Anti-Ki/anti-PA28γ autoantibodies contribute to the HEp-2 indirect immunofluorescence nuclear speckled pattern

Objectives: Antinuclear antibodies (ANAs) are associated with several autoimmune diseases. Indirect immunofluorescence (IIF) on human epithelial type 2 (HEp-2) cells is the golden standard for ANA detection in the clinic. In case of a positive HEp-2 IIF test result, follow-up tests are done to determine autoantibody specificity. For a fraction of the HEp-2 IIF-positive samples, the nature of the autoantigens remains uncharacterized. Our objective was to characterize autoantigens in such samples.

Methods: To characterize autoantigens in an unbiased way, we combined protein immunoprecipitation with liquid chromatography (LC) tandem mass spectrometry (MS/MS) sequencing.

Results: Using such approach we detected the Ki antigen, also referred to as PA28γ, in the immunoprecipitate of serum samples of three individuals with an autoimmune disease. The HEp-2 nuclear speckled IIF fluorescent signal of all three serum samples was abolished after pre-absorption of the serum with recombinant Ki antigen, confirming that autoantibodies against Ki underlie the HEp-2 IIF signal.

Conclusions: Our data suggest that anti-Ki autoantibodies can underlie a nuclear speckled HEp-2 IIF pattern.