Sexual transmission and propagation of SIV and HIV in resting and activated CD4+ T cells

HIV delays IFN-α production from human plasmacytoid dendritic cells and is associated with SYK phosphorylation

Plasmacytoid dendritic cells (pDC) are the major producers of type I interferons (IFNs) in humans and rapidly produce IFN-α in response to virus exposure. Although HIV infection is associated with pDC activation, it is unclear why the innate immune response is unable to effectively control viral replication. We systematically compared the effect of HIV, Influenza, Sendai, and HSV-2 at similar target cell multiplicity of infection (M.O.I.) on human pDC function. We found that Influenza, Sendai, HSV-2 and imiquimod are able to rapidly induce IFN-α production within 4 hours to maximal levels, whereas HIV had a delayed induction that was maximal only after 24 hours. In addition, maximal IFN-α induction by HIV was at least 10 fold less than that of the other viruses in the panel. HIV also induced less TNF-α and MIP-1β but similar levels of IP-10 compared to other viruses, which was also mirrored by delayed upregulation of pDC activation markers CD83 and CD86. BDCA-2 has been identified as an inhibitory receptor on pDC, signaling through a pathway that involves SYK phosphorylation. We find that compared to Influenza, HIV induces the activation of the SYK pathway. Thus, HIV delays pDC IFN-α production and pDC activation via SYK phosphorylation, allowing establishment of viral populations.

Vpx is critical for SIVmne infection of pigtail macaques

BACKGROUND

Viral protein X (Vpx) of SIV has been reported to be important for establishing infection in vivo. Vpx has several different activities in vitro, promoting preintegration complex import into the nucleus in quiescent lymphocytes and overcoming a block in reverse transcription in macrophages. Vpx interacts with the DDB1-CUL4-DCAF1 E3 ligase complex, which may or may not be required for the ascribed functions. The goal of the current study was to determine whether these activities of Vpx are important in vivo.

RESULTS

An infectious, pathogenic clone of SIVmne was used to examine correlations between Vpx functions in vitro and in vivo. Three previously described HIV-2 Vpx mutants that were shown to be important for nuclear import of the preintegration complex in quiescent lymphocytes were constructed in SIVmne: A vpx-deleted virus, a truncation of Vpx at amino acid 102 that deletes the C-terminal proline-rich domain (X(102)), and a mutant with tyrosines 66, 69, and 71 changed to alanine (X(y-a)). All mutant viruses replicated similarly to wild type SIVmne027 in primary pigtail macaque PBMCs, and were only slightly retarded in CEMx174 cells. However, all the vpx mutant viruses were defective for replication in both human and pigtail monocyte-derived macrophages. PCR assays demonstrated that the efficiency of reverse transcription and the levels of viral integration in macrophages were substantially reduced for the vpx mutant viruses. In vitro, the X(y-a) mutant, but not the X(102) mutant lost interaction with DCAF1. The wild type SIVmne027 and the three vpx mutant SIVs were inoculated by the intra-rectal route into pigtail macaques. Peak levels of plasma viremia of the vpx mutant SIVs were variable, but consistently lower than that observed in macaques infected with wild type SIVmne. In situ hybridization for SIV demonstrated that compared to wild type SIVmne infected macaques five of the six animals inoculated with the vpx mutant SIVs had only low levels of SIV-expressing cells in the rectum, most intestinal epithelial tissues, spleen, and mesenteric and peripheral nodes.

CONCLUSIONS

This work demonstrates that the activities of Vpx to overcome restrictions in culture in vitro are also likely to be important for establishment of infection in vivo and suggest that both the nuclear localization and DCAF1-interaction functions of Vpx are critical in vivo.

Selective transmigration of monocyte-associated HIV-1 across a human cervical monolayer and its modulation by seminal plasma

OBJECTIVE

To analyse the transmigration of immune cells infected by HIV-1 across the epithelial monolayer using the endometrial human endometrial carcinoma (HEC)-1A cell line and to study the influence of seminal plasma in this process.

DESIGN

After sexual intercourse involving a male partner infected by HIV-1, a selection process has been shown to lead to a predominant transmission of the R5 phenotype despite the presence of X4 and R5 strains in semen. Transmigration of HIV-infected monocytes present in semen may represent a pertinent mechanism that could explain this tropism selection.

METHODS

Epithelial monolayer crossing was studied by using HEC-1A epithelial cells cultured on permeable support and monocyte-enriched or lymphocyte-enriched populations of cells infected or not by HIV R5 or X4 strains. Transmigrating cells were quantified and analysed for their ability to transmit HIV infection to immune target cells. The effect of HIV-negative seminal plasma on cell transmigration was analysed.

RESULTS

A preferential passage of the R5 strain associated with monocyte-enriched populations was observed together with the ability of this strain to transmit infection. Seminal plasma was found able to decrease the epithelial crossing of immune cells by enhancing transepithelial resistance and by increasing the adherence of immune cells to the monolayer.

CONCLUSION

The preferential transmigration of HIV R5 strains associated with monocytes across the endocervical monolayer may explain the predominant transmission of the R5 strains after sexual intercourse. By its capacity to modulate the tightness of the epithelial structure, seminal plasma reinforces this selection process.

HIV reservoirs and strategies for eradication

Combination antiretroviral therapy (cART) has led to a reduction in morbidity and mortality in HIV-infected patients but therapy is lifelong and there is no cure for HIV. The major barriers to cure include HIV latency, which has been identified in different T-cell subsets, as well as persistence of HIV in anatomical reservoirs. We review recent developments in our understanding of the major reservoirs of HIV in patients on cART as well as how latency is established and maintained in T cells. Finally, we review the scientific rationale of and clinical experience with pharmacotherapeutic strategies aimed at eliminating latently infected cells.

Vaginal microbicides to prevent human immunodeficiency virus infection in women: perspectives on the female genital tract, sexual maturity and mucosal inflammation

Topically applied vaginal microbicides to protect against human immunodeficiency (HIV) virus infection offer an important female-controlled prevention strategy. Microbicides have been in development for more than 2 decades, and have included various agents that disrupt cellular and microbial membranes (surfactants), restore the natural acidic protective pH of the vagina (acid buffers), and those that interfere with interactions between HIV envelope proteins and cellular receptors (anionic polymers). Although none of these candidate microbicides have shown significant protection against HIV in clinical trials, a topical gel, including the antiretroviral drug tenofovir (TFV) 1% was the first microbicide to be tested to show some protection against HIV infection. This review explores the effect of female genital tract biology and anatomy, mucosal inflammation, and age on the effectiveness of microbicides to prevent HIV infection.

Differential innate immune responses to low or high dose oral SIV challenge in Rhesus macaques

Mucosal transmission of HIV predominately occurs during sexual intercourse or breast-feeding and generally results in a successful infection from just one or few founder virions. Here we assessed the impact of viral inoculum size on both viral and immune events within two groups of Rhesus macaques that were non-traumatically, orally inoculated with either multiple low (1000 to 4000 TCID(50)) or high (100,000 TCID(50)) doses of SIV. In agreement with previous studies, more diverse SIV variants were observed in macaques following infection with high dose oral SIV compared to a low dose challenge. In peripheral blood cells, the immune gene transcript levels of CXCL9, IFNγ, TNFα and IL10 remained similar to uninfected macaques. In contrast, OAS and CXCL10 were upregulated following SIV infection in both the high and low dosed macaques, with a more rapid kinetics (detectable by 7 days) following the high SIV dose challenge. In peripheral lymph nodes, an increase in CXCL10 was observed irrespective of viral dose while CXCL9 and OAS were differentially regulated in the two SIV dosed groups. Magnetic bead sorting of CD3+, CD14+ and CD3- /CD14- cells from peripheral blood identified the increase in OAS expression primarily within CD14+ monocytes, whereas the CXCL10 expression was primarily in CD3+ T cells. These findings provide insights into the impact of SIV challenge dose on viral and innate immune factors, which has the potential to inform future SIV/HIV vaccine efficacy trials in which vaccinated hosts have the potential to be infected with a range of viral challenge doses.

