Complement-mediated clearance of erythrocytes: mechanism and delineation of the regulatory roles of Crry and DAF. Decay-accelerating factor

New insights into red blood cells in tumor precision diagnosis and treatment

Red blood cells (RBCs), which function as material transporters in organisms, are rich in materials that are exchanged with metabolically active tumor cells. Recent studies have demonstrated that tumor cells can regulate biological changes in RBCs, including influencing differentiation, maturation, and morphology. RBCs play an important role in tumor development and immune regulation. Notably, the novel scientific finding that RBCs absorb fragments of tumor-carrying DNA overturns the conventional wisdom that RBCs do not contain nucleic acids. RBC membranes are excellent biomimetic materials with significant advantages in terms of their biocompatibility, non-immunogenicity, non-specific adsorption resistance, and biodegradability. Therefore, RBCs provide a new research perspective for the development of tumor liquid biopsies, molecular imaging, drug delivery, and other tumor precision diagnosis and treatment technologies.

Expansion of Anticomplement Therapy Indications from Rare Genetic Disorders to Common Kidney Diseases

Complement constitutes a major part of the innate immune system. The study of complement in human health has historically focused on infection risks associated with complement protein deficiencies; however, recent interest in the field has focused on overactivation of complement as a cause of immune injury and the development of anticomplement therapies to treat human diseases. The kidneys are particularly sensitive to complement injury, and anticomplement therapies for several kidney diseases have been investigated. Overactivation of complement can result from loss-of-function mutations in complement regulators; gain-of-function mutations in key complement proteins such as C3 and factor B; or autoantibody production, infection, or tissue stresses, such as ischemia and reperfusion, that perturb the balance of complement activation and regulation. Here, we provide a high-level review of the status of anticomplement therapies, with an emphasis on the transition from rare diseases to more common kidney diseases.

MASP3 Deficiency in Mice Reduces but Does Not Abrogate Alternative Pathway Complement Activity Due to Intrinsic Profactor D Activity

Complement factor D (FD) is a rate-limiting enzyme of the alternative pathway (AP). Recent studies have suggested that it is synthesized as an inactive precursor and that its conversion to enzymatically active FD is catalyzed by mannan-binding lectin-associated serine protease 3 (MASP3). However, whether MASP3 is essential for AP complement activity remains uncertain. It has been shown that Masp1/3 gene knockout did not prevent AP complement overactivation in a factor H-knockout mouse, and a human patient lacking MASP3 still retained AP complement activity. In this study, we have assessed AP complement activity in a Masp3-knockout mouse generated by CRISPR/Cas9 editing of the Masp1/3 gene. We confirmed specific Masp3 gene inactivation by showing intact MASP1 protein expression and absence of mature FD in the mutant mice. Using several assays, including LPS- and zymosan-induced C3b deposition and rabbit RBC lysis tests, we detected plasma concentration-dependent AP complement activity in Masp3 gene-inactivated mice. Thus, although not measurable in 5% plasma, significant AP complement activity was detected in 20-50% plasma of Masp3 gene-inactivated mice. Furthermore, whereas FD gene deletion provided more than 90% protection of CD55/Crry-deficient RBCs from AP complement-mediated extravascular hemolysis, Masp3 gene deletion only provided 30% protection in the same study. We also found pro-FD to possess intrinsic catalytic activity, albeit at a much lower level than mature FD. Our data suggest that MASP3 deficiency reduces but does not abrogate AP complement activity and that this is explained by intrinsic pro-FD activity, which can be physiologically relevant in vivo.

Recent advances in erythrocyte membrane-camouflaged nanoparticles for the delivery of anti-cancer therapeutics

INTRODUCTION

Red blood cell (or erythrocyte) membrane-camouflaged nanoparticles (RBC-NPs) not only have a superior circulation life and do not induce accelerated blood clearance, but also possess special functions, which offers great potential in cancer therapy.

AREAS COVERED

This review focuses on the recent advances of RBC-NPs for delivering various agents to treat cancers in light of their vital role in improving drug delivery. Meanwhile, the construction and in vivo behavior of RBC-NPs are discussed to provide an in-depth understanding of the basis of RBC-NPs for improved cancer drug delivery.

EXPERT OPINION

Although RBC-NPs are quite prospective in delivering anti-cancer therapeutics, they are still in their infancy stage and many challenges need to be overcome for successful translation into the clinic. The preparation and modification of RBC membranes, the optimization of coating methods, the scale-up production and the quality control of RBC-NPs, and the drug loading and release should be carefully considered in the clinical translation of RBC-NPs for cancer therapy.

Absence of complement component 3 does not prevent classical pathway-mediated hemolysis

Complement component 3 (C3) is emerging as a potential therapeutic target. We studied complement-mediated hemolysis using normal and C3-depleted human sera, wild-type (WT) and C3-deficient rat sera, and WT and C3 knockout rat models. In all of the in vitro and in vivo experiments, we found that the loss of C3 did not prevent classical pathway-mediated hemolysis, but it did almost abolish alternative pathway-mediated hemolysis. Experiments using preassembled classical pathway C3 convertases confirmed that C4b2a directly activated complement component 5 (C5), leading to membrane attack complex formation and hemolysis. Our results suggest that targeting C3 should effectively inhibit hemolysis and tissue damage mediated by the alternative pathway of complement activation, but this approach might have limited efficacy in treating classical pathway-mediated pathological conditions.

