Expression of retinoic acid-inducible gene-I in lupus nephritis

Sir, Retinoic acid-inducible gene-I (RIG-I) is a member of the DExH-box family of proteins, which is involved in inflammatory reactions related to RNA metabolisms, although the bioactivities of RIG-I still remain to be clarified in detail [1]. Previous studies have reported that expression of RIG-I is induced in various inflammatory diseases, such as viral infections, leukaemia and bladder carcinoma [1]. However, there are no reports on the expression of RIG-I in the kidneys of patients with lupus nephritis. In the present study, we examined the expression of RIG-I in kidney tissue specimens obtained from cases of human lupus nephritis, and evaluated the correlation between its expression and the histological activity of the renal disease in these patients. From January 2000 to August 2006, lupus nephritis was diagnosed in 15 children and adolescents at Hirosaki University Hospital. All met the International Society of Nephrology/Renal Pathology Society (ISN/RPS) criteria for the histological diagnosis of lupus nephritis. Tissue cylinders obtained by renal biopsy were divided into three portions and processed for routine light-microscopic, electron-microscopic and immunofluorescence examinations. After the renal tissue samples for the immunofluorescence study were embedded in optimal cutting temperature (OCT), routine studies to determine the deposition of IgG, IgA, IgM, C3 and C1q were performed, and the samples were stored at 308C until further use. Of these OCT-embedded kidney tissue specimens, 10 specimens obtained from eight patients, in good condition, were selected for this study. Repeat renal biopsy was performed in two of the eight patients (cases 3 and 6) who exhibited changes of proliferative lupus nephritis at presentation. Eight kidney tissue specimens obtained from patients with minimal-change (MC) disease were used as controls. Each of the kidney tissue specimens from the patients with lupus nephritis was examined by light microscopy in a blinded fashion by one of the examiners and the histological findings were scored. The activity index was evaluated semi-quantitatively using the scoring system described by Austin et al. [2]. The OCT-embedded tissue specimens were cut into 5 mm-thick sections in a cryostat, briefly fixed in cold acetone and then air-dried; the slides were then washed in PBS immediately before the immunohistochemical procedure. After blocking by incubating with 1% goat serum, the slides were incubated with anti-RIG-I antibody (1 : 1000), as described by Imaizumi et al. [1]. The samples were then incubated with fluorescein isothiocyanate (FITC)-conjugated secondary antibodies (Sigma, Saint Louis, USA). According to ISN/RPS criteria for light-microscopic histological classification, two patients (cases 2 and 8) were classified as class II nephritis, four (cases 1, 3, 4 and 6) as class IV nephritis, and two (cases 5 and 7) as class V nephritis. The mean activity index was 6.3, and two patients (cases 3 and 6) had a very high activity index of more than 10 (Table 1). Significant histological improvement was observed at the second renal biopsy in both cases 3 and 6. According to the ISN/RPS criteria, case 3 was classified as class II and case 6 as class III. Indeed, the activity index was also significantly decreased in both the cases (to three in case 3 T a b le 1 . S u m m a ry o f th e p a th o lo g ic a l a n d im m u n o h is to ch em ic a l fi n d in g s in th e ei g h t p a ti en ts w it h lu p u s n ep h ri ti s