Biosynthesis of complement

Current Understanding of Complement Proteins as Therapeutic Targets for the Treatment of Immunoglobulin A Nephropathy

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and a frequent cause of kidney failure. Currently, the diagnosis necessitates a kidney biopsy, with routine immunofluorescence microscopy revealing IgA as the dominant or co-dominant immunoglobulin in the glomerular immuno-deposits, often with IgG and sometimes IgM or both. Complement protein C3 is observed in most cases. IgAN leads to kidney failure in 20-40% of patients within 20 years of diagnosis and reduces average life expectancy by about 10 years. There is increasing clinical, biochemical, and genetic evidence that the complement system plays a paramount role in the pathogenesis of IgAN. The presence of C3 in the kidney immuno-deposits differentiates the diagnosis of IgAN from subclinical glomerular mesangial IgA deposition. Markers of complement activation via the lectin and alternative pathways in kidney-biopsy specimens are associated with disease activity and are predictive of poor outcome. Levels of select complement proteins in the circulation have also been assessed in patients with IgAN and found to be of prognostic value. Ongoing genetic studies have identified at least 30 loci associated with IgAN. Genes within some of these loci encode complement-system regulating proteins that can interact with immune complexes. The growing appreciation for the central role of complement components in IgAN pathogenesis highlighted these pathways as potential treatment targets and sparked great interest in pharmacological agents targeting the complement cascade for the treatment of IgAN, as evidenced by the plethora of ongoing clinical trials.

Endothelial cells: source, barrier, and target of defensive mediators

Endothelium is strategically located at the interface between blood and interstitial tissues, placing thus endothelial cell as a key player in vascular homeostasis. Endothelial cells are in a dynamic equilibrium with their environment and constitute concomitantly a source, a barrier, and a target of defensive mediators. This review will discuss the recent advances in our understanding of the complex crosstalk between the endothelium, the complement system and the hemostasis in health and in disease. The first part will provide a general introduction on endothelial cells heterogeneity and on the physiologic role of the complement and hemostatic systems. The second part will analyze the interplay between complement, hemostasis and endothelial cells in physiological conditions and their alterations in diseases. Particular focus will be made on the prototypes of thrombotic microangiopathic disorders, resulting from complement or hemostasis dysregulation-mediated endothelial damage: atypical hemolytic uremic syndrome and thrombotic thrombocytopenic purpura. Novel aspects of the pathophysiology of the thrombotic microangiopathies will be discussed.

Pig Liver Xenotransplantation: A Review of Progress Toward the Clinic

Experience with clinical liver xenotransplantation has largely involved the transplantation of livers from nonhuman primates. Experience with pig livers has been scarce. This brief review will be restricted to assessing the potential therapeutic impact of pig liver xenotransplantation in acute liver failure and the remaining barriers that currently do not justify clinical trials. A relatively new surgical technique of heterotopic pig liver xenotransplantation is described that might play a role in bridging a patient with acute liver failure until either the native liver recovers or a suitable liver allograft is obtained. Other topics discussed include the possible mechanisms for the development of the thrombocytopenis that rapidly occurs after pig liver xenotransplantation in a primate, the impact of pig complement on graft injury, the potential infectious risks, and potential physiologic incompatibilities between pig and human. There is cautious optimism that all of these problems can be overcome by judicious genetic manipulation of the pig. If liver graft survival could be achieved in the absence of thrombocytopenia or rejection for a period of even a few days, there may be a role for pig liver transplantation as a bridge to allotransplantation in carefully selected patients.

Complement: A primer for the coming therapeutic revolution

The complement system is an important part of the innate and adaptive immune systems. Originally characterized as a single serum component contributing to the killing of bacteria, we now know that there are close to sixty complement proteins, multiple activation pathways and a wide range of effector functions mediated by complement. The system plays a critical role in host defense against bacteria, viruses, fungi and other pathogens. However, inappropriate complement activation contributes to the pathophysiology of autoimmune diseases and many inflammatory syndromes. Over the last several decades, therapeutic approaches to inhibit complement activation at various steps in the pathways have met with initial success, particularly at the level of the terminal pathway. This success, combined with insight from animal model studies, has lead to an unprecedented effort by biotech and pharmaceutical companies to begin developing complement inhibitors. As a result, complement has been brought for the first time to the attention of pharmacologists, toxicologists, project managers and others in the drug development industry, as well as those in the investment world. The purpose of this primer is to provide a broad overview of complement immunobiology to help those new to complement understand the rationale behind the current therapeutic directions and the investment potential of these new therapeutics.

C1q as an autocrine and paracrine regulator of cellular functions

Most of the complement proteins in circulation are, by and large, synthesized in the liver. However data accumulated over the past several decades provide incontrovertible evidence that some if not most of the individual complement proteins are also synthesized extrahepatically by activated as well as non-activated cells. The question that is finally being addressed by various investigators is: are the locally synthesized proteins solely responsible for the myriad of biological functions in situ without the contribution of systemic complement? The answer is probably "yes". Among the proteins that are synthesized locally, C1q takes center stage for several reasons. First, it is synthesized predominantly by potent antigen presenting cells such as monocytes, macrophages and dendritic cells (DCs), which by itself is a clue that it plays an important role in antigen presentation and/or DC maturation. Second, it is transiently anchored on the cell surface via a transmembrane domain located in its A chain before it is cleaved off and released into the pericellular milieu. The membrane-associated C1q in turn, is able to sense danger patterns via its versatile antigen-capturing globular head domains. More importantly, locally synthesized C1q has been shown to induce a plethora of biological functions through the induction of immunomodulatory molecules by an autocrine- or paracrine- mediated signaling in a manner that mimics those of TNFα. These include recognition of pathogen- and danger- associated molecular patterns, phagocytosis, angiogenesis, apoptosis and induction of cytokines or chemokines that are important in modulating the inflammatory response. The functional convergence between C1q and TNFα in turn is attributed to their shared genetic ancestry. In this paper, we will infer to the aforementioned "local-synthesis-for-local function" paradigm using as an example, the role played by locally synthesized C1q in autoimmunity in general and in systemic lupus erythematosus in particular.