Differential contribution of C5aR and C5b-9 pathways to renal thrombic microangiopathy and macrovascular thrombosis in mice carrying an atypical hemolytic syndrome-related factor H mutation

Atypical hemolytic uremic syndrome (aHUS) is a form of thrombotic microangiopathy (TMA) caused by dysregulated complement activation. Clinically, aHUS is effectively treated by an anti-C5 monoclonal antibody (mAb) but whether the disease is mediated by the C5a receptor (C5aR) or C5b-9 pathway, or both, is unknown. Here we address this in a factor H mutant mouse (FH^R/R) which developed complement-mediated TMA as well as macrovascular thrombosis caused by an aHUS-related factor H point mutation (mouse W1206R, corresponding to human W1183R). C5 deficiency and anti-C5 mAb treatment blocked all disease manifestations in FH^R/R mice. C5aR1 gene deficiency prevented macrovascular thrombosis in various organs but did not improve survival or reduce renal TMA. Conversely, C6 or C9 deficiency significantly improved survival and markedly diminished renal TMA but did not prevent macrovascular thrombosis. Interestingly, as they aged both FH^R/R C6^-/- and FH^R/R C9^-/- mice developed glomerular disease reminiscent of C3 glomerulonephritis. Thus, C5aR and C5b-9 pathways drove different aspects of disease in FH^R/R mice with the C5aR pathway being responsible for macrovascular thrombosis and chronic inflammatory injury while the C5b-9 pathway caused renal TMA. Our data provide new understanding of the pathogenesis of complement-mediated TMA and macrovascular thrombosis in FH^R/R mice and suggest that C5 blockade is more effective for the treatment of aHUS than selectively targeting the C5aR or C5b-9 pathway alone.

Antibody Inhibition of Properdin Prevents Complement-Mediated Intravascular and Extravascular Hemolysis

Paroxysmal nocturnal hemoglobinuria (PNH) is a serious blood disorder characterized by dysregulated complement activation on blood cells. Eculizumab, the current standard therapy and a humanized anti-C5 mAb, relieves anemia and thrombosis symptoms of PNH patients by preventing complement-dependent intravascular hemolysis (IVH). However, up to 20% of PNH patients on long-term eculizumab treatment still suffer from significant anemia and are transfusion dependent because of extravascular hemolysis (EVH) of C3-opsonized PNH erythrocytes. In this study, we show that function-blocking anti-properdin (P) mAbs dose-dependently inhibited autologous, complement-mediated hemolysis induced by factor H dysfunction. Furthermore, anti-human P (hP) mAbs potently and dose-dependently inhibited acidified serum-induced hemolysis of PNH erythrocytes (Ham test). In contrast to erythrocytes rescued by anti-C5 mAb, nonlysed PNH erythrocytes rescued by anti-P mAb incurred no activated C3 fragment deposition on their surface. These results suggested that anti-P mAbs may prevent EVH as well as IVH of PNH erythrocytes. To test the in vivo efficacy of anti-hP mAbs in preventing EVH, we generated a P humanized mouse by transgenic expression of hP in P knockout mice (hP-Tg/P^-/-). In a murine EVH model, complement-susceptible erythrocytes were completely eliminated within 3 d in control mAb-treated hP-Tg/P^-/- mice, whereas such cells were protected and persisted in hP-Tg/P^-/- mice treated with an anti-hP mAb. Collectively, these data suggest that anti-P mAbs can inhibit both IVH and EVH mediated by complement and may offer improved efficacy over eculizumab, the current standard therapy for PNH.

Further evidence that paroxysmal nocturnal haemoglobinuria is a disorder of defective cell membrane lipid rafts

The glycolipid glycosylphosphatidylinositol anchor (GPI-A) plays an important role in lipid raft formation, which is required for proper expression on the cell surface of two inhibitors of the complement cascade, CD55 and CD59. The absence of these markers from the surface of blood cells, including erythrocytes, makes the cells susceptible to complement lysis, as seen in patients suffering from paroxysmal nocturnal haemoglobinuria (PNH). However, the explanation for why PNH-affected hematopoietic stem/progenitor cells (HSPCs) expand over time in BM is still unclear. Here, we propose an explanation for this phenomenon and provide evidence that a defect in lipid raft formation in HSPCs leads to defective CXCR4- and VLA-4-mediated retention of these cells in BM. In support of this possibility, BM-isolated CD34⁺ cells from PNH patients show a defect in the incorporation of CXCR4 and VLA-4 into membrane lipid rafts, respond weakly to SDF-1 stimulation, and show defective adhesion to fibronectin. Similar data were obtained with the GPI-A⁻ Jurkat cell line. Moreover, we also report that chimeric mice transplanted with CD55^−/− CD59^−/− BM cells but with proper GPI-A expression do not expand over time in transplanted hosts. On the basis of these findings, we propose that a defect in lipid raft formation in PNH-mutated HSPCs makes these cells more mobile, so that they expand and out-compete normal HSPCs from their BM niches over time.

Deletion of Crry and DAF on murine platelets stimulates thrombopoiesis and increases factor H-dependent resistance of peripheral platelets to complement attack

Complement receptor 1-related gene/protein y (Crry) and decay-accelerating factor (DAF) are two murine membrane C3 complement regulators with overlapping functions. Crry deletion is embryonically lethal whereas DAF-deficient mice are generally healthy. Crry(-/-)DAF(-/-) mice were viable on a C3(-/-) background, but platelets from such mice were rapidly destroyed when transfused into C3-sufficient mice. In this study, we used the cre-lox system to delete platelet Crry in DAF(-/-) mice and studied Crry/DAF-deficient platelet development in vivo. Rather than displaying thrombocytopenia, Pf4-Cre(+)-Crry(flox/flox) mice had normal platelet counts and their peripheral platelets were resistant to complement attack. However, chimera mice generated with Pf4-Cre(+)-Crry(flox/flox) bone marrows showed platelets from C3(-/-) but not C3(+/+) recipients to be sensitive to complement activation, suggesting that circulating platelets in Pf4-Cre(+)-Crry(flox/flox) mice were naturally selected in a complement-sufficient environment. Notably, Pf4-Cre(+)-Crry(flox/flox) mouse platelets became complement susceptible when factor H function was blocked. Examination of Pf4-Cre(+)-Crry(flox/flox) mouse bone marrows revealed exceedingly active thrombopoiesis. Thus, under in vivo conditions, Crry/DAF deficiency on platelets led to abnormal platelet turnover, but peripheral platelet count was compensated for by increased thrombopoiesis. Selective survival of Crry/DAF-deficient platelets aided by factor H protection and compensatory thrombopoiesis demonstrates the cooperation between membrane and fluid phase complement inhibitors and the body's ability to adaptively respond to complement regulator deficiencies.

