Antibodies directed against ribosomal P proteins cross‐react with phospholipids

Summary Anti‐ribosomal P protein (anti‐P) antibodies are marker antibodies in systemic lupus erythematosus (SLE). Their association with psychiatric or neurological manifestations has been proposed, but remains controversial. Anti‐phospholipid antibodies are the hallmark of a syndrome that may comprise a number of neurological manifestations. Thus, anti‐P and anti‐phospholipid antibodies have both been associated with central nervous system involvement and their co‐existence in the same sera was reported. We verified the ability of purified anti‐P antibodies to bind different phospholipids and phospholipid‐binding proteins in solid‐phase assays. Anti‐P antibodies from five of eight patients bound cardiolipin (CL) when saturated with fetal calf serum (FCS); in three cases anti‐CL antibodies were also detected in the flow‐through. No anti‐P eluate, nor any corresponding flow‐through, bound β 2 ‐glycoprotein I alone or prothrombin. Moreover, no anti‐P eluate bound CL when the plates were blocked with bovine serum albumin in the absence of FCS. Anti‐P antibodies with anti‐CL activity bound both ssDNA and dsDNA and also nucleosomes in three patients. Our data indicate a great heterogeneity of anti‐P antibodies that appear to be overlapped partially with the other autoantibody populations detected frequently in SLE.