The C1q family of proteins: insights into the emerging non-traditional functions

Research conducted over the past 20 years have helped us unravel not only the hidden structural and functional subtleties of human C1q, but also has catapulted the molecule from a mere recognition unit of the classical pathway to a well-recognized molecular sensor of damage-modified self or non-self antigens. Thus, C1q is involved in a rapidly expanding list of pathological disorders – including autoimmunity, trophoblast migration, preeclampsia, and cancer. The results of two recent reports are provided to underscore the critical role C1q plays in health and disease. First is the observation by Singh et al. (2011) showing that pregnant C1q−/− mice recapitulate the key features of human preeclampsia that correlate with increased fetal death. Treatment of the C1q−/− mice with pravastatin restored trophoblast invasiveness, placental blood flow, and angiogenic balance and, thus, prevented the onset of preeclampsia. Second is the report by Hong et al. (2009) which showed that C1q can induce apoptosis of prostate cancer cells by activating the tumor suppressor molecule WW-domain containing oxydoreductase (WWOX or WOX1) and destabilizing cell adhesion. Downregulation of C1q on the other hand, enhanced prostate hyperplasia and cancer formation due to failure of WOX1 activation. C1q belongs to a family of structurally and functionally related TNF-α-like family of proteins that may have arisen from a common ancestral gene. Therefore C1q not only shares the diverse functions with the tumor necrosis factor family of proteins, but also explains why C1q has retained some of its ancestral “cytokine-like” activities. This review is intended to highlight some of the structural and functional aspects of C1q by underscoring the growing list of its non-traditional functions.

An in vitro model of pig liver xenotransplantation--pig complement is associated with reduced lysis of wild-type and genetically modified pig cells

BACKGROUND

After pig liver transplantation in humans, the graft will produce pig complement (C). We investigated in vitro the lysis of wild-type (WT), α1,3-galactosyltransferase gene-knockout (GTKO), and CD46 transgenic (CD46) pig peripheral blood mononuclear cells (PBMC) caused by human anti-pig antibodies (Abs) + pig C.

METHODS

Human serum IgM/IgG binding to WT and GTKO PBMC was determined by flow cytometry, and lysis of pig PBMC by a C-dependent cytotoxicity assay using (i) human serum (human Abs + C), (ii) GTKO pig serum (anti-Gal Abs + pig C), (iii) heat-inactivated human serum (human Abs) + rabbit C, or (iv) human Abs + pig C (serum).

RESULTS

Binding of human IgM and IgG to GTKO PBMC was less than to WT PBMC (P < 0.05). In the presence of human Abs, lysis of WT and GTKO PBMC by rabbit C was 87 and 13%, respectively (WT vs. GTKO, P < 0.01), but was only 37 and 0.4% in the presence of pig C (WT vs. GTKO, P < 0.05). Human/rabbit C-induced lysis was greater than pig C-induced lysis for both WT and GTKO PBMC. CD46 pig PBMC reduced rabbit/human C- and pig C-mediated lysis (P < 0.05).

CONCLUSIONS

Pig livers, particularly from GTKO and CD46 pigs, are likely to have an immunologic advantage over other organs after transplantation into humans. In the absence of pig antibodies directed to human tissues, pig complement is unlikely to cause problems after liver xenotransplantation, especially if GTKO/CD46 pigs are used as the source of the livers.

Liver xenografts for the treatment of acute liver failure: clinical and experimental experience and remaining immunologic barriers

A critical element restricting the application of liver transplantation is the shortage of human deceased donor organs. Xenotransplantation using pig organs might be a solution to this shortage. Although the problems that still require resolution include the immunologic barrier, the potential risk of transferring infectious agents with the transplanted organ, and uncertainty about whether the transplanted organ will function satisfactorily in the human environment, recent progress in the genetic manipulation of pigs has led to the prospect that clinical xenografting, at least as a bridge to allotransplantation, may be possible in the foreseeable future. Experience with clinical auxiliary and orthotopic liver xenotransplantation and experimental liver xenotransplantation in nonhuman primate and other large animal models is reviewed, and the remaining immunologic problems are discussed. Evidence suggests that, in patients with hepatic failure, the pig liver may be less susceptible to antibody-mediated injury than other pig organs, such as the heart or kidney. Pig Kupffer cells and other macrophages will recognize and phagocytose primate red blood cells, but this problem should be overcome by pretransplant depletion of macrophages from the organ-source pig. From the evidence currently available, it does not seem unduly optimistic to anticipate that a liver from an alpha1,3-galactosyltransferase gene-knockout pig would survive at least long enough to function as a successful bridge to allotransplantation.

Infection-enhancing and -neutralizing activities of mouse monoclonal antibodies against dengue type 2 and 4 viruses are controlled by complement levels

Dengue viruses are distributed widely in the tropical and subtropical areas of the world and cause dengue fever and its severer form, dengue hemorrhagic fever. While neutralizing antibodies are considered to play a major role in protection from these diseases, antibody-dependent enhancement (ADE) of infection is an important mechanism involved in disease severity, in addition to the involvement of T lymphocytes. Here, we analyzed relationships between neutralizing and enhancing activities at a clonal level using models of dengue type 2 virus (DENV2) and dengue type 4 virus (DENV4). Totals of 33 monoclonal antibodies (MAbs) against DENV2 and 43 against DENV4 were generated, all directed to the envelope protein. In these MAbs, enhancing activities were shown at subneutralizing doses under normal ADE assay conditions where test samples were heat inactivated. However, the inclusion of commercial rabbit complement or fresh sera from healthy humans in the ADE assay system abolished the enhancing activities of all these MAbs. The reductive effect of fresh sera on enhancing activities was significantly reduced by their heat inactivation or the use of C1q- or C3-depleted sera. In some fresh sera, enhancing activities were shown within a range of 20 to 80% of normal complement levels in a dose-dependent fashion. These results indicate that a single antibody species possesses two distinct activities (neutralizing/enhancing), which are controlled by the level of complement, suggesting the involvement of complement in dengue disease severity. Fresh human sera also tended to reduce enhancing activities more effectively in homologous than heterologous combinations of viruses (DENV2/DENV4) and MAbs (against DENV2/DENV4).

