Interaction of C3 and C3b with immunoglobulin G

Diversity of complement activation in different thyroid diseases

OBJECTIVE

To investigate complement components expression in both thyroid tissues and serum from patients with Hashimoto's thyroiditis (HT), Graves' disease (GD), and papillary thyroid cancer (PTC).

METHODS

C1q, mannose binding lectin (MBL), Bb, C4d, C3d and membrane attack complex (MAC) (C5b-9) deposition and complement regulate proteins (CD46, CD55 and CD59) expression in thyroid tissues from HT, GD, PTC, and control groups were examined by IHC. C1q, MBL, Bb, C4d, C3a, and soluble C5b-9 (sC5b-9) serum levels in the HT, GD, PTC, and healthy donor (HD) groups were measured by ELISAs.

RESULTS

MAC deposition was detected in thyroid tissues in the HT, GD and PTC groups, but not the control group. MBL, Bb, C4d, C3d and MAC staining intensities in thyroid tissues were significantly higher in the HT and PTC groups than in the control group (all P < 0.05). The C1q level was higher in HT tissues than in control tissues (both P < 0.05). No complement component had a significant difference in staining intensities between the GD and control groups. CD55 and CD59 expression levels in thyroid tissues were higher in the PTC group than in the HT, GD and control groups (all P < 0.05). Similarly, CD46 levels were higher in HT tissues than in control tissues. Bb, C4d, C3a and sC5b-9 serum levels were significantly increased in HT, GD and PTC patients compared with HDs (all P < 0.05).

CONCLUSION

Complement is overactivated in HT and PTC, but not in GD. All the three pathways are activated in HT, and the MBL and alternative complement pathways are activated in PTC. These distinct complement activation profiles may participate in HT, GD and PTC pathogenesis.

Is there a Role of Intravenous Immunoglobulin in Immunologic Recurrent Pregnancy Loss?

Recurrent pregnancy loss (RPL) commonly refers to three or more miscarriages that occur before 20 weeks of pregnancy. The immunological cause of RPL could be either an auto- or alloimmune-related event or both. Because of the discovery of immunological abnormalities in RPL patients in clinical practice, several immunomodulatory therapies were introduced to maintain the immune balance at the maternal-fetal interface. Intravenous immunoglobulin (IVIg) is one of the immunomodulators. In recent years, several studies have analyzed the therapeutic effect of IVIg on RPL patients with antiphospholipid syndrome (APS) or unexplained RPL. However, their results are controversial. IVIg can be used in RPL patients with APS who have previously failed in other treatments. It is recommended that IVIg infusion could be considered used before conception in RPL patients who have cellular immune abnormalities such as increased natural killer (NK) cell counts, NK cell cytotoxicity, or increased T helper (Th)1/Th2 ratio, depending on the cut-off values of each hospital. The aim of this review was to summarize the mechanisms, efficacy, pharmacokinetics, and side effects associated with passive immunization using IVIg in immunologic RPL, according to the literature published in recent years. We hope that more obstetricians will be able to understand the timing and indication of IVIg properly in immunologic RPL patients and effectively enhance pregnancy outcomes for mothers and neonates.

Effect of Intravenous immunoglobulin on Th1 and Th2 lymphocytes and improvement of pregnancy outcome in recurrent pregnancy loss (RPL)

BACKGROUND

Women with elevated natural killer (NK) cell frequency and function during pregnancy, suffer from recurrent pregnancy loss (RPL). In the present study, the possible effect of intravenous immunoglobulin (IVIG) administration on Th1 and Th2 cell frequency, cytokine secretion, and expression of transcription factors is compared between RPL patients and control group.

MATERIALS AND METHODS

Totally, 44 women with a history of RPL (32 women as treated group and 12 as control group) were enrolled in the study. The frequency of Th1 and Th2 lymphocytes, the expression of transcription factors related to these cells and the serum levels of associated cytokines were assessed by flowcytometry, real-time PCR and ELISA, respectively. All, assessments were performed both before and after treatment with IVIG.

RESULTS

A significant reduction in Th1 lymphocyte frequency, transcription factor expression and cytokine levels were observed in IVIG-treated group, while all the above parameters indicated a significant increase for Th2 lymphocytes. Th1/Th2 ratio decreased significantly (p value<0.0001) at the end of treatment and 28 out of 32 (87.5%) women in IVIG-treated group had live birth in comparison with 5 out of 12 (41.6%) in untreated group.

CONCLUSION

IVIG administration proves to be an efficient therapeutic strategy which is able to enhance the success rate of pregnancy through a shift in Th2 responses. Furthermore, IVIG presents efficacy for the treatment of reproduction failures especially in subjects with immune cell abnormalities and increased NK cell level and function.