Combination of factor H mutation and properdin deficiency causes severe C3 glomerulonephritis

Factor H (fH) and properdin both modulate complement; however, fH inhibits activation, and properdin promotes activation of the alternative pathway of complement. Mutations in fH associate with several human kidney diseases, but whether inhibiting properdin would be beneficial in these diseases is unknown. Here, we found that either genetic or pharmacological blockade of properdin, which we expected to be therapeutic, converted the mild C3 GN of an fH-mutant mouse to a lethal C3 GN with features of human dense deposit disease. We attributed this phenotypic change to a differential effect of properdin on the dynamics of alternative pathway complement activation in the fluid phase and the cell surface in the fH-mutant mice. Thus, in fH mutation-related C3 glomerulopathy, additional factors that impact the activation of the alternative pathway of complement critically determine the nature and severity of kidney pathology. These results show that therapeutic manipulation of the complement system requires rigorous disease-specific target validation.

Absence of CD59 exacerbates systemic autoimmunity in MRL/lpr mice

CD59 is a GPI-anchored membrane regulator of complement expressed on blood cells as well as peripheral tissues. It protects host cells from complement injury by inhibiting formation of the membrane attack complex. Recent studies in mice have suggested also a role of CD59 in T cell immune response that was mechanistically independent of complement. In the present study, we investigated the function of CD59 in the MRL/lpr model of murine lupus. We backcrossed the Cd59a knockout (Cd59a(-/-)) mouse onto the MRL/lpr background and compared Cd59a(+/+)-MRL/lpr and Cd59a(-/-)-MRL/lpr littermates for the development of systemic autoimmunity. We found that CD59a deficiency significantly exacerbated the skin disease and lymphoproliferation characteristic of MRL/lpr mice. It also increased autoantibody titers and caused a higher level of proteinuria in male MRL/lpr mice. Bone marrow transfer experiments indicated that CD59a expression on both bone marrow-derived cells and peripheral tissues played a role in lymphoproliferation, whereas the skin disease phenotype is determined mainly by local CD59a expression. Importantly, C3 gene deletion or C5 neutralization with a blocking mAb in Cd59a(-/-)-MRL/lpr mice did not rescue the proautoimmune phenotype associated with CD59a deficiency. These results together suggest that CD59a inhibits systemic autoimmunity in MRL/lpr mice through a complement-independent mechanism.

Complement regulatory protein Crry deficiency contributes to the antigen specific recall response in experimental autoimmune myasthenia gravis

BACKGROUND

Myasthenia gravis (MG) and animal model of experimental autoimmune myasthenia gravis (EAMG) is the most common autoimmune disorder of neuromuscular transmission. The disease is caused by the breakdown of the acetylcholine receptor (AChR) which is largely due to complement activation at the neuromuscular junction (NMJ). Limited knowledge exists to the extent that complement receptor 1-related gene/protein y deficiency (Crry -/-) modulates the adaptive immune response and EAMG outcome.

METHODS

Mouse EAMG was induced by s.c. administrations of purified acetylcholine receptor (AChR) to Crry -/- and age- matched WT (C57BL/6) mice. Disease severity was assessed by clinical score assessment and muscle grip strength measurements. Serum complement activity was determined by hemolytic assay. ELISA was used to detect the level of AChR specific antibodies. Splenic cells were analyzed for T and B cells subsets distribution, release of cytokines and AChR specific recall responses. Deposition of complement components at the NMJ was assessed by immunofluorescence staining.

RESULTS

In comparison to WT EAMG, Crry -/- EAMG mice showed signs of augmented muscle weakness but differences, except for one time point, were not statistically significant. Serum complement activity was reduced in Crry -/- EAMG mice and no substantial changes in deposition of C3, C3b/iC3b and C5b-9 (MAC) at the NMJ between WT EAMG and Crry -/- EAMG mice were detected. Lack of Crry affected adaptive immune response. Crry -/- EAMG mice showed increases in the number of AChR specific splenic T-cells secreting IFN-γ and IL-4. Production of complement fixing antibodies (IgG2b, IgG2c) was also augmented. More Th1, Th2 and Th17 cytokines were released into the bloodstream of Crry -/- EAMG mice.

CONCLUSIONS

Data suggest that Crry deficiency modulates the adaptive immune response in EAMG, but its effect on disease outcome is limited. This was due to the generally lower serum complement level caused by increased C3 turnover. Modulation of complement activity with soluble or membrane bound regulators of complement activity represents a potentially effective approach to modify autoimmune processes in MG and EAMG.

Early and extensive CD55 loss from red blood cells supports a causal role in malarial anaemia

Background

Levels of complement regulatory proteins (CrP) on the surface of red blood cells (RBC) decrease during severe malarial anaemia and as part of cell ageing process. It remains unclear whether CrP changes seen during malaria contribute to the development of anaemia, or result from an altered RBC age distribution due to suppressive effects of malaria on erythropoiesis.

