Isolation of primary HIV-1 that target CD8+ T lymphocytes using CD8 as a receptor

Consequences of HIV infection in the bone marrow niche

Dysregulation of the bone marrow niche resulting from the direct and indirect effects of HIV infection contributes to haematological abnormalities observed in HIV patients. The bone marrow niche is a complex, multicellular environment which functions primarily in the maintenance of haematopoietic stem/progenitor cells (HSPCs). These adult stem cells are responsible for replacing blood and immune cells over the course of a lifetime. Cells of the bone marrow niche support HSPCs and help to orchestrate the quiescence, self-renewal and differentiation of HSPCs through chemical and molecular signals and cell-cell interactions. This narrative review discusses the HIV-associated dysregulation of the bone marrow niche, as well as the susceptibility of HSPCs to infection by HIV.

HIV co-receptor-tropism: cellular and molecular events behind the enigmatic co-receptor switching

Recognition of cell-surface receptors and co-receptors is a crucial molecular event towards the establishment of HIV infection. HIV exists as several variants that differentially recognize the principal co-receptors, CCR5 and CXCR4, in different cell types, known as HIV co-receptor-tropism. The relative levels of these variants dynamically adjust to the changing host selection pressures to infect a vast repertoire of cells in a stage-specific manner. HIV infection sets in through immune cells such as dendritic cells, macrophages, and T-lymphocytes in the acute stage, while a wide range of other cells, including astrocytes, glial cells, B-lymphocytes, and epithelial cells, are infected during chronic stages. A change in tropism occurs during the transition from acute to a chronic phase, termed as co-receptor switching marked by a change in disease severity. The cellular and molecular events leading to co-receptor switching are poorly understood. This review aims to collate our present understanding of the dynamics of HIV co-receptor-tropism vis-à-vis host and viral factors, highlighting the cellular and molecular events involved therein. We present the possible correlations between virus entry, cell tropism, and co-receptor switching, speculating its consequences on disease progression, and proposing new scientific pursuits to help in an in-depth understanding of HIV biology.

Recognition of Apoptotic Cells by Viruses and Cytolytic Lymphocytes: Target Selection in the Fog of War

Viruses and cytolytic lymphocytes operate in an environment filled with dying and dead cells, and cell fragments. For viruses, irreversible fusion with doomed cells is suicide. For cytotoxic T lymphocyte and natural killer cells, time and limited lytic resources spent on apoptotic targets is wasteful and may result in death of the host. We make the case that the target membrane cytoskeleton is the best source of information regarding the suitability of potential targets for engagement for both viruses and lytic effector cells, and we present experimental evidence for detection of apoptotic cells by HIV, without loss of infectivity.

Interaction of virus populations with their hosts

Viral population numbers are extremely large compared with those of their host species. Population bottlenecks are frequent during the life cycle of viruses and can reduce viral populations transiently to very few individuals. Viruses have to confront several types of constraints that can be divided into basal, cell-dependent, and organism-dependent constraints. Viruses overcome them exploiting a number of molecular mechanisms, with an important contribution of population numbers and genome variation. The adaptive potential of viruses is reflected in modifications of cell tropism and host range, escape to components of the host immune response, and capacity to alternate among different host species, among other phenotypic changes. Despite a fitness cost of most mutations required to overcome a selective constraint, viruses can find evolutionary pathways that ensure their survival in equilibrium with their hosts.

How HIV Nef Proteins Hijack Membrane Traffic To Promote Infection

The accessory protein Nef of human immunodeficiency virus (HIV) is a primary determinant of viral pathogenesis. Nef is abundantly expressed during infection and reroutes a variety of cell surface proteins to disrupt host immunity and promote the viral replication cycle. Nef counteracts host defenses by sequestering and/or degrading its targets via the endocytic and secretory pathways. Nef does this by physically engaging a number of host trafficking proteins. Substantial progress has been achieved in identifying the targets of Nef, and a structural and mechanistic understanding of Nef's ability to command the protein trafficking machinery has recently started to coalesce. Comparative analysis of HIV and simian immunodeficiency virus (SIV) Nef proteins in the context of recent structural advances sheds further light on both viral evolution and the mechanisms whereby trafficking is hijacked. This review describes how advances in cell and structural biology are uncovering in growing detail how Nef subverts the host immune system, facilitates virus release, and enhances viral infectivity.

Efficacy of antiviral treatment in cytomegalovirus detected ulcerative colitis: meta-analysis of available data

Aim: This meta-analysis aimed to pool available data regarding the efficacy of ganciclovir treatment among cytomegalovirus-detected ulcerative colitis patients.Methods: We screened PubMed, Ovid, Web of Science and Cochrane databases for relevant studies, and four investigators independently evaluated the studies for eligibility. The primary outcome was surgical resection or death from ulcerative colitis. The data were then pooled via DerSimonian-Laird estimator and Mantel-Haenszel (MH) method, two points added for continuity correction and random-effects model fitted in the Bayesian framework. We first constructed a Bugs model with Student t-distribution as prior for between-study heterogeneity. The model was fitted by Gibbs sampler (JAGS) to produce a marginal posterior distribution.Results: Our screening identified 15 eligible studies for final data synthesis and combined data from 191 ganciclovir-treated and 166 non-treated patients. Effect estimates from the fixed-effects meta-analysis model did not encourage ganciclovir treatment (OR, 1.43; 95% CIs [0-95, 2.16]), with a negligible unaccounted heterogeneity (I² = 0%). The Bayesian random-effects model generated high-density credible intervals, suggesting a high probability, that future studies will also not encourage ganciclovir treatment (mu, 1.028; 95% credible intervals [0.054, 2.238]; 80% credible intervals [0.401, 1.703]) which indicates that future studies will favor non-treatment of ulcerative colitis with ganciclovir.Conclusions: Data produced in this study do not encourage ganciclovir treatment for UC patients. However, studies included in this analysis were observational, and thus, inherited severe selection bias. We suggest randomized controlled studies be conducted to make firm recommendations in this context.

Interaction of Virus Populations with Their Hosts

Viral population numbers are extremely large compared with those of their host species. Population bottlenecks are frequent during the life cycle of viruses and can reduce viral populations transiently to very few individuals. Viruses have to confront several types of constraints that can be divided in basal, cell-dependent, and organism-dependent constraints. Viruses overcome them exploiting a number of molecular mechanisms, with an important contribution of population numbers and genome variation. The adaptive potential of viruses is reflected in modifications of cell tropism and host range, escape to components of the host immune response, and capacity to alternate among different host species, among other phenotypic changes. Despite a fitness cost of most mutations required to overcome a selective constraint, viruses can find evolutionary pathways that ensure their survival in equilibrium with their hosts.

The next generation of HIV/AIDS drugs: novel and developmental antiHIV drugs and targets

There are presently 42 million people worldwide living with HIV/AIDS, the majority of which have limited access to antiretrovirals. Even if worldwide penetration was possible, our current chemotherapeutic strategies still suffer from issues of cost, patient compliance, deleterious acute and chronic side effects, emerging single and multidrug resistance, and generalized treatment and economic issues. Even our best antiretroviral therapeutic strategy, highly active antiretroviral therapy (HAART), falls short of completely suppressing HIV replication. Therefore, expansion of current therapeutic options by discovering new antiretrovirals and targets will be critical in the coming years. This review addresses the current status of reverse transcriptase and protease inhibitor development, and summarizes the progress in emerging classes of HIV inhibitors, including entry (T-20, T-1249), coreceptor (SCH-C, SCH-D), integrase (beta-Diketos) and p7 nucleocapsid Zn finger inhibitors (thioesters and PATEs). In addition, the processes of virus entry, PIC transport to the nucleus, HIV interaction with nuclear pores, Tat function, Rev function and virus budding (Tsg101 and ubiquitination) are examined, and proof of concept inhibitors and potential antiviral targets discussed.