The immunoglobulin, IgG Fc receptor and complement triangle in autoimmune diseases

Immunoglobulin G (IgG)-mediated activation of complement and IgG Fc receptors (FcγRs) are important defense mechanisms of the innate immune system to ward off infections. However, the same mechanisms can drive severe and harmful inflammation, when IgG antibodies react with self-antigens in solution or tissues, as described for several autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, and immune vasculitis. More specifically, IgG immune complexes (ICs) can activate all three pathways of the complement system resulting in the generation of C3 and C5 cleavage products that can activate a panel of different complement receptors on innate and adaptive immune cells. Importantly, complement and FcγRs are often co-expressed on inflammatory immune cells such as neutrophils, monocytes, macrophages or dendritic cells and act in concert to mediate the inflammatory response in autoimmune diseases. In this context, the cross-talk between the receptor for the anaphylatoxin C5a, i.e. C5ar1 (CD88) and FcγRs is of major importance. Recent data suggest a model of bidirectional regulation, in which CD88 acts upstream of FcγRs and sets the threshold for FcγR-dependent effector responses by regulating the ratio between activating and inhibitory FcγRs. Vice versa, FcγR ligation can either amplify or block C5aR-mediated effector functions, depending on whether IgG IC aggregate activating or inhibitory FcγRs. Further, complement and FcγRs cooperate on B cells and on follicular dendritic cells to regulate the development of autoreactive B cells, their differentiation into plasma cells and, eventually, the production of autoantibodies. Here, we will give an update on recent findings regarding this complex regulatory network between complement and FcγRs, which may also regulate the inflammatory response in allergy, cancer and infection.

Opinion paper: Stimulation of complement amplification or activation of the alternative pathway of complement?

In this opinion paper, we suggest that the scheme of the complement system should be redrawn in order to better illustrate its potencies. This can be achieved by putting the amplification loop of the alternative complement pathway at the center of the complement system. This arrangement emphasizes that C3b molecules, generated by any pathway, can stimulate complement amplification. Furthermore, it allows one to differentiate between this type of stimulation of amplification and that driven by those immune complexes that capture dimeric C3b molecules, which are more potent C3 convertase precursors than C3b. Schemes similar to the one drawn may help to better illustrate the interplay of the pathways and convey a clearer comprehension of the mechanics of the complement system.

Anti-inflammatory effect of intravenous immunoglobulin mediated through modulation of complement activation

Complement activation by immune complexes is well-known. In the course of autoimmune disease, acute and chronic complement activation is the primary inducer of inflammation and tissue damage. Polyclonal, polyspecific intravenous immunoglobulin (IVIg) preparations are a therapy of choice in a variety of autoimmune and inflammatory diseases. This review describes mechanisms by which IgG reduces complement activation or interferes with the action of proinflammatory complement-derived proteins. The known interference of IVIg with the biological activity of complement-derived proinflammatory proteins does not affect the generation of these potentially dangerous products, but can limit their devastating effects. Therefore, we embarked on studies on IVIg's potential to attenuate complement activation and thus to prevent further generation of such dangerous molecules. We present here a revised view of how the central event of complement activation--namely, complement amplification--operates on a molecular level and how IVIg, with its physiological autoantibodies directed against some complement proteins, is able to downregulate amplification of complement C3 activation. Finally, we summarize results of a study in which clinical effects of IVIg and attenuation of complement activation were assessed. We propose that the anti-inflammatory effect of IVIg in a wide range of autoimmune diseases might be explained, at least in part, by attenuation of complement amplification.

Complement amplification revisited

Complement amplification in blood takes place not only on activating surfaces, but in plasma as well, where it is maintained primarily by C3b2-IgG complexes. Regular products of C3 activation in serum, these complexes are inherently very efficient precursors of the alternative pathway C3 convertase. Moreover, they can bind properdin bivalently, thus creating preferred sites for convertase formation. C3b2-IgG complexes have a half-life that is substantially longer than that of free C3b, since both C3b molecules are partially protected from inactivation by factor H and I. These complexes are preferentially generated on certain naturally occurring and induced antibodies that exhibit a paratope-independent affinity for C3/C3b. Such antibodies are known to stimulate alternative complement pathway activation. We have assembled the evidence for the generation and the functional potency of the C3b2-IgG complexes, which have been studied during the last two decades. We illustrate their roles in immune complex solubilization, phagocytosis, immune response, and their ability to initiate devastating effects in ischemia/reperfusion and in aggravating inflammation.

