Acute-phase serum amyloid A production by rheumatoid arthritis synovial tissue

Abstract

The known association between the acute-phase response and progressive joint damage may be the direct result of synovial A-SAA-induced effects on cartilage degradation. encodes constitutive SAA and is minimally inducible. A-SAA increases dramatically during acute inflammation and may reach levels that are 1000-fold greater than normal. A-SAA is mainly synthesized in the liver, but extrahepatic production has been demonstrated in many species, including humans. A-SAA mRNA is expressed in RA synoviocytes and in monocyte/macrophage cell lines such as THP-1 cells, in endothelial cells and in smooth muscle cells of atherosclerotic lesions. A-SAA has also been localized to a wide range of histologically normal tissues, including breast, stomach, intestine, pancreas, kidney, lung, tonsil, thyroid, pituitary, placenta, skin and brain. To identify the cell types that produce A-SAA mRNA and protein, and their location in RA synovium. Rheumatoid synovial tissue was obtained from eight patients undergoing arthroscopic biopsy and at joint replacement surgery. Total RNA was analyzed by reverse transcription (RT) polymerase chain reaction (PCR) for A-SAA mRNA. PCR products generated were confirmed by Southern blot analysis using human A-SAA cDNA. Localization of A-SAA production was examined by immunohistochemistry using a rabbit antihuman A-SAA polyclonal antibody. PrimaryRA synoviocytes were cultured to examine endogenous A-SAA mRNA expression and protein production. demonstrates RT-PCR products generated using synovial tissue from three representative RA patients. Analysis of RA synovial tissue revealed differences in A-SAA mRNA levels between individual RA patients. ). This study demonstrates that A-SAA mRNA is expressed in several cell populations infiltrating RA synovial tissue. A-SAA mRNA expression was observed in all eight unseparated RA tissue samples studied. A-SAA mRNA expression and protein production was demonstrated in primary cultures of purified RA synoviocytes. Using immunohistochemical techniques, A-SAA protein appeared to colocalize with both lining layer and sublining layer synoviocytes, macrophages and some endothelial cells. The detection of A-SAA protein in culture media supernatants harvested from unstimulated synoviocytes confirms endogenous A-SAA production, and is consistent with A-SAA mRNA expression and translation by the same cells. Moreover, the demonstration of A-SAA protein in RA synovial tissue, RA cultured synoviocytes, macrophages and endothelial cells is consistent with previous studies that demonstrated A-SAA production by a variety of human cell populations. The RA synovial lining layer is composed of activated macrophages and fibroblast-like synoviocytes. The macrophage is the predominant cell type and it has been shown to accumulate preferentially in the surface of the lining layer and in the perivascular areas of the sublining layer. Nevertheless, our observations strongly suggest that A-SAA is produced not only by synoviocytes, but also by synovial tissue macrophage populations. Local A-SAA protein production by vascular endothelial cells was detected in some, but not all, of the tissues examined. The reason for the variability in vascular A-SAA staining is unknown, but may be due to differences in endothelial cell activation, events related to angiogenesis or the intensity of local inflammation. The value of measuring serum A-SAA levels as a reliable surrogate marker of inflammation has been demonstrated for several diseases including RA, juvenile chronic arthritis, psoriatic arthropathy, ankylosing spondylitis, Behçet's disease, reactive arthritis and Crohn's disease. It has been suggested that serum A-SAA levels may represent the most sensitive measurement of the acute-phase reaction. In RA, A-SAA levels provide the strongest correlations with clinical measurements of disease activity, and changes in serum levels best reflect the clinical course. A number of biologic activities have been described for A-SAA, including several that are relevant to the understanding of inflammatory and tissue-degrading mechanisms in human arthritis. A-SAA induces migration, adhesion and tissue infiltration of circulating monocytes and polymorphonuclear leukocytes. In addition, human A-SAA can induce interleukin-1β, interleukin-1 receptor antagonist and soluble type II tumour necrosis factor receptor production by a monocyte cell line. Moreover, A-SAA can stimulate the production of cartilage-degrading proteases by both human and rabbit synoviocytes. The effects of A-SAA on protease production are interesting, because in RA a sustained acute-phase reaction has been strongly associated with progressive joint damage. The known association between the acute-phase response and progressive joint damage may be the direct result of synovial A-SAA-induced effects on cartilage degradation. In contrast to noninflamed synovium, A-SAA mRNA expression was identified in all RA tissues examined. A-SAA appeared to be produced by synovial tissue synoviocytes, macrophages and endothelial cells. The observation of A-SAA mRNA expression in cultured RA synoviocytes and human RA synovial tissue confirms and extends recently published findings that demonstrated A-SAA mRNA expression in stimulated RA synoviocytes, but not in unstimulated RA synoviocytes.

