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The complement protein C4, encoded by two genes (C4A and C4B) on chromosome 6p, is the most polymorphic among the MHC III gene products. We investigated the molecular basis of C4 deficiency in a Finnish woman with systemic lupus erythematosus. C4-specific mRNA was present at low concentrations in C4-deficient (C4D) patient fibroblasts, but no pro-C4 protein was detected. This defect in C4 expression was specific in that synthesis of two other complement proteins was normal. Analysis of genomic DNA showed that the proposita had both deleted and nonexpressed C4 genes. Each of her nonexpressed genes, a C4A null gene inherited from the mother, a C4A null gene, and a C4B null gene inherited from the father, all contained an identical 2-bp insertion (TC) after nucleotide 5880 in exon 29, providing the first confirmatory proof of the C4B pseudogene. This mutation has been previously found only in C4A null genes. Although the exon 29/30 junction is spliced accurately, this frameshift mutation generates a premature stop at codon 3 in exon 30. These truncated C4A and C4B gene products were confirmed through RT-PCR and sequence analysis. Among the possible genetic mechanisms that produce identical mutations is both genes, the most likely is a mutation in C4A followed by a gene conversion to generate the mutated C4B allele.

Deficiency of the complement protein C2 (C2D), one of the most common genetic deficiencies of the complement system, is associated with rheumatological disorders and increased susceptibility to infection. Two types of C2D have been recognized, each in the context of specific major histocompatibility complex (MHC) haplotypes; type I, a deletion, frameshift and premature stop codon resulting in absence of detectable C2 protein synthesis, and type II, missense mutations resulting in a block in secretion of C2 proteins. Analysis of C2 expression in a child with C2 deficiency, a MHC haplotype different from those associated with type I or II C2D, and recurrent infections revealed additional molecular heterogeneity among C2 deficient patients. No detectable C2 protein was synthesized in the child's fibroblasts under conditions supporting C2 synthesis and secretion in normals and the child's C2 mRNA was reduced to 42% of normal. Nucleotide sequencing of RT-PCR fibroblast mRNA and genomic DNA revealed a type I C2 deficiency (28 base-pair deletion) on one allele and a previously unrecognized two base-pair deletion in exon 2 on the other. Expression of the closely linked factor B gene was markedly decreased (Bf mRNA 25% of normal), though Bf was up-regulated appropriately by interferon-gamma and the flanking sequence containing the Bf promoter was normal in this C2-deficient patient. Moreover, the concentration of Bf protein was normal in the patient's plasma.

Factor B (Bf), a constituent of the alternative pathway of complement activation, is encoded by a single gene that is located within the MHC. In murine kidney and intestine, two Bf transcripts (Bf short and Bf long), generated from distinct transcriptional initiation sites, are expressed in approximately equal amounts. In the liver,95% of Bf mRNA is the short transcript. To ascertain the biologic consequences of this tissue-specific mRNA polymorphism, we quantitated the effect of structural differences between the two transcripts on net Bf protein synthesis. Cell-free translation of Bf mRNA in vitro revealed that the rate of translation of Bf short is about twice that of Bf long. The 5' extension of Bf long includes four short open reading frames upstream of the authentic translational initiation codons. Mutation of all four upstream AUGs generates a Bf long transcript with a translational rate about equal to that of Bf short. This effect was primarily accounted for by mutation of the second AUG from the 5' end. Similar studies of Bf expression in vivo showed an approximately twofold difference in translational rate between Bf long and the transcript mutated at the upstream AUGs. Because systemic and local inflammation in kidney alters the ratio of Bf long and short, net production of complement protein Bf in extrahepatic tissues is regulated by both transcriptional and translational control mechanisms.

The expression of the complement protein C3 in extrahepatic tissues is highly regulated during the course of inflammation. Hence, systemic acute phase stimuli such as bacterial lipopolysaccharide (LPS) and autoimmune nephritis in aged 'lupus mice' (MRL-lpr/lpr and NZB x NZW F1) both lead to increased C3 mRNA expression in whole kidney. In situ hybridization was used to determine the intrarenal cell type(s) capable of constitutive and regulated C3 mRNA expression. Normal mice injected with Escherichia coli LPS show a marked increase in whole kidney C3 mRNA over control (saline-injected) animals. The renal C3 mRNA in LPS-stimulated mice was found in cortical tubular epithelium. By contrast, in aged (18 week) MRL-lpr/lpr mice, which develop lupus nephritis, the increased intrarenal C3 messenger RNA was localized to perivascular inflammatory cells surrounding medium-sized arteries. Similar perivascular infiltrates were seen in the lungs of the MRL-lpr/lpr mice, and focal inflammatory cell infiltrates were also found in the myocardium. Leucocytes in these infiltrates accounted for the increased C3 expression in these tissues. These findings suggest cell as well as tissue specificity of the response to inflammatory stimuli in the local extrahepatic production of the third component of complement.