Foreskin T-cell subsets differ substantially from blood with respect to HIV co-receptor expression, inflammatory profile, and memory status

The foreskin is the main site of heterosexual human immunodeficiency virus (HIV) acquisition in uncircumcised men, but functional data regarding T-cell subsets present at this site are lacking. Foreskin tissue and blood were obtained from Ugandan men undergoing elective adult circumcision. Tissue was treated by mechanical and enzymatic digestion followed by T-cell subset identification and assessment of cytokine production using flow cytometry. Foreskin CD4(+) T cells were predominantly an effector memory phenotype, and compared with blood they displayed a higher frequency of CCR5 expression (42.0% vs. 9.9%) and interleukin-17 production. There was no difference in T-regulatory cell frequency, but interferon-γ and tumor necrosis factor-α production were increased in foreskin CD8(+) T cells. These novel techniques demonstrate that the foreskin represents a proinflammatory milieu that is enriched for HIV-susceptible T-cell subsets. Further characterization of foreskin T-cell subsets may help to define the correlates of HIV susceptibility in the foreskin.

The majority of freshly sorted simian immunodeficiency virus (SIV)-specific CD8(+) T cells cannot suppress viral replication in SIV-infected macrophages

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) primarily infect activated CD4(+) T cells but can infect macrophages. Surprisingly, ex vivo tetramer-sorted SIV-specific CD8(+) T cells that eliminated and suppressed viral replication in SIV-infected CD4(+) T cells failed to do so in SIV-infected macrophages. It is possible, therefore, that while AIDS virus-infected macrophages constitute only a small percentage of all virus-infected cells, they may be relatively resistant to CD8(+) T cell-mediated lysis and continue to produce virus over long periods of time.

Glut1-mediated glucose transport regulates HIV infection

Cell cycle entry is commonly considered to positively regulate HIV-1 infection of CD4 T cells, raising the question as to how quiescent lymphocytes, representing a large portion of the viral reservoir, are infected in vivo. Factors such as the homeostatic cytokine IL-7 have been shown to render quiescent T cells permissive to HIV-1 infection, presumably by transiently stimulating their entry into the cell cycle. However, we show here that at physiological oxygen (O(2)) levels (2-5% O(2) tension in lymphoid organs), IL-7 stimulation generates an environment permissive to HIV-1 infection, despite a significantly attenuated level of cell cycle entry. We identify the IL-7-induced increase in Glut1 expression, resulting in augmented glucose uptake, as a key factor in rendering these T lymphocytes susceptible to HIV-1 infection. HIV-1 infection of human T cells is abrogated either by impairment of Glut1 signal transduction or by siRNA-mediated Glut1 down-regulation. Consistent with this, we show that the susceptibility of human thymocyte subsets to HIV-1 infection correlates with Glut1 expression; single-round infection is markedly higher in the Glut1-expressing double-positive thymocyte population than in any of the Glut1-negative subsets. Thus, our studies reveal the Glut1-mediated metabolic pathway as a critical regulator of HIV-1 infection in human CD4 T cells and thymocytes.

HMGB1, an alarmin promoting HIV dissemination and latency in dendritic cells

Dendritic cells (DCs) initiate immune responses by transporting antigens and migrating to lymphoid tissues to initiate T-cell responses. DCs are located in the mucosal surfaces that are involved in human immunodeficiency virus (HIV) transmission and they are probably among the earliest targets of HIV-1 infection. DCs have an important role in viral transmission and dissemination, and HIV-1 has evolved different strategies to evade DC antiviral activity. High mobility group box 1 (HMGB1) is a DNA-binding nuclear protein that can act as an alarmin, a danger signal to alert the innate immune system for the initiation of host defense. It is the prototypic damage-associated molecular pattern molecule, and it can be secreted by innate cells, including DCs and natural killer (NK) cells. The fate of DCs is dependent on a cognate interaction with NK cells, which involves HMGB1 expressed at NK-DC synapse. HMGB1 is essential for DC maturation, migration to lymphoid tissues and functional type-1 polarization of naïve T cells. This review highlights the latest advances in our understanding of the impact of HIV on the interactions between HMGB1 and DCs, focusing on the mechanisms of HMGB1-dependent viral dissemination and persistence in DCs, and discussing the consequences on antiviral innate immunity, immune activation and HIV pathogenesis.

HIV-1 transmission and viral adaptation to the host

HIV-1 transmission predominantly occurs via mucosal transmission and blood–blood contact. In most newly infected individuals, outgrowth of a single virus variant has been described. This indicates that HIV-1 transmission is a very inefficient process and is restricted by an extensive transmission bottleneck. The transmission rate is directly correlated to the viral load in the donor and the susceptibility of the recipient, which is influenced by factors such as the integrity of mucosal barriers, target cell availability and genetic host factors. After establishment of infection in the new host, the viral population remains very homogenous until the host immune response drives evolution of the viral quasispecies. This review describes our current knowledge on HIV-1 transmission and recent insights in viral adaption to its host.

Multi-faceted role of naturally occurring autoantibodies in fighting pathogens

Naturally occurring antibodies (NAbs) play a vital role in the first line of defense against bacterial and viral infections. Most studies in mice and man have attributed this role to NAbs of the IgM isotype. However, there is also a significant amount of data on the anti-infectious function of NAbs of the IgG isotype. Most of these observations are derived from studies using a privileged source of NAbs, the pooled human IgG for intravenous application, IVIG. In addition to its use as a replacement in humoral immunodeficiencies, IVIG is extensively used in autoimmune and inflammatory diseases. The properties of NAbs, the principal components of IVIG, are considered crucial for their immune-regulatory properties, owing to their ability to recognize self-antigens and even autoantibodies. By virtue of these specificities for several cellular antigens, including exposed proteins that act as receptors for a variety of pathogens, certain NAbs in IVIG have a therapeutic role in preventing or modulating infections. We summarize in this chapter several examples that highlight the importance of NAbs in the control of certain bacterial and viral infections.

TLR2 activation enhances HIV nuclear import and infection through T cell activation-independent and -dependent pathways

TLR2 activation plays a crucial role in Neisseria gonorrheae-mediated enhancement of HIV infection of resting CD4(+) T cells. We examined signaling pathways involved in the HIV enhancing effect of TLR2. TLR2 but not IL-2 signals promoted HIV nuclear import; however, both signals were required for the maximal enhancing effect. Although TLR2 signaling could not activate T cells, it increased IL-2-induced T cell activation. Cyclosporin A and IkBα inhibitor blocked TLR2-mediated enhancement of HIV infection/nuclear import. PI3K inhibitor blocked HIV infection/nuclear import and T cell activation and exerted a moderate inhibitory effect on cell cycle progression in CD4(+) T cells activated by TLR2/IL-2. Blockade of p38 signaling suppressed TLR2-mediated enhancement of HIV nuclear import/infection. However, the p38 inhibitor did not have a significant effect on T cell activation or TCR/CD3-mediated enhancement of HIV infection/nuclear import. The cell cycle arresting reagent aphidicolin blocked TLR2- and TCR/CD3-induced HIV infection/nuclear import. Finally, cyclosporin A and IκBα and PI3K inhibitors but not the p38 inhibitor blocked TLR2-mediated IκBα phosphorylation. Our results suggest that TLR2 activation enhances HIV infection/nuclear import in resting CD4(+) T cells through both T cell activation-dependent and -independent mechanisms.

