Viral transfer from MDDCs to CD4⁺ lymphocytes. Immature or mature MDDCs were exposed to AT-2 SIV CP-mac, and cell-free virus was washed away. AT-2 SIV–bearing DCs were mixed with autologous CD4⁺ T cells, centrifuged, and incubated to allow conjugate formation. The cells were then incubated at 37°C for 1 or 2 hours (5-7 donors) before the cells were adhered to alcian blue–treated slides. Staining for SIV envelope (green) and cellular nuclei (blue) was performed as outlined in Figure 3. DCs were identified by staining with PE-conjugated anti–HLA-DR (red). Arrows show transfer of SIV envelope between DCs and T cells, and arrowheads denote cell-to-cell transmission between T cells. Asterisks show SIV envelope on T cells, often remote from the DC–T-cell synapse. Virus staining at the DC–T-cell synapses was seen in all donors after incubation at 37°C, but the extended virus-positive “bridges” between DCs and T cells were observed more infrequently. Murine and human Ig controls showed no staining. Original magnification, × 100.