Fibronectin and immune complexes in rheumatic diseases

Increased circulating fibronectin, depletion of natural IgM and heightened EBV, HSV-1 reactivation in ME/CFS and long COVID

Myalgic Encephalomyelitis/ Chronic Fatigue syndrome (ME/CFS) is a complex, debilitating, long-term illness without a diagnostic biomarker. ME/CFS patients share overlapping symptoms with long COVID patients, an observation which has strengthened the infectious origin hypothesis of ME/CFS. However, the exact sequence of events leading to disease development is largely unknown for both clinical conditions. Here we show antibody response to herpesvirus dUTPases, particularly to that of Epstein-Barr virus (EBV) and HSV-1, increased circulating fibronectin (FN1) levels in serum and depletion of natural IgM against fibronectin ((n)IgM-FN1) are common factors for both severe ME/CFS and long COVID. We provide evidence for herpesvirus dUTPases-mediated alterations in host cell cytoskeleton, mitochondrial dysfunction and OXPHOS. Our data show altered active immune complexes, immunoglobulin-mediated mitochondrial fragmentation as well as adaptive IgM production in ME/CFS patients. Our findings provide mechanistic insight into both ME/CFS and long COVID development. Finding of increased circulating FN1 and depletion of (n)IgM-FN1 as a biomarker for the severity of both ME/CFS and long COVID has an immediate implication in diagnostics and development of treatment modalities.

Anti-citrullinated fibronectin antibodies in rheumatoid arthritis are associated with human leukocyte antigen-DRB1 shared epitope alleles

Introduction

Fibronectin is one of the most abundant proteins present in the inflamed joint. Here, we characterized the citrullination of fibronectin in the joints of rheumatoid arthritis (RA) patients and studied the prevalence, epitope specificity and human leukocyte antigen (HLA) association of autoantibodies against citrullinated fibronectin in RA.

Methods

Citrullinated residues in fibronectin isolated from RA patient synovial fluid were identified by mass spectrometry. The corresponding citrullinated and non-citrullinated peptides were synthesized and used to analyze the presence of autoantibodies to these peptides in RA sera and sera from other diseases and healthy controls by ELISA. The data were compared with risk factors like shared epitope HLA alleles and smoking, and with clinical features.

Results

Five citrullinated residues were identified in fibronectin from RA synovial fluid. RA sera reacted in a citrulline-dependent manner with two out of four citrullinated fibronectin peptides, one of which contains two adjacent citrulline residues, in contrast to non-RA sera, which were not reactive. The most frequently recognized peptide (FN-Cit_1035,1036, LTVGLTXXGQPRQY, in which × represents citrulline) was primarily targeted by anti-CCP (cyclic citrullinated peptide) 2-positive RA patients. Anti-FN-Cit_1035,1036 autoantibodies were detected in 50% of established anti-CCP2-positive RA patients and in 45% of such patients from a early arthritis clinic. These antibodies appeared to be predominantly of the immunoglobulin G (IgG) isotype and to be associated with HLA shared epitope alleles (odds ratio = 2.11).

Conclusions

Fibronectin in the inflamed synovia of RA patients can be citrullinated at least at five positions. Together with the flanking amino acids, three of these citrullinated residues comprise two epitopes recognized by RA autoantibodies. Anti-citrullinated fibronectin peptide antibodies are associated with HLA shared epitope alleles.

Overcoming the barriers in micellar drug delivery: loading efficiency, in vivo stability, and micelle-cell interaction

IMPORTANCE OF THE FIELD

Spontaneously constructed from block copolymers in aqueous media, the polymer micelle has been extensively studied as a potential carrier of poorly water-soluble drugs, but cellular uptake pathways and stability of micelles in blood have not yet been clearly understood. An in-depth insight into the physical and biological behaviors of polymer micelles is necessitated for designing next-generation micelles.

AREAS COVERED IN THIS REVIEW

This review suggests possible solutions to improve micellar drug loading capacity, scrutinizes the parameters influencing the micelle stability in blood, and also discusses the fate of micelles in cellular and in vivo environment, respectively. Direct and indirect evidences from the literatures mostly published after 90's were collected, analyzed and summarized.

WHAT THE READER WILL GAIN

A critical analysis of micelle's stability in vivo and micelle-cell interaction is provided to highlight the key issues to be addressed to affirm that micelle can properly work as a drug carrier in clinical settings.

TAKE HOME MESSAGE

With a clear understanding of its behaviors in biological environment, the polymer micelle is a promising nanocarrier for chemotherapy.