Acquisition of CD4-dependence by CD4-independent SIV passaged in human peripheral blood mononuclear cells

Background

Chemokine receptors (CKRs), the primordial receptors for primate lentiviruses, are sufficient to mediate virus-cell fusion. Several different fusogenic CKRs and related receptors provide a broad potential host cell range, presumably advantageous for viral spread within a given infected individual, and across species. By contrast, the additional constraint of obligatory CD4 binding, just prior to CKR engagement, radically restricts potential host cells within an individual (or lymph node microenvironment), and might also limit xenotransmission, as CD4 sequences vary among primates. In spite of these potential drawbacks, CD4 dependent entry for SIV and HIV is the rule rather than the exception, and is generally thought to have evolved by selection for 1) stabilization of virus–cell surface interactions, and 2) conformational shielding of readily neutralized CKR binding epitopes. CD4 binding residues of SIV and HIV envelope are recessed, (relatively hidden from immune detection) and may exhibit a strong degree of automimicry, thus benefitting from self tolerance.

Documented evolution, within individual macaques, of neutralization-resistant CD4-dependent SIV, derived from CD4-independent inocula, supports these ideas, but does not explain CD4’s exclusive role as the penultimate receptor-even more striking, given the wide diversity of CKRs and other surface molecules that can serve as actual fusion receptors for SIV. We, therefore, explored the additional, non-exclusive, hypothesis that surface CD4 on leukocytes is a marker of a more favorable host cell environment, as compared to CD8, NK, or B cell surface markers.

Results

We demonstrate progressive in vitro evolution of two SIV strains to CD4-dependence (and CXCR4 tropism) in normal human PBMCs (hPBMCs). The two CD4-independent strains of SIV tested developed nearly complete CD4 dependence over several months of serial passage in hPBMCs, correlating with a limited number of non-synonymous env region mutations, some previously reported to be determinants of CD4-dependency. The initial ability of SIV stocks to grow to significant (albeit, relatively low) levels in CD4(−), CD14(−) cells was also lost with long term passage. Rapid emergence and subsequent prominence of G → A and A → G mutations within env regions associated with CD4 dependence was seen.

Conclusions

Progressive acquisition of strict CD4 tropism, independent of immunoselection, supports the idea that surface CD4 identifies optimal host cells having intracellular environments most favorable to viral replication. The prominence of mutations involving G to A, or A to G, suggests that APOBEC 3 mediated infidelity may facilitate rapid switching of cell surface receptor usage within SIV swarms encountering fluctuating availability of optimal CD4⁺CKR⁺ targets. These observations of non-immune selection are compatible with, and may accelerate, simultaneous selection for previously described CD4-dependent neutralization resistance in vivo.

Down-modulation of CD8αβ is a fundamental activity of primate lentiviral Nef proteins

It is well established that the Nef proteins of human and simian immunodeficiency viruses (HIV and SIV) modulate major histocompatibility complex class I (MHC-I) cell surface expression to protect infected cells against lysis by cytotoxic T lymphocytes (CTLs). Recent data supported the observation that Nef also manipulates CTLs directly by down-modulating CD8αβ (J. A. Leonard, T. Filzen, C. C. Carter, M. Schaefer, and K. L. Collins, J. Virol. 85:6867-6881, 2011), but it remained unknown whether this Nef activity is conserved between different lineages of HIV and SIV. In this study, we examined a total of 42 nef alleles from 16 different primate lentiviruses representing most major lineages of primate lentiviruses, as well as nonpandemic HIV-1 strains and the direct precursors of HIV-1 (SIVcpz and SIVgor). We found that the vast majority of these nef alleles strongly down-modulate CD8β in human T cells. Primate lentiviral Nefs generally interacted specifically with the cytoplasmic tail of CD8β, and down-modulation of this receptor was dependent on the conserved dileucine-based motif and two adjacent acidic residues (DD/E) in the C-terminal flexible loop of SIV Nef proteins. Both of these motifs are known to be important for the interaction of HIV-1 Nef with AP-2, and they were also shown to be critical for down-modulation of CD4 and CD28, but not MHC-I, by SIV Nefs. Our results show that down-modulation of CD4, CD8β, and CD28 involves largely overlapping (but not identical) domains and is most likely dependent on conserved interactions of primate lentiviral Nefs with cellular adaptor proteins. Furthermore, our data demonstrate that Nef-mediated down-modulation of CD8αβ is a fundamental property of primate lentiviruses and suggest that direct manipulation of CD8+ T cells plays a relevant role in viral immune evasion.

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.

HIV-1 Nef disrupts intracellular trafficking of major histocompatibility complex class I, CD4, CD8, and CD28 by distinct pathways that share common elements

The Nef protein is an important HIV virulence factor that promotes the degradation of host proteins to augment virus production and facilitate immune evasion. The best-characterized targets of Nef are major histocompatibility complex class I (MHC-I) and CD4, but Nef also has been reported to target several other proteins, including CD8β, CD28, CD80, CD86, and CD1d. To compare and contrast the effects of Nef on each protein, we constructed a panel of chimeric proteins in which the extracellular and transmembrane regions of the MHC-I allele HLA-A2 were fused to the cytoplasmic tails of CD4, CD28, CD8β, CD80, CD86, and CD1d. We found that Nef coprecipitated with and disrupted the expression of molecules with cytoplasmic tails from MHC-I HLA-A2, CD4, CD8β, and CD28, but Nef did not bind to or alter the expression of molecules with cytoplasmic tails from CD80, CD86, and CD1d. In addition, we used short interfering RNA (siRNA) knockdown and coprecipitation experiments to implicate AP-1 as a cellular cofactor for Nef in the downmodulation of both CD28 and CD8β. The interaction with AP-1 required for CD28 and CD8β differed from the AP-1 interaction required for MHC-I downmodulation in that it was mediated through the dileucine motif within Nef (LL(164,165)AA) and did not require the tyrosine binding pocket of the AP-1 μ subunit. In addition, we demonstrate a requirement for β-COP as a cellular cofactor for Nef that was necessary for the degradation of targeted molecules HLA-A2, CD4, and CD8. These studies provide important new information on the similarities and differences with which Nef affects intracellular trafficking and help focus future research on the best potential pharmaceutical targets.

CD4-independent human immunodeficiency virus infection involves participation of endocytosis and cathepsin B

During a comparison of the infectivity of mNDK, a CD4-independent human immunodeficiency virus type 1 (HIV-1) strain, to various cell lines, we found that HeLa cells were much less susceptible than 293T and TE671 cells. Hybridoma cells between HeLa and 293T cells were as susceptible as 293T cells, suggesting that cellular factors enhance the mNDK infection in 293T cells. By screening a cDNA expression library in HeLa cells, cystatin C was isolated as an enhancer of the mNDK infection. Because cathepsin B protease, a natural ligand of cystatin C, was upregulated in HeLa cells, we speculated that the high levels of cathepsin B activities were inhibitory to the CD4-independent infection and that cystatin C enhanced the infection by impairing the excessive cathepsin B activity. Consistent with this idea, pretreatment of HeLa cells with 125 µM of CA-074Me, a cathepsin B inhibitor, resulted in an 8-fold enhancement of the mNDK infectivity. Because cathepsin B is activated by low pH in acidic endosomes, we further examined the potential roles of endosomes in the CD4-independent infection. Suppression of endosome acidification or endocytosis by inhibitors or by an Eps15 dominant negative mutant reduced the infectivity of mNDK in which CD4-dependent infections were not significantly impaired. Taken together, these results suggest that endocytosis, endosomal acidification, and cathepsin B activity are involved in the CD4-independent entry of HIV-1.

