C3 cleaved by membrane proteases binds to C3b acceptors expressed on concanavalin A-stimulated human lymphocytes and enhances antibody-dependent cellular cytotoxicity

The versatile functions of complement C3-derived ligands

The complement system is a major component of immune defense. Activation of the complement cascade by foreign substances and altered self-structures may lead to the elimination of the activating agent, and during the enzymatic cascade, several biologically active fragments are generated. Most immune regulatory effects of complement are mediated by the activation products of C3, the central component. The indispensable role of C3 in opsonic phagocytosis as well as in the regulation of humoral immune response is known for long, while the involvement of complement in T-cell biology have been revealed in the past few years. In this review, we discuss the immune modulatory functions of C3-derived fragments focusing on their role in processes which have not been summarized so far. The importance of locally synthesized complement will receive special emphasis, as several immunological processes take place in tissues, where hepatocyte-derived complement components might not be available at high concentrations. We also aim to call the attention to important differences between human and mouse systems regarding C3-mediated processes.

NK cell-mediated lysis is essential to kill Epstein-Barr virus transformed lymphoblastoid B cells when using rituximab

Rituximab is a humanized chimeric monoclonal antibody, targeted against the pan B cell marker CD20. It is frequently used to treat a variety of B cell lymphomas and immunosuppression associated lymphoproliferations such as posttransplant lymphoproliferative disorder (PTLD). The response rate of rituximab treatment is 65%, but the exact in vivo mechanism of action is not yet fully understood, although antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and direct induction of apoptosis have been suggested as effector mechanism. Rituximab may affect different types of lymphomas through different mechanisms. As lymphoblastoid cell lines (LCLs) are well-established in vitro models of PTLD, we investigated the effect of rituximab on these cells using a custom built automated laser confocal fluorescent microscope. We found that rituximab alone was not effective at inducing cell death of EBV-transformed B cells. The antibody was effective in the complement-mediated CDC. Rituximab could induce NK cell-mediated ADCC but it was more effective in the presence of untreated fresh human plasma compared to heat-inactivated human plasma. Our data suggest that complement-enhanced NK-mediated ADCC is required for effective rituximab mediated killing of EBV-transformed B cells. Determining and monitoring of serum complement levels and in vitro killing efficacy of NK cells of PTLD patients might help to predict resistant cases to rituximab therapy. On the other hand our results suggest a possibility that rituximab should be combined only with cytotoxic drugs that spare NK function when treating PTLD patients.

C3 molecules internalize and enhance the growth of Lewis lung carcinoma cells

C3 molecules from normal murine serum are mainly bound to Lewis lung carcinoma cells (3LL) that do not express CRs, mainly through covalent binding as determined by the appearance of bands stained with anti-C3 and larger than 190 kD in immunoblots of proteins in whole cell extracts. Methylamine-treated, or zymosan-treated normal mouse serum, heat inactivated, or EDTA-treated murine serum resulted in low C3 deposition on 3LL cells, as indicated by fluorescence tests and immunoblotting. Cytofluorimetric studies showed that C3 molecules bound to 3LL cells were internalized in a time- and temperature-dependent process. This was confirmed by electronmicroscopic studies. The conditions allowing C3 fixation to acceptor sites and subsequent internalization increased cell proliferation. This was also true, when serum from mice genetically deficient in C5 was used which stresses the role of C3 in contrast to effects of membrane attack complex formation.

A trypsin-like serine protease activity on activated human B cells and various B cell lines

We have studied the trypsin-like serine protease activity of human tonsillar B lymphocytes. The lysate of the low-density, in vivo activated B cells as well as the lysate of cells stimulated with anti-human IgM F(ab')2 show elevated trypsin-like serine protease activity compared to the resting subset as monitored by the cleavage of Tos-Gly-Pro-Arg-pNA. The cleavage is sensitive to N-tosyl-L-lysyl-chloromethyl ketone and benzamidine but not to iodoacetamide. Experiments with intact cells give similar results. The finding that the intact cells hydrolyze the substrate, while their supernatant does not, suggests that the protease activity is cell membrane associated. It is possible that C3 is a substrate of the enzyme since the activated B cells cleave C3, whereas the resting B cells do not, and also C3 inhibits the enzyme-substrate reaction. In addition to the ex vivo B cells, we studied the serine protease activity of certain well-characterized B cell lines. The results show a correlation between the phenotype and the enzyme expression of the cell lines. BL41, an Epstein-Barr virus (EBV)-negative Burkitt lymphoma line, with a resting phenotype, has low activity, while its EBV genome-carrying convertants E95-A-BL41, E95-C-BL41, EHR-A-BL41 and BL41/95 that have the phenotype of activated B cells, have high proteolytic activity. The lymphoblastoid cell line WW-1-LCL which has the phenotype of an immunoblast, has the highest serine protease activity. On the basis of the above data, we suggest that a rather tight correlation exists between the degree of activation and the appearance of serine protease(s) on the surface of human B cells.

