Cytochrome P450 2B1 mediates complement-dependent sublytic injury in a model of membranous nephropathy

Membranous nephropathy is a disease that affects the filtering units of the kidney, the glomeruli, and results in proteinuria accompanied by loss of kidney function. Passive Heymann nephritis is an experimental model that mimics membranous nephropathy in humans, wherein the glomerular epithelial cell (GEC) injury induced by complement C5b-9 leads to proteinuria. We examined the role of cytochrome P450 2B1 (CYP2B1) in this complement-mediated sublytic injury. Overexpression of CYP2B1 in GECs significantly increased the formation of reactive oxygen species, cytotoxicity, and collapse of the actin cytoskeleton following treatment with anti-tubular brush-border antiserum (anti-Fx1A). In contrast, silencing of CYP2B1 markedly attenuated anti-Fx1A-induced reactive oxygen species generation and cytotoxicity with preservation of the actin cytoskeleton. Gelsolin, which maintains an organized actin cytoskeleton, was significantly decreased by complement C5b-9-mediated injury but was preserved in CYP2B1-silenced cells. In rats injected with anti-Fx1A, the cytochrome P450 inhibitor cimetidine blocked an increase in catalytic iron and ROS generation, reduced the formation of malondialdehyde adducts, maintained a normal distribution of nephrin in the glomeruli, and provided significant protection at the onset of proteinuria. Thus, GEC CYP2B1 contributes to complement C5b-9-mediated injury and plays an important role in the pathogenesis of passive Heymann nephritis.     

Evaluation of sequential glomerular eluates from rats with Heymann nephritis

This study was undertaken to characterize the antigen-antibody content of sequential glomerular eluates from rats with Heymann nephritis. Serum and renal tissue were harvested every 2 wk after immunization with renal tubular antigen (Fx1A). Circulating antibody to the tubular antigen was detectable in the circulation from days 7 to 98. Direct immunofluorescence of renal tissue demonstrated an increase in IgG deposits through day 49 with stabilization thereafter. Tubular antigen deposits peaked at day 49 and then declined. One-hour and 3-hr acid eluates of isolated glomeruli were analyzed for IgG content, antibody specificity, and antigen content. Antibody from the 1-hr eluate bound to the tubular brush border but not the glomerulus, whereas the 3-hr eluate demonstrated binding to the glomerulus and not to the tubular brush border. In addition to rat IgG, the 1-hr eluate demonstrated a 70 kD band and the 3-hr eluate demonstrated a 45 kD band by polyacrylamide gel electrophoresis. By Western blot, antibody to the brush border bound to the 70 kD band. Anti-idiotypic antibody to anti-Fx1A, which binds to the glomerulus by indirect immunofluorescence, bound to the 45 kD band. The 3-hr eluate, but not the 1-hr eluate, precipitates radiolabeled F(ab')2 fragments from anti-Fx1A antibody but not from normal rat IgG. Quantitative analysis of the sequential eluates demonstrated that the 70 kD-anti-Fx1A system predominated early in the course of disease, whereas the 45 kD-anti-idiotype antigen-antibody system predominated late in the course of the disease. These observations confirm that two antigen-antibody systems contribute to the immune deposits in Heymann nephritis.     

Heymann antibodies induce complement-dependent injury of rat glomerular visceral epithelial cells