To study the role of local production of complement proteins during the evolution of a naturally occurring immune complex disease, C3, C4, C2 and Factor B mRNA expression was assessed in several tissues of the inbred mouse strains NZB and (NZB x W) F1 hybrid. In the NZB/W F1 hybrid strain, coincident with the development of glomerulonephritis a marked increase in kidney C3 and C4 mRNA was observed; Factor B mRNA, which is expressed as a doublet in kidney and intestine, showed an increase in expression of the smaller transcript. This alteration of kidney C3, C4 and Factor B mRNA is identical to that noted in association with lupus nephritis in the MRL lpr/lpr strain and following in vivo administration of endotoxin to the BALB/c strain. The development of systemic lupus erythematosis (SLE) in the NZB/W F1 was not associated with a marked change in hepatic complement gene expression. These findings support the hypothesis that local production of complement may play a role in the pathogenesis of glomerulonephritis and other tissue injury in SLE.

Genetic deficiency of the second component of complement (C2) is the most common complement-deficiency state among Western Europeans and is frequently associated with autoimmune diseases. To examine the molecular basis of this deficiency, we established cultures of blood monocytes from four families with C2-deficient members. Using a hemolytic-plaque assay, [35S]methionine metabolic labeling of proteins in tissue culture and immunoprecipitation, RNA extraction and Northern blot analysis, and DNA restriction-enzyme digestion and Southern blot analysis, we found that C2 deficiency is not due to a major gene deletion or rearrangement but is the result of a specific and selective pretranslational regulatory defect in C2 gene expression. This leads to a lack of detectable C2 mRNA and a lack of synthesis of C2 protein. The approach used in this study should prove useful in examination of other plasma protein deficiencies, especially those in which the deficient gene is normally expressed in peripheral-blood monocytes or tissue macrophages and in which ethical considerations preclude the use of liver or other tissue for study.

The net production of the complement protein C2, C4, and factor B differ among mononuclear phagocytes from peripheral blood and different tissues in experimental animals and in humans. To examine the mechanisms that regulate these differences in humans, the proportions of C2-producing cells, the average single-cell production rate of C2, the posttranslational glycosylation and kinetics of secretion of C2 and factor B, and the amounts of C2 and factor B mRNA were examined in freshly isolated peripheral blood monocytes, monocytes maintained up to 2 wk in culture, and freshly isolated tissue macrophages from breast milk and bronchoalveolar lavage. In addition, the biosynthesis of two other proteins synthesized and secreted by mononuclear phagocytes, C3 and lysozyme, were examined. We report that despite comparable rates of C3 and lysozyme synthesis and similar processing and kinetics of secretion of C2 and factor B, the freshly isolated tissue macrophage differs from the monocyte-derived macrophage in the proportion of C2-producing cells, in the average single-cell production rate of complement, and in the amounts of specific C2 and factor B mRNA. These differences are tissue specific, because C2-specific mRNA content in bronchoalveolar macrophages is considerably greater than in breast milk macrophages, although the amounts of factor B mRNA are comparable. These data suggest that tissue-specific regulation of complement production in human mononuclear phagocytes occurs at a pretranslational level. These studies now provide a basis for investigation of the molecular effects of agents that modulate the biologic functions of monocytes and macrophages.

