Fibronectin-mediated uptake of gelatin-coated latex particles by peritoneal macrophages

Fibronectin--mediator between cells and connective tissue

Fibronectin, previously also termed LETS-protein, is a high-molecular-weight protein (mol. w. ca. 450,000) present in the form of thin fibrils in the pericellular space of fibroblasts and other adherent cells, as well as in distinct areas of the connective tissue. A soluble form, immunologically identical and chemically at least very similar to the cell-attached protein, is found in plasma in a concentration of about 300 micrograms/ml. It is also denominated cold-insoluble globulin. The protein has affinity both to cell surfaces and to various matrix substances such as fibrin and collagen and, therefore, is capable of mediating cell attachment to these substrates. In addition, it serves as an opsonin for the phagocytosis of gelatin-containing compounds and probably is essential for the removal of soluble fibrin from the circulating blood by the reticulo-endothelial system. Bacterial cell walls are also recognized by fibronectin. A conversion of soluble fibronectin to fibrils is achieved by heparin which also enhances the binding of soluble fibronectin to cells. Heparin or, as suggested, the related heparan sulfate present on the surface of various cells, appears to function as a cofactor in the formation of pericellular fibrils. The fibronectin fibrils precipitated with heparin, compared to soluble fibronectin, show a considerably improved affinity to native collagen, especially to type III. Hyaluronic acid has an antagonistic function which, at higher concentrations, prevents the fibronectin fibrils from interacting with collagen and cell surfaces. Masking of fibronectin fibrils was also achieved by sulfated proteoglycans of cartilage. Virus-transformed fibroblasts produce less fibronectin and are less capable of maintaining surface pericellular fibrils. A reasonable explanation is that they have an elevated secretion of hyaluronic acid. The transformed cells attach only weakly to a surface and exhibit a rounded shape in contrast to healthy ones. This phenotype can be corrected to a great extent with fibronectin. It is suggested that fibronectin also influences the formation of connective tissue by accumulating collagen precursors on the surface of fibroblasts and facilitating fibrillogenesis.

Fibronectin and cell shape in vivo: studies on the endometrium during pregnancy

The rat endometrium during pregnancy was used as a model system to study fibronectin in vivo. Fibronectin distribution on stromal fibroblasts, as determined by indirect immunofluorescence staining, was studied in relationship to cell shape during decidual transformation. Fibroblasts of the estrus endometrial stroma were elongated cells with a fibrillar pattern of fibronectin on their surfaces. During days 1-6 of pregnancy, as these elongated cells acquired a round morphology, fibronectin changed first to a patched distribution on the cells'a surfaces and then disappeared. The change in fibronectin was specific for the fibroblasts since over the same time period there was no decrease in fibronectin found associated with blood vessels or in the epithelial-stromal basement membrane. These results support the proposed relationship between cell surface fibronectin and cell shape that has been inferred from in vitro experiments. After implantation, fibronectin distribution was studied in relationship to the position of the conceptus. In the stroma proximal to the implanting conceptus, fibronectin was absent except around blood vessels, which may help explain how decidual tissue could act as a barrier to trophoblast invasion. Finally, fibronectin distribution was studied in the uterus after parturition. Debris in the uterine lumen was coated with fibronectin, which may be important in the rapid removal of this material by phagocytic cells. Also, fibronectin associated with the epithelial-stromal basement membrane was reorganized after reepithelialization had occurred.

Isolation of a fibronectin-binding protein from Staphylococcus aureus

Fibronectin ("cold-insoluble globulin") has been suggested to play a role in cell-to-cell and cell-to-substratum adhesions. The 70-kilodalton terminal part of human fibronectin has recently been shown to bind to Staphylococcus aureus. In the present study, a fibronectin-binding protein was purified from sonicated S. aureus strain E2371 by affinity chromatography on fibronectin-Sepharose. The fibronectin-binding protein was isolated from an extract of sonicated S. aureus containing at least 57 different proteins as determined by crossed immunoelectrophoresis in antibodies to sonicated S. aureus. The fibronectin-binding protein was released from fibronectin-Sepharose by carbamide (8 M). No impurities in the final preparation could be detected when tested in crossed immunoelectrophoresis. By polyacrylamide gel electrophoresis in both reduced and unreduced gels, the protein showed two bands with relative molecular masses of 197,000 and 60,000, respectively. A complex between the purified S. aureus protein and fibronectin could be demonstrated by crossed immunoelectrophoresis both in monospecific antibodies against fibronectin and in S. aureus polyspecific antibody.

