Role of C3 in in vitro lymphocyte cooperation

Contribution of animal models to the mechanistic understanding of Alternative Pathway and Amplification Loop (AP/AL)-driven Complement-mediated Diseases

This review aimed to capture the key findings that animal models have provided around the role of the alternative pathway and amplification loop (AP/AL) in disease. Animal models, particularly mouse models, have been incredibly useful to define the role of complement and the alternative pathway in health and disease; for instance, the use of cobra venom factor and depletion of C3 provided the initial insight that complement was essential to generate an appropriate adaptive immune response. The development of knockout mice have further underlined the importance of the AP/AL in disease, with the FH knockout mouse paving the way for the first anti-complement drugs. The impact from the development of FB, properdin, and C3 knockout mice closely follows this in terms of mechanistic understanding in disease. Indeed, our current understanding that complement plays a role in most conditions at one level or another is rooted in many of these in vivo studies. That C3, in particular, has roles beyond the obvious in innate and adaptive immunity, normal physiology, and cellular functions, with or without other recognized AP components, we would argue, only extends the reach of this arm of the complement system. Humanized mouse models also continue to play their part. Here, we argue that the animal models developed over the last few decades have truly helped define the role of the AP/AL in disease.

The Pentraxins 1975-2018: Serendipity, Diagnostics and Drugs

The phylogenetically ancient, pentraxin family of plasma proteins, comprises C-reactive protein (CRP) and serum amyloid P component (SAP) in humans and the homologous proteins in other species. They are composed of five, identical, non-covalently associated protomers arranged with cyclic pentameric symmetry in a disc-like configuration. Each protomer has a calcium dependent site that mediates the particular specific ligand binding responsible for all the rigorously established functional properties of these proteins. No genetic deficiency of either human CRP or SAP has been reported, nor even any sequence polymorphism in the proteins themselves. Although their actual functions in humans are therefore unknown, gene deletion studies in mice demonstrate that both proteins can contribute to innate immunity. CRP is the classical human acute phase protein, routinely measured in clinical practice worldwide to monitor disease activity. Human SAP, which is not an acute phase protein, is a universal constituent of all human amyloid deposits as a result of its avid specific binding to amyloid fibrils of all types. SAP thereby contributes to amyloid formation and persistence in vivo. Whole body radiolabelled SAP scintigraphy safely and non-invasively localizes and quantifies systemic amyloid deposits, and has transformed understanding of the natural history of amyloidosis and its response to treatment. Human SAP is also a therapeutic target, both in amyloidosis and Alzheimer's disease. Our drug, miridesap, depletes SAP from the blood and the brain and is currently being tested in the DESPIAD clinical trial in Alzheimer's disease. Meanwhile, the obligate therapeutic partnership of miridesap, to deplete circulating SAP, and dezamizumab, a humanized monoclonal anti-SAP antibody that targets residual SAP in amyloid deposits, produces unprecedented removal of amyloid from the tissues and improves organ function. Human CRP binds to dead and damaged cells in vivo and activates complement and this can exacerbate pre-existing tissue damage. The adverse effects of CRP are completely abrogated by compounds that block its binding to autologous ligands and we are developing CRP inhibitor drugs. The present personal and critical perspective on the pentraxins reports, for the first time, the key role of serendipity in our work since 1975. (345 words).

Multinucleated Giant Cells Are Specialized for Complement-Mediated Phagocytosis and Large Target Destruction

Multinucleated giant cells (MGCs) form by fusion of macrophages and are presumed to contribute to the removal of debris from tissues. In a systematic in vitro analysis, we show that IL-4-induced MGCs phagocytosed large and complement-opsonized materials more effectively than their unfused M2 macrophage precursors. MGC expression of complement receptor 4 (CR4) was increased, but it functioned primarily as an adhesion integrin. In contrast, although expression of CR3 was not increased, it became functionally activated during fusion and was located on the extensive membrane ruffles created by excess plasma membrane arising from macrophage fusion. The combination of increased membrane area and activated CR3 specifically equips MGCs to engulf large complement-coated targets. Moreover, we demonstrate these features in vivo in the recently described complement-dependent therapeutic elimination of systemic amyloid deposits by MGCs. MGCs are evidently more than the sum of their macrophage parts.

