus (Ad) vectors, based on Ad5, transduce these cells inefficiently. One reason for low permissiveness of hematopoietic cells to infection by species C Ads appears to be inefficient attachment. Vectors pseudotyped with species B fibers are clearly more efficient at transducing hematopoietic cells than Ad5. To evaluate which Ad species B type(s) would be the most efficient vector(s) for primary T-cells, B-cells and monocytes, attachment to and entry of the species B1 serotypes 3p and 7p and the species B2 serotypes 11p and 35 into primary PBMCs was studied. Ad11p and Ad35 were the only serotypes to show efficient binding and for which uptake by PBMCs could be detected. Infection of PBMCs by Ad5, Ad11p and Ad35 was compared. Expression of Ad hexons was detected in stimulated PBMCs, most frequently in T-cells, and in unstimulated monocytes, although B-cells appear to be refractory to productive infection. Replication of Ad DNA was severely restricted in most PBMCs. Neither hexon expression nor genome replication could be detected in unstimulated lymphocytes, but FISH and a real-time PCR-based assay suggested that Ad11p and Ad35 DNA reach the nucleus. Activation thus appears to be required for T-cells to be permissive to Ad gene expression. In summary, there are substantial differences between Ad3p and Ad7p on the one hand and Ad11p and Ad35 on the other, in their ability to interact with PBMCs. Ad11p and Ad35 probably represent vectors of choice for these cell types.

Evaluation of adenovirus vectors containing serotype 35 fibers for vaccination

In contrast to commonly used serotype 5-based adenovirus (Ad) vectors, Ad's containing fibers derived from B-group serotype 35 (Ad5/35) efficiently transduce human DCs ex vivo and appear to target antigen-presenting cells after intravenous injection into baboons. Based on this, Ad5/35 vectors could be valuable tools for immunotherapy and vaccination. On the other hand, a number of studies indicate that signaling through the B-group Ad receptor, CD46, can cause tolerance or immunosuppression. Since mice do not express CD46 in a human-like pattern, we studied the in vivo properties of Ad5/35 in transgenic mice that express CD46 in a pattern and at a level similar to those of humans. Hypersensitivity assays and analyses of frequencies of regulatory T cells and T cell responses did not indicate that Ad5/35 injection exerts detrimental effects on the host's immune system. An Ad5/35 vector expressing a model antigen was able to trigger a strong T cell response against the test antigen after intramuscular injection. Overall, compared to Ad5 vectors, Ad5/35 vectors had a better safety profile, reflected by lower serum levels of proinflammatory cytokines.

Compartmentalization in T‐cell signalling: Membrane microdomains and polarity orchestrate signalling and morphology

Lymphocyte function is regulated by complex signalling responses to diverse extracellular inputs, and a cell will often receive multiple, conflicting signals at one time. The mechanisms by which a lymphocyte integrates these signals into a single cellular response are not well understood. An important factor in the integration of signals likely involves the regulation of access of signalling molecules to cell surface receptors and of receptor signals to morphological determinants within the cell. Recent studies have led to important advances in our understanding of both the mechanisms by which signals are compartmentalized in T cells and the physiological role played by such compartmentalization. We review progress in the field, with a particular focus on membrane microdomains or lipid rafts and on cell polarity.

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

Complement is not only part of the innate immune system, but has also been implicated in adaptive immunity. The role of complement and its regulatory proteins in modulating T cell activity has been the focus of several recent studies. These, which have included work on the membrane co-factor protein (MCP or CD46), decay accelerating factor (DAF or CD55) and CD59, indicate that complement regulators can influence the proliferative capacity of T cells and their ability to produce cytokines, influencing the outcome of a T cell response to a given antigen. Here we review these studies, which reveal another important link between the innate and the adaptive immune system.

Receptors for complement C3 on T-lymphocytes: Relics of evolution or functional molecules?

Despite the fact that receptors for complement on T-cells have been described many years ago the function remains unclear as is the role of complement in the T-cell response. In this review we will evaluate how the accumulated wisdom concur with the current concepts of the adaptive T-cell response.

Complement and Toll-like receptors: Key regulators of adaptive immune responses

The innate immune system provides sophisticated defense mechanisms to protect complex macroorganisms from the attack of microorganisms. Among those, the complement system and Toll-like receptors are of paramount importance to discriminate between infectious non-self and non-infectious self and to provide critical danger signals instructing adaptive immune responses. Here, we will discuss recent advances in our understanding of the mechanisms underlying complement and TLR-mediated regulation of adaptive immunity. We will focus on the regulation of T cell immunity and discuss recent findings on the cross-talk between complement receptor and TLR signaling pathways. Such cross-talk is likely to affect the outcome of infections with intracellular pathogens, as well as the initiation and maintenance of aberrant immune responses leading to autoimmunity and atopy.