IgG naturally occurring antibodies stabilize and promote the generation of the alternative complement pathway C3 convertase

Normal human IgG contains naturally occurring anti-C3 antibodies (anti-C3 NAbs) that have been proposed to regulate complement amplification. Here, we report a novel procedure for anti-C3 NAb purification. Pooled human IgG was fractionated on a DEAE column prior to affinity chromatography on IgG and then on C3. Anti-C3 NAbs co-purified with anti-F(ab')2 NAbs. In a refined protocol, IgG fractions were absorbed on Fc, F(ab')2, and C3, which allowed to isolate the directly accessible NAbs and to remove IgG hinge-region-specific NAbs. Since a substantial fraction of total anti-C3 NAbs in whole IgG pre-existed as complexes, IgG that did not bind to the three affinity columns was treated with urea and the affinity chromatography repeated to collect the dissociated NAbs. The urea-accessible anti-F(ab')2 NAbs were rather pure but anti-C3 NAbs yet contained substantial amounts of anti-F(ab')2 NAbs. Anti-C3 NAbs showed up to 400-fold and anti-F(ab')2 NAbs, up to 30-fold enrichment as compared to pooled normal human IgG. Anti-C3 NAb preparations exhibited nephritic factor activity that was up to 60 times stronger than that of total IgG from a patient with membranoproliferative glomerulonephritis type 2. In addition, anti-C3 NAbs promoted C3 convertase generation, when added to the convertase precursor or during convertase assembly, suggesting a non-nephritic-factor mechanism. Factors H and I reduced the overall level of activity but had no influence on the NAb dose-response curve meaning that NAbs did not interfere with factor H binding. Convertase promoting activity during assembly correlated with the content of anti-C3 NAbs in NAb complexes. In conclusion, anti-C3 NAbs associated with framework-specific anti-idiotypic NAbs stabilize C3 convertase and promote its generation but their activity is compensated for in whole IgG.

Membranoproliferative glomerulonephritis type II (dense deposit disease): an update

Membranoproliferative glomerulonephritis type II (MPGN II) is a rare disease characterized by the deposition of abnormal electron-dense material within the glomerular basement membrane of the kidney and often within Bruch's membrane in the eye. The diagnosis is made in most patients between the ages of 5 and 15 yr, and within 10 yr, approximately half progress to end-stage renal disease, occasionally with the late comorbidity of visual impairment. The pathophysiologic basis of MPGN II is associated with the uncontrolled systemic activation of the alternative pathway (AP) of the complement cascade. In most patients, loss of complement regulation is caused by C3 nephritic factor, an autoantibody directed against the C3 convertase of the AP, but in some patients, mutations in the factor H gene have been identified. For the latter patients, plasma replacement therapy prevents renal failure, but for the majority of patients, there is no proven effective treatment. The disease recurs in virtually all renal allografts, and a high percentage of these ultimately fail. The development of molecular diagnostic tools and new therapies directed at controlling the AP of the complement cascade either locally in the kidney or at the systemic level may lead to effective treatments for MPGN II.

C3b2-IgG Complexes Retain Dimeric C3 Fragments at All Levels of Inactivation

C3b2-IgG complexes are formed during complement activation in serum by attachment of two C3b molecules (the proteolytically activated form of C3) to one IgG heavy chain (IgG HC) via ester bonds. Because of the presence of two C3b molecules, these complexes are very efficient activators of the alternative complement pathway. Likewise, dimeric C3b is known to enhance complement receptor 1-dependent phagocytosis, and dimeric C3d (the smallest thioester-containing fragment of C3) linked to a protein antigen facilitates CR2-dependent B-cell proliferation. Because the efficiency of all these interactions depends on the number of C3 fragments, we investigated whether C3b2-IgG complexes retained dimeric structure upon physiological inactivation. We used two-dimensional SDS-PAGE and Western blot to study the arrangement of the C3b molecules by analyzing the fragmentation pattern after cleavage of the ester bonds. Upon inactivation with factors H and I, a 185-kDa band was generated under reducing conditions. It released IgG HC and the 65-kDa fragment of C3b alpha' chain after hydrolysis of the ester bonds with hydroxylamine. The two C3b molecules were not 65-kDa-to-40-kDa linked, because neither ester-bonded 65 kDa HC nor 65 kDa-40 kDa fragments were observed, nor was a 40-kDa peptide released after hydroxylamine cleavage. Factor I and CR1 cleaved the C3b2-IgG molecule to its final physiological product, C3dg2-IgG, which migrated as a 133-kDa fragment in reduced form. This fragment released exclusively C3dg (the final physiological product of C3b inactivation by factor I) and IgG HC. C3dg2-HC appeared as a double band on SDS-PAGE only at low gel porosity, suggesting the presence of two conformers of the same composition. Our results suggest that, upon physiological inactivation, C3b2-IgG complexes retain dimeric inactivated C3b and C3dg, which allows bivalent binding to the corresponding complement receptors.

