Human immunodeficiency virus type 1 RNA outside the primary encapsidation and dimer linkage region affects RNA dimer stability in vivo.

To characterize the cis-acting determinants that function in RNA dimer formation and maintenance, we examined the stability of RNA dimers isolated from virus particles containing mutations in the encapsidation region of human immunodeficiency virus type 1 (HIV-1). The genomic RNAs of all mutants containing lesions in elements required for in vitro dimerization exhibited thermal stability similar to that of wild-type (WT) HIV-1. These data indicate that the eventual formation of stable dimeric RNA in vivo is not absolutely dependent on the elements that promote dimer formation in vitro. Surprisingly, mutants that lacked a large segment of the middle portion of the genome, outside the likely primary dimer linkage region, formed RNA dimers that were measurably more stable than WT. In addition, the insertion of one or multiple copies of a foreign gene, which resulted in a series of vectors that approached RNA length similar to that of WT RNA, still exhibited augmented dimer stability. These results suggest that there are regions in the HIV-1 genome outside the primary dimer initiation and dimer linkage regions that can negatively affect dimer stability.

Functional analysis of simian immunodeficiency virus SIVAGM long terminal repeat.

We have previously shown that long terminal repeats (LTRs) derived from various isolates of SIVAGM share a unique functional property. In the absence of viral Tat, all SIVAGM LTRs act as much more efficient promoters than any of the other LTRs derived from representative primate immunodeficiency viruses. In the presence of Tat, however, SIVAGM LTRs are activated relatively inefficiently. To map the elements that confer these features on the SIVAGM LTR, a number of deletion mutants were constructed, and their promoter activities were determined using a bacterial CAT gene as a marker. The results obtained indicated that various elements located in the U3 region may contribute to the high basal promoter activity and that no negative elements are present in the region. The Tat-responsive sequence TAR was localized to the R region as observed for the other LTRs. A mutant carrying a single nucleotide deletion in this region completely lost responsiveness to Tat protein.

Generation and characterization of a host cell-dependent gag gene mutant of human immunodeficiency virus type 1.

An in-frame gag gene mutant of human immunodeficiency virus type 1, which carries two amino acid substitutions in the center of the p24 coding region, was constructed in vitro, and its replication properties in several cell lines were examined. In CD4-negative SW480 cells transfected with the mutant clone, synthesis and processing of viral gag, pol, and env proteins occurred normally, and viral particles were produced. Virions derived from the transfection displayed a severe replication defect when inoculated into some CD4-positive cell lines (H9 and Molt4 clone 8), but in other lines (A3.01 and M8166), the mutant virus grew fairly well. The mutant was demonstrated to be defective at an early infection phase (from adsorption to integration) in Molt4 clone 8 cells but was normal in A3.01 cells. These results indicated that the Gag-p24 protein of human immunodeficiency virus type 1 plays an important role at the early infection phase in a cell-dependent manner.

Both SU and TM env proteins are responsible for monkey cell tropism of simian immunodeficiency virus SIV mac.

While simian immunodeficiency virus (SIV) derived from an infectious molecular clone pMA239 is tropic and pathogenic for monkeys, the virus derived from another infectious clone pMA142 does not replicate in monkey cells. To determine genetic sequences responsible for this tropism, a series of recombinant clones were constructed from pMA142 and pMA239. The determinant in pMA239 was mapped within regions encompassing the env gene. Viruses, which carry the 239 env gene encoding surface and/or transmembrane proteins, were tropic for monkey cells.

The feline immunodeficiency virus ORF-A gene facilitates efficient viral replication in established T-cell lines and peripheral blood lymphocytes.

Frameshift mutants corresponding to all of the identified open reading frames of feline immunodeficiency virus, including the ORF-A gene, which has an unknown function, were constructed in vitro. Upon transfection into cells, no significant difference between the phenotypes of ORF-A mutant clones and those of wild-type clones was demonstrated. Although only ORF-A mutant virus among all mutant viruses from transfected cells showed infectivity in established T-cell lines, the replication and propagation of the ORF-A mutant virus were efficiently reduced compared with those of the wild-type virus. Moreover, the loss of the function of the ORF-A gene resulted in a severe defect in productive infection in primary peripheral blood lymphocytes both in the amount of reverse transcriptase activity produced and in core protein expression. These findings demonstrate that the ORF-A gene of feline immunodeficiency virus is required for efficient viral replication and suggest that the ORF-A gene is likely to be important for natural infection.

