(A) HIV-1 virions with incorporated α4β7 were selectively captured by plate-immobilized MAdCAM-1 and transferred to susceptible target cells. Serial dilutions of HIV-1 THRO.18 viral stocks either positive (α4β7+) or negative (α4β7−) for α4β7 were incubated with plastic-coated rhMAdCAM-1 (MAdCAM+, green lines) or an irrelevant integrin ligand (PECAM+, pink lines), washed extensively to remove unbound virus, and overlaid with target cells (TZM-bl), as illustrated in the accompanying cartoon. (B) HIV-1 virions with incorporated α4β7 are selectively captured by MAdCAM-expressing cells, which mediate trans-infection of bystander target cells. Serial dilutions of HIV-1 THRO.18 viral stocks either positive (α4β7+) or negative (α4β7−) for α4β7 were incubated with MAdCAM-1-expressing cells (MAdCAM+, green lines) or with untransfected control cells (MAdCAM-, pink lines), washed extensively to remove unbound virus, and cocultured with TZM-bl cells. (C) HIV-1 virions with incorporated α4β7 infect MAdCAM-expressing susceptible cells more efficiently than α4β7− virions. Serial dilutions of HIV-1 BaL pseudoviruses either positive (α4β7+) or negative (α4β7−) for α4β7 were incubated with TZM-bl cells transfected to express MAdCAM-1 (MAdCAM+, green) or with untransfected control TZM-bl (MAdCAM-, pink). In all the experiments, HIV-1 infection was assessed by measuring activation of the luciferase reporter gene.