Complement research: biosynthesis, genetics, immunoregulatory role and clinical studies

Complementology is one of the major areas of immunological research in Hungary. Here, András Falus and colleagues describe studies on the control of biosynthesis of complement components and on the nature and function of receptors in the complement system.

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

Factors involved in macrophage: immune complex binding

The interaction of the immune complex (IC) composed of DNA and monoclonal anti-DNA antibody with thioglycollate-stimulated mouse peritoneal macrophages was investigated. The immune complex: macrophage interaction was shown to be highly time and temperature dependent; at 37 degrees C it proceeds faster than at 0 degrees C, although there is higher overall binding of IC to macrophages at 0 degrees C. The maximum bound IC detected was at a DNA/antibody ratio of 6.2ng/ml to 7.3 micrograms/ml. Higher densities of either DNA or antibody inhibited IC: phagocyte interaction. Binding of the IC to macrophages is through cell surface Fc receptors and is enhanced in the presence of 40 mg/ml albumin. Fresh human and mouse sera at the concentration of 10 percent, inhibited the IC binding to mouse peritoneal macrophages. Macrophage receptors for IC are not saturated even after 60 minutes. Addition of either chloroquine or cytochalasin B, resulted in increased binding of IC to macrophages.

Immunomodulatory effect of synthetic peptides corresponding to sequences within the CH2 and CH3 domain of human IgG1

The Fc region of IgG is known to be the source of small peptides possessing immunomodulatory function. Results are summarized showing the effect of synthetic peptides composed of surface exposed residues of C gamma 2 or C gamma 3 domains on different steps of human B lymphocyte activation cycle. Both the CH2 (289Thr-301Arg) and CH3 (407Tyr-416Arg) peptides as well as the whole Fc fragment enhanced the IgM synthesis of PWM or PMA + CaI activated lymphocytes. This effect was exerted at the early phase of B cell activation. The incubation of separated resting B cells with Fc fragments or CH2 peptides resulted in increase of cell volume and in expression of HLA-DR antigen. On the other hand, LIF production was induced both by CH2 and CH3 peptides. It was also shown that Fc peptides induce IL-1 release from monocytes. The results suggest that the CH2 and CH3 domain peptides exert their effect partly directly, by activating resting B cells, rendering the cells more susceptible to other stimuli; and moreover, by enhancing the humoral response by triggering the release of IL-1.

Lewis lung carcinoma cells enhance the synthesis of C3 and are opsonized by C3 secreted from murine macrophages

We investigated the effect of Lewis Lung carcinoma cells on the production of C3 by murine macrophages and examined the capacity of secreted C3 to opsonize Lewis Lung carcinoma cells. C3 released in culture from macrophages obtained from tumor-bearing C57Bl/6 mice as well as from normal macrophages exposed to Lewis Lung carcinoma cells in vitro was measured by hemolytic assays and by Western blot. We found that contact with tumor cells in vivo as well as in vitro enhanced the amount of C3 secreted by murine macrophages by a factor of 2-3. The inflammatory agent carrageenan caused only a small increase in the amount of secreted C3. On Western blots of concentrated macrophage supernatants, there was partial cleavage of secreted C3 which was, however, not more pronounced in the case of C3 from tumor-stimulated macrophages than from normal macrophages. Supernatants from normal as well as tumor-stimulated macrophages were capable of opsonizing Lewis Lung carcinoma cells as shown by their capacity to bind human erythrocyte in an immune adherence reaction. Pretreatment of the tumor cells with a protease inhibitor, PMSF, inhibited the capacity of the tumor cells to bind C3, suggesting that a tumor cell-associated protease might be involved in the binding of C3 to the tumor cell surface.