Distribution of HIV-1 infection in different T lymphocyte subsets: antiretroviral therapy-naïve vs. experienced patients

Memory CD4 T cells are the primary targets of HIV-1 infection, which then subsequently spreads to other T lymphocyte subsets. Antiretroviral therapy (ART) alters the pattern of HIV-1 distribution. Blood samples were collected from ART-naïve or -experienced HIV-1 patients, and the memory and naïve subsets of CD4(+) and CD8(+) T lymphocytes, respectively, were isolated by cell sorting. DNA was extracted and the HIV-1 env C2/V3 region PCR amplified. Amplicons were cloned and sequenced, and genetic relatedness among different HIV-1 compartments was determined by the phylogenetic analysis of clonal sequences. The viral V3 sequence of HIV-1 in each compartment was analyzed by using webPSSM to determine CCR5 or CXCR4 coreceptor binding property of the virus. The direction of viral migration among involved compartments was determined by using the MacClade program. In ART-naïve patients, HIV-1 was generally confined to the memory CD4 T (mT4) cell compartment, even though in a few cases, naïve CD4 T (nT4) cells were also infected. When this occurred, the HIV-1 gene migrated from nT4 to mT4. In contrast, HIV-1 was detected in nT4 and mT4 as well as in the memory CD8 T (mT8) compartments of ART-experienced patients. However, no clear pattern of directional HIV-1 gene flow among the compartments could be determined because of the small sample size. All HIV-1-infected T cell compartments housed the virus that used either CCR5 or CXCR4 as the coreceptor.

Properties of HTLV-I transformed CD8+ T-cells in response to HIV-1 infection

HIV-1 infection studies of primary CD8(+) T-cells are hampered by difficulty in obtaining a significant number of targets for infection and low levels of productive infection. Further, there exists a paucity of CD8-expressing T-cell lines to address questions pertaining to the study of CD8(+) T-cells in the context of HIV-1 infection. In this study, a set of CD8(+) T-cell clones were originated through HTLV-I transformation in vitro, and the properties of these cells were examined. The clones were susceptible to T-cell tropic strains of the virus and exhibited HIV-1 production 20-fold greater than primary CD4(+) T-cells. Productive infection resulted in a decrease in expression of CD8 and CXCR4 molecules on the surface of the CD8(+) T-cell clones and antibodies to these molecules abrogated viral binding and replication. These transformed cells provide an important tool in the study of CD8(+) T-cells and may provide important insights into the mechanism(s) behind HIV-1 induced CD8(+) T-cell dysfunction.

HIV reservoirs in vivo and new strategies for possible eradication of HIV from the reservoir sites

Even though the treatment of human immunodeficiency virus (HIV)-infected individuals with highly active antiretroviral therapy (HAART) provides a complete control of plasma viremia to below detectable levels (<40 copies/mL plasma), there is an unequal distribution of all antiretroviral drugs across diverse cellular and anatomic compartments in vivo. The main consequence of this is the acquisition of resistance by HIV to all known classes of currently prescribed antiretroviral drugs and the establishment of HIV reservoirs in vivo. HIV has a distinct advantage of surviving in the host via both pre-and postintegration latency. The postintegration latency is caused by inert and metabolically inactive provirus, which cannot be accessed either by the immune system or the therapeutics. This integrated provirus provides HIV with a safe haven in the host where it is incessantly challenged by its immune selection pressure and also by HAART. Thus, the provirus is one of the strategies for viral concealment in the host and the provirus can be rekindled, through unknown stimuli, to create progeny for productive infection of the host. Thus, the reservoir establishment remains the biggest impediment to HIV eradication from the host. This review provides an overview of HIV reservoir sites and discusses both the virtues and problems associated with therapies/strategies targeting these reservoir sites in vivo.

Could CD4 capture by CD8+ T cells play a role in HIV spreading?

CD8(+) T cells have been shown to capture plasma membrane fragments from target cells expressing their cognate antigen, a process termed "trogocytosis". Here, we report that human CD4, the Human Immunodeficiency Virus (HIV) receptor, can be found among the proteins transferred by trogocytosis. CD4 is expressed in a correct orientation after its capture by CD8(+) T cells as shown by its detection using conformational antibodies and its ability to allow HIV binding on recipient CD8(+) T cells. Although we could not find direct evidence for infection of CD8(+) T cells having captured CD4 by HIV, CD4 was virologically functional on these cells as it conferred on them the ability to undergo syncytia formation induced by HIV-infected MOLT-4 cells. Our results show that acquisition of CD4 by CD8(+) T cells via trogocytosis could play a previously unappreciated role for CD8(+) T cells in HIV spreading possibly without leading to their infection.

A mathematical model of HIV infection: Simulating T4, T8, macrophages, antibody, and virus via specific anti-HIV response in the presence of adaptation and tropism

A mathematical model of the host's immune response to HIV infection is proposed. The model represents the dynamics of 13 subsets of T cells (HIV-specific and nonspecific, healthy and infected, T4 and T8 cells), infected macrophages, neutralizing antibodies, and virus. The results of simulation are in agreement with published data regarding T4 cell concentration and viral load, and exhibit the typical features of HIV infection, i.e. double viral peaks in the acute stage, sero conversion, inverted T cell ratio, establishment of set points, steady state, and decline into AIDS. This result is achieved by taking into account thymic aging, viral and infected cell stimulation of specific immune cells, background nonspecific antigens, infected cell proliferation, viral production by infected macrophages and T cells, tropism, viral, and immune adaptation. Starting from this paradigm, changes in the parameter values simulate observed differences in individual outcomes, and predict different scenarios, which can suggest new directions in therapy. In particular, large parameter changes highlight the potentially critical role of both very vigorous and extremely damped specific immune response, and of the elimination of virus release by macrophages. Finally, the time courses of virus, antibody and T cells production and removal are systematically investigated, and a comparison of T4 and T8 cell dynamics in a healthy and in a HIV infected host is offered.

T-cell dysfunction in HIV-1 infection: targeting the inhibitors

Since AIDS emerged almost three decades ago, there have been considerable advances in the field of antiretroviral chemotherapy for those chronically infected with HIV-1. However, this therapy is noncurative and as our understanding of HIV-1 immunopathogenesis increases, it is becoming apparent that further therapeutic interventions are required to reverse the devastating effects of HIV-1 infection worldwide. While viral clearance remains the principle goal of HIV-1 treatment, this article describes immunotherapeutic options that target the immunological effects of the virus, to reduce its presence in the body and counteract viral-induced T-cell dysfunction and inhibition. Such approaches may augment existing antiretroviral therapy to overturn virus-induced T-cell anergy in the infected host, improving levels of immune control that reduce viremia and decrease the rate of transmission.

HIV immunopathogenesis and strategies for intervention

Therapeutic options aimed at tackling the HIV pandemic face many obstacles. The lack of readily accessible and affordable therapies means that most of those affected go untreated. The array of escape mechanisms used by HIV has undermined the efficiency of many antiviral products and continually represents a barrier to the development of an effective vaccine. Recent developments have seen a shift away from a cytopathic viral model of HIV pathogenesis towards the crucial role of immunopathogenic features--notably generalised immune activation--in the development of AIDS. As conventional vaccine strategies have sought to promote viral neutralisation and suppressive cellular responses, novel strategies that aim to address HIV immunopathogenesis should be sought. We review current opinion on HIV-induced pathogenic immune activation and strategies aimed at eliminating HIV, including a potential role for non-neutralising antibodies as part of a therapeutic vaccine option.