A quantitative method for comparison of expression of alternatively spliced genes using different primer pairs

In this paper we describe a novel two-step method for comparison of expression of alternatively spliced genes by quantitative PCR (QPCR) applying different primer pairs. As a model system we used rat decay accelerating factor (DAF; CD55) mRNA, which comprises three different isoforms: soluble (sDAF), transmembrane (tmDAF) and glycosyl-phosphatidylinositol (GPI) anchored (gpiDAF) forms. The first step was to prepare solid phase specific for each mRNA isoform and purify the three DAF-forms from total RNA. We then assessed amplification efficiency of primer pairs designed to recognise each of the isoforms using equimolar amounts of the three purified DAF mRNAs. The final step in our assay was to compare expression of the three DAF-isoforms in testis by QPCR taking into account the efficiency of their amplification to enable quantification. The RNA capture/QPCR method we described here can be used for quantifying the expression ratios of alternatively spliced mRNAs from a single gene or for direct comparison of expression of different genes.

Emerging roles and new functions of CD46

In the past 20 years, our understanding of the workings of complement regulatory protein, CD46 (membrane cofactor protein), has grown as has the impressive list of pathogens interacting with this membrane-bound complement inhibitor. Referred to as a "pathogen magnet," CD46 serves as a receptor for seven human pathogens. Initially discovered as a widely expressed C3b- and C4b-binding protein, it was subsequently shown to be a cofactor for the serine protease factor I to inactivate by limited proteolysis these two opsonins and components of the convertases. The involvement of CD46 in reproductive processes continues to be an emerging story. It is a protector of placental tissue, but it may also play a more direct role in reproduction through its expression on the inner acrosomal membrane of spermatozoa. Cross-linking CD46 with antibodies or natural or pathogenic ligands induces rapid turnover and signaling events. In this regard, much attention is currently focused on generating human T lymphocyte regulatory cells by cross-linking CD46. Finally, highlighting its importance in protecting cells against excessive complement activation is the discovery that even a heterozygous deficiency of CD46 predisposes to hemolytic uremic syndrome.

Allograft rejection: effect of local synthesis of complement

The complement system is known for its ability to participate in non-specific inflammation and membrane injury as well as contributing to antigen-specific immune stimulation. In renal transplantation, the complement cascade behaves true to form in that both non-immune- and immune-mediated destruction of the renal tubules are complement dependent. What is remarkable, however, is the extent of involvement of local synthesis of complement in both of these injuries, suggesting that the extravascular tissue compartment is the domain of local synthesis, whereas the effect of circulating complement is much less. This creates a new paradigm for studying the influence of local synthesis of complement in other organ-based diseases and underlines the need for tissue-targeting strategies in successful therapeutic development.

Complement-induced regulatory T cells suppress T-cell responses but allow for dendritic-cell maturation

Concurrent activation of the T-cell receptor (TCR) and complement regulator CD46 on human CD4+ T lymphocytes induces Tr1-like regulatory T cells that suppress through IL-10 secretion bystander T-cell proliferation. Here we show that, despite their IL-10 production, CD46-induced T-regulatory T cells (Tregs) do not suppress the activation/maturation of dendritic cells (DCs). DC maturation by complement/CD46-induced Tregs is mediated through simultaneous secretion of GM-CSF and soluble CD40L, factors favoring DC differentiation and reversing inhibitory effects of IL-10. Thus, CD46-induced Tregs produce a distinct cytokine profile that inhibits T-cell responses but leaves DC activation unimpaired. Such "DC-sparing" Tregs could be desirable at host/environment interfaces such as the gastrointestinal tract where their specific cytokine profile provides a mechanism that ensures unresponsiveness to commensal bacteria while maintaining reactivity to invading pathogens.