Effect of intravenous immunoglobulin treatment on the Th1/Th2 balance in women with recurrent spontaneous abortions

PROBLEM

The way by which intravenous immunoglobulin (IvIg) acts to prevent immunlogically mediated recurrent spontaneous abortions (RSA) has not been clarified. In the present study, a possible effect of IvIg on the T helper cell (Th1/Th2) balance was investigated in abortions of either alloimmune or autoimmune abnormalities.

METHOD OF STUDY

The study included 21 women treated with IvIg before conception because of a history of RSA characterized by alloimmune abnormalities (n = 15) or associated with anti-phospholipid antibodies (APA) (n = 6). Peripheral blood samples, collected before and 5 days after the first IvIg infusion, were stimulated, and Th1 and Th2 cells were detected by flow-cytometric analysis using a combination of monoclonal antibodies against T-cell surface markers and intracellular interferon (IFN)-gamma and interleukin (IL)-4. The percentage of IFN-gamma-producing (Th1) and IL-4-producing (Th2) cells and the Th1/Th2 ratio were compared between pre- and post-infusion samples.

RESULTS

A decrease of Th1 percentage in 66.6% of the cases and a concurrent Th2 percentage increase (47.61%) resulted in a decrease in the Th1/Th2 ratio in most of the cases (76.1%) (p < 0.01). Similar results were found in Group A (Th1/Th2 decreased in 60% of the cases, p < 0.05), while in Group B the effect of IvIg was not clear (Th1/Th2 increased in three and decreased in another three cases).

CONCLUSION

Our finding suggests that IvIg administration in women with alloimmune RSA enhances Th2 polarization. This is not always the case with APA-associated abortions.

Expression of rat complement control protein Crry on tumor cells inhibits rat natural killer cell-mediated cytotoxicity

Crry is a rodent membrane-bound inhibitor of complement activation and is a structural and functional analog of the human complement inhibitors decay-accelerating factor and membrane cofactor protein. We found previously that expression of rat Crry on a human tumor cell line enhances tumorigenicity in nude rats. In this study, we investigated the effect that rat Crry expressed on tumor cells has on rat cell-mediated cytotoxicity and antibody-dependent cell-mediated cytotoxicity (ADCC). The expression of rat Crry on the surface of different human tumor cell lines inhibited ADCC mediated by rat natural killer (NK) cells. C3 opsonization is known to enhance NK cell-mediated cytolysis, and a potential mechanism for Crry-mediated inhibition of NK cell lysis is through Crry modulation of C3 deposition on target cells. However, the transfection of tumor cell lines with Crry enhanced their resistance to NK cell-mediated lysis in the absence of exogenous complement. The resistance of Crry-expressing tumor cells to NK cell-mediated ADCC could be reversed by treatment with anti-Crry F(ab)(2). In addition, anti-Crry F(ab)(2) enhanced the susceptibility of 13762 rat mammary adenocarcinoma cells (that endogenously express Crry) to ADCC mediated by allogeneic rat NK cells in the absence of added complement. We found no evidence that rat NK cells were a source of complement for target cell deposition during the in vitro cytolysis assay. These data suggest a novel function for rat Crry in tumor immune surveillance that may be unrelated to complement inhibition.

Interaction of C3b(2)--IgG complexes with complement proteins properdin, factor B and factor H: implications for amplification

Nascent C3b can form ester bonds with various target molecules on the cell surface and in the fluid phase. Previously, we showed that C3b(2)--IgG complexes represent the major covalent product of C3 activation in serum [Lutz, Stammler, Jelezarova, Nater and Späth (1996) Blood 88, 184--193]. In the present report, binding of alternative pathway proteins to purified C3b(2)--IgG complexes was studied in the fluid phase by using biotinylated IgG for C3b(2)--IgG generation and avidin-coated plates to capture complexes. Up to seven moles of properdin 'monomer' bound per mole of C3b(2)--IgG at physiological conditions in the absence of any other complement protein. At low properdin/C3b(2)--IgG ratios bivalent binding was preferred. Neither factor H nor factor B affected properdin binding. On the other hand, properdin strongly stimulated factor B binding. Interactions of all three proteins with C3b(2)--IgG exhibited pH optima. An ionic strength optimum was most pronounced for properdin, while factor B binding was largely independent of the salt concentration. C3b(2)--IgG complexes were powerful precursors of the alternative pathway C3 convertase. In the presence of properdin, C3 convertase generated from C3b(2)--IgG cleaved about sevenfold more C3 than the enzyme generated on C3b. C3b(2)--IgG complexes could therefore maintain the amplification loop of complement longer than free C3b.