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MGCs are specialized for phagocytosis of large and complement-opsonized particles

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MGCs show extensive membrane ruffles containing pre-activated complement receptor 3

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Membrane ruffles provide excess membrane for ingestion of large materials

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MGCs eliminate systemic amyloid deposits after immunotherapeutic targeting

A Novel Mechanism for Complement Activation at the Surface of B Cells Following Antigen Binding

Coligation of CD21 with BCR on the surface of B cells provides a costimulatory signal essential for efficient Ab responses to T-dependent Ags. To achieve this, Ag must be directly linked to C3 fragments, but how this occurs in vivo is not fully understood. Using BCR transgenic mice, we demonstrated that C3 was deposited on the surface of B cells following both high- and moderate-affinity Ag binding. This was dependent on the specific binding of IgM to the BCR-bound Ag and can occur independently of soluble immune complex formation. Based on these data, we propose a novel model in which immune complexes can form directly on the surface of the B cell following Ag binding. This model has implications for our understanding of B lymphocyte activation.

The role of complement in the development of systemic lupus erythematosus

Complement has both beneficial and deleterious roles in the pathogenesis of systemic lupus erythematosus (SLE). On the one hand, patients with SLE present with decreased complement levels and with complement deposition in inflamed tissues, suggestive of a harmful role of complement in the effector phase of disease. On the other hand, homozygous deficiency of any of the classical pathway proteins is strongly associated with the development of SLE. There are two main hypotheses to explain these observations. The first invokes an important role for complement in the physiological waste-disposal mechanisms of dying cells and immune complexes. The second hypothesis is based around the role of complement in determining the activation thresholds of B and T lymphocytes, with the proposal that complement deficiency causes incomplete maintenance of peripheral tolerance. These two hypotheses are not mutually exclusive. In addition, there is evidence for a contribution from other genetic factors in determining the phenotype of disease in the absence of complement.

Interleukin 6 influences germinal center development and antibody production via a contribution of C3 complement component

Mice rendered deficient for interleukin (IL) 6 by gene targeting were evaluated for their response to T cell-dependent antigens. Antigen-specific immunoglobulin (Ig)M levels were unaffected whereas all IgG isotypes showed varying degrees of alteration. Germinal center reactions occurred but remained physically smaller in comparison to those in the wild-type mice. This concurred with the observations that molecules involved in initial signaling events leading to germinal center formation were not altered (e.g., B7.2, CD40 and tumor necrosis factor R1). T cell priming was not impaired nor was a gross imbalance of T helper cell (Th) 1 versus Th2 cytokines observed. However, B7.1 molecules, absent from wild-type counterparts, were detected on germinal center B cells isolated from the deficient mice suggesting a modification of costimulatory signaling. A second alteration involved impaired de novo synthesis of C3 both in serum and germinal center cells from IL-6-deficient mice. Indeed, C3 provided an essential stimulatory signal for wild-type germinal center cells as both monoclonal antibodies that interrupted C3-CD21 interactions and sheep anti-mouse C3 antibodies caused a significant decrease in antigen-specific antibody production. In addition, germinal center cells isolated from C3-deficient mice produced a similar defect in isotype production. Low density cells with dendritic morphology were the local source of IL-6 and not the germinal center lymphocytes. Adding IL-6 in vitro to IL-6-deficient germinal center cells stimulated cell cycle progression and increased levels of antibody production. These findings reveal that the germinal center produces and uses molecules of the innate immune system, evolutionarily pirating them in order to optimally generate high affinity antibody responses.

Immunomodulatory properties of Nocardia lysozyme digest (NLD) in complement normal and C5-deficient mice

The constantly increasing number of substances with adjuvant activity outpaces the elucidation of their mode of action. This problem is of great importance as the immunomodulatory action of an adjuvant is time- and route-dependent, which implies that administration at a different moment or site may result in a reduced immune response. In the present work the possibility to achieve dual effect (stimulatory or inhibitory) is regarded in the light of the complement system. The object of the study is a preparation obtained by lysozyme digestion of Nocardia opaca cell walls (NLD). According to the results, the administration of NLD to mice (i.p. at a daily dose of 0.5 mg kg-1) during 3 days prior to the antigen resulted in an inhibition of serum antibody level against sheep red blood cells (SRBC) and lipopolysaccharide (LPS). At the same time, the preparation stimulated the antibody response to SRBC if it was applied after the antigen. The ability of NLD to ensure protection against experimental infection with Klebsiella pneumoniae was comparatively studied in complement-normal mice (strain ICR) and in C5-deficient mice (strain DBA/2). Firstly, it was established that complement-deficient mice were more resistant to infection than complement-normal. Secondly, the preparation expressed a protective effect in C5-deficient animals; nevertheless the inoculation was done s.c. or i.v. The departure of the infection depended on the rate of opsonization of K. pneumoniae. Under certain conditions NLD can provoke excessive C3 activation, which might aggravate the course of the infection. The preparation augmented the host response to second challenge with K. pneumoniae of complement-normal and C5-deficient mice.