The aim of this study was to investigate the in vitro role of the complement membrane attack complex (MAC) in the injury induced by nephritogenic anti-brush border vesicle (Fx1A) antibodies on rat glomerular visceral epithelial cells (GEC). Both sheep and rabbit anti-rat brush border vesicle IgG-induced complement-dependent lysis of cultured GEC. Fab fragments of sheep anti-rat brush border vesicles and polyclonal or monoclonal gp330 IgG were devoid of lytic activity. Shedding of cell-surface antigens induced by sheep or rabbit anti-rat brush border vesicle IgG protected GEC from subsequent exposure to lytic antibodies and complement, an effect that was not obtained with Fab fragments. When GEC were incubated with sheep or rabbit anti-rat brush border vesicle IgG in capping conditions, the C3 component was co-redistributed with Heymann immune complexes; in contrast, the MAC remained diffusely bound to the cell surface, indicating that it was not associated with the antigen-antibody complexes. The MAC was demonstrated on the surface of GEC by immunofluorescence staining with anti-MAC neoantigen and by electron microscopy of negatively stained membranes showing focal clusters of 110 A MAC lesions. When GEC were treated with sheep IgG or rabbit IgG plus C6-deficient sera, the cells were not lysed and MAC was not demonstrable on the surface; however, lytic activity was restored when C6-deficient sera were reconstituted with purified C6. The results are consistent with the interpretation that injury induced by Heymann antibodies on GEC is MAC-dependent.     

Expression and modulation of surface antigens in cultured rat glomerular visceral epithelial cells

This study, using immunocytochemical light and electron microscopy techniques, characterizes the distribution of three antibodies bound to the surface of rat glomerular visceral epithelial cells (GEC) in culture, and tests their ability to redistribute corresponding antigens under conditions appropriate for antigenic modulation (antigen disappearance). At 4 degrees C or after fixation, anti-renal tubular brush border vesicle (BBV) IgG bound diffusely to the surface of GEC and to coated pits. Anti-gp330 IgG had a discrete distribution on the surface of GEC and reacted with coated pits. Anti-podocalyxin IgG was bound diffusely to the surface of GEC but not to coated pits. At 37 degrees C, anti-BBV IgG induced marked redistribution of immune complexes with both shedding and internalization. Anti-gp330 IgG induced weaker redistribution, with internalization of immune complexes predominating. Anti-podocalyxin IgG induced rapid redistribution of immune complexes and antigenic modulation but minimal internalization. Experiments of differential redistribution indicated that anti-BBV IgG modulated the expression of both gp330 and podocalyxin; anti-gp330 IgG had a weaker effect on BBV antigens and podocalyxin; and anti-podocalyxin failed to redistribute BBV antigens or gp330. The relevance of these immunocytochemical studies of antibody-cell surface antigen interaction in cultured GEC to understanding the pathogenesis of Heymann glomerulonephritis (HG) is discussed.     

Decay accelerating factor regulates complement activation on glomerular epithelial cells

Epithelial cells of the glomerular capillary are the site of C5b-9 mediated injury in rat membranous nephropathy. We investigated the regulation of C activation by cultured glomerular epithelial cells (GEC). Rat and human GEC were more resistant to C injury by homologous C than heterologous C. In human GEC homologous C cytotoxicity was enhanced by antiserum to decay accelerating factor (DAF) indicating that homologous C activation was, at least in part, restricted by membrane DAF. Anti-DAF immunoprecipitated a 67-kDa protein from human glomeruli. In rat GEC, pronase and phosphatidylinositol-specific phospholipase C (which are known to inactivate human DAF) enhanced cytotoxicity by homologous C. Thus, DAF is present on human GEC in culture and in human kidney glomeruli, and a DAF-like protein is present on cultured rat GEC. These proteins regulate C activation in vitro and may play a role in controlling C activation on GEC in vivo.     

Binding of antibromelain monomeric Fab' improves the stability of stem bromelain against inactivation

Antienzyme polyclonal antibodies against stem bromelain were raised in male albino rabbits and the Fab' monomers isolated from the IgG of the immune sera. Incubation of bromelain with the Fab' resulted in binding and gel filtration of the resulting complex suggested a 1:1 stoichiometry. Complexing with the Fab' resulted in significant stabilization of bromelain against thermal inactivation and alkaline pH.     