Methods

A cross sectional study was conducted in the north-east coast of Tanzania to investigate whether the changes in glycosylphosphatidylinositol (GPI)-anchored complement regulatory proteins (CD55 and CD59) contributes to malaria anaemia. Blood samples were collected from a cohort of children under intensive surveillance for Plasmodium falciparum parasitaemia and illness. Levels of CD55 and CD59 were measured by flow cytometer and compared between anaemic (8.08 g/dl) and non- anaemic children (11.42 g/dl).

Results

Levels of CD55 and CD59 decreased with increased RBC age. CD55 levels were lower in anaemic children and the difference was seen in RBC of all ages. Levels of CD59 were lower in anaemic children, but these differences were not significant. CD55, but not CD59, levels correlated positively with the level of haemoglobin in anaemic children.

Conclusion

The extent of CD55 loss from RBC of all ages early in the course of malarial anaemia and the correlation of CD55 with haemoglobin levels support the hypothesis that CD55 may play a causal role in this disorder.

[Murine models in blood transfusion: allo-immunization, hemolysis]

Mice represent an animal model that can be easily manipulated. Mice have been used to model many human diseases. This review addresses murine models of immunity directed against red blood cell antigens as well as models of antibody and non-antibody mediated hemolysis. These models allow for a better understanding of the side effects of transfusion, such as red blood cell allo-immunization and post-transfusional hemolytic reactions. They also help explore strategies to treat and prevent these side effects in ways that would not be available using clinical research alone.

The complement cascade as a mediator of tissue growth and regeneration

Recent evidence has demonstrated that the complement cascade is involved in a variety of physiologic and pathophysiologic processes in addition to its role as an immune effector. Research in a variety of organ systems has shown that complement proteins are direct participants in maintenance of cellular turnover, healing, proliferation and regeneration. As a physiologic housekeeper, complement proteins maintain tissue integrity in the absence of inflammation by disposing of cellular debris and waste, a process critical to the prevention of autoimmune disease. Developmentally, complement proteins influence pathways including hematopoietic stem cell engraftment, bone growth, and angiogenesis. They also provide a potent stimulus for cellular proliferation including regeneration of the limb and eye in animal models, and liver proliferation following injury. Here, we describe the complement cascade as a mediator of tissue growth and regeneration.

Use of mouse models to study the mechanisms and consequences of RBC clearance

Mice provide tractable animal models for studying the pathophysiology of various human disorders. This review discusses the use of mouse models for understanding red-blood-cell (RBC) clearance. These models provide important insights into the pathophysiology of various clinically relevant entities, such as autoimmune haemolytic anaemia, haemolytic transfusion reactions, other complications of RBC transfusions and immunomodulation by Rh immune globulin therapy. Mouse models of both antibody- and non-antibody-mediated RBC clearance are reviewed. Approaches for exploring unanswered questions in transfusion medicine using these models are also discussed.

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.

Complement-dependent T-cell lymphopenia caused by thymocyte deletion of the membrane complement regulator Crry

Although complement lysis is frequently used for the purification of lymphocyte subpopulations in vitro, how lymphocytes escape complement attack in vivo has not been clearly delineated. Here, we show that conditional gene targeting of a murine membrane complement regulator Crry on thymocytes led to complement-dependent peripheral T-cell lymphopenia. Notably, despite evidence of hypersensitivity to complement attack, Crry-deficient T cells escaped complement injury and developed normally in the thymus, because of low intrathymic complement activity. Crry-deficient T cells were eliminated in the periphery by a C3- and macrophage-mediated but C5-independent mechanism. Thus, Crry is essential for mature T-cell survival in the periphery but not for lymphogenesis in the thymus. The observation that the thymus is a complement-privileged site may have implications for complement-based antitumor therapies.

Crry deficiency in complement sufficient mice: C3 consumption occurs without associated renal injury

The rodent-specific complement regulator complement receptor 1-related gene/protein-y (Crry) is critical for complement homeostasis. Gene deletion is 100% embryonically lethal; Crry-deficient (Crry(-/-)) mice were rescued by back-crossing onto C3 deficiency, confirming that embryo loss was complement mediated. In order to rescue viable Crry(-/-) mice without deleting C3, we have tested inhibition of C5 during gestation. Crry(+/-) females were given neutralizing anti-C5 mAb immediately prior to mating with Crry(+/-) males and C5 inhibition maintained through pregnancy. A single, healthy Crry(-/-) female was obtained and mating with Crry(+/-) males yielded healthy litters containing equal numbers of Crry(+/-) and Crry(-/-) pups. Inter-crossing Crry(-/-) mice yielded healthy litters of expected size. Although the mice were not anemic, exposure of Crry(-/-) erythrocytes to normal mouse serum caused C3 deposition and lysis, while transfusion into normal or C6(-/-) mice resulted in rapid clearance. Complement activity and C3 levels in Crry(-/-) mice were markedly reduced. Comparison with factor H deficient (CfH(-/-)) mice revealed similar levels of residual C3; however, unlike the CfH(-/-) mice, Crry(-/-) mice showed no evidence of renal injury, demonstrating distinct roles for these regulators in protecting the kidney.

Expression levels of CD47, CD35, CD55, and CD59 on red blood cells and signal-regulatory protein-alpha,beta on monocytes from patients with warm autoimmune hemolytic anemia

BACKGROUND

Animal models have shown that CD47-deficient mice develop severe autoimmune hemolytic anemia (AIHA) because the binding of red blood cell (RBC) CD47 to signal-regulatory protein (SIRP-alpha) on macrophages contributes to the inhibition of phagocytosis. In contrast, complement-inhibitory proteins such as CD35, CD55, and CD59 may protect RBCs against the lysis by complement.

