UL33 and UL78 block HIV infection in THP-1 cells. (A-B) THP-1 cells expressing YFP, Flag-UL33-YFP, or Flag-UL78-YFP were infected 48 hours after transfection with the CCR5-tropic JR-CSF strain (A) or the CXCR4-tropic LAI strain (B). The percentage of HIV-infected cells in YFP, Flag-UL33-YFP or Flag-UL78-YFP positive and negative cell populations was assessed by flow cytometry using anti-HIV p24 antibodies. (C) Flow cytometry analysis of cell-surface CD4 by surface staining with anti-CD4 in THP-1 cells expressing mock, Flag-UL33-YFP or Flag-UL78-YFP. (D-E) Lysates prepared from THP-1 cells expressing the indicated proteins were immunoprecipitated with either anti-HA or anti-CXCR4 antibodies, resolved by SDS-PAGE and immunoblotted with anti-CD4 antibodies. Expression controls of transfected proteins are shown in Figure 2A through D (same lysates have been used for both experiments). Experiments were at least repeated twice with similar results. Mock = pcDNA3 expression vector. Statistical differences compared with mock nucleofected cells were assessed using the Student t test (***P < .001).