Fibronectin as an opsonic regulator of monocyte phagocytosis

Plasma fibronectin concentrations in patients with HIV infection and visceral leishmaniasis

Concentrations of circulating fibronectin were studied in plasma of 22 patients with acquired immunodeficiency syndrome and 24 healthy blood donors. There were significant differences between the plasma fibronectin values of the patient population with AIDS (mean 2 SD = 206 +/- 86 mg/l) and of the control group (mean +/- 2 SD = 297 +/- 92 mg/l). AIDS patients with visceral leishmaniasis showed a significant diminution of plasma fibronectin concentration (mean +/- 2 SD = 132 +/- 31 mg/l). A significant increase in plasma fibronectin concentrations was noted after 14 days of treatment with antimony (mean 2 SD = 239 +/- 67 mg/l). These results suggest that the measurement of fibronectin concentrations in patients with AIDS especially with concurrent infection, contributes to diagnosis. A significant decrease in plasma fibronectin concentrations was observed in patients with AIDS and visceral leishmaniasis.

Localization and characterization of fibronectin-binding to group A streptococci. An electron microscopic study using protein-gold-complexes

The location and nature of the binding sites for fibronectin (Fn) and its N-terminal 29 K fragment (FnF) on group A streptococci were studied by electron microscopy using these proteins labelled with colloidal gold. The investigated strains exhibited a different labelling intensity as well as a different labelling pattern varying from a strong regular distribution to a weak focal binding. Binding of Fn and FnF was inhibited by itself as well as by lipoteichoic acid (LTA), anti-LTA and concanavalin A. Simultaneous labelling of the bacteria with marker complexes of FnF, human serum albumin and fibrinogen revealed separate receptor sites for each protein. Our results confirmed LTA to be mainly responsible for the binding of Fn on group A streptococci.

Plasma levels of fibronectin in polymyalgia rheumatica giant cell arteritis

In order to verify whether measurement of plasma fibronectin (Fn) could represent a useful tool in acute-phase-response assessment, Fn was measured in 16 previously untreated patients (group A) affected by polymyalgia rheumatica giant cell arteritis (PMR-GCA), both before, during, and after 45 days of steroid therapy, and its course was compared with the behavior of some acute-phase reactants such as erythrocyte sedimentation rate (ESR), fibrinogen (Fng), and prealbumin (Preal). No difference was detected between the baseline Fn levels found in patients and those registered in a control group composed of 15 sex- and age-matched healthy subjects; no correlation was found with the other acute-phase parameters considered, and no significant variation of plasma Fn levels was registered as a result of the steroid therapy administered. On the contrary, all the other parameters revealed a good degree of correlation and tended progressively and homogeneously towards normalization as a result of the therapy administered. Plasma Fn was also measured in another group of 16 PMR-GCA patients (group B), all of whom had pathological retinal fluoroangiographic findings, and its levels were compared with those of the von Willebrand factor antigen (vWfAg), a biochemical index of vascular damage. While the levels of Fn continued to be the same as those detected in the control group, the values of vWfAg registered in group B proved to be significantly different from those found in another homogeneous control group of 25 healthy subjects. Finally, no correlation could be detected between Fn and vWfAg, and neither of them showed any significant correlation with the ESR.(ABSTRACT TRUNCATED AT 250 WORDS)

Composition of immune complexes and their relation to plasma fibronectin in chronic myeloproliferative disorders

High concentrations of circulating immune complexes were detected by polyethylene glycol precipitation in 11 of 20 patients with myelofibrosis secondary to chronic myeloproliferative disease. Circulating immune complexes showed a positive correlation with plasma IgG concentrations both in patients and controls. Covariance analysis of the two groups showed significantly increased polyethylene glycol precipitable IgG in patients when adjusted for plasma IgG concentrations, indicating that the patients had significantly increased concentrations of complexed IgG. The immune complexes contained IgG, C3, and fibronectin and were inversely correlated with plasma fibronectin concentrations, suggesting that this major non-specific opsonin is important for the normal clearance of immune complexes. Therapeutic plasmapheresis efficiently removed circulating complexes and produced an increase in plasma fibronectin. This suggests that plasmapheresis may be useful for controlling immune complex mediated complications of these disorders.

Fibronectin and immune complexes in rheumatic diseases

The relation between fibronectin and immune complexes in rheumatic disease was examined in a series of linked studies. Fibronectin was present in immune complexes formed in vitro in the absence of C1q. Gel filtration chromatography showed complexed fibronectin was present in the serum of a patient with rheumatoid vasculitis, but not in normal serum; the complexed fibronectin coeluted with IgA and C3. Two dimensional immunoelectrophoresis showed a single fibronectin component was present in normal serum, but a number of components were present in serum from a rheumatoid patient. Polyacrylamide gel electrophoresis followed by immunoblotting for fibronectin showed that polyethylene glycol precipitates of synovial fluid contained immunoreactive components of a variety of sizes, indicating the presence of fragments of the molecule. An analysis of fibronectin in polyethylene glycol precipitates of paired serum and synovial fluid samples from 17 patients with rheumatoid arthritis and 16 with osteoarthritis showed more fibronectin was present in rheumatoid samples, especially in synovial fluid. More fibronectin was also present in synovial fluid than in serum polyethylene glycol precipitates; there was no direct relationship with C1q levels. All these results suggest that fibronectin is an integral component of immune complexes. This has potential pathogenic significance because it shows that a product of connective tissue cells may influence the functions of the immune system.