Generation of transmitted/founder HIV-1 infectious molecular clones and characterization of their replication capacity in CD4 T lymphocytes and monocyte-derived macrophages

Genome sequences of transmitted/founder (T/F) HIV-1 have been inferred by analyzing single genome amplicons of acute infection plasma viral RNA in the context of a mathematical model of random virus evolution; however, few of these T/F sequences have been molecularly cloned and biologically characterized. Here, we describe the derivation and biological analysis of ten infectious molecular clones, each representing a T/F genome responsible for productive HIV-1 clade B clinical infection. Each of the T/F viruses primarily utilized the CCR5 coreceptor for entry and replicated efficiently in primary human CD4(+) T lymphocytes. This result supports the conclusion that single genome amplification-derived sequences from acute infection allow for the inference of T/F viral genomes that are consistently replication competent. Studies with monocyte-derived macrophages (MDM) demonstrated various levels of replication among the T/F viruses. Although all T/F viruses replicated in MDM, the overall replication efficiency was significantly lower compared to prototypic "highly macrophage-tropic" virus strains. This phenotype was transferable by expressing the env genes in an isogenic proviral DNA backbone, indicating that T/F virus macrophage tropism mapped to Env. Furthermore, significantly higher concentrations of soluble CD4 were required to inhibit T/F virus infection compared to prototypic macrophage-tropic virus strains. Our findings suggest that the acquisition of clinical HIV-1 subtype B infection occurs by mucosal exposure to virus that is not highly macrophage tropic and that the generation and initial biological characterization of 10 clade B T/F infectious molecular clones provides new opportunities to probe virus-host interactions involved in HIV-1 transmission.

Targeting HIV-1 innate immune responses therapeutically

PURPOSE OF REVIEW

The early stage of HIV-1 infection is when the virus is most vulnerable, and should therefore offer the best opportunity for therapeutic interventions. This review addresses the recent progress in the understanding of innate immune responses against HIV-1 with focus on the potential targets for prevention of viral acquisition, replication and dissemination.

RECENT FINDINGS

Research indicates that the host-derived factor trappin-2/elafin is protective against HIV, whereas semen-derived enhancer of viral infection and defensins 5 and 6 enhance viral transmission. Further, studies suggest that stimulation of TLR4 and inhibition of TLR7-9 pathways may be HIV suppressive. The regulation and function of viral restriction factors tetherin and APOBEC3G have been investigated and a molecule mimicking the premature uncoating achieved by TRIM5α, PF74, has been identified. Chloroquine has been shown to inhibit plasmacytoid dendritic cell activation and suppress negative modulators of T-cell responses. Blockade of HMBG1 has been found to restore natural-killer-cell-mediated killing of infected dendritic cells, normally suppressed by HIV-1. Interestingly, when used as adjuvants, EAT-2 and heat shock protein gp96 reportedly enhance innate immune responses.

SUMMARY

Several targets for innate immunity-mediated therapeutics have been identified. Nonetheless, more research is required to unveil their underlying mechanisms and interactions before testing these molecules in clinical trials.

Virus replication strategies and the critical CTL numbers required for the control of infection

Vaccines that elicit protective cytotoxic T lymphocytes (CTL) may improve on or augment those designed primarily to elicit antibody responses. However, we have little basis for estimating the numbers of CTL required for sterilising immunity at an infection site. To address this we begin with a theoretical estimate obtained from measurements of CTL surveillance rates and the growth rate of a virus. We show how this estimate needs to be modified to account for (i) the dynamics of CTL-infected cell conjugates, and (ii) features of the virus lifecycle in infected cells. We show that provided the inoculum size of the virus is low, the dynamics of CTL-infected cell conjugates can be ignored, but knowledge of virus life-histories is required for estimating critical thresholds of CTL densities. We show that accounting for virus replication strategies increases estimates of the minimum density of CTL required for immunity over those obtained with the canonical model of virus dynamics, and demonstrate that this modeling framework allows us to predict and compare the ability of CTL to control viruses with different life history strategies. As an example we predict that lytic viruses are more difficult to control than budding viruses when net reproduction rates and infected cell lifetimes are controlled for. Further, we use data from acute SIV infection in rhesus macaques to calculate a lower bound on the density of CTL that a vaccine must generate to control infection at the entry site. We propose that critical CTL densities can be better estimated either using quantitative models incorporating virus life histories or with in vivo assays using virus-infected cells rather than peptide-pulsed targets.

In the search for vaccines that provide reliable protection against major diseases such as HIV-AIDS, TB and Malaria, there is now a focus on generating populations of antigen-specific cytotoxic T lymphocytes (CTL), immune cells that recognise and kill infected cells. However, we have little idea of the number or density of CTL a vaccine would need to elicit to provide sterilizing immunity to an infection in a given tissue. In this study we use mathematical models to understand how a virus's replication strategy influences the minimum density of CTL needed to provide immunity at an infection site. We show that traditional models that neglect the viral lifecycle within infected cells will underestimate this density. To illustrate, we use our modelling framework to estimate the CTL density needed to control the spread of virus at the very earliest stages of primary SIV infection in rhesus macaques.

Expression and reactivation of HIV in a chemokine induced model of HIV latency in primary resting CD4+ T cells

Background

We recently described that HIV latent infection can be established in vitro following incubation of resting CD4+ T-cells with chemokines that bind to CCR7. The main aim of this study was to fully define the post-integration blocks to virus replication in this model of CCL19-induced HIV latency.

Results

High levels of integrated HIV DNA but low production of reverse transcriptase (RT) was found in CCL19-treated CD4+ T-cells infected with either wild type (WT) NL4.3 or single round envelope deleted NL4.3 pseudotyped virus (NL4.3- Δenv). Supernatants from CCL19-treated cells infected with either WT NL4.3 or NL4.3- Δenv did not induce luciferase expression in TZM-bl cells, and there was no expression of intracellular p24. Following infection of CCL19-treated CD4+ T-cells with NL4.3 with enhanced green fluorescent protein (EGFP) inserted into the nef open reading frame (NL4.3- Δnef-EGFP), there was no EGFP expression detected. These data are consistent with non-productive latent infection of CCL19-treated infected CD4+ T-cells. Treatment of cells with phytohemagluttinin (PHA)/IL-2 or CCL19, prior to infection with WT NL4.3, resulted in a mean fold change in unspliced (US) RNA at day 4 compared to day 0 of 21.2 and 1.1 respectively (p = 0.01; n = 5), and the mean expression of multiply spliced (MS) RNA was 56,000, and 5,000 copies/million cells respectively (p = 0.01; n = 5). In CCL19-treated infected CD4+ T-cells, MS-RNA was detected in the nucleus and not in the cytoplasm; in contrast to PHA/IL-2 activated infected cells where MS RNA was detected in both. Virus could be recovered from CCL19-treated infected CD4+ T-cells following mitogen stimulation (with PHA and phorbyl myristate acetate (PMA)) as well as TNFα, IL-7, prostratin and vorinostat.

Conclusions

In this model of CCL19-induced HIV latency, we demonstrate HIV integration without spontaneous production of infectious virus, detection of MS RNA in the nucleus only, and the induction of virus production with multiple activating stimuli. These data are consistent with ex vivo findings from latently infected CD4+ T-cells from patients on combination antiretroviral therapy, and therefore provide further support of this model as an excellent in vitro model of HIV latency.