STUDY DESIGN AND METHODS

With the use of flow cytometric analyses, the expression of CD47, CD35, CD55, and CD59 on RBCs and of SIRP-alpha,beta on peripheral monocytes of 36 patients with warm AIHA (wAIHA; 23 with active wAIHA, 13 with wAIHA in remission) and 20 healthy subjects was evaluated.

RESULTS

The mean fluorescence intensities (MFIs) of the expression of CD47, CD35, CD55, and SIRP-alpha,beta of active wAIHA patients, wAIHA in remission, and healthy subjects were not statistically different. Patients with active wAIHA showed significantly lower CD59 expression on RBCs than healthy individuals (MFI, 512.5 +/- 59.6 vs. 553.7 +/- 36.6; p = 0.009), while the CD59 expression in patients with wAIHA in remission was not significantly different from that of healthy controls (MFI, 538.4 +/- 48.3 vs. 553.7 +/- 36.6; p > 0.05). The expression of CD59 on RBCs of 3 patients who died from the wAIHA was lower than that seen on RBCs of healthy controls (MFI, 433.6 +/- 69.6 vs. 553.74 +/- 36.6; p = 0.0001).

CONCLUSIONS

Our data show that the expression of CD47 on RBCs and SIRP-alpha,beta on monocytes of patients with wAIHA is not different from that seen in healthy individuals. In addition, we detected that patients with active wAIHA present low expression of CD59 and normal expression of CD35 and CD55 on their RBCs. Complement-regulatory proteins may play an important role in protecting RBC destruction through the activation of complement.

Deficiency of decay-accelerating factor and complement receptor 1-related gene/protein y on murine platelets leads to complement-dependent clearance by the macrophage phagocytic receptor CRIg

Complement activation on human platelets is known to cause platelet degranulation and activation. To evaluate how normal platelets escape complement attack in vivo, we studied the fate of murine platelets deficient in 2 membrane complement regulatory proteins using an adoptive transfer model. We show here that deficiency of either decay-accelerating factor (DAF) or complement receptor 1-related gene/protein y (Crry) on murine platelets was inconsequential, whereas DAF and Crry double deficiency led to rapid clearance of platelets from circulation in a complement- and macrophage-dependent manner. This finding contrasted with the observation on erythrocytes, where Crry deficiency alone resulted in complement susceptibility. Quantitative flow cytometry showed DAF and Crry were expressed at similar levels on platelets, whereas Crry expression was 3 times higher than DAF on erythrocytes. Antibody blocking or gene ablation of the newly identified complement receptor CRIg, but not complement receptor 3 (CR3), rescued DAF/Crry-deficient platelets from complement-dependent elimination. Surprisingly, deficiency of CRIg, CR3, and other known complement receptors failed to prevent Crry-deficient erythrocytes from complement-mediated clearance. These results show a critical but redundant role of DAF and Crry in platelet survival and suggest that complement-opsonized platelets and erythrocytes engage different complement receptors on tissue macrophages in vivo.

Activator-specific requirement of properdin in the initiation and amplification of the alternative pathway complement

Properdin is a positive regulator of alternative pathway (AP) complement. The current understanding of properdin function is that it facilitates AP complement activation by stabilizing the C3 convertase C3bBb. Properdin-deficient patients are susceptible to lethal meningococcal infection, but the mechanism of this selective predisposition is not fully understood. By gene targeting in the mouse, we show here that properdin is essential for AP complement activation induced by bacterial lipopolysacharride (LPS) and lipooligosacharride (LOS) and other, but not all, AP complement activators. LPS- and LOS-induced AP complement activation was abolished in properdin-/- mouse serum, and properdin-/- mice were unable to clear Crry-deficient erythrocytes, which are known to be susceptible to AP complement-mediated extravascular hemolysis. In contrast, zymosan- and cobra venom factor-induced AP complement activation, and classical pathway-triggered AP complement amplification were only partially or minimally affected in properdin-/- mice. We further show that the ability of human properdin to restore LPS-dependent AP complement activity in properdin-/- mouse serum correlated with the human properdin-binding affinity of the LPS. These results reveal a novel role of properdin in AP complement initiation and have implications for understanding the selective predisposition of properdin-deficient patients to meningococcal infection.

IgM and IgA anti-erythrocyte autoantibodies induce anemia in a mouse model through multivalency-dependent hemagglutination but not through complement activation

By generating IgM and IgA switch variants of the 34-3C IgG2a anti-red blood cell (RBC) autoantibody, we evaluated the pathogenic activity of these 2 isotypes in view of the Fc-associated effector functions (ie, complement activation and polyvalency-dependent agglutination). We found that polymeric forms of 34-3C IgM and IgA anti-RBC autoantibody were as pathogenic as IgG2a, which was the most pathogenic among 4 different IgG subclasses, whereas their monomeric variants completely lacked pathogenic effects. Histological examination showed that 34-3C IgM and IgA autoantibodies caused anemia as a result of multivalency-dependent hemaggultination and subsequent sequestration of RBC in the spleen, in contrast to Fc receptor- and complement receptor-mediated erythrophagocytosis by Kupffer cells with IgG isotypes. In addition, the development of anemia induced by IgM and IgA isotypes of 34-3C antibody and by 2 additional IgM anti-RBC monoclonal autoantibodies was not inhibited at all in C3-deficient mice, indicating the lack of involvement of complement activation in the pathogenesis of IgM- and IgA-induced anemia. Our data demonstrate a remarkably high pathogenic potential of polymeric forms of IgM and IgA anti-RBC autoantibodies due to their ability to induce hemagglutination but completely independent of complement activation.

