HIV-1 is localized on membrane extensions at the DC-CD4⁺ T cell infectious synapse. Transmission electron microscopy of contacts between immature DC and T cells. (A-C) Projection electron microscopy images of a 100-nm-thick section from fixed, plastic-embedded cocultures of T cells and immature DCs either in the absence of added HIV-1 (A), after exposure to HIV-1 for 1 hour (B), and after treatment with the cdc42 inhibitor secramine A, followed by exposure to HIV-1 (C). (D-E) A 5-nm tomographic slice from a 3D image of a 175-nm-thick section prepared from fixed, plastic-embedded cocultures of T cells and DCs exposed to HIV-1 (as in B) showing viruses riding on the membrane extensions from the DCs. (F) Schematic rendering of tomographic slice in panel E highlighting contact between T cells (green) and immature DCs (blue) in the presence of HIV (red). Scale bars, original magnifications: (A-C) 2 μm; (D-E) 1 μm. (G) Representative image of membrane extensions on DCs with HIV-1 near extension tips. Bar represents 1 μm. (H) Quantification of fused HIV-1 viral particles in target CD4⁺ T lymphocytes after nucleofection of Cdc42 mutants in DCs. All values are normalized to a 100% value assigned to the nontreated condition. Arrows indicate HIV-1 viral particles on membrane extensions on DCs. Data are mean ±SD of 3 independent experiments. *P < .05 (Student t test). Bonferroni test with an α-error value of 5% has been applied to all panels with multiple comparisons.