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Primary Sjögren's syndrome (pSS) is characterised clinically by sicca symptoms of the eyes and mouth. However, a proportion of pSS patients also present with extraglandular symptoms, for which immunosuppressive treatments may be needed. An expert panel of the European League Against Rheumatism (EULAR) has very recently developed a EULAR Sjögren's syndrome disease activity index (ESSDAI) to evaluate these systemic symptoms of pSS.1 The index consists of 12 organ-specific domains, which are predominantly clinical; only 2 of them include haematological (cytopenias) or biological (clonal component, serum complement levels, serum IgG and cryoglobulins) findings.1
β2 Microglobulin is a low-molecular weight (11.8 kDa) polypeptide protein, which is one of the major histocompatibility class I molecules on the cell surfaces of all nucleated cells and is produced in particular by lymphocytes. We have previously reported that serum β2 microglobulin is an independent predictor of the development of pSS in subjects with sicca symptoms.2 Serum β2 microglobulin has also been related to renal3,–,5 and pulmonary manifestations of pSS6,–,8 and to extraglandular manifestations of pSS overall,9 and is associated with lymphoma development in pSS.3 10
The EULAR Sjögren's syndrome expert panel encouraged further studies in independent patient cohorts to assess the reliability and sensitivity of the ESSDAI.1 We therefore determined the ESSDAI from a previously gathered4 and well-characterised pSS patient cohort of 78 subjects (75 women and 3 men), aged 58±13 years, range 29–82 years, with a mean duration of disease of 9±4 years, who had undergone a careful clinical examination together with an in-depth interview with special emphasis on extraglandular manifestations of pSS. Seventy-five of the patients fulfilled the American-European Consensus Group Criteria.11 The study was approved by the Ethical Committee of Tampere University Hospital, Tampere, Finland. In particular, we aimed to establish whether serum β2 microglobulin, which is not included in the domains of the ESSDAI, correlates with this new activity index of the systemic manifestations of pSS. The serum β2 microglobulin determinations were performed upon informed consent by radioimmunoassay (Pharmacia beta-2-micro RIA kit; Pharmacia Diagnostics, Uppsala, Sweden).
The mean ESSDAI in our pSS patient cohort was 11.10±7.52, ranging from 0 to 37, which showed fairly good agreement with the values in the cohorts from the expert panels,1 where the mean ESSDAI score was 15.48 (range 2–47).1 The ESSDAI was, accordingly, also applicable in an independent pSS cohort, and it could be used without special training to the ratings, precisely as intended by the expert panel.1 In our pSS patient cohort, serum β2 microglobulin concentrations correlated significantly with the ESSDAI (r=0.383, p=0.001, Pearson correlation coefficient) (figure 1). The ESSDAI was significantly higher in patients in the highest (>2.9 mg/l) than the lowest (<2.4 mg/l) tertiles of serum β2 microglobulin (14.85±8.63 vs 10.38±6.47, p=0.04, t test). The ESSDAI also correlated significantly with erythrocyte sedimentation rate (r=0.247, p=0.029), but it did not correlate with serum IgG (r=0.014, p=0.906) or C4 (r=−0.105, p=0.359), although these two parameters are included in the ESSDAI. This is probably due to the fact that IgG and C4 are given a rather low weight in the calculation of ESSDAI.
Our results thus lend support to the previous findings that serum β2 microglobulin is a noteworthy biomarker of the systemic inflammatory activity of pSS and, in addition to ESSDAI, is useful in the follow-up of pSS patients, increased levels alerting vigilance to pulmonary,6,–,8 nephrological3,–,5 or lymphoproliferative complications of the disease.3 10 Moreover, we found that the ESSDAI was fairly easily applicable retrospectively in a carefully gathered patient cohort.
Funding This study was financially supported by the Competitive Research Funding of Tampere University Hospital, Tampere, Finland, and the Tampere Tuberculosis Foundation.
Ethics approval The study has been approved by the Ethical Committee of Tampere University Hospital, Tampere, Finland.
Provenance and peer review Not commissioned; externally peer reviewed.
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