Lysis of erythrocytes by complement in the absence of antibody

What is the role of complement in bystander hemolysis? Old concept, new insights

INTRODUCTION

Bystander hemolysis occurs when antigen-negative red blood cells (RBCs) are lysed by the complement system. Many clinical entities including passenger lymphocyte syndrome, hyperhemolysis following blood transfusion, and paroxysmal nocturnal hemoglobinuria are complicated by bystander hemolysis.

AREAS COVERED

The review provides data about the role of the complement system in the pathogenesis of bystander hemolysis. Moreover, future perspectives on the understanding and management of this syndrome are described.

EXPERT OPINION

Complement system can be activated via classical, alternative, and lectin pathways. Classical pathway activation is mediated by antigen-antibody (autoantibodies and alloantibodies against autologous RBCs, infectious agents) complexes. Alternative pathway initiation is triggered by heme, RBC microvesicles, and endothelial injury that is a result of intravascular hemolysis. Thus, C5b is formed, binds with C6-C9 compomers, and MAC (C5b-9) is formulated in bystander RBCs membranes, leading to cell lysis. Intravascular hemolysis, results in activation of the alternative pathway, establishing a vicious cycle between complement activation and bystander hemolysis. C5 inhibitors have been used effectively in patients with hyperhemolysis syndrome and other entities characterized by bystander hemolysis.

Soluble MAC is primarily released from MAC-resistant bacteria that potently convert complement component C5

The membrane attack complex (MAC or C5b-9) is an important effector of the immune system to kill invading microbes. MAC formation is initiated when complement enzymes on the bacterial surface convert complement component C5 into C5b. Although the MAC is a membrane-inserted complex, soluble forms of MAC (sMAC), or terminal complement complex (TCC), are often detected in sera of patients suffering from infections. Consequently, sMAC has been proposed as a biomarker, but it remains unclear when and how it is formed during infections. Here, we studied mechanisms of MAC formation on different Gram-negative and Gram-positive bacteria and found that sMAC is primarily formed in human serum by bacteria resistant to MAC-dependent killing. Surprisingly, C5 was converted into C5b more potently by MAC-resistant compared to MAC-sensitive Escherichia coli strains. In addition, we found that MAC precursors are released from the surface of MAC-resistant bacteria during MAC assembly. Although release of MAC precursors from bacteria induced lysis of bystander human erythrocytes, serum regulators vitronectin (Vn) and clusterin (Clu) can prevent this. Combining size exclusion chromatography with mass spectrometry profiling, we show that sMAC released from bacteria in serum is a heterogeneous mixture of complexes composed of C5b-8, up to three copies of C9 and multiple copies of Vn and Clu. Altogether, our data provide molecular insight into how sMAC is generated during bacterial infections. This fundamental knowledge could form the basis for exploring the use of sMAC as biomarker.

The allosteric modulation of complement C5 by knob domain peptides

Bovines have evolved a subset of antibodies with ultra-long heavy chain complementarity determining regions that harbour cysteine-rich knob domains. To produce high-affinity peptides, we previously isolated autonomous 3–6 kDa knob domains from bovine antibodies. Here, we show that binding of four knob domain peptides elicits a range of effects on the clinically validated drug target complement C5. Allosteric mechanisms predominated, with one peptide selectively inhibiting C5 cleavage by the alternative pathway C5 convertase, revealing a targetable mechanistic difference between the classical and alternative pathway C5 convertases. Taking a hybrid biophysical approach, we present C5-knob domain co-crystal structures and, by solution methods, observed allosteric effects propagating >50 Å from the binding sites. This study expands the therapeutic scope of C5, presents new inhibitors, and introduces knob domains as new, low molecular weight antibody fragments, with therapeutic potential.

Antibodies are proteins produced by the immune system that can selectively bind to other molecules and modify their behaviour. Cows are highly equipped at fighting-off disease-causing microbes due to the unique shape of some of their antibodies. Unlike other jawed vertebrates, cows’ antibodies contain an ultra-long loop region that contains a ‘knob domain’ which sticks out from the rest of the antibody. Recent research has shown that when detached, the knob domain behaves like an antibody fragment, and can independently bind to a range of different proteins.

Antibody fragments are commonly developed in the laboratory to target proteins associated with certain diseases, such as arthritis and cancer. But it was unclear whether the knob domains from cows’ antibodies could also have therapeutic potential. To investigate this, Macpherson et al. studied how knob domains attach to complement C5, a protein in the inflammatory pathway which is a drug target for various diseases, including severe COVID-19.

The experiments identified various knob domains that bind to complement C5 and inhibits its activity by altering its structure or movement. Further tests studying the structure of these interactions, led to the discovery of a common mechanism by which inhibitors can modify the behaviour of this inflammatory protein.

Complement C5 is involved in numerous molecular pathways in the immune system, which means many of the drugs developed to inhibit its activity can also leave patients vulnerable to infection. However, one of the knob domains identified by Macpherson et al. was found to reduce the activity of complement C5 in some pathways, whilst leaving other pathways intact. This could potentially reduce the risk of bacterial infections which sometimes arise following treatment with these types of inhibitors.

These findings highlight a new approach for developing drug inhibitors for complement C5. Furthermore, the ability of knob domains to bind to multiple sites of complement C5 suggests that this fragment could be used to target proteins associated with other diseases.