Characterization of a CD4-independent clinical HIV-1 that can efficiently infect human hepatocytes through chemokine (C-X-C motif) receptor 4

OBJECTIVE

HIV-1 isolates are prominently CD4-dependent and, to date, only a few laboratory-adapted CD4-independent strains have been reported. Therefore, whether CD4-independent viruses may exist in HIV-1-infected patients has remained unclear. Here, we report the successful isolation of a CD4-independent clinical HIV-1 strain, designated SDA-1, from the viral quasispecies of a therapy-naive HIV-1 and Pneumocystis jirovecii pneumonia patient in the late-stage of AIDS with extremely low CD4 cell count (CD4 = 1/microl). We characterized this virus and further explored whether it could infect or induce pathological effects in human hepatocytes.

DESIGN AND METHODS

To determine coreceptor usage and CD4-independent infection, the HIV-1 envelope (Env)-pseudotypes and Env-chimeric viruses were used.

RESULTS

SDA-1 was able to infect CD4 cell lines through either chemokine (C-X-C motif) receptor 4 or CCR5. It still maintained the ability to infect CD4 cells through multiple coreceptors of chemokine (C-X-C motif) receptor 4, chemokine (C-C motif) receptor 5, chemokine (C-C motif) receptor 3 and chemokine (C-C motif) receptor 8. Productive infection by SDA-1 was noted in both CD4-negative hepatoma cells and primary cultured human hepatocytes. Moreover, we demonstrated that SDA-1 could efficiently infect human hepatocytes on both static and mitotic phases through chemokine (C-X-C motif) receptor 4, without inducing apoptotic cell death.

CONCLUSION

The present study provides evidence that emergence of CD4-independent HIV-1 virus in vivo may occur in HIV-1-infected patients. In addition, these results shed light on the mechanisms involved in liver damage in HIV-1-infected individuals, which could have important implications concerning the range of mutability and the pathogenesis of AIDS.

Infection of CD8+CD45RO+ memory T-cells by HIV-1 and their proliferative response

CD8+ T-cells are involved in controlling HIV-1 infection by eliminating infected cells and secreting soluble factors that inhibit viral replication. To investigate the mechanism and significance of infection of CD8+ T-cells by HIV-1 in vitro, we examined the susceptibility of these cells and their subsets to infection. CD8+ T-cells supported greater levels of replication with T-cell tropic strains of HIV-1, though viral production was lower than that observed in CD4+ T-cells. CD8+ T-cell infection was found to be productive through ELISA, RT-PCR and flow cytometric analyses. In addition, the CD8+CD45RO+ memory T-cell population supported higher levels of HIV-1 replication than CD8+CD45RA+ naïve T-cells. However, infection of CD8+CD45RO+ T-cells did not affect their proliferative response to the majority of mitogens tested. We conclude, with numerous lines of evidence detecting and measuring infection of CD8+ T-cells and their subsets, that this cellular target and potential reservoir may be central to HIV-1 pathogenesis.

Antiviral NK cell responses in HIV infection: II. viral strategies for evasion and lessons for immunotherapy and vaccination

As is the case in other viral infections, humans respond to HIV infection by activating their NK cells. However, the virus uses several strategies to neutralize and evade the host's NK cell responses. Consequently, it is not surprising that NK cell functions become compromised in HIV-infected individuals in early stages of the infection. The compromised NK cell functions also adversely affect several aspects of the host's antiviral adaptive immune responses. Researchers have made significant progress in understanding how HIV counters NK cell responses of the host. This knowledge has opened new avenues for immunotherapy and vaccination against this infection. In the first part of this review article, we gave an overview of our current knowledge of NK cell biology and discussed how the genes encoding NK cell receptors and their ligands determine innate genetic resistance/susceptibilty of humans against HIV infections and AIDS. In this second part, we discuss NK cell responses, viral strategies to counter these responses, and finally, their implications for anti-HIV immunotherapy and vaccination.

HIV-1-infected CD8+CD4+ T cells decay in vivo at a similar rate to infected CD4 T cells during HAART

OBJECTIVES

To investigate the potential for CD4+CD8+ T cells [CD8 double positive (CD8 DP)] T cells to form a reservoir of HIV-1 following HAART through measurement of the rate of decay of infected CD4/CD8 DP T cells.

METHODS

HIV-1 proviral loads in highly pure CD4 and CD8 DP T cells were determined for study subjects before and after 200-400 days of therapy and HIV-1 DNA decay rates were calculated.

RESULTS

Before therapy, HIV-1 proviral load in CD8 DP correlated negatively with CD4 cell count. Decay rates of HIV-1-infected CD4 and CD8 DP T cells were similar. Rates for CD8 DP T cells correlated with the time to suppression of viral replication, whereas no such relationship was true for CD4 cell decay rates. A significant reduction in activated cells was observed for both cell types. The action of HAART on HIV-1 replication was similar for both CD4 cells and CD8 DP T cells, although the rate of clearance of infected CD8 DP T cells appeared more critical for a rapid reduction in plasma viral load. Although the size of the CD8 DP T cell reservoir in peripheral blood was smaller relative to that of CD4 cells, HAART did not completely clear HIV-1 infection from this cell subset.

CONCLUSION

This study confirmed that CD8 DP T cells are a major reservoir for HIV-1 in vivo and, therefore, represent a potential reservoir for HIV-1 during HAART, in a manner analogous to that of CD4 T cells.

Frequent detection of cell-associated HIV-1 RNA in patients with plasma viral load <50 copies/ml

Despite prolonged undetectable plasma viral load some HIV-1 infected patients have been reported to develop resistance-associated mutations leading to treatment failure. The mechanisms for this phenomenon and the point of origin for residual viral evolution are still not elucidated. In order to quantify cell-associated HIV-1 RNA in patients with different levels of plasma viremia paired cell-associated HIV-1 RNA loads and plasma viral loads were determined. Weak inverse correlation between these parameters and the amounts of CD4(+) T cells was observed, whereas there was no correlation between viral loads and CD8(+) T cells or CD14(+) monocytes, respectively. In a subset of patients, cell-associated and plasma HIV-1 env V3 sequences were analyzed. Plasma viral load and the amount of cell-associated HIV-RNA correlated strongly. However, in 62.3% of patients with undetectable plasma viral load cell-associated HIV-RNA could be detected. Analyses of HIV-RNA in plasma and blood cells showed identical sequences in 4/19 patients, whereas the majority of patients had differing HIV-1 RNA sequences in plasma and cells, respectively. In summary, this study shows that residual viral replication in peripheral blood still occurs in the majority of patients with undetectable plasma viral load. Since these replication events could lead to ongoing viral evolution it should be considered to optimize antiretroviral therapy in order to minimize the development of drug resistance.

Virus immunocapture provides evidence of CD8 lymphocyte-derived HIV-1 in vivo

OBJECTIVES

To demonstrate that HIV-1 immunocapture with an antibody against CD8 specifically captures virions derived from infected CD8 T cells, and to determine the proportion of HIV-1 derived from CD8 lymphocytes in plasma samples from HIV-infected individuals.

METHODS

A virus capture method was developed to enable the detection of HIV-1 virions based upon the presence of certain cell-specific host-derived proteins (CD8, CD3, CD36) within the viral envelope. HIV-1 virions were captured using antibodies against these proteins and levels of bound virus were determined by quantitative reverse transcriptase-polymerase chain reaction. Highly pure CD8 and CD3+CD8- T-cell cultures were used as in-vitro models to determine the specificity of the virus capture technique.

