The effect of antibody isotype on the activation of C3 and C4 by immune complexes formed in the presence of serum: correlation with the prevention of immune precipitation

Radioimmunoassays for C3a and C4a have been used to measure the activation of complement during the formation of immune complexes in human serum by the interaction of DNP-BSA and each of 11 mouse anti-DNP monoclonal antibodies of varied isotype and affinity. Those containing IgG2 or IgM were potent activators of C4, whilst IgG1 containing complexes were less efficient. C3 activation in normal serum was similar for complexes containing IgG1, IgG2a, IgG2b or IgM. IgA complexes did not activate C3 or C4. All complexes except those containing IgA precipitated more slowly in serum than in buffer. IgG2 antibodies were potent activators despite being very slow to precipitate in buffer. In serum containing EGTA activation of C4 was abolished and precipitation of complexes occurred at the same rate as in buffer. Nevertheless, C3 activation still occurred by the alternative pathway for all IgG and IgM complexes. Antibodies of the same isotype did not necessarily activate complement to the same extent. The ranking of the ability to activate complement was the same as that observed when performed complexes containing the same antibodies were added to serum. The levels of C4a generated were similar under both conditions but for most antibodies more C3a was generated by preformed complexes.

Regional assignment of the human C1-inhibitor gene to 11q11-q13.1

In situ hybridisation using a biotinylated 1.8 kb human cDNA clone in both normal and structurally abnormal chromosomes supports regional localisation of the gene for human C1-inhibitor to chromosome 11q11-q13.1.

Complement biosynthesis in human synovial tissue

Molecular biological and immunochemical techniques have been used to study the synthesis of complement components by synovial tissue from patients with rheumatoid arthritis or osteoarthritis and by normal synovial tissue from a patient undergoing patellectomy. Using Northern and dot-blot analyses, mRNAs coding for C1-inhibitor, C2, C3, C4 and factor B have been detected, but not for C5. Quantitative analyses of the data have not shown any significant differences in the steady state levels of any of the mRNAs in synovium from rheumatoid arthritis and osteoarthritis patients. When synovial membrane fragments from rheumatoid arthritis, osteoarthritis patients or normal synovium were cultured in vitro, synthesis of C1-inhibitor, C2, C3, C4 and factor B detected by ELISA and C2, C3 and factor B were shown to be functionally active. This study thus provides conclusive evidence that synthesis of complement components occurs locally within normal and inflamed synovial tissue. The local synthesis of complement within normal synovial joints may be of importance in their defence against infection, whereas in inflamed joints it may contribute to the inflammatory response.

Regulation of hepatic synthesis of C3 and C4 during the acute-phase response in the rat

Following the i.p. injection of casein, rats showed increased serum levels of C4 and C3. C4 levels peaked on day 2 and returned to normal by day 4, while C3 levels peaked on day 3 before returning to normal on day 4. These changes were paralleled by changes in the hepatocyte synthesis rates of these two proteins. Macrophages, isolated from the peritoneal cavities of rats on days 1 to 7 (day-1 to day-7 macrophages) following i.p. injection of casein, were cultured in vitro, and the peritoneal macrophage-conditioned media (PMCM) assayed for their abilities to stimulate synthesis of C4 and C3 by hepatocytes from control rats. Day-2 PMCM selectively stimulated synthesis of C4, while day-3 and day-4 PMCM selectively stimulated C3 synthesis. These activities were called C4-hepatocyte stimulating factor (C4-HSF) and C3-HSF, respectively. The addition of anti-interleukin (IL) 1, tumor necrosis factor (TNF)-alpha, TNF-beta, IL 6 or interferon (IFN)-gamma antibodies to day-2 PMCM did not affect C4-HSF activity, and none had any effect on C3-HSF activity in day-4 PMCM. However, the addition of anti-IL 1 to day-4 PMCM resulted in the re-expression of C4-HSF activity as well as loss of thymocyte proliferative activity. C4-HSF activity could also be detected in day-4 PMCM by separating it from IL 1 activity using gel filtration chromatography. Furthermore the addition of recombinant IL 1 beta to day-2 PMCM prevented the expression of C4-HSF activity. Thus IL 1 appears to play a regulatory role in the acute-phase response in the rat, by preventing the expression of C4-HSF activity. The identities of C4-HSF activity and C3-HSF are still unknown but we believe that C3-HSF activity could be IL 6 as it has a similar molecular weight (30 kDa) and purified human IL 6 was more effective than IL 1, TNF-alpha or TNF-beta in stimulating C3 synthesis by rat hepatocytes. C4-HSF activity appears to be a property of a previously undescribed cytokine. It is not IL 1 alpha or beta, TNF-alpha or -beta, IL 6 or IFN-gamma.

Control of immune complexes by the classical pathway

The association between inherited deficiencies of the classical pathway complement components (C1q, C1r, C1s, C4, C2 and C3) and immune complex disease shows that complement is involved in protection against the development of immune complex disease (ICD). This protection is conferred by the ability of the complement system to keep antigen antibody complexes (IC) small and soluble. Two mechanisms exist, prevention of immune precipitation (PIP), which inhibits the formation of large insoluble lattices when IC are formed in the presence of complement (nascent IC), and solubilisation of preformed immune precipitates (SOL). PIP is probably the more important as it is unlikely that, in vivo, IC are ever formed in the absence of complement. PIP displays an absolute dependency upon the classical pathway while SOL is alternative pathway dependent. However, for optimal efficiency SOL requires an intact classical pathway. Thus the classical pathway plays a role in both PIP and SOL. The end result of both processes is the covalent binding of C3b to the IC lattice, which not only keeps IC soluble, but permits binding to CR1 for removal from the circulation. The sera of patients with ICD contain a factor (s) which inhibits PIP. The sera of RA patients inhibits PIP and purified IgM-RF has been shown to inhibit this function. However a second inhibitor of PIP has recently been purified, a glycoprotein (Mr 60 kd) (gp60) which is present in normal serum and in increased concentration in RA sera. Gp60 binds to the Fc piece of IgG, but not to IgA or IgM, and competes with C1q for a binding site on IgG Fc. Thus gp60 appears to act by preventing binding and activation of C1 by IgG containing IC.

