There are some papers showing that syndecan 4 binds to DC-Hils, resulting in the suppression of T cells. Syndecan-1 is a marker of plasma cells and shedded form of syndecan -1 is increased in autoimmune disorders (but real function is not known).
Dear Prof Uede, I have not first-hand experience in this field. It seems the greatest impact was endowed by Kiyoshi Ariizumi from Department of Dermatology, UT Southwestern Medical Center, Texas.