Wild-type measles virus infection in human CD46/CD150-transgenic mice: CD11c-positive dendritic cells establish systemic viral infection

We generated transgenic (TG) mice that constitutively express human CD46 (huCD46) and/or TLR-inducible CD150 (huCD150), which serve as receptors for measles virus (MV). These mice were used to study the spreading and pathogenicity of GFP-expressing or intact laboratory-adapted Edmonston and wild-type Ichinose (IC) strains of MV. Irrespective of the route of administration, neither type of MV was pathogenic to these TG mice. However, in ex vivo, limited replication of IC was observed in the spleen lymphocytes from huCD46/huCD150 TG and huCD150 TG, but not in huCD46 TG and non-TG mice. In huCD150-positive TG mouse cells, CD11c-positive bone marrow-derived myeloid dendritic cells (mDC) participated in MV-mediated type I IFN induction. The level and induction profile of IFN-beta was higher in mDC than the profile of IFN-alpha. Wild-type IC induced markedly high levels of IFN-beta compared with Edmonston in mDC, as opposed to human dendritic cells. We then generated huCD46/huCD150 TG mice with type I IFN receptor (IFNAR1)-/- mice. MV-bearing mDCs spreading to draining lymph nodes were clearly observed in these triple mutant mice in vivo by i.p. MV injection. Infectious lymph nodes were also detected in the double TG mice into which MV-infected CD11c-positive mDCs were i.v. transferred. This finding suggests that in the double TG mouse model mDCs once infected facilitate systemic MV spreading and infection, which depend on mDC MV permissiveness determined by the level of type I IFN generated via IFNAR1. Although these results may not simply reflect human MV infection, the huCD150/huCD46 TG mice may serve as a useful model for the analysis of MV-dependent modulation of mDC response.

CD46-utilizing adenoviruses inhibit C/EBPbeta-dependent expression of proinflammatory cytokines

The majority of adenovirus serotypes utilize the coxsackievirus-adenovirus receptor (CAR) for virus-host cell attachment, but subgroup B and subgroup D (adenovirus type 37 [Ad37]) viruses recognize CD46. CD46 is a ubiquitously expressed receptor that serves as a cofactor for the inactivation of the complement components C3b and C4b, and it also serves as a receptor for diverse microbial pathogens. A reported consequence of CD46 engagement is a reduced capability of human immune cells to express interleukin-12 (IL-12), a cytokine involved in both the innate and adaptive immune responses. Studies were thus undertaken to determine whether CD46-utilizing Ads alter the expression of proinflammatory cytokines. Subgroup B (Ad16 and -35) and Ad37, but not Ad2 or -5, significantly reduced IL-12 production by human peripheral blood mononuclear cells stimulated with gamma interferon (IFN-gamma) and lipopolysaccharide. IL-12 mRNA (p35 and p40 subunits) levels as well as other cytokine mRNA levels (IL-1alpha and -beta, IL-1Ra, and IL-6) were decreased upon interaction with CD46-utilizing Ads. Analysis of transcription factor activity required for cytokine expression indicated that CD46-utilizing Ads preferentially inhibited IFN-gamma-induced C/EBPbeta protein expression, consequently reducing its ability to form DNA complexes. Interference with IFN-gamma signaling events by CD46-utilizing Ads, but not CAR-utilizing Ads, reveals a potentially critical difference in the host immune response against distinct Ad vectors, a situation that has implications for gene delivery and vaccine development.

Evaluation of biodistribution and safety of adenovirus vectors containing group B fibers after intravenous injection into baboons

Vectors containing group B adenovirus (Ad) fibers are able to efficiently transduce gene therapy targets that are refractory to infection with standard Ad serotype 5 (Ad5) vectors, including malignant tumor cells, hematopoietic stem cells, and dendritic cells. Preliminary studies in mice indicate that, after intravenous injection, B-group fiber-containing Ads do not efficiently transduce most organs and cause less acute toxicity than Ad5 vectors. However, biodistribution and safety studies in mice are of limited value because the mouse analog of the B-group Ad receptor, CD46, is expressed only in the testis, whereas in humans, CD46 is expressed on all nucleated cells. Unlike mice, baboons have CD46 expression patterns and levels that closely mimic those in humans. We conducted a biodistribution and toxicity study of group B Ad fiber-containing vectors in baboons. Animals received phosphate-buffered saline, Ad5-bGal (a first-generation Ad5 vector), or B-group fiber-containing Ads (Ad5/35-bGal and Ad5/11-bGal) at a dose of 2 x 10(12) VP/kg, and vector biodistribution and safety was analyzed over 3 days. The amount of Ad5/35-bGal and Ad5/11-bGal vector genomes was in most tissues one to three orders of magnitude below that of Ad5. Significant Ad5/35- and Ad5/11-mediated transgene (beta-galactosidase) expression was seen only in the marginal zone of splenic follicles. Compared with the animal that received Ad5-bGal, all animals injected with B-group fiber-containing Ad vectors had lower elevations in serum proinflammatory cytokine levels. Gross and histopathology were normal in animals that received B-group Ad fiber-containing Ads, in contrast to the Ad5-infused animal, which showed widespread endothelial damage and inflammation. In a further study, a chimeric Ad5/35 vector carrying proapoptotic TRAIL and Ad E1A genes under tumor-specific regulation was well tolerated in a 30-day toxicity study. No major clinical, serologic, or pathologic abnormalities were noticed in this animal.

