Purified IgM confers ABO serum sensitivity to virions. (A) Inhibition of serum inactivation by excess soluble blood group antigens. SF-2 virus grown in PBMCs from an A donor was assayed for serum sensitivity in O and A sera as before, in the presence or absence of 10 μg soluble A trisaccharide. (B) IgM from ABO sera differentially sensitizes virus to guinea pig complement. SF-2 derived from an A donor was incubated with varying concentrations of IgM purified from ABO-defined donor sera in the presence of active guinea pig complement (C′) or a heat-inactivated control. Diamonds (♦), squares (▪), and triangles (▴) represent viral infections in the presence of IgM from O, A, or B sera, respectively. HIV-1 NL4-3 raised in 293T cells transfected with αGal(1-3)galactosyltransferase was treated similarly as a control for antibody-dependent serum sensitivity. Both viruses were plated on NP2CD4X4 cells and processed as before.