Elevated NK sensitivity of Raji cells carrying acceptor-bound C3 fragments

The majority of cell lines derived from Burkitt lymphomas carry CR2 on their plasma membrane cell lines of haematopoietic origin can activate C3 present in human serum through the alternative pathway. However, only the lines that carry CR2 were shown to bind C3 fragments. This bond can be either fixation to acceptor sites or attachment to the CR. Our studies with Raji cells showed that when the possibility for the covalent acceptor bond was eliminated by using methylamine (MA)- or zymosan-treated serum, considerably lower amounts of C3 were bound. In the zymosan-treated serum C3 fragments are present that can bind to receptors but their capacity for acceptor bond is absent. These results indicate that when Raji cell are incubated in human serum some of the generated C3 fragments are bound to acceptors and a lower proportion through the specific interaction with complement receptors. Pretreatment of the CR2 carrying cell lines with human serum elevated their sensitivity to the lytic effect of human blood lymphocytes. We showed in this work that MA-treated serum did not induce this elevation. Zymosan-treated serum under conditions that excluded activation of the residual native C3 molecules, i.e., in the presence of EDTA, did not have the enhancing effect either. These results suggest that the increased lytic efficiency imposed by human serum was due to cleavage of C3 molecules by Raji and fixation of the C3 fragments by acceptor sites. Natural killer cells carry CR3; therefore it is likely that the attached C3 fragments bind also to the effector cells. The C3 molecules could elevate thereby the avidity between the target and the lytic lymphocytes. The observation that C3 fragments are not bound to the surface of CR2 negative lines in spite of their capacity to activate C3 suggests that the receptor molecule is either involved in the activation and/or serves also as an acceptor.

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.

Alternative pathway of complement activation by stimulated T lymphocytes. II. Elevation of cytotoxic potential against complement receptor-carrying cell lines

Exposure of lectin-stimulated (concanavalin A, phytohemagglutinin and pokeweed mitogen) blood lymphocytes to human serum or to purified C3 increased their cytotoxic capacity towards complement receptor positive targets such as Raji and Daudi cells. The lysis of complement receptor-negative lymphoblastoid cell lines was not influenced. The lytic capacity of lymphocytes exposed to 12-O-tetradecanoylphorbol 13-acetate was not elevated by human serum. Lectin-stimulated lymphocytes were previously shown to activate and bind C3. The results using lymphocytes activated in different ways and targets with or without complement receptor expression suggest that the C3b deposited on lymphocytes binds to the complement receptor on the targets. This contact elevates the avidity between the two cells as indicated also by the increased frequency of the lymphocyte-target conjugates. On the basis of immune adherence the C3 fragment bound on the lymphocytes was identified as C3b. The increase of the conjugate formation and cytotoxicity was abrogated when the target cells, Raji, were pre-exposed to purified C3d which occupy the CR2 receptor. The majority of lymphocytes responsible for the cytotoxicity were CD8+.

Alternative pathway of complement activation by stimulated T lymphocytes. I. Binding of C3 fragments

Human blood lymphocytes cultured for 3 days with concanavalin A (Con A), phytohemagglutinin or pokeweed mitogen, in mixed lymphocyte culture with added interleukin 2 and stimulated by a lymphoblastoid cell line were found to activate and bind C3 molecules when exposed to human serum. The split products of C3 were detected in the supernatants and on the surface of the activated cells. The surface-attached C3 fragment on the Con A blast was identified as C3b by immune adherence i.e. binding of CR1 carrying human erythrocytes. In the Con A-stimulated population the majority of cells that activated and bound C3 were CD3 and Fc gamma receptor (CD16)-positive but complement receptor-negative blasts. In this cell subset both CD4 and CD8-positive cells were detected but their frequency suggested that a proportion of them carried both markers.

Effector or target cell selection mediated by C3 bridges

Potential effector cells (including stimulated lymphocytes and cultured monocytes) and potential target cells of NK and AK type cytotoxic reactions (including several lymphoblastoid cell lines) cleave the third complement component (C3). As a result of expression of C3bA sites such cells are able to bind covalently the activated C3b through its metastable binding site and thereby become "armed" by the C3b. This permits C3b-bridge formation between these cells and CR1-bearing cells. The "effector selection" (i.e. when C3b is bound covalently to potential target cells) or "target selection" (when C3b is covalently bound to C3bA sites on potential effector cells) mediated by C3b bridges results in enhanced killing capacity. Macrophages activate and bind C3b as well; but the covalent binding of C3b by these cells inhibits Fc receptor mediated ADCC type killing.