Vaginal langerhans cells nonproductively transporting HIV-1 mediate infection of T cells

Although implied by other models, proof that Langerhans cells (LCs) in the human vagina participate in dissemination of infectious human immunodeficiency virus type 1 (HIV-1) has been lacking. Here, we show that LCs migrate from HIV-1-exposed vaginal epithelia and pass infectious virus to CD4+ T cells without being productively infected themselves, and we point to a pathway that might enable HIV-1 to avoid degradation in vaginal LCs. Transport by migratory LCs to local lymphatics in a nonproductive but infectious form may aid HIV-1 in evasion of topical microbicides that target its intracellular productive life cycle.

Innate immune mediator profiles and their regulation in a novel polarized immortalized epithelial cell model derived from human endocervix

The endocervix in the female reproductive tract (FRT) is susceptible to sexually transmitted pathogens such as Chlamydia trachomatis and Neisseria gonorrhoeae. Endocervical epithelial cells in vivo make innate immune mediators that likely aid in the protection from these pathogens. In vitro studies to investigate the innate epithelial cell immune response to endocervical pathogens have been hindered by the paucity of human endocervix-derived epithelial cell lines that display the differentiation proteins and functional characteristics of their site of origin. We have established an immortalized epithelial cell line (A2EN) derived from an endocervical tissue explant that can be polarized to exhibit distinct apical and basolateral membrane domains. Polarized A2EN cells secrete mucus at their apical surface, and express MUC5B, a mucin specific to the endocervix. Polarized A2EN cells also express hormone receptors that respond appropriately to female steroid hormones. Polarized A2EN cells can be stimulated with the toll-like receptor 3 agonist, polyI:C, to express anti-microbial peptides (AMPs) as well as pro-inflammatory cytokines and chemokines. Cytokines and chemokines are also differentially secreted depending on the hormone milieu in which the cells are exposed. We conclude that polarized A2EN cells maintain distinctive phenotypic and functional characteristics of the epithelial cells found in the endocervix and, hence, could provide a useful, new in vitro model system for investigations on the role of endogenous and exogenous factors that regulate endocervical epithelial cell immunity including studies on sexually transmitted infections and topical microbicides.

Immune activation in the female genital tract during HIV infection predicts mucosal CD4 depletion and HIV shedding

Plasma viral load predicts genital tract human immunodeficiency virus (HIV) shedding in HIV-infected women. We investigated whether local mucosal T-cell activation (HLA-DR, CD38, CCR5, and Ki67) contributed to HIV shedding in the genital tracts of HIV-infected women. We showed that cervical cytobrush-derived T cells expressed higher frequencies of T-cell activation markers (CD38+ and HLA-DR+) than blood-derived T cells. Expression was significantly higher in HIV-infected women than in uninfected women. We found that the frequency of activated proliferating cervical T cells (Ki67+; Ki67+CCR5+) broadly predicted HIV shedding in the genital tract in HIV-infected women, independently of plasma viral loads. Furthermore, activated cervical T cells (HLA-DR+CD38+ and HLA-DR+CCR5+) and local HIV shedding were independently associated with CD4 depletion in the genital tract. These data suggest that the presence of high frequencies of activated T cells in the female genital mucosa during HIV infection facilitates both local HIV shedding and CD4 T-cell depletion.

Mechanism of host cell MAPK/ERK-2 incorporation into lentivirus particles: characterization of the interaction between MAPK/ERK-2 and proline-rich-domain containing capsid region of structural protein Gag

The characteristic event that follows infection of a cell by retroviruses Including human immunodeficiency virus (HIV)/ simian immunodeficiency virus (SIV) is the formation of a reverse transcription complex in which viral nucleic acids are synthesized. Nuclear transport of newly synthesized viral DNA requires phosphorylation of proteins in the reverse transcription complex by virion-associated cellular kinases. Recently, we demonstrated that disruption of cellular mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 2 (ERK-2) incorporation into SIV virions inhibits virus replication in nonproliferating target cells, indicating that MAPK/ERK-2 plays an important role in HIV /SIV replication. The mechanism of incorporation of MAPK/ERK-2 into virus particles is not defined. In this regard, we hypothesized that a likely interaction of MAPK/ERK-2 with Gag(p55) may enable its packaging into virus particles. In the present investigation, we provided evidence for the first time that MAPK/ERK-2 interacts with the structural Gag polyprotein p55 using a combination of mutagenesis and protein-protein interaction analysis. We further show that MAPK/ERK-2 interacts specifically with the poly-proline motif present in the capsid region of Gag(p55). Utilizing virus-like particles directed by Gag, we have shown that the exchange of conserved proline residues within capsid of Gag(p55) resulted in impaired incorporation of MAPK/ERK-2. In addition, the deletion of a domain comprising amino acids 201 to 255 within host cell MAPK/ERK-2 abrogates its interaction with Gag(p55). The relevance of the poly-proline motif is further evident by its conservation in diverse retroviruses, as noted from the sequence analysis and structural modeling studies of predicted amino acid sequences of the corresponding Gag proteins. Collectively, these data suggest that the interaction of MAPK/ERK-2 with Gag polyprotein results in its incorporation into virus particles and may be essential for retroviral replication.

Timing of the components of the HIV life cycle in productively infected CD4+ T cells in a population of HIV-infected individuals

We estimate the time required for HIV to complete separate stages of its infection cycle in productively infected CD4+ T cells in vivo by comparing initial delays after administration of single antiretroviral drugs until HIV RNA reduction in peripheral blood. Data were obtained from monotherapy studies of eight antiretroviral drugs from all currently licensed HIV drug classes: CCR5 blockers (maraviroc), fusion inhibitors (enfuvirtide), nucleoside and nonnucleoside reverse transcriptase inhibitors (abacavir, tenofovir, and rilpivirine), integrase inhibitors (raltegravir), and protease inhibitors (ritonavir and nelfinavir). We find that HIV requires an average of 52 h between export of virions in one generation to export in the next, with most of this (33 h) taken up by reverse transcription. Reverse transcription in vivo was three times longer than in vitro and began soon after virion fusion, as we determined no difference in mean times for commencement of reverse transcription and virion fusion as calculated by timing of the effects for tenofovir and maraviroc. Approximately 7 h is required between HIV integration and virion production. First-phase HIV RNA decay (half-life of 17 h over all drugs) seemed to slow as the stage being inhibited by the drug was further from viral production. The mean estimated half-life of plasma virions was 5 min, significantly shorter than previous estimates.

HLA-DR+ CD38+ CD4+ T lymphocytes have elevated CCR5 expression and produce the majority of R5-tropic HIV-1 RNA in vivo

