3D visualization of HIV transfer at the virological synapse between dendritic cells and T cells

Abstract

The efficiency of HIV infection is greatly enhanced when the virus is delivered at conjugates between CD4 + T cells and virus-bearing antigen-presenting cells such as macrophages or dendritic cells via specialized structures known as virological synapses. Using ion abrasion SEM, electron tomography, and superresolution light microscopy, we have analyzed the spatial architecture of cell-cell contacts and distribution of HIV virions at virological synapses formed between mature dendritic cells and T cells. We demonstrate the striking envelopment of T cells by sheet-like membrane extensions derived from mature dendritic cells, resulting in a shielded region for formation of virological synapses. Within the synapse, filopodial extensions emanating from CD4 + T cells make contact with HIV virions sequestered deep within a 3D network of surface-accessible compartments in the dendritic cell. Viruses are detected at the membrane surfaces of both dendritic cells and T cells, but virions are not released passively at the synapse; instead, virus transfer requires the engagement of T-cell CD4 receptors. The relative seclusion of T cells from the extracellular milieu, the burial of the site of HIV transfer, and the receptor-dependent initiation of virion transfer by T cells highlight unique aspects of cell-cell HIV transmission.

HIV-1 activates Cdc42 and induces membrane extensions in immature dendritic cells to facilitate cell-to-cell virus propagation

“…19 However, in contrast to the findings with the synapses formed by mature DCs, the membrane extensions in the synapses formed by immature DCs do not wrap around the T cells ( Figure 4K-L). Thus, although the membrane extensions from both mature and immature DCs are sheet-like in nature, the major difference in the corresponding synapses is that there is no visible encasement of the T cells by these extensions in the case of the infectious synapses formed by the immature DCs.…”

Architecture and Regulation of the HIV-1 Assembly and Holding Compartment in Macrophages

“…If the membrane sheets are dynamic and of a large surface area, this may help to explain why, in our snapshots of cell ultrastructure, we have observed only one distinct cell surface-connected compartment per cell. Large membrane sheets associated with virions have also been observed in HIV-1-pulsed dendritic cells (DC), suggesting that this may be a general feature of APC (8), although it should be noted that in this study the DC were not infected, and therefore only trapped virus was visualized. We observed both RR-accessible structures close to the cell surface and RR-inaccessible membrane structures within the cell.…”

HIV Entry and Envelope Glycoprotein-mediated Fusion

“…In an interesting twist, Melikyan and co-workers (62) found that hemifusion occurred at the plasma membrane but that pore expansion occurred following endocytosis in the endosome. Although the endocytic pathway was revealed by observation on the micrometer scale (62,63), it was not supported by examination on the nanoscale in primary lymphocytes (64). Inhibitors of dynaminand clathrin-dependent endocytosis (62,65) have been invoked to argue for HIV entry via endocytosis, but such inhibitors have multifaceted effects on cellular processes, including cell-cell fusion (66).…”