FIG 3.

Macrophage-tropic SIVmac239 N173Q mediates CD4-independent cell-cell transmission and enhanced cell-cell fusion with CCR5-expressing cells compared to non-macrophage-tropic SIVmac239. (A) Expression of rhesus CD4/CCR5 on Cf2-luc target cells used in both cell-cell transmission and cell-cell fusion assays. Percentages and mean fluorescent intensities (MFI) of CD4⁺ or CCR5⁺ cells are shown. Unstained cells and CD4-negative cells, included as negative controls, yielded signals similar to those of the isotype controls (data not shown). (B) VSV-G-pseudotyped SIVmac239 or SIVmac251 with or without N173 was transmitted at similar levels from infected Cf2 donor cells to TZM-BL target cells expressing human CD4/CCR5 or Cf2-luc target cells expressing high levels of rhesus CD4/CCR5. RLU, relative light units. (C and D) Mutation of N173 in SIVmac239 enhances CD4-independent cell-cell transmission (C) and Env-mediated fusion activity (D). Results are representative of 2 to 3 independent experiments. Error bars represent standard deviations for means of duplicate samples. An asterisk indicates a significant difference by Student's t test (P < 0.05).