Rpp25 is a major target of autoantibodies to the Th/To complex as measured by a novel chemiluminescent assay
1 INOVA Diagnostics, INC. 9900 Old Grove Rd, San Diego, CA 92131-1638, USA
2 Rheumatology and Clinical Immunology, Humanitas Clinical and Research Center, via Manzoni 56, 20089 Rozzano, and BIOMETRA department, University of Milan, Via Vanvitelli 32, 20129, Milan, Italy
3 Faculty of Medicine:HRB414, University of Calgary, 3330 Hospital Dr NW, Calgary, AB, T2N 4N1, Canada
4 Center for Rheumatic Diseases Dr. Gürtler, Kaiser-Friedrich-Str.8, D-41460 Neuss, Germany
5 Department of Oral Biology, University of Florida, 1395 Center Dr, Gainesville, FL 32610-0424, USA
6 Division of Rheumatology and Clinical Immunology, Department of Medicine, and Pathology, Immunology and Laboratory Medicine, University of Florida, 1600 SW Archer Rd, Gainesville, FL 32610-0221, USA
Arthritis Research & Therapy 2013, 15:R50 doi:10.1186/ar4210Published: 12 April 2013
Autoantibodies to the Th/To antigen have been described in systemic sclerosis (SSc) and several proteins of the macromolecular Th/To complex have been reported to react with anti-Th/To antibodies. However, anti-Th/To has not been clinically utilized due to unavailability of commercial tests. The objective of the present study is to evaluate the newly developed ELISA and chemiluminescent immunoassay (CLIA) to measure autoantibodies to Rpp25 (a component of the Th/To complex) using immunoprecipitation (IP) as the reference method.
The first cohort consisted of 123 SSc patients including 7 anti-Th/To positive samples confirmed by IP. Additional seven anti-Th/To positive samples from non-SSc patients were also tested. For evaluation of the QUANTA Flash Rpp25 CLIA (research use only), 8 anti-Th/To IP positives, a cohort of 70 unselected SSc patients and sera from various disease controls (n = 357) and random healthy individuals (n = 10) were studied.
Anti-Rpp25 antibodies determined by ELISA were found in 11/14 anti-Th/To IP positive but only in 1/156 (0.6%) negative samples resulting in a positive percent agreement of 78.6% (95% confidence interval [CI] 49.2, 95.3%) and a negative percent agreement of 99.4% (95% CI 96.4, 100.0%). To verify the results using a second method, 53 samples were tested by ELISA and CLIA for anti-Rpp25 reactivity and the results were highly correlated (rho = 0.71, 95% CI 0.56, 0.81; P < 0.0001). To define the cutoff of the CLIA, anti-Th/To IP positive and negative sera were tested using the anti-Rpp25 CLIA. At the cutoff selected by receiver operating characteristic (ROC) analysis 8/8 (100.0%) of the anti-Th/To positive sera but only 2/367 (0.5%) of the controls were positive for anti-Rpp25 antibodies. The positive and negative percent agreements were 100.0% (95% CI 63.1, 100.0%) and 99.5% (95% CI 98.0, 99.9%), respectively. In the disease cohorts 2/70 (2.9%) of the SSc patients were positive for anti-Rpp25 antibodies compared to 2/367 (0.5%) of the controls (P = 0.032). ROC analysis showed discrimination between SSc patients and controls with an area under the curve value of 0.732 (95% CI 0.655, 0.809).
Rpp25 is a major target of autoantibodies to the Th/To autoantigen complex. Further studies are needed to evaluate the clinical utility of the new assays.