Abstract
The indirect immunofluorescence assay (IIFA) on HEp-2 cells has been widely used for screening anti-nuclear antibodies (ANA) that are associated with systemic autoimmune rheumatic diseases (SARD). Sera containing ANA display multiple distinct fluorescence patterns on HEp-2 cells. Among them, a dense fine speckled (DFS) pattern caused by anti-DFS70 antibodies has been reported to have higher prevalence in healthy individuals than in patients with SARD. This DFS pattern is often difficult to distinguish amongst other SARD-associated ANA patterns, in particular a mixed homogeneous and speckled pattern. Furthermore, a strong DFS pattern can mask other SARD-associated patterns. Hence, we developed a novel immunoprecipitation method using magnetic beads to remove anti-DFS70 antibodies in serum prior to running IIFA. We also aimed to confirm the presence of anti-DFS70 and to uncover any SARD-associated ANA patterns masked by a strong DFS pattern. The sera used in our study were from 70 individuals who had routine ANA screen, of which 35 sera had an isolated DFS pattern with monospecific anti-DFS70 antibodies confirmed by a complementary assay, and 35 were control sera without a DFS pattern. An immunoprecipitation method using magnetic beads coated with recombinant human full length DFS70 protein was developed. The performance of this new method was evaluated in comparison to an immunoadsorption method using the same DFS70 protein. Our newly developed immunoprecipitation method demonstrated excellent sensitivity (91.4%) and specificity (100%) in confirming the DFS pattern associated with anti-DFS70 in sera. Additionally, our method was able to remove anti-DFS70 and uncover SARD-associated ANA patterns masked by a strong DFS pattern. It also showed a clearer background on IIFA than that of the immunoadsorption method. The novel magnetic bead-based immunoprecipitation method demonstrated satisfactory performance in removing anti-DFS70 without interfering with the detection of other antibodies. It can be easily integrated with IIFA to confirm anti-DFS70 associated DFS pattern. Additionally, it can simultaneously unmask other ANA patterns, which cannot be achieved by a conventional protocol that requires a complementary anti-DFS70 specific assay to be performed. Therefore, the novel method provides a more effective and accurate solution for ANA screening.