Neutralization and virion capture by 33C6 mAbs. (a) Neutralization of the challenge virus, SHIV-1157ipEL-p, by 33C6-IgM and 33C6-IgG1. (b) Capture of physical virus (pVirus) and infectious virus (iVirus) particles by 33C6-IgM and 33C6-IgG1. Data are representative of two independent experiments. Error bars, mean ± SEM. VRC01-IgG1 was used as positive control, while Fm-6-IgG1 and IgM isotype control were used as negative controls. Because a different secondary anti-IgM capture antibody was required for 33C6-IgM to capture cell-free virus using Protein G micro-beads, virion capture by 33C6-IgM could not be compared directly to that of 33C6-IgG1.