Characterization of a monoclonal anti-capsid antibody that cross-reacts with three major primate lentivirus lineages

HIV-1 Capsid Uncoating Is a Multistep Process That Proceeds through Defect Formation Followed by Disassembly of the Capsid Lattice

The HIV-1 core consists of a cone-shaped capsid shell made of capsid protein (CA) hexamers and pentamers encapsulating the viral genome. HIV-1 capsid disassembly, referred to as uncoating, is important for productive infection; however, the location, timing, and regulation of uncoating remain controversial. Here, we employ amber codon suppression to directly label CA. In addition, a fluid phase fluorescent probe is incorporated into the viral core to detect small defects in the capsid lattice. This double-labeling strategy enables the visualization of uncoating of single cores in vitro and in living cells, which we found to always proceed through at least two distinct steps─the formation of a defect in the capsid lattice that initiates gradual loss of CA below a detectable level. Importantly, intact cores containing the fluid phase and CA fluorescent markers enter and uncoat in the nucleus, as evidenced by a sequential loss of both markers, prior to establishing productive infection. This two-step uncoating process is observed in different cells, including a macrophage line. Notably, the lag between the release of fluid phase marker and terminal loss of CA appears to be independent of the cell type or reverse transcription and is much longer (>5-fold) for nuclear capsids compared to cell-free cores or cores in the cytosol, suggesting that the capsid lattice is stabilized by capsid-binding nuclear factors. Our results imply that intact HIV-1 cores enter the cell nucleus and that uncoating is initiated through a localized defect in the capsid lattice prior to a global loss of CA.

Direct translation of incoming retroviral genomes

Viruses that carry a positive-sense, single-stranded (+ssRNA) RNA translate their genomes soon after entering the host cell to produce viral proteins, with the exception of retroviruses. A distinguishing feature of retroviruses is reverse transcription, where the +ssRNA genome serves as a template to synthesize a double-stranded DNA copy that subsequently integrates into the host genome. As retroviral RNAs are produced by the host cell transcriptional machinery and are largely indistinguishable from cellular mRNAs, we investigated the potential of incoming retroviral genomes to directly express proteins. Here we show through multiple, complementary methods that retroviral genomes are translated after entry. Our findings challenge the notion that retroviruses require reverse transcription to produce viral proteins. Synthesis of retroviral proteins in the absence of productive infection has significant implications for basic retrovirology, immune responses and gene therapy applications.

Vpx enhances innate immune responses independently of SAMHD1 during HIV-1 infection

Background

The genomes of HIV-2 and some SIV strains contain the accessory gene vpx, which carries out several functions during infection, including the downregulation of SAMHD1. Vpx is also commonly used in experiments to increase HIV-1 infection efficiency in myeloid cells, particularly in studies that investigate the activation of antiviral pathways. However, the potential effects of Vpx on cellular innate immune signaling is not completely understood. We investigated whether and how Vpx affects ISG responses in monocytic cell lines and MDMs during HIV-1 infection.

Results

HIV-1 infection at excessively high virus doses can induce ISG activation, although at the expense of high levels of cell death. At equal infection levels, the ISG response is potentiated by the presence of Vpx and requires the initiation of reverse transcription. The interaction of Vpx with the DCAF1 adaptor protein is important for the enhanced response, implicating Vpx-mediated degradation of a host factor. Cells lacking SAMHD1 show similarly augmented responses, suggesting an effect that is independent of SAMHD1 degradation. Overcoming SAMHD1 restriction in MDMs to reach equal infection levels with viruses containing and lacking Vpx reveals a novel function of Vpx in elevating innate immune responses.

Conclusions

Vpx likely has as yet undefined roles in infected cells. Our results demonstrate that Vpx enhances ISG responses in myeloid cell lines and primary cells independently of its ability to degrade SAMHD1. These findings have implications for innate immunity studies in myeloid cells that use Vpx delivery with HIV-1 infection.

Analysis of the functional compatibility of SIV capsid sequences in the context of the FIV gag precursor

The formation of immature lentiviral particles is dependent on the multimerization of the Gag polyprotein at the plasma membrane of the infected cells. One key player in the virus assembly process is the capsid (CA) domain of Gag, which establishes the protein-protein interactions that give rise to the hexagonal lattice of Gag molecules in the immature virion. To gain a better understanding of the functional equivalence between the CA proteins of simian and feline immunodeficiency viruses (SIV and FIV, respectively), we generated a series of chimeric FIV Gag proteins in which the CA-coding region was partially or totally replaced by its SIV counterpart. All the FIV Gag chimeras were found to be assembly-defective; however, all of them are able to interact with wild-type SIV Gag and be recruited into extracellular virus-like particles, regardless of the SIV CA sequences present in the chimeric FIV Gag. The results presented here markedly contrast with our previous findings showing that chimeric SIVs carrying FIV CA-derived sequences are assembly-competent. Overall, our data support the notion that although the SIV and FIV CA proteins share 51% amino acid sequence similarity and exhibit a similar organization, i.e., an N-terminal domain joined by a flexible linker to a C-terminal domain, their functional exchange between these different lentiviruses is strictly dependent on the context of the recipient Gag precursor.