Regulation of C1-inhibitor synthesis by interferons and other agents

C1-inhibitor (C1-inh) is synthesised and secreted by at least four cell types: hepatocytes, mononuclear phagocytes, fibroblasts and umbilical vein endothelial cells. The production of this protein by monocytes/macrophages and Hep G2 cells has been studied in great detail. Environmental factors alter C1-inh synthesis by these cells. A number of agents which inhibit monocyte C1-inh production (such as histamine, PGE2 C5a des arg and serum treated immune complexes) bind to membrane receptors, activate adenylate cyclase, elevate intracellular cAMP and activate cAMP-dependent protein kinase. Elevation of monocyte cAMP levels is associated with decreased C1-inh secretion by these cells and reduced C1-inh mRNA levels. These changes can be seen within 8 hours of exposure. Stimulation of monocyte C1-inh synthesis occurs after the addition of agents which induce the formation of sodium ion and calcium ion channels, activate the phosphatidyl inositol cycle and activate protein kinase C (immune-complexes, carbamylcholine and phenylephrine). Agents which act directly on protein kinase C (phorbol myristate acetate) also stimulate C1-inh synthesis. Amongst the most potent stimulators of monocyte and Hep G2 C1-inh synthesis are the interferons (Ifns) Ifn alpha, Ifn beta and Ifn gamma. These are known to bind to specific receptors on cells (Ifn alpha and beta binding to Type I Ifn receptors and Ifn-gamma binding to type II Ifn receptors). At least two mechanisms by which Ifn receptor-ligand interaction elicit their effects exist. These are: 1) binding of an activated receptor transducer/regulatory component to specific DNA sequences on Ifn sensitive genes; 2) the activation of protein kinase C and binding of its regulatory components to specific DNA sequences. Ifn alpha, beta and gamma cause a dose related increase in monocyte and Hep G2 cell C1-inh mRNA abundance and protein synthesis. Ifn-gamma is the most potent of the interferons on monocyte C1-inh synthesis. Ifn alpha and beta being less effective but equipotent. These cytokines elicit their maximum effect on monocyte C1-inh synthesis after 1-2 hours treatment. This rapid stimulation of monocyte C1-inh synthesis suggests that this increases the transcription of the C1-inh gene. After removal of Ifns from monocytes the elevated C1-inh mRNA levels subside towards control levels of expression in Ifn alpha and beta-treated cells but remain elevated in Ifn-gamma-treated monocytes. This binding suggests that Ifn-gamma alters the stability of C1-inh mRNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Complement and related clinical disorders

The complement system, composed of 20 plasma proteins and several membrane receptors, plays an essential role in humoral immune responses. The activation of the classical and/or alternative pathways by specific and non-specific stimuli leads to the generation of chemotactic and anaphylatoxic inflammatory mediators, the formation of the cytolytic membrane attack complex, and the formation of C3 breakdown fragments which are opsonic. There is now evidence that the membrane receptors play important immunoregulatory functions. The link between various disease states and deficiencies of complement components and membrane receptors further supports the important immunological role this system plays. This review briefly introduces the components of the system, their biological roles and diseases associated with deficiency states.

Familial properdin deficiency associated with chronic discoid lupus erythematosus

A large family comprised of 18 members is described. Four male members are properdin-deficient, all are healthy bar the index patient who presented with chronic discoid lupus erythematosus. Serum from properdin-deficient males had a reduced ability to lyse rabbit erythrocytes via the alternative pathway or solubilize pre-formed immune complexes. Addition of purified properdin restored these activities. Classical pathway activity was normal. Definite, probable and possible female carriers had normal classical and alternative pathway activities.

Modulation of monocyte complement synthesis by lymphocytes and lymphocyte-conditioned media