A network of PDZ-containing proteins regulates T cell polarity and morphology during migration and immunological synapse formation

T cell shape is dictated by the selective recruitment of molecules to different regions of the cell (polarity) and is integral to every aspect of T cell function, from migration to cytotoxicity. This study describes a mechanism for the regulation of T cell polarity. We show that T cells contain a network of asymmetrically distributed proteins with the capacity to dictate the subcellular localization of both cell surface receptors and morphological determinants in T cells. Proteins from the Scribble, Crumbs3, and Par3 complexes, previously shown to regulate epithelial polarity, were polarized in T cells containing either uropods or immunological synapses. Reduction in Scribble expression prevented the polarization of cell surface receptors and prevented morphological changes associated with uropod formation, migration, and antigen presentation. By dynamically coordinating molecular distribution throughout the T cell, this network provides a mechanism by which T cell function and polarity are linked.

Altered CD46-mediated T cell co-stimulation in haemodialysis patients

While most of our understanding of immune dysfunction in dialysis patients involves alterations in CD28-CD80/86 signalling, nothing is known of CD46-mediated co-stimulation of T cells in these patients. Because C3b/C4b bind to CD46 and complement activation occurs during haemodialysis (HD), we addressed whether CD46-mediated T cell activation is altered in HD (n = 9), peritoneal dialysis (PD) (n = 10) and predialysis patients (n = 8) compared to healthy controls (HC) (n = 8). T cell surface markers, T cell proliferation and interleukin (IL)-10 production were studied in CD4(+)T cells. In addition, CD46 splice-variants and IL-10 promoter gene polymorphisms were studied by reverse transcription (RT) or amplification refractory mutation system-polymerase chain reaction (ARMS-PCR), respectively. In all uraemic patients, irrespective of the stage of renal insufficiency or dialysis modality, a significant increase in the percentage of CD25 positivity in naive CD4(+)T cells was found (64% +/- 21%versus 23% +/- 18%, P < 0.001). Lymphocytes of HD patients proliferated in greater numbers and produced more IL-10 after co-stimulation with anti-CD46 than after co-stimulation with anti-CD28. This was also found in CD4(+)T cells of PD patients, albeit to a lesser extent. In contrast, with T cells of predialysis patients and of HC, co-stimulation via CD28 was more efficient. The observed alterations in T cell proliferation and IL-10 production were associated neither with CD46 splice variants nor with IL-10 promoter gene polymorphisms. Lymphocytes of HD patients show an increased response on CD46 co-stimulation. These data suggest that ongoing complement activation in HD patients may lead to alterations in acquired immunity.

Viral replication-independent blockade of dendritic cell maturation and interleukin-12 production by human herpesvirus 6

Human herpesvirus 6 (HHV-6) is a potentially immunosuppressive CD4(+)-T-lymphotropic betaherpesvirus that causes severe human thymocyte depletion in heterochimeric SCID-hu thy/liv mice and has been implicated as a potential cofactor in the progression of AIDS. However, the mechanisms of HHV-6-mediated immunosuppression have not yet been fully elucidated. We investigated the phenotypic and functional alterations induced by HHV-6 on peripheral blood-derived human dendritic cells (DC). The infection of DC with HHV-6 A or B was nonproductive, as revealed by calibrated real-time PCR measuring the accumulation of viral genome equivalents over time. Nevertheless, preexposure to HHV-6 markedly impaired the maturation of DC driven by gamma interferon and lipopolysaccharide, as shown by the reduced surface expression of major histocompatibility complex class I molecules, HLA-DR, CD40, and CD80. Moreover, HHV-6, but not the closely related betaherpesvirus HHV-7, dramatically suppressed the secretion of interleukin-12 (IL-12) p70 by DC, while the production of other cytokines that influence DC maturation, i.e., IL-10 and tumor necrosis factor alpha, was not significantly modified. Likewise, the secretion of the CC chemokines macrophage inflammatory protein 1beta and RANTES was unaltered. Functionally, a pretreatment with HHV-6 impaired the ability of DC to stimulate allogeneic T-cell proliferation. Altogether, these data identify interference with the functional maturation of DC as a potential mechanism of HHV-6-mediated immunosuppression.