Assembly and regulation of the complement amplification loop in blood: the role of C3b-C3b-IgG complexes

Amplification of complement activation in blood and serum starts on multi-protein complexes that act as precursors of an alternative C3 convertase. Among these covalently linked C4b-, C3b-, and IgG-containing complexes C3b-C3b-IgG complexes represent the major species containing C3b and IgG. Recent work on their purification and characterization is discussed. Special emphasis is placed on the arrangement of ester bonds in these complexes and their dual type of partial protection from inactivation. Partial protection from inactivation is mediated by properdin which binds to these complexes in the complete absence of any other complement protein. High dose IgG, known to stimulate inactivation of these complexes, appears to lower properdin binding in a process that also involves factor H. Properdin stimulates factor B binding to these complexes and renders them far better precursors of a C3 convertase than C3b. The available information allows a suggestion for a new scheme on how the amplification loop is assembled and regulated in blood and serum.

The covalent interaction of C3 with IgG immune complexes

Antigens (Ags) are converted into immune complexes (antigen-antibody complexes, IC) as soon as they encounter their specific antibodies (Abs). In fluids containing complement, the process of IC formation and fixation of complement components occur simultaneously. Hence, the formation of Ag-Ab-complement complexes is the normal way of eliminating Ags from a host. C3b-C3b-IgG covalent complexes are immediately formed on interaction of serum C3 with IgG-IC. These C3b-C3b dimers constitute the core for the assembly of C3/C5-convertase on the IC, which are subsequently converted into iC3b-iC3b-IgG by the complement regulators. These complexes are detected on SDS-PAGE by two bands of molecular composition, C3alpha65-C3alpha43 (band A) and C3alpha65-heavy chain of the Ab (band B), which correspond to C3b-C3b and C3b-IgG covalent interaction respectively, and that identify opsonized IC (C3b-IC). C3b can attach to Fab and Fc regions of the Ab molecule with similar efficiency. The presence of multiple C3b binding regions on IgG is considered an advantageous characteristic that facilitates the elimination of Ags in the form of C3b(n)-IC. Ab molecules on the IC recognize the Ag, and also serve as a very good acceptor for C3b binding. In this way, Ags, even if they have no acceptor sites for C3b, can be efficiently processed and removed. When C3 is activated in serum by IC or other activators, secondary C3b-IgG covalent complexes are generated, with bystander monomeric circulating IgG, and thus constitute, physiological products of complement activation. These complexes gain importance when IgG concentration is extremely high as in cases of infusion of intravenous IgG (IVIG) in several pathologies. The covalent attachment of activated complement C3 (C3b, iC3b, C3 d,g) to Ags or IC links innate and adaptative immunity by targeting Ags to different cells of the immune system (follicular dendritic cells, phagocytes, B cells). Hence C3b marks Ags definitively, from the earliest contact with the innate immune system until their complete elimination from the host.

How pre-existing, germline-derived antibodies and complement may help induce a primary immune response to nonself

In the hypothesis that pre-existing, germline-encoded antibodies (naturally occurring antibodies, NAb) bind to conserved epitopes on invading nonself antigens, bound NAbs may initiate complement deposition and become targets of nascent C3b, which generates C3b-C3b-NAb complexes that remain associated with the nonself antigen (C3b-C3b-NAb...antigen). The inactivated form of these complexes (C3dg-C3dg-NAb...nonself antigen) may bind bivalently and thus firmly to B cells via CR2, a process stimulating antigen presentation. In some cases, CR2-bound 'C3dg-C3dg-NAb...antigen complexes' may further be recognized by immunoglobulin (Ig) determinants on B cells, whereby an immune response is elicited. As conserved epitopes on the nonself antigen are already complexed to NAbs, only B cells carrying Ig determinants specific for nonself epitopes may be stimulated. This hypothesis can explain directed affinity maturation towards nonself, protection from a strong immune response to conserved epitopes, down-regulation of antibody formation and unresponsiveness to high-dose antigen.

New therapeutic avenues in autoimmunity

During the last decade, much progress has been made in the understanding of processes that lead to autoimmunity. Cellular interactions mediated through cytokines and adhesion molecules were found to play a major role in the genesis of autoimmune conditions. During this period, we learned to recruit monoclonal antibodies to manipulate these delicate processes and to divert their outcome to a path we control better. Our comprehension of IVIG (intravenous immunoglobulin therapy) has broadened, and new indications for the implementation of this promising therapy have been pursued. In this review, we shed light on new therapeutic modalities that have been published since our previous report and discuss new data concerning the old modalities.