Expression of Complement Receptors 1 and 2 on Follicular Dendritic Cells Is Necessary for the Generation of a Strong Antigen-Specific IgG Response

Two mechanisms could account for the impaired humoral immune response found in Cr2−/− mice. The absence of complement receptors 1 and 2 (CR1, CR2) on B cells could affect their activation. Alternatively, impaired Ag trapping by follicular dendritic cells (FDC) could affect B cell maturation into Ig-secreting or memory B cells. To compare the roles of CR1 and CR2 on B cells vs FDC in this abnormal response, bone marrow (BM) chimeric mice were generated and immunized with specific T-dependent Ags. The primary and secondary Ab response was measured. Cr2+/+ animals reconstituted with a Cr2−/− BM generated a diminished but detectable humoral immune response compared with controls. When injected with preformed immune complexes (IC), these mice maintained follicular IC localization. Cr2−/− animals reconstituted with a Cr2+/+ BM had an initial rise in the Ab titer, but were unable to maintain it as shown by a pronounced decrease in the IgG titer. This defect persisted during the secondary immune response. Follicular IC trapping was also impaired. Despite the abnormal Ab response, germinal center formation was retained in all of the chimeric animals. These experiments are the first to demonstrate an absolute requirement for CR1 and CR2 expression on FDC in the generation of a normal humoral immune response.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Antibody response to a T-dependent antigen requires B cell expression of complement receptors

Several lines of evidence indicate that antibody responses to T-dependent antigens require complement receptors expressed on either B lymphocytes or follicular dendritic cells. We have used RAG-2 deficient blastocyst complementation to create mice specifically lacking B cell complement receptors. Despite normal expression of complement receptor 1 (CR1[CD35]) and CR2 (CD21) on follicular dendritic cells, these mice have a profound defect in their capacity to mount a T-dependent antibody response. This is the first direct demonstration in vivo that B cell expression of complement receptors is required for a humoral immune response. This is the first direct demonstration in vivo that B cell expression of complement receptors is required for a humoral immune response. This suggests that CD21 and/or CD35 on B lymphocytes may be required for cellular activation, adsorptive endocytosis of antigen, recruitment to germinal centers, and/or protection from apoptosis during the humoral response to T-dependent antigens.

Human natural anti-Gal IgG regulates alternative complement pathway activation on bacterial surfaces

One percent of circulating IgG in humans recognizes galactose alpha 1,3 galactose residues (anti-Gal) and is synthesized in response to stimulation by enteric bacteria. In this study, we found that the prevalence of binding of anti-Gal to blood isolates is significantly higher than its binding to normal stool isolates. When anti-Gal bound onto the lipopolysaccharide of a representative blood isolate, Serratia marcescens #21, it blocked its alternative complement pathway (ACP) lysis and made the organism serum resistant. In contrast, when anti-Gal bound to the capsular polysaccharide of a serum sensitive Serratia, #7, it increased ACP killing of this strain. The mechanism of blockade of ACP lysis by anti-Gal did not involve a decrease in the number of C3 molecules deposited onto Serratia #21 or an inhibition of the binding of C3b to its LPS, nor did it change the iC3b and C3d degradation products of bound C3b or prevent membrane attack complex formation on this organism. Our findings suggest that the effect of anti-Gal on immune lysis is dependent on the bacterial outer membrane structure to which it binds. We postulate that anti-Gal may play a role in the survival of selected Enterobacteriacae in Gram-negative sepsis by blocking ACP-mediated lysis of such bacteria by the nonimmune host, and that this effect depends on where anti-Gal finds its epitope on the bacterial outer membrane.

