Lupus (2014) 23, 1486–1493 http://lup.sagepub.com

PAPER

Urine b2-microglobulin is associated with clinical disease activity and renal involvement in female patients with systemic lupus erythematosus J-Y Choe, S-H Park and S-K Kim Division of Rheumatology, Department of Internal Medicine, Arthritis & Autoimmunity Research Center, Catholic University of Daegu School of Medicine, Daegu, Republic of Korea

Objective: We investigated the association of serum and urine b2-microglobulin (b2MG) with renal involvement and clinical disease activity in systemic lupus erythematosus (SLE). Methods: Sixty-four female patients with SLE were enrolled. We assessed SLE disease activity (SLEDAI)-2K and measured serum and urine b2MG levels, as well as complement (C3 and C4) and anti-dsDNA levels. According to the SLEDAI scores, two groups were categorized: low (0–5 of SLEDAI) and high (6–19 of SLEDAI) disease activity groups. The presence of renal involvement was determined by renal SLEDAI score. Statistical analysis was performed using Spearman’s correlation analysis, Mann-Whitney U test, multivariate regression analysis, and logistic regression analysis. Results: Urine b2MG levels were significantly different between low and high SLEDAI groups (p ¼ 0.001), but not for serum b2MG levels (p ¼ 0.579). Patients with renal involvement showed higher urine b2MG levels compared to those without renal involvement (p < 0.001), but again there was not a difference in serum b2MG levels (p ¼ 0.228). Urine b2MG was closely associated with SLEDAI (r ¼ 0.363, p ¼ 0.003), renal SLEDAI (r ¼ 0.479, p < 0.001), urine protein/Cr (r ¼ 0.416, p ¼ 0.001), and ESR (r ¼ 0.347, p ¼ 0.006), but not serum b2MG (r ¼ 0.245, p ¼ 0.051). Urine b2MG level was identified as a surrogate for renal involvement (p ¼ 0.009, OR ¼ 1.017, 95% CI 1.004–1.030) and overall disease activity (p ¼ 0.009, OR ¼ 1.020, 95% CI 1.005–1.036). Conclusions: We demonstrated that urine b2MG levels are associated with renal involvement and overall clinical disease activity in SLE. Lupus (2014) 23, 1486–1493. Key words: b2-microglobulin; disease activity

renal

involvement;

Introduction Systemic lupus erythematosus (SLE) is a chronic multisystem autoimmune disease with periods of remittance and relapse; it affects multiple organs and frequently requires steroid and immunosuppressive agents.1,2 Currently, there is a lack of understanding of the pathogenesis of SLE. A variety of signs and symptoms may present from mild to life-threatening including fever, malar rash, polyarthritis, lung involvement, and cardiovascular or Correspondence to: S-K Kim, Division of Rheumatology, Department of Internal Medicine, Arthritis and Autoimmunity Research Center, Catholic University of Daegu School of Medicine, 33, Duryugongwonro 17-gil, Nam-gu, Daegu 705-718, Republic of Korea. Email: [email protected] Received 23 March 2014; accepted 25 July 2014

systemic

lupus

erythematosus;

central nerve system damage. Lupus nephritis is a clinically apparent manifestation characterized by abnormal urinalysis such as proteinuria and/or hematuria resulting in decreased creatinine clearance and then irreversible renal failure in 10%–20% of patients,3,4 and it remains a strong predictor for increased morbidity and mortality in lupus patients in combination with toxicity of drug therapy against renal disease.2 Considering the clinical significance of lupus nephritis, it is imperative to assess renal disease activity, and early detection of renal involvement in lupus patients. b2-microglobulin (b2MG), a low-molecularweightprotein (11 kDa) associated with the lightchain molecule of major histocompatibility complex class I antigens, is primarily released from immune-related cells such as activated T- and B-lymphocytes and macrophages5 and is