gamma-Interferon (IFN-gamma) is a well characterized lymphokine known to regulate many mononuclear phagocyte functions, including expression of class I and class II major histocompatibility complex genes. The second component of complement (C2) and factor B are major histocompatibility complex class III gene products synthesized in mononuclear phagocytes. Recombinant IFN-gamma increased the synthesis of C2 and factor B in primary cultures of human mononuclear phagocytes and in murine fibroblasts transfected with cosmid DNA bearing the human C2 and factor B genes. In both cell types the increases in C2 and factor B protein synthesis were detected at concentrations of IFN-gamma less than 1 unit/ml and the regulation of each was pretranslational. The IFN-gamma-induced increases in C2 and factor B mRNA did not require new protein synthesis. In primary cultures of human monocytes, the kinetics of induction of C2 and factor B synthesis differed, but in the transfected L-cells the kinetics were similar, suggesting differences in transduction of the IFN-gamma signal, transcriptional, and/or post-transcriptional events in the two cell types. The small size of the factor B 5' flanking region, which is bounded by the 3' terminus of the IFN-gamma-regulated C2 gene, provides a well defined region to probe the structural basis for IFN-gamma regulation of gene expression.

To assess the potential role of membrane interleukin 1 (mIL-1) in modulating expression of acute phase proteins, we studied the effect of fixed mouse peritoneal macrophages and isolated cell membranes on the synthesis of C3 and albumin in human hepatoma Hep 3B cells. An increase in C3 synthesis and decrease in albumin synthesis were detected after incubation of Hep 3B cells with fixed stimulated macrophages or with membrane preparations. Estimates of hepatocellular C3 and albumin mRNA indicated that the mIL-1 regulation was exerted at a pretranslational level. The changes in C3 and albumin expression induced by mIL-1 were inhibited by addition of anti-interleukin 1 (IL-1) antiserum to the macrophages, supporting the hypothesis that a form of IL-1 is present on the outer cell membrane of macrophages. Moreover, cell-surface iodination of macrophages allowed the detection of a 33-kDa IL-1 molecule, suggesting that an IL-1 species, similar to the intracellular precursor of IL-1 is present at the outer cell surface of macrophages. The expression of mIL-1 was temporally dissociated from IL-1 release, indicating that the cell-surface associated IL-1 activity is not the result of adsorbed soluble IL-1. These studies provide a basis for further investigation of the role of mIL-1 in modulating monocytes and macrophages function via cell-cell interactions.

The concentration in plasma of the female protein (FP) of the golden Syrian hamster is regulated by sex steroids and by mediators of the acute-phase response to tissue injury or inflammation. A complementary DNA (cDNA) clone corresponding to FP was isolated from a hamster liver cDNA library and used to determine the nucleotide sequence and derived amino acid sequence of native FP. The primary sequence of FP is 69 percent identical to human serum amyloid P component and 50 percent identical to human C-reactive protein. Evidence showed that sex-limited and acute-phase control of the FP gene is pretranslational. The FP protein is thus a useful model for investigating dual regulation of expression of a single gene.

Complementary DNA clones corresponding to the human serum amyloid P component (SAP) were isolated, and the complete nucleotide and derived amino acid sequence of preSAP was determined. PreSAP biosynthesis is directed by a 1.1-kilobase mRNA. Synthesis and postsynthetic processing of preSAP in Xenopus oocytes result in secretion of a protein with mobility similar to native purified SAP when analyzed by sodium dodecyl sulfate gel electrophoresis. The human SAP gene is on chromosome 1, probably closely linked to the gene for C-reactive protein which encodes the related acute phase reactant found in human plasma.

Many of the serum complement components are synthesized primarily in the liver but extrahepatic synthesis in mononuclear phagocytes has been noted. This local production of complement, particularly at sites of inflammation, may be of importance in host defenses and immunopathological reactions. Recent studies of complement biosynthesis in vitro have revealed some of the genetic and microenvironmental factors that affect the rates of complement synthesis, posttranslational modification, and secretion of these proteins. The third, fourth, and fifth complement components are synthesized as single chain, procomplement proteins that require proteolytic cleavage to generate the native C4 protein is mediated by a plasmin-like enzyme. These studies, plus the most recent developments in analysis of the complement genes at a molecular level, will provide more complete understanding the mechanisms of regulation of complement production.