Rapid isolation and characterization of native mouse complement components C3 and C5

A rapid, 1 day procedure for the purification of mouse complement factors C3 and C5 is described. The method is based on fractionated precipitation by polyethylene glycol 6000, followed by Mono Q anion exchange chromatography on a system for fast protein liquid chromatography (FPLC). For C3 isolation, an additional FPLC separation step using Superose 12 (gel filtration) was used. C3 was purified 71-fold with a yield of 32% as measured by biological activity; the preparation contained no detectable contaminants as judged by SDS-PAGE. A comparable procedure for the isolation of C5 resulted in a preparation with a considerable contamination which could be easily removed by affinity chromatography using antibodies directed against these contaminants. With this combined procedure C5 was purified 536-fold with a yield of 28% based on biological activity. SDS-polyacrylamide gel electrophoresis revealed that mouse C3 and C5 had apparent Mrs of 170,000 and 190,000, respectively. Under reducing conditions the alpha and beta chains showed Mrs of 107,000 and 62,000 for C3, and 104,000 and 85,000 for C5.

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.

Enhancement of human B cell proliferation by an antibody to the C3d receptor, the gp 140 molecule

The C3d receptor is a specific marker of B lymphocytes. Recently we have shown that C3d receptor activity is carried by a gp 140 membrane antigen. A polyclonal antibody has been prepared by immunizing a rabbit with highly purified gp 140 molecule isolated from membranes of the human B lymphoblastoid cell line Raji and its high specificity was previously demonstrated. We tested the effect of this antibody to the C3d receptor on the B cell proliferative response. Purified B cells from human blood were induced to proliferate by a B cell growth factor (BCGF)-containing partially purified supernatant from activated T cells. The anti-C3d receptor F(ab')2 enhanced the BCGF-dependent B cell proliferation. This effect was dose dependent, was observed in the presence of different concentrations of BCGF and did not correspond to a change in the time course of the response. The anti-C3d receptor F(ab')2 had no mitogenic effect in the absence of T cell supernatant. In contrast the undigested anti-C3d receptor IgG suppressed the BCGF-dependent B cell proliferation. These results emphasize the potentialities of anti-gp 140 F(ab')2 to explore the involvement of the C3d receptor in the regulation of B cell response to T cell products.

Immunoregulatory effects of C3 and its major cleavage fragments

The immunoregulatory effects of highly purified C3 and its major cleavage fragments on in vitro responses of human peripheral blood mononuclear cells (PBMC) were examined. Polyclonal immunoglobulin production induced by PWM and T-cell proliferation stimulated by tetanus toxoid in human PBMC were inhibited by human C3b. This effect occurred in a dose range of 10(-6) to 10(-8) M and was observed in cultures containing human serum but not under serum-free conditions. This noncytotoxic suppression did not appear to be mediated by prostaglandin release nor by interference with antigen uptake and presentation by monocytes. In contrast, the addition of C3a resulted in a mild enhancement of immunoglobulin production and T-cell proliferation. This effect, however, was observed only under serum-free conditions and, thus, may not have physiologic importance. The native C3, from which the above fragments were generated, had no effect on these lymphocyte responses. These results suggest that regulation of immune responses may be one consequence of C3 cleavage during complement activation in vivo.

Complement and its fractions (C3-C4) pattern in subjects with neoplasia

The total complement (CH50) and its fractions C3 and C4 have been assayed for up to two years after surgery in subjects with breast, gastric and colon-rectum carcinomas. In all three types of pathology a constant pattern has been observed. Before surgery the CH50 stayed below the normal range but was increased following surgery. After a month it was again within the normal range and subsequently, according to the clinical evolution of the disease, it remained normal in those patients without relapse or any apparent metastasis, whereas in those patients who displayed metastasis and/or approached the terminal phase it fell below the normal range. The C3 fraction followed the CH50 pattern whereas the C4 did not show any variation correlated with the stages of the disease.

Anaphylatoxin-mediated regulation of the immune response. I. C3a-mediated suppression of human and murine humoral immune responses

The C3a fragment of the third component of complement was found to have immunosuppressive properties. C3a is capable of suppressing both specific and polyclonal antibody responses. In contrast, C3a had no effect on antigen- or mitogen-induced B or T cell proliferative responses. The carboxy-terminal arginine is essential for C3a to exhibit its immunosuppressive properties. The serum carboxypeptidase inhibitor 2-mercaptomethyl-5-guanodinopentanoic acid, which prevents cleavage of the terminal arginine that would produce C3ades Arg-77, allowed us to assay the effects of C3a on in vitro immune response systems where serum is required. When the terminal arginine is removed from C3a, the resulting C3ades Arg-77 molecule is nonsuppressive. Helper T lymphocytes are the target of C3a-mediated suppression of the immune response. Substitution of T cells by soluble T cell factors was found to abrogate the C3a suppressive activity.