Bystander mechanism for complement-initiated early oligodendrocyte injury in neuromyelitis optica

Neuromyelitis optica spectrum disorder (herein called NMO) is an autoimmune inflammatory disease of the central nervous system in which immunoglobulin G antibodies against astrocyte water channel aquaporin-4 (AQP4-IgG) cause demyelination and neurological deficit. Injury to oligodendrocytes, which do not express AQP4, links the initiating pathogenic event of AQP4-IgG binding to astrocyte AQP4 to demyelination. Here, we report evidence for a complement 'bystander mechanism' to account for early oligodendrocyte injury in NMO in which activated, soluble complement proteins following AQP4-IgG binding to astrocyte AQP4 result in deposition of the complement membrane attack complex (MAC) on nearby oligodendrocytes. Primary cocultures of rat astrocytes and mature oligodendrocytes exposed to AQP4-IgG and complement showed early death of oligodendrocytes in close contact with astrocytes, which was not seen in pure oligodendrocyte cultures, in cocultures exposed to AQP4-IgG and C6-depleted serum, or when astrocytes were damaged by a complement-independent mechanism. Astrocyte-oligodendrocyte cocultures exposed to AQP4-IgG and complement showed prominent MAC deposition on oligodendrocytes in contact with astrocytes, whereas C1q, the initiating protein in the classical complement pathway, and C3d, a component of the alternative complement pathway, were deposited only on astrocytes. Early oligodendrocyte injury with MAC deposition was also found in rat brain following intracerebral injection of AQP4-IgG, complement and a fixable dead-cell stain. These results support a novel complement bystander mechanism for early oligodendrocyte injury and demyelination in NMO.

Automated Two-Dimensional Immunoelectrophoresis and its Application to the Analysis of C3 and C4 in Rheumatoid Arthritis and Systemic Lupus Erythematosus (S.L.E.)

A semi-automated two-dimensional immunoelectrophoretic method has been developed which is quicker and cheaper to use than the original system, making it more suitable for routine use. It is accurate and reproducible and is suitable for complement conversion studies where C3 and C4 conversion can be measured with 5 and 10% reproducibility respectively. Conversion of C3 and C4, observed in both S.L.E. and rheumatoid arthritis, is an early and sensitive index of disease activity. Total C3 levels, although fluctuating widely in the course of the disease, were not shown to be as sensitive an index.

Structural Basis for Eculizumab-Mediated Inhibition of the Complement Terminal Pathway

Eculizumab is a humanized mAb approved for treatment of patients with paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome. Eculizumab binds complement component C5 and prevents its cleavage by C5 convertases, inhibiting release of both the proinflammatory metabolite C5a and formation of the membrane attack complex via C5b. In this study, we present the crystal structure of the complex between C5 and a Fab fragment with the same sequence as eculizumab at a resolution of 4.2 Å. Five CDRs contact the C5 macroglobulin 7 domain, which contains the entire epitope. A complete mutational scan of the 66 CDR residues identified 28 residues as important for the C5-eculizumab interaction, and the structure of the complex offered an explanation for the reduced C5 binding observed for these mutant Abs. Furthermore, the structural observations of the interaction are supported by the reduced ability of a subset of these mutated Abs to inhibit membrane attack complex formation as tested in a hemolysis assay. Our results suggest that eculizumab functions by sterically preventing C5 from binding to convertases and explain the exquisite selectivity of eculizumab for human C5 and how polymorphisms in C5 cause eculizumab-resistance in a small number of patients with paroxysmal nocturnal hemoglobinuria.

Supported Lipid Bilayer Platform To Test Inhibitors of the Membrane Attack Complex: Insights into Biomacromolecular Assembly and Regulation

Complement activation plays an important role in innate immune defense by triggering formation of the membrane attack complex (MAC), which is a biomacromolecular assembly that exhibits membrane-lytic activity against foreign invaders including various pathogens and biomaterials. Understanding the details of MAC structure and function has been the subject of extensive work involving bulk liposome and erythrocyte assays. However, it is difficult to characterize the mechanism of action of MAC inhibitor drug candidates using the conventional assays. To address this issue, we employ a biomimetic supported lipid bilayer platform to investigate how two MAC inhibitors, vitronectin and clusterin, interfere with MAC assembly in a sequential addition format, as monitored by the quartz crystal microbalance-dissipation (QCM-D) technique. Two experimental strategies based on modular assembly were selected, precincubation of inhibitor and C5b-7 complex before addition to the lipid bilayer or initial addition of inhibitor followed by the C5b-7 complex. The findings indicate that vitronectin inhibits membrane association of C5b-7 via a direct interaction with C5b-7 and via competitive membrane association onto the supported lipid bilayer. On the other hand, clusterin directly interacts with C5b-7 such that C5b-7 is still able to bind to the lipid bilayer, and clusterin affects the subsequent binding of other complement proteins involved in the MAC assembly. Taken together, the findings in this study outline a biomimetic approach based on supported lipid bilayers to explore the interactions between complement proteins and inhibitors, thereby offering insight into MAC assembly and regulation.

Membrane attack complex generation increases as a function of time in stored blood

OBJECTIVE

To determine if the complement system, a potent mediator of inflammation, contributes to haemolysis during red blood cell (RBC) storage.

BACKGROUND

RBCs in storage undergo structural and biochemical changes that may result in adverse patient outcomes post-transfusion. Complement activation on leukodepletion and during storage may contribute to the RBC storage lesion.

METHODS/MATERIALS

We performed a cross-sectional analysis of aliquots of leukoreduced RBC units, stored for 1-6 weeks, for the levels of C3a, C5a, Bb, iC3b, C4d and C5b-9 [membrane attack complex (MAC)] by enzyme-linked immunosorbent assay (ELISA).

RESULTS

We observed that only MAC levels significantly increased in RBC units as a function of storage time. We also observed that the level of C5b-9 bound to RBCs increased as a function of storage time.

CONCLUSION

MAC levels increased over time, suggesting that MAC is the primary complement-mediated contributor to changes in stored RBCs. Inhibition of the terminal complement pathway may stabilise RBC functionality and extend shelf life.

Solution structure of factor I-like modules from complement C7 reveals a pair of follistatin domains in compact pseudosymmetric arrangement

Factor I-like modules (FIMs) of complement proteins C6, C7, and factor I participate in protein-protein interactions critical to the progress of a complement-mediated immune response to infections and other trauma. For instance, the carboxyl-terminal FIM pair of C7 (C7-FIMs) binds to the C345C domain of C5 and its activated product, C5b, during self-assembly of the cytolytic membrane-attack complex. FIMs share sequence similarity with follistatin domains (FDs) of known three-dimensional structure, suggesting that FIM structures could be reliably modeled. However, conflicting disulfide maps, inconsistent orientations of subdomains within FDs, and the presence of binding partners in all FD structures led us to determine the three-dimensional structure of C7-FIMs by NMR spectroscopy. The solution structure reveals that each FIM within C7 contains a small amino-terminal FOLN subdomain connected to a larger carboxyl-terminal KAZAL domain. The open arrangement of the subdomains within FIMs resembles that of first FDs within structures of tandem FDs but differs from the more compact subdomain arrangement of second or third FDs. Unexpectedly, the two C7-FIMs pack closely together with an approximate 2-fold rotational symmetry that is rarely seen in module pairs and has not been observed in FD-containing proteins. Interfaces between subdomains and between modules include numerous hydrophobic and electrostatic contributions, suggesting that this is a physiologically relevant conformation that persists in the context of the parent protein. Similar interfaces were predicted in a homology-based model of the C6-FIM pair. The C7-FIM structures also facilitated construction of a model of the single FIM of factor I.

