Affinity of different GpL-18D1-IgG1 fusion proteins for FcγRs. (A) FACS analysis of HEK293 cells transiently transfected with expression plasmids encoding CD16, CD32A, CD32B and CD64. (B) HEK293 cells transiently expressing the indicated FcγRs and mock transfected control HEK293 cells were aliquoted and incubated with increasing concentrations of GpL(CT-HC)-18D1-IgG1, GpL(CT-LC)-18D1-IgG1 and GpL(NT-LC)-18D1-IgG1 (1.5 h, 37°C). After removal of the unbound GpL-18D1-IgG1 molecules cell-associated luciferase activity was determined and specific binding values were calculated by correcting the total binding values obtained from the FcγR transfectants for the unspecific binding values obtained from the mock transfected cells. The KD values were calculated with the “nonlinear regression to a one-site specific binding curve” function of the GraphPad Prism5 software. One representative binding experiment is shown for each of the various interactions. (C) Analysis of the interaction of 18D1 with CD64 by heterologous competition binding experiments with 7.5 ng/ml GpL(CT-HC)-18D1-IgG1, GpL(CT-LC)-18D1-IgG1 and GpL(NT-LC)-18D1-IgG1. Average values and statistics of all experiments are summarized in Table 3. RLU, relative light units.