β2 integrin adhesion complexes maintain the integrity of HIV-1 assembly compartments in primary macrophages

In human monocyte-derived macrophages (MDM), human immunodeficiency virus type 1 (HIV-1) assembly takes place primarily on complex intracellular plasma membrane domains connected to the cell surface by closely apposed membrane sheets or narrow channels. Some of the membranes associated with these compartments are decorated by thick (≈30 nm), electron-dense, cytoplasmic coats. Here we show by immunolabelling of ultrathin cryosections that the β2 integrin CD18, together with the αM and αX integrins (CD11b and CD11c), is clustered at these coated domains, and that the coats themselves contain the cytoskeletal linker proteins talin, vinculin and paxillin that connect the integrin complexes to the actin cytoskeleton. Intracellular plasma membrane-connected compartments (IPMC) with CD18-containing focal adhesion-like coats are also present in uninfected MDM. These compartments become more prominent as the cells mature in tissue culture and their appearance correlates with increased expression of CD18, CD11b/c and paxillin. Depletion of CD18 by RNA interference leads to parallel down-regulation of CD11b and CD11c, as well as of paxillin, and the disappearance of the adhesion-like coats. In addition, CD18 knockdown alters the appearance of virus-containing IPMC in HIV-infected MDM, indicating that the β2 integrin/focal adhesion-like coat structures are involved in the organization of these compartments.