Douglas T. Fearon

The role of antibody in the activation of the alternative complement pathway

The alternative complement pathway can be activated in the absence of specific antibody by the surface of certain cells and particles that provide a microenvironment in which the affinity of the regulatory protein H for surface-bound Cab is low. There is now abundant evidence that antibody can enhance activation. Evidence for a role of antibody independent of its classical-pathway function has accrued from experiments with purified proteins of the alternative pathway, sera genetically deficient in C4 and C2, and chelating agents specific for calcium so as to prevent activation of C1. Three models have been employed to demonstrate a function of antibody in the alternative pathway: complexes of antibody with soluble antigen; antibody directed against naturally occurring particulate activators, including zymosan, rabbit and mouse erythrocytes, the pneumococcus, and several types of virus-infected, mammalian cells [17]; and antibody directed against certain nonactivating particles, such as the streptococcus [17]. Because antibody may exert its effect in these systems by different mechanisms, each system will be discussed separately.

Acquired deficiency of the inhibitor of the first component of complement: Report of five additional cases with commentary on the syndrome

The association of late onset recurrent angioedema with a deficiency of the inhibitor of the first component of complement (C1INH) and of the binding subunit of the first component, Clq, defines the syndrome of acquired C1INH deficiency. The description of five new cases, along with the original two and the 18 others in the literature, brings the total reported cases to 25 and highlights the associated B cell abnormalities that are present in 23 and are of a malignant nature in 19 cases. In three of the five newly reported cases, the occurrence of angioedema, which prompted recognition of the acquired deficiency of C1INH, C1q, and C4, preceded the delineation of the underlying B cell malignancy by 2 to 3 yr despite efforts to recognize neoplastic disease in two of these patients throughout the interval. Because the acquired C1INH deficiency reflects increased catabolism rather than impaired biosynthesis, only high-dose attenuated androgens elicit a measurable increment in serum C1INH. The occurrence of the syndrome with multiple myeloma is noted for the first time.

Tranexamic acid: Preoperative prophylactic therapy for patients with hereditary angioneurotic edema

Abstract Short-term therapy of 96-hr duration with tranexamic acid was prophylactically effective as defined by the absence of attacks of angioedema in 14 patients with hereditary angioedema undergoing 10 dental and 4 general surgical procedures. Eight of the 14 patients had previously undergone dental extractions without prophylactic therapy with antifibrinolytic agents and each had experienced one or more attacks of angioedema. Seven of these 8 patients had a cumulative experience of 13 episodes of laryngeal edema after dental extractions and the eighth had a bout of cutaneous angioedema. Although the number of dental extractions conducted without prophylactic antifibrinolytic therapy cannot be accurately defined in retrospect, the prominence of laryngeal edema in this circumstance is striking when compared with the absence of attacks in the presence of prophylaxis with tranexamic acid. Methyltestosterone and impeded androgens are now known to be effective prophylaxis for spontaneous and, presumably, postoperative attacks when employed chronically because their administration is associated with correction of the biochemical defect of hereditary angioneurotic edema, but their chronic administration to children and women of childbearing age requires further definition because of their potential pituitary suppressive action. Tranexamic acid prophylaxis makes it possible to offer to untreated patients with hereditary angioneurotic edema dental work and other operative procedures that in the past were withheld or conducted with considerable risk.

The Human C3b Receptor

SummaryThe C3b receptor of human erythrocytes, neutrophils, monocytes, all mature B cells, a subpopulation of T cells, and glomerular podocytes is a single chain glycoprotein that exists in two allotypic forms having Mrs of approximately 250,000 (F) and 260,000 (S). The number of receptors present on erythrocytes varies by eight-fold among different individuals and is genetically regulated by two codominant alleles that are distinct from the alleles determining the structural polymorphism. The number of receptors expressed by neutrophils is subject to rapid increases from 5000 per cell to 40,000 per cell by exposure to nanomolar concentrations of C5adesArg, in vitro, and a similar mechanism is probably the basis for observing increased receptor expression on neutrophils in patients undergoing hemodialysis. Cytoskeletal association of the C3b receptor on monocytes and neutrophils is suggested by experiments demonstrating receptor-mediated phagocytosis, adsorptive endocytosis through coated pits, and restricted lateral diffusion, and by the reciprocal co-redistribution of cross-linked C3b and Fc receptors, and the detergent-insolubility of cross-linked C3b receptors. The factor H-like cofactor activity of the C3b receptor promotes the cleavage of bound C3b to iC3b, C3c and C3d, g, reactions that may enhance the clearance of circulating immune complexes and the generation of ligands for CR2 and CR3. The inherited partial deficiency of erythrocyte C3b receptors in patients with SLE, and the absence of glomerular C3b receptors in these patients with proliferative glomerulonephritis may contribute to systemic and organ-specific abnormalities in the clearance of immune complexes that contribute to the pathogenesis of this disease.

Clinical and biochemical effects of impeded androgen (oxymetholone) therapy of hereditary angioedema.