Culture supernatants from mitogen- and antigen-stimulated human peripheral blood lymphocytes (PBL), stimulated synthesis of the second complement component (C2) by human monocytes, but not as effectively as the stimulated PBL themselves, which adhered to the monocytes and caused marked spreading. In contrast to PBL, lymphocytes isolated from the synovial membranes (SML) of patients with rheumatoid arthritis and their culture supernatants were able to stimulate C2 synthesis without exposure to mitogens or antigens. Depletion of B and T populations showed that T cells were responsible for stimulation of C2 synthesis. Further studies of synthesis rates of C2, C3 factor B (B), C1 inhibitor, and properdin (P) were undertaken, and it was found that lymphocytes and their supernatants increased synthesis of C2, B and C1 inhibitor, and reduced synthesis of C3 and P. This profile of activity was identical to that produced by the addition of recombinant gamma-interferon (rIFN-gamma) to the cultures. Furthermore the addition of a monoclonal antibody to rIFN-gamma to cultures abrogated the effects of rIFN-gamma, and almost completely reversed the effects of lymphocytes and their supernatants. Thus it appears that gamma-interferon is the lymphocyte product which is responsible for the modulation of monocyte complement synthesis. The results of studies with synovial membrane lymphocytes raise the possibility that this process occurs in vivo. Monocyte C2 had a higher specific functional activity (SpFA) than serum C2 isolated from serum or C2 produced by HepG2 cells. Monocyte C2 formed a C3 convertase which had a longer half-life than that found with both serum C2 or HepG2 C2. Thus monocyte C2 behaves like oxidized C2. Monocytes exposed to rIFN-gamma, lymphocytes or lymphocyte-conditioned medium (LCM) produced C2 which had an even higher SpFA. Although antibody to IFN-gamma prevented any increase in C2 synthesis in monocyte cultures containing lymphocytes or LCM, C2 SpFA was still increased. Thus a second lymphocyte product is responsible for this 'oxidation' effect. This production of 'oxidized' C2 by monocytes and further 'oxidation' by the action of either lymphocytes or gamma-interferon might play a significant role in the perpetuation of complement activation at sites of inflammation.

Autoantibody facilitated cleavage of C1-inhibitor in autoimmune angioedema

C1-inhibitor (C1-Inh) is an important inhibitor of the inflammatory response and deficiency of this inhibitor, which may be hereditary or acquired, is associated with recurrent episodes of edema. Recently, an autoimmune form of angioedema has been described that is associated with functional deficiency of C1-Inh and an autoantibody that impedes C1-Inh function. In this report we describe the isolation of C1-Inh from the monocytes and plasma of a patient with autoimmune angioedema and demonstrate that the patient's monocytes secrete structurally and functionally normal C1-Inh, but show that this protein circulates in the patient's plasma in an inactive, structurally altered form. Furthermore, using analytic gel electrophoresis techniques it is demonstrated that the patient's autoantibody facilitates cleavage of normal C1-Inh, by its target proteases, to the same species of C1-Inh that is found circulating in the patient's plasma. This autoantibody facilitated cleavage of normal C1-Inh is apparently a consequence of destabilization of protease/inhibitor complexes. These findings contribute to our understanding of protease/C1-Inh interactions and document important observations on pathogenic mechanisms in autoimmune disease.

Monocyte C1-inhibitor synthesis in patients with C1-inhibitor deficiency

Monocytes of seven out of eight patients with type 1 C1-inhibitor (C1-inh) deficiency (HAE) produced 40% as much C1-inh as monocytes from normal donors (controls). In contrast, monocytes from three patients with type 2 and three patients with acquired C1-inh deficiency produced similar amounts of C1-inh as controls. Recombinant gamma-interferon (gamma-interferon 10 ng/ml) stimulated C1-inh production of C1-inh (eight-10-fold) by control and patients' monocytes. Monocytes from patients with type 1 HAE contained 40% the level of C1-inh messenger ribonucleic acid (mRNA) found in control monocytes. Gamma-interferon increased the abundance of C1-inh mRNA by the same extent in both control and patients' monocytes. C1-inh protein and mRNA were undetectable in the monocytes of one patient, unless stimulated by gamma-interferon. Under these conditions, his monocytes produced comparable amounts of C1-inh (protein and mRNA) as gamma-interferon-stimulated monocytes of the other type 1 HAE patients. The data suggest that in most type 2 HAE patients there is a lesion in the C1-inh gene such that mRNA is transcribed by a single allele.

The plasma protein which inhibits complement-mediated prevention of immune precipitation is an Fc binding protein

A glycoprotein (gp60) that inhibits complement-mediated prevention of immune precipitation (PIP) has been purified from normal serum. [125I]gp60 binds to IgG but not to IgA or IgM. The binding site has been shown to be localized on the Fc piece. The binding of radiolabelled gp60 to IgG has been analysed by direct binding and Scatchard, double-reciprocal and Hill plots. The mean affinity constant of gp60 for IgG is 1.56 x 10(8) l/mol and there appears to be a single class of binding sites for gp60 on IgG. Saturation was achieved when one molecule of gp60 was bound to each molecule of IgG. In competition inhibition assays, gp60 was shown to compete with C1q and IgM and IgG rheumatoid factors. The ability to inhibit C1q binding suggests that gp60 inhibits PIP by preventing binding and activation of C1. The possibility that gp60 is a fluid-phase Fc gamma receptor is discussed. See also the note added in proof.

Measurement of complement activation in clinical practice

Many new tests which measure complement activation are now available, although it is unclear which tests or group of tests are appropriate for monitoring complement activation in clinical samples. Carefully planned prospective studies are required to resolve this issue, and to identify which groups of patients in which the monitoring of complement activation improves the quality of their medical management.

The activation of C3 and C4 in human serum by immune complexes containing mouse monoclonal antibodies of different isotype and affinity: effects on solubilisation

Radioimmunoassays for C3a and C4a have been used to measure the activation of complement in human serum by immune complexes containing DNP-BSA and each of 11 mouse anti-DNP monoclonal antibodies of varied isotype and affinity. When preformed complexes were added to serum, those containing IgG2 or IgM were potent activators of C4, whilst IgG1 complexes were less efficient. C3 activation in normal serum was similar for complexes containing IgG1, IgG2a, IgG2b or IgM. IgA complexes did not activate C3 or C4. Solubilisation of complexes was greatest for IgM and IgG2b and least for IgG2a and IgA. In serum containing Mg2+ EGTA C4 activation was abolished and the amount of C3 activation was lower for all IgG and IgM complexes. Antibodies of the same isotype did not necessarily activate complement to the same extent. Unexpectedly, three of the four IgMs activated C3 in EGTA. For IgMs, neither complement activation nor solubilisation correlated with affinity. For IgG1 antibodies, solubilisation was inversely proportional to affinity. C3 or C4 activation did not correlate with affinity.