Interactions of liposomes with the reticuloendothelial system. II: Nonspecific and receptor-mediated uptake of liposomes by mouse peritoneal macrophages

Liposomes are taken up as intact vesicles by mouse peritoneal macrophages in a process which is temperature sensitive and is affected by inhibitors of glycolytic metabolism and of microfilament activity. Macrophages take up negatively charge vesicles more readily than positively charged vesicles (2-fold) or neutral vesicles (4-fold). Macrophages take up similar amounts of multilamellar liposomes, reversed phase liposomes and small unilamellar liposomes in terms of lipids, however this corresponds to vastly different numbers of particles and amounts of trapped volume. Coating the liposomes with macromolecular ligands capable of interacting with macrophage surface receptors can markedly promote liposome uptake. Thus, formation of an IgG-antigen complex on the liposome surface results in a 10(2)-fold enhancement of liposome uptake, while coating the vesicles with fibronectin results in a 10-fold augmentation of uptake. Uptake via IgG-mediated and fibronectin-mediated processes seem to be independent since excess unlabelled, IgG-coated liposomes will inhibit the uptake of radioactively-labelled IgG-coated liposomes much more effectively than the uptake of radioactively-labelled fibronectin-coated liposomes. Cell-bound liposomes can readily be visualized on and inside of the macrophages using fluorescence microscopy techniques.

Effect of glycosaminoglycans on fibronectin-mediated cell attachment

In this study, the role of glycosaminoglycans in fibronectin-mediated cell attachment to collagen has been investigated. While it has been established that fibronectin possesses binding sites for several glycosaminoglycans, it was found that only dextran sulfate and macromolecular heparin could decrease the initial rate of cell attachment to collagen. Low molecular weight heparin was inactive. This study suggests that the glycosaminoglycan binding site of fibronectin plays a role in the mechanism of cell attachment.

Fibronectin in exudative pleural effusions

Fibronectin is a glycoprotein found in body fluids, loose connective tissue matrix and in basement membranes. Fibronectin in pleural effusion was found to be immunologically indistinguishable from the plasma form, as shown by double-diffusion analysis. Fibronectin isolated from pleural fluid by affinity chromatography on gelatin-Sepharose had a polypeptide pattern similar to that of plasma fibronectin in SDS-polyacrylamide gel electrophoresis. In 28 patients with infectious or non-specific pleural effusion fibronectin concentrations in pleural fluid were 335 +/- 104 micrograms/ml (mean +/- SD), in 15 patients with malignant disease the concentrations were 369 +/- 173 micrograms/ml and in 26 patients with tuberculosis 441 +/- 103 micrograms/ml. The highest concentrations, 605 +/- 252 micrograms/ml, of fibronectin in pleural fluid were detected in 14 patients with connective tissue diseases. The results suggest that increased fibronectin concentrations reflect the presence of a pleurisy due to connective tissue disease or tuberculosis rather than other infectious or malignant disease.

Opsonic fibronectin deficiency and sepsis. Cause or effect?

Opsonic fibronectin is known to modulate macrophage (RE cell) and neutrophil Phagocytic function. Its depletion has been documented following trauma, burn, and operation in patients with rapid restoration of normal levels unless bacteremia and/or wound sepsis intervenes. Sepsis is associated with a secondary phase of opsonic fibronectin deficiency. We have observed in burn patients that this secondary phase of opsonic fibronectin depletion following trauma and burn is seen two to three days prior to the onset of clinical sepsis, raising the question of whether this deficiency sensitized the host to the subsequent development of sepsis or whether its deplection was merely an unsuspected sensitive indication of preclinical sepsis. To address the possibility that opsonic fibronectin deficiency might lower resistance to sepsis, Sprague-Dawley rats (200 gm) were partially depleted (35%) of their opsonic fibronectin prior to intraperitoneal inoculation with Staphylococcus aureus. Mortality to S. aureus peritonitis was significantly (p < 0.05) increased in animals with fibronectin deficiency. Furthermore, in control animals, nonsurvival was also associated with significantly (p < 0.05) lower initial fibronectin levels than survival. However, peritonitis itself also resulted in an early (within one hour) depletion of opsonic fibronectin followed by a marked "hyperopsonemia" within 12 hours in both groups. Thus, opsonic fibronectin depletion decreases resistance to sepsis, and the development of sepsis itself will initiate opsonic fibronectin deficiency. Host defense against infection may depend on early restoration and maintenance of normal opsonic fibronectin levels following trauma, burn, and operation, as well as the ability of the host to mount an appropriate hyperopsonemic elevation of fibronectin levels in response to infection.

