Gal-9–mediated up-regulation of p21 reduces the susceptibility of CD4⁺ T cells to HIV-1 infection. (A) Percentages of CD4⁺p24⁺ cells in activated CD4⁺ T cells in the presence of control or p21-specific siRNA; then stimulated with Gal-9 (1 μg/mL) 2 hours before HIV-1 infection with R-5-tropic (HIV-1_JR-CSF) or (B) X-4 tropic (HIV-1_LAI) isolate. (C) Representative example of dot plots from activated CD4⁺ T cells infected in the presence of p21-specific or control siRNA with R5-tropic HIV-1 isolate or (D) X4-tropic HIV-1 isolate in the presence (0.5 μg/mL or 1 μg/mL) or absence of Gal-9. Viral infection was measured by viral p24 antigen staining on CD4⁺ T cells using flow cytometry. Plots are representative of independent experiments performed on 3 different donors for each viral isolate. Significance was tested using paired t test.