Percentages of activated T cells correlate with HIV-1 disease progression, but the underlying mechanisms are not fully understood. We hypothesized that HLA-DR(+) CD38(+) (DR(+) 38(+)) CD4(+) T cells produce the majority of HIV-1 due to elevated expression of CCR5 and CXCR4. In phytohemagglutinin (PHA)-stimulated CD8-depleted peripheral blood mononuclear cells (PBMC) infected with HIV-1 green fluorescent protein (GFP) reporter viruses, DR(-) 38(+) T cells constituted the majority of CCR5 (R5)-tropic (median, 62%) and CXCR4 (X4)-tropic HIV-1-producing cells (median, 61%), although cell surface CCR5 and CXCR4 were not elevated in this subset of cells. In lymph nodes from untreated individuals infected with R5-tropic HIV-1, percentages of CCR5(+) cells were elevated in DR(+) 38(+) CD4(+) T cells (median, 36.4%) compared to other CD4(+) T-cell subsets (median values of 5.7% for DR(-) 38(-) cells, 19.4% for DR(+) 38(-) cells, and 7.6% for DR(-) 38(+) cells; n = 18; P < 0.001). In sorted CD8(-) lymph node T cells, median HIV-1 RNA copies/10(5) cells was higher for DR(+) 38(+) cells (1.8 × 10(6)) than for DR(-) 38(-) (0.007 × 10(6)), DR(-) 38(+) (0.064 × 10(6)), and DR(+) 38(-) (0.18 × 10(6)) subsets (n = 8; P < 0.001 for all). After adjusting for percentages of subsets, a median of 87% of viral RNA was harbored by DR(+) 38(+) cells. Percentages of CCR5(+) CD4(+) T cells and concentrations of CCR5 molecules among subsets predicted HIV-1 RNA levels among CD8(-) DR/38 subsets (P < 0.001 for both). Median HIV-1 DNA copies/10(5) cells was higher in DR(+) 38(+) cells (5,360) than in the DR(-) 38(-) (906), DR(-) 38(+) (814), and DR(+) 38(-) (1,984) subsets (n = 7; P ≤ 0.031). Thus, DR(+) 38(+) CD4(+) T cells in lymph nodes have elevated CCR5 expression, are highly susceptible to infection with R5-tropic virus, and produce the majority of R5-tropic HIV-1. PBMC assays failed to recapitulate in vivo findings, suggesting limited utility. Strategies to reduce numbers of DR(+) 38(+) CD4(+) T cells may substantially inhibit HIV-1 replication.

Herpes simplex virus-induced epithelial damage and susceptibility to human immunodeficiency virus type 1 infection in human cervical organ culture

Normal human premenopausal cervical tissue has been used to derive primary cell populations and to establish ex vivo organ culture systems to study infections with herpes simplex virus (HSV-1 or HSV-2) and human immunodeficiency virus type 1 (HIV-1). Infection with either HSV-1 or HSV-2 rapidly induced multinuclear giant cell formation and widespread damage in mucosal epithelial cells. Subsequent exposure of the damaged mucosal surfaces to HIV-1 revealed frequent co-localization of HSV and HIV-1 antigens. The short-term organ culture system provides direct experimental support for the epidemiological findings that pre-existing sexually transmitted infections, including primary and recurrent herpes virus infections at mucosal surfaces, represent major risk factors for acquisition of primary HIV-1 infection. Epithelial damage in combination with pre-existing inflammation, as described here for overtly normal human premenopausal cervix, creates a highly susceptible environment for the initiation and establishment of primary HIV-1 infection in the sub-mucosa of the cervical transformation zone.

Coming of age: reconstruction of heterosexual HIV-1 transmission in human ex vivo organ culture systems

Heterosexual transmission of human immunodeficiency virus-1 (HIV-1), from men to women, involves exposure to infectious HIV-1 in semen. Therefore, the cellular and molecular processes that underlie HIV-1 transmission are closely interconnected with fundamental principles of human reproductive biology. Human ex vivo organ culture systems allow experimental reconstruction of HIV-1 transmission, using human semen and premenopausal cervicovaginal mucosal tissue, with specific emphasis on the progression from exposure to development of primary HIV-1 infection. Clearly, an isolated piece of human tissue cannot duplicate the full complexity of events in natural infections, but with correct observation of conventional medical and ethical standards, there is no opportunity to study HIV-1 exposure and primary infection in young women. Human mucosal organ cultures allow direct study of HIV-1 infection in a reproducible format while retaining major elements of complexity and variability that typify community-based HIV-1 transmission. Experimental manipulation of human mucosal tissue both allows and requires acquisition of new insights into basic processes of human mucosal immunology. Expanding from the current foundations, we believe that human organ cultures will become increasingly prominent in experimental studies of HIV-1 transmission and continuing efforts to prevent HIV-1 infection at human mucosal surfaces.

Phenotypic and immunologic comparison of clade B transmitted/founder and chronic HIV-1 envelope glycoproteins

Sexual transmission of human immunodeficiency virus type 1 (HIV-1) across mucosal barriers is responsible for the vast majority of new infections. This relatively inefficient process results in the transmission of a single transmitted/founder (T/F) virus, from a diverse viral swarm in the donor, in approximately 80% of cases. Here we compared the biological activities of 24 clade B T/F envelopes (Envs) with those from 17 chronic controls to determine whether the genetic bottleneck that occurs during transmission is linked to a particular Env phenotype. To maximize the likelihood of an intact mucosal barrier in the recipients and to enhance the sensitivity of detecting phenotypic differences, only T/F Envs from individuals infected with a single T/F variant were selected. Using pseudotyping to assess Env function in single-round infectivity assays, we compared coreceptor tropism, CCR5 utilization efficiencies, primary CD4(+) T cell subset tropism, dendritic cell trans-infections, fusion kinetics, and neutralization sensitivities. T/F and chronic Envs were phenotypically equivalent in most assays; however, T/F Envs were modestly more sensitive to CD4 binding site antibodies b12 and VRC01, as well as pooled human HIV Ig. This finding was independently validated with a panel of 14 additional chronic HIV-1 Env controls. Moreover, the enhanced neutralization sensitivity was associated with more efficient binding of b12 and VRC01 to T/F Env trimers. These data suggest that there are subtle but significant structural differences between T/F and chronic clade B Envs that may have implications for HIV-1 transmission and the design of effective vaccines.

Increases in human papillomavirus detection during early HIV infection among women in Zimbabwe

BACKGROUND

Individuals who acquire human immunodeficiency virus (HIV) may experience an immediate disruption of genital tract immunity, altering the ability to mount a local and effective immune response. This study examined the impact of early HIV infection on new detection of human papillomavirus (HPV).

METHODS

One hundred fifty-five Zimbabwean women with observation periods before and after HIV acquisition and 486 HIV-uninfected women were selected from a cohort study evaluating hormonal contraceptive use and risk of HIV acquisition. Study visits occurred at 3-month intervals. Cervical swab samples available from up to 6 months before, at, and up to 6 months after the visit when HIV was first detected were typed for 37 HPV genotypes or subtypes.

RESULTS

We observed ∼5-fold higher odds of multiple (≥2) new HPV detections only after HIV acquisition, relative to HIV-negative women after adjusting for sexual behavior and concurrent genital tract infections. We also observed ∼2.5-fold higher odds of single new HPV detections at visits before and after HIV acquisition, relative to HIV-uninfected women in multivariable models.

CONCLUSIONS

These findings suggest that HIV infection has an immediate impact on genital tract immunity, as evidenced by the high risk of multiple new HPV detections immediately after HIV acquisition.

The immunosuppressive role of IL-32 in lymphatic tissue during HIV-1 infection

One pathological hallmark of HIV-1 infection is chronic activation of the immune system, driven, in part, by increased expression of proinflammatory cytokines. The host attempts to counterbalance this prolonged immune activation through compensatory mediators of immune suppression. We recently identified a gene encoding the proinflammatory cytokine IL-32 in microarray studies of HIV-1 infection in lymphatic tissue (LT) and show in this study that increased expression of IL-32 in both gut and LT of HIV-1-infected individuals may have a heretofore unappreciated role as a mediator of immune suppression. We show that: 1) IL-32 expression is increased in CD4(+) T cells, B cells, macrophages, dendritic cells, and epithelial cells in vivo; 2) IL-32 induces the expression of immunosuppressive molecules IDO and Ig-like transcript 4 in immune cells in vitro; and 3) in vivo, IL-32-associated IDO/Ig-like transcript 4 expression in LT macrophages and gut epithelial cells decreases immune activation but also may impair host defenses, supporting productive viral replication, thereby accounting for the correlation between IL-32 levels and HIV-1 replication in LT. Thus, during HIV-1 infection, we propose that IL-32 moderates chronic immune activation to avert associated immunopathology but at the same time dampens the antiviral immune response and thus paradoxically supports HIV-1 replication and viral persistence.