The James Blundell Award Lecture 2007: do we really understand immune red cell destruction?

We have learned a great deal about immune red blood cell (RBC) destruction since the elaboration of biochemical/immunological interactions of antibodies, complement and macrophages during the past 50 years. We first learned about the direct lysis of RBCs involving complement. We then learned of the role of the macrophage (particularly in the spleen and the liver) in initiating phagocytosis and antibody-dependent cytotoxicity of antibody-coated RBCs. Later, as the complexities of the human complement system were unravelled, we learned that complement-coated RBCs that were not directly haemolysed could interact with macrophages and that specific complement molecules on the RBC membrane could lead to a phagocytic event or the RBC (although heavily coated with complement) could survive normally. The application of isotope-labelling procedures (e.g. (51)Cr) for RBC survival (starting in the 1950s) advanced our knowledge considerably. Advances in knowledge in immunology helped us understand the complexity of the immunoglobulins (e.g. subclasses) and the specific receptors on macrophages and their role in immune haemolysis. Nevertheless, after more than 30 years researching this area, I am sometimes embarrassed to realize how much I cannot explain. Why do some patients have severe haemolytic transfusion reactions because of antibodies that are only detectable by one technique or not detectable by any? How do we explain autoimmune haemolytic anaemia with negative direct antiglobulin tests (DATs)? Why do RBCs strongly coated with immunoglobulin (Ig)G1 or IgG3 sometimes have normal survival? Are cells, other than macrophages, involved in immune RBC destruction? Could the relative amount of cytotoxicity vs. phagocytosis explain different clinical findings and response to treatment? How do we explain 'hyperhaemolysis' in sickle cell disease? Could novel mechanisms involving IgG glycosylation, CD47, 'armed' macrophages, bystander lysis, antibody activated reactive oxygen species, natural killer cells or antibody perturbation of RBC membrane be involved? Why do RBCs die after circulating for 100-120 days in healthy individuals? How should we define a 'clinically significant' antibody; how do we evaluate this? So many questions, so little time!

Bystander Immune Cytolysis

In addition to alloimmune and autoimmune cell lysis, a third category of immune destruction of blood cells should be recognized. This additional immunologic response occurs when cells or tissues are injured by immunologic reactions in which the cells act as "innocent bystanders." One mechanism by which an immune response to an exogenous antigen leads to the destruction of autologous blood cells is the temporary development of autoantibodies. This is actually an alloimmune reaction which results in a temporary state of "pseudo"-autoimmunity. Although originally described as a type of hemolysis of autologous cells, the concept of bystander immune cytolysis has been extended to include other instances in which immune destruction of cells is caused by antibody that is not developed in response to intrinsic antigens on the cell being lysed. In recent years, compelling data have been presented documenting bystander immune cytolysis in a number of different clinical settings, and efforts have been made to define the mechanisms by which this occurs. Physicians must be aware that some examples of immune lysis of autologous cells are, in reality, examples of temporary bystander immune cytolysis rather than true autoimmune disease. Furthermore, some alloimmune hemolytic reactions can result in lysis of bystander cells.

Complement Activation During Hemorrhagic Shock and Resuscitation in Swine

Activation of the complement (C) cascade is known to play a key role in the adverse immune consequences of hemorrhagic trauma with subsequent shock and resuscitation. However, it is not clear whether hypovolemia per se, without trauma and resuscitation, can also lead to C activation. To address this question, we studied the presence, kinetics, and cause of C activation in a porcine model of hemorrhagic shock and resuscitation in the absence of trauma. Pigs were bled to and kept at 35 mmHg for 90 min, followed by hypotensive resuscitation with different fluids and, finally, with shed blood. The animals developed severe lactic acidosis between 30 and 90 min, which was accompanied by a trend for initial rise and subsequent 40% drop of CH50/mL, indicating massive C activation even before resuscitation, i.e., before reperfusion damage could have occurred. Resuscitation with plasma expanders caused 20% additional C consumption, whereas whole blood raised CH50/mL. Plasma C5a decreased initially and then significantly increased at 60 and 180 min, whereas thromboxane B2 showed a 3-fold increase at 30 and 60 min. Plasma LPS was also increased above baseline at 90 and 180 min. In in vitro studies with pig blood, spontaneous C5a formation, as well as zymosan-induced C consumption, was significantly enhanced under the conditions of lactic acidosis. Our data suggest that lactic acidosis, endotoxemia, and possibly other ischemia-related tissue alterations act in a vicious cycle in inducing C activation and, hence, aggravation of shock. The biphasic course of CH50/mL and C5a changes may reflect yet unrecognized physiological responses to hemorrhage-related C activation.