Cutting edge: treatment of complement regulatory protein deficiency by retroviral in vivo gene therapy

Gene therapy is an attractive means to replace a deficient or defective protein. Using a murine retroviral vector, we provide an example of reconstituting a C regulator by neonatal in vivo gene transfer. The fusion gene containing the mouse C receptor 1-related gene/protein y (Crry) and a single chain Ab fragment with specificity for mouse glycophorin A was placed under transcriptional control of a liver-specific promoter. Shortly after birth, Crry KO mice were injected with the retroviral vectors. Protein expression progressively increased over the next 6-8 wk after which an equilibrium was established. Coating levels on RBCs were obtained that inhibited C activation similar to wild-type cells and remained constant for > 1 year. Thus, gene therapy with targeted regulators represents a treatment option to provide a long-term and sustained protein supply for the site-specific blockade of undesirable complement activation.

Retrovirus-mediated over-expression of decay-accelerating factor rescues Crry-deficient erythrocytes from acute alternative pathway complement attack

Decay-accelerating factor (DAF) and complement receptor 1-related gene/protein y (Crry) are two membrane-bound complement regulators on murine erythrocytes that inhibit C3/C5 convertases. Previously, we found that Crry- but not DAF-deficient erythrocytes were susceptible to alternative pathway complement-mediated elimination in vivo. To determine whether it is a unique activity or a higher level expression of Crry makes it indispensable on murine erythrocytes, we over-expressed DAF on Crry-deficient (Crry(-/-)) erythrocytes by retroviral vector-mediated DAF gene transduction of bone marrow stem cells. DAF retrovirus-transduced erythrocytes expressed 846 +/- 127 DAF molecules/cell (DAF(high)) compared with 249 +/- 94 DAF molecules/cell (DAF(low)) and 774 +/- 135 Crry molecules/cell on control mouse erythrocytes. DAF(high)-Crry(-/-) erythrocytes were significantly more resistant than either DAF(low)-Crry(-/-), DAF(-/-) -Crry(+/+) or wild-type erythrocytes to classical pathway complement-mediated C3 deposition in vitro. Furthermore, increased DAF expression rescued Crry(-/-) erythrocytes from acute alternative pathway complement attack in vivo. Notably, long term monitoring revealed that DAF(high)-Crry(-/-) erythrocytes were still more susceptible than wild-type erythrocytes to complement-mediated elimination as they had a shorter half-life in complement-sufficient mice but survived equally well in complement-deficient mice. These results suggest that both a high level expression and a more potent anti-alternative pathway complement activity of Crry contributed to its indispensable role on murine erythrocytes. Additionally, they demonstrate the feasibility of using stem cell gene therapy to correct membrane complement regulator deficiency on blood cells in vivo.

Complement regulatory proteins and autoimmunity

The complement system is known to be involved in autoimmunity at several levels. Activated complement contributes to the inflammatory tissue injury characteristic of many autoimmune disease settings. On the other hand, early components of the classical pathway, including C1q, C4 and C2, are thought to be important for disposing apoptotic cellular autoantigens and/or the induction of B cell tolerance in the bone marrow, and their deficiency is a strong risk factor for systemic autoimmunity. Recent studies using transgenic mice have revealed membrane complement regulatory proteins as important modulators in the pathogenesis and manifestation of autoimmune injury. Available evidence suggests that these regulatory proteins may act to suppress autoimmunity via both complement-dependent and -independent mechanisms.

Molecular and cellular basis for pathogenicity of autoantibodies: lessons from murine monoclonal autoantibodies

The pathogenesis of autoantibody-mediated cellular and tissue lesions in autoimmune diseases is most straightforwardly attributable to the combined action of self-antigen binding properties and effector functions associated with the Fc regions of the different immunoglobulin (Ig) isotypes. The analysis of two different sets of monoclonal autoantibodies derived from lupus-prone mice revealed remarkable differences in the pathogenic potentials of different IgG subclasses: (1) the IgG2a and IgG2b subclasses of anti-red blood cell (RBC) autoantibodies are the most pathogenic and efficiently activate two classes of activating IgG Fc receptors (FcgammaRIII and FcgammaRIV) and complement; (2) the IgG3 subclass is less pathogenic and activate only complement; and (3) the IgG1 subclass is the least pathogenic and interact only with FcgammaRIII. In addition, because of the unique property of IgG3 to form self-associating complexes and generate cryoglobulins, this subclass of rheumatoid factor and anti-DNA autoantibodies became highly pathogenic and induced lupus-like nephritis and/or vasculitis. Since the switch to IgG2a and IgG3 is promoted by Th1 cytokine interferon gamma, these results strongly suggest that Th1 autoimmune responses could be critically involved in the generation of more pathogenic autoantibodies in systemic lupus erythematosus. This finding is consistent with the observation that the progression of murine lupus nephritis is correlated with the relative dominance of Th1 autoimmune responses. Finally, the analysis of IgG glycosylation pattern revealed that more sialylated IgG autoantibodies remained poorly pathogenic because of limited Fc-associated effector functions and loss of cryoglobulin activity. This suggests that the terminal sialylation of the oligosaccharide side chains of IgG could be a significant factor determining the pathogenic potential of autoantibodies. Our results thus underline the importance of subpopulations of autoantibodies, induced by the help of Th1 cells, in the pathogenesis of autoantibody-mediated cellular and tissue injuries.