Acute HIV-1 Infection

In 2009, the United Nations Estimated that 33.2 Million People worldwide were living with human immunodeficiency virus type 1 (HIV-1) infection and that 2.6 million people had been newly infected.1 The need for effective HIV-1 prevention has never been greater. In this review, we address recent critical advances in our understanding of HIV-1 transmission and acute HIV-1 infection. Fourth-generation HIV-1 testing, now available worldwide,2 ,3 will allow the diagnosis of infection in many patients and may lead to new treatments and opportunities for prevention.

Early events in sexual transmission of HIV and SIV and opportunities for interventions

To constrain the growth of the HIV/AIDS pandemic and ultimately end it, effective measures must be developed to prevent sexual mucosal transmission, the major route by which new infections are acquired. I review sexual mucosal transmission of HIV and SIV, with a focus on vaginal transmission in the SIV rhesus macaque animal model, and the evidence for small founder populations of infected cells and the local expansion at the portal of entry necessary to establish systemic infection. These early events represent windows of maximum opportunity for interventions to prevent systemic infection. I highlight the paradoxical role the innate immune response plays in actually facilitating transmission, and a novel microbicide strategy that targets this innate response to prevent systemic infection, and I conclude with an agenda for future research that emphasizes mucosal immunology, virology and pathogenesis studies at each anatomic site of entry.

Functional impairment of central memory CD4 T cells is a potential early prognostic marker for changing viral load in SHIV-infected rhesus macaques

In HIV infection there is a paucity of literature about the degree of immune dysfunction to potentially correlate and/or predict disease progression relative to CD4(+) T cells count or viral load. We assessed functional characteristics of memory T cells subsets as potential prognostic markers for changing viral loads and/or disease progression using the SHIV-infected rhesus macaque model. Relative to long-term non-progressors with low/undetectable viral loads, those with chronic plasma viremia, but clinically healthy, exhibited significantly lower numbers and functional impairment of CD4(+) T cells, but not CD8(+) T cells, in terms of IL-2 production by central memory subset in response to PMA and ionomycine (PMA+I) stimulation. Highly viremic animals showed impaired cytokine-production by all T cells subsets. These results suggest that functional impairment of CD4(+) T cells in general, and of central memory subset in particular, may be a potential indicator/predictor of chronic infection with immune dysfunction, which could be assayed relatively easily using non-specific PMA+I stimulation.

Barriers to mucosal transmission of immunodeficiency viruses

Lentiviruses such as HIV have a daunting challenge in gaining access to a new host predominantly through the penile, rectal, or vaginal/cervical mucosal tissue after sexual exposure. Multiple mechanisms have evolved to help prevent such infections, including anatomical barriers, innate inhibitors, and adaptive immune responses. For lentiviruses, it appears that in naive or even conventionally vaccinated hosts, typical adaptive immune responses are generally too little and too late to prevent infection. Nevertheless, a combination of anatomical barriers and innate immune responses may limit transmission, especially in patients without predisposing conditions such as mucosal lesions or preexisting sexually transmitted infections. Furthermore, when infection does occur, most often the primary viremia of the acute infection can be traced back genetically to a single founder virus. Unfortunately, even a single virion can establish an infection that will ultimately lead to the demise of the host. This review seeks to describe the biology of and barriers to establishment of systemic, disseminated productive infection with HIV after sexual exposure and to discuss the possible mechanisms leading to infection by a single viral variant. Understanding the initial events of infection, before systemic spread, could provide insights into strategies for reducing acquisition or ameliorating clinical outcome.

Rapid dissemination of SIV follows multisite entry after rectal inoculation

Receptive ano-rectal intercourse is a major cause of HIV infection in men having sex with men and in heterosexuals. Current knowledge of the mechanisms of entry and dissemination during HIV rectal transmission is scarce and does not allow the development of preventive strategies. We investigated the early steps of rectal infection in rhesus macaques inoculated with the pathogenic isolate SIVmac251 and necropsied four hours to nine days later. All macaques were positive for SIV. Control macaques inoculated with heat-inactivated virus were consistently negative for SIV. SIV DNA was detected in the rectum as early as four hours post infection by nested PCR for gag in many laser-microdissected samples of lymphoid aggregates and lamina propria but never in follicle-associated epithelium. Scarce SIV antigen positive cells were observed by immunohistofluorescence in the rectum, among intraepithelial and lamina propria cells as well as in clusters in lymphoid aggregates, four hours post infection and onwards. These cells were T cells and non-T cells that were not epithelial cells, CD68⁺ macrophages, DC-SIGN⁺ cells or fascin⁺ dendritic cells. DC-SIGN⁺ cells carried infectious virus. Detection of Env singly spliced mRNA in the mucosa by nested RT-PCR indicated ongoing viral replication. Strikingly, four hours post infection colic lymph nodes were also infected in all macaques as either SIV DNA or infectious virus was recovered. Rapid SIV entry and dissemination is consistent with trans-epithelial transport. Virions appear to cross the follicle-associated epithelium, and also the digestive epithelium. Viral replication could however be more efficient in lymphoid aggregates. The initial sequence of events differs from both vaginal and oral infections, which implies that prevention strategies for rectal transmission will have to be specific. Microbicides will need to protect both digestive and follicle-associated epithelia. Vaccines will need to induce immunity in lymph nodes as well as in the rectum.

Curing HIV: Pharmacologic approaches to target HIV-1 latency

HIV-1 infection persists even after years of antiretroviral therapy (ART). Although ART can halt viral replication and thereby reduce viremia to clinically undetectable levels, proviral latency established within the host genome remains largely unaffected by ART and can replenish systemic infection following interruption of therapy. Pharmacologic strategies, which not only target viral replication but also deplete proviral infection, are required for successful clearance of HIV-1 infection. This review highlights the current understanding of molecular mechanisms that establish and maintain HIV-1 latency in its major reservoir, the resting memory CD4(+) T cell. We also identify the molecular targets that might be exploited to induce HIV-1 expression, remove epigenetic restrictions, or enhance effective transcription. Finally, we discuss the potential pharmacologic approaches toward targeting viral persistence in different cellular and anatomical reservoirs to achieve a cure of HIV-1 infection.

HIV reservoirs and latency models

The main impediment to a cure for HIV is the existence of long-lasting treatment resistant viral reservoirs. In this review, we discuss what is currently known about reservoirs, including their formation and maintenance, while focusing on latently infected CD4+ T cells. In addition, we compare several different in vivo and in vitro models of latency. We comment on how each model may reflect the properties of reservoirs in vivo, especially with regard to cell phenotype, since recent studies demonstrate that multiple CD4+ T cell subsets contribute to HIV reservoirs and that with HAART and disease progression the relative contribution of different subsets may change. Finally, we focus on the direct infection of resting CD4+ T cells as a source of reservoir formation and as a model of latency, since recent results help explain the misconception that resting CD4+ T cells appeared to be resistant to HIV in vitro.