Introduction:

encodes constitutive SAA and is minimally inducible. A-SAA increases dramatically during acute inflammation and may reach levels that are 1000-fold greater than normal. A-SAA is mainly synthesized in the liver, but extrahepatic production has been demonstrated in many species, including humans. A-SAA mRNA is expressed in RA synoviocytes and in monocyte/macrophage cell lines such as THP-1 cells, in endothelial cells and in smooth muscle cells of atherosclerotic lesions. A-SAA has also been localized to a wide range of histologically normal tissues, including breast, stomach, intestine, pancreas, kidney, lung, tonsil, thyroid, pituitary, placenta, skin and brain.

Aims:

To identify the cell types that produce A-SAA mRNA and protein, and their location in RA synovium.

Materials and methods:

Rheumatoid synovial tissue was obtained from eight patients undergoing arthroscopic biopsy and at joint replacement surgery. Total RNA was analyzed by reverse transcription (RT) polymerase chain reaction (PCR) for A-SAA mRNA. PCR products generated were confirmed by Southern blot analysis using human A-SAA cDNA. Localization of A-SAA production was examined by immunohistochemistry using a rabbit antihuman A-SAA polyclonal antibody. PrimaryRA synoviocytes were cultured to examine endogenous A-SAA mRNA expression and protein production.

Results:

demonstrates RT-PCR products generated using synovial tissue from three representative RA patients. Analysis of RA synovial tissue revealed differences in A-SAA mRNA levels between individual RA patients.

).

Discussion:

This study demonstrates that A-SAA mRNA is expressed in several cell populations infiltrating RA synovial tissue. A-SAA mRNA expression was observed in all eight unseparated RA tissue samples studied. A-SAA mRNA expression and protein production was demonstrated in primary cultures of purified RA synoviocytes. Using immunohistochemical techniques, A-SAA protein appeared to colocalize with both lining layer and sublining layer synoviocytes, macrophages and some endothelial cells. The detection of A-SAA protein in culture media supernatants harvested from unstimulated synoviocytes confirms endogenous A-SAA production, and is consistent with A-SAA mRNA expression and translation by the same cells. Moreover, the demonstration of A-SAA protein in RA synovial tissue, RA cultured synoviocytes, macrophages and endothelial cells is consistent with previous studies that demonstrated A-SAA production by a variety of human cell populations.

The RA synovial lining layer is composed of activated macrophages and fibroblast-like synoviocytes. The macrophage is the predominant cell type and it has been shown to accumulate preferentially in the surface of the lining layer and in the perivascular areas of the sublining layer. Nevertheless, our observations strongly suggest that A-SAA is produced not only by synoviocytes, but also by synovial tissue macrophage populations. Local A-SAA protein production by vascular endothelial cells was detected in some, but not all, of the tissues examined. The reason for the variability in vascular A-SAA staining is unknown, but may be due to differences in endothelial cell activation, events related to angiogenesis or the intensity of local inflammation.

The value of measuring serum A-SAA levels as a reliable surrogate marker of inflammation has been demonstrated for several diseases including RA, juvenile chronic arthritis, psoriatic arthropathy, ankylosing spondylitis, Behçet's disease, reactive arthritis and Crohn's disease. It has been suggested that serum A-SAA levels may represent the most sensitive measurement of the acute-phase reaction. In RA, A-SAA levels provide the strongest correlations with clinical measurements of disease activity, and changes in serum levels best reflect the clinical course.

A number of biologic activities have been described for A-SAA, including several that are relevant to the understanding of inflammatory and tissue-degrading mechanisms in human arthritis. A-SAA induces migration, adhesion and tissue infiltration of circulating monocytes and polymorphonuclear leukocytes. In addition, human A-SAA can induce interleukin-1β, interleukin-1 receptor antagonist and soluble type II tumour necrosis factor receptor production by a monocyte cell line. Moreover, A-SAA can stimulate the production of cartilage-degrading proteases by both human and rabbit synoviocytes. The effects of A-SAA on protease production are interesting, because in RA a sustained acute-phase reaction has been strongly associated with progressive joint damage. The known association between the acute-phase response and progressive joint damage may be the direct result of synovial A-SAA-induced effects on cartilage degradation.