Effect of immunoglobulin variable region structure on C3b and C4b deposition

Many of the biological activities of immunoglobulins, including interaction with the complement system, are attributed to the structure of the heavy chain constant domains. However, previous studies indicated that immune complexes formed with independently derived isotype-matched pairs of monoclonal antibodies vary with respect to their capacity to activate complement and to serve as targets for C3b and C4b deposition. The goal of the present study was to provide a structural basis for explaining how variable domains influence C3b and C4b deposition on immunoglobulins. Heavy and light chain variable domains from a pair of IgG2a antibodies previously shown to differ in terms of complement activation and C3b and C4b deposition were cloned and sequenced. The two clones utilize distinct heavy and light variable region genes and the translated amino acid sequence reveals several residues that could serve as potential targets for complement deposition which differs between the two antibodies. Molecular modeling suggests that many of the relevant differences between the two antibodies are located in solvent exposed portions of the heavy and light chain variable domains and that some of the relevant sites are located within the complementarity determining regions. Differences in antibody affinity do not provide an explanation for the previously observed role of variable domains on interactions with the complement system. These data suggest that sequence variations within solvent-exposed variable domain residues may play a key role in C3b and C4b deposition on immunoglobulins.

Intravenous immunoglobulin for treatment of recurrent pregnancy loss

PROBLEM

Efficacy of immunotherapy for treatment of recurrent spontaneous abortion (SA) has been controversial. The low treatment effect of white blood cell immunization lead to investigations of alternative treatments including intravenous (i.v.) immunoglobulin (Ig). To evaluate the efficacy of IVIg for treatment of recurrent SA, a prospective, randomized, double blinded, placebo-controlled trial was performed.

METHODS

Ninety-five women experiencing two or more consecutive spontaneous abortions, with no known cause were randomized and received either IVIg 500 mg/kg/month or placebo (albumin).

RESULTS

Of 95 women participating in the study, 47 received IVIg and 48 received placebo. Medication was discontinued in 34 women who failed to conceive within four cycles. The remaining 61 women achieved pregnancy. Pregnancy outcomes included 29 deliveries and 32 recurrent SA. Among women delivering live births 18 (62%) received IVIg and 11 (38%) received placebo. By contrast, 21 (66%) women experiencing recurrent SAs received placebo and 11 (34%) received IVIg. Among 61 women who conceived, 29 received IVIg and 32 received placebo. Of the 29 women who conceived and received IVIg, 18 (62%) delivered live births and 11 (38%) experienced recurrent SA. Of 32 women who conceived and received placebo 11 (34%) delivered live births and 21 (66%) had recurrent SA. The difference in live birth rates between women receiving IVIg and placebo was significant (P = 0.04, odds ratio 3.1).

CONCLUSION

IVIg is effective in enhancing the percentage of live births among women experiencing unexplained recurrent SA.

Covalent binding of C3b to monoclonal antibodies selectively up-regulates heavy chain epitope recognition by T cells

Protein C3 of the complement system is known for its role in the nonspecific immune response. Covalent binding of C3b to antigen upon complement activation also plays a significant role in specific T cell immune response. C3b-antigen complexes can bind to complement receptors on the antigen-presenting cell, and the C3b antigen link (most often an ester link) remains fairly stable inside the cells. In this study, IgG1,kappa and IgG2a,kappa murine monoclonal antibodies (mAb) were used as antigens; covalent complexes between mAb and C3b were produced and purified in vitro from purified proteins; human B cell lines and T cell clones were raised from tumor patients who received mAb injections for cancer therapy or diagnosis. Recognition of epitopes of these mAb by T cell clones when the mAb were processed alone or bound to C3b was compared. IgG or IgG-C3b complexes presented by B cell lines were able to stimulate proliferation of kappa light chain-specific T cell clones at similar concentrations. In contrast, IgG-C3b complex recognition by heavy chain-specific T cell clones required 100-fold less IgG-C3b than uncomplexed IgG. As C3b was shown to be covalently bound only to the IgG heavy chains in the complexes, C3b chaperoning is restricted to only the IgG heavy chain and selectively influences intracellular steps of IgG heavy chain processing. This differential modulation of C3b suggests an early dissociation of IgG heavy and light chains in antigen-presenting cells.

Ultrasonographic findings of pregnancy losses after treatment for recurrent pregnancy loss: intravenous immunoglobulin versus placebo

OBJECTIVE

To describe the ultrasonographic findings of pregnancies that terminated in repeat abortion in women participating in an ongoing randomized placebo-controlled clinical trial evaluating the efficacy of intravenous (IV) immunoglobulin (Ig) in the treatment of recurrent spontaneous abortion (SA).

DESIGN

A prospective descriptive study of ultrasonographic findings of pregnancies from 27 women experiencing repeated recurrent SAs after entry into a clinical trial.

SETTING

Clinical practice at the Genetics & IVF Institute in Fairfax, Virginia.

PATIENTS

Women experiencing two or more consecutive recurrent SAs received either 500 mg/kg per mo IV Ig or placebo (albumin). To date 90 women have been enrolled in the clinical trial and 52 have achieved pregnancy. The outcome of the 52 pregnancies include 16 deliveries, 9 ongoing pregnancies, and 27 losses.