Reduction in alternative complement pathway mediated C3 conversion following burn injury

Studies were performed to further investigate an abnormality of the alternative complement pathway associated with burn injury. C3 conversion by inulin and cobra venom factor was measured in serial serum samples from 18 burned patients and in the sera from 25 normal controls. C3 conversion by both activating substances in the patients' sera was significantly decreased during 50 days postburn in comparison to C3 conversion in the normal sera. The most marked reduction in C3 conversion occurred in patients with large full-thickness injuries and infectious complications. However, the abnormality was not more severe in patients with pneumonia and bacteremia in comparison to patients with bacteremia only. It was also not predictive of fatal outcome resulting from infectious complications. Preliminary evidence was presented to suggest that the abnormality is caused by a serum inhibitor, which has an affinity for cellulose and/or is inactivated by it.

Pathopysiological aspects of immune complex diseases. Part II. Phagocytosis, exocytosis, and pathogenic depositions

Elimination of IC by the phagocytic system occurs mainly by macrophages and contrarotates to the pathogenic effect. Decisive to prevent systemic IC disease is the capacity of the phagocytic system. In the case of its saturation, the danger of the occurrence of IC disease is greatly enhanced. Conclusive evidence seems to exist that IC of extremely small or extremely high lattice structure (precipitates) are less pathogenic than soluble IC of medium network. Small IC in extreme antigen and antibody excess or precipitates exhibit a reduced complement activating potency. Small IC in extreme antigen or antibody excess hardly interact in vitro with membrane receptors and do not induce IC disease when injected or formed in vivo. Highly lattices IC, especially precipitates, are eliminated extremely quickly from the circulation, mainly by macrophages and there deposition in the kidney is significantly reduced. Thus, lack of quality of the antibody to precipitate the antigen and a reduced capacity and effectivity of the phagocytic system to eliminate the IC may be extremely important in the generation of IC diseases. Facing the overwhelming and partly even inconsistant data of this topic, one may doubt whether IC diseases may be regarded to be a defined and coherent disease. Too many variables and questions exist concerning the nature of the antigen, especially in tumor and autoimmune diseases, concerning the quality of the antibody and the characteristics of the pathogenic IC and concerning localization and the elimination process. Nevertheless, common pathophysiological pathways of IC diseases may be recognized.

A sensitive hemolytic assay for mouse C3

We have developed a sensitive hemolytic assay for mouse C3, utilizing readily available guinea pig complement components and standard techniques, which allows C3 to be detected in mouse serum diluted many thousand-fold. Mouse serum also contains a potent, relatively heat-stable C3-inactivator system, which is largely blocked by Suramin.

Activation of B cell subsets by T-dependent and T-independent antigens

The capacity of the trinitrophenyl haptenic group coupled to a series of chemically dissimilar carriers to cross-stimulate putative T-dependent and T-independent B-cell subpopulations was determined by using an in vitro limiting dilution technique to generate primary IgM responses. TNP-Ficoll and TNP-dextran, two T-independent antigens with little or no polyclonal mitogenicity, stimulate the same population of anti-TNP precursors, which is distinct from the precursor population activated by TNP-LPS, a T-independent polyclonal mitogen, or by TNP-HRBC, a T-dependent antigen. TNP-LPS and TNP-HRBC activate the same precursor population, indicating that LPS can substitute for the T cell signal in T-dependent B-cell responses, whereas nonmitogenic T-independent antigens cannot. However, the cumulative evidence from this and other laboratories suggests that LPS and T-dependent antigens activate B cells by different mechanisms. TNP conjugates of Ficoll and dextran, which are relatively poor inducers of polyclonal B cell activation, induced larger anti-TNP clones than did TNP-LPS, a strong polyclonal mitogen. Macrophages are required for the anti-TNP-Ficoll/anti-TNP-dextran response, whereas, a similar requirement has not been shown for the anti-TNP-LPS response. Thus, macrophages may function as polyclonal B cell activators in T-independent responses. Experiments in which TNP was coupled directly onto the macrophage surface support this hypothesis. B-cell heterogenity in T-dependent responses is suggested by experiments using the C3 receptor as a marker for functional subpopulations of B cells. Murine T cells cooperate with B cells that carry a receptor for C3 and with at least some B cells which lack the C3 receptor in a primary in vitro antibody response. In vitro culture experiments using populations of B cells fractionated on the basis of the C3 receptor showed that CR+ cells were unable to make T-dependent antibody responses in the presence of anti-C3 antibody, whereas the response of CR- B cells was unaffected. Using irradiated, carrier-primed spleen cells from B10.A mice as a source of helper cells for B cells derived from various congenic strains in an in vitro primary IgM response to TNP-KLH, CR+ B cells cooperated across haplotype differences in the I region of the MHC, whereas CR- B cells did not. Preliminary mapping experiments for the genetic restriction of CR- B cells suggest complementation between the I-A and I-C subregions of the MHC. These findings tentatively suggest the existence of alternative cooperative pathways between T cells and B cell subpopulations.