Immunomodulation to combat atherosclerosis: the potential role of immune regulatory cells

Atherosclerosis is an inflammatory disease of the arterial wall that carries an important socio-economic burden. The available data strongly suggest that both innate and adaptive immuno-inflammatory mechanisms are the major determinants of plaque complications (called instability). Therefore, most of the important advances in the comprehension of the mechanisms of atherosclerosis came from studies that aimed at the elucidation of the critical components involved in the modulation of the immuno-inflammatory response in experimental models of atherosclerosis. As the T helper (Th)1-driven immune response has been consistently shown to promote atherosclerosis, the general belief is that immunomodulation through Th2 may be suitable to halt the disease process. Here, a novel view of the immuno-inflammatory response in atherosclerosis is presented, in which the natural and/or adaptive regulatory responses modulate both Th1 and Th2 responses, and play a central role in counteracting disease initiation and progression.

Expression of CD46 in developing rat spermatozoa: ultrastructural localization and utility as a marker of the various stages of the seminiferous tubuli

Identification of the various stages of the seminal tubule epithelium that are important in spermatogenesis in humans and rodents requires considerable expertise for analysis of ultrastructural appearance under light microscopy. Few good stage-specific markers have been reported to facilitate the process. We recently described characterization of the expression of CD46 (membrane cofactor protein) in the rat using a novel monoclonal antibody. Expression of CD46 was restricted to spermatozoa and their immediate precursors in the testis. In the present study, we used a combination of morphological analyses, known acrosome markers, actin staining, direct nuclear staining, and staining for CD46 to delineate precisely the subcellular location of CD46. Staining of CD46 colocalized with known acrosome markers in late spermatids and mature spermatozoa and was confirmed by electron microscopy to be acrosome-restricted. Expression was first detected in step 7 spermatids, whereas known markers were not expressed until step 9. The CD46 staining pattern differed through spermatid development, and distinct patterns of staining could be identified that, when combined with 4'-6-diamino-2-phenylindole-2HCl nuclear staining, enabled the accurate staging of the seminiferous tubule epithelium in different profiles. This detailed description of the spatiotemporal expression patterns of CD46 provides a valuable tool for analysis of spermatogenesis in the rat. Furthermore, this information will aid ongoing studies regarding the roles of CD46 in acrosome-related spermatozoal functions.

Complement: central to innate immunity and bridging to adaptive responses

The complement system, a pillar of innate immunity, has belatedly become recognised as a key modulator of adaptive immunity, acting to direct, modulate and modify the responses of lymphocytes to stimuli. These effects are mediated by interactions between complement components or activation-derived fragments and specific binding proteins--complement receptors and regulators--on the target cells. This review will describe the current state of knowledge in this swiftly moving field. It is hoped that the recognition of these properties will help to establish complement in the role it richly deserves as the lynchpin of immunity.

The role of rat Crry, a complement regulatory protein, in proliferation of thymocytes

In our previous work we showed that 3F10 monoclonal antibody (mAb), which recognizes the rat complement receptor 1-related/gene protein y (Crry), induces homotipic aggregation of thymocytes. In this work we studied the effect of 3F10 mAb on proliferation of rat thymocytes stimulated with concanavalin A (ConA) or by cross-linking the T cell receptor (TCR) by anti-alphabetaTCR mAb (R73), in vitro, and the mechanisms involved in the process. Our results show that 3F10 mAb stimulates proliferation of total thymocytes triggered by suboptimal concentrations of ConA or TCR cross-linking, in a dose-dependent manner. Maximal stimulation was observed using 10 microg/ml and 20 microg/ml of 3F10 mAb, respectively. The 3F10-induced stimulation of thymocytes proliferation in the presence of ConA, that was followed by increased production of interleukin-2 (IL-2), up-regulation of the expression of IL-2 receptor alpha (IL-2Ralpha) and was inhibited by anti-CD11a and anti-CD18 mAbs. Purified thymocytes did not respond by proliferation to 3F10 mAb, either alone or in combination with R73 mAb or ConA. Proliferation of these cells was achieved only in the presence of OX-6+ antigen-presenting cells (APC) and additional signals transmitted by TCR or ConA. These results suggest that Crry is involved in the LFA-1 dependent proliferation of thymocytes, a phenomenon that has not been recognized so far.

CD46: expanding beyond complement regulation

During the 1980s CD46 was discovered in a search for C3b binding proteins of human peripheral blood cells. Its role as an inactivator of C3b and C4b deposited on self-tissue is highlighted by the observation that partial deficiency of CD46 is a predisposing factor to hemolytic uremic syndrome. This discovery has an impact on the treatment options for these patients. Other new findings have expanded the role of CD46 in immunity and disease. For example, signaling through CD46 on human T lymphocytes drives them to become regulatory cells, indicating a novel link between the complement system and cellular immunity. Also, CD46 interacts with at least seven human pathogens and participates in reproduction/fertilization, further suggesting that dissecting its multi-faceted activities will have important clinical implications.

