New Insights into the Functionality of a Virion-Anchored Host Cell Membrane Protein: CD28 Versus HIV Type 1

It is now well established that the HIV type 1 (HIV-1) incorporates a vast array of host-encoded molecules in its envelope during the budding process. Interestingly, it was demonstrated that the attachment process is accentuated by supplementary interactions between virion-anchored host molecules and their cognate ligands. Such an enhancement of the viral attachment process was found to result in an increase of infectivity for both T and macrophage-tropic strains of HIV-1. Given that previous work indicates that HIV-1 is budding at the site of cell-to-cell contact, a location rich in the costimulatory CD28 glycoprotein, we investigated whether CD28 could be efficiently acquired by HIV-1. We have been able to generate progeny viruses bearing or not bearing on their surfaces host-derived CD28 using our previously described transient transfection and expression system. The physical presence of CD28 was found to markedly increase virus infectivity in a CD28/B7-dependent manner following infection of two human lymphoid cell lines expressing high levels of surface B7-1/B7-2, two natural ligands of CD28. The physiological significance of CD28 incorporation was provided by the observation that an anti-CD28 Ab decreased replication in primary human mononuclear cells of clinical isolates of HIV-1 propagated in such cells. A virus precipitation assay revealed that M-, T-, and dual-tropic clinical strains of HIV-1 produced in primary human mononuclear cells do indeed incorporate CD28. These results show for the first time that HIV-1 can incorporate CD28 and the acquisition of this specific host surface glycoprotein modulates the virus life cycle.