Methods for the identification of human B lymphocytes

Complement-receptor lymphocytes and monocytes were identified by a rosette method using trypsin treated sheep erythrocytes (Et), sensitized with affinity column purified IgM (19S) antibodies against sheep erythrocytes (E) and mouse complement deficient in C5. Fc receptor mononuclear cells were identified by a rosette method using Et and IgG (7S) antibodies against E. Identification of the cell-type rosetting was facilitated by myeloperoxidase staining of dry mounted rosetted preparations. Comparison of lymphocytes with receptors for activated mouse complement and lymphocytes with stable surface immunoglobulin detected by immunofluorescent assay, strongly suggests that these cells constitute the same population in the peripheral blood of healthy humans.

Complement receptors and cell associated complement components

Membrane receptors for activated complement components are widely distributed amongst tissue cells of most mammalian species. Common amongst these are receptors for C3b which mediate many of the biological functions of C3. In addition, the genetic control of certain complement components is linked to the genes which code for the major histocompatibility complex. Many of these components are also present on cell surfaces. This suggests that the function of the complement system and the major histocompatibility complex may be related.

The expression of differentiated functions in somatic cell hybrids: retention and activation of C3 production

Rat and mouse fibroblasts which do not secrete the third component of complement (or secrete it at a very low rate), were respectively hybridized with mouse and rat hepatoma cells which actively produce C3. These interspecific hybrids actively secrete both mouse and rat C3. The production of C3 coded by the hepatoma cell genome is thus retained, but in addition, the secretion of C3 coded by the fibroblast genome is activated. On the other hand, intraspecific hybrids have been isolated from normal diploid C3 producing cells and non-producing cells. These cell hybrids also retain the capacity to secret C3. Retention of C3 production thus does not appear to be peculiar to hybrides derived from hepatoma cells.

The requirement for C3 receptors on the precursors of 19S and 7S antibody-forming cells

The main conclusion from this study is that C3 receptors are not required for the generation from B cells of a thymus-dependent 7S antibody response. The requirement for C3 receptors on the precursors of antibody-forming cells was studied in an adoptive transfer system using thoracic duct lymphocytes (TDL) from primed rats as a source of precursors and irradiated recipients as hosts. 7S precursors were found in both the CR+ and the CR- fractions of TDL and it was established that the response transferred by CR- cells did not arise from either a raidoresistant B cell in the host or from CR+ cells contaminating the CR- population. Thus, the C3 receptor is not obligatory for B-cell-T-cell cooperation in the 7S response. The precursors of 19S antibody-forming cells were found only in the CR+ subpopulation. The CR-Ig+ subpopulation was shown to contain all the B blasts in rat TDL and a very small number (approximately 1% of all TDL) of small lymphocytes. This latter population contained the CR- 7S precursors and contributed approximately 20% of the total adoptive secondary 7S response transferred by CR+ and CR- subpopulations combined. This observation suggests that the percentage of rat TDL committed to carry 7S memory is small, a conclusion which is confirmed and extended in the following paper.

Defective opsonization. A common immunity deficiency

Serum opsonization of yeasts for phagocytosis by normal polymorphonuclear leucocytes was defective in 11 of 43 children with unexplained frequent infections. The children had a range of infections, largely bacterial, and only 3 had diarrhoea and rash in infancy. A similar defect in at least 6 of the 9 mothers of these children (of either sex), with normal function in the fathers, suggests that the defect was primary and was transmitted by an unusual form of dominant inheritance. Four of 72 healthy adults and 1 of 11 children with unrelated disease showed similar defective function, but the incidence of the defect in the patients with frequent infection was significantly greater than this. The defective function can be corrected, in vitro and in vivo, by normal plasma at concentrations too low to be effective alone. This suggests that there is a defective factor rather than an inhibitor, and that different factors are limiting in normal and in defective plasma. Sera from affected members of the same family do not correct each other, but defective sera from different families usually do.