IFN-gamma upregulates expression of the mouse complement C1rA gene in keratinocytes via IFN-regulatory factor-1

We examined the expression of the mouse complement component C1rA (mC1rA) in IFN-gamma-stimulated mouse keratinocytes (Pam 212) and found that it was upregulated. To analyze the mechanism involved, we cloned the 2,150 bp 5'-flanking region of mC1rA by the vectorette-PCR technique, and identified the transcription start site of mC1rA by rapid amplification of complementary DNA ends. Analysis of the 5' sequence revealed putative binding sites for activator protein 1, CCAAT/enhancer binding protein (C/EBP), signal transducer and activator of transcription 1 (STAT-1), IFN-regulatory factor-1 (IRF-1), and others. We detected transcriptional activation dependent on this upstream region in reporter gene assays and Northern blots. To identify the cis-acting regulatory elements involved, we analyzed serial deletion constructs of the promoter using luciferase reporters. The -80 to -19 bp region, which contains a putative IRF-1 binding site, was required for both basal promoter activity and responses to IFN-gamma. The use of site-directed point mutations, electrophoresis mobility shift assays, and supershift assays indicated that the putative IRF-1 binding site was essential for both IFN-gamma-dependent and -independent transcriptional activity of the mC1rA promoter. We conclude that IFN-gamma stimulates mC1rA gene expression via IRF-1 in mouse keratinocytes.

A decrease in complement is associated with increased renal and hematologic activity in patients with systemic lupus erythematosus

OBJECTIVE

To determine the degree to which changes in C3 and C4 precede or coincide with changes in systemic lupus erythematosus (SLE) activity, as measured by 5 global activity indices, the physician's global assessment (PGA), modified SLE Disease Activity Index (M-SLEDAI), modified Lupus Activity Index (M-LAI), Systemic Lupus Activity Measure (SLAM), and the modified British Isles Lupus Assessment Group (M-BILAG), and to evaluate the association between changes in C3 and C4 levels and SLE activity in individual organ systems.

METHODS

Fifty-three lupus patients were observed monthly for 1 year in a longitudinal study. Lupus disease activity and complement levels were measured at each visit. Lupus flare was defined as a 1.0 (or greater) increase in the PGA, a 3-point increase in the M-SLEDAI, a 0.1 increase in the M-LAI, a 3-point increase in the SLAM, or a 4-point increase in the M-BILAG within a 1-month period. Flare rates were calculated for subgroups defined by previous (1 month before) or concurrent changes in complement levels. Logistic regression models were used to determine the significance of the association between recent changes in complement levels and flare, controlling for prednisone dosage. Similar models were used to assess the association between changes in C3 or C4 levels and increased SLE activity in specific organ systems.

RESULTS

Lupus flares occurred at 12% of visits based on the PGA, 19% based on the M-SLEDAI, 25% based on the M-LAI, 13% based on the SLAM, and 12% based on the M-BILAG. Recent changes in C3 and C4 levels were not associated with flares based on 3 of the 5 activity indices. Flares defined by the M-LAI were more frequent when there was a concurrent decrease in C3 (odds ratio [OR] 1.9, 95% confidence interval [95% CI] 1.1-3.1) or C4 (OR 2.1, 95% CI 1.3-3.6). Higher flare rates, as defined by the SLAM, were associated with previous increases in C3 (OR 1.6, 95% CI 1.0-2.6) and C4 (OR 2.2, 95% CI 1.2-3.9). When individual organ systems were analyzed, decreases in C3 and C4 were associated with a concurrent increase in renal disease activity (OR 2.2, 95% CI 1.4-3.5 and OR 1.9, 95% CI 1.1-3.4, respectively). Decreases in C3 were also associated with concurrent decreases in the hematocrit (OR 4.6, 95% CI 1.7-12.3), platelet (OR 2.5, 95% CI 1.5-4.1), and white blood cell (OR 2.2, 95% CI 1.3-3.6) counts. Previous increases in C3 levels were associated with a decrease in platelets (OR 1.7, 95% CI 1.1-2.7). A decrease in C4 was associated with a concurrent decrease in the hematocrit level (OR 3.2, 95% CI 1.3-7.5) and platelet count (OR 1.6, 95% CI 1.0-2.5).

CONCLUSION

Decreases in complement levels were not consistently associated with SLE flares, as defined by global measures of disease activity. However, decreasing complement was associated with a concurrent increase in renal and hematologic SLE activity.

Cytokine-mediated up-regulation of CD55 and CD59 protects human hepatoma cells from complement attack

Hepatic parenchymal cells respond in many different ways to acute-phase cytokines. Some responses may protect against damage by liver-derived inflammatory mediators. Previous investigations have shown that cytokines cause increased secretion by hepatoma cells of soluble complement regulatory proteins, perhaps providing protection from complement attack. More important to cell protection are the membrane complement regulators. Here we examine, using flow cytometry and Northern blotting, the effects of different cytokines, singly or in combination, on expression of membrane-bound complement regulators by a hepatoma cell line. The combination of tumour necrosis factor-alpha, IL-1beta, and IL-6 caused increased expression of CD55 (three-fold) and CD59 (two-fold) and decreased expression of CD46 at day 3 post-exposure. Interferon-gamma reduced expression of CD59 and strongly antagonized the up-regulatory effects on CD59 mediated by the other cytokines. Complement attack on antibody-sensitized hepatoma cells following a 3-day incubation with the optimum combination of acute-phase cytokines revealed increased resistance to complement-mediated lysis and decreased C3b deposition. During the acute-phase response there is an increased hepatic synthesis of the majority of complement effector proteins. Simultaneous up-regulation of expression of CD55 and CD59 may serve to protect hepatocytes from high local concentrations of complement generated during the acute-phase response.

Detection of two variants of complement component C3 in C3-deficient guinea pigs distinguished by the absence and presence of a thiolester

The complement system is an essential part of the innate defense, and C3 is an integral part of this powerful system. In previously identified complement C3 deficient guinea pigs only approx. 5% of the normal serum C3 level is detectable. No differences were found between in vitro C3 protein synthesis and C3 mRNA levels of cells from C3-deficient and wild-type animals and the amino acid sequences of both C3 proteins are identical as deduced from cDNA sequencing. Previously, the principal inability to form a C3 thiolester was discussed as a possible reason for this C3-deficiency. Here we report the isolation of two functionally different C3 species from the C3-deficient animals. Only one of these C3 proteins exhibits normal hemolytic activity and contains a thiolester group. The second C3 species is exclusively present in C3-deficient animals and lacks a thiolester, explaining its failure to express hemolytic activity. The presence of a second C3 species lacking a thiolester structure only in C3-deficient animals indicates that the stability of the thiolester may play a role in C3 deficiency. However further analysis of the in vitro stability of the thiolesters of C3 from normal and C3-deficient guinea pigs revealed no differences. A decreased in vivo thiolester stability might lead to the presence of C3 with and without a thiolester or alternatively the expression of two isoforms of C3 in these animals. Considering the central role of C3 in host defense, the mechanisms of C3 thiolester formation require further analysis.