Access of HIV-2 to CD169-dependent dendritic cell-mediated trans infection pathway is attenuated

The mechanisms behind the low viral loads and lower mortality rates of HIV-2(+) individuals remain unknown. We hypothesized that reduced interaction of HIV-2 with CD169, the primary HIV-1 attachment factor on monocyte-derived dendritic cells (DCs) that targets captured virus particles to the trans infection pathway, contributes to its diminished pathogenic phenotype in vivo. We observed a significant decrease in capture of HIV-2 Gag-eGFP virus-like particles (VLPs) and infectious GFP-containing HIV-2 particles compared to corresponding HIV-1 particles by CD169(+) mature DCs. Interestingly, there was decreased co-localization of HIV-2 with HIV-1 Gag at plasma membrane microdomains in virus producer cells which correlated with reduced incorporation of GM3, the CD169 ligand, in HIV-2 virions, and reduction in mature DC-mediated HIV-2 trans infection compared to HIV-1. We conclude that limited interaction of HIV-2 with CD169 diminishes virus access to the mature DC-mediated trans infection pathway and might result in attenuated HIV-2 dissemination in vivo.

Development of an antigen-capture ELISA for the detection of the p27-CA protein of HERV-K(HML-2)

The detection or quantification of retroviruses is often achieved using an antigen-capture ELISA (AC-ELISA) that targets the Gag capsid (CA) protein. We report here the development of an AC-ELISA specific for the p27-CA protein of HERV-K(HML-2). A monoclonal p27-specific antibody is used for capture and a polyclonal anti-p27-CA immune serum generated in rabbits serves for detection. The assay was shown to be specific for HERV-K(HML-2), showing no evidence of cross reactivity with the human retroviruses HIV-1, HIV-2 and HTLV-1 or with XMRV (as a model non-human gammaretrovirus). Using purified recombinant antigen, the limit of detection was shown to be 130pg/ml. The AC-ELISA can be used to quantify HERV-K(HML-2) expression in teratocarcinoma cell lines and to normalize HERV particles generated by transfecting HEK 293T cells with full-length molecular clones. This novel AC-ELISA also proved useful in studies of virus regulation, for example in demonstrating that HERV-K(HML-2) expression is dramatically enhanced by overexpression of Staufen-1, a binding partner of the HERV-K(HML-2) Rec protein. This specific and sensitive HERV-K(HML-2) AC-ELISA will be a useful tool for investigating many aspects of endogenous retroviruses, from basic research to the role they may play in human diseases or as a surrogate marker for particular diseases.

Suppressing active replication of a live attenuated simian immunodeficiency virus vaccine does not abrogate protection from challenge

Although safety concerns preclude the use of live attenuated HIV vaccines in humans, they provide a useful system for identifying the elusive correlates of protective immunity in the SIV/macaque animal model. However, a number of pieces of evidence suggest that protection may result from prior occupancy of susceptible target cells by the vaccine virus rather than the immune response. To address this, we developed a Nef-deletion variant of an RT-SHIV whose active replication could be shut off by treatment with RT-inhibitors. Groups of macaques were inoculated with the ∆Nef-RT-SHIV and immune responses allowed to develop before antiretroviral treatment and subsequent challenge with wild-type SIVmac239. Vaccinated animals either resisted infection fully or significantly controlled the subsequent viremia. However, there was no difference between animals undergoing replication of the vaccine virus and those without. This strongly suggests that competition for available target cells does not play a role in protection.

Impact of naturally occurring amino acid variations on the detection of HIV-1 p24 in diagnostic antigen tests

BACKGROUND

The detection of HIV-1 p24 antigen in diagnostic tests relies on antibodies binding to conserved areas of the protein to cover the full range of HIV-1 subtypes. Using a panel of 43 different virus-like particles (VLPs) expressing Gag from clinical HIV-1 isolates, we previously found that some highly sensitive tests completely failed to detect p24 of certain VLPs, seemingly unrelated to their subtype. Here we aimed to investigate the reason for this failure, hypothesising that it might be due to single amino acid variations in conserved epitopes.

METHODS

Using amino acid alignment, we identified single amino acid variations at position 16 or 170 of p24, unique to those VLPs that failed to be detected in certain diagnostic tests. Through DNA-mutagenesis, these amino acids were changed to ones more commonly found at these positions. The impact of these changes on p24 detection was tested in commercial diagnostic tests as well as by Western Blot and ELISA, using epitope-specific antibodies.

RESULTS AND CONCLUSIONS

Changing positions 16 or 170 to consensus amino acids restored the detection of p24 by the investigated diagnostic tests as well as by epitope-specific antibodies in Western Blot and ELISA. Hence, single amino acid changes in conserved epitopes can lead to the failure of p24 detection and thus to false-negative results. To optimise HIV diagnostic tests, they should also be evaluated using isolates which harbour less-frequent epitope variants.

4-Quinolone alkaloids from Melochia odorata

The methanol extract of Melochia odorata yielded three 4-quinolone alkaloids including waltherione A (1) and two new alkaloids, waltherione C (2) and waltherione D (3). Waltheriones A and C showed significant activities in an in vitro anti-HIV cytoprotection assay at concentrations of 56.2 and 0.84 μM and inhibition of HIV P24 formation of more than 50% at 1.7 and 0.95 μM, respectively. The structures of the alkaloids were established by spectroscopic data interpretation.