Electron tomography visualization of HIV-1 virions trapped by fusion inhibitors to host cells in infected tissues

HIV-1 delivers its genetic material to infect a cell after fusion of the viral and host cell membranes, which takes place after the viral envelope (Env) binds host receptor and co-receptor proteins. Binding of host receptor CD4 to Env results in conformational changes that allow interaction with a host co-receptor (CCR5 or CXCR4). Further conformational rearrangements result in an elongated pre-hairpin intermediate structure in which Env is anchored to the viral membrane by its transmembrane region and to the host cell membrane by its fusion peptide. Although budding virions can be readily imaged by electron tomography (ET) of HIV-1-infected tissues and cultured cells, virions that are fusing (attached to host cells via pre-hairpin intermediates) are not normally visualized, perhaps because the process of membrane fusion is too fast to capture by ET. To image virions during fusion, we used fusion inhibitors to prevent downstream conformational changes in Env that lead to membrane fusion, thereby trapping HIV-1 virions linked to target cells by pre-hairpin intermediates. ET of HIV-1 pseudovirions bound to CD4⁺/CCR5⁺ TZM-bl cells revealed presumptive pre-hairpin intermediates as 2-4 narrow spokes linking a virion to the cell surface. To extend these results to a more physiological setting, we used ET to image tissues and organs derived from humanized bone marrow/liver/thymus mice infected with HIV-1 and then treated with CPT31, a high-affinity D-peptide fusion inhibitor linked to cholesterol. Trapped HIV-1 virions were found in all tissues studied (small intestine, mesenteric lymph nodes, spleen, and bone marrow), and spokes representing pre-hairpin intermediates linking trapped virions to cell surfaces were similar in structure and number to those seen in the previous pseudovirus and cultured cell ET study.IMPORTANCETrapped and untrapped HIV-1 virions, both mature and immature, were distinguished by localizing spokes via 3D tomographic reconstructions of HIV-1 infected and fusion-inhibitor-treated tissues of humanized mice. The findings of trapped HIV-1 virions in all tissues examined demonstrate a wide distribution of the CPT31 inhibitor, a desirable property for a potential therapeutic. In addition, the presence of virions trapped by spokes, particularly in vascular endothelial cells, demonstrates that the fusion inhibitors can be used as markers for potential HIV-1-target cells within tissues, facilitating the mapping of HIV-1 target cells within the complex cellular milieu of infected tissues.