Rheumatology: a close encounter with proteomics

Proteomics is a fast-growing discipline in biomedicine that can be defined as the large-scale characterization of the entire protein complement of a cell, tissue or organism. Because protein levels and function may be critically dependent upon post-transcriptional mechanisms (e.g. post-translational modifications), there has been significant interest in directly examining protein structure and function. It is now clear that proteomics studies may unmask previously unknown functions of proteins or protein interactions. However, proteomics in the field of rheumatology is still in its infancy. This review guides the reader through the consecutive steps of a proteomics study and provides an outline of the applications in the field of rheumatology, which may range from proteome analyses of biological fluids of rheumatic diseases to identify possible new diagnostic tools, towards more pathophysiological studies on target tissues, such as synovial tissue or articular cartilage. Proteomics has great potential in the field of rheumatology and will no doubt have a great impact on our molecular understanding of these complex diseases.

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.

A monoclonal antibody enhances ligand binding of fibronectin MSCRAMM (adhesin) from Streptococcus dysgalactiae

A monoclonal antibody 3A10, generated from a mouse immunized with the Streptococcus dysgalactiae fibronectin (Fn) binding protein FnbA, was isolated, and its effect on ligand binding by the antigen was examined. The epitope for 3A10 was localized to a previously unidentified Fn binding motif (designated An) just N-terminal of the repeat domain which represents the primary ligand binding site on FnbA. Fn binding to Au was enhanced by 3A10 rather than inhibited. This effect was demonstrated in two different assays. First, in the presence of 3A10 the Au-containing proteins and synthetic peptide more effectively competed with bacterial cells for binding to Fn. Second, 3A10 dramatically increased the binding of biotin-labeled forms of the Au-containing proteins to Fn immobilized on a blotting membrane. Pure 3A10 IgG did not recognize the antigen by itself, and Fn was required for the immunological interaction between the antibody and the epitope. This induction effect of Fn was shown in both Western blot and enzyme-linked immunosorbent assay in which immobilized Au-containing molecules were probed with 3A10 in the presence of varying concentrations of Fn. Specificity analyses of 3A10 revealed that the monoclonal also recognized a ligand binding motif in a Streptococcus pyogenes Fn binding MSCRAMM but not the corresponding motifs in two related adhesins from Staphylococcus aureus and S. dysgalactiae. Furthermore, 3A10 stimulated Fn binding by S. pyogenes cells. These results together with subsequent biophysical studies presented in the accompanying paper (House-Pomepeo, K., Xu, Y., Joh, D., Speziale, P., and Höök, M. (1996) J. Biol. Chem. 271, 1379-1384) indicate that the ligand binding sites of Fn binding MSCRAMMs have little or no secondary structure. However, on binding to Fn, they appear to undergo a structural rearrangement resulting in a defined structure rich in beta sheet and expressing a ligand-induced binding site for antibodies such as 3A10.

Adhesion and costimulation of proliferative responses of human gamma delta T cells by interaction of VLA-4 and VLA-5 with fibronectin

The very late antigens, VLA-4 and VLA-5 belong to the beta 1 subfamily of integrins and have been identified as receptors for different binding regions of fibronectin (FN). We have detected VLA-4 and VLA-5, but not VLA-3 and VLA-6 expressed on human CD3+CD4-CD8- gamma delta TCR T cells by flow cytometry. Binding assays, performed on FN-coated plates, showed that activated CD25high (IL-2 receptor) but not resting CD25low gamma delta T cells specifically adhere to FN. The binding capacity is inhibited by the synthetic peptide GRGDSP which inhibits adhesion mediated by VLA-5 and a functional mAb directed against the alpha 4 subunit. Most FN binding is mediated by VLA-4. Additionally, resting gamma delta T cells cultured on coimmobilized anti-TCR delta 1 mAb and FN or the 40 kDa fragment (which contains the adhesion site in the IIICS domain recognized by VLA-4) for 96 h in the absence of exogeneous IL-2 showed significant increase in proliferation when compared to that of resting gamma delta T cells cultured on immobilized anti-TCR delta 1 mAb alone. Also expression of CD25 was significantly enhanced on cells cultured on coimmobilized anti-TCR delta 1 mAb and FN, indicative of T cell activation. Cross-linking of VLA-4 and VLA-5 molecules costimulated expansion of resting gamma delta T cells induced by cross-linked TCR delta 1. These results suggest that the gamma delta T cell beta 1 integrins, VLA-4 and VLA-5, may function in a dual capacity as signalling and adhesion molecules.

