Complement activation and regulator expression after anoxic injury of human endothelial cells

The complement alternative pathway and hemostasis

The complement and hemostatic systems are complex systems, and both involve enzymatic cascades, regulators, and cell components-platelets, endothelial cells, and immune cells. The two systems are ancestrally related and are defense mechanisms that limit infection by pathogens and halt bleeding at the site of vascular injury. Recent research has uncovered multiple functional interactions between complement and hemostasis. On one side, there are proteins considered as complement factors that activate hemostasis, and on the other side, there are coagulation proteins that modulate complement. In addition, complement and coagulation and their regulatory proteins strongly interact each other to modulate endothelial, platelet and leukocyte function and phenotype, creating a potentially devastating amplifying system that must be closely regulated to avoid unwanted damage and\or disseminated thrombosis. In view of its ability to amplify all complement activity through the C3b-dependent amplification loop, the alternative pathway of complement may play a crucial role in this context. In this review, we will focus on available and emerging evidence on the role of the alternative pathway of complement in regulating hemostasis and vice-versa, and on how dysregulation of either system can lead to severe thromboinflammatory events.

C3 Deficiency Leads to Increased Angiogenesis and Elevated Pro-Angiogenic Leukocyte Recruitment in Ischemic Muscle Tissue

The complement system is a potent inflammatory trigger, activator, and chemoattractant for leukocytes, which play a crucial role in promoting angiogenesis. However, little information is available about the influence of the complement system on angiogenesis in ischemic muscle tissue. To address this topic and analyze the impact of the complement system on angiogenesis, we induced muscle ischemia in complement factor C3 deficient (C3-/-) and wildtype control mice by femoral artery ligation (FAL). At 24 h and 7 days after FAL, we isolated the ischemic gastrocnemius muscles and investigated them by means of (immuno-)histological analyses. C3-/- mice showed elevated ischemic damage 7 days after FAL, as evidenced by H&E staining. In addition, angiogenesis was increased in C3-/- mice, as demonstrated by increased capillary/muscle fiber ratio and increased proliferating endothelial cells (CD31+/BrdU+). Moreover, our results showed that the total number of leukocytes (CD45+) was increased in C3-/- mice, which was based on an increased number of neutrophils (MPO+), neutrophil extracellular trap formation (MPO+/CitH3+), and macrophages (CD68+) displaying a shift toward an anti-inflammatory and pro-angiogenic M2-like polarized phenotype (CD68+/MRC1+). In summary, we show that the deficiency of complement factor C3 increased neutrophil and M2-like polarized macrophage accumulation in ischemic muscle tissue, contributing to angiogenesis.

Complement-mediated thrombotic microangiopathy as a link between endothelial damage and steroid-refractory GVHD

Transplant-associated thrombotic microangiopathy (TA-TMA), a complication of hematopoietic cell transplant (HCT), is associated with significant morbidity and mortality. The pathophysiology and overlap of TA-TMA with other posttransplant complications such as graft-versus-host disease (GVHD) is poorly understood. We retrospectively identified cases of TA-TMA among patients with grade 3/4 gastrointestinal (GI) GVHD, reviewed intestinal biopsy specimens, and performed correlative testing of biomarkers associated with TA-TMA. TA-TMA was more common in patients with steroid-refractory GVHD compared with steroid-responsive GVHD (79.3% vs 42.1%; P = .001). Among patients surviving 100 days post-HCT, 1-year survival from day 100 was significantly better for patients who had not developed TA-TMA in the first 100 days (69.5% vs 36.7%; P < .001). Only 1 of 7 proposed TA-TMA histology criteria (mucosal hemorrhage) differed significantly based on GVHD steroid response. In multivariable modeling, steroid-refractory GVHD was a risk factor for development of TA-TMA (hazard ratio, 3.09; 95% confidence interval, 1.68-5.67; P < .001). There were no differences in complement activation at GVHD onset; however, 2 to 6 weeks later, patients with TA-TMA had higher levels of BBPlus and C5b-9, markers of alternative and terminal pathway activation (BBPlus: median, 600 vs 209.3 ng/mL; P = .0045) (C5b-9: median, 425.9 vs 258.4 ng/mL; P = .029). TA-TMA is associated with poor overall survival (OS) following HCT and may be detected early by histologic findings and may be differentiated from GVHD by measurement of alternative and terminal complement pathway activation. It is unknown whether treatment of TA-TMA will improve survival in steroid-refractory GVHD.