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10.1177/0961203314547797

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reabsorbed in the proximal renal tubule following glomerular filtration.6 Serum b2MG levels are mainly determined by turnover and activity of lymphocytes in lymphoproliferative and autoimmune diseases.7 Increased detection of urinary b2MG has been found in tubulointerstitial renal injuries.8 Some studies have demonstrated that serum b2MG could be used as an indicator for monitoring disease activity in SLE,9–13 Sjo¨gren’s syndrome,14 and rheumatoid arthritis.15 In addition, increased b2MG levels in lupus nephritis patients have been found to be closely associated with renal disease activity.16,17 These data suggest that b2MG might be a potent marker for overall clinical activity and renal disease status in lupus patients. Currently, there arenot enough data regarding concomitant b2MG levels detected in serum and urine for the prediction of renal involvement and disease activity in SLE. In this study, we investigated the potential association of serum and urine b2MG with clinical disease activity and renal involvement in SLE.

Participants and methods

complete blood count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), serum complement (C3 and C4) levels, blood urea nitrogen (BUN), serum creatinine, spot urine protein/creatinine ratio, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and anti-doublestranded DNA (anti-dsDNA) titers. Lupus activity was classified on the basis of Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) scores: no activity (SLEDAI ¼ 0), mild activity (SLEDAI ¼ 1–5), moderate activity (SLEDAI ¼ 6–10), high activity (SLEDAI ¼ 11–19), and very high activity (SLEDAI  20).19 In this study, we arbitrarily classified disease activity into two groups, low (SLEDAI  5) and high (SLEDAI  6) groups, by SLEDAI-2K.20 Renal disease activity was measured using renal SLEDAI score of SLEDAI-2K consisting of hematuria, proteinuria, pyuria, and urinary casts. Renal SLEDAI scores range from 0 (non-active renal disease) to a maximum score of 16, because each item can give a maximum of four points. Patients with non-renal involvement (score ¼ 0) and renal involvement (scores  1) were categorized in this study on the basis of their renal SLEDAI scores. Measurement of serum and urine 2MG levels

Participants In total, 64 females with SLE, each of whom fulfilled at least four of the 1982 revised American College of Rheumatology criteria for the diagnosis of SLE,18 were consecutively enrolled in our outpatient clinic. All of the women were older than 18 years of age at the time of recruitment. Patients were excluded from this study if they had other rheumatic diseases such as rheumatoid arthritis, Sjo¨gren’s syndrome, ankylosing spondylitis, and fibromyalgia. Patients who used more than 0.5 mg/kg of prednisolone, cyclophosphamide, or mycophenolate mofetil for newly developed active lupus nephritis within the pastthree months were also excluded from this study. All patients provided informed consent at the time of enrollment in the study. The Institutional Review Board/Ethics Committee of Catholic University Medical Center approved the protocol of this study. Clinical data collection and assessment of disease activity Age and disease duration for each patient were identified through individual interview and review of medical records. Routine laboratory tests were performed at the time of enrollment and included a

Serum and urine samples from each patient were collected at the time of enrollment in the study and were stored at 70 C until analysis. Concentrations of b2MG in urine and serum were measured using an enzyme-linked immunosorbent assay with an anti-b2MG antibody (Abcam, Cambridge, United Kingdom). Statistical analysis Data were expressed as mean  standard deviation (SD) for continuous variables. Normality test for data distribution was performed by KolmogorovSmirnov and Shapiro-Wilk tests. The results of this study were used by non-parametric analysis. The Mann-Whitney U test was used to compare the mean differences between two groups (low vs. high SLEDAI groups and non-renal vs. renal involvement groups). In order to determine the correlation between serum and urine b2MG levels and other clinical and laboratory parameters, Spearman’s correlation analysis was performed. Multivariate logistic regression was used to evaluate the association of high SLEDAI or renal involvement with clinical and laboratory parameters including age, disease duration, ESR, CRP, serum b2MG, urine b2MG, Lupus

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anti-dsDNA titer, C3, and C4 and was applied to calculate odds ratios (ORs) and 95% confidence intervals (CIs). A two-sided significance level of p < 0.05 was used to evaluate statistical significance. All statistical analyses were performed using the Statistical Package for the Social Sciences, version 13.0 (SPSS Inc, Chicago, IL, USA).