HIV vaccines: progress to date

The quest for an effective and safe HIV-1 vaccine has been and still is the aspiration of many scientists and clinicians worldwide. Until recently, the hopes for an effective vaccine were thwarted by the disappointing results and early termination in September 2007 of the STEP study, which saw a subgroup of male vaccine recipients at an increased risk of HIV-1 infection, and the failure of earlier trials of vaccines based on recombinant envelope proteins to provide any level of protection. The results of the STEP study raised important questions in the field of HIV vaccines, including the use of recombinant adenovirus vectors as immunogens, the rationale for the development of T-cell-based vaccines and the development pathway for these vaccines, in terms of assessment of immunogenicity and the challenge models used. The study of neutralizing antibodies has demonstrated that the induction of high-titre, broadly neutralizing antibodies in the majority of recipients is likely to be highly problematic. However, the results of the RV144 Thai trial released in September 2009 have brought new optimism to the field. This study employed envelope-based immunogens delivered as a priming vaccination with a recombinant poxvirus vector and boosting with recombinant proteins. This regimen provided modest protection to HIV-1 infection in a low-risk population. Although the correlates of protection are currently unknown, extensive studies are underway to try to determine these. Neutralizing antibodies were not induced in the RV144 study; however, considerable titres of binding antibodies to HIV-1 viral envelope (Env) were. It is speculated that these antibodies may have provided a means of protection by a mechanism such as antibody-dependent cell-mediated cytotoxicity. In addition, no CD8+ T-cell responses were induced, but robust CD4+ T-cell responses were, and correlates of protection are being sought by analysing the quality of this aspect of the vaccine-induced immune response. The current paradigm for an optimal HIV-1 vaccine is to design immunogens and vaccination protocols that allow the induction of both broadly neutralizing humoral and broadly reactive and effective cell-mediated immunity, to act at sites of possible infection and post-infection, respectively. However, this is challenged by the results of the RV144 trial as neither of these responses were induced but modest protection was observed. Understanding the biology and immunopathology of HIV-1 early following infection, its modes of transmission and the human immune system's response to the virus should aid in the rational design of vaccines of increased efficacy.

Attachment and fusion inhibitors potently prevent dendritic cell-driven HIV infection

Dendritic cells (DCs) efficiently transfer captured (trans) or de novo-produced (cis) virus to CD4 T cells. Using monocyte-derived DCs, we evaluated entry inhibitors targeting HIV envelope (BMS-C, T-1249) or CCR5 (CMPD167) for their potency to prevent DC infection, DC-driven infection in T cells in trans and cis, and direct infection of DC-T-cell mixtures. Immature DC-T-cell cultures with distinct mechanisms of viral transfer yielded similar levels of infection and produced more proviral DNA compared with matched mature DC-T-cell cultures or infected immature DCs. Although all compounds completely blocked HIV replication, 16 times more of each inhibitor (250 vs 15.6 nM) was required to prevent low-level infection of DCs compared with the productive DC-T-cell cocultures. Across all cell systems tested, BMS-C blocked infection most potently. BMS-C was significantly more effective than CMPD167 at preventing DC infection. In fact, low doses of CMPD167 significantly enhanced DC infection. Elevated levels of CCL4 were observed when immature DCs were cultured with CMPD167. Viral entry inhibitors did not interfere with Candida albicans-specific DC cytokine/chemokine responses. These findings indicate that an envelope-binding small molecule is a promising tool for topical microbicide design to prevent the infection of early targets needed to establish and disseminate HIV infection.

Early HIV-1 target cells in human vaginal and ectocervical mucosa

After translocation through the pleuristratified epithelium of the lower female genital tract, HIV-1 encounters potential target mononuclear cells in the lamina propria of the vagina and ectocervix. Here we show that each major type of genital mononuclear cells, including dendritic cells (DCs), macrophages and lymphocytes, are susceptible to HIV-1 in vitro. Among suspensions of vaginal and ectocervical mononuclear cells, DCs were the first cells to take up virus, containing GFP-tagged virions as early as 15 min after exposure. At 2 hr after exposure, DCs still contained the largest proportion of HIV-1(+) cells compared to lamina propria macrophages and lymphocytes from the same mucosal compartment. By 4 days, however, lymphocytes from both vaginal and ectocervical mucosa supported the highest level of HIV-1 replication. Genital macrophages from the same mucosal tissues also were permissive to HIV-1, in sharp contrast to intestinal macrophages, which we have shown previously do not support HIV-1 replication. Thus, among human vaginal and ectocervical mononuclear target cells, DCs are the first to take up HIV-1 and T cells support the most robust viral replication. Further characterization of the parameters of HIV-1 infection in genital mononuclear cells will enhance our understanding of HIV-1 infection in the female genital tract.

Immune reconstitution of the female reproductive tract of humanized BLT mice and their susceptibility to human immunodeficiency virus infection

An HIV vaccine capable of providing sterilizing immunity from vaginal infection would reduce the spread of HIV to women. Unfortunately, only one of the four HIV-1 vaccine clinical trials has demonstrated any level of protection (31%) against HIV-1 transmission. Additionally, only one topical microbicide clinical trial has reported an overall reduction in HIV transmission (39%). Developing even more effective vaccines and microbicides will require a better understanding of the key events involved in HIV infection and dissemination at the site of exposure. Novel immunodeficient mice capable of being systemically reconstituted with human hematopoietic stem cells have provided new systems where HIV transmission studies can be performed. Specifically, a humanized mouse model of vaginal HIV transmission has been developed that utilizes the humanized bone marrow-liver-thymus (BLT) mouse. The female reproductive tract (FRT) of humanized BLT mice is reconstituted with functional human immune cells rendering them susceptible to vaginal HIV-1 infection. In this review we focus on four aspects of BLT mice for the study of vaginal HIV-1 transmission: (1) we discuss methods for creating humanized BLT mice and their reconstitution with human hematopoietic cells, (2) we describe reconstitution of the BLT mouse FRT with human immune cells, (3) we highlight the work done regarding vaginal HIV-1 transmission and (4) we summarize the efficacy of systemic pre-exposure prophylaxis (PrEP) to prevent vaginal HIV-1 transmission in BLT mice. BLT mice are a highly relevant small animal model for studying vaginal HIV-1 transmission, prevention and therapy.

AIDS and optic neuritis in a rhesus monkey infected with the R5 clade C SHIV-1157ipd3N4

A Chinese rhesus macaque infected with the pathogenic CCR5-tropic clade C simian-human immunodeficiency virus, SHIV-1157ipd3N4, had persistent viremia, depletion of CD4(+) T cells to <200 cells/μl, opportunistic infections, coagulopathy, and gradual development of bilateral blindness. MRI revealed marked thickening of both optic nerves. Histopathological evaluation showed diffuse cellular infiltration at necropsy and a focus of SHIV-infected cells. This is the first report of CNS pathology following chronic infection with an obligate R5 SHIV.

The genotype of early-transmitting HIV gp120s promotes α (4) β(7)-reactivity, revealing α (4) β(7) +/CD4+ T cells as key targets in mucosal transmission

Mucosal transmission of HIV is inefficient. The virus must breach physical barriers before it infects mucosal CD4⁺ T cells. Low-level viral replication occurs initially in mucosal CD4⁺ T cells, but within days high-level replication occurs in Peyer's patches, the gut lamina propria and mesenteric lymph nodes. Understanding the early events in HIV transmission may provide valuable information relevant to the development of an HIV vaccine. The viral quasispecies in a donor contracts through a genetic bottleneck in the recipient, such that, in low-risk settings, infection is frequently established by a single founder virus. Early-transmitting viruses in subtypes A and C mucosal transmission tend to encode gp120s with reduced numbers of N-linked glycosylation sites at specific positions throughout the V1-V4 domains, relative to typical chronically replicating isolates in the donor quasispecies. The transmission advantage gained by the absence of these N-linked glycosylation sites is unknown. Using primary α₄β₇⁺/CD4⁺ T cells and a flow-cytometry based steady-state binding assay we show that the removal of transmission-associated N-linked glycosylation sites results in large increases in the specific reactivity of gp120 for integrin- α₄β₇. High-affinity for integrin α₄β₇, although not found in many gp120s, was observed in early-transmitting gp120s that we analyzed. Increased α₄β₇ affinity is mediated by sequences encoded in gp120 V1/V2. α₄β₇-reactivity was also influenced by N-linked glycosylation sites located in C3/V4. These results suggest that the genetic bottleneck that occurs after transmission may frequently involve a relative requirement for the productive infection of α₄β₇⁺/CD4⁺ T cells. Early-transmitting gp120s were further distinguished by their dependence on avidity-effects to interact with CD4, suggesting that these gp120s bear unusual structural features not present in many well-characterized gp120s derived from chronically replicating viruses. Understanding the structural features that characterize early-transmitting gp120s may aid in the design of an effective gp120-based subunit vaccine.