Comparative Analysis of Whey N-Glycoproteins in Human Colostrum and Mature Milk Using Quantitative Glycoproteomics

Glycosylation is a ubiquitous post-translational protein modification that plays a substantial role in various processes. However, whey glycoproteins in human milk have not been completely profiled. Herein, we used quantitative glycoproteomics to quantify whey N-glycosylation sites and their alteration in human milk during lactation; 110 N-glycosylation sites on 63 proteins and 91 N-glycosylation sites on 53 proteins were quantified in colostrum and mature milk whey, respectively. Among these, 68 glycosylation sites on 38 proteins were differentially expressed in human colostrum and mature milk whey. These differentially expressed N-glycoproteins were highly enriched in "localization", "extracellular region part", and "modified amino acid binding" according to gene ontology annotation and mainly involved in complement and coagulation cascades pathway. These results shed light on the glycosylation sites, composition and biological functions of whey N-glycoproteins in human colostrum and mature milk, and provide substantial insight into the role of protein glycosylation during infant development.

Microbial Neuraminidase Induces a Moderate and Transient Myelin Vacuolation Independent of Complement System Activation

AIMS

Some central nervous system pathogens express neuraminidase (NA) on their surfaces. In the rat brain, a single intracerebroventricular (ICV) injection of NA induces myelin vacuolation in axonal tracts. Here, we explore the nature, the time course, and the role of the complement system in this damage.

METHODS

The spatiotemporal analysis of myelin vacuolation was performed by optical and electron microscopy. Myelin basic protein-positive area and oligodendrocyte transcription factor (Olig2)-positive cells were quantified in the damaged bundles. Neuronal death in the affected axonal tracts was assessed by Fluoro-Jade B and anti-caspase-3 staining. To evaluate the role of the complement, membrane attack complex (MAC) deposition on damaged bundles was analyzed using anti-C5b9. Rats ICV injected with the anaphylatoxin C5a were studied for myelin damage. In addition, NA-induced vacuolation was studied in rats with different degrees of complement inhibition: normal rats treated with anti-C5-blocking antibody and C6-deficient rats.

RESULTS

The stria medullaris, the optic chiasm, and the fimbria were the most consistently damaged axonal tracts. Vacuolation peaked 7 days after NA injection and reverted by day 15. Olig2+ cell number in the damaged tracts was unaltered, and neurodegeneration associated with myelin alterations was not detected. MAC was absent on damaged axonal tracts, as revealed by C5b9 immunostaining. Rats ICV injected with the anaphylatoxin C5a displayed no myelin injury. When the complement system was experimentally or constitutively inhibited, NA-induced myelin vacuolation was similar to that observed in normal rats.

CONCLUSION

Microbial NA induces a moderate and transient myelin vacuolation that is not caused either by neuroinflammation or complement system activation.