INTERVENTIONS

Ultrasonographic examinations performed in 27 women experiencing pregnancy loss are the subject of this study.

MAIN OUTCOME MEASURES

The frequency of ultrasonographic findings of empty gestation sac (blighted ovum) and intrauterine fetal demise (IUFD) is compared between patients receiving IV Ig and placebo.

RESULTS

Ultrasonographic findings of the 27 pregnancies losses included 11 blighted ova and 16 IUFDs. Of 11 blighted ova, 8 (73%) were in women receiving IV Ig and 3 (27%) were receiving placebo. Sixteen IUFDs were observed: 3 (19%) in women receiving IV Ig and 13 (81%) in women receiving placebo. Of 11 pregnancy losses occurring in women receiving IV Ig, 8 (73%) were blighted ova, 3 (27%) were IUFDs. Sixteen pregnancy losses occurred in women receiving placebo: 3 (19%) were blighted ova and 13 (81%) were IUFDs. The differences in frequency of blighted ova between IV Ig- and placebo-treated women was significant.

CONCLUSION

IV Ig is not effective in preventing blighted ova and may be effective in preventing IUFDs.

Formation of covalent C3b-tetanus toxin complexes: a tool for the in vitro study of antigen presentation

A novel method is described for the formation and purification of covalent complexes between the complement component C3b and an antigen (tetanus toxin, TT), using purified proteins in fluid phase. C3b is generated in situ by tryptic cleavage of C3 after co-precipitation of C3 and TT in the presence of polyethylene glycol. Various parameters were analysed to optimize complex formation; under conditions which minimized the formation of covalent C3b multimers, 30% and 8% respectively of C3b and TT were incorporated into covalent one-to-one complexes which were purified using gel filtration chromatography. The linkage was localized between the alpha' chain of C3b and either the H or L chain of TT; it required the in situ formation of C3b and was partially destroyed by 1 M hydroxylamine. Spontaneous dissociation of the complex could be partly avoided by HgCl2, a thiol reagent which inhibits the esterase-like activity of bound C3b. These findings suggest the involvement of the reactive carbonyl of nascent C3b with hydroxyl groups of TT. Such C3b-TT complexes provide a defined tool to analyse the influence of antigen-bound C3b on antigen addressing and intracellular processing by antigen-presenting cells.

Complement-mediated inactivation of interferon-gamma in ELISA systems

The recovery of a predetermined amount of interferon-gamma (IFN-gamma) added to normal serum was studied in two independent sandwich ELISA systems specific for rat and human IFN-gamma. In both assays the ELISA activity was rapidly lost in fresh but not in heat-inactivated (30', 56 degrees C) serum. Ninety percent of the initial activity had disappeared within 30 minutes upon incubation at 37 degrees C. Serum-mediated inhibition was not species-specific as the ELISA activity of rat IFN-gamma diminished equally well in rat and human sera. Inhibition was critically dependent on the isotype of the solid-phase monoclonal antibody (mAb) used in the ELISA systems. IgG1 and IgG2a mAbs efficiently inhibited the ELISA activity of IFN-gamma, whereas an IgA mAb was ineffective. The inhibition was not influenced by a wide variety of anti-proteolytic agents but was effectively blocked by anti-complementary substances or treatments directed to the first (C1) and third (C3) component of complement. Our results indicate that activation of the classical pathway of complement (CPC) and the concomitant covalent binding of C3 to the IFN-gamma molecule play a major role in the inhibitory process. It is concluded that reduction of the ELISA activity is attributable to diminished accessibility of the detector antibody for the IFN-gamma protein as a consequence of C3 binding.

Mechanism of therapeutic effect of high-dose intravenous immunoglobulin. Attenuation of acute, complement-dependent immune damage in a guinea pig model

Studies were performed in in vitro and in vivo models to assess the effect of intravenous immunoglobulin (IVIG) on the development of acute complement-mediated tissue damage. IVIG significantly increased the duration of survival and frequently prevented the death of guinea pigs injected with anti-Forssman antiserum to cause lethal Forssman shock; no control animal treated with albumin and/or maltose vehicle survived. The most pronounced effect was achieve by delivering IVIG as one slow injection at 1,800 mg/kg 3 h before Forssman shock was elicited. Infusion of guinea pig IgG at the same dosage was similarly protective. A strong positive correlation was found between IgG plasma levels and survival time in guinea pigs treated with graded doses of IVIG. Therapy itself did not affect C3 and C4 levels nor the capacity to activate these components. In vitro studies showed almost complete inhibition of C3 uptake onto IgG-sensitized erythrocytes using serum from an IVIG-treated animal. We suggest that supraphysiologic levels of IVIG act in part by preventing active C3 fragments from binding to target cells. Infusion of high dose IVIG may be a rational approach to modulating acute, complement-dependent tissue damage in a variety of diseases in man.