Autoimmunity to tropomyosin isoforms in ulcerative colitis (UC) patients and unaffected relatives

Autoimmunity to cytoskeletal protein tropomyosin (TM) has been demonstrated in UC. However, the TM isoforms involved in this IgG-mediated autoimmune response in UC and the possible presence of serum IgG antibodies against TM (hTMs IgG) in unaffected UC relatives are unknown. The aim of this study was to investigate the human TM (hTM) isoforms recognized by serum IgG from UC and to explore whether hTM IgG antibodies are present in healthy UC relatives. We studied 33 UC patients with 58 unaffected relatives, 31 Crohn's disease (CD) patients with 31 unaffected relatives and 20 controls (C). Serum IgG against four recombinant hTM isoforms (hTM1, 2, 3, 5) were tested by ELISA. p-ANCA were tested by ELISA and immunofluorescence. Serum hTM1 and hTM5 IgG were higher in UC patients than in CD and C (P<0.005). Among UC patients 52% were seropositive for hTM1 and 64% for hTM5 (P<0.001 versus CD and C). In UC, hTM5 IgG were higher in p-ANCA+ than in ANCA- patients (P=0.04). In UC relatives hTM1 IgG were higher than in CD relatives and C (P<0.01). UC relatives were more frequently seropositive for hTM1 than hTM5 IgG (P=0.001). while probands were more frequently seropositive for hTM5 IgG (P=0.008). We conclude that autoimmunity to hTMI and hTM5 is a feature of UC, while hTM1 IgG differentiate UC relatives from controls. A genetic susceptibility to immune recognition of hTM isoforms in UC is suggested.

Specificity of the thioester-containing reactive site of human C3 and its significance to complement activation

The specificity of the thioester-containing site in three plasma proteins is regulated by elements of their protein structures other than the thioester bond itself. Human C4A and alpha 2-macroglobulin preferentially form amide linkages while human C3 primarily forms ester linkages with hydroxyl groups. We have examined the thioester in C3 and found evidence of strong preferences for certain carbohydrates, indications of selectivity for specific positions on those carbohydrates and a preference for terminal sugars in polysaccharides. A testable set of rules are derived from these findings which predict preferred attachment sites on polysaccharides. A computer model of the effect of different reactivities on activation of the alternative pathway of complement suggested that organisms might greatly alter their susceptibility to complement with small changes in carbohydrate structure. While a random selection of 20 biological particles showed no correlation between activation and C3b attachment efficiency, subsets of related organisms differing primarily in their surface polysaccharide exhibited stronger correlations. The strongest correlation occurred in a series of the yeasts (Cryptococcus neoformans) possessing capsular polysaccharides with one, two, three or four branching xylose sugars per repeating unit. These organisms exhibited capture efficiencies for metastable C3b from 12% (one-xylose strain) to 41% (four-xylose strain).

C5b-9 increases albumin permeability of isolated glomeruli in vitro

Deposition of antibody and activation of the complement cascade are important in both naturally occurring glomerulonephritis and in experimental models including passive Heymann nephritis. We studied the effect of antibody and complement on albumin permeability of isolated glomeruli to determine the role of the terminal complement components (C5-C9) in mediating the proteinuria in nephritis. Isolated glomeruli were treated with anti-Fx1a (Heymann antibody) and then incubated them with pooled human serum, serum in which complement had been inactivated by heat, or serum deficient in C6 or C7. The albumin reflection coefficient (sigma albumin) was calculated from the volumetric response of glomeruli to transcapillary oncotic gradients produced by albumin or high molecular weight neutral dextran (252 kD). Convectional permeability to albumin (Palbumin) was calculated as 1-sigma albumin. Albumin permeability of control glomeruli was not different from 0. Albumin permeability was not altered by antibody alone but was increased to 0.65 +/- 0.04 when antibody treated glomeruli were incubated for 10 minutes with pooled serum as a source of complement. Heat treatment of serum to inactivate complement prevented the increase in permeability. Incubation for 10 minutes with serum without antibody pretreatment caused a lesser increase in permeability of isolated glomeruli (0.18 +/- 0.06). Serum deficient in either C6 or C7 did not cause an increase in albumin permeability of antibody pre-treated glomeruli, but incubation with a combination of these sera (now containing the complete cascade) increased permeability to the same extent as did pooled normal serum (0.58 +/- 0.04).(ABSTRACT TRUNCATED AT 250 WORDS)

Mutations in the putative lipid-interaction domain of complement C9 result in defective secretion of the functional protein

Complement protein C9 assembles with C5, C6, C7, C8 on the surface of target cells to form the lytic membrane attack complex (MAC). During MAC assembly and insertion into the target membrane, the hydrophilic, globular C9 partially unfolds to expose a hydrophobic lipid interaction domain. Several copies of amphiphilic C9 subsequently polymerize to form the characteristic ring-like MAC. Using a combined photoaffinity label and computer modeling approach, two amphipathic helices in a segment encompassing the amino acids 293-334 have been predicted to interact with membrane lipids. To elucidate the mechanism of C9 lipid binding and insertion, site-directed mutagenesis was used to change the amphipathic character of the helices. While some conservative amino acid replacements such as Thr307 by a Leu were tolerated and yielded fully active C9 when expressed in COS cells, successive changes of Leu305 into Val, Ala, and Glu on the hydrophobic site of the first helix gave rise to only partly or not secreted C9. All non-conservative amino acid replacements introduced on either side of the helices resulted in non-secreted C9 that was subsequently degraded intracellularly, indicating the importance of the correct folding of the presumptive transmembrane domain during biosynthesis. A natural secretion-incompetent mutant was found in which Val293, located in the proposed lipid-binding region, was lacking. Taken together, these findings suggest that the high incidence of homozygous C9 deficiencies may be due to a blockage in intracellular transport and secretion due to point mutations in this 'hot spot' region of the molecule.

Local complement activation in inflammatory bowel disease

To trace sites for local complement activation in inflammatory bowel disease, an indirect two-colour immunofluorescence method was applied on prewashed and directly ethanol-fixed mucosal specimens from patients with ulcerative colitis, Crohn's colitis, or terminal ileitis. Monoclonal antibodies to the IgG subclasses and to neoepitopes of activated complement C3b and the terminal complement complex (TCC) were used in combination with rabbit antiserum to immunoglobulins and various complement components. Deposits of activated C3b were found on the luminal face of the surface epithelium in the most affected ulcerative colitis specimens from 91% of 23 studied patients, together with cytolytic TCC in 81%. Furthermore, there was a selective deposition of the immunoglobulin G subclass 1 (IgG1) within the epithelial immune complexes in 63% of 11 studied patients. These results suggested that IgG1 autoantibodies to brush-border antigen(s) induce a complement-mediated attack on the epithelium in ulcerative colitis. The epithelial complement deposition seen in Crohn's disease tended to be more granular and was observed in 5 of 10 patients with colitis and in 4 of 10 with ileitis. No co-localization of IgG was observed, suggesting that complement activation had been induced by the alternative pathway. Type III immune reaction may, in addition, take place in both diseases since there was evidence of continuous vascular complement activation in submucosal blood vessels.