Differences in susceptibility of mature and immature mouse B lymphocytes to anti-immunoglobulin-induced immunoglobulin suppression in vitro. Possible implications for B-cell tolerance to self

Purified goat antibodies against mouse mu-chains and rabbit antibodies against mouse Ig determinants, and their Fab fragments, inhibited the development of IgM-bearing B cells in explant cultures of 14-day mouse fetal liver, and caused the disappearance of cell surface IgM in explant and dissociated cell cultures of more developed lymphoid tissues. While treatment of cultures of fetal or newborn liver, or adult bone marrow, with low concentrations (less than or equal to 10 mug/ml) of anti-Ig for less than or equal to 24 h caused the complete, but reversible, disappearance (modulation) of cell surface IgM, treatment for greater than or less than 48 h produced irreversible IgM suppression. In contrast, anti-Ig-induced suppression of cell surface IgM in cultures of adult spleen or lymph nodes required much higher concentrations of antibody (greater than or equal to 100 mug/ml) and was always reversible. These differences between immature and mature IgM-bearing cells could not be related to differences in the amount of surface IgM on the cells. The remarkable sensitivity of newly formed B cells to IgM modulation and irreversible IgM suppression when ligands bind to their Ig receptors, may have important implications for B-cell tolerance to self antigens.

A mouse hepatoma cell line which secretes several serum proteins including albumin and alpha-foetoprotein

A permanent cell line (BW) was established from a transplantable mouse hepatoma, BW7756, which produces alpha-foetoprotein (AFP). Three clones were isolated from the uncloned culture: BW1, BW2 and BWTG3. The cells of the latter clone, which was isolated after selection in the presence of thioguanine, are deficient in the enzyme hypoxanthine-guanine-phosphoribosyl transferase. Both BW1 and BWTG3 cells have mean chromosome number of 64 (60 telocentric and 4 metacentric chromosomes). All three clones secrete at least five serum proteins into the culture medium: albumin, AFP, and alpha 2 globulin, transferrin and C3, the third component of complement. The approximate rate of albumin secretion by BW1 and BWTG3 cells is 10 mug/24 h/10(6) cells. Both albumin and AFP can easily be detected in cell extracts. The simultaneous production of AFP and a hepatocyte specific marker (albumin) by cloned hepatoma cells show that the production of AFP by the tumour is due to the tumoural hepatocytes themselves.

Stimulation of murine B lymphocytes by isolated C3b

Addition of isolated B3b to murine lymph node cell cultures induced increased DNA synthesis. The stimulation is dependent on the dose of C3b added and is potentiated by fetal calf serum present in the medium. Isolated C3 is less stimulatory than C3b; C3a and C3c had no effect on DNA synthesis in these cultures. 10-20% large immunoglobulin containing blastlike cells developed in lymph node cell cultures stimulated by 50 mug C3b in the presence of 3% fetal calf serum. The stimulation by C3b was observed in cultures of lymph node and spleen cells of several mouse strains including C3H/HeJ and congenitally thymus-deficient (nu/nu) mice. The results suggest that B lymphocytes are the main target of the stimulatory effect of C3b. Two mechanisms which may be involved in the stimulation of lymphocytes by C3b are discussed: (a) cross-linking of receptors on the cell surface or between cells, and (b) the binding of C3b to receptors of B lymphocytes and the formation of the complement enzyme C3bB. The results are compatible with the suggestion that activation of C3 is part of the events triggering the B lymphocytes.