RESULTS

The in-vitro model demonstrates that incorporation of the CD8 molecule into released virions is specific to infection of CD8 T cells. Levels of HIV-1 immunocaptured from plasma of infected individuals using the anti-CD8 antibody indicate that up to 15% (range 10-33) of the plasma viral load is derived from CD8 lymphocytes.

CONCLUSION

This study demonstrates for the first time that HIV-1-infected CD8 T cells can contribute substantially to levels of circulating virus during the course of infection. Levels of CD8-derived virus did not correlate with the level of infection of circulating CD8 T cells, but do show a significantly good fit to plasma viral loads based on a power model. The extensive infection of CD8 T cells implied by these results may contribute towards immune dysfunction and disease progression to AIDS.

Productive human immunodeficiency virus type 1 infection in peripheral blood predominantly takes place in CD4/CD8 double-negative T lymphocytes

Human immunodeficiency virus type 1 (HIV-1) transcription is subject to substantial fluctuation during the viral life cycle. Due to the low frequencies of HIV-1-infected cells, and because latently and productively infected cells collocate in vivo, little quantitative knowledge has been attained about the range of in vivo HIV-1 transcription in peripheral blood mononuclear cells (PBMC). By combining cell sorting, terminal dilution of intact cells, and highly sensitive, patient-specific PCR assays, we divided PBMC obtained from HIV-1-infected patients according to their degree of viral transcription activity and their cellular phenotype. Regardless of a patient's treatment status, the bulk of infected cells exhibited a CD4+ phenotype but transcribed HIV-1 provirus at low levels, presumably insufficient for virion production. Furthermore, the expression of activation markers on the surface of these CD4+ T lymphocytes showed little or no association with enhancement of viral transcription. In contrast, HIV-infected T lymphocytes of a CD4-/CD8- phenotype, occurring exclusively in untreated patients, exhibited elevated viral transcription rates. This cell type harbored a substantial proportion of all HIV RNA+ cells and intracellular viral RNAs and the majority of cell-associated virus particles. In conjunction with the observation that the HIV quasispecies in CD4+ and CD4-)/CD8- T cells were phylogenetically closely related, these findings provide evidence that CD4 expression is downmodulated during the transition to productive infection in vivo. The abundance of viral RNA in CD4-/CD8- T cells from viremic patients and the almost complete absence of viral DNA and RNA in this cell type during antiretroviral treatment identify HIV+ CD4-/CD8 T cells as the major cell type harboring productive infection in peripheral blood.

How does hepatitis C virus enter cells?

Hepatitis C virus (HCV) exists in different forms in the circulation of infected people: lipoprotein bound and lipoprotein free, enveloped and nonenveloped. Viral particles with the highest infectivity are associated with lipoproteins, whereas lipoprotein-free virions are poorly infectious. The detection of HCV's envelope proteins E1 and E2 in lipoprotein-associated virions has been challenging. Because lipoproteins are readily endocytosed, some forms of HCV might utilize their association with lipoproteins rather than E1 and E2 for cell attachment and internalization. However, vaccination of chimpanzees with recombinant envelope proteins protected the animals from hepatitis C infection, suggesting an important role for E1 and E2 in cell entry. It seems possible that different forms of HCV use different receptors to attach to and enter cells. The putative receptors and the assays used for their validation are discussed in this review.

Positivity of cytomegalovirus antibodies predicts a better clinical and radiological outcome in multiple sclerosis patients

OBJECTIVES

To establish the relationship between the presence and titer of virus-specific serum- and cerebrospinal fluid (CSF)-antibodies in multiple sclerosis (MS) patients and disease severity measured with different quantitative magnetic resonance imaging (MRI) techniques.

METHODS

We investigated an association between clinical and MRI measures of disease activity and the presence and titer of IgG antibodies against seven common viruses (measles, rubella, herpes simplex virus type 1 and 2, varicella zoster virus, cytomegalovirus (CMV) and Epstein-Barr virus). One hundred and forty (90 female/50 male) patients with definite MS and 131 age and sex-matched controls participated in the study. Antibody positivity and titer were ascertained by the enzyme linked immunosorbent assay (ELISA) technique and clinical assessment was performed by evaluating the expanded disability status scale (EDSS) score and the lifetime relapse rate (LRR). T1- and T2-lesion loads (LL) and the brain parenchymal fraction (BPF) were calculated.

RESULTS

Multiple analyses showed that there was an association between antibody positivity against CMV and higher titer and better clinical and MRI outcomes. The cluster analyses indicated that patients positive for antibodies against CMV had significantly older age at onset (uncorr p = 0.001 and corr p = 0.009), lower LRR (uncorr p = 0.003 and corr p = 0.03) and higher BPF (uncorr p = 0.004 and pcorr p = 0.04). CMV-positive patients who had higher antibody titer showed lower T2-LL (uncorr p = 0.003 and corr p = 0.03) and higher BPF (uncorr p = 0.006 and corr p = 0.05).

DISCUSSION

Surprisingly, our results focused attention on the 'protective' role of a particular virus. CMV is probably capable of triggering some immunomodulating/immune evasion mechanisms which may decrease immune reactivity in MS patients. Further studies are needed to confirm and elucidate our study results on a larger sample of MS patients and in animal model studies.

Specific interaction of CXCR4 with CD4 and CD8alpha: functional analysis of the CD4/CXCR4 interaction in the context of HIV-1 envelope glycoprotein-mediated membrane fusion

We investigated possible interactions between HIV-1 receptor (CD4) and the main coreceptors CXCR4 and CCR5. We found that CD4 and CXCR4 coexpressed in 293T cells form a complex that can be immunoprecipitated with antibodies directed against the extracellular domain of either protein. Mutagenesis revealed that the CD4/CXCR4 interaction maps to two previously uncharacterized basic motifs in the cytoplasmic domain of CD4. HIV-1 envelope glycoprotein-mediated membrane fusion was found to be independent of the ability of CD4 and CXCR4 to interact, whether fusion was studied in a virus-cell or a cell-cell model. However, this interaction might explain the adaptation of HIV-1 to CXCR4 as an alternative to CCR5. We found that CXCR4 also interacts with the cytoplasmic domain of CD8alpha in a way that is similar to the CD4/CXCR4 interaction. The CD4/CXCR4 and CD8alpha/CXCR4 interactions may thus be involved in cellular signaling pathways shared by the CD4 and CD8alpha molecules.

Persistent abnormalities in peripheral blood dendritic cells and monocytes from HIV-1-positive patients after 1 year of antiretroviral therapy

Antiretroviral therapy (ART) has led to marked decreases in morbidity and mortality rates among HIV-1-positive patients; however, immune recovery is not complete. Although dendritic cells (DCs) were shown to be involved in HIV-1 pathogenesis, few studies have investigated the effect of ART on DCs. We have analyzed the effect of ART on numerical distribution, expression of chemokine receptors, and ex vivo production of inflammatory cytokines by peripheral blood (PB) monocytes and DCs in a cohort of chronically infected HIV-1-positive patients. Patients were tested before therapy and at weeks +2, +4, +8, +12, and +52 after starting ART.Our results show an incomplete T-cell immune reconstitution in chronically infected patients who had undetectable plasma viremia while taking ART for 1 year. This was associated with persistent abnormalities at week +52 of ART, corresponding to increased numbers of CD16 DCs and monocytes, as well as altered expression of CXC chemokine receptors, in the form of increased CXCR1 expression on monocytes and decreased reactivity for CXCR2 and/or CXCR4 on myeloid and plasmacytoid DCs. In addition, an abnormally high spontaneous ex vivo secretion of inflammatory cytokines by CD16 DCs and monocytes was still detected after 1 year of ART. These abnormalities were especially pronounced in patients with less than 200 CD4 T cells/microL, which could be related to the persistence of undetected viral replication and sustained immune activation.