Use of antisera to the isolated alpha and beta subunits of C3 as probes to study functional sites present on particle-bound C3b but absent on native soluble forms of C3

The effect of antisera to the isolated alpha and beta chains of C3 on certain C3b-dependent reactions has been studied. C5-mediated haemolysis of EAC1423b was inhibited preferentially by antiserum to the alpha chain, whereas antiserum to the beta chain inhibited the formation of C3bBb. The anti-beta chain antiserum also stabilised C3bBbP, and rendered the enzyme relatively resistant to accelerated decay in the presence of factor H. These and previous findings that anti-alpha and anti-beta IgG bind to restricted subsets of antigenic determinants on C3/C3b suggest that these antisera affect C3b function through the binding of antibodies to active binding sites exclusively exposed by bound C3b. The anti-alpha and anti-beta antibody probes are currently being further developed to verify this interpretation.

The effects of immunoglobulin isotype and antibody affinity on complement-mediated inhibition of immune precipitation and solubilization

We have studied complement-mediated prevention of precipitation (PIP) and solubilization of immune complexes (IC) formed with DNP19-BSA and murine monoclonal antibodies (MCAs) of different isotypes and affinities. PIP was effective for IC formed with IgG1 and IgM antibodies, but not for IgA MCA. For IgG1 MCAs, affinity appeared to exert a minor effect on PIP, and IC formed in antibody excess or at equivalence were retained in solution more readily than those formed in antigen excess. For IgM MCAs affinity and antigen-antibody ratio did not affect PIP. As PIP did not occur in Mg-EGTA, it was concluded that PIP was entirely classical pathway dependent. Solubilization of IC containing IgG1 MCAs occurred more rapidly and to a greater extent with low affinity antibodies and an inverse relationship between affinity and the extent of solubilization was observed. Complexes formed with IgG1 MCAs were solubilized relatively poorly when formed in antigen excess. In contrast, affinity and antigen-antibody ratio did not influence the rate and extent of solubilization of IC containing IgM MCAs. IC formed with IgG2b were solubilized rapidly whereas those formed with IgG2a or IgA were solubilized poorly. The relative contributions of the classical and the alternative pathways to solubilization varied with each antibody and the effect of antigen-antibody ratio on these relative contributions was inconsistent.

The role of ion channels and protein kinase C activation in the stimulation of complement protein synthesis

Monocyte C2 synthesis is stimulated by antigen-antibody complexes (IC), carbamylcholine (C-Ch), phenylephrine (PE) and gamma-interferon. Tetrodotoxin or nifedipine abrogated the effects of IC, C-Ch and PE but did not influence the effect of gamma-interferon on C2 synthesis. Thus stimulation of C2 synthesis by IC, C-Ch and PE is dependent upon activation of Na+/K+ and Ca2+ channels, whereas gamma-interferon operates independently of these ion channels. Calcium channel agonists (CG28392 and BK8644) stimulated C2 synthesis, and this effect was prevented by nifedipine but not by tetrodotoxin. Thus Na+/K+ channels are activated prior to Ca2+ channels. Stimulation of C2 synthesis occurred when phospholipase C or phorbol myristate acetate (PMA) were added to the monocyte cultures, suggesting that PI cycle turnover and protein kinase-C (PK-C) activation are involved in the stimulation of C2 synthesis in monocytes. PMA, an activator of PK-C, stimulated the synthesis of C2, C3, B, P and C1-inhibitor approximately two-fold. In contrast gamma-interferon reduced synthesis of C3 and P by 44% and 22% respectively, and stimulated C1-inhibitor synthesis twelve-fold. These data suggest that the action of gamma-interferon complement synthesis is, at least partially, independent of PK-C activation. The effects of IC, C-Ch, PE, PI, CG28392, BK8644 and gamma-interferon were inhibited by trifluoperazine implying that calmodulin and/or other calcium binding proteins play a role in the modulation of complement protein production.

The relative roles of genetic and environmental factors in the regulation of erythrocyte C3b receptor (ECR1) numbers in normal individuals

Erythrocyte C3b receptors (ECR1) have been measured in 122 pairs of twins (60 monozygotic, 62 dizygotic) using an [125I]-labelled monoclonal antibody (E11). The range of ECR1 numbers was wide, 99-4179 sites/cell, with a log-normal distribution around a geometric mean of 837 sites/cell. The intra-pair variance in dizygotic twins was no greater than that in monozygotic twins. These data indicate that ECR1 numerical expression is governed by environmental rather than genetic factors.

Modulation of monocyte complement synthesis by interferons

Recombinant Escherichia coli-derived gamma-interferon has been shown to stimulate synthesis of the second component of complement (C2), factor B and C1 inhibitor, but to inhibit synthesis of the third component (C3). alpha- and beta-interferons stimulate synthesis of factor B and C3 inhibitor, inhibit C5 synthesis but do not alter synthesis of C2. alpha- and beta-interferons act synergistically with gamma-interferon to enhance both factor B and C1-inhibitor synthesis.