Morphological localization of a major lysosomal membrane glycoprotein in the endocytic membrane system

We have raised specific polyclonal immunoglobulin G (IgG) against a major lysosomal membrane sialoglycoprotein (LGP107) taken from rat liver and have prepared a conjugate of its Fab' fragment with horseradish peroxidase (HRP-anti LGP107 Fab') as a probe for the subcellular antigen. Electron immunocytochemistry in primary cultured rat hepatocytes showed that LGP107 resided primarily within lysosomes and was associated with luminal amorphous materials as well as limiting membranes. In addition, LGP107 was shown to be substantially distributed throughout the endocytic vacuolar system. The glycoprotein was found clustered in coated pits at the cell surface and localized along the surrounding membranes in endocytic vesicles. When cultured cells were exposed to HRP-anti LGP107 Fab', the antibody which was bound to its antigen within the coated pits was internalized via a system of endocytic vesicles and transported to lysosomes. During 20 min of incubation at 37 degrees C, the HRP tracer appeared at an early stage in small vesicles and moved progressively to larger vesicles, including multivesicular bodies. After 1 h, the tracer could be clearly seen in lysosomes heterogeneous in shape and size. The existence of LGP107 in endocytic compartments and the uptake of anti LGP107 antibody by hepatocytes were not blocked by prior treatment of the cells with cycloheximide and excess amounts of anti LGP107 IgG. These data suggest that LGP107 circulates between the cell surface and lysosomes through the endocytic membrane traffic in hepatocytes.     

Studies of the mechanism of bacterial resistance to complement-mediated killing. V. IgG and F(ab')2 mediate killing of E. coli 0111B4 by the alternative complement pathway without increasing C5b-9 deposition

The mechanism of antibody-dependent complement-(C) mediated killing of Escherichia coli 0111B4, strain 12015 (12015), was examined. 12015 was resistant to serum killing when incubated in hypogammaglobulinemic serum (H gamma S) or pooled normal human serum (NHS) that had been previously adsorbed to remove specific antibody (Abs NHS). Presensitization with immune rabbit serum or purified immune rabbit IgG resulted in 1 to 3 log killing when 5 X 10(8) colony forming units (CFU)/ml were incubated in 10 to 40% Abs NHS. Binding of 125I-C3 and 131I-C9 to the bacterial surface of the presensitized and the nonpresensitized strain was quantitated when these organisms were incubated in 10, 20, and 40% Abs NHS. Stable binding of up to 3.0 X 10(5) molecules of C3 and 8.0 X 10(4) molecules of C9 to presensitized and nonpresensitized isolates occurred in the highest concentration of serum, but there was no killing without presensitization. Similar results were found when Abs NHS was chelated with ethylene bis glycoltetraacetic acid containing 2 mM MgCl2 (Mg EGTA) to block classical pathway activation, indicating that antibody mediated the bactericidal reaction through the alternative pathway. Deposition of C3 and C9 and killing of 120 15 in 10% Abs NHS or 10% H gamma S was measured after presensitization with increasing amounts of IgG, F(ab')2, or Fab'. There was a dose-dependent increase in C3 deposition and killing, but only minimal change in C9 binding when 1.0 X 10(3) to 3.2 X 10(4) IgG or F(ab')2/CFU were bound to the bacterial surface. In contrast, there was no increase in C3 or C9 binding and no bacterial killing when 1 X 10(3) to 3.4 X 10(4) molecules Fab'/CFU were bound to the bacterial surface. These experiments show that immune IgG and F(ab')2 can mediate killing of E. Coli 0111B4 by the alternative pathway without changing the extent of terminal C component attachment to the bacterial surface.     

Generation of a novel decay accelerating factor (DAF) knock-out rat model using clustered regularly-interspaced short palindromic repeats, (CRISPR)/associated protein 9 (Cas9), genome editing