Immunosuppression in Trauma Patients

An immunosuppressed state develops following traumatic injury, which makes patients more prone to develop infection. A variety of disturbances accompany injury that affect both specific and nonspecific components of host defense. Many clinical studies have attempted to evaluate the many deficits that follow injury and place the patient at a higher risk for infection. Several components of host defense are affected simultaneously and include (1) cellular changes (decreased activation of T-lymphocyte subsets with decreased helper cells, increased suppressor T-cell function, increased but abnormal activity of macrophages, activation of polymorphonuclear leukocytes with depressed chemotaxis and killing); (2) depressed nonspecific and specific serum immunity (e.g., depressed fibronectin and immunoglobulin levels); (3) the presence of altered cytokine levels (interleukin-1 [IL-1], IL-2, IL-6, tumor necrosis factor) levels; (4) ongoing serum proteolytic activity; and (5) the generation of serum suppressive peptides. An in-depth understanding of the deficits that occur following injury in host defense will provide the basis for therapeutic intervention.

Lysis of complement-sensitive Entamoeba histolytica by activated terminal complement components. Initiation of complement activation by an extracellular neutral cysteine proteinase

Activation of complement by Entamoeba histolytica may be initiated by the extracellular 56-kD neutral cysteine proteinase which cleaves the alpha chain of C3. To determine the relationship between the fluid-phase activation of complement and our observation that only strains isolated from patients with invasive disease are resistant to complement-mediated lysis, we investigated the fate of C3 with recent amebic isolates. When 125I-C3 was incubated with trophozoites in serum, C3 in the fluid phase was cleaved to C3b or C3bi, but the alpha chain of the C3 molecules on the cell surface appeared intact. Since the lysis of nonpathogenic strains takes place in the absence of bound C3b, we demonstrated that this reaction occurs by reactive lysis initiated in the fluid phase: (a) the killing of nonpathogenic strains was enhanced when alternative pathway activation was accelerated by the addition of cobra venom factor; (b) non-pathogenic strains were lysed by purified terminal components; and (c) sera incubated with pathogenic E. histolytica produced passive lysis of chicken erythrocytes. These results demonstrate for the first time that complement-sensitive E. histolytica are lysed by activation of the terminal complement components in the fluid phase where the 56-kD neutral cysteine proteinase cleaves C3, and not by the surface deposition of activated C3.

Epithelial deposition of immunoglobulin G1 and activated complement (C3b and terminal complement complex) in ulcerative colitis

The epithelial destruction seen in ulcerative colitis remains unexplained. Complement activation has been proposed to be involved, but no definite evidence has been available to this end. In the present study, we examined immunohistochemically ulcerative colitis lesions with monoclonal antibodies to activation neoepitopes in the complement component C3b and in the cytolytically active terminal complement complex. Colonic tissue specimens from 23 patients with ulcerative colitis were examined by indirect two-color immunofluorescence staining with monoclonal antibodies to the four human immunoglobulin G subclasses and to activated complement C3b or terminal complement complex. All except two patients had activated C3b deposited apically on the surface epithelium of involved mucosa. Immunoglobulin G1 was found on the epithelium in extensively prewashed specimens from 7 of 11 patients, and a striking colocalization of immunoglobulin G1, C3b, and terminal complement complex was observed in 4. Immune deposits were not observed in 31 noninflamed specimens from the same ulcerative colitis patients. Only 1 of 44 histologically normal mucosae from 17 controls and 1 of 10 colonic adenomas contained some epithelial complement deposits. It is concluded that activated complement is often deposited along the brush border of the surface epithelium in active ulcerative colitis lesions and may be associated with immunoglobulin G1 autoantibody.

Studies on serum resistance in Escherichia coli

Serum-sensitive mutants and their serum-resistant smooth parental E. coli strains (Wf8, Wf26, and WF 52) have been investigated in respect to their binding of different complement components. These pairs consisting of a wild-type and its mutants represent a better model for the investigation of the mechanism of serum resistance than the comparison of unrelated strains. Both strains of a pair bind equivalent amounts of C3. In binding assays using radiolabeled terminal components C6, C7, C8, and C9, the serum-sensitive strains do bind more late acting components than their resistant parental strains. An active membrane attack complex stably bound to the cell surface was found on the mutants, whereas with wild-type bacteria a complex could be isolated from the supernatant which is composed of the late acting complement components and S-protein. This complex is released from the surface of the wild-type bacteria without participation of C9.

Specific deposition of complement protein C3b on abnormal PNH erythrocytes permits their separation by partitioning. Possible general approach for isolation of specific cell populations

The deposition of complement proteins on a cell surface has previously been shown to reduce the cell's partition ratio in a two-polymer aqueous phase system. This phenomenon has now been extended to segregate, by partitioning, subpopulations of erythrocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH). Purified components of the complement system were employed to deposit the protein C3b specifically on abnormal erythrocytes which lacked the membrane-associated complement regulatory protein DAF. As few as 2100 C3b/cell reduced the partition ratio and 24,000 C3b/cell resulted in resolution of the C3b-bearing and non-bearing human red cells. It was found that the proportion of cells separated did not equal the proportion of cells lysed by complement in the acidified serum lysis test when blood from three of the five patients was examined. The results indicate that the defect giving rise to DAF- cells may be, but is not necessarily, coexpressed with defects affecting other membrane-associated regulatory factors. A broader application of the method using monoclonal antibodies to direct purified complement components to specific cell populations should permit their isolation in large quantities.