Ischemia/Reperfusion

Ischemic disorders, such as myocardial infarction, stroke, and peripheral vascular disease, are the most common causes of debilitating disease and death in westernized cultures. The extent of tissue injury relates directly to the extent of blood flow reduction and to the length of the ischemic period, which influence the levels to which cellular ATP and intracellular pH are reduced. By impairing ATPase-dependent ion transport, ischemia causes intracellular and mitochondrial calcium levels to increase (calcium overload). Cell volume regulatory mechanisms are also disrupted by the lack of ATP, which can induce lysis of organelle and plasma membranes. Reperfusion, although required to salvage oxygen-starved tissues, produces paradoxical tissue responses that fuel the production of reactive oxygen species (oxygen paradox), sequestration of proinflammatory immunocytes in ischemic tissues, endoplasmic reticulum stress, and development of postischemic capillary no-reflow, which amplify tissue injury. These pathologic events culminate in opening of mitochondrial permeability transition pores as a common end-effector of ischemia/reperfusion (I/R)-induced cell lysis and death. Emerging concepts include the influence of the intestinal microbiome, fetal programming, epigenetic changes, and microparticles in the pathogenesis of I/R. The overall goal of this review is to describe these and other mechanisms that contribute to I/R injury. Because so many different deleterious events participate in I/R, it is clear that therapeutic approaches will be effective only when multiple pathologic processes are targeted. In addition, the translational significance of I/R research will be enhanced by much wider use of animal models that incorporate the complicating effects of risk factors for cardiovascular disease. © 2017 American Physiological Society. Compr Physiol 7:113-170, 2017.

Complement system activation contributes to the ependymal damage induced by microbial neuraminidase

Background

In the rat brain, a single intracerebroventricular injection of neuraminidase from Clostridium perfringens induces ependymal detachment and death. This injury occurs before the infiltration of inflammatory blood cells; some reports implicate the complement system as a cause of these injuries. Here, we set out to test the role of complement.

Methods

The assembly of the complement membrane attack complex on the ependymal epithelium of rats injected with neuraminidase was analyzed by immunohistochemistry. Complement activation, triggered by neuraminidase, and the participation of different activation pathways were analyzed by Western blot. In vitro studies used primary cultures of ependymal cells and explants of the septal ventricular wall. In these models, ependymal cells were exposed to neuraminidase in the presence or absence of complement, and their viability was assessed by observing beating of cilia or by trypan blue staining. The role of complement in ependymal damage induced by neuraminidase was analyzed in vivo in two rat models of complement blockade: systemic inhibition of C5 by using a function blocking antibody and testing in C6-deficient rats.

Results

The complement membrane attack complex immunolocalized on the ependymal surface in rats injected intracerebroventricularly with neuraminidase. C3 activation fragments were found in serum and cerebrospinal fluid of rats treated with neuraminidase, suggesting that neuraminidase itself activates complement. In ventricular wall explants and isolated ependymal cells, treatment with neuraminidase alone induced ependymal cell death; however, the addition of complement caused increased cell death and disorganization of the ependymal epithelium. In rats treated with anti-C5 and in C6-deficient rats, intracerebroventricular injection of neuraminidase provoked reduced ependymal alterations compared to non-treated or control rats. Immunohistochemistry confirmed the absence of membrane attack complex on the ependymal surfaces of neuraminidase-exposed rats treated with anti-C5 or deficient in C6.

Conclusions

These results demonstrate that the complement system contributes to ependymal damage and death caused by neuraminidase. However, neuraminidase alone can induce moderate ependymal damage without the aid of complement.

Targeted complement inhibition and microvasculature in transplants: a therapeutic perspective

Active complement mediators play a key role in graft-versus-host diseases, but little attention has been given to the angiogenic balance and complement modulation during allograft acceptance. The complement cascade releases the powerful proinflammatory mediators C3a and C5a anaphylatoxins, C3b, C5b opsonins and terminal membrane attack complex into tissues, which are deleterious if unchecked. Blocking complement mediators has been considered to be a promising approach in the modern drug discovery plan, and a significant number of therapeutic alternatives have been developed to dampen complement activation and protect host cells. Numerous immune cells, especially macrophages, develop both anaphylatoxin and opsonin receptors on their cell surface and their binding affects the macrophage phenotype and their angiogenic properties. This review discusses the mechanism that complement contributes to angiogenic injury, and the development of future therapeutic targets by antagonizing activated complement mediators to preserve microvasculature in rejecting the transplanted organ.