In the first days following sexual transmission, HIV replication occurs initially at relatively low levels in mucosal tissues because of a paucity of CD4⁺ T cell targets for the virus to infect. After a period of days, virus accesses specific gut tissues that are enriched in activated CD4⁺ T cells, where near-exponential replication ensues. The period of time before HIV accesses gut tissues represents a window of opportunity where a microbicide, pre-exposure and/or post-exposure antiretroviral prophylaxis or a vaccine-induced immune response could block infection. We previously reported that the HIV envelope protein gp120 binds to integrin α₄β₇ on the surface of CD4⁺ T cells. α₄β₇ mediates the homing of CD4⁺ T cells into the gut tissues where HIV can replicate exponentially. Here we report that the genotypic features that distinguish viruses isolated within the first month after infection, termed early-transmitting isolates, promote increased steady-state reactivity with α₄β₇. This property likely provides these viruses with enhanced transmission-fitness. These results suggest that the infection of α₄β₇⁺/CD4⁺ T cells can play an important role early in HIV transmission. These findings have potentially important implications in the design of interventions to block the mucosal transmission of HIV.

Gaining a foothold: how HIV avoids innate immune recognition

During the first week after sexual exposure to HIV, HIV infection does not appear to trigger a strong innate immune response. Here we describe some recent studies that show that HIV may avoid triggering antiviral innate immune responses by not replicating efficiently in dendritic cells and by avoiding detection in infected CD4 T cells and macrophages by harnessing a host cytoplasmic DNase TREX1 to digest nonproductive HIV reverse transcripts.

Selective transmission of R5 HIV-1 variants: where is the gatekeeper?

To enter target cells HIV-1 uses CD4 and a coreceptor. In vivo the coreceptor function is provided either by CCR5 (for R5) or CXCR4 (for X4 HIV-1). Although both R5 and X4 HIV-1 variants are present in body fluids (semen, blood, cervicovaginal and rectal secretions), R5 HIV-1 appears to transmit infection and dominates early stages of HIV disease. Moreover, recent sequence analysis of virus in acute infection shows that, in the majority of cases of transmission, infection is initiated by a single virus. Therefore, the existence of a "gatekeeper" that selects R5 over X4 HIV-1 and that operates among R5 HIV-1 variants has been suggested. In the present review we consider various routes of HIV-transmission and discuss potential gatekeeping mechanisms associated with each of these routes. Although many mechanisms have been identified none of them explains the almost perfect selection of R5 over X4 in HIV-1 transmission. We suggest that instead of one strong gatekeeper there are multiple functional gatekeepers and that their superimposition is sufficient to protect against X4 HIV-1 infection and potentially select among R5 HIV-1 variants. In conclusion, we propose that the principle of multiple barriers is more general and not restricted to protection against X4 HIV-1 but rather can be applied to other phenomena when one factor has a selective advantage over the other(s). In the case of gatekeepers for HIV-1 transmission, the task is to identify them and to decipher their molecular mechanisms. Knowledge of the gatekeepers' localization and function may enable us to enhance existing barriers against R5 transmission and to erect the new ones against all HIV-1 variants.

Finding a cure for HIV: will it ever be achievable?

Combination antiretroviral therapy (cART) has led to a major reduction in HIV-related mortality and morbidity. However, HIV still cannot be cured. With the absence of an effective prophylactic or therapeutic vaccine, increasing numbers of infected people, emerging new toxicities secondary to cART and the need for life-long treatment, there is now a real urgency to find a cure for HIV.

There are currently multiple barriers to curing HIV. The most significant barrier is the establishment of a latent or "silent" infection in resting CD4+ T cells. In latent HIV infection, the virus is able to integrate into the host cell genome, but does not proceed to active replication. As a consequence, antiviral agents, as well as the immune system, are unable to eliminate these long-lived, latently infected cells. Reactivation of latently infected resting CD4+ T cells can then re-establish infection once cART is stopped. Other significant barriers to cure include residual viral replication in patients receiving cART, even when the virus is not detectable by conventional assays. In addition, HIV can be sequestered in anatomical reservoirs, such as the brain, gastrointestinal tract and genitourinary tract.

Achieving either a functional cure (long-term control of HIV in the absence of cART) or a sterilizing cure (elimination of all HIV-infected cells) remains a major challenge. Several studies have now demonstrated that treatment intensification appears to have little impact on latent reservoirs. Some potential and promising approaches that may reduce the latent reservoir include very early initiation of cART and the use of agents that could potentially reverse latent infection.

Agents that reverse latent infection will promote viral production; however, simultaneous administration of cART will prevent subsequent rounds of viral replication. Such drugs as histone deacetylase inhibitors, currently used and licensed for the treatment of some cancers, or activating latently infected resting cells with cytokines, such as IL-7 or prostratin, show promising results in reversing latency in vitro when used either alone or in combination. In order to move forward toward clinical trials that target eradication, there needs to be careful consideration of the risks and benefits of these approaches, agreement on the most informative endpoints for eradication studies and greater engagement of the infected community.

Biological factors that may contribute to regional and racial disparities in HIV prevalence

Despite tremendous regional and subregional disparities in HIV prevalence around the world, epidemiology consistently demonstrates that black communities have been disproportionately affected by the pandemic. There are many reasons for this, and a narrow focus on socio-behavioural causes may be seen as laying blame on affected communities or individuals. HIV sexual transmission is very inefficient, and a number of biological factors are critical in determining whether an unprotected sexual exposure to HIV results in productive infection. This review will focus on ways in which biology, rather than behaviour, may contribute to regional and racial differences in HIV epidemic spread. Specific areas of focus are viral factors, host genetics, and the impact of co-infections and host immunology. Considering biological causes for these racial disparities may help to destigmatize the issue and lead to new and more effective strategies for prevention.

Mucosal immunity and HIV-1 infection: applications for mucosal AIDS vaccine development

Natural transmission of human immunodeficiency virus type 1 (HIV-1) occurs through gastrointestinal and vaginal mucosa. These mucosal tissues are major reservoirs for initial HIV replication and amplification, and the sites of rapid CD4(+) T cell depletion. In both HIV-infected humans and SIV-infected macaques, massive loss of CD4(+) CCR5(+) memory T cells occurs in the gut and vaginal mucosa within the first 10-14 days of infection. Induction of local HIV-specific immune responses by vaccines may facilitate effective control of HIV or SIV replication at these sites. Vaccines that induce mucosal responses, in particular CD8(+) cytotoxic T lymphocytes (CTL), have controlled viral replication at mucosal sites and curtailed systemic dissemination. Thus, there is strong justification for development of next generation vaccines that induce mucosal immune effectors against HIV-1 including CD8(+) CTL, CD4(+) T helper cells and secretory IgA. In addition, further understanding of local innate mechanisms that impact early viral replication will greatly inform future vaccine development. In this review, we examine the current knowledge concerning mucosal AIDS vaccine development. Moreover, we propose immunization strategies that may be able to elicit an effective immune response that can protect against AIDS as well as other mucosal infections.