CD48 and HIV-1 infection: The activating ligands included NTB-A, which binds NTB-A on the NK cell surface and leads to activation and secretion of interferon-γ, CD48, which binds 2B4, ULBP-1 and -2, ligands that bind NKG2D and that have been demonstrated to be upregulated in the setting of HIV-1 infection by the viral protein vpr (32), and CD155, a NK ligand that binds DNAM-1 on NK cells and induces activation (33).