Interactions of the platelets in paroxysmal nocturnal hemoglobinuria with complement. Relationship to defects in the regulation of complement and to platelet survival in vivo

The blood cells of patients with paroxysmal nocturnal hemoglobinuria (PNH) have abnormal interactions with complement. The activity of the alternative pathway C3 convertase on the platelets of 9 out of 19 patients with PNH was elevated. 10 patients had C3 convertase activity within the normal range even though 80-95% of their platelets lacked the complement regulatory protein decay accelerating factor (DAF) that is absent from the affected blood cells in PNH. PNH and normal platelets released factor H when C3 was bound to their surfaces. This may account for the apparent regulation of C3 convertase activity on platelets that lack DAF. The abnormal uptake of the membrane attack complex of complement by PNH III erythrocytes was not seen in PNH platelets. 111Indium-labeled platelet survival times were normal in five of eight patients, which suggests that the lack of the membrane attack complex defect results in normal platelet survival in PNH.

Acute myeloblastic leukemia in paroxysmal nocturnal hemoglobinuria. Evidence of evolution from the abnormal paroxysmal nocturnal hemoglobinuria clone

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematopoietic stem cell disorder in which the blood cells demonstrate aberrant interactions with serum complement. In part, this is due to the absence of the complement regulatory protein, decay accelerating factor (DAF). A small number of patients with PNH have gone on to develop acute nonlymphocytic leukemia, which is thought to arise from the injured marrow as a second hematopoietic disorder. We have studied a patient with PNH who developed acute myeloblastic leukemia (AML); the blasts from this patient were found to lack DAF as measured by polyclonal antibody binding and fluorescence flow cytometry as well as by immunoblotting. The blasts from 11 other patients with AML bound anti-DAF antibody in amounts similar to normal mononuclear cells from healthy donors. Cells of the human leukemia cell lines HL-60, K562, U937, and HEL also bound anti-DAF antibody. In addition to DAF deficiency, blasts from the PNH patient had undetectable alkaline phosphatase activity, in contrast to human leukemia cell lines. These data suggest that the leukemic cells of the PNH patient arose out of the PNH clone and that AML in the setting of PNH is not a separate disorder.

The activation of C5 in the fluid phase and in the absence of C3 through the classical pathway of the complement system

Unsensitized guinea-pig erythrocytes (Egp) were lysed by a combination of eight isolated, human-derived complement components, Cls, C4, C2, C5, C6, C7, C8 and C9 (Cls-C9exC3), even in the presence of anti-C3. It was determined that a factor was generated in the reaction mixture of Cls, C4, C2, C5 and C6, which had a lytic activity against Egp when C7, C8 and C9 were added. The lytic factor was similar to C56 in the following properties: the activity of the lytic factor decreased when incubated with C7 prior to its reaction with Egp, the lytic factor did not bind to Egp by itself but it did bind in the presence of C7, EDTA did not have any inhibitory effect on the lytic factor, and the activity of the lytic factor was lost by treatment with anti-C5 or anti-C6 but not by treatment with anti-C4. Furthermore, C5a, a cleavage product of C5, was clearly detected in the reaction mixture of Cls, C4, C2 and C5. These findings indicate that C5 can be activated proteolytically into C5a and C5b in the fluid phase solely by the classical pathway C3 convertase, C42, without any participation of C3.

Adsorption on B cell hybridomas removes suppressor cells from spleen cells of neonatally tolerized mice

Spleen cells from CBA mice neonatally tolerized to Dd MHC (major histocompatibility complex) determinants were adsorbed on monolayers of Dd-specific B cell hybridomas. Adsorption on 4 different Dd-specific hybridomas but not on a Kd-specific hybridoma removed suppressor cells and resulted in generation of cytotoxic T lymphocyte (CTL) effector cells against the tolerogen. Responses of normal CBA CTL against Dd as well as anti-third party responses were not influenced by adsorption. Successful adsorption was also achieved on monolayers of fixed hybridoma cells. Monolayers of Dd-specific hybridomas specifically removed suppressor cells from CBA mice tolerant to Dd; they failed to adsorb suppressor cells from AKR mice tolerant to H-2b. Immunofluorescence analysis with antibodies specific for determinants on tolerizing cells showed the presence of donor-derived F1 cells on the monolayer. Under the experimental circumstances described suppressor cells are most likely removed by recognition of their MHC determinants by the monolayer population. These data suggest that neonatal tolerance is actively maintained by donor-derived suppressor cells.

The acute phase response of C3, C5, ceruloplasmin, and C-reactive protein induced by turpentine pleurisy in the rabbit

Concentrations of five serum proteins, C3, C5, ceruloplasmin, C-reactive protein, and albumin, have been measured during the acute phase response in rabbits with turpentine-induced pleurisy. C-reactive protein concentrations in the circulation rose abruptly between 12 and 36 hours to a level greater than 50 times the pretreatment concentration, then returned to undetectable amounts by 96 hours. C3 and ceruloplasmin both showed some increase in concentration by 12 hours and reached their maximum concentrations of two to three times the baseline levels 48-72 hours after the turpentine treatment. Concentrations were still elevated at 120 hours, after which time they gradually returned to normal. C5 and albumin concentrations in the turpentine-treated rabbits did not differ from the baseline concentrations. The same five proteins were measured in the inflammatory exudate. C-reactive protein was not detectable at any of the time points. C3, C5, ceruloplasmin, and albumin were present in normal pleural fluid at roughly half their serum concentrations. The activities of C3, C5, and ceruloplasmin were low in the early exudate, but C3 and C5 activity rose relative to their concentrations in the later samples of pleural fluid. The specific activities of C3 and C5 were higher in the pleural fluid at 72 hours than in plasma, while that of ceruloplasmin remained less in the pleural fluid than in plasma throughout the experiment. The involvement of these proteins and their relation to the inflammatory response are discussed.