The alternative complement pathway regulates pathological angiogenesis in the retina

A defining feature in proliferative retinopathies is the formation of pathological neovessels. In these diseases, the balance between neovessel formation and regression determines blindness, making the modulation of neovessel growth highly desirable. The role of the immune system in these retinopathies is of increasing interest, but it is not completely understood. We investigated the role of the alternative complement pathway during the formation and resolution of aberrant neovascularization. We used alternative complement pathway-deficient (Fb(-/-)) mice and age- and strain-matched control mice to assess neovessel development and regression in an oxygen-induced retinopathy (OIR) mouse model. In the control mice, we found increased transcription of Fb after OIR treatment. In the Fb(-/-) mice, we prepared retinal flatmounts and identified an increased number of neovessels, peaking at postnatal day 17 (P17; P=0.001). Subjecting human umbilical vein endothelial cells (HUVECs) to low oxygen, mimicking a characteristic of neovessels, decreased the expression of the complement inhibitor Cd55. Finally, using laser capture microdissection (LCM) to isolate the neovessels after OIR, we found decreased expression of Cd55 (P=0.005). Together, our data implicate the alternative complement pathway in facilitating neovessel clearance by down-regulating the complement inhibitor Cd55 specifically on neovessels, allowing for their targeted removal while leaving the established vasculature intact.-Sweigard, J. H., Yanai, R., Gaissert, P., Saint-Geniez, M., Kataoka, K., Thanos, A., Stahl, G. L., Lambris, J. D., Connor, K. M. The alternative complement pathway regulates pathological angiogenesis in the retina.

Complement component 3 inhibition by an antioxidant is neuroprotective after cerebral ischemia and reperfusion in mice

Oxidative stress after stroke is associated with the inflammatory system activation in the brain. The complement cascade, especially the degradation products of complement component 3, is a key inflammatory mediator of cerebral ischemia. We have shown that pro-inflammatory complement component 3 is increased by oxidative stress after ischemic stroke in mice using DNA array. In this study, we investigated whether up-regulation of complement component 3 is directly related to oxidative stress after transient focal cerebral ischemia in mice and oxygen-glucose deprivation in brain cells. Persistent up-regulation of complement component 3 expression was reduced in copper/zinc-superoxide dismutase transgenic mice, and manganese-superoxide dismutase knock-out mice showed highly increased complement component 3 levels after transient focal cerebral ischemia. Antioxidant N-tert-butyl-α-phenylnitrone treatment suppressed complement component 3 expression after transient focal cerebral ischemia. Accumulation of complement component 3 in neurons and microglia was decreased by N-tert-butyl-α-phenylnitrone, which reduced infarct volume and impaired neurological deficiency after cerebral ischemia and reperfusion in mice. Small interfering RNA specific for complement component 3 transfection showed a significant increase in brain cells viability after oxygen-glucose deprivation. Our study suggests that the neuroprotective effect of antioxidants through complement component 3 suppression is a new strategy for potential therapeutic approaches in stroke.

The complement system in ischemia-reperfusion injuries

Tissue injury and inflammation following ischemia and reperfusion of various organs have been recognized for many years. Many reviews have been written over the last several decades outlining the role of complement in ischemia/reperfusion injury. This short review provides a current state of the art knowledge on the complement pathways activated, complement components involved and a review of the clinical biologics/inhibitors used in the clinical setting of ischemia/reperfusion. This is not a complete review of the complement system in ischemia and reperfusion injury but will give the reader an updated view point of the field, potential clinical use of complement inhibitors, and the future studies needed to advance the field.

