Antibody binding to gp140 constructs in ELISA experiments. The antigenic profiles of the gp140 constructs NL4-3 (X4), ADA (R5) and the hybrid NL4-3/ADA were evaluated in ELISA experiments with selected monoclonal antibodies (mAbs) against the gp120 V3 loop (447-52D and D19) [36, 37] and the gp41 region (Md-1) . Functional exposure of coreceptor binding epitopes upon CD4 activation was monitored with strictly CD4 dependent mAb CG10 . Where applicable, nonlinear regression fits are shown in the diagrams instead of connected data points. Derived KD values (PRISM software) are listed in the table of Additional file 5. The purified gp140 timers (NL4-3, ADA and NL4-3/ADA) were compared with gp120 monomers (X4 HXBc2, R5 Bal, R5X4 89.6; Immune Technology) regarding their reactivity with CD4bs mAb b13 (5 nM) and trimer specific gp41 mAb Md-1 (1:2000 dilution). The data is representative for at least three independent replicate experiments with each data point of the binding curves determined in triplicates with indicated mAb concentrations. Error bars indicate the standard deviation.