Human autoantibodies to diacyl‐phosphatidylethanolamine recognize a specific set of discrete cytoplasmic domains

Summary The aim of this study was to characterize a novel human autoantibody–autoantigen system represented as cytoplasmic discrete speckles (CDS) in indirect immunofluorescence (IIF). A distinct CDS IIF pattern represented by 3–20 discrete speckles dispersed throughout the cytoplasm was identified among other cytoplasmic speckled IIF patterns. The cytoplasmic domains labelled by human anti‐CDS‐1 antibodies did not co‐localize with endosome/lysosome markers EEA1 and LAMP‐2, but showed partial co‐localization with glycine–tryptophan bodies (GWB). CDS‐1 sera did not react with several cellular extracts in immunoblotting and did not immunoprecipitate recombinant GW182 or EEA1 proteins. The typical CDS‐1 IIF labelling pattern was abolished after delipidation of HEp‐2 cells. Moreover, CDS‐1 sera reacted strongly with a lipid component co‐migrating with phosphatidylethanolamine (PE) in high performance thin‐layer chromatography (HPTLC)‐immunostaining of HEp‐2 cell total lipid extracts. The CDS‐1 major molecular targets were established by electrospray ionization–mass spectrometry (ESI‐MS), HPTLC‐immunostaining and chemiluminescent enzyme‐linked immunosorbent assay as diacyl‐PE species, containing preferentially a cis ‐C18 : 1 fatty acid chain at C‐2 of the glycerol moiety, namely 1,2‐ cis ‐C18 : 1‐PE and 1‐C16 : 0‐2‐ cis ‐C18 : 1‐PE. The clinical association of CDS‐1 sera included a variety of systemic and organ‐specific autoimmune diseases but they were also observed in patients with no evidence of autoimmune disease.