Background/Purpose:   The natural history of systemic lupus erythematosus (SLE) is felt to evolve from a state of normal immunity to serologic autoimmunity and then to pathologic autoimmunity.  This process may take several years, and can include a time when only one or two clinical features are present.  We have used the term incomplete lupus erythematosus (ILE) to describe such individuals.  This study was undertaken to search for biomarkers that characterize ILE, specifically autoantibodies that differentiate these subjects from healthy controls (HC) with moderate to high levels of antinuclear antibodies.

Methods:   A library of 7-mer peptoids (polymers of N-substituted glycine) was synthesized on Tentagel beads using 10 different monomer amines, leading to a theoretical complexity of 10⁷different species.  This bead-coupled library was then depleted of compounds that bound to pooled IgG from HC as well as HC with ANA but no evidence of SLE.  The bead library was probed with pooled IgG from individuals with ILE.  Peptoids from the positively selected beads were purified, identified by tandem MS, re-synthesized and coupled to ELISA plates.  Individual serum samples from subjects with ILE, SLE, RA and controls were then tested for peptoid reactivity.

Results: Screening 375,000 compounds yielded 100 beads that bound ILE IgG.  12 of these were chosen for further analysis and 8 had appropriate full-length peptoid structures.  The peptoids with the greatest discrimination a pool of HC sera, designated ILE-2, and ILE-7, were chosen for ELISA analyses.  Using individual samples from the training serum pool, the mean ELISA reactivity for 38 SLE patients was significantly different than all 40 HC sera (2.98 vs. 0.638, p < 0.0001).  The area under the ROC curve was 0.96 with a sensitivity of 89% and specificity of 97%.  The mean values for individual ILE patients using the ILE-2 ELISA were also significantly different from HC (1.81 vs. 0.492, p = 0.0025).  A validation set of 40 HC, 18 ILE, and 64 SLE subjects was characterized using the mean + 2SD value for HC.  Even with a more stringent cut-off, ILE patients were still significantly different than controls (p = 0.0007), although a proportion of ILE patients fell into the ILE negative range.  ILE-2 and ILE-7 reactivity was found at a low level in the subset of ANA+ RA patients, but not in ANA- RA patients or other disease controls.

Conclusion:   A novel, un-biased chemical library can serve as practical mimics for unknown autoantigens.  The ligands can be used immediately in analytic assays.  The peptoids identified in this screen have excellent ability to identify subjects with lupus-related autoimmunity and discriminate them from ANA-positive control subjects.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/using-a-library-of-synthetic-autoantigen-mimics-to-discover-biomarkers-of-systemic-lupus-erythematosus/