Genetic characterization of simian immunodeficiency virus isolated from an African mandrill

Human MX2 is an interferon-induced post-entry inhibitor of HIV-1 infection

Animal cells harbour multiple innate effector mechanisms that inhibit virus replication. For the pathogenic retrovirus human immunodeficiency virus type 1 (HIV-1), these include widely expressed restriction factors, such as APOBEC3 proteins, TRIM5-α, BST2 (refs 4, 5) and SAMHD1 (refs 6, 7), as well as additional factors that are stimulated by type 1 interferon (IFN). Here we use both ectopic expression and gene-silencing experiments to define the human dynamin-like, IFN-induced myxovirus resistance 2 (MX2, also known as MXB) protein as a potent inhibitor of HIV-1 infection and as a key effector of IFN-α-mediated resistance to HIV-1 infection. MX2 suppresses infection by all HIV-1 strains tested, has equivalent or reduced effects on divergent simian immunodeficiency viruses, and does not inhibit other retroviruses such as murine leukaemia virus. The Capsid region of the viral Gag protein dictates susceptibility to MX2, and the block to infection occurs at a late post-entry step, with both the nuclear accumulation and chromosomal integration of nascent viral complementary DNA suppressed. Finally, human MX1 (also known as MXA), a closely related protein that has long been recognized as a broadly acting inhibitor of RNA and DNA viruses, including the orthomyxovirus influenza A virus, does not affect HIV-1, whereas MX2 is ineffective against influenza virus. MX2 is therefore a cell-autonomous, anti-HIV-1 resistance factor whose purposeful mobilization may represent a new therapeutic approach for the treatment of HIV/AIDS.

Further investigation of simian immunodeficiency virus Vif function in human cells

Primate lentivirus Vif proteins function by suppressing the antiviral activity of the cell-encoded apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like (APOBEC) proteins APOBEC3G and APOBEC3F. It has been hypothesized that species-specific susceptibilities of APOBEC proteins to Vif proteins may help govern the transmission of primate lentiviruses to new host species. Consistent with this view and with previous results, we report that the Vif proteins of several diverse simian immunodeficiency viruses (SIVs) that are not known to infect humans are not effective inhibitors of human APOBEC3G or APOBEC3F when assessed in transient-transfection experiments. Unexpectedly, this lack of SIV Vif function did not prevent the replication of two vif-deficient SIVs (SIVtan and SIVmnd1; isolated from tantalus monkeys and mandrills, respectively) in a human T-cell line, HUT78, that expresses both APOBEC 3G and APOBEC3F, a finding which demonstrates that some SIVs are partially resistant to the antiretroviral effects of these enzymes irrespective of Vif function. Additional virus replication studies also revealed that the Vif protein of SIVtan is, in fact, active in human T cells, as it substantially enhanced the replication of its cognate virus and human immunodeficiency virus type 1. In sum, we now consider it improbable that species-specific restrictions to SIV Vif function can explain the lack of human infection with certain SIVs. Instead, our data reveal that the species-specific modulation of Vif function is more complex than previously envisioned and that additional (as-yet-unidentified) viral or host factors may be involved in regulating this dynamic interaction between host and pathogen.

Inhibition of HIV/SIV replication by dominant negative Gag mutants

There are several major strategies against HIV/AIDS. Of these, the gene therapy is a novel, challenging, and promising one. The target genes, which have been extensively studied for the potential gene therapy of HIV/AIDS, include those of cellular and viral origins. Especially, trans-dominant negative Tat, Rev, Env, Pol, and Gag mutants of HIV have currently attracted considerable attention. In this brief review, we summarize the nature of the HIV/SIV mutants of this category and discuss their future use for gene therapy with special reference to the dominant negative Gag mutants of HIV-1.

Compatibility of Vpu-like activity in the four groups of primate immunodeficiency viruses

Env-minus mutants of the viruses of major four human and simian immunodeficiency viruses (HIVs and SIVs) were monitored for their progeny virion production upon transfection into the cells, which are dependent on the HIV-1 Vpu for efficient particle release. Of the env mutants of HIV-1 (one mutant), HIV-2/SIVmac (three mutants), SIVagm (one mutant), and SIVmnd (one mutant) examined, the mutant of SIVmnd generated a very low level of progeny virions similar to that by the HIV-1 Vpu-minus mutant. This effect of the mutation was not observed in the cells which are independent on the Vpu for virion release. The Env of SIVmnd efficiently enhanced virion release of heterologous viruses like the HIV-1 Vpu.

Functional analysis of vif genes derived from various primate immunodeficiency viruses

Replication property in cells of human and simian immunodeficiency viruses (HIVs and SIVs) lacking intact vif gene was evaluated. Of 10 vif mutants constructed in vitro of the major four HIV/SIV groups, only those derived from HIV-1 and HIV-2/SIVmac displayed replication defect. The cell lines non-permissive for the vif mutants of HIV-1 and SIVmac were found to be different. To determine whether Vif is exchangeable between HIV-1 and SIVmac, chimeric virus clones with respect to the vi gene were constructed and virus replication in the cells non-permissive for the vif mutant viruses was monitored. Productive infection in these cells of chimeric viruses clearly indicated that Vif is functionally exchangeable, and that Vifs of different virus origin act through a similar mechanism.

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.

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.