Genetic analyses reveal structured HIV-1 populations in serially sampled T lymphocytes of patients receiving HAART

HIV-1 infection and compartmentalization in diverse leukocyte targets significantly contribute to viral persistence during suppressive highly active antiretroviral therapy (HAART). Longitudinal analyses were performed on envelope sequences of HIV-1 populations from plasma, CD4+ and CD8+ T lymphocytes in 14 patients receiving HAART and 1 therapy-naive individual. Phylogenetic reconstructions and analysis of molecular variance revealed that HIV-1 populations in CD4+ and CD8+ T cells remained compartmentalized over time in most individuals. Analyses of viral genetic variation demonstrated that, despite compartmentalization remaining over time, viral subpopulations tended not to persist and evolve but instead broke down and became reconstituted by new founder viruses. Due to the profound impact of HAART on viral evolution, it was difficult to discern whether these dynamics were ongoing during treatment or predominantly established prior to the commencement of therapy. The genetic structure and viral founder effects observed in serially sampled T lymphocyte populations supported a scenario of metapopulation dynamics in the tissue(s) where different leukocytes become infected, a factor likely to contribute to the highly variable way that drug resistance evolves in different individuals during HAART.

HIV replication elicits little cytopathic effects in vivo: Analysis of surrogate markers for virus production, cytotoxic T cell response and infected cell death

Several potential mechanisms for viral destruction of HIV-infected cells have been described. The hypothesis was examined that if HIV were cytopathic, a positive relation between the in vivo virus production or CTL activity and infected cell death should be observed. In a regression analysis no significant relation was found between surrogate markers for in vivo virus production or the virus-specific CTL response and death rates of productively infected cells. In a subgroup of patients the hypothesis is rejected that HIV replication elicits a large (R(2) > 0.25) cytopathic effect (P < 0.05, N = 36). It is concluded that HIV replication elicits little cytopathic effect in productively infected cells and that CD4(+) T lymphocytes are eroded by other mechanisms.

HIV type 1 persistence in CD4- /CD8- double negative T cells from patients on antiretroviral therapy

The establishment of reservoirs of latently infected cells is thought to contribute to the persistence of HIV-1 infection in the host. Studies so far have mainly focused on the long-lived reservoir of HIV-infected resting CD4+ T cells. A discrete population of HIV-infected CD4-/CD8- double negative (DN) T cells has recently been shown to exist and may also play a role in HIV-1 persistence. DN T cells are CD3 positive, either TCRalphabeta or TCRgammadelta positive, but lack both CD4 and CD8 surface markers. We developed a novel, magnetic bead column-based cell fractionation procedure for isolating >99% pure DN T cells. CD4+, CD8+, and DN T cells were purified from 23 samples of a cohort of 18 HIV-1-infected patients. Each cell fraction was analyzed for levels of total and integrated HIV-1 DNA. A correlation was observed between the presence of HIV-1 DNA in the DN T cell fraction and plasma viral load (VL). Using a micrococulture technique, we saw an initial release of virus from DN T cells of a patient with high VL. Analysis of env and nef sequence data suggested that the HIV-1 present in CD4+ and DN T cells originated from a common infecting strain. Different from the published literature, we have demonstrated the presence of HIV-1 DNA in DN T cells only in patients who are experiencing HAART failure. While these cells may have a limited role in viral persistence in high VL patients, our results suggest DN T cells are unlikely to be a major reservoir in patients on HAART with clinically undetectable plasma viral RNA.

Infection of human and non-human cells by a highly fusogenic primary CD4-independent HIV-1 isolate with a truncated envelope cytoplasmic tail

Truncation of the envelope cytoplasmic tail has enabled FIV, SIV, and some laboratory HIV-1 strains to acquire broader cellular tropism and enhanced fusogenicity. Here we have characterized a primary CD4-independent HIV-1 isolate (92UG046-T8) with a truncated cytoplasmic tail that was able to infect and induce syncytia in primary lymphocytes from human, chimpanzee, and monkey, as well as CD4-negative cell lines from human and monkey. Increased syncytia were also noticeable with 293 cells expressing the cloned envelope from the 92UG046-T8 isolate suggesting envelope-mediated cellular fusion. Except pooled serum from HIV-1-infected individuals, monoclonal anti-envelope antibodies or antibodies/antagonists against CD4, CXCR4, and CCR5 were not able to prevent infection by the 92UG046-T8 isolate. This is the first report showing a primary HIV-1 variant with truncated cytoplasmic tail which is highly fusogenic and can infect a broad range of cells from human and non-human origins. In vivo evolution of similar HIV-1 mutants may have important implications in AIDS pathogenesis.

HIV type 1 cervicovaginal reservoirs in the era of HAART

Highly active antiretroviral therapy (HAART) does not lead to viral eradication, due to HIV-1 residual disease. We investigated whether the cervicovaginal tract serves as a viral reservoir. Seven out of eight cervicovaginal fluids were positive for cell-free HIV-1, by supersensitive reverse transcriptase-polymerase chain reactions (RT-PCR), with a detection limit of 1 copy/ml. No viral outgrowth, intracellular proviral DNA, or viral RNA was detected from cervicovaginal lavage and ecto- and endocervical cells. The cervicovaginal tract of patients on HAART is likely not a major solid tissue reservoir for HIV-1. Nonetheless, the presence of even low cell-free HIV-1 RNA in cervicovaginal secretions continues to suggest the importance of practicing protected sex, even in the era of HAART.

Therapeutic role of CD8+ T cells in HIV-1 infection: targets and suppressors of viral replication

CD8+ T cells are pivotal in controlling viral replication in HIV-1-infected subjects. However, in chronic infection, HIV-1-specific CD8+ T cells fail to adequately control infection, presenting incomplete maturation and more severe functional impairment with advanced disease. Accumulating evidence has shown that CD8+ T cells can also be productively infected by HIV-1. Whether HIV-1 infection of CD8+ T lymphocytes impacts on their antiviral activity remains to be determined. This review explores the potential mechanisms of HIV-1 infection of CD8+ T cells, its likely contribution to the immunopathogenesis of HIV-1 infection and potential therapeutic interventions.

CXCR4-dependent infection of CD8+, but not CD4+, lymphocytes by a primary human immunodeficiency virus type 1 isolate

We recently isolated from an infant an X4-syncytium-inducing (SI) human immunodeficiency virus type 1 (HIV-1) variant (92US143-T8) that was able to infect CD8+ lymphocytes independently of CD4. Although it was CD4 independent, the 92US143-T8 isolate also maintained the ability to infect CD4+ cells. In the present study, we investigated the role of CXCR4 in the infection of CD4+ and CD8+ cells by this primary isolate. The expression of CXCR4 was down modulated in CD8+ lymphocytes after infection with the 93US143-T8 isolate. Infection of CD8+ lymphocytes by the 93US143-T8 isolate was prevented by treatment with AMD3100, a specific antagonist for CXCR4, indicating CXCR4-dependent infection. Interestingly, AMD3100 treatment had no inhibitory role in the infection of purified CD4+ lymphocytes by the same isolate. Furthermore, AMD3100 treatment failed to prevent infection of known CD4+ CXCR4+ T-cell lines (MT-2 and CEM) by the 93US143-T8 isolate. In fact, virus replication in the CD4+ cells was often enhanced in the presence of AMD3100. Viruses produced from the infected CD4+ cells in the presence of AMD3100 maintained an unchanged envelope genotype and an SI phenotype. For the first time, these results provide evidence of CXCR4-dependent infection of CD8+ lymphocytes by a primary HIV-1 isolate. This study also shows a different mode of infection for the CD4+ and CD8+ lymphocytes by the same HIV-1 variant. Finally, our findings suggest that a more careful evaluation is necessary before the random use of AMD3100 as a new entry inhibitor in patients harboring SI HIV-1 strains.