Factor B (Bf) gene expression is regulated independently in hepatic and extrahepatic tissues. In studying murine factor B expression, we found two mature mRNA species for Bf in kidney and intestine (2.7 and 2.4 kb) but in other tissues, including liver, lung, spleen, heart, brain, and skeletal muscle, only a single Bf transcript (2.4 kb) was observed. These two Bf transcripts in kidney or intestine are identical in structure except for a 300 bp 5' extension found in the 2.7-kb mRNA. The long Bf transcript originates from a tissue-specific alternative site of transcriptional initiation upstream of the initiation site for the 2.4-kb mRNA, as determined by primer extension and nuclease protection assays. The upstream initiation site lacks a typical promoter motif and is only 80 bp downstream from the terminus of the murine C2 gene. After stimulation in vivo with endotoxin, only the 2.4-kb mRNA increases in kidney indicating independent regulation of the two Bf transcripts. The differences in 5' untranslated region generated in these two Bf mRNA species may affect local concentrations of factor B protein in kidney and intestine by mechanisms depending on differential translation rates or stability of mRNA.

The structure of a human serum amyloid A (SAA) genomic clone (SAAg9) has been analyzed and the nucleotide sequence of the coding regions is compared with that of the cDNA for apoSAA1. The leader and coding sequences of exons 2 and 3 are identical to SAA1. However, there are 10 nucleotide and 7 derived amino acid substitutions in exon 4. These changes are identical to the amino acid sequence of the amyloid protein associated with familial Mediterranean fever. In particular, the amino acid substitution (Thr to Phe) at residue 69 of SAA1 may have an important role in this type of hereditary amyloidosis. The genomic clone SAAg9 has been transfected into mouse L cells, and constitutive expression of human specific mRNA and protein were observed in stable transfected clones. The expression of both SAA mRNA and protein were increased by incubation of the transfected cells with purified human interleukin-1 (IL-1), both human and mouse recombinant IL-1, and recombinant human tumor necrosis factor alpha. The induction of SAA is pretranslational and is likely to be mediated by protein factor(s) since incubation with cycloheximide diminished IL-1-dependent increase in SAA mRNA.

Human C-reactive protein (CRP) is one of several plasma proteins that increase after tissue injury or inflammation. The magnitude of the increase in CRP (100 to 1000-fold over the resting state) makes this an excellent model for studies of eukaryotic gene control. Accordingly, the biosynthesis and postsynthetic processing of human CRP was examined under cellfree conditions and in Xenopus oocytes injected with liver mRNA. The primary translation product is larger than native serum CRP by an 18-amino acid amino terminal extension. The sequence of this signal peptide was derived from nucleotide sequencing of a 1.8-kilobase (Kb) CRP-specific cDNA clone. Northern blot analysis revealed a CRP mRNA of approximately 2.2 Kb, a size more than three times that required (615 bases) to code for the primary translation product. These data form the basis for study of molecular control of the acute phase response.

Several well-differentiated human hepatoma cell lines (HepG2, Hep3B) have been used to identify factors which regulate hepatic gene expression during the host response to inflammation/tissue injury (acute phase response). Studies in these cell lines, as well as in primary cultures of rat, rabbit, and mouse hepatocytes, have demonstrated that interleukin-1 beta (IL-1 beta), tumor necrosis factor (TNF-alpha), and interferon-beta 2 (IFN-beta 2) each mediate changes in expression of several hepatic acute phase genes. In this study we identify a subclone of the HepG2 cell line in which there is a selective defect in IL-1 beta-mediated acute phase gene expression. Recombinant human IL-1 beta mediates an increase in synthesis of the positive acute phase complement protein factor B and a decrease in synthesis of negative acute phase protein albumin in the parent uncloned HepG2 cell line (HG2Y), but not in the subclone HG2N. Recombinant human IFN-beta 2 and TNF-alpha, however, regulate acute phase protein synthesis in the subclone HG2N; i.e. IFN-beta 2 and TNF-alpha increase synthesis of factor B and decrease synthesis of albumin in both HG2Y and HG2N cells. Equilibrium binding analysis with 125I-rIL-1 beta at 4 degrees C showed that both HG2N and HG2Y cells bind IL-1 beta specifically and saturably. HG2N and HG2Y possess 3.8 and 4.0 x 10(3) plasma membrane receptors/cell with affinities of 0.96 and 1.07 x 10(-9) M, respectively. Thus, the defect in this subclone of the HepG2 cell line is likely to involve the signal transduction pathway for the biological activity of IL-1 beta and will be useful in elucidation of this signal transduction pathway.