Integration is essential for efficient gene expression of human immunodeficiency virus type 1.

A mutant of human immunodeficiency virus type 1 which carries a frameshift insertion in the integrase/endonuclease region of pol gene was constructed in vitro. Upon transfection into cells, although this mutant exhibited a normal phenotype with respect to expression of gag, pol, and env genes and to generation of progeny virions, no replication-competent virus in CD4-positive cells emerged. An assay for the single-step replication of a defective viral genome dependent on trans complementation by rev protein was established and used to monitor the early phase of viral infection process. Viral clones with a mutation in the vif, vpr, or vpu gene displayed no abnormality in the early phase. In contrast, the integrase mutant did not direct a marker gene expression after infection. Together with an observation that the mutant lacked the ability to integrate, these results indicated that the integration was required for efficient viral gene expression and productive infection of human immunodeficiency virus type 1.

Cell-dependent requirement of human immunodeficiency virus type 1 Vif protein for maturation of virus particles.

A highly sensitive single-round infection assay using a bacterial chloramphenicol acetyltransferase was developed to analyze an early stage of human immunodeficiency virus type 1 replication. By a combination of transfection and single-round infection assay, a virus with a vif mutation, depending on host cells from which the virus was derived, was demonstrated to be defective at the early phase of infection cycle. Analysis of viral proteins synthesized in cells indicated that incorporation of the Env surface protein into virions of the vif mutant, again in a cell-dependent way, was greatly restricted. Taken together, it is concluded that the Vif protein acts through modulation of the Env protein in the virions, directly or indirectly, to enhance viral infectivity in a certain cell type.

Compatibility of Tat and Rev transactivators in the primate lentiviruses.

Primate immunodeficiency viruses carry a unique set of transacting regulator genes, which are essential for viral replication. The exchangeability of these Tat and Rev transactivators derived from viruses of the four major subgroups identified to date was assessed in transient transfection and infection assay systems. The human immunodeficiency virus type 1 (HIV-1), a major causative virus of human AIDS, efficiently activated the other viruses. In contrast, the tat and rev gene products of HIV-2, SIVAGM (virus of the African green monkey), and SIVMND (virus of the mandrill) did not fully transactivate the HIV-1. In particular, the rev of HIV-1 was not substantially replaced by those of the other viruses. The result that HIV-1 is distinct from the other immunodeficiency viruses with respect to the compatibility of two transactivators gives a firm functional basis for the unique phylogenetic position of HIV-1.

Infection of macaque monkeys with a chimeric human and simian immunodeficiency virus.

Two macaque monkeys were inoculated with a chimeric human and simian immunodeficiency virus carrying the tat, rev, vpu and env genes of human immunodeficiency virus type 1. Infectious virus was recovered from one of the monkeys at 2 and 6 weeks post-infection. The hybrid nature of the isolated viruses was verified by Southern and Western blotting analyses. Both of the monkeys infected with the chimera elicited a humoral antibody response against the virus.

Sequences responsible for efficient replication of simian immunodeficiency virus SIVMND in cells of the monocyte/macrophage lineage.

We determined the susceptibility of monocytic cell lines to infection with viral strains derived from two infectious clones of simian immunodeficiency virus isolated from a mandrill. One of the strains, which replicates poorly in T cell lines, was found to grow more rapidly than the other in these cells. The viral determinant for this property was genetically mapped within the env gene encoding a surface protein. Six amino acid substitutions identified appeared to be located outside of the domains corresponding to human immunodeficiency virus type 1 env functional domains such as the CD4-binding and V3 loop regions.

Functional analysis of biologically distinct genetic variants of simian immunodeficiency virus isolated from a mandrill.

We examined the biological properties of two infectious clones of a simian immunodeficiency virus, SIVMND, which were designated as pMD121 and pMD122. Upon transfection into CD4-negative cells, pMD122 generated virions much less efficiently than pMD121. Likewise, the growth kinetics in CD4-positive cells of virus derived from pMD122 were remarkably delayed relative to those of virus from pMD121. The cytocidal activity of the MD122 virus was also low. A series of recombinant clones were constructed from pMD121 and pMD122 to determine the sequence responsible for the low virulence of the MD122 virus. The genetic determinant in pMD122 responsible for its properties was mapped to within a region (316 base pairs) encompassing the tat, rev, and env coding sequences. Sequence analysis revealed that the two clones differed by only one nucleotide in this region. A nucleotide substitution G (pMD121) to T (pMD122) altered an arginine codon to a serine codon in the first tat coding exon. Transient transfection experiments showed that the tat activity of pMD122 was about twofold less than that of pMD121. These findings indicate that small differences in tat activity can have a dramatic effect on the biological behavior of SIVMND.