High levels of human immunodeficiency virus infection of CD8 lymphocytes expressing CD4 in vivo

Human immunodeficiency virus (HIV)-infected CD8 lymphocytes have been reported in vivo, but the mechanism of infection remains unclear. Experiments using the thy/hu mouse model support export of intrathymically infected CD8 precursors, while recent in vitro data suggest that mature CD8 lymphocytes upregulate CD4 upon activation (generating a CD8bright CD4dim phenotype) and are susceptible to HIV infection. To determine whether these mechanisms operate in vivo and to assess their relative importance in the generation of circulating HIV-infected CD8 lymphocytes, we quantified HIV long terminal repeat (LTR) DNA in CD8+ CD4- and CD8bright CD4dim lymphocytes isolated from HIV-infected individuals by fluorescence-activated cell sorting. HIV infection of CD8 lymphocytes was demonstrated in 17 of 19 subjects, with a significant inverse relationship between level of infection and CD4 lymphocyte count (R = -0.73; P < 0.001). The level of HIV infection of CD8bright CD4dim lymphocytes was significantly higher (median, 1,730 HIV LTR copies/10(6) cells; n = 9) than that of CD8+ CD4- lymphocytes (undetectable in seven of nine individuals; P < 0.01) and approached that of CD4 lymphocytes from the same individuals (median, 3,660 HIV LTR copies/10(6) cells). CD8bright CD4dim lymphocytes represented 0.8 to 3.3% of total CD8 lymphocytes and were most prevalent in the memory subset. Thus, HIV-infected CD8 lymphocytes commonly circulate in HIV-infected individuals and are generated through infection of activated CD8 lymphocytes rather than through export of intrathymically infected precursors. The high level of infection of CD8bright CD4dim lymphocytes could have a direct role in the decline in CD8 lymphocyte function that accompanies HIV disease progression.

HIV-1 compartmentalization in diverse leukocyte populations during antiretroviral therapy

CD4+ T lymphocytes are the primary target of human immunodeficiency virus type 1 (HIV-1), but there is increasing evidence that other immune cells in the blood, including CD8+ T lymphocytes and monocytes, are also productively infected. The extent to which these additional cellular reservoirs contribute to ongoing immunodeficiency and viral persistence during therapy remains unclear. In this study, we conducted a detailed investigation of HIV-1 diversity and genetic structure in CD4+ T cells, CD8+ T cells, and monocytes of 13 patients receiving highly active antiretroviral therapy (HAART). Analysis of molecular variance and nonparametric tests performed on HIV-1 envelope sequences provided statistically significant evidence of viral compartmentalization in different leukocyte populations. Signature pattern analysis and predictions of coreceptor use provided no evidence that selection arising from viral tropism was responsible for the genetic structure observed. Analysis of viral genetic variation in different leukocyte populations demonstrated the action of founder effects as well as significant variation in the extent of genetic differentiation between subpopulations among patients. In the absence of evidence for leukocyte-specific selection, these features were supportive of a metapopulation model of HIV-1 replication as described previously among HIV-1 populations in the spleen. Compartmentalization of the virus in different leukocytes may have significant implications for current models of HIV-1 population genetics and contribute to the highly variable way in which drug resistance evolves in different individuals during HAART.

CD4 on CD8(+) T cells directly enhances effector function and is a target for HIV infection

Costimulation of purified CD8(+) T lymphocytes induces de novo expression of CD4, suggesting a previously unrecognized function for this molecule in the immune response. Here, we report that the CD4 molecule plays a direct role in CD8(+) T cell function by modulating expression of IFN-gamma and Fas ligand, two important CD8(+) T cell effector molecules. CD4 expression also allows infection of CD8 cells by HIV, which results in down-regulation of the CD4 molecule and impairs the induction of IFN-gamma, Fas ligand, and the cytotoxic responses of activated CD8(+) T cells. Thus, the CD4 molecule plays a direct role in CD8 T cell function, and infection of these cells by HIV provides an additional reservoir for the virus and also may contribute to the immunodeficiency seen in HIV disease.

When integrated in a subepithelial mucosal layer equivalent, dendritic cells keep their immature stage and their ability to replicate type R5 HIV type 1 strains in the absence of T cell subsets

Many potential targets of human immunodeficiency virus type 1 (HIV-1) reside in the human reproductive tract, including dendritic cells (DC). The ability of these cells to replicate HIV-1 is dependent on many factors such as their differentiation/maturation stage. Nevertheless, precise mechanisms underlying the early steps of transmucosal infection are still unknown. Our purpose was to investigate DC/HIV-1 interactions in a subepithelial mucosal layer equivalent (SEMLE) reconstructed in vitro. We used mixed interstitial DC (IntDC)/Langerhans cell (LC)-like cell subpopulations generated in vitro from CD34(+) progenitors. These cells were either integrated in SEMLE or maintained in suspension. Experimental infections were performed with a type X4 strain (HIV-1(LAI)) and a type R5 strain (HIV-1(Ba-L)). Proviral DNA was detected by in situ polymerase chain reaction (PCR) and viral replication was quantified by measuring p24 core protein release in the culture media. Our results showed that SEMLE enable DC to retain immature stage and reproduce the tropic selection that occurs in vivo. Indeed, IntDC/LC were infected by both types of HIV-1 strains, regardless of the infection schedule, whereas only type R5 virus replicated in DC in the absence of T cell subsets. Furthermore, the ability of DC to replicate HIV-1(BaL) was lost after 14 days of culture unless the cells had previously been integrated in SEMLE. These results suggest that this 3D model maintains the ability of DC to replicate type R5 virus by delaying their maturation. In conclusion, this in vitro model mimics human submucosa and can be considered as relevant for studying the preliminary steps of transmucosal HIV-1 infection.

Highly uneven distribution of tenofovir-selected simian immunodeficiency virus in different anatomical sites of rhesus macaques

Antiviral tenofovir monotherapy was used to determine whether drug-selected simian immunodeficiency virus (SIV) variants replaced their wild-type progenitors at the same rate in different tissues of six rhesus macaques. The relative frequencies of drug-resistant and wild-type genotypes were measured longitudinally in blood and in 23 lymphoid and nonlymphoid tissues collected at necropsy. The mutant/wild-type genotype ratio was measured using a heteroduplex tracking assay targeting tenofovir-selected SIV reverse transcriptase codons. After the initiation of tenofovir treatment in animals with high steady-state viremia levels, resistant genotypes emerged in the plasma within 1 to 8 weeks and in five of six cases reached frequencies of nearly 100% within 4 to 25 weeks. The appearance of tenofovir-resistant genotypes in peripheral blood mononuclear cell (PBMC) DNA was generally delayed by 1 to 2 weeks and in one case was completely absent. Necropsies performed 8 to 55 weeks after the initiation of tenofovir treatment showed the frequency of resistant SIV genotypes to be generally higher in tissue RNA than DNA fractions. The frequency of drug-resistant genotypes varied widely between anatomical sites, including different lymph nodes of the same animal. Except for the epidydimis, the tissues with the lowest rates of proviral replacement by tenofovir-resistant genotypes differed between animals. The highly uneven distribution of tenofovir-resistant genotypes in different tissues seen shortly after the initiation of tenofovir monotherapy may reflect differences in local antiviral drug selection pressures and/or the stochastic effect of small effective populations of drug-resistant variants randomly seeding different anatomical sites early in therapy.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