Decay accelerating factor (DAF), a key complement activation control protein, is a 70 kDa membrane bound glycoprotein which controls extent of formation of the C3 and C5 convertases by accelerating their decay. Using clustered regularly-interspaced short palindromic repeats, (CRISPR)/associated protein 9 (Cas9) genome editing we generated a novel DAF deficient (Daf^?/?) rat model. The present study describes the renal and extrarenal phenotype of this model and assesses renal response to complement-dependent injury induced by administration of a complement-fixing antibody (anti-Fx1A) against the glomerular epithelial cell (podocyte). Rats generated were healthy, viable and able to reproduce normally. Complete absence of DAF was documented in renal as well as extra-renal tissues at both protein and mRNA level compared to Daf^+/+ rats. Renal histology in Daf^?/? rats showed no differences regarding glomerular or tubulointerstitial pathology compared to Daf^+/+ rats. Moreover, there was no difference in urine protein excretion (ratio of urine albumin to creatinine) or in serum creatinine and urea levels. In Daf^?/? rats, proteinuria was significantly increased following binding of anti-Fx1A antibody to podocytes while increased C3b deposition was observed. The DAF knock-out rat model developed validates the role of this complement cascade regulator in immune-mediated podocyte injury. Given the increasing role of dysregulated complement activation in various forms of kidney disease and the fact that the rat is the preferred animal for renal pathophysiology studies, the rat DAF deficient model may serve as a useful tool to study the role of this complement activation regulator in complement-dependent forms of kidney injury.

     

Crystallohydrodynamics for solving the hydration problem for multi-domain proteins: open physiological conformations for human IgG.

Hydrodynamic methods provide a route for studying the low-resolution conformation--in terms of time-averaged spatial orientation of the Fab' and Fc domains relative to each other--of the human IgG subclasses, IgG1, IgG2, IgG3 and IgG4 in the environment in which many exist naturally---a solution. Representative modelling strategies are now available using 'shell-bead' or 'shell' modelling of the surface of the molecules with the size-independent programme SOLPRO [J. Garcia de la Torre, S.E. Harding, B. Carrasco, Eur. Biophys. J. 28 (1999) 119-132]. The shell model fits to the equivalent inertial surface ellipsoids of the published crystal structures for the Fab' and Fc domains of IgG are made and an apparent hydration delta(app) of 0.51g/g for Fab' and 0.70 g/g for the glycoprotein Fc are obtained, which yield an average value of (0.59+/-0.07) g/g for the intact antibody (2 Fab'+1 Fc). The relative orientations of these domains for each of the IgG subclasses is then found (using where appropriate a cylindrical hinge) from SOLPRO by modelling the Perrin function, P (i.e. 'frictional ratio due to shape') using this delta(app) and experimentally measured sedimentation coefficients. All the IgG subclasses appear as open, rather than compact structures with the degree of openness IgG3>IgG1>(IgG2, IgG4), with IgG3 and IgG1 non-coplanar. The hingeless mutant IgGMcg, with s degrees (20,w) approximately 6.8 S yields a coplanar structure rather similar to IgG2 and IgG4 and consistent with its crystallographic structure. The extension of this procedure for representing solution conformations of other antibody classes and other multi-domain proteins is indicated.     

The crystal structure of C2a, the catalytic fragment of classical pathway C3 and C5 convertase of human complement

The multi-domain serine protease C2 provides the catalytic activity for the C3 and C5- convertases of the classical and lectin pathways of complement activation. Formation of these convertases requires the Mg(2+)-dependent binding of C2 to C4b, and the subsequent cleavage of C2 by C1s or MASP2, respectively. The C-terminal fragment C2a consisting of a serine protease (SP) and a von Willebrand factor type A (vWFA) domain, remains attached to C4b, forming the C3 convertase, C4b2a. Here, we present the crystal structure of Mg(2+)-bound C2a to 1.9 A resolution in comparison to its homolog Bb, the catalytic subunit of the alternative pathway C3 convertase, C3bBb. Although the overall domain arrangement of C2a is similar to Bb, there are certain structural differences. Unexpectedly, the conformation of the metal ion-dependent adhesion site and the position of the alpha7 helix of the vWFA domain indicate a co-factor-bound or open conformation. The active site of the SP domain is in a zymogen-like inactive conformation. On the basis of these structural features, we suggest a model for the initial steps of C3 convertase assembly.     