Human T Regulatory Cells Can Use the Perforin Pathway to Cause Autologous Target Cell Death

Cytotoxic T lymphocytes and natural killer cells use the perforin/granzyme pathway to kill virally infected cells and tumor cells. Mutations in genes important for this pathway are associated with several human diseases. CD4(+) T regulatory (Treg) cells have emerged as important in the control of immunopathological processes. We have previously shown that human adaptive Treg cells preferentially express granzyme B and can kill allogeneic target cells in a perforin-dependent manner. Here, we demonstrate that activated human CD4(+)CD25(+) natural Treg cells express granzyme A but very little granzyme B. Furthermore, both Treg subtypes display perforin-dependent cytotoxicity against autologous target cells, including activated CD4(+) and CD8(+) T cells, CD14(+) monocytes, and both immature and mature dendritic cells. This cytotoxicity is dependent on CD18 adhesive interactions but is independent of Fas/FasL. Our findings suggest that the perforin/granzyme pathway is one of the mechanisms that Treg cells can use to control immune responses.

The role of complement in the development of systemic lupus erythematosus

Complement has both beneficial and deleterious roles in the pathogenesis of systemic lupus erythematosus (SLE). On the one hand, patients with SLE present with decreased complement levels and with complement deposition in inflamed tissues, suggestive of a harmful role of complement in the effector phase of disease. On the other hand, homozygous deficiency of any of the classical pathway proteins is strongly associated with the development of SLE. There are two main hypotheses to explain these observations. The first invokes an important role for complement in the physiological waste-disposal mechanisms of dying cells and immune complexes. The second hypothesis is based around the role of complement in determining the activation thresholds of B and T lymphocytes, with the proposal that complement deficiency causes incomplete maintenance of peripheral tolerance. These two hypotheses are not mutually exclusive. In addition, there is evidence for a contribution from other genetic factors in determining the phenotype of disease in the absence of complement.

CD46: A complement regulator and pathogen receptor that mediates links between innate and acquired immune function

In the last 10 years, the human cell-surface molecule, CD46, has evolved from 'just another complement regulator' to a receptor for a striking array of pathogens. CD46 not only protects cells from complement-mediated attack and facilitates infection by a large number of pathogens, but also exerts complex effects on cellular immune function. It has been proposed that CD46 links innate and adaptive immunity by affecting cellular immune function in response to complement binding, and the role of CD46 in the pathogenesis of many infectious pathogens is now the subject of intense investigation. So far, the flood of information that implicates CD46 in modifying a host response to measles, Neisseria, human herpes virus 6, and pathogens that activate complement has not yet been matched with a comprehensive understanding of the molecular mechanisms by which CD46 affects immune function. This review summarizes the evidence that points to a significant role for CD46 in a range of pathological processes and describes how CD46 might exert its effects by altering signal transduction and antigen presentation pathways.

Rat membrane cofactor protein (MCP; CD46) is expressed only in the acrosome of developing and mature spermatozoa and mediates binding to immobilized activated C3

The rat analogue of the complement regulator membrane cofactor protein (MCP; CD46) was recently cloned and analysis at the mRNA level suggested that expression was restricted to testis. In light of the proposed roles of human MCP in sperm-egg interaction, we undertook to analyze rat MCP expression at the protein level in order better to address its putative role in fertilization. Recombinant fusion proteins comprising antibody Fc and specific domains of rat MCP were generated and used to develop a monoclonal antibody, MM.1, specific for rat MCP. Immunohistochemistry using these reagents confirmed the reported testis-specific expression of MCP in sexually mature rats and demonstrated that MCP was expressed only by spermatozoa and their immediate precursors in spermiogenesis, spermatids. Prepubertal male rats did not express MCP, and there was no evidence of MCP expression at any site in the embryo. Spermatozoal MCP expression was restricted to the inner acrosomal membrane, exposed only after fixation or induction of the acrosome reaction. Acrosome-reacted but not unreacted spermatozoa bound methylamine-activated C3 immobilized on plastic. The retention of MCP at this subcellular site, which is probably crucial to sperm-egg interaction, and the functional demonstration of binding to activated C3 strengthen suggestions from human studies that MCP may play an important role in fertilization. The reagents and results described here will enable studies of the role of spermatozoal MCP in sperm-egg interaction using a relevant animal model system.

