HIV-1 Buds Predominantly at the Plasma Membrane of Primary Human Macrophages

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Abstract
HIV-1 assembly and release are believed to occur at the plasma membrane in most host cells with the exception of primary macrophages, for which exclusive budding at late endosomes has been reported. Here, we applied a novel ultrastructural approach to assess HIV-1 budding in primary macrophages in an immunomarker-independent manner. Infected macrophages were fed with BSA-gold and stained with the membrane-impermeant dye ruthenium red to identify endosomes and the plasma membrane, respectively. Virus-filled vacuolar structures with a seemingly intracellular localization displayed intense staining with ruthenium red, but lacked endocytosed BSA-gold, defining them as plasma membrane. Moreover, HIV budding profiles were virtually excluded from gold-filled endosomes while frequently being detected on ruthenium red–positive membranes. The composition of cellular marker proteins incorporated into HIV-1 supported a plasma membrane–derived origin of the viral envelope. Thus, contrary to current opinion, the plasma membrane is the primary site of HIV-1 budding also in infected macrophages. Macrophages are one of the major target cells for HIV-1 infection and play an important role in viral pathogenesis. Previous studies indicated that the pathway of HIV-1 particle morphogenesis is distinct in primary human macrophages, and this has been suggested to play a role in viral persistence. Early reports indicated that HIV-1 accumulates within apparently intracellular vacuolar structures, which were later identified as being of late endosomal origin. Endosomes were therefore suggested to comprise the budding and storage compartment for HIV-1 in primary human macrophages, from which infectious virus can be released in a regulated manner. In the present study, we show that HIV-1 budding occurs predominantly at the plasma membrane also in primary human macrophages. Using electron microscopy, we observed that the cell surface of macrophages displays an unexpectedly complex morphology with many protrusions and deep invaginations. HIV-1 budding occurs primarily at these invaginations that are clearly connected to the cell surface and do not belong to the endocytic compartment. Mature virus particles can remain trapped within such invaginations giving the appearance of an intracellular budding compartment. These results suggest a general pathway of HIV-1 morphogenesis with the plasma membrane as viral budding site.