Maturation and enucleation of primitive erythroblasts during mouse embryogenesis is accompanied by changes in cell-surface antigen expression

Primitive erythroblasts (EryPs) are the first hematopoietic cell type to form during mammalian embryogenesis and emerge within the blood islands of the yolk sac. Large, nucleated EryPs begin to circulate around midgestation, when connections between yolk sac and embryonic vasculature mature. Two to 3 days later, small cells of the definitive erythroid lineage (EryD) begin to differentiate within the fetal liver and rapidly outnumber EryPs in the circulation. The development and maturation of EryPs remain poorly defined. Our analysis of embryonic blood at different stages reveals a stepwise developmental progression within the EryP lineage from E9.5 to E12.5. Thereafter, EryDs are also present in the bloodstream, and the 2 lineages are not easily distinguished. We have generated a transgenic mouse line in which the human epsilon-globin gene promoter drives expression of green fluorescent protein exclusively within the EryP lineage. Here, we have used this line to characterize changes in cell morphology and surface-marker expression as EryPs mature and to track EryP numbers and enucleation throughout gestation. This study identifies previously unrecognized synchronous developmental stages leading to the maturation of EryPs in the mouse embryo. Unexpectedly, we find that EryPs are a stable cell population that persists through the end of gestation.

DAF/Crry double deficiency in mice exacerbates nephrotoxic serum-induced proteinuria despite markedly reduced systemic complement activity

Decay-accelerating factor (DAF) and complement receptor 1-related gene/protein y (Crry) are two membrane-anchored complement regulatory proteins in rodent. Although both proteins are broadly distributed and exert complement regulation at the same steps of the complement cascade, DAF knockout mice are viable whereas Crry knockout mice die in utero as a result of maternal complement attack. The latter outcome has prevented the dissection of overlapping functions of DAF and Crry in adult mouse tissues in vivo. By crossing female DAF(-/-)/Crry(-/-)/C3(-/-) mice with male DAF(-/-)/Crry(+/-)/C3(+/-) mice, we circumvented maternal complement attack during fetal development and generated viable DAF(-/-)/Crry(-/-)/C3(+/-) mice to address the consequence of DAF/Crry double deficiency. DAF(-/-)/Crry(-/-)/C3(+/-) mice were born at the expected frequency and survived to adulthood. However, they were found to have greatly reduced systemic complement activity due, at least in part, to spontaneous C3 activation and consumption. Plasma C3 proteins in DAF(-/-)/Crry(-/-)/C3(+/-) mice were 30% of that of wild-type mice, and serum complement activity, as assessed by zymosan and immune complex C3 opsonization assays, was 90% reduced in DAF(-/-)/Crry(-/-)/C3(+/-) mice. Remarkably, despite greatly reduced systemic complement activity, DAF(-/-)/Crry(-/-)/C3(+/-) mice developed more severe proteinuria after induction of nephrotoxic serum nephritis as compared with DAF(-/-)/Crry(+/-)/C3(+/-) and DAF(-/-)/Crry(-/-)/C3(-/-) littermate controls. The results highlight the critical and overlapping role of Crry and DAF in vivo in preventing complement activation and tissue injury.

Suppression of complement regulatory proteins (CRPs) exacerbates experimental autoimmune anterior uveitis (EAAU)

This study was undertaken to explore the role of complement regulatory proteins (CRPs) in experimental autoimmune anterior uveitis (EAAU). We observed that the levels of CRPs, Crry and CD59, in the eyes of Lewis rats increased during EAAU and remained elevated when the disease resolved. The in vivo role of these CRPs in EAAU was explored using neutralizing mAbs, antisense oligodeoxynucleotides (AS-ODNs), and small interfering RNAs against rat Crry and CD59. Suppression of Crry in vivo at days 9, 14, or 19 by neutralizing mAb or AS-ODNs resulted in the early onset of disease, the exacerbation of intraocular inflammation, and delayed resolution. Suppression of CD59 was only effective when the Abs and ODNs were given before the onset of disease. The most profound effect on the disease was observed when a mixture of Crry and CD59 mAbs or AS-ODNs was administered. A similar effect was observed with a combination of Crry and CD59 small interfering RNA. There was no permanent histologic damage to ocular tissue after the inflammation cleared in these animals. Increased complement activation as determined by increased deposition of C3, C3 activation fragments, and membrane attack complex was observed in the eyes of Lewis rats when the function and/or expression of Crry and CD59 was suppressed. Thus, our results suggest that various ocular tissues up-regulate the expression of Crry and CD59 to avoid self-injury during autoimmune uveitis and that these CRPs play an active role in the resolution of EAAU by down-regulating complement activation in vivo.

Efficacy of high-dose therapy and hematopoietic stem cell transplantation for mantle cell lymphoma

Conventional treatment of mantle cell lymphoma (MCL) yields modest responses and short remissions. We report 30 hematopoietic stem cell transplants (HSCT) for MCL: 13 autologous, 10 allogeneic myeloablative, and 7 nonablative. After a median 1.2 years from diagnosis (range 0.5 to 4.7) and a median of 2 pre-HSCT chemotherapeutic regimens (range 1 to 5), their median age at HSCT was 52 years (range 37 to 67). Eleven patients (41%) were in first remission, 11 (41%) were in second remission, and 7 (25%) had resistant disease. Four died early. Nineteen achieved CR (83%) and 4 PR (17%). With median 2.7 years of follow-up, 5-year overall survival (OS) was 42% (95% CI 11-73%) after autologous versus allogeneic at 49% (95% CI 22-76%). Five-year progression-free survival (PFS) was 31% (95% CI 3-59%) and 50% (95% CI 24-76%) for autologous and allogeneic HSCT, respectively. Fourteen died: 3 from sepsis, 1 acute GVHD, 10 MCL. No autologous transplant-related deaths occurred. Allogeneic transplant-related mortality was 29% (95% CI 6-52%) at 1 and 5 years. HSCT for MCL can yield extended disease control and long-term survival.

