Striking inverse correlation between IgG anti-F(ab')2 and autoantibody production in patients with cold agglutination

Previous experiments showed that the physiologic IgG anti-F(ab')2 antibody suppresses the response of human autoreactive B cells. In the present study, we analyzed the IgG anti-F(ab')2 antibody in 293 patients with cold agglutination (CA). Their average IgG anti-F(ab')2 titer was not much different (211 +/- 8.3) from that of 279 healthy persons (195 +/- 6.7). However, CA patients with high anti-F(ab')2 titers had low CA autoantibody titers and vice versa (P = .0028; rho = -0.175). The stratification of patients according to the auto-antibody's specificity (anti-I, anti-i, anti-Pr) showed an inverse correlation between anti-F(ab')2 and CA in the anti-I group (P = .0057; rho = -0.180). Interestingly, the association was present only in patients whose disease was caused by noninfectious agents (P < .0001; rho = -0.423). The inverse correlation argues for an important role of the IgG anti-F(ab')2 in the regulation of autoantibody production in CA patients.

Influence on myoadenylate deaminase function in rat skeletal muscle after homologous and heterologous immunization with the purified enzyme

The aim of the study was to shed some light on the possibility of anti-MAD autoantibodies in cases of rhabdomyolysis or microtraumatic muscular damage causing the symptoms of the acquired myoadenylate deaminase deficiency. Therefore a homologous and a heterologous immunization of rats was carried out with myoadenylate deaminase (MAD; EC 3.5.4.6.) isolated from rat white gastrocnemius muscle (type IIb) and rabbit skeletal muscle. Antibody response to homologous and heterologous antigens was quantified by ELISA and by the inhibition of MAD activity in vitro. Anti-MAD IgG antibodies obtained by homologous immunization caused an inhibition rate of 80% when using MAD from rat white gastrocnemius muscle. Furthermore, the function of MAD in homologously and heterologously immunized rats was investigated after maximal swimming exercise in comparison to non-immunized animals by determination of muscle ammonia and by HPLC detection of inosine 5'-monophosphate (IMP) in skeletal muscle tissues. Both IMP and ammonia accumulation were significantly reduced in type IIb muscle fibers after homologous immunization of rats compared to non-immunized controls (n = 7), whereas the concentration of muscle lactate showed no differences. According to these results we propose that by homologous immunization with purified MAD, anti-MAD IgG impairs the function of cytoplasmic MAD during vigorous exercise in vivo.

[Pharmacological modulation of the complement system: an opportunity for successful xenotransplantation]

Hyperacute graft rejection triggered by the activation of the recipient's complement system represents the major obstacle to a successful xenotransplantation. Inhibition of complement activation is, therefore, considered as a prerequisite for xenograft survival. Support of the physiological regulation of the complement system appears to be the most promising strategy as indicated by first results from animal xenograft experiments. The transfer of human membrane-bound complement regulatory proteins offers new chances to protect the xenograft against the cytolytic complement attack. Another approach aims at interfering with receptor/ligand interaction of the adhesion molecules CR3 and CR4 (CD11b,c/CD18). All strategies of complement intervention have to consider the important function of complement within the immunological defense.

Extracorporeal circulation: in vivo and in vitro analysis of complement activation by heparin-bonded surfaces

Complement activation was analyzed during extracorporeal CO2 removal to compare heparin-coated with standard surfaces where systemic heparinization was required. In vivo studies were performed in adult sheep for up to 5 days under standardized conditions using a capillary membrane oxygenator. Applying assays for hemolytic complement function (CH50, APH50) and C3-derived split products, we found that complement activation was markedly reduced in sheep connected to an extracorporeal circuit where heparin was covalently bound by end-point attachment. In addition, incubation of human serum in a miniaturized circulation system revealed less complement activation by heparin-bonded surfaces, as evaluated by enzyme-linked immunosorbent assays for C3a and the activation-specific protein-protein complexes, C1rsC1 inhibitor (classical pathway) and C3b(Bb)P (alternative pathway). Our results provide further evidence that biocompatibility can be improved by end-point attachment of heparin to the surfaces of the extracorporeal circuit.

Complement activation by recombinant HIV-1 glycoprotein gp120

The mechanism of CD4+ cell depletion in HIV-infected patients is poorly understood. In this study we investigated whether rgp120 can activate the complement system in the absence of anti-gp120 Abs. We found that the complement proteins C4, C3d, C5b-9, and properdin bind to rgp 120-coated CD4+ T cells of healthy individuals when incubated in autologous serum. Activation of the complement system occurred primarily via the classical pathway and was abolished in sera deficient in C1q and C4 as well as in the presence of EDTA. No cell lysis was observed in a lymphocytotoxicity assay using human serum, possibly because of homologous restriction of complement lysis. In contrast, addition of rabbit sera induced lysis of the rgp 120-precoated cells. Cell lysis by rabbit serum was found to be because of naturally occurring IgM anti-gp 120 Abs. The rgp 120, which was immobilized on the surface of microtiter plates activated complement in the absence of lymphocytes. Complement activation by cell-bound HIV-1 envelope glycoprotein gp120 with subsequent opsonization may be relevant for the elimination of noninfected CD4+ T cells in HIV-infected patients.