Functional classification of simian immunodeficiency virus isolated from a chimpanzee by transactivators.

In reporter-based transient expression systems, we characterized simian immunodeficiency virus from a chimpanzee (SIVCPZ), with special reference to the human immunodeficiency virus type 1 (HIV-1). SIVCPZ was not equally activated by tat and rev transactivators derived from representative primate lentiviruses. HIV-1 alone activated SIVCPZ to the full extent in both tat and rev assays. The tat and rev gene products of SIVCPZ, as well as those of HIV-1, efficiently transactivated the other viruses. These results indicate that SIVCPZ is identical to HIV-1 with regard to the compatibility of tat and rev gene activities among four subgroups of primate lentiviruses.

Production of feline immunodeficiency virus in feline and non-feline non-lymphoid cell lines by transfection of an infectious molecular clone.

An infectious molecular clone of the TM1 strain of feline immunodeficiency virus (FIV) was transfected into each of one feline (CRFK), two simian (COS and Vero) and two human (SW480 and HeLa) non-lymphoid cell lines, and virus production was assayed on feline T lymphoblastoid MYA-1 cells by monitoring reverse transcriptase activity. Infectious virus was produced in CRFK, Vero and HeLa cells, but not in COS and SW480 cells. When the basal promoter activity of the FIV long terminal repeat (LTR) was examined in these cell lines by using a chloramphenicol acetyltransferase assay, the activity correlated with the virus production in each cell line. Furthermore, when the activity of the FIV LTR was compared with those of three primate lentivirus LTRs, the highest activity in all the cell lines examined was produced by the LTR of simian immunodeficiency virus from African green monkey (SIVAGM), suggesting that it has a wide expression range. In COS and SW480 cells, the activity of the FIV LTR was much lower than that of the SIVAGM LTR. These results indicate that, whereas the primary block to FIV infection of certain cells may occur at the cell surface, the FIV LTR may also participate in controlling virus replication, as an intracellular mechanism.

Isolation and characterization of a highly divergent HIV-2[GH-2]: generation of an infectious molecular clone and functional analysis of its rev-responsive element in response to primate retrovirus transactivators (Rev and Rex).

A highly divergent HIV-2 designated as HIV-2[GH-2] was obtained from an AIDS-related complex (ARC) patient in Ghana. A full-length molecular clone of this isolate was obtained and a biologically active clone was constructed. Its restriction pattern differed from that of prototype HIV-2[GH-1] in 25 of 35 restriction sites, but was strikingly similar to a previously characterized HIV-2 isolate from a Ghanaian (HIV-2ALT). The conserved integrase region (288-bp fragment) previously displayed 95% identity with that of ALT but 17-20% divergence from the HIV-2 prototype member, and a new distinct subgroup (HIV-2b) of HIV-2 consisting of GH-2 and ALT was postulated (Miura et al. 1991.) These isolates, however, were biologically distinguishable from each other by its replication capacity in a monocyte line, U937, in which GH-2 could not grow but ALT grew well. In addition, the nucleotide sequence of the LTR of this new isolate displays 21% divergence from that of prototype HIV-2[GH-1], but the core enhancer, Sp1 binding sites and TATA box were conserved. Although the 3' half of the env gene sequence which is deleted in HIV-2ALT clone showed 27% diversity from the prototype, functional differences in the rev-responsive element were not observed.

Biological characterization of human immunodeficiency virus type 1 and type 2 mutants in human peripheral blood mononuclear cells.

