Normal blood plasma (A) with HIV-1_SF162 suspended at a concentration similar to that found in the infected donors (10⁴–10⁵ copies per milliliter) and (B) plasma from an HIV-1–infected donor. Because of the low concentration, virus was first concentrated from 40-ml samples by capturing it on fluorescence-labeled VRC01-MNPs followed by separation on magnetic columns. Then, captured virions were stained with anti–gp120 2G12 antibodies and once again separated from free antibodies on magnetic columns. Note that as a result of the concentration procedure, these preparations contain substantially more MNPs than those analyzed in Figure 2. (A) VRC01-MNPs suspended in control virus-free plasma and incubated with anti-gp120 antibodies (left panel); MNP-captured HIV-1 from virus-spiked normal plasma incubated with anti–gp120 isotype control antibody (center panel) or with anti–gp120 2G12 antibodies (right panel). (B) HIV-1 captured from the plasma of an infected individual with VRC01-MNPs was incubated with either gp120 isotype control (left panel) or visualized with anti–gp120 2G12 antibodies (right panel). (C and D) HIV-1 captured from the plasma of an infected individual was stained for HLA-DR (C) or LFA-1 (D). Left panels: isotype controls. Right panels: specific staining of HIV-1 captured by VRC01-MNPs with anti–HLA-DR (C) and anti–LFA-1 (D) antibodies. On each plot, the fraction of events in their respective gates is expressed as a percentage of the total events in the plot.