Relation between human decay-accelerating factor (hDAF) expression in pig cells and inhibition of human serum anti-pig cytotoxicity: value of highly expressed hDAF for xenotransplantation

BACKGROUND

Although the successful production of alpha1,3-galactosyltransferase-knockout (GT-KO) pigs has increased expectations of clinical xenotransplantation, additional modifications of genetically engineered pigs are still being explored, because even GT-KO pigs are incapable of inhibiting the host's immunological response completely. One of the potential candidates is a complement-regulatory protein, such as human decay-accelerating factor (hDAF). However, there are few reports on how high the expression level of hDAF in pig cells would be required for suppression of complement activation. The purpose of this study was to examine the relationship between the level of hDAF expression and its inhibitory effect on human serum cytotoxicity.

METHODS

An expression (pCAGGS) vector containing the hDAF gene was transfected into pig fibroblasts using an electroporation system (Gene Pulser II). Forty-eight to fifty-two hours after transfection, the cells were stained with FITC-labeled anti-hDAF antibody and then applied to the cell sorter. hDAF-transfected cells with various expression levels were collected by gating on fluorescence intensity. The level of hDAF expression was determined relative to that in human control endothelial cells. Collected cells expressing x1, x5, x10, x15 and x30 hDAF were incubated into 96-well plates for 16 h, and the cells were subjected to 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay.

RESULTS

hDAF expression levels in transfected cells at the time of MTT assay (16 h after sorting) were comparable to those immediately after sorting. hDAF expression in pig cells five times higher than in human endothelial cells was effective in inhibiting complement-dependent cytotoxicity of most human sera. However, 15- to 30-fold expression of hDAF was required for effective inhibition of human sera with the highest cytotoxic capacity.

CONCLUSIONS

A much higher level of hDAF expression in pig cells than previously considered necessary might be required to provide additional benefit in inhibiting antibody-mediated rejection. Genetically engineered pigs that express very high levels of hDAF would be beneficial for xenotransplantation.

Expression of complement regulators and receptors on human NT2-N neurons--effect of hypoxia and reoxygenation

Complement activation can cause tissue damage in cerebral stroke by the release of biologically potent activation products and impaired function of regulatory proteins. We investigated the constitutive and hypoxia-reoxygenation-dependent expression of complement receptor 1 (CD35), membrane cofactor protein (CD46), decay-accelerating factor (CD55), protectin (CD59), and complement C3a and C5a receptors (C3aR and C5aR) on human NT2-N neurons. The effect of hypoxia-reoxygenation on C3d-deposition on neurons and endothelial cells was also investigated. NT2-N neurons were examined by cellular enzyme-linked immunosorbent assay and immunofluorescence microscopy. Endothelial cells were examined by flow cytometry. Three hours 1% or 0.1% hypoxia and 21h reoxygenation with 50% AB-serum were used to investigate the effect of hypoxia-reoxygenation on regulators and C3d-deposition. NT2-N neurons expressed significant amounts of CD59 (Clone H19/Clone BRIC229: p=0.000006/p=0.000003), CD46 (p=0.00006), CD55 (p=0.003) and C3aR (p=0.00003). CD35 and C5aR were not significantly expressed. There were no effects of hypoxia-reoxygenation on any of the regulators or receptors after 1% hypoxia and reoxygenation. However, CD55 (p=0.02) was down-regulated after 0.1% hypoxia and subsequent reoxygenation with AB-serum. There were no difference observed in the C3d-deposition during hypoxia-reoxygenation in either neurons or endothelial cells. In conclusion, human NT2-N neurons constitutively express C3aR, CD46, CD55 and, in particular, CD59. The cells may respond to locally produced C3a and, at the same time, be well protected against complement attack. Although severe hypoxia-reoxygenation may down-regulate CD55 expression, it does not seem to influence C3d-deposition.