Mutants of human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2), which have been shown to be infectious in established cell lines, were tested for ability to replicate and induce syncytium formation in human peripheral blood mononuclear cells (PBMC). The vpu mutant of HIV-1 showed depressed kinetics of replication in an established T cell line, as reported previously, but in PBMC, its replication was similar to that of the wild type virus. The vpx gene of HIV-2 was required for efficient virus propagation in PBMC, but not in an established T cell line, as previously reported. However, the growth rates of the vpx mutant in PBMC preparations from two individuals were different. The results of experiments on infection of PBMC with the vif and vpr mutants of HIV-1 and HIV-2 were essentially consistent with previous results of infection of established T cell lines. No negative effect of the nef gene products of HIV-1 and HIV-2 was observed. The abilities of the wild type virus and the mutants of HIV-1 to induce syncytium formation in both PBMC and established cell lines were similar. In contrast, neither the wild type nor any of the mutants of HIV-2 induced syncytium formation in PBMC. These results suggest that the functions of some genes can be detected only in mixed populations or primary cells such as PBMC. Studies on the roles of these genes in PBMC may provide a better understanding of their functions in vivo.

Genetic characterization of simian immunodeficiency virus isolated from an African mandrill.

We constructed an infectious molecular clone of simian immunodeficiency virus from an African mandrill (SIVMND). Upon transfection, this clone directed the production of progeny virus particles infectious to and cytopathic for CD4+ human leukemia cells. Thirteen frameshift proviral mutants with an alteration in the eight open reading frames of SIVMND were generated by recombinant DNA techniques, and were analyzed biologically and biochemically. While mutations in the structural genes gag, pol, and env abolished viral growth and induction of cytopathology, mutants of the vif, vpr, and nef genes were fully biologically active. Of the tat and rev mutants, only one rev mutant grew in CD4+ cells with delayed kinetics. In reporter-based transient expression systems, transactivation potentials of the tat and rev mutants were evaluated. A mutant lacking 2nd coding exon of tat gene exhibited tat activity similar to that of the wild type clone. The infectious rev mutant was partially defective for rev gene activity.

Functional analysis of long terminal repeats derived from four strains of simian immunodeficiency virus SIVAGM in relation to other primate lentiviruses.

The promoter activity of long terminal repeats (LTRs) of four strains of the simian immunodeficiency virus isolated from African green monkeys (SIVAGM) was compared with those of various LTRs derived from the other representative primate lentiviruses: human immunodeficiency virus type 1 (HIV-1), type 2 (HIV-2), SIV from a rhesus monkey (SIVMAC), and SIV from a mandrill (SIVMND). The expression of the LTRs was evaluated by monitoring chloramphenicol acetyltransferase production after transfection of reporter plasmid clones. In the absence of viral tat, all SIVAGM LTRs acted as much more efficient promoters than any of the other LTRs. When tat gene products were supplied in trans, LTRs of SIVAGM and SIVMND were activated inefficiently relative to high responder LTRs of HIV-2 and SIVMAC. The LTR of HIV-1 was highly activated by HIV-1 tat, but not so much by HIV-2, SIVAGM, and SIVMND tat.

Compatibility of rev gene activity in the four groups of primate lentiviruses.

The compatibility of rev genes derived from various primate immunodeficiency viruses of all distinct subgroups identified was assessed in three experimental systems: complementation experiments between proviral rev and gag mutants, evaluation of the ability of the rev gene products to activate proviral reporters, and examination of the capacity of various viruses to augment marker gene expression in the infected reporter cell lines. In all systems, human immunodeficiency virus type 1 (HIV-1) rev was not substantially substituted or was extremely poorly substituted by the rev of the other viruses. The rev of simian immunodeficiency virus (SIV) from a mandrill could be exchanged by HIV-1 rev. In contrast, the rev gene products of all viruses efficiently activate HIV-2 and SIV from an African green monkey.

Identification of feline immunodeficiency virus rev gene activity.

We constructed 16 deletion mutants from an infectious molecular clone of feline immunodeficiency virus (FIV) and a reporter plasmid carrying the bacterial chloramphenicol acetyltransferase (CAT) gene to identify the rev transactivator activity of the virus. Cotransfections of various mutants and the rev reporter clone bearing a portion of FIV env in addition to the CAT gene revealed that the sequence important for the augmentation of CAT production was located in three separate parts of the virus genome. This enhancement was FIV specific in that the human retrovirus rev and rex gene products did not activate the reporter. The phenotypic properties of an FIV proviral mutant containing a small deletion in the genome were similar to those of rev mutants derived from primate immunodeficiency viruses. These results indicate that FIV, like the other lentiviruses, contains the rev gene in its genome.