CXCR4 and HIV-1 infection: To determine the IFN-I sensitivity of transmitted viruses, we developed a high-throughput reporter assay using the IFN-I-sensitive U87 human glioblastoma cell line modified to express CXCR4 and CCR5 and stably transduced with a lentivirus vector to express secreted nanoluciferase in response to HIV-1 infection and Tat expression.