Synthesis of the first component of complement by primary cultures of human tumors of the colon and urogenital tract and comparable normal tissue

Decidual endothelial cells express surface-bound C1q as a molecular bridge between endovascular trophoblast and decidual endothelium

This study was prompted by the observation that decidual endothelial cells (DECs), unlike endothelial cells (ECs) of blood vessels in normal skin, kidney glomeruli and brain, express surface-bound C1q in physiologic pregnancy. This finding was unexpected, because deposits of C1q are usually observed in pathologic conditions and are associated with complement activation. In the case of DECs, we failed to detect immunoglobulins and C4 co-localized with C1q on the cell surface. Surprisingly, DECs expressed mRNA for the three chains of C1q and secreted detectable level of this component in serum-free medium. The ability to synthesize C1q is acquired by DECs during pregnancy and is not shared by ECs obtained from endometrium and from other sources. Cell-associated C1q has a molecular weight similar to that of secreted C1q and is released from DECs following treatment with heparinase or incubation at low pH. This suggests that C1q binds to DECs and it is not constitutively expressed on the cell surface. C1q is localized at contact sites between endovascular trophoblast and DECs and acts as an intercellular molecular bridge because adhesion of endovascular trophoblast to DECs was inhibited by antibodies to C1q and to a receptor recognizing its globular portion expressed on trophoblast.

Human keratinocytes and A-431 cells synthesize and secrete factor B, the major zymogen protease of the alternative complement pathway

Biosynthetic radiolabeling studies demonstrate that human keratinocytes and A-431 cells, a human epidermoid carcinoma cell line, synthesize and secrete factor B as a monomeric 105-kD protein. Epithelial cell-derived factor B comigrates in SDS-PAGE with that produced by HepG2 cells, a human hepatoma cell line traditionally employed in studies of complement component biosynthesis. Comparative pulse-chase studies in A-431 and HepG2 cells show that this alternative pathway complement component is produced as co-migrating 100-kD intracellular proteins that are processed in both cell types to 105-kD extracellular factor B. Quantitatively, immunoprecipitable factor B accounts for 0.05% of radiolabeled proteins in A-431 cell culture media. Treatment of biosynthetically radiolabeled A-431 cell culture media with cobra venom factor and factor D for 60 min at 37 degrees C produces the specific factor B cleavage products Ba and Bb. These fragments are not identifiable in control culture media subjected to similar treatment in the absence of alternative pathway activators. Northern blot analysis of total cellular RNA from human keratinocytes, A-431 cells, and HepG2 cells reveals qualitative identity of a 2.8-kb factor B mRNA species in these three cell types. The relative level of factor B mRNA expression in these cells parallels their level of factor B protein synthesis (i.e., HepG2 cells greater than A-431 cells greater than human keratinocytes). Epithelial cell-derived factor B may play an important role in local inflammatory reactions and also directly interact with epithelial cell derived C3--a key classical and alternative pathway complement component recently shown to be produced by human keratinocytes.

Biosynthesis of normal and low-molecular-mass complement component C1q by cultured human monocytes and macrophages

High levels of low-molecular-mass complement component C1q (LMM-C1q), a haemolytically inactive form of C1q, are found in serum of individuals with inherited complete (functional) C1q deficiency and in serum of patients with systemic lupus erythematosus, whereas lower levels are present in normal serum [Hoekzema, Hannema, Swaak, Paardekooper & Hack (1985) J. Immunol. 135, 265-271]. To investigate whether LMM-C1q is a (by-)product of C1q synthesis or the result of degradation of C1q, cultures of blood monocytes and of alveolar macrophages, which secrete functional C1q, were studied. A considerable portion of C1q-like protein secreted by these cells was found to be LMM-C1q. In contrast with the C1q fragments that resulted from degradation of normal C1q during phagocytosis, culture-derived LMM-C1q appeared to be identical with LMM-C1q found in serum, as judged by sedimentation behaviour, subunit structure and recognition by poly- and mono-clonal antibodies raised against C1q. The presence of LMM-C1q in cytoplasmic organelles compatible with the Golgi apparatus and the inability to generate LMM-C1q by impeding hydroxylation and triple-helix formation of C1q further argues against degradation as its source. Monocyte cultures of homozygous probands from two families with complete functional C1q deficiency reflected the abnormalities in serum, i.e. absence of functional C1q, but increased levels of LMM-C1q. By contrast, secretion of C1q and LMM-C1q by cells from healthy individuals was clearly co-ordinate, indicating that LMM-C1q in serum may provide a unique marker of C1q synthesis in vivo.