Daily therapy and alternate-day therapy with the attenuated androgen oxymetholone were compared in patients with hereditary angioedema (HAE). Fifteen of 16 patients who experienced at least monthly attacks of HAE without treatment were asymptomatic on administration of 5 mg oxymetholene daily. When 13 of the patients who had been maintained asymptomatically on 5 mg oxymetholone daily were advanced to a treatment schedule of 5 mg every other day, seven attacks occurred during a cummulative 50 mo of therapy. The adverse effects that occurred with daily oxymetholone therapy largely subsided when the patients received alternate-day therapy, while a significant mean rise in C4 protein and function occurred only on daily therapy. Statistically significant mean increases in serum levels of C1INH occurred with daily therapy and were maintained with alternate-day therapy. Clinical benefit can be obtained with a treatment program that does not produce a statistically significant rise in C4 protein or function and does not raise C1INH to the lower limit of normal. The finding that alternate-day therapy diminished the side effects of the drug while affording a substantial reduction in the incidence and severity of attacks indicates the feasibility of this therapeutic approach.

The natural modulation of the amplification phase of complement activation.

As C3 cleavage represents the most critical step in the elaboration of the biologic effects of the complement system, the modulation of this reaction by formation and function of the C3b-dependent C3 convertase may well determine whether the initial activation of the complement sequence eventuates in beneficial or detrimental effects to the host. Stabilization of the amplification C3b-dependent convertase, C3bBb, is achieved with P and C3NeF, respectively, after their binding which exhibits different molecular and temperature requirements. Control of this amplifying step occurs at three levels: intrinsic decay of the inherently labile C3bBb convertase; extrinsic decay by displacement of Bb from the convertase with beta1H; and inactivation by C3bINA of C3b after its generation from native C3 or removal of protective Bb by intrinsic or extrinsic decay. In the presence of the stabilizing factors the control proteins must function in sequence with beta1H-mediated decay preceding C3b inactivation.

INHIBITION OF COMPLEMENT-DERIVED ENZYMES*

To examine the effects of naturally occurring and synthetic inhibitors of complement-derived enzymes. it is necessary to consider the pathways in which these enzymes act. Two distinct protein sequences lcad to activation of the terminal five components of complement (C3-C9) : the classic sequence, which consists of the first (CI ). fourth ( C 4 ) , and second (C2) complement components, and the properdin (alternative) pathway, which consists of the C3 nephritic factor (C3Nef) , properdin ( P ) , factor D. factor B, and C3, and possibly other unidentified factors (FIGURE 1 ) . Initiation of the classic pathway occurs with binding of CI to antigen-antibody complexes in which the antibody is of the IgM class or certain IgG subclasses. CI consists of three subunits, Clq, C l r , and CIS. which form a trimolecular complex in the presence of Ca2+.* After the Clq subunit has bound to the immune complex, C l r is activated to c i r , which, in turn, uncovers the enzymatic site of Cis. C i s had been characterized as a serine esterasc based on its esterolytic activity, susceptibility to irreversible inhibition by diisopropyl phosphorofluoridate ( D F P ) ,, and competitive inhibition of the inactivation of one of its natural substrates, C2, by synthetic amino acid esters. C i s , isolated or as part of the C1 macromolecule, enzymatically cleaves its two-natural substrates, C 4 and C2, the major cleavage products of w h i c h form C42, the bimolecular complex termed the classic C3 convertase. C42 cleaves C3 and then C5 with formation of the terminal pentamolecular complex, C56789, which is responsible for membrane attack.8 The properdin pathway is less well understood, but activation has been shown to occur with complex polysaccharides, such as zymosan and endotoxin,! with aggregated IgA myeloma immunoglobulin lo and an IgA-containing cryoglobulin.ll The precise mechanism of activation is unclear but appears to involve the normal plasma counterpart of C3Nef 12. l 3 and eventuates in conversion of C3 to C3b. Properdin then interacts with C3b, which action possibly results in enhanced reversible binding of factor B to C3b and more efficient cleavage offactor B by factor d t o generate the properdin pathway C 3 convertase, C3B, that is functionally analogous to C42 in its capacity to activate C3-C9.I5 A precursor form of factor b, factor D, that is activated to factor D by the action of trypsin has been isolated.1 Proteolysis of C 3 by either C4? or C% produces C3a. a low-molecularweight anaphylatoxin fragment, and C3b, which is able to interact with factors D and B to form the properdin pathway C 3 convertase.l: This interaction

Demonstration of Receptor Function of Membrane Proteins by Selection and Immobilization with Monoclonal Antibodies

Monoclonal antibodies (MAbs) that are specific for certain types of cells and for stages of ontogenetic development can be produced without prior knowledge of antigenic differences between the cell types. Once identified, however, the elucidation of the function of these interesting antigens may represent a formidable task. In this chapter, we discuss the technique of selection and immobilization of membrane proteins by MAbs bound to Staphylococcus aureus bacteria. This technique allows visualization of receptor interactions with ligands on cells or in solution. Since multivalent interactions between S. aureus bacteria-MAb-receptor complexes and ligand-coated particles can be established, the method is particularly useful for studies on receptors where univalent reactions are of low affinity. The use of this method to identify two different membrane proteins as receptors for fragments of the third component of complement (C3) and one of these additionally as an Epstein-Barr virus (EBV) receptor, is described. To introduce these studies, the biology of C3, C3 receptors, and EBV is first briefly reviewed.