Results Basic characteristics of enrolled patients Demographic, clinical, and laboratory characteristics are illustrated in Table 1. In total, 64 female patients with a mean age of 41.2 years (SD 10.9) and a mean disease duration of 7.5 years (SD 4.9) were recruited for this study. On the basis of their renal SLEDAI score, enrolled participants were categorized into non-renal (n ¼ 36) and renal (n ¼ 28) involvement groups. The mean levels of hemoglobin, ESR, urine protein/Cr, SLEDAI, and renal SLEDAI score were different between the two groups (p < 0.05 for each parameter) (Table 1). Other parameters were statistically

similar between the two groups (p > 0.05 for each parameter, Table 2). Among 64 patients, 36 were classified as low SLEDAI, and 28 patients were classified as high SLEDAI. Hemoglobin, platelet counts, ESR, C3, C4, urine protein/Cr, renal SLEDAI, and extra-renal SLEDAI scores were significantly different between the high and low SLEDAI groups (p < 0.05 of each parameter). Other parameters such as age, disease duration, and anti-dsDNA titer were not statistically different between the two groups (p > 0.05 for each parameter). The mean level of serum b2MG was 1786.7  575.2 pg/ml for the low SLEDAI group and 2173.3  1591.3 pg/ml for the high SLEDAI group, which was not significantly different (p ¼ 0.579) (Figure 1(a)). However, the difference in urine b2MG level was significant between the low SLEDAI (66.5  54.9 pg/ml) and high SLEDAI (511.1  1121.2 pg/ml) groups (p ¼ 0.001) (Figure 1(b)). A difference in serum b2MG levels was noted between the non-renal and renal involvement groups (1745.3  560.7 vs. 2226.5  1582.9, respectively, p ¼ 0.228) (Figure 1(c)). In contrast, the mean level of urine b2MG was significantly different

Table 1 Baseline characteristics of enrolled participants

Parameters

Total (n ¼ 64)

Non-renal involvement (n ¼ 36)

Age (year) Disease duration (year) WBC (103/l) Hemoglobin (g/dl) Platelet (103/l) AST (IU/l) ALT (IU/l) BUN (mg/dl) Cr (mg/dl) ESR (mm/hour) CRP (mg/l) Urine protein/Cr (mg/mg) Anti-dsDNA (IU/ml) C3 (mg/dl) C4 (mg/dl) SLEDAI Renal SLEDAI Extrarenal SLEDAI

41.2  10.9 7.5  4.9 5.5  2.1 12.1  1.6 222.3  80.0 25.3  16.0 24.6  19.5 14.7  7.9 0.9  0.9 22.0  15.0 1.9  3.4 15.9  84.0 48.5  91.7 92.1  45.4 20.2  14.7 5.1  3.9 2.3  2.9 2.9  2.0

42.0  9.9 7.6  4.9 5.6  2.1 12.6  1.3 230.8  60.3 24.5  9.4 25.5  20.4 13.5  4.7 0.7  0.2 18.0  13.6 1.4  2.1 0.2  0.1 32.3  48.3 99.6  55.1 20.2  9.1 2.5  2.0 0.0  0.0 2.5  2.0

Renal involvement (n ¼ 28)

p valuesa

40.1  12.3 7.4  5.0 5.4  2.0 11.5  1.7 211.3  99.9 26.4  22.0 23.5  18.5 16.2  10.6 1.1  1.4 27.5  15.4 2.4  4.7 36.1  125.3 69.3  125.7 82.5  26.7 20.2  20.0 8.5  3.0 5.1  2.1 3.3  1.9

0.405 0.775 0.705 0.010 0.261 0.860 0.734 0.766 0.713 0.010 0.491