Isolation and characterization of macrophage-derived C1q and its similarities to serum C1q

Recently, we have shown that the collagen-like, Fc-recognizing subcomponent C1q of the first complement component is synthesized by human, guinea pig and mouse peritoneal macrophages. To test whether macrophages may contribute to the serum pool of C1q, C1q was purified from guinea pig serum and from guinea pig peritoneal macrophage supernatants and compared for similarities. Both molecules had a similar sedimentation rate (macrophage C1q: 11.3 S, serum C1q: 11.2 S) and showed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions three identical bands with molecular weights of Mr, 29 000, Mr, 27 000 and Mr 23 000 for the A, B and C chains, respectively. Both C1q molecules migrated by immunoelectrophoresis in the gamma region and, in Ouchterlony analysis, showed complete antigenic identity with rabbit anti-serum C1q. These experiments demonstrate the antigenic and protein chemical similarities between serum C1q and C1q secreted by macrophages supporting the idea that macrophages have to be considered as one potential source of serum C1q. Furthermore, macrophage-derived C1q may be of importance in the local microenvironment at an inflammatory site involving macrophages.

Increase in the haemolytic complement activity of dogs affected with cyclic haematopoiesis

Cyclic haematopoiesis (CH) is an inherited disorder which occurs in both humans and Grey Collie dogs. The disease is characterized by fluctuations in the numbers of peripheral blood leucocytes, reticulocytes and platelets at regular intervals and is associated with a variety of clinical signs. The most prominent cycle observed is that of neutrophils. The 12-day neutropenic cycle includes a period of relatively normal neutrophil counts, a period of neutropenia and generally a period in which neutrophil counts greatly exceed the normal range. In this study the daily serum haemolytic complement activity (classical pathway CH50) of CH and normal Collie dogs was assayed. The serum CH50 of normal Collie dogs was relatively stable throughout the test period. In contrast, the serum CH50 of CH dogs fluctuated extensively and the mean serum CH50 of CH dogs during the neutropenic cycle greatly exceeded that of normal Collie dogs over the same test period. A close temporal relationship between the stage of the CH neutropenic cycle and the serum CH50 was observed. The mean serum CH50 during neutropenia was not significantly different from that observed when neutrophil counts were within normal range, both values being significantly higher than that of normal dogs. However, the mean serum CH50 during the period of neutrophil rebound was significantly (P less than 0.01) higher than that during the period of neutropenia or normal neutrophil counts. These data suggest that alterations in the production of complement components or regulatory proteins occur at regular intervals in CH dogs.

The functions of endogenous C1q, a subcomponent of the first component of complement, as a receptor on the membrane of macrophages

C1q, the Fc-recognizing subcomponent of the first component of complement is synthesized by peritoneal macrophages. During the secretion phase C1q serves as an Fc-binding protein in the membrane of macrophages. The Fc-mediated rosette formation was inhibited in a dose-dependent manner when macrophages were pretreated with anti-C1q F(ab')2, whereas C3b rosette formation was not affected. Furthermore, preincubation of peritoneal macrophages with anti-C1q F(ab')2 abolished, dose- and time-dependently, the polyanion-mediated stimulation of secretion of lysosomal enzymes. Polyanion-induced enzyme release was prevented after incubation of polyanions with highly purified C1q. The inhibition of Fc receptor activity by polyanions (i.e. dextran sulfate, liquoid, polyvinyl sulfate) is completely reversed upon treatment of these macrophages with protamine. These findings are compatible with the hypothesis that C1q produced by macrophages serves in the macrophage membrane as an endogenous receptor for Fc and polyanionic molecules. Thus, C1q mediates cell-bound biological receptor functions before it is released from these cells and is incorporated into the macromolecular C1 complex.