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

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

Altered renal tubular expression of the complement inhibitor Crry permits complement activation after ischemia/reperfusion

Ischemia/reperfusion (I/R) of several organs results in complement activation, but the kidney is unique in that activation after I/R occurs only via the alternative pathway. We hypothesized that selective activation of this pathway after renal I/R could occur either because of a loss of complement inhibition or from increased local synthesis of complement factors. We examined the relationship between renal complement activation after I/R and the levels and localization of intrinsic membrane complement inhibitors. We found that loss of polarity of complement receptor 1-related protein y (Crry) in the tubular epithelium preceded activation of the alternative pathway along the basolateral aspect of the tubular cells. Heterozygous gene-targeted mice that expressed lower amounts of Crry were more sensitive to ischemic injury. Furthermore, inhibition of Crry expressed by proximal tubular epithelial cells in vitro resulted in alternative pathway-mediated injury to the cells. Thus, altered expression of a complement inhibitor within the tubular epithelium appears to be a critical factor permitting activation of the alternative pathway of complement after I/R. Increased C3 mRNA and decreased factor H mRNA were also detected in the outer medulla after I/R, suggesting that altered synthesis of these factors might further contribute to complement activation in this location.

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.

C5a causes limited, polymorphonuclear cell-independent, mesenteric ischemia/reperfusion-induced injury

C5 is critical in the development of local mucosal damage and inflammation as well as in the development of remote organ injury after mesenteric ischemia/reperfusion (IR). To define the role of C5a in tissue injury, we treated wild-type mice with a cyclic hexapeptide C5a receptor antagonist (C5aRa) and administered recombinant C5a to C5 deficient (C5(-/-)) mice subjected to mesenteric IR. We demonstrate that at 2-h postreperfusion, C5a administered to C5-/- mice during IR induces limited intestinal mucosal injury but failed to cause remote lung injury despite the fact that it upregulated adhesion molecule expression. C5aRa treatment of C5+/+ mice undergoing IR limited local injury and prevented distant organ injury. We conclude that although C5a can trigger certain components of the IR induced injury, other mediators such as C5b-9 and local factors are needed for the complete expression of IR tissue damage.

Locally produced complement and its role in renal allograft rejection

The role of innate immunity in allograft injury is just beginning to become clear, and complement is probably one of a number of factors that are activated very early in the course of transplantation. Kidney transplantation into complement-inhibited rats reduces subsequent inflammation of the graft, probably as a result of reduction of ischemia reperfusion damage as well as diminution of immune mediated damage. Closer analysis of the role of locally synthesised components in mice has suggested that regional synthesis of complement proteins, in particular by the renal tubule, may play a more important role than circulating components. A marked effect on the antidonor T cell response may be explained by the triggering of complement receptors present on antigen presenting cells or T cells infiltrating the graft, or by a more direct effect of complement on the liaison between proximal tubule cells and T cells. Therapeutic control is likely to require a shift to a more targeted approach, directed at complement components produced in the extravascular tissue compartment.

C5 is required for CD49d expression on neutrophils and VCAM expression on vascular endothelial cells following mesenteric ischemia/reperfusion

Complement activation is critical in the development of local mucosal damage and inflammation as well as of remote organ injury after mesenteric ischemia/reperfusion. To further define the role of C5 activation in local and remote tissue injury, C5 deficient (C5(-/-)) and wild-type control (C5(+/+)) mice treated with an anti-C5 mAb were subjected to sham or ischemia followed by up to 4 h of reperfusion. The development of local (intestinal) and remote (lung) injury was associated with the expression of CD49d on the surface of circulating blood neutrophils and with VCAM on the endothelial cells of intestinal and lung vessels. Because CD49d heterodimerizes with integrin beta1 on the surface of neutrophils and can bind VCAM on the endothelium, we propose that complement activation causes organ damage by upregulating molecules that lead to inappropriate homing of neutrophils.