Heparin facilitates the extraction of tissue fibronectin

Extraction of fibronectin from two human tissues, lung parenchyma and placental villi, was facilitated by the incorporation of heparin into extraction media. The effect of heparin was additive to the effect of urea which is known to extract fibronectin. These experiments provide further evidence that fibronectin and glycosaminoglycans are associated in connective tissues and the use of heparin forms the basis for a simple method for extraction and quantitation of tissue fibronectin.

Human fibronectin binding to staphylococcal surface protein and its relative inefficiency in promoting phagocytosis by human polymorphonuclear leukocytes, monocytes, and alveolar macrophages

The interaction between human fibronectin and 17 strains of staphylococci was studied in an attempt to elucidate the staphylococcal cell wall component(s) involved in fibronectin binding and to determine the influence of fibronectin upon phagocytosis by three types of phagocytic cells. Purified, radiolabeled fibronectin bound to a similar degree to six laboratory strains and three fresh clinical isolates of Staphylococcus aureus; similar binding of fibronectin was found with S. aureus strains deficient in cell wall teichoic acid or clumping factor and coagulase, as well as with three strains of S. epidermidis. There was minimal binding of fibronectin to encapsulated S. aureus and to Escherichia coli. Fibronectin bound to intact cells and to a crude cell wall preparation of S. aureus H, but not to purified cell walls or peptidoglycan. Trypsinization of staphylococci prevented subsequent fibronectin binding, but binding did not correlate well with the protein A content in S. aureus cell walls. At physiological concentrations, fibronectin binding to staphylococci did not promote phagocytosis of bacteria by human polymorphonuclear leukocytes, monocytes, or alveolar macrophages. Also, depletion of fibronectin from normal human serum did not result in a measurable loss of opsonic activity for staphylococci. It is concluded that fibronectin binding to staphylococci involves a surface protein shared among strains of S. aureus and S. epidermidis, and that in comparison to C3b and IgG, fibronectin plays a relatively minor role as an opsonin for staphylococci.

Human alveolar macrophage fibronectin: synthesis, secretion, and ultrastructural localization during gelatin-coated latex particle binding

Human pulmonary alveolar macrophages synthesized and secreted several characteristic high molecular weight proteins for at least 7 d in vitro. Immunoprecipitates of medium and cell lysates from metabolically labeled cultures with specific anti-human plasma fibronectin IgG contained one major labeled polypeptide of molecular weight 440,000 (unreduced) or 220,000 (reduced). An identical polypeptide in conditioned medium from radiolabeled macrophages bound specifically to gelatin-Sepharose, demonstrating that alveolar macrophages synthesized and secreted a molecule immunologically and functionally similar to fibronectin. Fibronectin was the major newly synthesized and secreted polypeptide of freshly harvested alveolar macrophages. Pulse-chase experiments revealed that newly synthesized fibronectin was rapidly secreted into medium, approximately 50 percent appearing by 1 h and 80 percent by 8 h. Immunoperoxidase staining using antifibronectin F(ab')(2)-peroxidase conjugates revealed the majority of immunoreactive fibronectin to be intracellular, localized to endoplasmic reticulum and Golgi apparatus. No extracellular matrix fibronectin was visualized, and cell surface staining was rarely seen, usually appearing only at sites where cells were closely apposed and not at sites of macrophage-substrate attachment. Similar immunostaining of fibroblast cultures revealed cell surface-associated fibrillar fibronectin. Ultrastructural localization of fibronectin during binding and phagocytosis of gelatin-coated and plain latex particles revealed fibronectin only on gelatin-latex beads and at their cell binding sites. Neigher plain latex beads nor their cell membrane binding sites stained for fibronectin. These results demonstrate that fibronectin is a major product of human alveolar macrophages, is rapidly secreted, and is localized at cell membrane binding sites for gelatin-coated particles. In view of the known binding properties of fibronectin, it may serve as an endogenous opsonic factor promoting the binding of staphylococcus, denatured collagen, fibrin, or other macromolecules to macrophages in the lower respiratory tract.

A difference between plasma and cellular fibronectins located with monoclonal antibodies

We describe the properties of two monoclonal antibodies to hamster cellular fibronectin. One of them exhibits a marked specificity for cellular fibronectin, whereas the other recognized both cellular and plasma fibronectins. However, both antibodies recognize determinants in the same restricted region of cellular fibronectin, located near to but not at the C-terminal and of the intact molecule. Tryptic and chymotryptic digestions release this portion of the molecule in 40 kd fragments that contain a free sulfhydryl group. Recognition of fibronectin by these two monoclonal antibodies does not require the presence of carbohydrate residues on the fibronectin. These monoclonal antibodies allow location of a structural difference between the two forms of fibronectin and should permit further analysis of this difference.