C3b receptor (CR1) expression on the polymorphonuclear leukocytes from patients with systemic lupus erythematosus

Polymorphonuclear leukocytes (PMN) C3b receptor (CR1) numbers have been measured in 14 normal individuals and 15 patients with SLE. The results in the normals showed that PMN possess three distinct pools of CR1. CR1 expression was lowest at 0 degrees C (mean 86,000 +/- s.e.m. 7,000), but increased when the cells were incubated at 37 degrees C (125,000 +/- 16,000) or when the cells were exposed to the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP, 10(-5) mol 1) at 37 degrees C (207,000 +/- 21,000). The increased expression at 37 degrees C was not dependent upon protein synthesis, an intact cytoskeleton or energy. Although the response to FMLP did not require de novo protein synthesis, increased CR1 expression was dependent upon an intact cytoskeleton and energy. All three PMN CR1 pools were reduced in patients with active SLE, but were normal in those in whom the disease was inactive. Serial studies performed on three SLE patients showed that PMN CR1 numbers were low during periods of disease activity and increased during remission. These data suggest that low PMN CR1 numbers in SLE are a consequence of the disease.

Inhibition of complement-mediated solubilisation of antigen-antibody complexes by serum factor(s) in patients with various connective tissue diseases

Control of immune complex formation is important to limit disease resulting from their deposition in tissues. Any inhibition of immune complex solubilisation is thus significant in the pathogenesis of immune complex diseases. More than half of our patients with various rheumatic connective tissue diseases were demonstrated to have serum inhibition of immune complex solubilisation (12/16 rheumatoid arthritis, 22/37 systemic lupus erythematosus, 16/29 primary Sjogren's syndrome, and eight of nine with mixed connective tissue disease). This serum inhibitory activity did not correlate well with serum levels of IgM rheumatoid factor or circulating immune complexes, and its nature remains to be elucidated.

Complement and immune complex diseases

Research during the past decade has led to a much greater understanding of the activation and control, as well as a more complete delineation, of the complement system of proteins. There has been definition of the roles of individual components in modulation of immune complex formation, the deposition of which leads to tissue injury in the autoimmune connective tissue diseases. The ability of serum to render immune complexes more soluble is complement-mediated and appears to be an important protective mechanism against immune complex diseases. Inherited deficiencies and production of non-functional variants of complement components, decreased synthesis, hypercatabolism, and the presence of serum inhibitors may all contribute to the reduced immune complex solubilisation which has been found in the connective tissue diseases. More work is required to define further the role of complement and immune complexes in the basic pathogenesis of these diseases.

Circulating immune complexes associated with decreased complement-mediated inhibition of immune precipitation in sera from patients with bacterial endocarditis

The sera of patients with bacterial endocarditis frequently contain high levels of circulating immune complexes. In in vitro assays these sera have been shown to be deficient in the complement mediated inhibition of immune precipitation (immune complex solubilization) although C3 and C4 levels were often normal. The deficiency is due to the presence of a factor which also inhibits the ability of normal serum to solubilize immune complexes. This inhibitor is possibly rheumatoid factor which is frequently detected in endocarditis. Serial studies on 16 patients showed the levels of immune complexes, the ability to prevent immune precipitation and rheumatoid factor to correlate with disease activity. The similarity of the findings to those in rheumatoid arthritis are discussed.

An inhibitor of complement-mediated prevention of immune precipitation in rheumatoid arthritis--relationship to disease activity and systemic manifestations

In normal serum complement prevents precipitation of antigen-antibody complexes (PIP). However rheumatoid arthritis (RA) serum contains an inhibitor of this complement-mediated function. We have undertaken two prospective studies in order to look for any relationship between the presence and levels of inhibitory activity in sera and synovial fluids (SF) of patients with RA and disease activity (study A), and the presence of systemic manifestations (nodules and vasculitis) of RA (study B). In study A, levels of inhibitory activity were highest in the sera and synovial fluids of patients with seropositive RA. However there was no correlation between the inhibitory levels and indices of generalised disease activity (articular index, erythrocyte sedimentation rate (ESR), haemoglobin, white cell and platelet counts). Local joint tenderness score correlated weakly with the inhibitory level in SF (P less than 0.05). There was no correlation, however, with either the SF protein concentration or white cell count. In study B, PIP was shown to be lower in patients with the systemic manifestations of RA than in those with purely articular manifestations. PIP was particularly low in those patients with vasculitis compared to those with subcutaneous nodules. Serum levels of inhibitory activity were highest in patients with vasculitis and lowest in those with articular disease only, whereas patients with nodules had intermediate levels. Our conclusion is that inhibition of immune precipitation is not associated with disease activity, but is associated with the extra-articular manifestations of RA. The inhibitory factor may play a role in the pathogenesis of RA.

Inhibition of C1q binding to antigen-antibody complexes by a factor in rheumatoid arthritis serum

The sera and synovial fluids of patients with rheumatoid arthritis (RA) contain a factor which decreases the binding of C1q to antigen-antibody complex (IC). Several lines of evidence suggest that this factor is distinct from the documented C1q inhibitor which is a chondroitin sulphate. It binds to IC rather than to C1q. It is resistant to digestion with chondroitinase ABC. The addition of chondroitin sulphate to serum does not inhibit the binding of IC to C1q. The observation that three purified IgM and IgG rheumatoid factors (RF) did not reduce C1q binding to IC indicates that the factor is not RF. The ability of RA sera to reduce IC binding to C1q was inversely correlated with their ability to prevent immune precipitation (PIP), and directly with levels of an inhibitor of PIP. These data suggest that the factor which binds to IC and reduces C1q binding may be responsible for the excessive immune precipitation which occurs in RA sera.

