Ribosomal P autoantibodies are present before SLE onset and are directed against non-C-terminal peptides

Abstract

Autoantibodies to ribosomal P (ribo P) are found in 15-30% of systemic lupus erythematosus (SLE) patients and are highly specific for SLE. The goal of this study is to assess the temporal association of anti-ribosomal P (anti-P) responses with SLE disease onset, as well as to characterize select humoral ribo P epitopes targeted in early, pre-diagnostic SLE samples. Patients with stored serial serum samples available prior to SLE diagnosis were identified from a military cohort. Each sample was tested for antibodies against ribo P utilizing standard C terminus ribo P enzyme-linked immunosorbent assays (ELISA) and a solid phase, bead-based assay with affinity-purified ribo P proteins. In this study, antibodies to ribo P were more common in African American SLE patients (p = 0.026), and anti-P-positive patients comprised a group with more measured autoantibody specificities than did other SLE patients (3.5 vs 2.2, p < 0.05). Antibodies against ribo P were present on average 1.7 years before SLE diagnosis and were detected an average of 1.08 years earlier in pre-diagnostic SLE samples using affinity-purified whole protein rather than C-terminal peptide alone (p = 0.0019). Furthermore, 61% of anti-P-positive patients initially had antibodies to aa 99-113, a known ribosomal P0 antigenic target, at a time point when no antibodies to the clinically used C terminus were detected. Our findings provide evidence that antibodies against ribosomal P frequently develop before clinical SLE diagnosis and are more broadly reactive than previously thought by targeting regions outside of the C terminus.

Figure 1

Anti-ribosomal P antibodies commonly target non-C-terminal epitopes. (a) Venn diagram depicting the percentage of patients positive during the pre-clinical and early disease period for antibodies to affinity purified ribosomal P only, C-terminal peptide only, and both. Numbers shown represent absolute number of patients within each group. (b) Twenty patients positive for antibodies to ribosomal P protein but not to the C-terminal peptide had antibodies directed against Epitope 3. Four of these twenty patients later developed antibodies to the C-terminal region while the patients were under observation. (c) A representative patient is shown with the ELISA data shown in the line graphs (corresponding the the Y axis on the left) and the BioRad data shown in the bar graph (corresponding to the Y axis on the right). Dx denotes SLE diagnosis.

Figure 2

SLE patients are positive by affinity-purified ribosomal P earlier than by the C-terminus. In patients with samples positive for anti-ribosomal P by either the Bioplex assay or C-terminal peptide ELISA, the Bioplex assay showed a positive result an average of 1.08 years prior to a positive test by C-terminus ELISA (p = 0.0019 by Wilcoxon Signed Rank Test).