Antivimentin (anti-Vim) can immunoprecipitate Sa antigen from placental extract. Vimentin was immunoprecipitated from semipurified placental extract with monoclonal antibody RV202 (lanes 3). Immunoprecipitated vimentin was stained by anti-modified citrulline (anti-MC) antibodies (upper panel), anti-Sa serum (middle panel), or polyclonal antivimentin (anti-Vim) antibodies (lower panel), indicating that Sa is citrullinated vimentin. An immunoprecipitation (IP) with an isotype-matched monoclonal control antibody 4G3 (lanes 4) served as a negative control. Lanes 1 contain human recombinant vimentin citrullinated in vitro (cit-Vim) (50 ng) as a positive control. Lanes 2 show 10% input. Molecular weight markers are indicated on the left.