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

Human immunodeficiency virus type 1 (HIV-1) isolates derived directly from clinical samples are usually unable to grow in cytokine-independe nt continuous cell lines, thus hindering the study of their biological features and their sensitivity to humoral and cellular protective imm unity. To overcome these limitations, we have derived from the Hut78 T -cell line a CD4(+) clone (PM1) characterized by a unique susceptibili ty to a wide range of HIV-1 isolates, including primary and biological ly pure macrophage (M phi)-tropic isolates (e.g., HIV-1(BaL), which ar e unable to infect other human T- or promonocytic cell lines, Both pri mary and M phi-tropic HIV-1 establish persistent infection in PM1, wit h sustained levels of virus replication for prolonged periods, Experim ents with chimeric viruses containing envelope fragments of HIV-1(BaL) inserted into the genetic framework of HXB2, a molecular clone derive d from the cell-line-tropic isolate HIV-1(IIIB), showed the third hype rvariable domain (V3) of gp 120 to be a critical determinant of the ce ll 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. Th e biological characteristics of HIV-1(BaL) and of a primary isolate (H IV-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 b y 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 HI V-1(IIIB), suggesting that the affinity of HIV-1(BaL) for CD4 is signi ficantly lower, Infection of PM1 with either HIV-1(BaL) or HIV-1(573) failed to induce downregulation of surface CD4 expression and syncytiu m formation. Analogous results were obtained with a chimeric virus (HX B2[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 l ack of CD4 downregulation, persistent infection of PM1 by HIV-1(BaL) o r HIV-1(573) failed to interfere with HIV-1(IIIB) superinfection, as r evealed by the expression of a type-specific V3 loop epitope (M77) and by the induction of extensive syncytium formation. This lack of inter ference suggests that a direct viral interaction may occur in vivo bet ween biologically diverse HIV-1 strains. The PM1 clone represents a re producible and efficient cellular system for the in vitro propagation of primary and M phi-tropic isolates of HIV-1 and may therefore provid e a precious tool for studies aimed at developing broadly active vacci nes against HIV-1.