In vivo correction of complement regulatory protein deficiency with an inhibitor targeting the red blood cell membrane

Because of the complement system's involvement in many human diseases and potential complications associated with its systemic blockade, site-specific regulation of this effector system is an attractive concept. We report on further developments of such an approach using a single-chain Ab fragment as a vehicle to deliver complement regulatory proteins to a defined cell type. In a model system in which RBCs deficient in complement receptor 1-related gene/protein y (Crry) are rapidly cleared after injection into wild-type animals by a complement-dependent mechanism, we selectively reconstituted these cells with N- and C-terminally targeted recombinant forms of Crry. Transfusion of Crry-coated knockout RBCs into C57BL/6 mice extended their in vivo half-life from <5 min to approximately 2 days. Maintenance of protective levels of Crry (by a combined treatment of donor and recipient RBCs) led to nearly normal RBC survival. Uniform in vitro and in vivo coating of the RBCs and the more efficient complement inhibitory capacity of C-terminally tagged Crry were other interesting features of this experimental system. These results suggest the possibility of using the single-chain Ab fragment-mediated targeting concept of complement regulatory proteins to restrict complement inhibition to the site of its excessive activation.

Membrane complement regulatory proteins

A number of proteins anchored on the cell surface function to protect host tissues from bystander injury when complement is activated. In humans, they include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46), complement receptor 1 (CR1, CD35) and CD59. Although disease conditions directly attributable to abnormal function of these proteins are relatively rare, it has become evident from recent studies using animal models that membrane complement regulatory proteins are important modulators of tissue injury in many autoimmune and inflammatory disease settings. Evidence is also emerging to support a role of these proteins in regulating cellular immunity. In this article, we highlight recent advances on the in vivo biology of membrane complement regulatory proteins and discuss their relevance in human disease pathogenesis and therapeutics.

The complement inhibitory protein DAF (CD55) suppresses T cell immunity in vivo

Decay-accelerating factor ([DAF] CD55) is a glycosylphosphatidylinositol-anchored membrane inhibitor of complement with broad clinical relevance. Here, we establish an additional and unexpected role for DAF in the suppression of adaptive immune responses in vivo. In both C57BL/6 and BALB/c mice, deficiency of the Daf1 gene, which encodes the murine homologue of human DAF, significantly enhanced T cell responses to active immunization. This phenotype was characterized by hypersecretion of interferon (IFN)-γ and interleukin (IL)-2, as well as down-regulation of the inhibitory cytokine IL-10 during antigen restimulation of lymphocytes in vitro. Compared with wild-type mice, Daf1^−/− mice also displayed markedly exacerbated disease progression and pathology in a T cell–dependent experimental autoimmune encephalomyelitis (EAE) model. However, disabling the complement system in Daf1^−/− mice normalized T cell secretion of IFN-γ and IL-2 and attenuated disease severity in the EAE model. These findings establish a critical link between complement and T cell immunity and have implications for the role of DAF and complement in organ transplantation, tumor evasion, and vaccine development.

A novel mechanism of complement-independent clearance of red cells deficient in glycosyl phosphatidylinositol–linked proteins

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic anemia characterized by the increased sensitivity of red blood cells (RBCs) to complement, leading to intravascular hemolysis and hemoglobinuria. PNH is due to the expansion of a cell clone that has acquired a mutation in the PIGA gene. Mice with targeted Piga gene inactivation genetically mimic the human disease and have phosphatidylinositol glycan class A-negative (PIGA-) RBCs with a reduced half-life in circulation. Although PIGA-RBCs are hypersensitive to complement in vitro, their complement sensitivity in vivo is barely detectable. Here we show that the shortened survival of PIGA- RBCs is independent of complement either by using inhibitory C5 antibodies or by transfusion into C5-, C4-, C3-, or factor B-deficient mice. Splenectomy or high-dose cortisone treatment had no effect on the shorter survival of PIGA- RBCs. However, treatment with liposome-encapsulated clodronate, an agent that depletes macrophages in vivo, normalized the half-life of PIGA- RBCs. This indicates that the shortened survival of PIGA- RBCs is due to a novel pathway of PIGA- RBC clearance that is mediated by macrophages, but occurs independently of complement. Future investigations will show whether this novel pathway of PIGA- RBC destruction identified in mice may also operate in patients with PNH. (Blood. 2004;103:2827-2834)

An update on the role of high-dose therapy with autologous or allogeneic stem cell transplantation in mantle cell lymphoma

PURPOSE OF REVIEW

Early trials of high-dose therapy with autologous stem cell transplantation in mantle cell lymphoma were discouraging, with no clear survival advantage attributable to the procedure. Most early series were plagued by small numbers, retrospective designs, and short follow-up. Also, until recently, allogeneic stem cell transplantation was not an option for most mantle cell lymphoma patients who were too old or infirm to tolerate standard conditioning regimens.

RECENT FINDINGS

New advances in allogeneic transplantation, particularly reduced-intensity conditioning regimens, have increased the availability of this procedure to patients with mantle cell lymphoma. New evidence has emerged during the last several years that suggests autologous stem cell transplantation in first complete remission may provide a survival advantage over conventional chemotherapy in patients with mantle cell lymphoma. Additionally, investigational strategies such as in vivo purging with rituximab and the use of radioimmunotherapy in conditioning regimens may further increase response rates and, hopefully, survival in mantle cell lymphoma patients. Finally, recent studies suggest the existence of a graft-versus-lymphoma effect in mantle cell lymphoma providing strong scientific rationale for the possible curative potential of allogeneic stem cell transplantation in this disease.

SUMMARY

This review focuses on recent advances in allogeneic and autologous transplantation for mantle cell lymphoma. Particular emphasis is placed on the role of autologous transplantation in first complete remission, the role of in vivo purging with rituximab, the utility of radioimmunotherapy and, finally, the evolving strategy of reduced-intensity allogeneic stem cell transplantation.