[Significance of the complement system for xenotransplantation: strategies for therapeutic intervention]

Hyperacute graft rejection triggered by the activation of the recipient's complement system represents the major obstacle to successful xenotransplantation. After the binding of preformed antibodies to vascular glycoproteins complement-induced activation and injury of endothelial cells with subsequent thrombosis leads to rapid destruction of foreign tissues. Inhibition of complement activation is therefore considered as a prerequisite for xenograft survival. Recent animal and cell culture experiments suggest that support of the physiological regulation of the complement system appears to be most promising. Besides the application of soluble complement inhibitors (e.g. soluble complement receptor 1, sCR1; C1 inhibitor) the genetic transfer of human membrane-bound complement regulatory proteins (e.g. DAF, CD59) offers new chances to protect the xenograft against the cytolytic complement attack. Results from the authors' experiments shall be included in a short overview to the issue.

Secreted chondroitin sulfate proteoglycan of human B cell lines binds to the complement protein C1q and inhibits complex formation of C1

We recently characterized a species of proteochondroitin sulfate (CSPG) secreted by human B cell lines that closely resembles in its structure the serum-derived C1q inhibitor (C1qI). These proteoglycans have in common a molecular mass of approximately 130 to 150 kDa with a core protein of 30 kDa to which up to four chondroitin sulfate chains each of approximately 26 kDa are attached. Since this B cell-derived CSPG is a potential source for serum C1qI, we measured its capacity to interact with C1q in solid-phase binding and complex electrophoresis assays. B cell CSPG purified from culture supernatants of the two human B cell lines JOK-1 and U266 strongly bound to C1q. In contrast to the secreted form, cellular proteoglycan of the myeloma cell line U266 did not interact with C1q. Binding of C1q to CSPG was competitively inhibited by free glycosaminoglycans (GAG) in the order dextran sulfate > heparin > heparan sulfate > chondroitin-6-sulfate (CS-C) > dermatan sulfate (CS-B) > chondroitin-4-sulfate (CS-A). B cell CSPG inhibited the hemolytic activity of C1q and C1. In addition, B cell CSPG blocked C1q receptor binding in a dose-dependent manner. The proteoglycans did not influence the activity of C1 complex already bound to EAC4 target cells. By interaction of CSPG with solid-phase-bound C1q, formation of the C1 complex upon the addition of C1r and C1s was impaired. Strong binding of B cell CSPG to C1q, its inhibition of C1q activity, and its structural similarities to the previously described human serum C1qI indicate that B cells produce a soluble CSPG, which may act as C1qI under physiologic conditions.

Identification of complement activation sites in human immunodeficiency virus type-1 glycoprotein gp120

Recombinant glycoprotein 120 (rgp120) of human immunodeficiency virus type-1 (HIV-1) activates the human complement system in the absence of anti-gp120 antibodies. HIV-1 glycoprotein gp120 can dissociate from the viral envelope either spontaneously or after binding of HIV-1 to the CD4 molecule. As a consequence, gp120 can circulate in the patient's serum and attach to the surface of uninfected CD4+ T cells. Complement activation by cell-bound HIV-1 glycoprotein gp120 with subsequent opsonization may represent a mechanism for the elimination of uninfected CD4+ cells by the reticuloendothelial system, thereby enhancing the progression of HIV disease. In the current study, the complement proteins C4,C3,C5,C9, and properdin were found to bind to a synthetic peptide covering positions 233-251 of the gp120BRU sequence on incubation with normal human serum. Complement activation by the peptide was comparable with that induced by aggregated IgG, complete rgp120, and the previously described complement-activating gp41-peptide 609-623. Activation occurred via the classical pathway and was abrogated in the presence of EDTA, Mg2+/EGTA, or C4-deficient human serum. Peptides partly overlapping the sequence 233-251 activated complement to a lesser extent. The complement-activating capacity of the gp120 sequence 233-251 was not restricted to the HIV-1BRU isolate, because a peptide from the corresponding sequence of the HIV-1MN strain was also capable of activating complement. An additional strong complement-activating site was identified in the gp120 sequence 321-360 of the HIV-1MN strain. These data indicate that distinct sites in gp120 are able to activate human serum complement via the classical pathway in the absence of anti-gp120 and independent of glycosylation.

Secreted chondroitin sulfate proteoglycan of human B cell lines binds to the complement protein C1q and inhibits complex formation of C1

We recently characterized a species of proteochondroitin sulfate (CSPG) secreted by human B cell lines that closely resembles in its structure the serum-derived C1q inhibitor (C1qI). These proteoglycans have in common a molecular mass of approximately 130 to 150 kDa with a core protein of 30 kDa to which up to four chondroitin sulfate chains each of approximately 26 kDa are attached. Since this B cell-derived CSPG is a potential source for serum C1qI, we measured its capacity to interact with C1q in solid-phase binding and complex electrophoresis assays. B cell CSPG purified from culture supernatants of the two human B cell lines JOK-1 and U266 strongly bound to C1q. In contrast to the secreted form, cellular proteoglycan of the myeloma cell line U266 did not interact with C1q. Binding of C1q to CSPG was competitively inhibited by free glycosaminoglycans (GAG) in the order dextran sulfate &gt; heparin &gt; heparan sulfate &gt; chondroitin-6-sulfate (CS-C) &gt; dermatan sulfate (CS-B) &gt; chondroitin-4-sulfate (CS-A). B cell CSPG inhibited the hemolytic activity of C1q and C1. In addition, B cell CSPG blocked C1q receptor binding in a dose-dependent manner. The proteoglycans did not influence the activity of C1 complex already bound to EAC4 target cells. By interaction of CSPG with solid-phase-bound C1q, formation of the C1 complex upon the addition of C1r and C1s was impaired. Strong binding of B cell CSPG to C1q, its inhibition of C1q activity, and its structural similarities to the previously described human serum C1qI indicate that B cells produce a soluble CSPG, which may act as C1qI under physiologic conditions.