Decreased C3b receptors (CR1) on erythrocytes from patients with systemic lupus erythematosus

Using 125I-F(ab')2 anti-CR1 we have measured C3b receptors (CR1) on the erythrocytes of 56 normal individuals 26 patients with systemic lupus erythematosus (SLE) and 24 with rheumatoid arthritis (RA). The mean number of CR1 sites in SLE (1150/cell) but not RA (1460/cell) was significantly lower (P less than 0.01) than normal (2200/cell). Although the cumulative frequency curve for normals showed minor inflections at frequencies of 18% and 64%, these were not sufficiently marked to permit us to conclude that they distinguished subpopulations of different CR1 phenotypes. Measurement of CR1 numbers of two normal families and four families of SLE patients indicated that low CR1 numbers aggregated in families as did high CR1 numbers, a finding which suggests that CR1 numbers are under genetic control. However, certain observations in SLE patients indicated that low CR1 numbers could be an acquired abnormality. These included, (a) absent CR1 phenotype in a patient whose family had moderate and high CR1 numbers, (b) increasing CR1 numbers as SLE patients went into remission, (c) CR1 numbers were lower in patients with active compared with inactive disease and (d) CR1 numbers were different in each of two sets of identical twins (Fig. 4A). Our conclusions are that, (a) genetic factors probably influence CR1 numbers in normal individuals, (b) that our findings were not inconsistent with the two codominant allele models (Wilson et al., 1982), and (c) the low CR1 phenotype of SLE patients may be secondary to the disease process.

Comparative study of endralazine and hydralazine for the treatment of hypertension uncontrolled by a beta-blocker and diuretic

Endralazine and hydralazine were compared in a randomized double-blind, parallel group study lasting 1 year in 30 patients with essential hypertension inadequately controlled by a beta-blocker and a diuretic. Dosage ranged from 10 mg to 30 mg endralazine per day and from 75 mg to 200 mg hydralazine per day according to patient response. The results showed that endralazine was at least as effective as hydralazine in reducing blood pressure. Patients' tolerance, assessed by drop-out rate, was significantly better (p less than 0.05) in the endralazine group. No cases were found of drug-induced lupus-like syndrome on endralazine as opposed to 2 cases with hydralazine. The dose of endralazine required much less adjustment than that of hydralazine.

Biosynthesis of complement components by cultured rat hepatocytes

Hepatocytes which had been isolated from the livers of Charles River rats were cultured in vitro. The cells were shown to synthesize albumin and the complement components C4, C2, C3 and B. Pulse-label studies with [35S]methionine showed that C4 and C3 were synthesized as single polypeptide chains. Pro-C4 did not appear to be converted into the plasma form of C4 intracellularly, whereas cell lysates contained the alpha- and beta-chains of plasma C3 as well as pro-C3. It is concluded that culture of rat hepatocytes in vitro provides a useful technique for studies of the synthesis of complement components.

Prevention of immune precipitation by purified components of the alternative pathway

The role of the alternative pathway in complement-mediated prevention of immune precipitation has been investigated by the use of BSA-anti-BSA immune complex (IC) purified components. For immune precipitation to be prevented all six alternative pathway components (C3, factors B D, P, H and I) were required. In the absence of one or both of the control proteins H and I, excessive fluid phase turnover of C3 occurred with precipitation of IC. Kinetic studies showed that in the presence of the control proteins, an initial phase of precipitation occurred, and was followed by a phase of resolubilization of IC. When the efficiency of classical and alternative pathways in the prevention of immune precipitation was compared it was found that the classical pathway proteins were more effective than the alternative pathway components. A reaction mixture containing the components of both pathways was no better than the classical pathway protein alone. 125I-C3 was bound to IC which had been rendered soluble in the presence of classical or alternative pathway components. A molar ratio of two molecules C3b:five molecules IgG was calculated. Other complement components which were bound to IC which had been formed in the presence of serum were C1q, C4, C2, C3, C5, P and H. Factors B and I were not detected. Our findings suggest that the alternative pathway is of secondary importance to the classical pathway in the prevention of immune precipitation.

Requirement for cations in the stimulation of C2 synthesis by human monocytes

C2 synthesis by monocytes is stimulated by carbamylcholine acting on nicotinic receptors, phenylephrine acting on alpha 1 adrenergic receptors, and antigen-antibody complexes (IC) acting on Fc receptors. Stimulation of C2 synthesis is reversed by agents which block calcium (LaCl2, CoCl2, verapamil, nifedipidine, diltiazem) and sodium channels (tetrodotoxin) and calmodulin antagonists (trifluoperazine and W7). The changes in intracellular cyclic nucleotide levels that follow these receptor-ligand interactions (decreased cAMP, increased cGMP) do not occur in the presence of calcium and sodium channel blockers. These results suggest that the transmembrane signal which is involved in the stimulation of C2 synthesis is the entry of sodium and calcium ions. Whether this influx occurs by separate channels or a common channel has not been determined. The intracellular events involved in the stimulation of C2 synthesis appear to be calmodulin-dependent.