Integrins and other adhesion molecules on lymphocytes from synovial fluid and peripheral blood of rheumatoid arthritis patients

Cell and matrix adhesion of lymphocytes participates in homing, migration and accumulation of these cells in inflamed tissues as well as in the generation of immune and inflammatory responses. In inflamed joints of rheumatoid arthritis (RA) patients, lymphocytes accumulate in the synovial membrane and the synovial fluid. In the present study we have analyzed the expression of integrins and other adhesion molecules in synovial fluid lymphocytes (RA-SFL) and paired peripheral blood lymphocytes (RA-PBL) from 21 RA patients by immunofluorescence flow cytometry. We have also investigated the expression of these adhesion molecules on peripheral blood lymphocytes obtained from 13 sex- and age-matched healthy controls (CO-PBL). RA-SFL, which consisted mostly of T cells, showed higher expression of the integrin subunits beta 1 (CD29), VLA-1 alpha, -3 alpha, -4 alpha, -5 alpha and -6 alpha when compared to RA-PBL. In turn, RA-PBL showed lower expression of these molecules than CO-PBL. The expression of the immunoglobulin-related molecules CD2, CD54 (ICAM-1) and CD58 (LFA-3) was higher on RA-SFL when compared to RA-PBL or CO-PBL, and similar results were obtained with the beta 2 integrin subunits CD11a and CD18. In contrast, L-selectin (LECAM-1) and ICAM-2 were expressed at much lower levels on RA-SFL than on RA-PBL or CO-PBL. CD44, a receptor for hyaluronic acid and collagen, was expressed by most RA-SFL, RA-PBL and CO-PBL cells but at higher density on RA-SFL. The results indicate that RA-SFL express a distinct array of adhesion molecules, similar to the one of memory T lymphocytes. This characteristic phenotype may contribute to the lymphocytic infiltration of the synovium and to the pathogenesis of RA.

VLA family in rheumatoid arthritis: evidence for in vivo regulated adhesion of synovial fluid T cells to fibronectin through VLA-5 integrin

Adhesion of T cells to extracellular matrix (ECM) proteins through VLA integrin receptors is crucial for lymphocyte trafficking, tissue localization and inflammatory function. We have investigated the expression of different VLA integrins (VLA-1-5) on peripheral blood (PB) and synovial fluid (SF) T lymphocytes from patients with rheumatoid arthritis (RA). Their expression on different cell types from synovial membrane (SM) is also reported. The role of VLA-4 fibronectin (FN) receptors in the interaction of activated SF T cells from RA patients with a 38-kD fragment of FN has been previously demonstrated. Here we have focused functional studies on VLA-5 as an alternative FN receptor for RA T cells. A significant higher proportion of SF T cells were able to bind to an 80-kD fragment of FN, containing the Arg-Gly-Asp (RGD) cell binding site, compared with PB T cells. This attachment was almost completely inhibited by anti-VLA-5 MoAbs as well as by RGD peptides. This enhanced capability by SF T cells appears to be independent of the level of the surface expression of the receptor and correlates better with their activation state as determined by the expression of the activation molecule AIM (CD69). The evidence for the expression of VLA heterodimers on both SF and SM cells from RA patients suggests the possible implication of ECM proteins in mediating and perpetuating inflammation in vivo.

Increased binding of synovial T lymphocytes from rheumatoid arthritis to endothelial-leukocyte adhesion molecule-1 (ELAM-1) and vascular cell adhesion molecule-1 (VCAM-1)

The infiltration of the synovial membrane (SM) by mononuclear cells, mostly T cells, is a typical histopathological feature associated with rheumatoid arthritis (RA). The entry of T lymphocytes into the SM is believed to be mediated by a number of molecules in the endothelium that are induced in response to a series of inflammatory mediators. In this study, we have investigated the adhesion of synovial T cells from RA patients to two endothelial ligands: endothelial-leukocyte adhesion molecule-1 (ELAM-1), the only selectin known to function as a vascular addressin for T cells, and vascular cell adhesion molecule-1 (VCAM-1), the cellular ligand of VLA-4. Our results clearly demonstrate that synovial T cells isolated from both SM and synovial fluid (SF), bearing an activated and memory phenotype, displayed an enhanced capacity to interact with these two endothelial molecules as compared with T cells from peripheral blood (PB) either of the same RA patients or healthy donors. A further enhancement of VLA-4-mediated T cell binding to VCAM-1 and fibronectin could be observed when already in vivo-activated synovial T cells were stimulated in vitro with phorbol esters, suggesting the existence of several cellular affinity levels for both very late activation-4 (VLA-4) ligands. Moreover, both PB and synovial T cells from RA patients exhibited strong proliferative responses when they were cultured with either fibronectin or VCAM-1 in combination with submitogenic doses of anti-CD3 mAb. This increased endothelial binding ability of synovial T lymphocytes together with their proliferation in response to the interaction with VCAM-1 and fibronectin may represent important mechanisms in the regulation of T cell penetration and persistence in the chronically inflamed SM of RA.