Contribution of Renal Secreted Complement C3 to the Circulating Pool in Humans

Complement C3 produced within the kidney may be an important mediator of local inflammatory and immunological injury. The overall level of renal C3 production and consequently its contribution to the total circulating C3 level are, however, unknown. This was investigated by using the conversion of C3 from recipient to donor allotype following renal transplantation. The C3 F and S allotypes of 80 consecutive renal donor-recipient pairs (148 individuals) were determined by amplification refractory mutation system analysis. The extent of allotype conversion in C3 F/S mismatched recipients was quantified at different stages after transplantation, using an enzyme-linked immunosorbent assay specific for the HAV 4-1 polymorphism of C3 that is strongly associated with C3F. Twenty-one of the eighty recipients were potentially informative, i.e., were C3 SS recipients of C3 FF or FS donor kidneys. In the early postoperative period, donor-derived C3 (HAV 4-1-positive) was undetectable, increasing to 9.6% of the total circulating C3 at times of acute allograft rejection. When graft dysfunction occurred from causes other than rejection, donor C3 remained undetectable. After stable graft function was attained (3–13 mo after transplantation), donor C3 made up 4.5% of the total circulating C3 pool. Our findings demonstrate that human transplant kidney in the resting state is a significant source of extrahepatic C3. Its heightened local synthesis during rejection episodes suggests a possible pathogenic role for C3 in this immunological process.

Endothelial cells cultured from human brain microvessels produce complement proteins factor H, factor B, C1 inhibitor, and C4

The inflammatory mediators, cytokines and complement proteins are believed to regulate the sequential events during the development of lesions secondary to ischaemia and reperfusion. The endothelial cell monolayer of the brain microvasculature is the critical interface between the blood-borne mediators and brain tissue. The involvement of these cells in complement production and regulation has not been well documented. In the present study, expression of complement proteins (C1 inhibitor, factor H, factor B, C4) by cultured endothelial cells obtained from human brain microvessels has been characterized. Interferon gamma upregulates the production of all the complement factors studied. Serine proteases, plasmin and miniplasmin induce the expression of C4, decrease the level of ELISA detectable C1 inhibitor, and do not affect the production of factors H and B. These data indicate that complement proteins are expressed locally by the brain microvessels, and may modulate the inflammatory responses of brain tissue.

Complement biosynthesis in the central nervous system

Complement is an important effector arm of the human immune response. Binding of proteolytic fragments derived from activation of complement by specific receptors leads to responses as diverse as inflammation, opsonization, and B-cell activation. The importance of characterizing the expression and regulation of complement in the CNS is highlighted by growing evidence that complement plays a significant role in the pathogenesis of a variety of neurological diseases, such as multiple sclerosis and Alzheimer's disease. In vitro studies have demonstrated that astrocytes, the predominant glial cell type in the brain, are capable of expressing or producing a majority of the components of the complement system. Expression of many complement proteins synthesized by astrocytes is regulated by both pro- and anti-inflammatory cytokines, many of which are also produced by several cell types in the CNS. In addition to astrocytes, ependymal cells, endothelial cells, microglia, and neurons have recently been shown to synthesize various complement proteins or express complement receptors on their cell surfaces. Together, these studies demonstrate that several cell types throughout the brain have the potential to express complement and, in many cases, increase expression in response to mediators of the acute phase response. These studies suggest that complement may play a greater role in CNS immune responses than previously thought, and pave the way for better understanding of the dynamics of complement expression and regulation in vivo. Such understanding may lead to therapeutic manipulation of complement host defense functions in a variety of inflammatory and degenerative diseases in the CNS.

Role of protein kinase C activation in synthesis of complement components C2 and factor B in interferon-gamma-stimulated human fibroblasts, glioblastoma cell line A172 and monocytes

The synthesis of C2 and factor B, the key components of complement system, is performed by various kinds of cells and is also up-regulated by interferon-gamma (IFN-gamma). By using human fibroblasts, human glioblastoma cell line A172 and monocytes, we investigated the signal-transduction mechanism for IFN-gamma-induced synthesis of C2 and factor B. The C2 and factor B synthesis induced by IFN-gamma in all three cell types was inhibited by a protein kinase C (PKC) inhibitor, 1-(5-isoquinolinyl-sulphonyl)-2-methylpiperazine (H-7). The depletion of PKC in these cell types after treatment with phorbol 12-myristate 13-acetate (PMA) resulted in inhibition of IFN-gamma-induced C2 production. In addition, IFN-gamma treatment elicited a decrease in cytoplasmic PKC in A172 cells, indicating that PKC is activated by IFN-gamma. These results suggest that PKC is crucial for IFN-gamma-induced C2 and factor B synthesis. Northern-blot analysis showed that the effects at H-7 were at least partly mediated by modulation of C2 and factor B mRNA abundance in A172 cells. Since treatment of fibroblasts and A172 cells with IFN-gamma had no effect on intracellular Ca2+ concentration, and since neither EGTA nor nifedipine inhibited C2 or factor B synthesis induced by IFN-gamma, we concluded that intracellular Ca2+ mobilization was not involved in the effect of IFN-gamma. In addition, genistein, herbimycin A and N-(6-aminohexyl)-5-chloro-1-naphthalene-sulphonamide (W-7) had no inhibitory effect on IFN-gamma-mediated action in any of the three cell types, which suggests that IFN-gamma acts independently of tyrosine kinases and calmodulin-dependent protein kinases.