Phagocytosis of gelatin-latex particles by a murine macrophage line is dependent on fibronectin and heparin

It has been suggested that fibronectin plays a role in clearing particles from the circulation by promoting binding to phagocytes of the reticuloendothelial system. By use of a well-defined system to investigate the possible opsonic role of fibronectin, we have studied the uptake of gelatin-coated latex particles by a murine macrophage cell line (P388D1). Fibronectin promotes binding of gelatin-coated beads to these cells in both suspension and monolayer cultures. In both cases there is a requirement for heparin as a cofactor. Other glycosaminoglycans (chondroitin sulfates A and C, dermatan sulfate, and keratan sulfate) were inactive, whereas heparan sulfate was somewhat active. Proof that beads were actually endocytosed was obtained by electron microscopy, which showed beads internalized in membrane-bounded vesicles, and by immunofluorescence analyses, using antibodies to fibronectin to stain external beads. Two rapid assays for the opsonic activity of fibronectin were developed based on differential centrifugation of cell-associated beads and on the immunofluorescence procedure. Binding and endocytosis were time- and temperature-dependent and varied with the amount of gelatin on the beads and with the concentrations of fibronectin and heparin added, and could be inhibited by F(ab')2 antifibronectin. These studies provide a sound basis for a detailed analysis of the interaction of fibronectin with the cell surface and of its involvement in endocytosis.

Phagocytosis-promoting activity of avian plasma and fibroblastic cell surface fibronectins

The purpose of this investigation was to determine whether fibronectin preparations from both chicken plasma and cell surface of fibroblasts can promote phagocytosis of gelatin-coated latex particles. Chicken plasma fibronectin was isolated (a) by ammonium sulfate fractionation, chromatography on Sepharose-4B followed by purification on a Sepharose-4B-heparin column; (b) by affinity chromatography on a Sepharose-4B-rat-antifibronectin column; (c) by affinity chromatography on Sepharose-4B-gelatin followed by molecular sieve separation on Sepharose-CL4B; (d) by a dual affinity chromatographic method using a Sepharose-4B-gelatin column and a Sepharose-4B-heparin column. Chicken cell surface fibronectin from fibroblast cultures was purified by ammonium sulfate precipitation followed by chromatography on Sepharose-CL4B. The purity of preparations was examined by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate; all samples showing high purity. The opsonic activities of the preparations were measured by the uptake of 125I-labeled gelatin coated latex particles in conjunction with rat liver slice, and peritoneal macrophage monolayer systems. Both the plasma fibronectin and cell surface fibronectin preparations showed substantial opsonic activities in the test systems. Fresh chicken plasma did not reveal any phagocytosis promoting activity due to the presence of some unidentified inhibitor(s). The results showed that an opsonically active protein can be isolated from chicken plasma or serum and this protein is identical to plasma fibronectin. Furthermore, it could be concluded that cell surface fibronectin from chicken fibroblasts also can serve as an opsonin for gelatin coated particles.

A role for anti-inflammatory agents and cyclic AMP in regulating fibronectin-mediated phagocytosis

The present study deals with the effects of anti-inflammatory drugs and agents known to elevate intracellular levels of cyclic AMP (cAMP) on plasma fibronectin-mediated (PFn) phagocytosis of radiolabeled, gelatin-coated latex particles (g-Ltx*) by inflammatory macrophages. Monolayers of casein-elicited peritoneal macrophages were preincubated with the specified agents for either 1 or 24 hrs at 37 degrees C prior to the measurements of phagocytosis in the presence of human plasma fibronectin (47 microgram/ml) and heparin (6.7 U/ml). Under these conditions, prostaglandin E1, colchicine, vincristine, and cytochalasin B were all effective in inhibiting g-Ltx* phagocytosis by macrophages in a dose-dependent fashion. More potent inhibition of phagocytosis was manifested by agents known to increase intracellular levels of cAMP in phagocytic cells. Dibutyryl cyclic AMP (dbcAMP), d,1-isoproterenol and aminophylline (10(-5) to 10(-3) M) were all effective in reducing the uptake of g-Ltx* by macrophages. The combination of dbcAMP and aminophylline acted additively. These studies demonstrate that anti-inflammatory drugs and cAMP-elevating agents exert potent inhibitory effects on fibronectin-mediated phagocytosis of gelatin-coated particles by macrophages. Thus, our system provides a suitable in vitro model for further investigations into the humoral regulation of phagocytosis of denatured collagen-coated particles and tissue debris by inflammatory phagocytic cells.