Syndecan-1 and syndecan-4 can also bind to and activate TACI. (A) TAg Jurkat cells were electroporated with either empty vector (broken line) or one of the indicated syndecan expression plasmids (solid line). Binding of TACI-Fc to syndecan-expressing cells was determined by flow cytometry. TACI-Fc binds to syndecans-1, -2, and -4 similarly. (B) TAg Jurkat cells were transiently transfected with empty vector plus NFAT reporter plasmid or a TACI cDNA with reporter plasmid, or with syndecan cDNAs. The TACI/NFAT cells were cocultured with either no stimulants, empty vector, or syndecan-expressing cells. Transfection of Jurkat cells with TACI/NFAT alone results in basal activation.40 Syndecan-1 and syndecan-4 activated NFAT-SEAP reporter similarly to syndecan-2. Expression of the syndecans in TAg Jurkat cells was verified by flow cytometry using heparan sulfate antibodies (bottom panel). (C) Heparin does not interfere with BAFF binding to TACI. TACI-Fc was incubated alone or with BAFF in the absence or presence of saturating levels of heparin (10 μg/mL). TACI-Fc was purified from solution by passage over protein A-Sepharose, and adsorbed BAFF eluted with SDS, and then detected by Western blotting. Recombinant BAFF migrates as 2 bands on SDS-PAGE, as indicated by arrows.