The biological basis of and strategies for clinical xenotransplantation

Recent discoveries have suggested that the exchange of multiple leukocyte lineages between grafts and host and subsequent long-term chimerism in both is the seminal mechanism of the acceptance of organs transplanted from the same (allografts) or different species (xenografts). This insight suggests new strategies which may allow xenotransplantation, the principal obstacle to which has been humoral rejection. We have defined humoral rejection as a family of complement activation syndromes afflicting allografts and xenografts in which there is a strong (but not invariable) association with performed antigraft antibodies, invariable evidence of complement activation, histopathologic stigmas of vascular endothelial damage, and a concomitant local or systemic coagulopathy. The generic descriptive term hyperacute rejection is a misnomer because a slow-motion version of the same "humoral" process can occur with some allografts and is the rule with the so-called concordant species xenotransplantations. The pathway of experience and discovery leading to this conclusion shows clearly that the distinction frequently made between allograft versus xenograft humoral rejection does not actually exist in principle, but only in details and intensity. Breaking down this barrier to xenotransplantation, whether or not it is associated with antibodies, is unrealistic. However, the possibility of avoiding the barrier has been exposed by showing that animal organs can be humanized, with a mixed donor and recipient cell population similar to the chimerism seen in long surviving allografts or even with complete leukocyte replacement. Pilot experiments in rodents suggest that organs from fully xenogeneic chimeras can be made into xenogeneic targets that are no more provocative of complement activation than allografts when they are transplanted into the donor bone marrow species. Although the validity of this concept of organ xenograft preparation is only at the pilot stage of verification, there is reason to suspect that the complement trigger of humoral rejection can be thereby disarmed. If this can be accomplished, independent evidence suggests that cellular rejection can be controlled with conventional T-cell directed immunosuppression, perhaps even with surprising ease. The potential subtle liability of synthetic products of xenogeneic parenchymal cells is not yet known.

Donor species complement after liver xenotransplantation. The mechanism of protection from hyperacute rejection

Hamster hearts transplanted into stable rat recipients of hamster livers (OLT rats) were hyperacutely rejected after transfer with unaltered rat antihamster hyperimmune serum (HS). This was followed by immediate liver xenograft rejection in 4 of 5 rats. In contrast, simple heat inactivation of the rat HS resulted in prolonged survival of hamster hearts to 25 days without deterioration effect in the liver xenografts. This effect was species-specific because third-party mouse heart grafts in OLT rats were hyperacutely rejected in minutes if either active or heat inactivated antimouse HS was given. In cytotoxicity experiments, the complement in OLT serum produced weak lysis of hamster lymphocytes, while efficiently doing so with mouse cell targets. Because normal hamster serum caused no lysis at all of hamster target cells, the residual low-grade lysis of OLT serum was possibly being mediated by extrahepatic sources of rat C. In conclusion, the homology of C and target cells represents a mechanism of protection that the liver confers to other organs, and that is mostly easily seen in xenografts but may be allospecifically operational with allografts as well within the limits of MHC restriction.

Complement biosynthesis by mononuclear phagocytes

Mononuclear phagocytes are an important in vivo source of a wide range of complement components. They are able to rapidly up-regulate or down-regulate complement synthesis in response to many different pharmacological and biological stimuli. This ability is likely to make a significant contribution to maintaining host defences particularly in peripheral tissues. The important role of molecular biology in the study of complement biosynthesis by mononuclear phagocytes will be emphasised.

Interleukin-1 and tumor necrosis factor-mediated regulation of C3 gene expression in human astroglioma cells

In this report, we show that in the human astroglioma cell line D54-MG, both interleukin-1 (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) enhance C3 gene expression in a time- and dose-dependent manner. Kinetic analysis demonstrates that after 96 h, C3 mRNA levels increase approximately 30-fold and 20-fold in response to IL-1 beta or TNF-alpha, respectively. C3 protein production increases proportionally, reaching levels 36-fold and 18-fold higher than untreated controls upon exposure to IL-1 beta or TNF-alpha, respectively. D54-MG cells require a minimal 1 h exposure to IL-1 beta in order to enhance C3 gene expression significantly, while 4 to 8 h are required for TNF-alpha. Simultaneous treatment of D54-MG cells with IL-1 beta and interferon-gamma (IFN-gamma) resulted in an additive increase in both C3 mRNA and protein expression, a finding not seen with the combination of TNF-alpha and IFN-gamma. Primary rat astrocytes also express increased C3 mRNA levels after 48 h in response to IL-1 beta (5.3-fold increase) and TNF-alpha (7-fold increase), while an additive effect was observed upon simultaneous treatment with both IL-1 beta and IFN-gamma. In the central nervous system (CNS), endogenous complement and cytokine production by astrocytes, and enhancement by IFN-gamma, a product of activated T cells often seen in the CNS in neural autoimmune disease, may contribute to the pathogenesis of inflammatory demyelinating diseases such as multiple sclerosis.

Hereditary deficiency of C3 in animals and humans

Inherited deficiency of complement C3 has been described in guinea pigs, dogs and 20 humans. Homozygous deficiency of C3 is associated with recurrent pyogenic infections by encapsulated bacteria, especially H. influenzae, S. pneumoniae and N. meningitidis. In dogs and humans there is also an association with development of glomerulonephritis of the mesangiocapillary type. Some patients also develop transient erythematous rashes in association with pyogenic infections, with histology showing predominantly neutrophil infiltration and small vessel vasculitis. Studies of antibody responses, mainly in experimental animals have shown impaired primary and secondary responses to both thymus-dependent and -independent antigens at low immunizing doses, with a reduced switch from IgM to IgG production. The molecular basis of C3 deficiency has been established in two humans with C3 deficiency. In one it was due to a splice junction mutation and in another, to a partial gene deletion. These mutations are not compatible with the production of functional C3 in any tissue. Such patients with absolute C3 deficiency are a valid model for understanding the physiological role of C3 in vivo.

Presentation of a circulating self protein (C5) to MHC class II restricted T cells

Presentation of a soluble self antigen, the fifth component of complement (C5), is discussed with emphasis on the differential ability of presentation by subpopulations of APC (dendritic cells, macrophages, B cells, fibroblasts, B cell lines and bone marrow macrophages). Constitutive presentation of C5 in C5 sufficient mice is a prerequisite for tolerance induction in MHC class II restricted T cells and can be directly demonstrated by the ability of ex-vivo APC from C5 sufficient, but not C5 deficient mice, to activate C5 specific T cells in vitro in the absence of added antigen. C5 presentation and tolerance induction in MHC class II restricted T cells is strictly dependent on an exogenous source of self antigen. C5 biosynthesized, but not secreted by macrophages is ignored by MHC class II restricted cells and induces neither tolerance nor autoimmunity. C5 presentation for tolerance induction depends largely on the efficiency of antigen uptake by APC, a property which varies within different APC subpopulations and with the nature of the antigen.