Nucleotide and AP5A complexes of porcine adenylate kinase: A 1H and 19F NMR study

Proton and fluorine nuclear magnetic resonance spectroscopies (NMR) were used as methods to investigate binary complexes between porcine adenylate kinase (AK1) and its substrates. We also studied the interaction of fluorinated substrate analogues and the supposed bisubstrate analogue P1,P5-bis(5'-adenosyl) pentaphosphate (AP5A) with AK1 in the presence of Mg2+. The chemical shifts of the C8-H, C2-H, and ribose C1'-H resonances of both adenosine units in stoichiometric complexes of AK1 with AP5A in the presence of Mg2+ could be determined. The C2-H resonance of one of the adenine bases experiences a downfield shift of about 0.8 ppm on binding to the enzyme. The chemical shift of the His36 imidazole C2-H was changed in the downfield direction on ATP-Mg2+ and, to a lesser extent, AMP binding. 19F NMR chemical shifts of 9-(3-fluoro-3-deoxy-beta-D-xylofuranosyl)adenine triphosphate (3'-F-X-ATP)-Mg2+ and 9-(3-fluoro-3-deoxy-beta-D-xylofuranosyl)adenine monophosphate (3'-F-X-AMP) bound to porcine adenylate kinase could be determined. The different chemical shifts of the bound nucleotides suggest that their mode of binding is different. Free and bound 3'-F-X-AMP are in fast exchange with respect to their 19F chemical shifts, whereas free and bound 3'-F-X-ATP are in slow exchange on the NMR time scale in the absence as well as in the presence of Mg2+. This information could be used to determine the apparent dissociation constants of the nucleotides and the 3'-F-X analogues in the binary complexes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Targeting weak antigens to CD64 elicits potent humoral responses in human CD64 transgenic mice

Previous studies have documented that targeting foreign Ags to IgG FcgammaR leads to enhanced Ag-specific responses in vitro and in vivo. However, the ability to overcome immunologic nonresponsiveness by targeting poorly immunogenic Ags to FcgammaR has not been investigated. To address this question in a simple model, we immunized transgenic mice expressing human CD64 (FcgammaRI) and their nontransgenic littermates with Fab' derived from the murine anti-human CD64 mAb m22. The m22 Fab' served as both the targeting molecule and the Ag. We found that only CD64-expressing mice developed anti-Id titers to m22. Furthermore, chemically linked multimers of m22 Fab', which mediated efficient internalization of the human CD64, were significantly more potent than monomeric m22 F(ab')(2) at inducing anti-Id responses. In all cases, the humoral responses were specific for m22 Id and did not react with other murine IgG1 Fab' fragments. Chemical addition of a second murine Fab' (520C9 anti-human HER2/neu) to m22 Fab' multimers demonstrated that IgG1 and IgG2a anti-Id titers could be generated to 520C9 only in the CD64-expressing mice. These results show that targeting to CD64 can overcome immunological nonresponsiveness to a weak immunogen. Therefore, targeting to CD64 may be an effective method to enhance the activity of nonimmunogenic tumor vaccines.     

Effect of divalent metal ions on soluble and membrane-bound alkaline phosphatase activity in suckling rat intestine.

EDTA treatment of intestinal brush border membranes (BBM) and epithelial cell supernatant completely inhibited alkaline phosphatase (AP) activity in suckling rat intestine. AP activity was fully regained upon dialysis of the preparations against Zn2+ and to a lesser extent against Co2+, Ca2+ and Mn2+ ions. Other metal ions (Cd2+ and Mg2+) tested were essentially ineffective in restoring the enzyme activity. Considerable differences were observed in kinetic characteristics of the membrane-bound and soluble AP activities in response to various metal ions. There were apparent differences in Km, Vmax, energy of activation (Ea) and thermal stability of the soluble and membrane-bound AP activities, after metal ion substitutions. Nearly 35% AP activity was solubilized on sodium dodecyl sulphate treatment of brush borders (membrane protein: detergent ratio 1:3; w/w). Dialysis of detergent solubilized BBM against different metal ions reconstituted AP activity in the particulate fraction: the order of effectiveness was Zn greater than Ca greater than Mn greater than Co. The kinetic properties of the reconstituted AP were essentially similar to the non-integrated enzyme activity in response to various divalent metal ions examined. But there were apparent differences in Km, Vmax, Ea and thermal stability of the reconstituted AP activity compared to native brush border enzyme. The results suggest the unique requirement of Zn ions for stability and catalytic activity of the soluble and membrane-bound AP activity in suckling rat intestine.     

