DISTINCT MODES OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 PROVIRAL LATENCY REVEALED BY SUPERINFECTION OF NONPRODUCTIVELY INFECTED CELL-LINES WITH RECOMBINANT LUCIFERASE-ENCODING VIRUSES

To study the basis of cellular latency of human immunodeficiency virus (HIV), we have used a recombinant luciferase-encoding HIV (HXB-Luc) t o superinfect nonproductively HIV-1-infected human leukemic cell lines . HXB-Luc contains the Photinus pyralis luciferase gene in place of th e nef gene and provides a highly sensitive, simple assay for HIV infec tion and expression. To circumvent any superinfection block in latentl y infected cells, we also generated viruses pseudotyped with murine le ukemia virus amphotropic envelope (HXB-Lucampho). The parental uninfec ted lines, U937 and A3.01, from which the latently infected cell lines U1 and ACH-2, respectively, were derived could he readily infected wi th pseudotyped or nonpseudotyped reporter viruses. However, superinfec tion of U1 cells with either HXB-Luc or HXB-Lucampho resulted in only low levels of luciferase activity. Like the endogenous provirus, HXB-L uc provirus could be efficiently activated by phorbol ester treatment of HXB-Lucampho-superinfected U1 cells. In contrast, superinfection of ACH-3 cells resulted in active expression of the secondarily introduc ed virus even in unstimulated cells and luciferase production higher t han in the parental cell line A3.01. Thus, the proviral latency in U1 cells appears to result from a defect in the cellular environment (a t rans effect), whereas the latency in ACH-2 is specific to the integrat ed provirus and is probably a cis effect due to the site of integratio n. These results demonstrate distinct modes of proviral latency in the se two cell Line models and may have implications in our understanding of the regulation and significance of cellular latency in HIV infecti on.