TACI-Fc binds specifically to 293T cells. (A) Recombinant proteins containing (TACI-Fc; lanes 1 and 2) or lacking (0-Fc; lanes 3 and 4) the extracellular domain of TACI fused to the Fc region of human IgG1 were purified as described in “Materials and methods.” Proteins were analyzed by SDS-PAGE and detected by Coomassie blue staining. (B) Cells were examined for expression of putative TACI ligands by incubation with TACI-Fc, the control protein 0-Fc, or an antibody to the CD45 molecule as a positive control. Antibody and Fc binding were detected by flow cytometry. Numerous cell lines tested, including CEM cells, did not bind TACI-Fc, whereas (C) 293T cells showed significant binding to TACI-Fc but not to 0-Fc.