Ligand binding determines whether CD46 is internalized by clathrin-coated pits or macropinocytosis

CD46 is a ubiquitous human cell surface receptor for the complement components C3b and C4b and for various pathogens, including the measles virus and human herpes virus 6. Ligand binding to CD46 affects (i) protection of autologous cells from complement attack by breakdown of complement components, (ii) intracellular signals that affect the regulation of immune cell function, (iii) antigen presentation, and (iv) down-regulation of cell surface CD46. Recent evidence indicates that CD46 signaling can link innate and acquired immune function. The molecular mechanisms for these processes and the importance of intracellular trafficking of the receptor have not yet been elucidated. We demonstrate here that, in nonlymphoid cells, CD46 is constitutively internalized via clathrin-coated pits, traffics to multivesicular bodies, and is recycled to the cell surface. However, cross-linking of CD46 at the cell surface, by either multivalent antibody or by measles virus, induces pseudopodia that engulf the ligand in a process similar to macropinocytosis, and leads to the degradation of cell surface CD46. Thus, we have elucidated two pathways for CD46 internalization, which are regulated by the valence of cross-linking of CD46 and which utilize either clathrin-coated pits or pseudopodial extension. This has important implications for CD46 signaling, antigen presentation, CD46 down-regulation, and engulfment of pathogens.

Selective suppression of IL-12 production by human herpesvirus 6

Human herpesvirus 6 (HHV-6) is a potentially immunosuppressive agent that has been suggested to act as a cofactor in the progression of HIV disease. Exposure of human macrophages to HHV-6A or HHV-6B profoundly impaired their ability to produce interleukin 12 (IL-12) upon stimulation with interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS). By contrast, the production of tumor necrosis factor-alpha (TNF-alpha); regulated on activation, normal T-cell expressed and secreted (RANTES); and macrophage inflammatory protein 1 beta (MIP-1 beta) was not negatively affected. To exclude the involvement of IL-12-suppressive cytokines, such as IL-10 and TNF-alpha, the viral stocks were fractionated by ultra-centrifugation. The bulk of the suppressive activity was recovered within the virion-rich pelleted fraction that was virtually devoid of such cytokines. IL-12 suppression was independent of viral replication, and the effect was not abrogated upon ultraviolet-light inactivation of the viral inoculum. The mechanism of HHV-6-mediated IL-12 suppression was investigated by RNase protection assays, which demonstrated unaltered levels of IL-12 p35 mRNA and only a modest reduction in p40 mRNA, which was insufficient to account for the near-complete loss of both extracellular and intracellular IL-12 protein. Moreover, both the IFN-gamma and the LPS signaling pathways were intact in HHV-6-treated cells. These data suggest that HHV-6 can dramatically affect the generation of effective cellular immune responses, providing a novel potential mechanism of HHV-6-mediated immunosuppression.

Role of the complement system in rejection

The complement system plays a complex role in transplantation, beginning with effects on reperfusion injury and continuing with stimulation of the adaptive immune response. Recent evidence has emphasised the importance of the late components of the complement cascade in the mediation of post-ischaemic damage, which are apparently triggered by the classical, alternative or lectin pathways of complement activation, depending on the organ affected. In studies of renal allograft rejection, the local synthesis of complement component C3 seems to influence the T-cell response more strongly than circulating complement protein, raising the possibility that there is co-operation between locally derived C3 and antigen presentation in the graft. Class switching of alloantibody to a high-affinity IgG response is also highly dependent on C3. In addition, the finding that capillary-bound C4d is a robust marker for humoral rejection has started a new investigation into the significance of alloantibodies in acute and chronic allograft rejection. There are several selective and nonselective inhibitors suitable for clinical development; clearly it is time for more concerted effort to evaluate their role in clinical transplantation.

CD46 in meningococcal disease

The human-specific bacterial pathogen Neisseria meningitidis is a major cause of sepsis and/or meningitis. The pili of N. meningitidis interact with CD46, a human cell-surface protein involved in regulation of complement activation. Transgenic mice expressing human CD46 were susceptible to meningococcal disease, because bacteria crossed the blood-brain barrier in these mice. Development of disease was more efficient with piliated bacteria after intranasal, but not intraperitoneal, challenge of CD46 transgenic mice, suggesting that human CD46 facilitates pilus-dependent interactions at the epithelial mucosa. Hence, the human CD46 transgenic mice model is a potentially useful tool for studying pathogenesis and for vaccine development against meningococcal disease.

