Siglec-1 mediates HIV-1 capture by IFNα-treated myeloid cells. A. Representative profiles of Siglec-1 staining in distinct myeloid cells cultured with or without 1000 U/ml of IFNα and assessed by FACS. Staining of matched-isotype control is also shown. The mean fold increase in fluorescence after IFNα treatment of cells derived from three donors is shown in red numbers. B. Mean number of Siglec-1 antibody binding sites per cell displayed by different myeloid cells exposed to 1000 U/ml of IFNα for 48 h and assessed by quantitative FACS analysis. Data show mean values and SEM from four experiments including cells from 12 donors. C. Comparative binding of HIV-1NL4–3 to different myeloid cells previously exposed to 1000 U/ml of IFNα for 48 h. Cells were cultured with HIV-1NL4–3 for 4 h at 4°C, washed and lysed to measure p24Gag by ELISA. Data show mean values and SEM from two experiments including cells from six donors. D. Relative binding of HIV-1NL4–3 by different IFNα-treated myeloid cells that had been pre-incubated with 10 μg/ml of the indicated mAbs before HIV-1 exposure for 4 h at 4°C as described in C. To compare the effect of the mAbs in different myeloid cells, values were normalized to the level of HIV-1 binding by mock-treated cells (set to 100%). Data show mean values and SEM from two experiments including cells from six donors. Statistical differences were assessed with a paired t test in B and C, and with a one sample t-test in D.