Solution structure of human and mouse immunoglobulin M by synchrotron X-ray scattering and molecular graphics modelling. A possible mechanism for complement activation

The pentameric 71-domain structure of human and mouse immunoglobulin M (IgM) was investigated by synchrotron X-ray solution scattering and molecular graphics modelling. The radii of gyration RG of human IgM Quaife and its Fc5, IgM-S, Fab'2 and Fab fragments were determined as 12.2 nm, 6.1 nm, 6.1 nm, 4.9 nm and 2.9 nm in that order. The RG values were similar for mouse IgM P8 and its Fab'2 and Fab fragments, despite the presence of an additional carbohydrate site. The IgM scattering curves, to a nominal resolution of 5 nm, were compared with molecular graphics models based on published crystallographic alpha-carbon co-ordinates for the Fab and Fc structures of IgG. Good curve fits for Fab were obtained based on the crystal structure of Fab from IgG. A good curve fit was obtained for Fab'2, if the two Fab arms were positioned close together at their contact with the C mu 2 domains. The addition of the Fc fragment close to the C mu 2 domains of this Fab'2 model, to give a planar structure, accounted for the scattering curve of IgM-S. The Fc5 fragment was best modelled by a ring of five Fc monomers, constrained by packing considerations and disulphide bridge formation. A position for the J chain between two C mu 4 domains rather than at the centre of Fc5 was preferred. The intact IgM structure was best modelled using a planar arrangement of these Fab'2 and Fc5 models, with the side-to-side displacement of the Fab'2 arms in the plane of the IgM structure. All these models were consistent with hydrodynamic simulations of sedimentation data. The solution structure of IgM can therefore be reproduced quantitatively in terms of crystallographic structures for the fragments of IgG. Putative Clq binding sites have been identified on the C mu 3 domain. These would become accessible for interaction with Clq when the Fab'2 arms move out of the plane of the Fc5 disc in IgM, that is, a steric mechanism exposing pre-existing Clq sites. Comparison with a solution structure for Clq by neutron scattering shows that two or more of the six globular Clq heads in the hexameric head-and-stalk structure are readily able to make contacts with the putative Clq sites in the C mu 3 domains of free IgM if if the Clq arm-axis angle in solution is reduced from 40 degrees-45 degrees to 28 degrees. This could be the trigger for Cl activation.     

Mg2+ binding to alkaline phosphatase correlates with slow changes in protein lability

The in vitro reactivation of unfolded Escherichia coli alkaline phosphatase (AP) in the presence of the two natively bound metals Zn2+ and Mg2+ produces two protein species, characterized by different guanidine hydrochloride denaturation kinetics. The high-lability AP form slowly converts to the low-lability form in a first-order reaction with a characteristic lifetime (inverse rate constant) of approximately 300 h at pH 8.0 and 25 degrees C. Addition of Zn2+ and Mg2+ ligands to (folded) apo-AP also produces two protein species, with denaturation kinetics and a long conversion lifetime similar to those found in refolding AP. In contrast, adding Zn2+ alone to apo-AP produces only the high-lability species with no subsequent structural change, suggesting that Mg2+ binding is the event which is responsible for the production of the low-lability AP. The rate of conversion from high- to low-lability AP was found to be linearly dependent on Mg2+ concentration, indicating that Mg2+ binding is rate limiting for this reaction. Experiments where either Zn2+ or Mg2+ was added first, with the second metal added later, show that Mg2+ binding is slowed by the prior presence of bound Zn2+. Mg2+ binding to Zn-AP also slightly increases the enzymatic activity; however, the extent of formation of the low-lability species is related to the square of the Mg2+-induced activity increase. Thus the binding of two Mg2+ to AP produces the dramatic reduction in the rate of denaturation that characterizes the low-lability species. The data suggest the possibility of long distance intersubunit interactions and a role for Mg2+ in providing "kinetic stability" for AP.     