A theory of regulation and self-nonself discrimination in an immune network

A mechanism is suggested for the regulation of the immune response that involves antigen-binding and anti-idiotypic lymphocytes. The cross-linking of receptors is postulated to be a general mechanism for triggering T cells, B cells and cytotoxic effector cells (e.g. macrophages). A key role in regulation is ascribed to antigen-specific T cell-dependent factors, which are assumed to be able to block the receptors of both T cells and B cells of the anti-idiotypic specificities. The simple postulates of the model lead to feasible mechanisms for the helper, suppressor and killer roles of T cells, cellular and humoral immune responses, low zone tolerance and the tolerogenic effects of monomers, immune memory, antigenic competition, the abrogation of tolerance with cross-reacting antigens, the usefulness of the switch from IgM to IgG, and self-tolerance to both serum antigens and cell surface antigens. Biological roles are suggested for one of the Ia antigens and beta 2-microglobulin. The theory leads to a number of predictions, which can be tested experimentally. A simple mathematical model is included, which provides an indication of how the theory may be further developed on a quantitative basis.

Separation and functional analysis of subpopulations of lymphocytes bearing complement and Fc receptors

A highly versatile procedure is described in this review which can be used to separate and obtain in pure form subpopulations of lymphoid cells which express different cell surface structures. The method is based on the observation that when rosetting and non-rosetting leukocytes are centrifuged on a cushion of Isopaque/Ficoll, the rosetting leukocytes and red cells sink whereas the non-rosetting leukocytes float. Thus, any subpopulation of leukocytes can be separated providing they can be identified by rosetting. The earlier sections of this review describe the method, its efficiency of separation and its advantages compared with other fractionation procedures. Subsequent sections describe experiments in which the procedure was specifically applied to separating Fc receptor (Fc+) and complement receptor (CR+) lymphocytes. On the basis of these two receptors it was possible to subdivide T and B lymphocytes into distinct subpopulations. Four subclasses of B lymphocytes were identified in mouse spleen (Fc+CR+,Fc+CR-,Fc-CR+ and Fc-CR-) and two subclasses of T cells were also detected (Fc+ and Fc-). The functional relevance of these subpopulations of lymphocytes was examined. It was found that in all cases examined, antigens could successfully activate CR+ B cells to produce antibody. However, only polymeric antigens, whether T-dependent or T-independent, were capable of triggering CR- B cells to synthesize antibody. Furthermore, preliminary experiments suggest that Fc receptors are present on functional B cells and helper T cells but are not expressed on cytotoxic T cells. On the basis of these results it is proposed that complement receptors on B lymphocytes provide an additional binding site which stabilizes the union between the antigen-specific receptors and soluble antigen. In contrast, due to their multi-determinant nature, polymeric antigens can avidly bind to B cells without involvement of the complement receptors. The possibility of Fc receptors playing a similar role in stabilizing the interaction of antigen with specific receptors on lymphocytes, particularly on T helper cells, is also discussed.

T- and B-cell interactions in autoimmune syndromes

We have reviewed briefly some of the individual capabilities of T and B cells and how these can be modified by interactions between the two cell lines. Evidence that T cells have their own distinct receptors for antigens, yet under certain circumstances may bind IgM or IgG produced by B cells, has been particularly emphasized. The probability that B cells, in turn, may bind the antigen receptor of T-cell origin reflects the balanced nature of an intricate communication system in which interactions between antigens, antigen receptors, and binding sites for these receptors all serve to modulate the integrated functioning of T and B cells. It is suggested that this communication system is disturbed in patients with rheumatoid arthritis; specifically, it is proposed that defective feedback activity of IgG-antigen complexes on activated T cells may exist in some patients and could result in unchecked and harmful T-cell activity in the joint. Therapeutic implications of this idea are mentioned.

Role of C'3 and Fc receptors in B-lymphocyte activation

Attempts were made to identify the non-Ig lymphocyte receptor responsible for B-cell induction by antigen and polyclonal B-cell activators (PBA). As a first step, the role of C'3 and Fc receptors was analyzed. It was shown that complement could be fixed onto B cells to such an extent that the lymphocytes could not bind complement-coated red cells, but this did not result in induction of polyclonal antibody synthesis, nor did it inhibit the lymphocytes response to PBA. However, the C'3 receptros possessed a passive focussing role in the induction of polyclonal antibody responses. Thus, PBA that had fixed complement activated polyclonal responses at lower concentrations than the same substances that had not fixed complement. Most likely the dual binding of PBA molecules to B cells by the PBA and the C'3 receptors caused more PBA molecules to be bound to each cell. However, the focussing function of the C'3 receptors was several orders of magnitude smaller than that of the Ig receptors. Analogous studies were carried out with Fc receptors. Binding of different types of antigen-antibody complexes did not cause activation of polyclonal or specific antibody synthesis, nor did it significantly interfere with induction of antibody synthesis by PBA substances.