1 Tampere University Hospital, Tampere and 2 University of Tampere Medical School, Tampere, Finland
Correspondence to: M Pertovaara, Department of Internal Medicine, Section of Rheumatology, Tampere University Hospital, PO Box 2000, FIN-33521 Tampere, Finland. E-mail: marja.pertovaara{at}pshp.fi
SIR, Interstitial nephritis is the most common renal lesion encountered in primary Sjögren's syndrome (pSS). Clinically it presents most frequently as distal renal tubular acidosis (dRTA). We have investigated the occurrence of renal involvement in 78 patients with pSS and found mild proteinuria (0.150.42 g/24 h) in up to 44% and dRTA in 33% of the 55 patients investigated by the ammonium chloride loading test [1]. Increased urinary excretion rates of albumin (20 µg/min), alpha-1 microglobulin (
7.0 µg/min) or IgG (
5.0 µg/min) were detected in 12, 12 and 14% of the patients respectively [1].
Matrix metalloproteinases (MMP) are enzymes located in the interstitium or cell membrane, which degrade the structural proteins of the tissue and cell membranes. MMP type IV collagenase-gelatinase is known to be involved in the pathogenesis of SS [2]. The local activity of MMP-9 has been found to be elevated in the saliva [3] and tear fluid [4] in patients with pSS and abnormal expression has also been found in their labial salivary glands [5]. Interleukin-6 (IL-6) stimulates MMP-9 production in human lymphoma tissue specimens [6], and elevated IL-6 concentrations have been found in plasma [7], saliva [8] and tear fluid [9] in patients with SS. The levels of urinary IL-6 in patients with pSS have not been investigated.
To investigate the possible role of MMP-9 and IL-6 in the pathogenesis of renal manifestations in pSS, we measured urinary MMP-9 and IL-6 concentrations and calculated timed overnight excretions of them in 74 patients with pSS (71 female, 3 male; mean age 59 ± 12 yr, mean duration of disease 9 ± 4 yr). Commercially available enzyme-linked immunosorbent assays (Quantikine human total MMP-9 ELISA kit, RD systems, MN, USA and Pelikine human IL-6 ELISA kit, CLB, Amsterdam) were used in measuring urinary MMP-9 and IL-6 concentrations. The optical density in individual wells was determined with a Multiscan Biochromatic 348 spectrophotometer (Labsystems, Helsinki, Finland). The detection limit of the MMP-9 assay was 0.156 ng/ml and of the IL-6 assay 0.6 pg/ml. The results were analysed in relation to the presence of dRTA and to the level of 24-h urinary excretion of protein as well as to albumin (cU-alb), IgG (cU-IgG) and alpha-1 microglobulin (cU-1m) measured from samples obtained in timed overnight urine collection and analysed by nephelometry as previously described [1].
Timed overnight urinary excretions of MMP-9 and IL-6 did not correlate with age, duration of disease or creatinine clearance. Urinary MMP-9 excretion correlated significantly with 24-h urinary protein excretion (r = 0.380, P = 0.001; Spearman rank correlation coefficient) (Fig. 1a), and with cU-alb (r = 0.281, P = 0.016) and cU-IgG (r = 0.316, P = 0.006) excretion, but not with excretion of cU-1m. Urinary IL-6 excretion correlated significantly with 24-h urinary protein excretion (r = 0.352, P = 0.002) (Fig. 1b), cU-alb (r = 0.441, P<0.0001), cU-IgG (r = 0.367, P = 0.001) and cU-
1m excretions (0.313, P = 0.007). Moreover, MMP-9 and IL-6 excretions correlated significantly with each other (r = 0.369, P = 0.001). The mean excretion rate (± S.D.) of MMP-9 was 0.36 ± 0.98 ng/min and of IL-6 1.87 ± 0.53 pg/min. There was a tendency to higher excretion of MMP-9 (0.57 ± 1.32 ng/min vs 0.23 ± 0.54 ng/min, P = 0.140, MannWhitney U-test) and IL-6 (3.27 ± 7.31 pg/min vs 0.81 ± 2.52 pg/min, P = 0.076) in pSS patients with proteinuria (n = 32) than in those with normal urinary protein excretion (n = 42). Similarly, urinary excretions of MMP-9 (0.68 ± 1.67 ng/min vs 0.23 ± 0.52 ng/min, P = 0.302) and IL-6 (2.50 ± 5.55 pg/min vs 0.69 ± 1.66 pg/min, P = 0.217) were higher in pSS patients with latent dRTA (n = 16) than in those with normal renal acidification capacity (n = 36), though the differences were not statistically significant.
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This study was supported by the Medical Research Fund of Tampere University Hospital and the Maud Kuistilas Foundation.
The authors have declared no conflicts of interest.
References