Mutations of the type A domain of complement factor B that promote high-affinity C3b-binding

Factor B is a zymogen that carries the catalytic site of the complement alternative pathway convertases. During C3 convertase assembly, factor B associates with C3b and is cleaved at a single site by factor D. The Ba fragment is released, leaving the active complex, C3bBb. During the course of this process, the protease domain becomes activated. The type A domain of factor B, also part of Bb, is similar in structure to the type A domain of the complement receptor and integrin, CR3. Previously, mutations in the factor B type A domain were described that impair C3b-binding. This report describes "gain of function" mutations obtained by substituting factor B type A domain amino acids with homologous ones derived from the type A domain of CR3. Replacement of the betaA-alpha1 Mg2+ binding loop residue D254 with smaller amino acids, especially glycine, increased hemolytic activity and C3bBb stability. The removal of the oligosaccharide at position 260, near the Mg2+ binding cleft, when combined with the D254G substitution, resulted in increased affinity for C3b and iC3b, a C3b derivative. These findings offer strong evidence for the direct involvement of the type A domain in C3b binding, and are suggestive that steric effects of the D254 sidechain and the N260-linked oligosaccharide may contribute to the regulation of ligand binding.     

Functional and structural insight into properdin control of complement alternative pathway amplification

Properdin (FP) is an essential positive regulator of the complement alternative pathway (AP) providing stabilization of the C3 and C5 convertases, but its oligomeric nature challenges structural analysis. We describe here a novel FP deficiency (E244K) caused by a single point mutation which results in a very low level of AP activity. Recombinant FP E244K is monomeric, fails to support bacteriolysis, and binds weakly to C3 products. We compare this to a monomeric unit excised from oligomeric FP, which is also dysfunctional in bacteriolysis but binds the AP proconvertase, C3 convertase, C3 products and partially stabilizes the convertase. The crystal structure of such a FP-convertase complex suggests that the major contact between FP and the AP convertase is mediated by a single FP thrombospondin repeat and a small region in C3b. Small angle X-ray scattering indicates that FP E244K is trapped in a compact conformation preventing its oligomerization. Our studies demonstrate an essential role of FP oligomerization in vivo while our monomers enable detailed structural insight paving the way for novel modulators of complement.     

The different roles of two distinct Fc gamma receptors on guinea pig macrophages in the phagocytosis of sensitized sheep erythrocytes

The functional roles of two distinct types of Fc gamma receptors (Fc gamma 1/gamma 2R specific for both IgG1 and IgG2, and Fc gamma 2R specific for IgG2 alone) on the surface of guinea pig macrophages in the phagocytosis of sensitized sheep erythrocytes (EA) were investigated by the use of two Fab's of monoclonal anti-Fc gamma 1/gamma 2R and anti-Fc gamma 2R antibodies. The binding and subsequent ingestion of IgG1 antibody-sensitized erythrocytes (EA gamma 1) by macrophages were completely inhibited by anti-Fc gamma 1/gamma 2R Fab', indicating that the reactions are mediated only by Fc gamma 1/gamma 2R. On the other hand, the binding and subsequent ingestion of IgG2 antibody-sensitized erythrocytes (EA gamma 2) were substantially inhibited by anti-Fc gamma 2R Fab', but not by anti-Fc gamma 1/gamma 2R Fab'. The inhibitory activities of anti-Fc gamma 2R Fab' were dependent upon the amount of IgG2 antibody bound on erythrocytes; increasing the amount of bound IgG2 antibody from 0.15 to 0.91 micrograms/2 X 10(8) erythrocytes resulted in a decrease in the inhibition of binding of EA gamma 2 by anti-Fc gamma 2R Fab' from 50 to 0%, and also a decrease in the inhibition of ingestion of EA gamma 2 from 100 to 50%.(ABSTRACT TRUNCATED AT 250 WORDS)