Immunofluorescence studies on the subcomponents of the first component of complement (C1): detection of C1q and C1s in different cells of biopsy material and on human as well as on guinea pig peritoneal macrophages

The first component of complement (C1) is a macromolecule consisting of three distinct subcomponents, C1q, C1r, and C1s. In regard to its production site and its role in phagocytic processes it was of interest to find out whether these different subcomponents could be detected in human biopsy material only as a complex in individual cells or whether C1 subcomponents could be found on different cells. To study this question, monospecific fluorescein-labelled anti-human-C1q IgG and monospecific rhodamine-labelled anti-human C1q IgG were used. Biopsy material from human rectum was stained with fluoresceinated antisera, either by use of one antiserum or by double staining. Using this technique, these observations were made: C1q as well as C1s were detectable in individual cells in the subepithelial area of the gut. Furthermore, C1q and C1s could be found together in the same cell or separately in different cells. These findings were supported by experiments with cultured peritoneal macrophages either from human or from guinea pig. The examination of the cultured cells with the two antisera revealed that individual cells were stained either by anti-C1q or by anti-C1s antibodies. The specificity of the detection of the individual subcomponents was also proven by the peroxidase technique and by using fluoresceinated anti-human C1q F(ab')2. The membrane immunofluorescent staining revealed the presence of C1q on the membrane of the macrophage.

The first component of complement. A quantitative comparison of its biosynthesis in culture by human epithelial and mesenchymal cells

Epithelial and mesenchymal cells synthesized and secreted all three subcomponents of the first component of complement (C1): C1q, C1r, and C1s. Quantitatively, however, columnar and transitional epithelial cells secreted 400--3,700 times more hemolytically active C1 than monocytes or fibroblasts. Only columnar epithelial cells synthesized C1 subcomponents with subunit structures similar to their serum counterparts. Transitional epithelial cells, fibroblasts, and monocytes produced C1q and C1s with subunits of apparent molecular weights larger than reported values. C1r from all cell lines was physiochemically similar to serum C1r.

Biosynthesis of the first component of complement by human fibroblasts

1. Haemolytic activity corresponding to that of the first component of complement (C1) was synthesized and secreted by all nine human fibroblast cell lines examined. No activity was found in the culture media of a variety of other human cell lines. 2. The component-C1 haemolytic activity secreted by the fibroblast lines behaved in an identical manner, in most respects, with that of the component-C1 haemolytic activity of human serum. The component-C1 haemolytic activity secreted by fibroblasts, however, was less susceptible to inhibition by rabbit fragment F(ab')(2) anti-(human subcomponent C1q) than was the component-C1 haemolytic activity of human serum. 3. Biosynthesis of fibroblast component-C1 haemolytic activity was inhibited by the presence of cycloheximide and regained on its removal. 4. Incorporation of radioactivity into proteins secreted by the fibroblasts and release of component-C1 haemolytic activity by the fibroblasts both increased in a linear manner until several days after the cultures had reached a state of confluent growth. 5. Radioactivity was incorporated into subcomponents C1q, C1r and C1s, as judged by the formation of specific immunoprecipitates and by absorption with immune aggregates. 6. The immunoprecipitates formed by using antisera against subcomponents C1r and C1s were run on polyacrylamide gels in sodium dodecyl sulphate, and this provided convincing physiochemical evidence for the biosynthesis of these subcomponents de novo. 7. The results obtained with immunoprecipitates formed by using anti-(subcomponent C1q) suggest that subcomponent C1q may be synthesized and secreted by fibroblast cell lines in vitro, in a form with a higher molecular weight than that of subcomponent C1q which is isolated by conventional techniques of protein fractionation from fresh serum.

Circulating immune complexes after repeated halothane anaesthesia

A patient developed hepatitis after receiving three halothane anaesthetics in 22 days. Twenty-four hours after the onset of jaundice she developed an acute serum sickness syndrome with polyarthralgia, proteinuria, and transient impairment of renal function. Serum concentrations of complement components C1q, C4, and C3 were substantially reduced, and immune complexes capable of activating the complement system via the classical pathway were present in the serum and synovial fluid. A metabolite of halothane was associated with these complexes. Fourteen months after exposure to halothane her lymphocytes were stimulated in vitro by this metabolite. The conditions under which stimulation occurred were unusual--namely, a 7S fraction of the serum, presumably IgG, was necessary. Our results provide strong evidence that halothane may be immunogenic and that its immunogenicity is dependent on the non-covalent binding of one of its metabolites to plasma proteins.