Upregulated expression and function of VLA-4 fibronectin receptors on human activated T cells in rheumatoid arthritis

The VLA-4 (CD49d/CD29) integrin is a cell surface receptor involved in the interaction of lymphoid cells with both extracellular matrix (ECM) and endothelial cells. We have investigated the expression and function of VLA-4 fibronectin (FN) receptors on T cells localized in the inflammed synovium of patients with rheumatoid arthritis (RA). A high proportion of T cells in both synovial membrane (SM) and synovial fluid (SF) expressed the activation antigens AIM (CD69) and gp95/85 (Ea2) as well as an increased number of VLA-4 alpha and beta 1 adhesion molecules, as compared with peripheral blood (PB) T cells from the same patients. Furthermore, the majority of these activated SF T cells were able to adhere to a 38-kD FN proteolytic fragment containing the connecting segment-1 (CS-1) specifically through VLA-4 receptors, whereas a significantly lower proportion of PB T cells displayed this capacity. Therefore, our results show that activated T cells selectively localize at sites of tissue injury in RA disease and provide evidence for the in vivo regulation of the expression and function of the VLA-4 integrin. This regulatory mechanism may enable T cells either to facilitate migration or to persist at sites of inflammation.

Characterization of the extracellular matrix of the primate temporomandibular joint

The distribution of type I and II collagens, fibronectin and the fibronectin-integrin receptor, tenascin, laminin, link protein, and cartilage-specific glycosaminoglycans was examined in the primate temporomandibular joint complex using an immunohistochemical approach. In general, type I collagen, fibronectin, and the fibronectin-integrin receptor were found to co-distribute throughout the joint complex. Immunostaining for these proteins was notably intense in the prechondroblastic and mineralization zones of the articular cartilages of the joint. Tenascin was identified in several structures of the joint, including the articular cartilages, where intense staining was observed in the prechondroblastic and cartilagenous zones. Laminin was detected only in the adventitia of blood vessels located in the attachment tissues of the disc and joint synovium. Cartilage-specific glycosaminoglycans and type II collagen were observed in the cartilagenous zones of the articular cartilages of the mandibular condyle and temporal bone. In addition, immunostaining for cartilage-specific glycosaminoglycans also was detected throughout the extracellular matrix surrounding "chondrocyte-like" cells located in the joint disc. Despite the localization of cartilage-specific glycosaminoglycans in the disc, type II collagen was not detected in this structure. It is suggested that a fibronectin-integrin receptor mechanism may be involved in the regulation of growth of the articular cartilages of the temporomandibular joint.

Diagnostic relevance of fibronectin in cryoprecipitates

Fibronectin is a known component of plasma cryoprecipitates. It is seen in cryoglobulins from patients with monoclonal gammopathies and also from rheumatoid arthritis, and patients with systemic lupus erythematosus and other connective tissue diseases. We evaluated the clinical relevance of measures of cryoprecipitable fibronectin from the sera of 88 patients with rheumatic diseases and 27 healthy controls. There were 28 patients with rheumatoid arthritis, 19 with systemic vasculitis, 5 with cutaneous vasculitis, and 36 with a systemic connective tissue disorder. We measured total and cryoprecipitable fibronectin and for comparison immunoglobulins G, A, and M and complement C3 and C4. Cryoprecipitable fibronectin was detected in 33% control sera and 42% patient sera. The mean levels were higher in the sera of patients in all diagnostic groups. The highest levels were seen in rheumatoid patients with systemic disease, systemic vasculitis, and connective tissue diseases. The presence of cryoprecipitable fibronectin was related to the clinical activity of systemic vasculitis; none of the 6 patients with clinically inactive vasculitis had detectable fibronectin in their cryoprecipitates; but it was seen in 7 to 13 cases with active vasculitis. There were only weak relationships between cryoprecipitable fibronectin and immunoglobulin and complement levels in cryoprecipitates. We conclude that routinely measuring fibronectin levels in cryoprecipitates is generally of doubtful diagnostic value. However, it appears to be a useful marker of the clinical activity of systemic vasculitis and we recommend its use in the laboratory assessment of vasculitis.

