The observation that antibody binding to CD30 is influenced by Trx1 suggested a redox-dependent conformational change within the CD30 ectodomain. To test whether Trx1-mediated reduction of CD30 influences binding of CD30 to its ligand (CD30L), we analyzed the interaction between CD30 and recombinant soluble CD30L (sCD30L) on the cell surface by flow cytometry. A brief incubation of CD30 þ Hodgkin's lymphoma HDLM-2 cells with wild-type Trx1 led to a substantial loss in CD30L binding to the cell surface ( Figure 6A) . A similar result was obtained for the large granular lymphoma Figure S4) . The same effect was evident in the absence of an exogenously added reducing system (Supplementary Figure S5) , thus demonstrating that under the given conditions Trx1-substrate interactions are not limited by oxidative inactivation. As shown by fluorescence microscopy, sCD30L binds to the surface of CD30-transfected HeLa cells and colocalizes with its receptor ( Figure 6B , upper row). Treatment of HeLa cells with Trx1(CCAAA) ( Figure 6B , middle row) but not a redox-inactive mutant ( Figure 6B , lower row) strongly interferes with CD30L binding and colocalization.