Characterization of the complement sensitivity of paroxysmal nocturnal hemoglobinuria erythrocytes

The affected erythrocytes of paroxysmal nocturnal hemoglobinuria (PNH II and PNH III cells) are abnormally sensitive to complement-mediated lysis. Normal human erythrocytes chemically modified by treatment with 2-amino-ethylisothiouronium bromide (AET) have been used as models for PNH cells inasmuch as they also exhibit an enhanced susceptibility to complement. To investigate the bases for the greater sensitivity of these abnormal cells to complement-mediated lysis, we compared binding of C3 and constituents of the membrane attack complex to normal, PNH II, PNH III, and AET-treated cells after classical pathway activation by antibody and fluid-phase activation by cobra venom factor complexes. When whole serum complement was activated by antibody, there was increased binding of C3 and C9 to PNH II, PNH III, and AET-treated cells, although the binding of these complement components to PNH II and PNH III cells was considerably greater than their binding to the AET-treated cells. In addition, all of the abnormal cell types showed a greater degree of lysis per C9 bound than did the normal erythrocytes. PNH III and AET-treated cells were readily lysed by fluid-phase activation of complement, whereas normal and PNH II erythrocytes were not susceptible to bystander lysis. The greater hemolysis of PNH III and AET-treated cells in this reactive lysis system was due to a quantitative increase in binding of constituents of the membrane attack complex. This more efficient binding of the terminal components after fluid-phase activation of whole serum complement was not mediated by cell-bound C3 fragments. These investigations demonstrate that the molecular events that characterize the enhanced susceptibility of PNH II, PNH III, and AET-treated erythrocytes to complement-mediated lysis are heterogeneous.

Covalent binding efficiency of the third and fourth complement proteins in relation to pH, nucleophilicity, and availability of hydroxyl groups

The binding of [3H]glycerol and [3H]putrescine to C3 was studied in a fluid-phase system using trypsin as the C3 convertase. The binding of glycerol showed little variation in the pH range between 6.0 and 10.0. The binding of putrescine (pKa = 9.0) is rather ineffective below pH 7.5 but becomes more efficient as the pH of the reaction mixture increases. These results agree with the contention that the final step of the binding reaction is the transfer of the acyl group of the exposed thio ester of C3 to a nucleophile since the nucleophilicity of hydroxyl groups is rather independent of pH whereas only the unprotonated form of amino groups is nucleophilic. The inefficient reaction of amino groups with the exposed thio ester of C3 is also supported by the study of the inhibitory activity of serine and its two derivatives, N-acetylserine and O-methylserine, to the binding of [3H]glycerol to C3. N-Acetylserine showed an inhibitory activity equivalent to that of serine, whereas O-methylated serine showed only minimal activity. It can be concluded, therefore, that serine reacts with the thio ester of C3 by its hydroxyl group but not by its alpha-amino group. The ability of the alcohol group of various alkanes to inhibit the binding of [3H]glycerol to C3 was also studied. The primary alcohols inhibit the binding reaction with an efficiency that is similar to glycerol, and there are no significant differences in the binding efficiencies of methanol, ethanol, 1-propanol, and 1-butanol.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane attack by complement

Membrane attack by complement involves the self-assembly on membranes of five hydrophilic proteins (C5b, C6, C7, C8 and C9) to an amphiphilic tubular complex comprising approximately 20 subunits. The hydrophilic-amphiphilic transition of the precursor proteins is achieved by restricted unfolding and exposure of previously hidden hydrophobic domains. Restricted unfolding, in turn, is driven by high-affinity protein-protein interactions resulting in the formation of amphilic complexes. Circular polymerization of C9 to a tubular complex (poly C9) constitutes the molecular mechanism for transmembrane channel assembly and formation of ultrastructural membrane lesions.

Abnormality of glycophorin-alpha on paroxysmal nocturnal hemoglobinuria erythrocytes

To investigate the greater enzymatic activity of the alternative pathway convertase (and the subsequent greater fixation of C3b) on paroxysmal nocturnal hemoglobinuria (PNH) erythrocytes, we have examined the topography of binding of C3b to PNH and normal erythrocytes. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, the alpha-chain of C3b was found to bind via predominantly ester bonds to free hydroxyl groups on glycophorin-alpha, the major erythrocyte sialoglycoprotein. The pattern of binding of nascent C3b was the same for normal and PNH erythrocytes. Thus, although C3b binding to a different membrane constituent did not appear to account for the greater enzymatic activity of the alternative pathway convertase when affixed to PNH erythrocytes, it seemed possible that the glycoproteins to which C3b bound might be qualitatively abnormal on the PNH cells, and that structural differences in these molecules might impose modifications in the enzyme-substrate interactions of the alternative pathway convertase. Using methods for radiolabeling both protein and carbohydrate residues, we therefore compared the electrophoretic pattern of the cell-surface glycoproteins on PNH and normal erythrocytes. The glycophorin-alpha dimer was found to be qualitatively abnormal on the PNH cells as evidenced by its greater susceptibility to trypsin-mediated proteolysis. In addition, the abnormal erythrocytes from patients with PNH had fewer periodate oxidizable constituents than did normal erythrocytes, indicating a relative deficiency of cell-surface sialic acid. These investigations suggest that abnormalities in membrane glycoproteins may underlie the aberrant interactions of complement with the hematopoietic elements of PNH.

Deficiency of an erythrocyte membrane protein with complement regulatory activity in paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic anemia in which the erythrocytes are abnormally sensitive to lysis by complement. A functional deficiency of membrane-associated complement regulators has been demonstrated on PNH erythrocytes. The two factor H-like proteins, the C3b receptor (CR1) and the decay-accelerating factor (DAF), were isolated from normal human erythrocytes, and specific antisera were prepared. Selective inhibition of the two proteins on normal erythrocytes by the antisera demonstrated (i) that the factor responsible for accelerated decay of erythrocyte-bound C3 convertase is DAF and (ii) that the cofactor required for inactivation of erythrocyte-bound C3b by factor I is CR1. PNH erythrocytes were deficient in both of these activities. Erythrocytes deficient in CR1, which were obtained from an apparently healthy individual, exhibited normal DAF activity but no factor I cofactor activity. These cells were not susceptible to complement-mediated lysis in acidified human serum, whereas PNH erythrocytes and Pronase-treated human erythrocytes (which lack DAF and CR1 activities) were lysed by this treatment. It is suggested that the protein primarily responsible for preventing complement activation on normal human erythrocytes is DAF. AMr 73,000 protein isolated from the normal erythrocyte membranes of one PNH patient by using anti-DAF IgG was largely absent from the abnormal erythrocytes of this individual, suggesting that PNH cells lack the DAF protein. CR1 antigen, however, was present on the abnormal PNH erythrocytes. The results suggest that the primary molecular defect underlying the clinical manifestations of PNH may be the lack of the membrane-associated DAF protein and that the abnormal cells may also exhibit impaired CR1 function.