Isolation of CD4-independent primary human immunodeficiency virus type 1 isolates that are syncytium inducing and acutely cytopathic for CD8+ lymphocytes

Previous studies have established the existence of CD4-independent simian immunodeficiency virus, human immunodeficiency virus type 2 (HIV-2), and laboratory strains of HIV-1. However, whether CD4-independent viruses may also exist in HIV-1-infected patients has remained unclear. We have recently reported the isolation of viruses from an AIDS patient that were able to infect CD8(+) cells independent of CD4, using CD8 as a receptor. Using a similar approach, here we examined viruses from 12 randomly selected patients (obtained from the AIDS Research and Reference Program, National Institutes of Health) for the presence of CD4-independent HIV-1. CD4-independent variants were isolated from infected CD8(+) cells from the viral quasispecies of 7 of 12 patients. The CD4-independent isolates were able to infect primary CD8(+) cells as well as a CD4(-) CD8(+) T-cell line. Soluble CD4 and blocking anti-CD4 or -CD8 antibody had no effect on infection of CD8(+) cells. Remarkably, two of the seven CD4-independent isolates, but not their parental bulk viruses, induced syncytia and caused acute death of infected CD8(+) cells. Some of the CD4-independent variants were also able to infect U87 cells that were negative for CD4, CD8, and common HIV coreceptors, suggesting a novel entry mechanism for these isolates. The CD4-independent isolates were derived from adults and children infected with subtypes A, B, and D. Although no common motif for CD4 independence was found, novel sequence changes were observed in critical areas of the envelopes of the CD4-independent viruses. These results demonstrate that HIV-1-infected patients can frequently harbor viruses that are able to mediate CD4-independent infection of CD8(+) cells. In addition, this study also provides evidence of primary HIV-1 variants that are syncytium inducing and acutely cytopathic for CD8(+) lymphocytes.

T-cell subsets that harbor human immunodeficiency virus (HIV) in vivo: implications for HIV pathogenesis

Identification of T-cell subsets that are infected in vivo is essential to understanding the pathogenesis of human immunodeficiency virus (HIV) disease; however, this goal has been beset with technical challenges. Here, we used polychromatic flow cytometry to sort multiple T-cell subsets to 99.8% purity, followed by quantitative PCR to quantify HIV gag DNA directly ex vivo. We show that resting memory CD4(+) T cells are the predominantly infected cells but that terminally differentiated memory CD4(+) T cells contain 10-fold fewer copies of HIV DNA. Memory CD8(+) T cells can also be infected upon upregulation of CD4; however, this is infrequent and HIV-specific CD8(+) T cells are not infected preferentially. Naïve CD4(+) T-cell infection is rare and principally confined to those peripheral T cells that have proliferated. Furthermore, the virus is essentially absent from naïve CD8(+) T cells, suggesting that the thymus is not a major source of HIV-infected T cells in the periphery. These data illuminate the underlying mechanisms that distort T-cell homeostasis in HIV infection.

Peripheral blood Dendritic cells are not a major reservoir for HIV type 1 in infected individuals on virally suppressive HAART

Dendritic cells (DCs) are potent antigen-presenting cells, and their physiological localization in tissues that interact with the external environment is important as a first barrier against pathogens such as human immunodeficiency virus type I (HIV-1). Several models have been proposed to explain the possible role of DCs as a reservoir for HIV-1 in patients on virally suppressive highly active antiretroviral therapy (HAART). However, the low yield of cell isolates has made this evaluation a difficult task. The present study analyzes whether peripheral blood DCs from HIV-1-infected individuals on virally suppressive HAART, with plasma HIV-1 RNA levels of less than 50 copies/ml, carry either HIV-1 provirus and/or HIV-1 virions. Peripheral blood DCs were isolated from a cohort of 10 HIV-1-seropositive men taking suppressive HAART. In five patients, plasmacytoid and myeloid dendritic cells were isolated to attempt to identify their respective roles in HIV-1 residual disease. Viral out-growth assays were performed in vitro, as well as gag and R/U5 polymerase chain reaction (PCR) amplification of viral RNA and DNA, respectively, from DC and peripheral blood mononuclear cell (PBMC) extracts. Fluorescence activated cell-sorting (FACS) data revealed cellular yields from 85.90 to 95.18%, of relatively pure DCs isolated from patients' PBMCs. Although HIV-1 RNA gag and DNA RU/5 were detected in all PBMC samples isolated from the patients, proviral DNA and viral RNA forms were not detected in any of the DC isolates. In addition, no replication-competent virus was demonstrated in DC coculture assays, while virus was isolated from each patients' CD8+ T-lymphocyte-depleted PBMC cocultures. Furthermore, HIV-1 gag proviral DNA was not detected in either plasmacytoid or myeloid DC subfractions. The current study suggests that in HIV-1-infected individuals treated with suppressive HAART, peripheral blood DCs do not carry HIV-1 proviral DNA or viral particles attached to their surface. These populations of peripheral blood DCs are likely not a major HIV-1 reservoir in patients on HAART with clinically undetectable plasma viral RNA.

Identification and characterization of HIV-2 strains obtained from asymptomatic patients that do not use CCR5 or CXCR4 coreceptors

In vivo, human immunodeficiency virus type 2 (HIV-2) infection reveals several unique characteristics when compared to HIV-1 infection, the most remarkable of which is the extraordinarily long asymptomatic period. Here we describe two HIV-2 primary isolates, obtained from asymptomatic individuals, which do not infect any coreceptor-expressing cell lines tested. In those cells, we show that the absence of replication is directly related to cell entry events. Furthermore, productive infection observed in peripheral blood mononuclear cells (PBMC) was not inhibited by natural ligands and monoclonal antibodies directed to CCR5 and CXCR4. Finally, viral entry efficiency and viral progeny production of these viruses are markedly impaired in PBMC, indicating a reduced replicative fitness of both viruses. In conclusion, our data suggest that in some HIV-2 asymptomatic individuals, the circulating viruses are unable to use the major coreceptors to infect PBMC. This fact should have important implications in HIV-2 pathogenesis and transmission.

Quasispecies and the development of new antiviral strategies

RNA virus populations consist of complex and dynamic mutant distributions, rather than defined genomic sequences. This feature confers great adaptability on viruses and is partly responsible for current difficulties of viral disease prevention and control. Mutant distributions, also termed mutant swarms or mutant clouds, were first proposed in a theory of molecular evolution termed quasispecies theory. The theoretical formulation of quasispecies and its links to present day RNA viruses are discussed. The need to accommodate antiviral strategies to the dynamic nature of viral populations is emphasized. In particular, recent results on viral extinction associated with enhanced mutagenesis (virus entry into error catastrophe) are reviewed and presented as an example of how the understanding of viruses as quasispecies could lead to a potential practical application in medicine.

A multiplex real-time PCR for quantification of HIV-1 DNA and the human albumin gene in CD4+ cells

We have established a simple system for measuring HIV-1 DNA load in CD4+ cells. In a multiplex configuration, a conserved region in the HIV-1 pol gene and a section of the human albumin gene were simultaneously amplified to estimate the number of HIV-1 DNA copies per cellular genome. An established Epstein-Barr virus (EBV) standard system was used to calibrate the HIV-1 quantification. Our multiplex PCR system was tested on different in vitro developed HIV-1 strains and on longitudinal samples from eight patients. The system was able to amplify both in vitro and in vivo samples of various genetic compositions. In all eight patients, HIV-1 DNA was detected and ranged between 0.17 and 51x10-3 copies per CD4+ cell and could be monitored longitudinally, including long-term PI-ART and STI. The measured HIV-1 DNA load may be used to select the best time for the institution or re-institution of therapy.