Involvement of complement in B-cell, T-cell and monocyte/macrophage activation

In the early 70's it had been shown, that for the immune response against T-dependent antigens C3 was necessary, while T-independent antigens, although activating the alternative pathway of complement, triggered antibody formation also in C-deficient mice. During recent years functional and biochemical knowledge about complement binding structures on B-cells and monocytes/macrophages continuously increased and, also, on T-cells C3 binding entities have been detected. In the case of B-cells and, at least in special experimental conditions, in the case of T-cells C3 can exert a proliferative response as long as the cells are prestimulated (excited) by anti-Ig or IL-2, respectively. Monocytes can bind C3b- or iC3b-carrying particles, but only when progressed to macrophages can they phagocytose such particles. Thus the concept evolves that B-cells, T-cells and monocytes can acquire competence for a C3-driven response when excited properly. The involvement of molecules such as CR1, CR2, factor H, IL-2-receptor and others with a basic structure of repeating units of 61 amino acids in the triggering processes is a surprising finding and certainly suggests their functional importance. In the case of T-independent antigens the structures triggering the alternative pathway of complement are the structures triggering monocytes directly. Whether these two functions have a causal relationship has to be shown.

A possible self-regulating mechanism mediated by C3b-acceptor-bound C3b generated by stimulated macrophages

Macrophages have been shown to produce C3 and to bear Fc receptors (FcR), and besides the various C3 receptors, they possess C3b acceptors (C3bA) as well as surface proteases capable of cleaving C3. Using the immune adherence method, we demonstrated that the amount of covalently fixed (i.e., C3bA-bound) C3b is markedly increased upon cell stimulation by phorbol myristate acetate or aggregated IgG, even in the absence of C3. The enhancement of nascent C3b (C3bx) binding to C3bA on these cells could be reversed by inhibiting the process at different stages, using either cycloheximide, phenyl-methyl-sulphonyl-fluoride, salycil hydroxamic acid, or methylamine. On the basis of our present results and earlier results, we propose a self-regulatory mechanism by which activated, C3-producing macrophages cleave C3 by their surface proteases. C3bx generated in this way fixes covalently to C3bA of the producer cells, resulting in the inhibition of FcR on these cells.

Mapping the functional topography of Fc gamma with monoclonal antibodies: localization of epitopes interacting with the binding sites of Fc receptor on human K cells

A panel of monoclonal antibodies (mAb) specific for the C gamma 2, C gamma 3 or inter C gamma 2/C gamma 3 domain epitopes was tested for inhibition of antibody-dependent cellular cytotoxicity (ADCC) specific for anti-D IgG-coated erythrocytes. Significant inhibition of ADCC was demonstrable for some antibodies having specificity for C gamma 2 or C gamma 3 domain epitopes, while others gave no inhibition. Fab fragments of a representative C gamma 2-specific antibody (A55) and C gamma 3-specific antibody (x3a8) retained their inhibitory capacity in lymphocyte-mediated ADCC, but only A55 Fab inhibited monocyte-mediated lysis. Furthermore, the Fab portion of A55 completely abolished the complement-dependent enhancement of ADCC mediated by concanavalin A-stimulated cells, while x3a8 Fab had no effect in this system. On the other hand, x3a8 Fab inhibited the binding of anti-D IgG-sensitized erythrocytes to lymphocytes while A55 Fab did not influence this latter interaction. The results suggest that C gamma 2 domain-FcR interaction is essential for the triggering of lytic process both in lymphocyte and in monocyte-mediated ADCC, while C gamma 3 domain has no role in the latter but is responsible for the appropriate contact between effector lymphocytes and target cells. A site in the region of Lys274 appears to be critical for triggering of both lymphocyte and monocyte-mediated ADCC.

Disialoganglioside GD3 on human melanoma serves as a relevant target antigen for monoclonal antibody-mediated tumor cytolysis

