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Human immunodeficiency virus type 1 (HIV-1) carries virus-encoded and host-derived proteins. Recent advances in the functional characterization of host molecules inserted into mature virus particles have revealed that HIV-1 biology is influenced by the acquisition of host cell membrane components. The CD28/B7 receptor/ligand system is considered one of the fundamental elements of the normal immune response. Two major cell types that harbor HIV-1 in vivo, i.e., monocytes/macrophages and CD4+ T cells, express the costimulatory molecules CD80 (B7.1) and CD86 (B7.2). We investigated whether CD80 and CD86 are efficiently acquired by HIV-1, and if so, whether these host-encoded molecules can contribute to the virus life cycle. Here we provide the first evidence that the insertion of CD80 and CD86 into HIV-1 increases virus infectivity by facilitating the attachment and entry process due to interactions with their two natural ligands, CD28 and CTLA-4. Moreover, we demonstrate that NF-kappaB is induced by CD80- and CD86-bearing virions when they are combined with the engagement of the T-cell receptor/CD3 complex, an event that is inhibited upon surface expression of CTLA-4. Finally, both CD80 and CD86 were found to be efficiently incorporated into R5- and X4-tropic field strains of HIV-1 expanded in cytokine-treated macrophages. Thus, besides direct interactions between the virus envelope glycoproteins and cell surface constituents, such as CD4 and some specific chemokine coreceptors, HIV-1 may attach to target cells via interactions between cell-derived molecules incorporated into virions and their natural ligands. These findings support the theory that HIV-1-associated host proteins alter virus-host dynamics.

The immunosuppressive macrolide rapamycin is used in humans to prevent graft rejection. This drug acts by selectively repressing the translation of proteins that are encoded by an mRNA bearing a 5'-polypyrimidine tract (e.g., ribosomal proteins, elongation factors). The human immunodeficiency virus type 1 (HIV-1) carries a polypyrimidine motif that is located within the tat exon 2. Treatment of human T lymphoid cells with rapamycin resulted in a marked diminution of HIV-1 transcription when infection was performed with luciferase reporter T-tropic and macrophage-tropic viruses. Replication of fully infectious HIV-1 particles was abolished by rapamycin treatment. The rapamycin-mediated inhibitory effect on HIV-1 production was reversed by FK506. The anti-HIV-1 effect of rapamycin was also seen in primary human cells (i.e., peripheral blood lymphocytes) from different healthy donors. Rapamycin was shown to diminish basal HIV-1 long terminal repeat gene expression, and the observed effect of rapamycin on HIV-1 replication seems to be independent of the virus-specific transactivating Tat protein. A constitutive beta-actin promoter-based reporter gene vector was unaffected by rapamycin treatment. Kinetic virus infection studies and exposure to reporter viruses pseudotyped with heterologous envelope proteins (i.e., amphotropic murine leukemia virus and vesicular stomatitis virus G) suggested that rapamycin is primarily affecting the life cycle of HIV-1 at a transcriptional level. Northern blot analysis confirmed that this compound is selectively targeting HIV-1 mRNA synthesis.

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.

Using virions harvested from 293T cells stably expressing either low or high levels of surface ICAM-1, we determined that the number of virus-embedded host ICAM-1 proteins is positively influenced by the expression level of ICAM-1 on virus producer cells. Moreover, the increase in virion-bound host cell membrane ICAM-1 led to a concomitant enhancement of virus infectivity when a T-cell-tropic strain of human immunodeficiency virus type 1 (HIV-1) was used. The phenomenon was also seen when primary human cells were infected with virions pseudotyped with the envelope protein from a macrophage-tropic HIV-1 isolate, thus ruling out any envelope-specific effect. We also observed that target cells treated with NKI-L16, an anti-LFA-1 antibody known to increase the affinity of LFA-1 for ICAM-1, were markedly more susceptible to infection with HIV-1 particles bearing on their surfaces large numbers of host-derived ICAM-1 proteins. Given that cellular activation of leukocytes is known to modify the conformational state of LFA-1 and induce ICAM-1 surface expression, it is tempting to speculate that activation of virus-infected cells will lead to the production of HIV-1 particles bearing more host ICAM-1 on their surfaces and that such progeny virions will preferentially infect and replicate more efficiently in activated cells which are prevalent in lymphoid organs.