Complement-subcomponent-C1-inhibitor synthesis by human monocytes

By using a radioimmunoassay, C1-inhibitor was found to accumulate in the supernatants of human monocyte cultures. The production of this protein was inhibited reversibly by cycloheximide. When C1-inhibitor synthesis was compared with C2 synthesis, it was found that C1-inhibitor synthesis continued, whereas synthesis of C2 appeared to cease after about 7 days in culture. Immunoprecipitation of supernatants of monocyte cultures that had been pulsed with [35S]methionine showed a specific band with an Mr of 105 000. Immunoprecipitates of the lysates revealed a band of Mr 83 000; this was thought to represent a partially or non-glycosylated precursor of C1-inhibitor. C1-inhibitor produced by the monocytes was shown, by using a haemolytic assay, to be functionally active. However, the functional activity of C1-inhibitor was reduced by only 44% in the presence of cycloheximide, whereas the concentration of this protein in cycloheximide-treated culture supernatants fell by more than 93%. This finding suggests that monocytes secrete a second molecule, which inhibits C1 activity but is distinct from classical C1-inhibitor.

Prevention of immune precipitation by purified classical pathway complement components

The role of the classical pathway of complement in the prevention of immune precipitation has been investigated using purified complement components and immune complexes (IC) consisting of rabbit anti-BSA and BSA. C1 reduced the rate of immune precipitation. As C1q, EDTA treated C1 or C1-inhibitor treated C1 were unable to retard the precipitation of IC, it was concluded that the intact C1 molecule was required for this function. Use of phenylmethylsulphonyl fluoride and benzamidine showed that the enzymatic site on C1 was not required for this activity. C4 and C2 did not affect immune precipitation significantly when C1 was present at the concentrations present in serum. When C3 was added to C1, C4 and C2 precipitation of IC did not occur. These data demonstrate that classical pathway activation alone is sufficient for the prevention of immune precipitation.

Cyclic nucleotides and their relationship to complement-component-C2 synthesis by human monocytes

The time courses of changes in cyclic nucleotide levels in monocytes have been studied. Histamine and prostaglandin E2 (PGE2) produced a rapid rise in cyclic AMP (peak 15 min) levels, which returned to normal within 4h, whereas cholera toxin, NaF and phosphodiesterase inhibitors produced slow sustained rises lasting over 24h. With the exception of isobutylmethylxanthine (10 mumol X 1(-1), none of these reagents altered cyclic GMP levels. alpha 1-Adrenergic and nicotinic cholinergic receptor-ligand interactions and imidazole produced rapid and relatively short-lived falls in cyclic AMP, and rises in cyclic GMP. In contrast, prostaglandin synthetase inhibitors produced delayed but more sustained falls in cyclic AMP but no rises in cyclic GMP. Agents that increased cyclic AMP decreased complement-component-C2 production, and those that decreased cyclic AMP increased C2 production. Agents that increased cyclic GMP alone (ascorbate, nitroprusside and prostaglandin F2 alpha) did not affect C2 production. Antigen-antibody complexes that stimulate C2 synthesis produced falls in cyclic AMP and rises in cyclic GMP similar to those produced by adrenergic and cholinergic ligands. Serum-treated complexes and anaphylatoxins, which inhibited C2 production, were associated with changes in cyclic AMP similar to those produced by histamine and PGE2. These data suggest that there are two transmembrane signals involved in the regulation of C2 production by monocytes. The inhibitory signal is adenylyl cyclase activation. The stimulatory signal is not so obvious, but may be Ca2+ influx, since the time courses of changes in cyclic nucleotides produced by agents that stimulate C2 synthesis are identical, and alpha 1-adrenergic agonists cause the formation of Ca2+ channels.

Adenosine A2 receptors on human monocytes modulate C2 production

Adenosine inhibits C2 production by human monocytes. The use of synthetic analogues of adenosine suggested that the action was mediated by receptors of A2 type. EHNA which inhibits the enzyme adenosine deaminase and increases the concentration of intracellular adenosine also reduces C2 production. It is therefore possible that endogenous adenosine regulates C2 synthesis.

IgM-RF prevents complement-mediated inhibition of immune precipitation

Rheumatoid arthritis (RA) serum inhibits complement-mediated inhibition of immune precipitation (solubilization). We have isolated inhibitory activity from RA sera and shown that it is a property of IgM-rheumatoid factor (IgM-RF) and, to a lesser extent, IgG-RF.

Inhibition of immune precipitation in rheumatic disease. A clinical and laboratory study

Antigen-antibody complexes formed in the presence of serum do not precipitate. This complement-dependent function is impaired in approximately half of all patients with seropositive rheumatoid arthritis (RA) but not in patients with other chronic inflammatory arthropathies. As patients with seropositive RA have normal or elevated serum complement levels, this findings suggests that an inhibitor is present in the serum of these patients. Although degree of impairment of solubilization is correlated with rheumatoid factor (RF) titre, decreases in RF titre in patients receiving gold therapy were not always accompanied by improvement of the solubilization process. Thus we can conclude that impaired solubilization is related to, but may be distinct form, RF. Impaired solubilization was associated with the presence of subcutaneous nodules, but not with other systemic features of RA. Thus this phenomenon may be of pathogenetic importance.

