Signicance of anti-ribonucleoprotein (RNP) antibody and anti-Smith (anti-Sm) antibody among patients with SLE or MCTD

Background The signicance of anti-ribonucleoprotein (RNP) antibody and anti-Smith (anti-Sm) antibody remains unclear in patients with systemic lupus erythematosus (SLE) or mixed connective tissue disease (MCTD). Methods Thirty patients were retrospectively enrolled in this study. They were all positive for antinuclear antibody (ANA) and anti-RNP antibody, were diagnosed with SLE(n = 25) or MCTD(n = 5), and underwent renal biopsy at our hospital between January 1990 and December 2014. These 30 patients were classied into 4 groups based on their anti-dsDNA and anti-Sm status. Renal histology was classied into 3 patterns, including pure subepithelial membranous nephropathy (MN), mesangial and/or subendothelial lesions (MES), and MN combined with MES.

Renal involvement is one of the major complications of mixed connective tissue disease (MCTD), and a variety of renal pathologies have been described in 10-50% of MCTD patients (3). In patients with MCTD, renal biopsy usually reveals membranous glomerulonephritis or mesangial glomerulonephritis, although focal or diffuse proliferative glomerulonephritis can sometimes be detected (3,4).
Pulmonary arterial hypertension is the leading cause of death in these patients, and that studies of inception cohorts with MCTD have described a cumulative incidence of renal disease of only 6% at 10 years (5,6). Besides the renal biopsy results described in MCTD, autopsy studies in these patients have shown intimal thickening of renal arteries resembling scleroderma kidney ndings (7).
In this study, we clarify the signi cance of anti-ribonucleoprotein (RNP) antibody and anti-Smith (anti-Sm) antibody in patients with systemic lupus erythematosus (SLE) or mixed connective tissue disease (MCTD).

Methods
We retrospectively reviewed patients who were positive for anti-RNP antibody, were diagnosed as having SLE or MCTD, and underwent renal biopsy at our nephrology center between January 1990 and December 2014. All patients ful lled the 1997 American College of Rheumatology (ACR) criteria for classi cation of SLE (8,9) or the 2004 Japanese Ministry of Health, Labor and Welfare criteria for classi cation of MCTD, which are the revised Kasukawa criteria (7). The criteria require and all of the following three major conditions to be ful lled: (i) at least one of three ndings (Raynaud's phenomenon, swollen hands, or pulmonary hypertension), (ii) anti-RNP antibody positivity, and (iii) mixed ndings of at least two of connective tissue diseases (SLE, systemic sclerosis, or polymyositis). Clinical characteristics, laboratory data, and histological ndings were obtained from the medical records of the patients. All clinical data were measured at the time of renal biopsy. Anti-dsDNA antibody was de ned as negative when the titer was ≤ 15 IU/ml. Proteinuria was measured by 24-hour urine collection.
Renal biopsy samples were processed by using standard techniques for light microscopy (LM), immuno uorescence microscopy (IF), and electron microscopy (EM) according to the published methods (10). Three pathologists at our hospital assessed the renal biopsy ndings. Renal disease was divided into three patterns: (1) membranous nephropathy (MN) was de ned as nephropathy with subepithelial deposits represented by spikes and a bubble-like appearance (Fig. 1a), (2)
In Comparison between groups A and B showed that anti-Sm antibody (-) status was closely related with normocomplementemia and MN in patients who were anti-RNP antibody (+) and anti-ds-DNA antibody (-), while anti-Sm antibody (+) status was closely related with hypocomplementemia and MES.
Comparison between groups C and D showed that anti-Sm antibody (+) status was closely related to hypocomplementemia, but did not have any signi cant in uence on renal histology.
Among 10 patients in group A, all 5 patients with MCTD were categorized as pure MN, while the 5 patients with SLE were categorized as MN + MES or pure MES. In groups B, C, and D, all 20 patients were classi ed as having SLE.

Discussion
In SLE, various autoantibodies can be positive, including ANA, anti-dsDNA antibody, anti-Sm antibody, anti-RNP antibody, anti-Ro/SS-A antibody, and anti-La/SS-B antibody. Among them, anti-ds DNA antibody is most commonly associated with lupus nephritis (LN), and the antibody titer is usually correlated with disease activity (11). Anti-Sm antibody has also been reported to show a close relation with renal disease, and this association is stronger when anti-Sm antibody is positive together with anti-dsDNA antibody (12). Anti-RNP antibody is detected in 25-47% of SLE patients, with high anti-RNP antibody titers being diagnostic of MCTD. Both anti-Sm and anti-RNP antibodies are more important for diagnosis of SLE than for monitoring disease progression, although anti-RNP antibody shows a higher prevalence in patients with Raynaud's phenomenon and is associated with milder renal involvement, while anti-Sm antibody re ects the severity and of renal involvement and renal disease activity (2). Kitridou et al. performed renal biopsy in 10 MCTD patients with renal disease, and reported that six patients (60%) had membranous nephropathy, two (20%) had mesangial glomerulonephritis, one (10%) had local sclerosing glomerulonephritis, and one (10%) had membranoproliferative nephritis (3). In addition, Bennet et al.
performed renal biopsy in four of 20 patients with MCTD, and found that three patients had membranous glomerulonephritis (13) .
In SLE, overproduction of type I interferons (IFNs) and autoantibodies targeting nucleic acids has been reported (14). Kirou et al. suggested that activation of the IFN pathway de nes a subgroup of SLE patients whose have more severe disease, including more severe renal disease, and increased disease activity, which is re ected by activation of complement and the presence of autoantibodies targeting RNA binding proteins (RBP), i.e., antibodies speci c for Ro, U1 RNP, and Sm (15). Lövgren et al. reported that the induction of IFN production by SLE serum was associated with the presence of anti-RBP antibodies with or without anti-DNA antibodies, but not with anti-DNA antibodies alone (16) .
Two distinct mechanisms for the formation of immune deposits in glomerulonephritis have been suggested, which are passive trapping of immune complexes associated with mesangial or subendothelial deposits, or in situ formation of immune complexes at subepithelial, subendothelial, and mesangial sites (17). The latter mechanism might be considered in MCTD patients with concomitant membranous nephropathy because they predominantly have subepithelial deposits. However, the precise immunological processes underlying membranous nephropathy in MCTD patients are still unclear.
In conclusion, the present study indicated that 5 patients out

The Limitations Of This Study
This retrospective study is made on only 30 patients, but these patients are even rarer than the two rare diseases (SLE and MTCD). The true evaluation is a future problem. This study cannot assess the association of anti-RNP with anything, as all patients had anti-RNP, therefore there is no comparison group without anti-RNP.