Population biology of HIV-1 infection: viral and CD4+ T cell demographics and dynamics in lymphatic tissues

CD4+ CCR5+ T-cell dynamics during simian immunodeficiency virus infection of Chinese rhesus macaques

Simian immunodeficiency virus (SIV) infection of rhesus macaques (RMs) provides a reliable model to study the relationship between lentivirus replication, cellular immune responses, and CD4+ T-cell dynamics. Here we investigated, using SIVmac251-infected RMs of a Chinese genetic background (which experience a slower disease progression than Indian RMs), the dynamics of CD4+ CCR5+ T cells, as this subset of memory/activated CD4+ T cells is both a preferential target of virus replication and a marker of immune activation. As expected, we observed that the number of circulating CD4+ CCR5+ T cells decreases transiently at the time of peak viremia. However, at 60 days postinfection, i.e., when set-point viremia is established, the level of CD4+ CCR5+ T cells was increased compared to the baseline level. Interestingly, this increase correlated with faster disease progression, higher plasma viremia, and early loss of CD4+ T-cell function, as measured by CD4+ T-cell count, the fraction of memory CD4+ T cells, and the recall response to purified protein derivative. Taken together, these data show a key difference between the dynamics of the CD4+ CCR5+ T-cell pool (and its relationship with disease progression) in Chinese RMs and those described in previous reports for Indian SIVmac251-infected RMs. As the SIV-associated changes in the CD4+ CCR5+ T-cell pool reflect the opposing forces of SIV replication (which reduces this cellular pool) and immune activation (which increases it), our data suggest that in SIV-infected Chinese RMs the impact of immune activation is more prominent than that of virus replication in determining the size of the pool of CD4+ CCR5+ T cells in the periphery. As progression of HIV infection in humans also is associated with a relative expansion of the level of CD4+ CCR5+ T cells, we propose that SIV infection of Chinese RMs is a very valuable and important animal model for understanding the pathogenesis of human immunodeficiency virus infection.

Strategies for the optimal sequencing of antiretroviral drugs toward overcoming and preventing drug resistance

Drug regimens now offer more potent, less toxic and more durable choices in the treatment of HIV disease than ever before. This has led to a need to consider the convenient, sequential use of active antiretroviral combinations. Ritonavir-boosted protease inhibitors (PIs) can now be potentially sequenced in a manner that uses the least cross-resistance-prone PI at the start of therapy while leaving the most cross-resistance-prone drug for later, if the latter retains activity against commonly observed drug-resistant forms. Similarly, such new drugs as tenofovir, abacavir and emtricitabine, which make up current nucleoside backbone options, can be potentially sequenced, since each of them selects for an individual pattern of resistance mutations that are generally distinct from those selected by previously popular thymidine analogs such as zidovudine and stavudine.

Regulatory T-cell markers, indoleamine 2,3-dioxygenase, and virus levels in spleen and gut during progressive simian immunodeficiency virus infection

High levels of viral replication occur in gut-associated lymphoid tissue (GALT) and other lymphoid tissues (LT) since the early phase of human/simian immunodeficiency virus (HIV/SIV) infection. Regulatory T cells (T(reg)), a subset of immunosuppressive T cells expressing CTLA-4 and the FoxP3 transcription factor, accumulate in LT during HIV/SIV infection. Here we show that FoxP3 and CTLA-4 mRNA are increased in leukocytes from the spleens, lymph nodes (LN), and mucosal sites of chronically SIV-infected macaques with high viremia (SIV(HI)) compared to animals with low viremia (SIV(LO)). FoxP3 and CTLA-4 correlated with SIV RNA levels in tissues; SIV virus levels in the spleen, inguinal LN, mesenteric LN, colon, and jejunum directly correlated with the plasma virus level. Importantly, CTLA-4 and FoxP3 mRNA were predominantly increased in the CD25(-) subpopulation of leukocytes from SIV(HI), further challenging the classical definition of T(reg) as CD4(+) CD25(+) T cells. Similar to CTLA-4 and FoxP3, expression of indoleamine 2,3-dioxygenase (IDO), an immunosuppressive enzyme induced by T(reg) in antigen-presenting cells, was increased in the spleens, mesenteric LN, colons, and jejuna from SIV(HI) compared to SIV(LO) and directly correlated to SIV RNA in the same tissues. Accordingly, plasma kynurenine/tryptophan, a marker for IDO enzymatic activity, was significantly higher in SIV(HI) compared to SIV(LO) and correlated with plasma viral levels. Increased T(reg) and IDO in LT of SIV-infected macaques may be the consequence of increased tissue inflammation and/or may favor virus replication during the chronic phase of SIV infection.