Localization of self antigen: implications for antigen presentation and induction of tolerance

The fifth component of complement (C5) is a self antigen expressed in serum of normal mice at a concentration of about 50 micrograms/ml. We have previously shown that C5 is constitutively processed and presented by antigen-presenting cells (APC) in normal mice to induce and maintain complete tolerance in major histocompatibility complex (MHC) class II-restricted T cells. This report addresses the question of whether C5 presentation involves exogenous antigen which has been internalized for processing or whether intracellular, biosynthesized C5 is being presented with MHC class II. Macrophages were found to synthesize, but not secrete C5 in bone marrow chimeras made from irradiated C5-deficient [C5(-)] hosts reconstituted with C5-sufficient [C5(+)] bone marrow [C5(+)-->C5(-)]. In these mice, macrophages are the only source of C5. [C5(+)-->C5(-)] chimeras are not tolerant of C5 and generate C5-specific T and B cell responses upon immunization indistinguishable from those of C5(-) mice. Macrophages from [C5(+)-->C5(-)] chimeras are unable to activate C5-specific T cell hybrids in vitro unlike macrophages from a C5(-) strain that has matured in a C5-expressing environment [C5(-)-->C5(+) chimeras]. This shows that under physiological conditions in vivo intracellular C5 does not get access to the class II presentation pathway and thus, does not induce tolerance in class II-restricted T cells.

Effect of interferon-gamma on complement gene expression in different cell types

We have studied the expression of the complement components C2, C3, factor B, C1 inhibitor (C1-inh), C4-binding protein (C4-bp) and factor H in human peripheral blood monocytes, skin fibroblasts, umbilical vein endothelial cells (HUVEC) and the human hepatoma cell line G2 (Hep G2) in the absence and the presence of interferon-gamma (IFN-gamma). E.l.i.s.a. performed on culture fluids, run-on transcription assays, Northern blot and double-dilution dot-blot techniques confirmed that monocytes expressed all six components, whereas fibroblasts, HUVEC and HepG2 each expressed five of the six components. Fibroblasts and HUVEC did not synthesize C4-bp, and Hep G2 did not produce factor H. In addition to these differences, the synthesis rates of C3, C1-inh and factor H were not the same in all cell types. However, the synthesis rates of C2 and factor B were similar in all four cell types. The half-lives of the mRNAs were shorter in monocytes than in other cell types. Monocyte factor H mRNA had a half-life of 12 min in monocytes, compared with over 3 h in fibroblasts and HUVEC. The instability of factor H mRNA in monocytes may contribute to their low factor H secretion rate. IFN-gamma produced dose-dependent stimulation of C2, factor B, C1-inh, C4-bp and factor H synthesis by all cell types expressing these proteins, but decreased C3 synthesis in all four cell types. Cell-specific differences in the response to IFN-gamma were observed. The increased rates of transcription of the C1-inh and factor H genes in HUVEC were greater than in other cell types, while the increased rate of transcription of the C2, factor B and C1-inh genes in Hep G2 cells was less than in other cell types. IFN-gamma did not affect the stability of C3, factor H or C4 bp mRNAs, but increased the stability of factor B and C1-inh mRNAs and decreased the stability of C2 mRNA. Although these changes occurred in all four cell types studied, the half-life of C1-inh mRNA in monocytes was increased almost 4-fold, whereas the increases in the other cell types were less than 30%. These data show that the constitutive synthesis rates of complement components may vary in the different cell types. They also show that the degree of change in synthesis rates in response to IFN-gamma in each of the cell types often varies due to differences in transcriptional response, sometimes in association with changes in mRNA stability.

Evidence that C4b-binding protein (proline-rich protein) is synthesized by hepatocytes

C4b-binding protein (C4bp), a glycoprotein involved in regulating the classical pathway of the complement system, binds the activated form of C4b and accelerates the decay rate of the C4b, C2a complex. Recently, sequence analysis of the cDNA for proline-rich protein (PRP) demonstrated that PRP is identical with C4bp. We measured the concentration of C4bp in serum by single radial immunodiffusion in patients with various liver diseases. Concentration of C4bp was significantly lower in hepatic cirrhosis (P = 0.001) and higher in fatty liver (P = 0.0002) than the control values, after adjusting for age, sex, and concentration of total cholesterol, triglyceride, and C-reactive protein. Significant positive correlations were observed between the concentration of C4bp in serum and total protein, albumin, cholinesterase level, and lecithin-cholesterol acyltransferase activity. Immunohistochemical analysis of human liver with specific antiserum to human C4bp demonstrated reaction endproducts in the hepatocytes around the central veins. These observations provide evidence that C4bp is synthesized by hepatocytes.

Effects of plasmapheresis and autotransfusion on complement recovery in dogs

To study selectively the dilutional aspects of severe blood loss on the serum complement system, we developed an animal model consisting of isovolumic phlebotomy and reinfusion of washed autologous erythrocytes. This model avoided hypoperfusion, ischemia, and transfusion of foreign antigen. We measured total serum protein, C3 antigen, total complement hemolytic activity, and alternative pathway hemolytic activity. Each of the first three parameters dropped to 55% of initial value (P less than 0.005) by the end of the phlebotomy/reinfusion procedure and returned to normal levels by Day 1 or 2. C3 antigen and total complement hemolytic activity then rose to 150% of normal by Day 4 and gradually returned to normal within 2 weeks. Alternative pathway activity, by contrast, fell by more than 80% (P less than 0.005) within the first 6 hr, recovered by Day 4, and gradually rose to about 140% of normal by Day 21. Trauma patients treated for heavy blood loss who suffer depletion of hemolytic complement during the first few hours may be at greater risk of infection due to immune deficiencies. The implication of the results presented here is that the alternative pathway may be particularly weakened during blood loss and transfusion by simple dilution in addition to the effects of processes omitted in this model. Knowledge of the kinetics of complement recovery, independent of other effects usually accompanying trauma, may be helpful in determining whether these patients might benefit from exogenous manipulation of the complement system.

