Only monospecific anti-DFS70 antibodies aid in the exclusion of antinuclear antibody associated rheumatic diseases: an Italian experience

Background: The dense fine speckled (DFS) is one of the most common patterns that can be observed as a result of the anti-nuclear antibodies (ANA) test on HEp-2 cells and is mostly caused by antibodies to DFS70 as the main antigenic target. As was recently demonstrated, isolated anti-DFS70 positivity can be used as an aid in the exclusion of ANA associated rheumatic diseases (AARD) due to the opportunity to better interpret unexplained positive IIF ANA results. Methods: Our study included 333 subjects with AARD, 51 undifferentiated connective tissue disease (UCTD) patients, 235 disease controls and 149 healthy blood donors from an Italian cohort. All samples were tested for anti-DFS70 and anti-ENA antibodies using QUANTA Flash assays (Inova Diagnostics, San Diego, CA, USA). Results: No differences in the prevalence of anti-DFS70 antibodies were seen among AARD, non-AARD and UCTD (2.1% [7/333] vs. 2.3% [9/384] vs. 5.9% [3/51], respectively; p-value= 0.188). AARD patients positive for anti-DFS70 antibodies showed in all cases an accompanying anti- ENA specificity. In contrast, monospecific anti-DFS70 antibodies showed a significantly different distribution with a clear trend across the main groups (AARD vs. non-AARD vs. UCTD: [0/7] vs. 22% [2/9] vs. 100% [3/3], p= 0.007). Anti-DFS70 antibody levels among AARD, non-AARD and UCTD patients were not significantly different (p= 0.094). Within the anti-DFS70 antibody positive cases, AARD cohort showed a higher variability (median [min-max]: 3.2 [3.2 450.8] CU) compared to non-AARD (median [min max]: 3.2 [3.2- 75.7] CU) and UCTD patients (median [min max]: 3.2 [3.2 59.0] CU). Conclusions: Our preliminary data showed a similar frequency of anti-DFS70 antibodies in AARD, UCTD and non-AARD cohorts. Monospecificity of anti-DFS70 antibodies but not their mere presence is the key element in the diagnostic algorithm. Mono-specific anti-DFS70 antibodies might be a helpful biomarker to discriminate individuals with AARD from non-AARD presenting with a positive ANA.