CTL fail to accumulate at sites of HIV-1 replication in lymphoid tissue

The inability of HIV-1-specific CTL to fully suppress virus replication as well as the failure of administration of exogenous CTL to lower viral loads are not understood. To evaluate the hypothesis that these phenomena are due to a failure of CTL to localize at sites of HIV-1 replication, we assessed the distribution of HIV-1 RNA and HIV-1-specific CTL identified by HIV-1 peptide/HLA class I tetrameric complexes (tetramers) within lymph nodes of 14 HIV-1-infected individuals who were not receiving antiretroviral therapy. A median of 0.04% of follicular compared with 0.001% of extrafollicular CD4(+) cells were estimated to be producing HIV-1 RNA, a 40-fold difference (p = 0.0001). Tetramer-stained cells were detected by flow cytometry in disaggregated lymph node cells from 11 subjects and constituted a significantly higher fraction of CD8(+) cells in lymph node (mean, 2.15%) than in PBMC (mean, 1.52%; p = 0.02). In situ tetramer staining in three subjects' lymph nodes, in which high frequencies of tetramer-stained cells were detected, revealed that tetramer-stained cells were primarily concentrated in extrafollicular regions of lymph node and were largely absent within lymphoid follicles. These data confirm that HIV-1-specific CTL are abundant within lymphoid tissues, but fail to accumulate within lymphoid follicles where HIV-1 replication is concentrated, suggesting that lymphoid follicles may be immune-privileged sites. Mechanisms underlying the exclusion of CTL from lymphoid follicles as well as the role of lymphoid follicles in perpetuating other chronic pathogens merit further investigation.

Role for CD21 in the establishment of an extracellular HIV reservoir in lymphoid tissues

Follicular dendritic cells (FDC) represent a major extracellular reservoir for HIV. A better understanding of the mechanisms of virion attachment to FDC may offer new avenues for reducing viral burdens in infected individuals. We used a murine model to investigate the establishment of extracellular HIV reservoirs in lymph nodes (LN). Consistent with findings in human tissues, CD21 was required for trapping of HIV to LN cells, as evidenced by significantly reduced virion binding when mice were pretreated with a C3 ligand-blocking anti-CD21 mAb and absence of virion trapping in CD21 knockout mice. Also consistent with findings in human tissues, the majority of HIV virions were associated with the FDC-enriched fraction of LN cell preparations. Somewhat surprisingly, HIV-specific Abs were not essential for HIV binding to LN cells, indicating that seeding of the FDC reservoir may begin shortly after infection and before the development of HIV-specific Abs. Finally, the virion-displacing potential for anti-CD21 mAbs was investigated. Treatment of mice with anti-CD21 mAbs several days after injection of HIV significantly reduced HIV bound to LN cells. Our findings demonstrate a critical role for CD21 in HIV trapping by LN cells and suggest a new therapeutic avenue for reducing HIV reservoirs.

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.

HIV-1 Vpr-induced apoptosis is cell cycle dependent and requires Bax but not ANT

The HIV-1 accessory protein viral protein R (Vpr) causes G₂ arrest and apoptosis in infected cells. We previously identified the DNA damage–signaling protein ATR as the cellular factor that mediates Vpr-induced G₂ arrest and apoptosis. Here, we examine the mechanism of induction of apoptosis by Vpr and how it relates to induction of G₂ arrest. We find that entry into G₂ is a requirement for Vpr to induce apoptosis. We investigated the role of the mitochondrial permeability transition pore by knockdown of its essential component, the adenine nucleotide translocator. We found that Vpr-induced apoptosis was unaffected by knockdown of ANT. Instead, apoptosis is triggered through a different mitochondrial pore protein, Bax. In support of the idea that checkpoint activation and apoptosis induction are functionally linked, we show that Bax activation by Vpr was ablated when ATR or GADD45α was knocked down. Certain mutants of Vpr, such as R77Q and I74A, identified in long-term nonprogressors, have been proposed to inefficiently induce apoptosis while activating the G₂ checkpoint in a normal manner. We tested the in vitro phenotypes of these mutants and found that their abilities to induce apoptosis and G₂ arrest are indistinguishable from those of HIV-1_NL4–3vpr, providing additional support to the idea that G₂ arrest and apoptosis induction are mechanistically linked.

