Oncology, St George's Hospital Medical School, 1 Centre for Paediatric Rheumatology, University College London,2 Rheumatology, GKT School of Medicine and3 Rheumatology Unit, Institute of Child Health, University College London, London, UK
Correspondence to: M. D. Bodman-Smith; E-mail: mbodmans{at}sghms.ac.uk
SIR, We have previously described the human heat shock protein BiP (immunoglobulin binding protein; glucose regulated protein 78) as an autoantigen in rheumatoid arthritis [1]. Antibodies to BiP have been described in the serum of RA patients by ourselves and other groups [2, 3] and we have recently described anti-BiP antibodies in the synovial fluid of RA patients and the serum of patients with primary Sjögren's syndrome [3]. Moreover anti-BiP antibodies have been shown to be elevated in animal models of arthritis [1]. The functions of these autoantibodies have yet to be elucidated.
Juvenile idiopathic arthritis (JIA) is a heterogeneous collection of chronic inflammatory diseases of unknown aetiology and pathology. JIA subgroups are classified by differences in clinical expression and prognosis and therefore may require different medical interventions. The clinical features and genetic epidemiology of juvenile idiopathic arthritis distinguish it from adult RA. Children with JIA also suffer from extra-articular complications, including growth retardation, osteoporosis, delayed puberty and increased risk of macrophage activation syndrome and amyloidosis. Although some subtypes of JIA may fully resolve, approximately half of children following a polyarticular disease course will continue to have active disease beyond 10 yr from onset [4]. JIA is still a major cause of physical disability, carrying a huge financial, socio-economic and personal burden. Current drug treatments demonstrate limited efficacy in certain subgroups of the disease, many children still experiencing early joint destruction necessitating surgical intervention [4]. Despite a plethora of studies attempting to identify biological markers of disease subtypes, there is no diagnostic test for JIA and it remains a diagnosis of exclusion [5 6]. In this study we examined each of the subgroups of JIA for the presence of anti-BiP antibodies to determine if this marker of RA is also shared in JIA. This study examined the anti-BiP antibody levels in a total of 154 serum samples from children with JIA, as defined by the ILAR criteria [7]. The subgroups consisted of 37 systemic JIA patients (mean age 8.8, range <122); 41 oligoarticular (persistent) patients (mean age 7.7, range 215); 33 oligoarticular (extended) patients (mean age 8.9, range 215); 43 polyarticular patients (mean age 9.4, range 216); and 16 age-matched controls. The anti-BiP ELISA was carried out as previously described [8]. Briefly, human recombinant BiP was bound to an Immunosorp 96-well ELISA plate and the plate blocked with 10% goat serum. Diluted sera were incubated on the plate overnight and the plate was then developed with an anti-human IgG. A reference serum was incorporated into each plate to allow meaningful comparisons between assays. The ELISA has been extensively verified and no correlation has been seen between anti-BiP antibody levels and rheumatoid factors (A, G and M) or antibodies to the irrelevant proteins ß-galactosidase (produced and purified in the same E. coli expression system) and tetanus toxoid. RA sera screened against Western blots of purified recombinant BiP confirmed binding and specificity of the proprietary monoclonal antibody to BiP. No correlation between anti-BiP and immunoglobulin levels (IgA, IgG, and IgM) was seen [9]. Age-appropriate patient consent and parental consent were obtained prior to the collection and processing of all blood samples.
Our results show that, when taken as a whole, JIA patients do not have elevated anti-BiP antibody levels compared with age-matched control individuals. However, when they were separated into clinical subgroups we found a slight, but non-significant, increase in the polyarticular patients (Fig. 1A) (systemic, mean 15.97% reference serum, S.D. 14.76; oligo, 15.40%, S.D. 12.96; oligo, ext, 13.19%, S.D. 12.21; poly, 23.42%, 26.87; control, 13.32%, S.D. 8.88). As we have previously presented data to show that RA patients have elevated anti-BiP levels [8, 10], we stratified the polyarticular JIA patients for RF positivity as clinically they are, unlike the other subtypes, indistinguishable from adult RA patients. Patients with polyarticular JIA with elevated RF levels had a significantly higher level of anti-BiP antibodies than those with no detectable RF by latex screening (Fig. 1B) (RF+, mean 36.61%, S.D 28.2; RF, mean 16.90, S.D 14.05; P = 0.01, Student's t-test). Interestingly, anti-BiP antibody levels were generally lower in normal age-matched controls than in older normal controls (data not shown). This supports existing evidence that anti-heat shock protein antibody levels increase with age [11].
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In summary, we have shown that anti-BiP antibody levels are not raised in JIA patients as a whole but remain specific for RA and primary Sjögren's syndrome [3]. We have, however, shown that those polyarticular JIA patients who are RF-positive have a significant increase in anti-BiP levels, consistent with the clinical similarity between this subgroup of JIA and adult RA patients.
M.D.B.-S., M.S.F. and V.C. have equity in Immune Regulation Ltd, which holds patent rights in BiP for diagnostic tests. G.P. is a director of and has equity in Immune Regulation Ltd, which holds patent rights in BiP for diagnostic tests. The other three authors have declared no conflicts of interest.
Notes
The first two authors contributed equally to this work.
References
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