Background: Cell-to-cell virus transmission is one of the mechanisms of viral spread and involves the transfer of HIV particles from infected to permissive cells in the absence of effective cell-to-cell fusion. Here, we describe a similar process, in which HIV can be passively transferred to resting or even nonpermissive cells.

Methods: Cells producing infectious or non-infectious HIV particles (from X4 and R5 isolates) were presented to unstimulated PBMC or CD4 cells. The transfer of HIV antigens (p24) was assessed by flow cytometry and fluorescence microscopy.

Results: Unstimulated PBMC incubated with HIV-producing cells showed high levels of p24 staining, which were abolished by CD4 blockade, but not by CXCR4 or CCR5 antagonists or by gp41 inhibitors. This suggests that p24 staining was not the consequence of a fusion-dependent infectious process, but was rather associated to a transfer of HIV particles from infected to uninfected cells. HIV-infected cells transferred p24 mainly to CD4 T-cells, which showed 75 ±8 % of p24⁺ cells, while CD8 T-cells, B-cells or NK-cells showed lower transfer levels (7 ±1, 4 ±1 and 5 ±1% of p24⁺ cells, respectively). The non-infectious transfer of HIV was confirmed by the efficient transfer of p24 from HIV-1 BaL-infected cells to CCR5 negative primary CD4 T-cells (73 ±6 % of p24⁺ cells) and by using 8E5 cells, which produce RT defective HIV particles, but retain the ability to transfer HIV to CD4 T-cells (49 ±5 % of p24⁺ cells). Transfer of HIV was not observed when cell-free virus preparations were incubated with primary cells, suggesting the requirement of cell-to-cell contacts. Moreover, intracellular p24 staining could be observed in unstimulated primary CD4 T-cells even 5 days after presentation by infected cells suggesting some degree of stability of transferred virus. The transfer of HIV p24 antigen was concomitant to the disappearance of CD4 molecules from the cell surface as probed with a panel of mAbs, suggesting a process of CD4 dependent endocytosis, which was consistent with intense patched staining near the plasmatic membrane observed by fluorescence microscopy analysis of p24⁺ CD4 T-cells.

Conclusions: Productively infected cells transfer HIV particles to resting cells by a CD4 dependent and coreceptor independent mechanism. HIV loaded cells may become passive HIV carriers, favoring HIV spread either by releasing loaded HIV particles or by undergoing a productive infectious process after cellular stimulation.