Human factor I is a two-chain plasma glycoprotein composed of disulfide-linked 50,000- and 38,000-dalton subunits. Analysis of its biosynthesis and postsynthetic processing demonstrated that factor I is synthesized as a single chain precursor (pro-I) that undergoes glycosylation and limited proteolysis to generate the native protein. One of three human hepatoma cell lines, HepG2 , secreted factor I predominantly (70-90%) in a single chain pro-I form. The other cell lines secrete factor I predominantly in its two chain native form. The defect in conversion of pro-I to I in HepG2 was protein specific since other multichain proteins, derived from single chain precursors, the third, fourth, and fifth components of complement were processed normally. Further analysis of the inefficient pro-I to I conversion by HepG2 revealed that Xenopus oocytes injected with HepG2 mRNA secreted factor I in a predominantly two-chain form. In addition, the apparent sizes of native factor I, transferrin, and alpha-1-antitrypsin secreted by the three hepatoma lines differed due to differences in postsynthetic processing.

The second component of complement (C2), is a class III major histocompatibility complex gene product and a glycoprotein in the classical complement activating system. Synthesis in the human hepatoma-derived cell line HepG2 results in three intracellular forms: an 84-kDa form secreted in 1-2 h; 79-kDa and 70-kDa forms that remain cell-associated for intervals up to 12 h. All three forms are C2 polypeptides as demonstrated by inhibition of immunoprecipitation with unlabeled C2 and the presence of common major peptide fragments following chymotryptic digestion. The cell-associated forms of C2 are not products of proteolysis as demonstrated by experiments with multiple proteinase inhibitors and by observations of the kinetics of synthesis. Inhibition of core glycosylation by tunicamycin and deglycosylation by acid hydrolysis indicate that the three intracellular C2 polypeptides are glycosylated to a similar extent. Although the 84-kDa form of C2 is susceptible to C1s cleavage, the two cell-associated forms are not. Cell-free biosynthesis by mRNA from HepG2 or human liver results in three primary translation products corresponding to the three unglycosylated forms of C2. These results indicate that HepG2 cells synthesize C2 protein in both secreted and cell-associated forms and that each form is derived from a separate primary translation product.

Inflammation or acute tissue injury results in a programmed change in the concentration of several plasma proteins. Among these proteins, two--C-reactive protein (CRP) and serum amyloid A protein (SAA)--increase up to 1000-fold after an acute-phase stimulus in humans and rabbits. To determine the mechanism for regulation of acute-phase gene expression, we examined changes in the rates of transcription and specific hepatic mRNA content for rabbit CRP, SAA, and some complement protein mRNA during an acute-phase response. Induction of a sterile inflammatory reaction with intramuscular injection of turpentine resulted in an increase in the hepatocellular content of CRP, SAA, C3, and factor B mRNA and the transcription of CRP, SAA, and C3 genes. These data suggest that the increase in CRP, SAA, and C3 serum concentrations observed during an acute-phase reaction is due to an increase in biosynthesis and is, at least in part, under transcriptional control.

To study the molecular mechanisms accounting for strain- and tissue-specific variations in the production of complement proteins, complementary DNA probes were used to assess qualitative and quantitative differences in specific mRNA content of complement proteins C2, factor B, and C3 in extracts of tissues (liver, lung, spleen, kidney, and peritoneal macrophages) isolated from various mouse strains. Northern blot analysis of total hepatic RNA revealed differences in C2, factor B, and C3 mRNA levels in strains that share B10 background but differ in the H-2 region (e.g., H-2k, H-2u, H-2d, H-2f). In each instance, hepatic mRNA specific for the individual gene product corresponded in amount to the serum levels. By contrast, specific mRNA content of C2 and factor B in macrophages differed significantly from those observed in liver for each strain. Modulation of C2, factor B, and C3 expression was studied after in vivo administration of recombinant IL 1 or endotoxin to H-2k (B10.AKM) or H-2u (B10.PL) strain mice. As assessed by Northern blot analysis, neither endotoxin nor IL 1 affected liver C2-specific mRNA but increased specific C2 mRNA levels in kidney and lung. For both strains, IL 1 increased specific factor B mRNA in all tissues examined except for the H-2u strain liver factor B mRNA content, which was not affected by IL 1, whereas that of H-2k mice was increased. The lack of factor B modulation by IL 1 in the H-2u lines was specific to that gene and not a reflection of a generalized IL 1 unresponsiveness. Differences in tissue and strain specific constitutive and IL 1-regulated expression of the C3 gene were also observed in the H-2u and H-2k strains.