Evidence for the recycling nature of the fibronectin receptor of macrophages

Plasma fibronectin (pFN) has been shown to mediate phagocytosis of several types of artificial particles and tissue debris by macrophages. In the present investigation some of the dynamic aspects of this receptor-mediated cellular process have been studied. Plasma fibronectin did not bind specifically to fibronectin (FN)-receptors of rat peritoneal macrophages at either 4 degrees C or 37 degrees C. On the other hand, pFN aggregated on the surface of gelatin-coated latex beads (gLtx) and 125I-labeled pFN covalently coupled to latex beads (pFN-Ltx) bound strongly to macrophages at both temperatures. Both of these particles were also internalized at 37 degrees C. Treatment of macrophages by chymotrypsin, thermolysin, or trypsin in a protein-free tissue culture medium did not affect either of the above reactions; however, pronase treatment strongly reduced both the binding and internalization of the pFN-coated particles. The pronase-treated macrophage monolayers in time regained their ability to bind and internalize pFN-gLtx when incubated in fresh tissue culture medium. Such recovery, however, did not take place when the medium contained cycloheximide. On the other hand, phagocytosis of pFN-gLtx was not affected directly by cycloheximide with untreated macrophages; this suggests that the FN-receptor recycles during sustained phagocytosis. This assumption was substantiated by the observations that some of the established lysosomotropic amines--i.e., chloroquine, dansylcadaverine, and dimethyldansylcadaverine--caused total inhibition of internalization without affecting the binding of particles to macrophages. Furthermore, chloroquine protected the FN-receptors against destruction by pronase. Together these results suggest that macrophage receptors for FN are protein, present both on the cell surface and intracellularly, and recycle between the plasma membrane and intracellular sites during phagocytosis.

Fibronectin: applications to clinical medicine

In summary, the role of fibronectin in clinical medicine is not yet certain. Correlation of sepsis and organ failure with decreased fibronectin levels is still to some degree questionable; controlled clinical trials are urgently needed. The risk of hepatitis, AIDS, and other transfusion-transmitted diseases must be balanced by data substantiating the clinical efficacy of fibronectin therapy. To date, no results from controlled trials using purified fibronectin have been reported. Final judgement must be reserved pending results of appropriate human studies. It is likely, however, that even if fibronectin is proven to be clinically useful, the patient population which will achieve some benefit from its use will be restricted to septic and/or critically ill patients. As noted by Mosher and Grossman however, physicians treating such patients would likely welcome any new and effective therapeutic intervention.

Fibronectin in polyethylene glycol precipitates: evidence for a role in immune complexes

Fibronectin is involved in the opsonic clearance of particulate material. It is present in plasma and synovial fluid and thus might be expected to have a role in the clearance of immune complexes. We have investigated this in a study of polyethylene glycol (PEG) precipitable material from the serum of patients with rheumatoid arthritis, systemic lupus erythematosus, and other connective tissue disorders. Fibronectin is a significant component of PEG precipitates but the amount present is influenced by the method of preparation: more precipitates at 4 degrees C than at 20 degrees C. Fibronectin precipitation by PEG was considered to be related to immune complexes because: there was no direct relationship between serum fibronectin levels and the amount present in PEG precipitates; radiolabelled purified isolated fibronectin did not precipitate in 4% PEG; there was a direct relationship between the amount of fibronectin in PEG precipitates and the amounts of immunoglobulin G, A, and M. These results indicate that fibronectin is involved in immune complexes in rheumatic diseases, though they do not show it has an important biological role in these circumstances.

Changes in plasma fibronectin during allogeneic bone marrow transplantation

Plasma fibronectin concentrations were measured serially in nine patients undergoing allogeneic bone marrow transplantation. Concentrations were reduced by conditioning treatment and episodes of bacterial infection. Acute graft versus host disease may exacerbate or prolong this process. Reduced plasma fibronectin concentrations impair the function of the mononuclear phagocyte system and the maintenance of capillary endothelial integrity and may thus contribute to the morbidity and mortality in patients undergoing bone marrow transplantation.

Fibronectin on the surface of human lymphocytes

Fibronectin was detected by immunofluorescence technique on the surface of one part of separated normal peripheral blood lymphocytes by using FITC-conjugated anti-human fibronectin antibodies. Approximately one-fifth of isolated B cells and 7% of O cells contained surface-bound fibronectin but T cells failed to stain. There were no detectable free receptors for fibronectin on the surface of lymphocytes of different subsets as it was studied with FITC-labelled purified fibronectin. The percent of B and O cells bearing surface bound fibronectin was markedly decreased in patients with acute and chronic lymphocytic leukemias.