Enhanced local production of complement components in the small intestines of patients with Crohn's disease

There is evidence that complement components may be formed locally in inflammatory lesions containing monocytes and macrophages. To investigate the role of complement in Crohn's disease we measured jejunal-fluid concentrations of the complement components C4, C3, and factor B by perfusion of a closed segment of the jejunum in 22 patients with Crohn's disease thought to be limited to the terminal ileum. The mean (+/- SEM) jejunal-fluid C4 concentration was 2.0 +/- 0.3 mg per liter, significantly higher than the mean level in 35 healthy controls (0.7 +/- 0.1 mg per liter; P less than 0.001). The mean C3 concentration was 1.0 +/- 0.1 mg per liter in the patients and 0.7 +/- 0.1 mg per liter in the controls (P less than 0.05). The factor B levels were similar in the two groups. Calculated rates of intestinal secretion of these components showed differences of the same magnitude. Leakage of protein from plasma was not increased. The jejunal-fluid:serum ratios of these complement proteins indicated that their appearance in the lumen of the jejunum was due to at least in part to local mucosal synthesis. The increased jejunal secretion of C4, but not C3 or factor B, paralleled the clinical activity of Crohn's disease. Values were normal in first-degree relatives of the patients (n = 13), patients with celiac disease (n = 8), and patients with ulcerative colitis (n = 4). We conclude that increased secretion of complement by clinically unaffected jejunal tissue in patients with Crohn's disease reflects the systemic nature of this disorder and may be due to the stimulated synthesis of complement by activated intestinal monocytes and macrophages.

Synthesis of complement component C5 by human B and T lymphoblastoid cell lines

Human B and T lymphoblastoid cell lines were shown to synthesize C5. C5 synthesis was quantitated with an enzyme-linked immunosorbent assay (ELISA) that utilized a pool of C5-specific monoclonal antibodies (mAbs). Some level of C5 synthesis was detected in all eight of the B and T cell lines examined. In three of the cell lines, C5 was detected in both culture supernatants and whole cell detergent lysates, whereas in the other five cell lines, C5 was detected only in the cell lysates. Lymphoblastoid cells with both distributions of C5 were shown to synthesize a messenger RNA that was similar in size to the C5 mRNA expressed by the HepG2 hepatoma cell line. Estimates of the concentration of the C5 transcript in poly(A)+ RNA from lymphoblastoid and HepG2 cells suggested that C5 mRNA levels in the lymphoblastoid cell lines were comparable and about one-tenth of the levels in HepG2 cells. Lymphoblastoid C5, isolated by immunoaffinity chromatography from the supernatants of 35S-labeled cultures, had the same subunit composition as plasma-derived C5, but had an alpha subunit of slightly smaller relative mass.

Complement levels in patients with hepatic dysfunction

Total hemolytic complement activity and serum complement protein concentrations were compared in 17 hospitalized patients with normal hepatic function and 16 patients with liver disease due to alcohol (15 patients) or acetaminophen toxicity (one patient). In contrast to the control patients, individuals with hepatic dysfunction had decreased total CH50 levels and low concentrations of total C3, C4, C5, factor B, and the regulatory proteins factor I and beta-1H. These patients also had increased C4d/C4 ratios, indicating classical pathway activation. The level of complement deficiency appears to correlate with either prolongation of the prothrombin time or depression of serum albumin concentration. These results indicate that patients with hepatic disease have severe complement depletion that is probably multifactorial in origin. This impairment in complement function will contribute to the impaired antibacterial host defense of the patient with chronic hepatic disease.

Biosynthesis of complement factor P (properdin) by the human pre-monocyte cell line (U-937)

The human monocyte-like cell line, U-937, is known to differentiate into macrophage-like cells following stimulation with phorbol myristate acetate (PMA) or interferon-gamma (IFN-gamma). The activated cells have been reported to have enhanced capacity to synthesize C2, C3, Factors B and H. Here, U-937 cells were used as a model system to investigate the effects of immunomodulatory agents on the biosynthesis of Factor P by monocytoid cells. Non-stimulated U-937 cells progressively secreted increasing amounts of Factor P over a 72-hr culture period. The secreted Factor P was hemolytically active. The daily production of Factor P was nearly linear (approx. 2.1 +/- 0.2 ng/10(6) cells; mean +/- SEM). Factor P synthesis was reversibly inhibited by cycloheximide indicating de novo synthesis. Both secreted Factor P and Factor P in normal plasma contained Factor P of heterogeneous molecular sizes and eluted from Sephacryl S-300 gel filtration column as a broad peak (mol. wt 250-800 kDa). The synthesis of Factor P by U-937 cells was augmented 1.8-, 2.1- and 2.5-fold respectively following induction with PMA (30 ng/ml), IFN-gamma (100 U/ml) and LPS (0.1 microgram/ml). Metabolic labeling of U-937 cells and autoradiograms of SDS-PAGE analysis of Factor P immunoprecipitates demonstrated a 54 kDa band in the culture supernate, co-migrating with purified 125I Factor P. Intracellular Factor P however had an apparent mol. wt that was 4000 kDa smaller than secreted Factor P. Thus U-937 cells synthesize a precursor Factor P subunit polypeptide chain which undergoes post-synthetic glycosylation and polymerization to give rise to the oligomers characteristic of native Factor P in fresh plasma. Our data also demonstrate that Factor P synthesis by monocytic cells can be enhanced by immunomodulatory factors or mediators that are generally found at sites of inflammation and immune response.

Purification of the fifth component of murine complement from ascites fluid

The murine complement component C5 was purified on an affinity column using a monoclonal anti-mouse C5 antibody. We describe in this paper that ascites fluid from normal (C5-sufficient) mice contains almost as much C5 protein as mouse serum. Since ascites fluid is much easier to obtain in large quantities it is a convenient source for the purification of this mouse serum protein.

Tissue-specific variation in C4 and Slp gene regulation

C4 and Slp are highly homologous mouse genes that differ in function and regulation. Allelic variants exist in quantitative regulation of C4 and in hormonal regulation of Slp. We have examined expression in several tissues, including liver and peritoneal macrophages which are the major sites of synthesis, using a probe that allows direct comparison of C4 and Slp mRNAs. Correctly-sized and initiated RNA, within an order of magnitude of liver levels, is found in mammary gland, lung, spleen, and kidney; lower levels are detectable in testis, brain, heart and submaxillary gland. By comparing expression in congenic mouse strains differing in C4 and Slp loci, regulation of these genes is seen to vary in different tissues. This provides a well-defined genetic system in which to examine cis-acting sequences and trans-acting factors that result in tissue-specific patterns of gene regulation.