Research on complement: old issues revisited and a novel sphere of influence

Immunology in recent years has taken a somewhat surprising turn, expressed by a renewed interest in innate immunity. Especially intriguing is the regulatory role exerted by the innate components on the adaptive response, with Toll receptors and complement components being the most investigated. This function has been firmly established for complement protein CR2 (CD21) as part of the BCR co-receptor CD19/CD21/CD81. New findings are now providing a broader picture of complement and its tuning of the immune response; for example, complement proteins have been implicated in the control of T-cell-mediated responses. We will review some of these data here and summarize new discoveries in areas of research on more traditional topics within the complement literature, such as complement and renal diseases, and the therapeutic use of C1-Inhibitor. We cover papers selected from studies presented at the XIX International Complement Workshop, held in Palermo in September 2002, and published within the past six months.

Tissue injury and repair in allografts: novel perspectives

PURPOSE OF REVIEW

Recent research achievements might considerably alter scientific concepts of pathways involved in tissue injury and repair.

RECENT FINDINGS

Accumulating evidence for an important role of alloantibodies in acute and chronic allograft rejection led to a renewed interest in humoral kidney transplant rejection. Studies reassessing the mechanisms of antibody- and complement-mediated injury now shed new light on the pathogenic mechanisms underlying acute or chronic graft dysfunction and injury. A closer look at humoral effector mechanisms revealed that endothelial cell activation and injury may play a key role in humoral rejection, and further uncovered an important interplay between humoral and cellular alloimmunity. Regeneration of cells after injury has been thought to rely on activation of local progenitor cells. Recent investigation indicates that regeneration of grafted solid organs is not exclusively based on self-renewal of tissues but obviously also involves repopulation of the graft by recipient cells, creating chimerism in the vasculature and other compartments. Besides reparative compensation of cell loss, chimerism of endothelial cells might also alter immunologic properties of the graft, thus favoring adaptation and graft survival. On the other hand, however, myofibroblasts mediating deleterious arterial intimal proliferation may also be of recipient origin. A possible source of graft-repopulating recipient cells are bone marrow-derived adult stem cells with the amazing capacity of differentiating into cell types of all three germ cell layers.

SUMMARY

Reliable diagnosis of humoral mechanisms in allograft rejection and identification of involved effector mechanisms should provide the basis for development and targeted application of specific anti-humoral treatment. Recently emerged new concepts of mechanisms underlying tissue regeneration might pave the way for entirely new therapeutic approaches in human disease.

Activation of human CD4+ cells with CD3 and CD46 induces a T-regulatory cell 1 phenotype

The immune system must distinguish not only between self and non-self, but also between innocuous and pathological foreign antigens to prevent unnecessary or self-destructive immune responses. Unresponsiveness to harmless antigens is established through central and peripheral processes. Whereas clonal deletion and anergy are mechanisms of peripheral tolerance, active suppression by T-regulatory 1 (Tr1) cells has emerged as an essential factor in the control of autoreactive cells. Tr1 cells are CD4+ T lymphocytes that are defined by their production of interleukin 10 (IL-10) and suppression of T-helper cells; however, the physiological conditions underlying Tr1 differentiation are unknown. Here we show that co-engagement of CD3 and the complement regulator CD46 in the presence of IL-2 induces a Tr1-specific cytokine phenotype in human CD4+ T cells. These CD3/CD46-stimulated IL-10-producing CD4+ cells proliferate strongly, suppress activation of bystander T cells and acquire a memory phenotype. Our findings identify an endogenous receptor-mediated event that drives Tr1 differentiation and suggest that the complement system has a previously unappreciated role in T-cell-mediated immunity and tolerance.

Complement component 3 is required for optimal expansion of CD8 T cells during a systemic viral infection

In addition to its established role in innate immune mechanisms, complement component C3 is also of critical importance in B cell activation and T cell-dependent Ab responses. In this study, we have examined the requirement for C3 in the generation of primary CD8 T cell responses to an acute systemic viral infection. We compared Ag-specific CD8 T cell responses to lymphocytic choriomeningitis virus (LCMV) between wild-type (+/+) and C3-deficient (C3(-/-)) mice on both 129/B6 and B6 backgrounds. These studies revealed that C3 activity is required for optimal expansion of LCMV-specific effector CD8 T cells in an epitope-dependent fashion, which is influenced by the genetic background of the mice. Studies in complement receptor 1/2 (CR1/CR2)-deficient mice showed that regulation of LCMV-specific CD8 T cell responses by C3 is not dependent upon CR1/CR2. These findings may have implications in vaccine development, therapy of autoimmune diseases, and prevention of graft rejection.