Role of C3 in the control of monocyte C2 production

Serum-treated antigen-antibody complexes (IC) and DTSP-polymerized C3b and C3c inhibited the production of the second complement component (C2) by monocytes in culture, whereas monomeric C3, C3b, C3bi, C3c or C3d had no effect. The degree of inhibition of dithiobissuccinimidyl propionate (DTSP)-polymerized C3 was proportional to the degree of polymerization, dimer exhibiting less inhibitory activity than larger molecular weight polymers. The inhibitory effect of serum-treated IC and DTSP-polymerized C3b was abrogated by their pretreatment with Fab fragments of anti-C3, or pretreatment of monocytes with Fab fragments of antiserum to the C3b receptor (CR1). We have concluded that the inhibition of C2 production produced by serum-treated IC is due to bound C3b or C3bi, and is mediated by CR1. Furthermore cross linking of receptors is required for this effect; two or more than two receptors must be cross-linked for a significant effect to be observed.

Anaphylatoxins inhibit C2 production

Anaphylatoxins C5a and C3a and their des Arg derivatives inhibited C2 production by mononuclear phagocytes. C5a and C5adesArg which were approximately equipotent (IC50 = 10(-10) mol/l) were more effective than C3a (IC50 = 5 X 10(-8) mol/l) which was approximately 10-20-fold more potent than C3adesArg IC50 = 5 X 10(-6) mol/l). Inhibition of C2 production was only reversed slightly by the addition of either indomethacin or ETYA to the cultures. Intracellular levels of cAMP, were increased by anaphylatoxins. The level of cAMP showed a good inverse correlation with C2 levels in the culture supernatants. The data suggest that the reduction in C2 production produced by anaphylatoxins may be mediated by an increase in intracellular cAMP.

Inhibition of complement-mediated solubilization of antigen-antibody complexes by sera from patients with rheumatoid arthritis

Sera and synovial fluids from patients with seropositive rheumatoid arthritis inhibit the ability of normal serum to prevent immune precipitation. Sera from patients with seronegative forms of arthritis contain little inhibitory activity. Studies of the mechanism of action of the inhibitor show that it reduces C4 consumption by antigen-antibody complexes. These findings suggest that the binding of C1 to complexes or activation of C1 is impaired. The possibility that the inhibitory activity may be mediated by rheumatoid factor is discussed. As inhibitory activity is more prevalent and of a higher level in patients with extra-articular features, it is possible that it may play a pathogenetic role.

Control of monocyte C2 production by cyclic AMP

We have measured the changes in C2 production and in cyclic nucleotide levels in mononuclear leucocytes cultured in the presence of agents which change intracellular cAMP. Adenylcyclase activators, cholera toxin and phosphodiesterase inhibitors increased cAMP levels and reduced C2 production. Imidazole, hydrocortisone and prostaglandin synthetase inhibitors reduced cAMP and increased C2 synthesis. The level of cAMP after 2 hr incubation showed an inverse correlation with C2 levels in the culture supernatants. When histamine, which increases cAMP and reduces C2 production, and imidazole, which reduces cAMP and increases C2 synthesis, were added together, levels of cAMP and C2 did not differ significantly from those found in control cultures. On the basis of these observations we have concluded that changes in monocyte cAMP alter the production of C2.

A comparison of the effects of heat-aggregated and chemically cross-linked IgG on monocyte C2 production

Heat or alkali-aggregated IgG was found to inhibit C2 production by monocytes, whereas chemically cross-linked IgG and antigen-antibody complexes stimulated C2 synthesis. Chemically cross-linked IgG was shown to inhibit monocyte EA-rosette formation presumably because it blocked monocyte Fc receptors. Furthermore stimulation of C2 synthesis was limited to polymers of the IgG1 and IgG3 subclasses. In contrast, heat-aggregated IgG failed to inhibit monocyte EA-rosette formation significantly, and all the heat-aggregated IgG subclasses inhibited C2 production. It therefore appears that physically aggregated IgG does not bind effectively to Fc receptors. As the effects of physically aggregated IgG C2 production are similar to those of the hydrophobic proteins casein and alkali-denatured human serum albumin (HSA), it is suggested that hydrophobic residues in the aggregates bind preferentially to the lipid component of the cell membrane.

Complement-mediated inhibition of immune precipitation in patients with immune complex diseases

The ability of human sera to prevent the precipitation of antigen-antibody complexes has been investigated. The early complement components including C3 are required for optimal prevention of immune precipitation, whereas the later components are not required. The sera of 36 of 75 patients with seropositive rheumatoid arthritis (RA), 14 of 32 with SLE and four of 17 with glomerulonephritis exhibited reduced capacities to prevent immune precipitation. In contrast sera from patients with seronegative RA, ankylosing spondylitis, psoriatic arthritis or degenerative joint disease were normal in this respect. In SLE and GN sera hypocomplementaemia was frequently associated but not always with failure to prevent immune precipitation, whereas only a small proportion of the patients with seropositive RA and reduced capacity to retain complexes in a soluble form were hypocomplementaemic. Thus the failure of sera to prevent the precipitation of antigen-antibody complexes is not always associated with hypocomplementaemia.

Serum-treated antigen-antibody complexes inhibit the production of C2 and factor B by mononuclear phagocytes

Antigen-antibody complexes enhanced the synthesis of C2 and factor B by human monocytes and macrophages, and C2 by guinea-pig macrophages. In contrast complexes that had been treated with serum inhibited the production of these components. The inhibitory effect of serum-treated complexes was abrogated by Fab fragments of anti-C3, anti-C3c and anti-C3d. It is therefore probable that inhibition was mediated by a C3 fragment bound to the complex. The enhancing effect of untreated complexes was reversible by serum-treated complexes, and the inhibitory action of serum-treated complexes was counteracted by untreated complexes. Such a system may be important in the regulation of the synthesis of complement components in response to local requirements.