Pulmonary alveolar type II epithelial cells synthesize and secrete proteins of the classical and alternative complement pathways

The serum complement system is a major mediator of inflammation reactions. Two of the complement proteins, the third (C3) and fifth (C5) components, are precursors of potent phlogistic molecules, C3a and C5a. C5a has potent chemotactic activity and plays an active role in pulmonary inflammation. We present evidence suggesting that several complement proteins, including C5, are synthesized locally in the lung in alveolar type II epithelial cells. Lung tissue from normal mice synthesized and secreted C5 protein similar to the C5 protein in mouse serum, whereas lung tissue from C5-deficient mice did not. Lung tissues from both normal and C5-deficient mice synthesized C3. Rat lung tissue synthesized and secreted C5, as well as C2, C4, C3, and factor B. Cultures of type II cells (95% type II cells, 5% macrophages) regularly synthesized all these proteins. In contrast, cultures of macrophages alone synthesized large amounts of C2 and factor B, and in some experiments C3 and C4, but never C5. The C5 synthesized by the rat cells was slightly larger than serum C5 (200 kD compared with 180 kD) and was not processed to the two-chain molecule seen in serum. Rat lung tissue and purified type II cells contained C5 mRNA with the same molecular mass as the C5 mRNA in rat liver and in mouse lung and liver. Human type II cells also synthesized C5, as well as C2, C4, C3, and factor B. Human pulmonary macrophages synthesized only C2, factor B, and, in some experiments, C3. Synthesis of complement proteins in cells that line the alveolar wall may provide a local source of these proteins for inflammatory responses in the lung. Local synthesis of complement proteins could be regulated independently of the synthesis in the liver.

Increased plasma levels of leukotriene B4 and prostaglandin E2 in cats experimentally inoculated with feline infectious peritonitis virus

Specific-pathogen-free kittens experimentally infected with feline infectious peritonitis virus (FIPV) subsequently demonstrated increased plasma levels of the arachidonic acid metabolites, leukotriene (LT) B4 and prostaglandin (PG) E2. Significant increases (P less than 0.025) in LTB4 plasma levels occurred in all (5/5) FIPV-inoculated kittens on postchallenge-exposure days (PCD) 7 and 14 vs PCD 0. Significant increases (P less than 0.05) in PGE2 plasma levels occurred in 80% (4/5) of FIPV-infected kittens on PCD 7 and 14. Maximal mean plasma levels of LTB4 and PGE2 occurred on PCD 7 (502.5 +/- 45.6 pg/ml and 1108.0 +/- 247.9 pg/ml, respectively). A positive correlation was found between LTB4 plasma levels and body temperature (r = 0.609, P less than 0.01). Mean survival time in FIPV-inoculated kittens was 19.4 +/- 3.2 days. Gross lesions, including peritoneal or pleural effusions (or both) and connective tissue edema, indicated an increased vascular permeability in the FIPV-infected kittens. Histologically, lesions were characterized by vasculitis or perivasculitis, vasodilatation, perivascular edema, and fibrinonecrotizing and pyogranulomatous inflammation. Immunofluorescent studies of tissues from FIPV-infected kittens demonstrated foci of polymorphonuclear leukocytes and FIPV-positive macrophages oriented around dilated blood vessels. Seemingly, arachidonic acid metabolites, including LTB4 or PGE2 released from macrophages, neutrophils or other cells, may be involved in the pathogenesis of FIP vascular and inflammatory lesions and in some of the clinical disease manifestations.

Altered C1 inhibitor genes in type I hereditary angioedema

Hereditary angioedema is an inherited disease transmitted as an autosomal dominant trait and characterized by deficient activity of C1 inhibitor, a glycoprotein that limits intravascular activation of complement. The unavailability of markers for the C1 inhibitor locus has so far precluded access to the genetic bases of the disorder. Using a recombinant-DNA probe for the C1 inhibitor gene, we identified a cluster of four distinctive DNA restriction sites in a multigeneration family. The strict cosegregation of these markers with a low C1 inhibitor level supports the conclusion that a defective structural gene is responsible for the disease. The occurrence of distinct DNA markers in patients from unrelated kinships indicates that, like forms of the condition displaying dysfunctional C1 inhibitor variants, those characterized by protein deficiency are also genetically heterogeneous. Although multiple restriction-site changes in the C1 inhibitor gene are carried by certain patients, DNA markers for this locus are strikingly infrequent among normal persons. These findings indicate that the gene alterations responsible for hereditary angioedema arise more often from DNA rearrangements than from nucleotide substitutions. Such variations in the structure of the C1 inhibitor gene may provide the basis for early and direct identification of persons at risk.

Relationship between secretory IgA, IgA-containing (C3-fixing) circulating immune complexes, and complement components (C3, C4) in patients with obstructive jaundice

Serum secretory IgA, IgA-containing circulating immune complexes (IgA-CIC), complement components, and major immunoglobulins were measured in patients with biliary tract stones and/or tumors of the biliary tract or pancreas. The levels of secretory IgA and total IgA were increased in patients with and without obstructive jaundice. The levels of both C3 and C4 were significantly higher in patients with or without obstructive jaundice than in healthy controls. In patients with obstructive jaundice the increased levels of secretory IgA, total IgA, and IgA-CIC were correlated with the increase of C3 but not with that of C4.

Complement subcomponent C1q secreted by cultured human monocytes has subunit structure identical with that of serum C1q

An enzyme-linked immunosorbent assay (e.l.i.s.a.) that is capable of quantifying C1q concentrations as low as 2 ng/ml and a sensitive haemolytic assay were used to study the appearance of material that cross-reacts with human serum C1q as well as C1q haemolytic activity in human monocyte culture media. This material was detected in the medium after 10-14 days and continued to be secreted through to day 28 of culture, at which time the cultures were terminated. Material specifically immunoabsorbed with Sepharose-anti-C1q antibody from a culture medium of cells that was metabolically labelled with [3H] proline or [35S] methionine demonstrated a polypeptide pattern identical with that of serum C1q on SDS/polyacrylamide-gel electrophoresis. Under non-reducing conditions two protein bands were detected migrating with the same Rf values as the serum C1q A-B and C-C dimers. On reduction three bands were evident, which migrated identically with the A, B and C chains of serum C1q. The amount of radioactivity in these bands increased with time in culture, consistent with the e.l.i.s.a. and haemolytic C1q assays. These bands were reactive with monospecific anti-C1q antibody after transfer to nitrocellulose.

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.