Neoantigen of the polymerized ninth component of complement. Characterization of a monoclonal antibody and immunohistochemical localization in renal disease

A monoclonal antibody to a neoantigen of the C9 portion of the membrane attack complex (MAC) of human complement has been developed and characterized. The distribution of this neoantigen was assessed by indirect immunofluorescence microscopy in nephritic and nonnephritic renal diseases. The antibody (Poly C9-MA) reacted on enzyme-linked immunosorbent assay (ELISA) with a determinant in complement-activated serum that was undetectable in normal human serum (NHS). Zymosan particles incubated in NHS had positive immunofluorescent staining with Poly C9-MA; however, binding of Poly C9-MA was not observed with zymosan particles incubated in sera deficient in individual complement components C3, C5, C6, C7, C8, or C9. Reconstitution of C9-deficient sera with purified C9 restored the fluorescence with Poly C9-MA. Poly C9-MA reacted positively by ELISA in a dose-dependent manner with purified MC5b-9 solubilized from membranes of antibody-coated sheep erythrocytes treated with NHS but not with intermediate complement complexes. Poly C9-MA also reacted in a dose-dependent manner on ELISA and in a radioimmunoassay with polymerized C9 (37 degrees C, 64 h) (poly C9) but not with monomeric C9. Increasing amounts of either unlabeled poly C9 or purified MC5b-9 inhibited the 125I-poly C9 RIA in an identical manner. These studies demonstrate that Poly C9-MA recognizes a neoantigen of C9 common to both the MAC and to poly C9. By immunofluorescence, Poly C9-MA reacted minimally with normal kidney tissue in juxtaglomerular loci, the mesangial stalk, and vessel walls. Poly C9-MA stained kidney tissue from patients with glomerulonephritis in a pattern similar to that seen with polyclonal anti-human C3. In tissue from patients with nonnephritic renal disease--diabetes, hypertension, and obstructive uropathy--Poly C9-MA was strongly reactive in the mesangial stalk and juxtaglomerular regions, tubular basement membranes, and vascular walls. Poly C9-MA binding was especially prominent in areas of advanced tissue injury. Poly C9-MA frequently stained loci where C3 was either minimally present or absent. These studies provide strong evidence for complement activation not only in nephritic but also in nonnephritic renal diseases.

Localization of the membrane attack complex (MAC) in experimental immune complex glomerulonephritis

The role of the membrane attack complex (MAC) as a mediator of renal tissue injury was evaluated in rats affected by bovine serum albumin (BSA)-induced immune complex glomerulonephritis. Immunofluorescence studies revealed concurrent deposits of IgG, BSA, C3, and the MAC along glomerular capillary walls, although the MAC manifested a more restricted distribution than that observed for immune complexes. Immunoelectron microscopic techniques were utilized to demonstrate immune complexes, C3, and the MAC within dense deposits in the subepithelial aspect of the basement membrane. Visceral epithelial foot processes were fused in areas overlying large dense deposits and exhibited intense staining for the MAC, lesser reactivity for C3 but IgG was absent from the foot process membranes. Smaller granular deposits of immune complexes, C3, and the MAC were observed in the subendothelial region of the lamina rara interna and the lamina densa. Immune complexes may activate the classical complement pathway causing diffuse injury to the glomerular basement membrane (GBM), allowing subepithelial accumulation of complexes. These observations implicate the MAC as a mediator of GBM and juxtaposed podocyte membrane injury, thereby contributing to disruption of the glomerular filtration barrier. IgG and C3 were demonstrated within tubulointerstitial regions on the surface of collagen fibers in close proximity to the tubular basement membrane (TBM) of proximal convoluted tubules. Within the TBM, C3 localization was prominent with diminished reactivity for the MAC, but IgG was not detectable. The demonstration of C3 and scant MAC deposits in the TBM of nonimmunized control rats without evidence of interstitial IgG and C3 deposits suggests that both nonimmune and immune processes play a role in the pathogenesis of extraglomerular lesions. Evidence derived from these morphologic studies indicates that the MAC is associated with injury to the GBM, foot process membranes of visceral epithelium, and the TBM. Further experiments designed to selectively enhance or inhibit the deposition of MAC and assess consequent renal dysfunction are required to substantiate hypotheses concerning the in vivo membranolytic potential of the MAC in experimental immune complex glomerulonephritis.

Paroxysmal nocturnal hemoglobinuria: deficiency in factor H-like functions of the abnormal erythrocytes

Erythrocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH) contained a subpopulation that lacked membrane-associated Factor H-like activity present on normal human erythrocytes. Initial deposition of C3b on the erythrocytes was effected using a fluid phase C3 convertase. The cells were then treated with fluorescein-labeled C3 and the cell-bound C3 convertase. Analysis utilizing the fluorescence-activated cell sorter revealed two distinct cell populations, one of which was highly fluorescent, indicating a large number of C3b molecules per cell. Only this population (43%) was susceptible to lysis (44%) when exposed to acidified serum before C3b deposition. The less fluorescent population resembled normal human erythrocytes and was not affected by prior treatment with acidified serum. Since C3b deposition occurred almost exclusively on the complement-sensitive cells in the PNH erythrocyte population, these cells could be examined for the Factor H-like regulatory activities without prior isolation. These functions include enhancement of inactivation of erythrocyte-bound C3b by Factor I and acceleration of the decay of erythrocyte-bound C3 convertase, C3b,Bb. It was found that C3b on PNH erythrocytes was 100-fold less susceptible to inactivation by Factor I than C3b on normal human erythrocytes. The half-life at 22 degrees C of C3b,Bb on PNH erythrocytes was threefold greater than on normal human erythrocytes and similar to that of the enzyme bound to particles that do not possess Factor H-like activity. These observations suggest that the abnormal susceptibility of PNH erythrocytes to lysis by complement is due to a functional deficiency in one or more of the Factor H-like proteins present on normal human erythrocytes.