Herpes simplex virus 1 infected neuronal and skin cells differ in their susceptibility to complement attack

Herpes simplex virus type 1 (HSV-1) infection in neurons is lifelong and generally asymptomatic. Reactivation of this latent infection results in skin blistering whereas the respective peripheral neurons are rarely affected. Why the neuronal cells are spared while the skin cells are sacrificed is not well understood. In the present study our aim was to study whether neuronal and skin cells differ in their ability to control complement attack during HSV-1 infection. Human embryonal skin (HES) cells and neuronal Paju cells were infected by HSV-1 in vitro. Both types of infected cells activated complement but were initially resistant to membrane attack complex (MAC) deposition. During the first hours of infection the expression of the endogenous complement regulators decay accelerating factor (DAF) and CD59 increased on both HES and Paju cells. By 12 hr the infected HES cells had lost their ability to control complement attack. The expression of DAF and CD59 decreased and the cells became targets for MAC attack. In contrast, complement regulator expression on the Paju cells did not decrease below the initial level and complement C5b-9 deposition was found only on 10% of the Paju cells at 12 hr. The results suggest that HSV-infected neuronal cells are better than skin cells in protecting themselves against complement attack. This may contribute to the persistence of a latent HSV-1 infection in neuronal cells for prolonged periods.

Therapeutic potential of complement inhibitors in myocardial ischaemia

Under normal conditions, the complement system functions to eradicate microbes and other membrane bound pathogens. In other situations, complement activation comprises a pivotal mechanism for mediating tissue demolition in inflammatory disorders, including ischaemia/reperfusion injury. Complement-mediated tissue damage has long been recognised as a significant contributor to myocardial reperfusion injury. However, clinical use of complement inhibitors to reduce the extent of irreversible tissue injury related to reperfusion, remains in the early stages of development. Activation of the complement system generates anaphylatoxins, opsonins and the lytic moiety known as the membrane attack complex (MAC). In addition, fragments of the complement cascade proteins (e.g., C3a and C5a) secondarily initiate processes deleterious to myocytes by recruiting and stimulating inflammatory cells, such as neutrophils and macrophages, within the area of reperfusion. Damaged tissue itself, is capable of upregulating the genes that encode the formation of complement proteins leading to assembly of the MAC, which in turn further advances tissue injury. All of these factors contribute to the development of myocardial infarction subsequent to ischaemia and reperfusion. This paper provides an overview of how the complement system operates and examines the various inhibitors, both endogenous and exogenous, that regulate the complement cascade. Activation and inhibition of the complement system will be discussed primarily in the context of myocardial ischaemia and reperfusion injury.

Complement activation after oxidative stress: role of the lectin complement pathway

The complement system plays an important role in mediating tissue injury after oxidative stress. The role of mannose-binding lectin (MBL) and the lectin complement pathway (LCP) in mediating complement activation after endothelial oxidative stress was investigated. iC3b deposition on hypoxic (24 hours; 1% O(2))/reoxygenated (3 hours; 21% O(2)) human endothelial cells was attenuated by N-acetyl-D-glucosamine or D-mannose, but not L-mannose, in a dose-dependent manner. Endothelial iC3b deposition after oxidative stress was also attenuated in MBL-deficient serum. Novel, functionally inhibitory, anti-human MBL monoclonal antibodies attenuated MBL-dependent C3 deposition on mannan-coated plates in a dose-dependent manner. Treatment of human serum with anti-MBL monoclonal antibodies inhibited MBL and C3 deposition after endothelial oxidative stress. Consistent with our in vitro findings, C3 and MBL immunostaining throughout the ischemic area at risk increased during rat myocardial reperfusion in vivo. These data suggest that the LCP mediates complement activation after tissue oxidative stress. Inhibition of MBL may represent a novel therapeutic strategy for ischemia/reperfusion injury and other complement-mediated disease states.

Immune evasion of tumor cells using membrane-bound complement regulatory proteins

Membrane-bound complement regulatory proteins (mCRPs) play an important role in the protection of cells from complement-mediated injury. It is now apparent that malignant tumor cells also express these proteins to escape complement attack. Here, Arko Gorter and Seppo Meri discuss the implications of complement resistance for the immunotherapeutic treatment of solid tumors with monoclonal antibodies.