Conclusion:

In contrast to noninflamed synovium, A-SAA mRNA expression was identified in all RA tissues examined. A-SAA appeared to be produced by synovial tissue synoviocytes, macrophages and endothelial cells. The observation of A-SAA mRNA expression in cultured RA synoviocytes and human RA synovial tissue confirms and extends recently published findings that demonstrated A-SAA mRNA expression in stimulated RA synoviocytes, but not in unstimulated RA synoviocytes.

Introduction

].

Patients

= 1) was obtained from the knee joint of a patient undergoing lower limb amputation.

Isolation and culture of synovial cells

Synovial cells were obtained by enzymatic digestion of synovial membrane with 1mg/ml collagenase type I (Worthington Biochemical, Freehold, NJ, USA) in RPMI (GibcoBRL, Paisley, UK) for 4 h at 37°C in 5% carbon dioxide. Dissociated cells were plated in RPMI supplemented with 10% foetal calf serum (GibcoBRL), 10 ml of 1 mmol/l HEPES (GibcoBRL), penicillin (100 units/ml), streptomycin (100 units/ml) and fungizone (0.25 μg/ml). The cells were grown to confluency (approximately 10days) at 37°C in a 5% carbon dioxide atmosphere, then harvested with trypsin and passaged. Synoviocytes were found to be morphologically homogenous fibroblast-like cells and were used between the third and seventh passage. To confirm synoviocyte cultures were not contaminated by monocytes, staining for the monocyte marker CD14 was carried out. Cells were placed in serum-free medium 24 h before total RNA extraction.

Reverse transcription-polymerase chain reaction

(A-SAA); 2.5 μl 10 ×PCRII buffer (Perkin Elmer) and 20 ng of each specific PCR primer pair in a 25 μl total volume. Specific primers for human A-SAA were used to amplify a 335 base-pair (bp) A-SAA product: sense primer (5' -AAG CTT CTT TCC GTT CCT TGG-3') and antisense primer(5' -GAG AGC AGA GTG AAG AGG AAG C-3'). The A-SAA primers used span an intron. Thus, the PCR generates an unequivocally RNA-derived band, based on its size. GAPDH primers were designed to generate a 635-bp product: sense primer (5' -CCA CCC ATG GCA AAT TCC ATG GCA-3') and antisense primer (5' -TCT AGA CGG CAG GTC AGG TCC ACC-3'). After preincubation (94°C, 10 min) each PCR sample underwent a 35-cycle amplification regimen of denaturation (94°C, 1 min), primer annealing (60-56°C, 1 min) and extension (72°C, 1 min), with a final extension (72°C, 10 min) in a thermal cycler (MJ Research, Inc, Cambridge, MA, USA).

Northern blot analysis

P] dCTP and a random primer labelling system (Promega). All membranes were probed under high stringency conditions. Blots were exposed to film at -80°C using intensifying screens and autoradiographic intensity was quantified using an imaging densitometer.

Southern blot analysis

P] dCTP and a random labelling system (Promega). All membranes were probed under high stringency conditions. Blots were exposed to film at -80°C using intensifying screens.

Measurement of acute-phase serum amyloid A by ELISA

].

Immunohistology

Synovial tissue was placed in the cryopreservative embedding media OCT compound (Tissue Tek, Sakura, Finetek, Europe BV, Zoeterwoude, The Netherlands) and immediately frozen in liquid nitrogen. Sections (7 μ m) were cut on a microtome (Microm HM 505N, GmbH 69190 Walldorf, Germany), placed on glass slides coated with 2% 3-amino-propyl-triethoxy-silane (Sigma-Aldrich Ireland Ltd, Dublin, Ireland) in acetone and dried overnight at room temperature. Isolated RA synoviocytes were trypsinized and placed into a six-well plate with apyrogenic cell culture coverslips. Once grown to confluency, the medium was removed and the cells were treated with methanol for 15 min. Synoviocytes were stained essentially as described for the tissue sections. Tissue sections were allowed to reach room temperature, fixed in acetone, air-dried and incubated for 1 h at room temperature with blocking serum (Vectastain Rabbit Elite Kit, Vector Laboratories Ltd, Peterborough, UK). The slides were incubated with avidin for 15 min, rinsed and then incubated with biotin for 15 min. The primary polyclonal antibody for A-SAA (1:1200-1:1600; rabbit antihuman) was incubated for 1 h at room temperature. Secondary antibodies (antirabbit and antimouse; Vectastain) were prepared and added to the relevant sections and incubated for 30 min. The secondary antibody was washed off and the slides were incubated with Avidin:Biotinylated enzyme complex solution for 30 min and incubated for 6 min with 3,3' -diaminobenzidine and counterstained in haemotoxylin stain for 1 min.