Monoclonal antibody MB3.6 (IgG3) recognizes disialoganglioside GD3, which represents a major surface marker on most human melanoma cells. We demonstrate that this antibody effectively lyses four human melanoma cell lines expressing significant levels of GD3 on their surface by either of two mechanisms: antibody-dependent cellular cytotoxicity (ADCC) or complement-mediated cytotoxicity. However, a melanoma cell line that expresses minimal levels of GD3 on 13% of the cells shows insignificant lysis by MB3.6 by either of these two mechanisms, suggesting that a threshold level of antigen expression may be required for effective in vitro cytolysis. In addition, monoclonal antibody (MAb) MB3.6 effectively inhibits establishment and growth of human melanoma tumors in the nude mouse when injected 24 hr after subcutaneous inoculation of tumor cells. Furthermore, MB3.6 produces specific regression of established melanoma tumors when injected 7 days after the subcutaneous inoculation of tumor cells. In contrast, tumor growth in animals injected with the melanoma cell line expressing minimal levels of GD3 was not affected by MAb MB3.6. These data indicate that once appropriate levels of the GD3 ganglioside are expressed on human melanoma cells, MAb MB3.6 can mediate tumor cell killing in vitro and in vivo and, thus, may prove useful for effective immunotherapy of human malignant melanoma.

Complement-dependent cellular cytotoxicity: lymphoblastoid lines that activate complement component 3 (C3) and express C3 receptors have increased sensitivity to lymphocyte-mediated lysis in the presence of fresh human serum

Lymphocyte-mediated lysis of cells of the Raji, Daudi, Jijoye, and Bjab lines was elevated when fresh human serum was added to the assay. A higher proportion of effector-target conjugates was observed in the presence of human serum. In similar experiments lysis of 1301, Rael, and P3HR-1 cells was unaltered. All cell lines activated the alternative pathway of complement but they varied in the expression of receptors for complement component 3 (C3) and in the ability to fix the C3 cleavage products on their membrane. The enhancement of lysis in the presence of human serum occurred only with those cells that bound C3. This characteristic was correlated to the expression of C3 receptors. Analysis of the nature of the deposited C3 was performed with Raji cells. Raji cells exposed to human serum bound C3b as indicated by the immunoadherence test. The C3b was further processed to C3bi, because the immunoadherence declined with time and conjugate formation increased with Daudi cells, which carry the C3 receptors CR2 and CR3. This suggests that in the lytic assay lymphocytes with C3bi receptors are recruited in the presence of human serum. We assume that the bridge of C3 molecules between targets and effectors increases the avidity of their interaction.

The selective localization of B lymphocytes in the spleen and the role of complement receptors

The role of complement receptors on the localization of T and B cells in the spleen of mice was studied using short-term homing experiments in cobra venom factor (CoF)-treated animals. The localization ratio of B and T cells in the spleen of CoF-treated mice decreased significantly compared to control recipients. No changes could be found in the relative distribution of resident T and B cells in the spleen or other lymphoid organs of CoF-treated animals and when their spleen or lymph node cells were transferred, the localization pattern was normal. When cells were incubated in serum prior to transfer a disturbed localization ratio in the spleen of untreated recipients was observed. This was due to a blockade of complement receptors as determined by the inability of the incubated cells to form EAC rosettes. No blockade of EAC rosettes and no changes in localization ratios upon transfer could be observed when the cells were incubated in functionally C3-depleted serum. The results suggest a role for the complement-receptor on B cells in the initial localization in the spleen, whereas no influence upon the selective localization in high endothelial venules-bearing organs was found.

Functional cooperation of C3b-acceptors, Fc gamma-receptors and cell-surface proteases on macrophages

Macrophages are FcR-positive cells, synthetize complement components and express proteolytic enzymes on their surface. In this paper a functional cooperation of C3b acceptor (C3bA) sites, which bind covalently nascent C3b molecules via their metastable binding site, IgG FcRs and cell surface proteases are described and the possible importance of this cooperation in regulation of immune response is discussed. It was found that isolated monocytes did not express C3bA in contrast to cultured macrophages which showed immune adherence positivity. Stimulation of macrophages resulted in enhanced expression of C3bA. C3 synthetized by macrophages was shown to be cleaved by cellular proteases which resulted in the binding of nascent C3b to C3bA. C3bA-nascent C3b interaction inhibited FcR-dependent effector functions, such as immune complex phagocytosis and antibody-dependent cellular cytotoxicity.

Contribution of activated C3 to lymphocyte-mediated target lysis: complement-dependent cellular cytotoxicity

Lysis of Raji cells by human lymphocytes was found to be enhanced if human serum was added to the assay. This was not due to the contribution of antibodies because hypogammaglobulinemic serum also augmented cytotoxicity. The results suggest that the mechanism of the enhancement was due to activation of C3 by the Raji cells. We assume that the cleavage products are deposited on cell surfaces in such a way that they contribute to contact between effector and target. Previous reports in two antibody-dependent cellular cytotoxicity systems provided other examples for the existence of this phenomenon.