Interaction between C3 and IL-2; inhibition of C3b binding to CR1 by IL-2

We previously reported that C3 has a role in the enhancement of the IL-2 dependent proliferation of helper T cells. Because the IL-2R has a structural homology with the complement proteins, such as CR1 and CR2, we studied the possible ligand crossreactions on CR1 and IL-2-receptor, and the direct interaction between C3 and IL-2. While C3 has an enhancing effect on the IL-2 dependent proliferation of HT-2, a CR1-positive mouse T-cell line, the growth of the CTLL-16 line (CR1-negative) is not affected by C3. It has been proven that neither the insolubilized C3 nor the soluble C3b-like C3 react with the IL-2 binding epitope of the IL-2 receptor. However, using human RBC we have demonstrated that the binding of aggregated C3 to CR1 is inhibited by rIL-2, in a dose-dependent manner. When RBC were incubated with rIL-2 and FITC-labelled Fab-anti-CR1 simultaneously, there was no inhibition in the fluorescence intensity. As detected by ELISA, rIL-2 was bound to the same extent by insolubilized C3, C3b, and C3c, while C3d coat had lower binding capacity. The receptor-binding epitope of IL-2 is intact in the complex of complement proteins and rIL-2, as demonstrated by the binding of DMS1, a monoclonal antibody reacting with the receptor site of IL-2. It is strongly suggested that C3b may play a role in the growth of CR1 positive T cells.

Appearance of acceptor-bound C3b on HLA-DR positive macrophages and on stimulated U937 cells; inhibition of Fc gamma-receptors by the covalently fixed C3 fragments

The appearance and the functional role of acceptor-bound C3b during differentiation of human monocytes into macrophages were studied. Acceptor-bound C3b could be detected by the immune adherence (IA) test parallel to the expression of antigenic determinants specific to mature cells--i.e. on days 4-5 of culture. Consequently, the capacity of these phagocytes to fix C3b covalently via C3b-acceptors (C3bAs) can be considered as one of the signs of their activation/differentiation. All the mature macrophages positive in the IA test were also found to express HLA-DR antigens on their membrane. Using solubilized extracts of stimulated, 35S-cysteine-labelled cells of the human monocytic cell line, U937, we demonstrate that C3 synthesized by these cells can bind to C3bAs of the same cells. Covalently fixed C3 fragments were found to inhibit Fc gamma-receptor-mediated ingestion of immune complexes and also antibody-dependent cellular cytotoxicity of monocyte-derived macrophages.

Appearance of covalently bound antigen in immune complexes formed during the activation of complement

The effects of complement activation on the antigenic component of immune complexes have been studied, using 125I-BSA-rabbit anti-BSA-IgG complexes as models. Polyethylene glycol precipitates of 4 types of IC (those formed in native normal human serum, or NHS-containing EDTA, NHS-EDTA, and preformed soluble IC incubated in NHS or NHS-EDTA immediately after preparation) were analysed by sodium dodecyl sulphate-polyacrylamide slab gel electrophoresis. A characteristic difference in the distribution of 125I-BSA in the gel was observed between the NHS- and NHS-EDTA-treated samples. With the former type of IC, a significant part (16-23%) of the label was found in gel fractions of mol. wt. exceeding that of the antigen (80-300 kDa vs. 69 kDa), whereas with NHS-EDTA-treated IC only a minimal amount (4-5%) of the radioactivity was detected in these fractions. These findings indicate that complement activation results in the covalent binding of complement to the antigenic component of IC. The practical importance of these observations is discussed. In addition, a marked difference in precipitability was observed between IC formed in NHS and preformed IC incubated in NHS.

C3b covalently bound to IgG demonstrates a reduced rate of inactivation by factors H and I

We have prepared C3b covalently linked to IgG via a hydroxylamine-sensitive bond between the C3b alpha' chain and sites predominantly, but not exclusively, located in the IgG heavy chain. This C3b species displays relative resistance to inactivation by factors H and I when compared with free C3b. This resistance appears to be due entirely to reduced affinity of C3b-IgG for factor H. Resistance to inactivation is not conferred on C3b by binding to another serum glycoprotein of similar size, ceruloplasmin, and may be a special property of IgG. C3b-IgG demonstrates an enhanced capacity to consume serum C3 relative to C3b. These alterations of the behavior of C3b when bound to IgG may in part explain the augmentation of alternative pathway activity by IgG. In addition, IgG-induced protection of C3b might influence both complement-mediated killing and phagocytosis of bacteria, as well as modify the in vivo handling of IgG-containing soluble immune complexes.