Acute-phase serum amyloid A mRNA in inflamed human synovial

			tissue

; lane 1). Analysis of RA synovial tissue revealed differences in A-SAA mRNA levels between individual RA patients. Expression levels of the house-keeping gene (GAPDH) were similar in all patients.

Acute-phase serum amyloid A mRNA expression in cultured human

			synovial cells

).

Acute-phase serum amyloid A protein in primary synoviocyte culture supernatants

).

Immunohistochemical localization of acute-phase serum amyloid A in

			human synovial tissue and cultured synoviocytes

) treated with IgG.

Discussion

The aim of the present study was to examine synovial A-SAA production in RA and to identify the cell populations expressing A-SAA in inflamed tissue. In contrast to non-inflamed synovium, A-SAA mRNA expression was identified in all RA tissues examined. A-SAA appeared to be produced by synovial tissue synoviocytes, macrophages and endothelial cells.

] demonstrating A-SAA production by a variety of human cell populations.

]. As demonstrated in the present study, not all lining layer and sublining layer macrophages appeared to produce A-SAA. Futher studies of isolated synovial tissue macrophages, and immunohistological studies employing double-labelling techniques, will elucidate this observation. Nevertheless, the observations reported in this study strongly suggest that A-SAA is produced not only by synoviocytes, but also by synovial tissue macrophage populations. Local A-SAA protein production by vascular endothelial cells was detected in some, but not all, of the tissues examined. The reason for the variability in vascular A-SAA staining is unknown, but this variability may be due to differences in endothelial cell activation, events relating to angiogenesis or the intensity of local inflammation.

] quantified serum A-SAA levels in 140 patients with various inflammatory joint diseases with duration of less than 2 years, and demonstrated significant correlations with other acute-phase measurements such as C-reactive protein and the erythrocyte sedimentation rate. The magnitude of the A-SAA response was greatest, and the highest levels occurred in RA. In RA, A-SAA levels provided the strongest correlations with clinical measurements of disease activity, and changes in serum levels best reflected the clinical course.

]. The known association between the acute-phase response and progressive joint damage may be the direct result of synovial A-SAA-induced effects on cartilage degradation.

Acknowledgments

Rosemary O'Hara is supported by a grant from the Health Research Board of Ireland; Evelyn P Murphy is the Pfizer Newman Scholar, University College, Dublin. Synovial tissue samples were provided by Eithne Murphy, Leanne Stafford and David Kane. Nicola Cassidy prepared tissue sections for immunohistochemical analysis. Emer Cunningham (Biotrin International, Dublin, Ireland) provided the A-SAA ELISA kits as a gift and the assays were performed by John Paul Doran, during a Summer Studentship sponsored by the Health Research Board of Ireland.

Figures and Tables

= 3, lanes 1-3) was performed using primers for human A-SAA and GAPDH. The 500 bp molecular weight marker (MW) is highlighted.

Detection of A-SAA mRNA expression in primary human synoviocytes. RT-PCR analysis was performed using total RNA from individual rheumatoid primary synoviocytes with primers to human A-SAA and GAPDH (lanes 1, 2). The 500 bp molecular weight marker (MW) is highlighted.

.

Detection of A-SAA mRNA expression in inflamed human synovial tissue by RT-PCR. Southern blot analysis of A-SAA and GAPDH cDNAs generated using total RNA from normal human synovium (lane 1) and rheumatoid synovium (lane 2).

Agarose gel electrophoresis demonstrating the integrity of total RNA used for Northern analysis.

Levels of A-SAA detected in culture primary RA synoviocyte supernatants. Mean values of control and individual RA samples (RA1-RA4) are indicated. N, number of samples measured per group.

The specificity of staining was confirmed using synoviocytes stained with isotype-matched IgG.

Keywords

• acute-phase response
• rheumatoid arthritis
• serum amyloid A
• synovial tissue