Elevated plasma fibronectin levels in rats with immune and toxic glomerular diseases

We measured plasma fibronectin levels by a rocket immunoelectrophoresis in rats with chronic serum sickness induced by repeated injections of ovalbumin and in rats with epithelial nephropathy induced by a single injection of adriamycin. In the early phases of the immune model, rats presented granular deposits of IgG in the mesangial area with no or descrete proteinuria (less than 40 mg/24 h). Fibronectin levels in that group were significantly higher (450 +/- 90 micrograms/mL) than in normal rats of the same age (350 +/- 46; p less than 0.01). When animals presented IgG deposits in the capillary wall, an important nephrotic syndrome developed in most of them. Fibronectin levels then increased very significantly (863 +/- 153 micrograms/mL; p less than 0.0005). In the model of adriamycin nephropathy, fibronectin significantly increased (580 +/- 110 micrograms/mL; p less than 0.0005) from the first week, when proteinuria was in a range 40-60 mg/24 h. However, the levels were higher (860 +/- 175 micrograms/mL; p less than 0.0005) when a complete nephrotic syndrome developed. At this time, plasma fibronectin levels correlated directly in both models with the degree of proteinuria and inversely with the total serum protein concentration. Our results show that plasma fibronectin levels increased very early in animals with immune and toxic damage of the kidney. The highest elevated values found thereafter, when a full nephrotic syndrome was present, suggest an increased synthetic rate of that glycoprotein linked to that situation.

Fragmented fibronectin and other synovial fluid proteins in chronic arthritis: their relation to immune complexes

Fibronectin, an opsonic glycoprotein has been shown to exist in fragmented forms in serum and synovial fluid. Some fragments in synovial fluid appear to be polyethylene glycol (PEG) precipitable, suggesting incorporation into immune complexes (IC). PEG precipitation, SDS-PAGE and immunoblotting were used to determine whether PEG precipitable fragments are real or artefactual. Disease specificity of fragmentation and IC incorporation of fibronectin and other proteins were also studied using these techniques. PEG precipitable fragments do not appear to be artefactual, although some fibronectin fragments are cryoprecipitable. Protein fragments showed similar distributions in whole serum and synovial fluid, disease specific differences being confined to PEG precipitates. Rheumatoid arthritis (RA) synovial fluid PEG precipitates displayed the greatest array of fragmented immunoglobulins and fibronectin. No PEG precipitates contained albumin fragments. Protein fragments in IC may impair their effective removal from RA joints. Accumulated IC could lead to tissue damage via complement activation.

What proteins are present in polyethylene glycol precipitates from rheumatic sera?

The proteins present in 4% polyethylene glycol (PEG) precipitates of 10 normal sera and 60 samples from patients with rheumatic diseases were studied. A variety of immunochemical methods were used, including estimation of the percentages of total serum proteins precipitated by PEG, gel filtration analyses of the precipitates, and affinity chromatography with protein A and anti-immunoglobulin columns. Substantial amounts of protein were precipitated from normal sera. Many non-immunoglobulin proteins were precipitated from patients' sera, including fibronectin, haptoglobin, albumin, transferrin, and alpha 1-antitrypsin. Affinity chromatography with anti-immunoglobulin columns bound non-immunoglobulin proteins from PEG precipitates, but the protein A affinity column did not do so. The view that circulating antibody-antigen complexes alone are precipitated by 4% PEG is too simplistic; many non-immunoglobulin proteins are involved. They may either bind to immune complexes or be coprecipitated owing to non-specific protein aggregation.

Disposition of IgA-containing circulating immune complexes

Two fundamentally different mechanisms may account for the glomerular immune deposits in IgA nephropathy (IgAN): (1) deposition of circulating immune complexes and (2) the in situ formation of immune complexes. In this review the experimental evidence for and against an important role of circulating IgA-containing immune complexes in the pathogenesis of IgAN is summarized. Several physical characteristics, including size, lattice composition, and electrical charge, may influence the deposition of immune complexes in the renal mesangium. Furthermore, the likelihood of deposition of circulating IgA-containing immune complexes in vulnerable locations (such as the kidney) may be increased because of their impaired removal from the circulation by macrophages of the liver and spleen and the erythrocyte-immune complex clearing mechanism. However, the relative contributions of these factors to the pathogenesis of IgAN remain speculative.