Lack of binding between cryoimmunoglobulins, immunoglobulins and fibronectin: implications for immune complex vasculitis

Binding of polymeric IgG to fibronectin in extracellular matrices: an in vitro paradigm for immune-complex deposition

We have previously shown a biochemical interaction between fibronectin (Fn) and polymeric immunoglobulins (Igs), that we localized to the fourth and fifth N-terminal type I repeats (4F1.5F1) in Fn and the Fc portion of IgG. Therefore, we hypothesized that Fn, as a constituent of the extracellular matrix (ECM) may directly bind circulating immune complexes (ICs) causing their deposition, thereby contributing to the pathogenesis of IC diseases. As an in vitro paradigm to test this idea, we have generated Fn-containing ECMs from varied cells in culture and demonstrated a saturable dose-dependent binding of aggregated (agg) IgG, as a prototype of ICs, as well as the binding of both heat and cold aggregated purified type I cryoglobulins (CGs) to these ECMs. No binding was observed to ECMs (Matrigel) that do not contain Fn. Characteristic of our previous findings, polymeric but not monomeric IgG bound to the acellular Fn-containing ECMs. To further demonstrate the specificity of the interaction and implicate matrix Fn in the binding of aggIgG, complete inhibition of binding of aggIgG to Fn was achieved by blocking Fn on the ECMs with anti-Fn antiserum and by preincubation of the Ig aggregates with anti-human IgG antibodies. By competing the binding interaction with fluid phase Fn and the Ig-binding site on Fn, 4F(1).5F(1), 70% inhibition was obtained. Additional experiments performed with purified CGs show that an identical dose-dependent increase in Fn binding occurred using both preformed and forming cryoprecipitates suggesting that Fn does not confer cryoprecipitation of CGs and that the specific association of Fn with cryoprecipitates probably results from their polymeric configuration. Our results support the notion that Fn, as it exits in expanding ECMs characteristic of glomerulonephropathies and rheumatoid synovial disease, specifically interacts with complexed/polymeric Igs, thereby perpetuating IC deposition and playing a role in the pathogenesis of IC diseases.

Cryofibrinogenaemia: a study of 49 patients

The purpose of this study was to characterize the clinical features and components of 30 patients with isolated cryofibrinogen (CF) versus those of 19 patients with combined CF and cryoglobulins (CG). Secondary forms of cryofibrinogenaemia associated with collagen disorders, infectious or malignant diseases, were significantly more frequent in patients with combined CF and CG than those with isolated CF (79 versus 47%, P = 0.02). Both groups of CF patients presented predominantly cutaneous symptoms (77% in isolated CF; 58% in combined CF + CG), and less frequently venous and/or arterial thrombosis (13% in isolated CF; 3% in combined CF + CG). Patients with idiopathic forms of CF, and particularly those without CG, suffered essentially from recurrent painful skin ulcers, mainly triggered by cold exposure. Patients with isolated CF had higher mean plasma concentrations of CF than those with combined CF + CG (1. 61 +/- 1.26 versus 0.82 +/- 1.18 g/l, respectively; P = 0.004), but there was no correlation between the CF plasma level and either the severity of symptoms or the sensitivity to cold. In patients with isolated CF, fibronectin was suggested (by precipitation analysis) to be a major component of the cryoprecipitate, whereas immunoglobulins were rarely present (in only three out of 30 patients). By contrast, in the majority of patients (78%) with combined CF and CG, the CF consisted mainly of immunoglobulins of the same class as those characterizing the associated CG. Analysis of the CG precipitate revealed the presence of fibronectin but not fibrinogen, alpha1-antitrypsin and alpha2-macroglobulin. In conclusion, isolated and combined cryofibrinogenaemia are associated with different clinical signs requiring different clinical management, but there is no evidence as yet for a causal role of the cryoprecipitates in the differences observed.

Biochemical analysis of the interaction of fibronectin with IgG and localization of the respective binding sites

Fibronectin (Fn), a mosaic protein composed of multiple copies of three different module types (Fl, F2 and F3), has been found associated with circulating immune complexes (ICs) and immunoglobulin (Ig) aggregates in a variety of IC diseases and myeloproliferative disorders. We have previously shown that a proteolytic fragment of Mr = 25,900 Da, from the NH2-terminal domain of Fn, composed of five type 1 modules (1Fl -5Fl) binds to the major Ig classes under physiologic conditions, suggesting that the presence of Fn in ICs and cryoglobulins results from a physicochemical binding interaction between these two molecules. Using an ELISA, we now show that the interaction between Fn and IgG is: (1) not influenced by any other constituent of plasma; (2) unaffected by temperature; and (3) has an estimated Kd of 3.77 x 10(-9) M. In addition, we have further delineated the respective sites involved in the interaction between Fn and IgG. Recombinant type l module pairs (1Fl.2Fl and 4Fl.5Fl) from the NH2-terminus of Fn, expressed in yeast, were employed in an ELISA and affinity chromatography and compared with the 25.9 kDa (1Fl - 5Fl) fragment and intact Fn for binding to IgG. The 4Fl.5Fl and the 25.9 kDa fragment bound to immobilized IgG and inhibited Fn binding to IgG to nearly the same extent as the intact molecule (IC50: Fn = 6.77 x 1O(-9) M; 25.9 kDa fragment = 5 x 10(-9) M; 4Fl.5Fl = 7.6 x 10(-9) M). Thus, the binding site for IgG on the Fn molecule is localized to and completely conferred by the 4Fl.5Fl module pair (residues 151-244). Similar experiments using papain-generated Fab and Fc fragments of IgG localized the Fn binding site on IgG to the Fe region of the IgG molecule. Fn bound to the Fc fragment with a nearly identical Kd of 3.69 x 10(-9) M, as to intact IgG (3.77 x 10(-9) M). These studies support the hypothesis that the interaction between Fn and Ig may contribute to the pathophysiology of immune complex related disorders.

Fibronectin and circulating immune complexes in Behet's disease

Being a high-molecular-weight adhesive glycoprotein, fibronectin (Fn) is suggested to be a component of immune complexes and may participate in the clearance of immune complexes. In Behçet's disease (BD), a multisystem disorder of unknown etiology, immune complexes have been shown to be deposited in affected tissue during disease activity, suggesting an immune mechanism. This study investigates the relationship between Fn and circulating immune complexes (CIC) and evaluates the changes in the levels of Fn and CIC along with disease activity. In 63 patients (31 active, 32 inactive) with BD, plasma Fn and serum CIC, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and the third and fourth components of the complement system (C3, C4) were studied. The mean ESR, CRP, C3 and C4 levels of active BD patients were found to be significantly higher than those of the inactive BD patient group. Although the mean Fn and CIC levels of BD patients were not significantly different from those of the healthy control group, Fn and CIC values of active BD patients were significantly lower than in the inactive group. Moreover, no significant correlation was observed among Fn levels and ESR, CRP, C3, C4 and CIC levels in BD patients. The result of this study suggest that the variation in Fn concentration is independent of the acute-phase response. The lack of relationship between the CIC and Fn concentrations indicates that IC deposition in the vessel wall is independent of the CIC levels. In order to determine the exact roles of Fn and IC, further studies in tissue specimens are required.