In brief, PBMC samples of vaccine recipients in RV144 (ALVAC) and HVTN204 (Ad5 vector) were first stained with CFSE, a fluorescent dye used to track T-cell proliferation, and then stimulated with the corresponding empty vector for three days to induce the expansion of vector-reactive CD4 T cells, followed by infection with either CCR5-tropic (R5; US-1 strain) or CXCR4-tropic (X4; 92/UG/029 strain) HIV. Here, CD4 is linked to infection.