GROWTH OF MACROPHAGE-TROPIC AND PRIMARY HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 (HIV-1) ISOLATES IN A UNIQUE CD4(+) T-CELL CLONE (PM1) - FAILURE TO DOWN-REGULATE CD4 AND TO INTERFERE WITH CELL-LINE-TROPIC HIV-1

Human immunodeficiency virus type 1 (HIV-1) isolates derived directly from clinical samples are usually unable to grow in cytokine-independent continuous cell lines, thus hindering the study of their biological features and their sensitivity to humoral and cellular protective immunity. To overcome these limitations, we have derived from the Hut78 T-cell line a CD4(+) clone (PM1) characterized by a unique susceptibility to a wide range of HIV-1 isolates, including primary and biologically pure macrophage (M phi)-tropic isolates (e.g., HIV-1(BaL), which are unable to infect other human T- or promonocytic cell lines, Both primary and M phi-tropic HIV-1 establish persistent infection in PM1, with sustained levels of virus replication for prolonged periods, Experiments with chimeric viruses containing envelope fragments of HIV-1(BaL) inserted into the genetic framework of HXB2, a molecular clone derived from the cell-line-tropic isolate HIV-1(IIIB), showed the third hypervariable domain (V3) of gp 120 to be a critical determinant of the cell line tropism of HIV-1. Nevertheless, the V3 loop of HIV-(BaL), was not sufficient to confer on the chimeras a bona fide M phi tropism. The biological characteristics of HIV-1(BaL) and of a primary isolate (HIV-1(573)) were investigated by using the PM1 clone. Infection of PM1 by HIV-1(BaL) was critically dependent on the CD4 receptor, as shown by competition experiments with an anti-CD4 monoclonal antibody (OKT4a) or with soluble CD4. However, the amount of soluble CD4 required for inhibition of HIV-1(BaL) was approximately 100-fold higher than for HIV-1(IIIB), suggesting that the affinity of HIV-1(BaL) for CD4 is significantly lower, Infection of PM1 with either HIV-1(BaL) or HIV-1(573) failed to induce downregulation of surface CD4 expression and syncytium formation. Analogous results were obtained with a chimeric virus (HXB2[BaLPvuII-BamHI]) encompassing a large portion of gp120 and gp41 of HIV-1(BaL) indicating that the env genes contain critical determinants for CD4 downregulation and syncytium formation. Consistent with the lack of CD4 downregulation, persistent infection of PM1 by HIV-1(BaL) or HIV-1(573) failed to interfere with HIV-1(IIIB) superinfection, as revealed by the expression of a type-specific V3 loop epitope (M77) and by the induction of extensive syncytium formation. This lack of interference suggests that a direct viral interaction may occur in vivo between biologically diverse HIV-1 strains. The PM1 clone represents a reproducible and efficient cellular system for the in vitro propagation of primary and M phi-tropic isolates of HIV-1 and may therefore provide a precious tool for studies aimed at developing broadly active vaccines against HIV-1.