HIV-1 encodes a small gene known as vpr (viral protein regulatory) whose product is a 96–amino acid protein. HIV-1 infects cells of the immune system, such as CD4-positive lymphocytes. When cells become infected with HIV-1, two deleterious effects result from expression of the vpr gene. One effect of vpr is to manipulate the cell cycle by blocking the cells in G₂ (the phase of the cell cycle immediately preceding mitosis). Thus, cells infected with HIV-1 cease to proliferate, due to the action of vpr. A second effect of vpr is the induction of cell death by a process known as apoptosis or programmed cell death. When cells die by apoptosis, they do so following activation of a cellular set of genes and proteins whose primary function is to inactivate various cellular functions that are needed in order to maintain cellular viability. In this study, Andersen et al. demonstrate that the above two effects of vpr are linked. In particular, the authors show that the blockade in cell proliferation in G₂ is a requirement toward the onset of programmed cell death. Programmed cell death can be accomplished by a number of cellular proteins known as “executioners.” Various executioner proteins reside on the mitochondrial membranes and may trigger release of factors from the mitochondria, which in turn will precipitate the onset of apoptosis. In this work Anderson et al. identify the mitochondrial protein, Bax, as the key executioner of apoptosis in the context of HIV-1 vpr. The authors' findings provide important mechanistic understanding of how the vpr gene contributes to HIV-1–induced cell death.

Vpr cytopathicity independent of G2/M cell cycle arrest in human immunodeficiency virus type 1-infected CD4+ T cells

The mechanism of CD4(+) T-cell depletion in human immunodeficiency virus type 1 (HIV-1)-infected individuals remains unknown, although mounting evidence suggests that direct viral cytopathicity contributes to this loss. The HIV-1 Vpr accessory protein causes cell death and arrests cells in the G(2)/M phase; however, the molecular mechanism underlying these properties is not clear. Mutation of hydrophobic residues on the surface of its third alpha-helix disrupted Vpr toxicity, G(2)/M arrest induction, nuclear localization, and self-association, implicating this region in multiple Vpr functions. Cytopathicity by virion-delivered mutant Vpr protein correlated with G(2)/M arrest induction but not nuclear localization or self-association. However, infection with whole virus encoding these Vpr mutants did not abrogate HIV-1-induced cell killing. Rather, mutant Vpr proteins that are impaired for G(2)/M block still prevented infected cell proliferation, and this property correlated with the death of infected cells. Chemical agents that inhibit infected cells from entering G(2)/M also did not reduce HIV-1 cytopathicity. Combined, these data implicate Vpr in HIV-1 killing through a mechanism involving inhibiting cell division but not necessarily in G(2)/M. Thus, the hydrophobic region of the third alpha-helix of Vpr is crucial for mediating G(2)/M arrest, nuclear localization, and self-association but dispensable for HIV-1 cytopathicity due to residual cell proliferation blockade mediated by a separate region of the protein.

T cell receptor excision circles (TRECs) analysis during acute intrarectal infection of cynomolgus monkeys with pathogenic chimeric simian human immunodeficiency virus

Several studies have shown the importance of evaluating Recent Thymic Emigrants (RTEs) by quantification of T cell receptor-rearrangement excision circles (TRECs), as a measure of de novo T cell generation during human immunodeficiency virus-1 (HIV-1) infection. To determine whether acute viral infection may have an impact on TRECs, cynomolgus monkeys (Macaca fascicularis) were infected intrarectally with simian human immunodeficiency virus (SHIV) 89.6P(cy11) and the number of signal-joint (sj) TRECs was determined in purified CD4+ and CD8+ populations for up to 28 weeks post-infection. Four weeks after infection, TRECs levels significantly decreased in both CD3+ CD4+ and in CD3+ CD8+ T lymphocytes of infected monkeys, whereas they remained unchanged in uninfected animals. This reduction was followed by a progressive TRECs number recovery in CD3+ CD4+ T lymphocytes that positively correlated with changes in the levels of circulating CD3+ CD4+ T cells. In the CD3+ CD8+ T cell subset, TRECs number remained significantly low and inversely correlated with the increase in the percentages of CD3+ CD8+ T cells. These data suggest that SHIV89.6P(cy11) intrarectal infection of cynomolgus monkeys differently affects TRECs content in CD3+ CD4+ and CD3+ CD8+ T cell subsets.

Determining thymic output quantitatively: using models to interpret experimental T-cell receptor excision circle (TREC) data

T cells develop in the thymus and then are exported to the periphery. As one ages, the lymphoid mass of the thymus decreases, and a concomitant decrease in the ability to produce new T cells results. Human immunodeficiency virus (HIV) infects CD4(+) T cells and, hence, can also affect thymic function. Here we discuss experimental techniques and mathematical models that aim to quantify the rate of thymic export. We focus on a recent technique involving the quantification of T-cell receptor excision circles (TRECs). We discuss how proper interpretation of TREC data necessitates the critical development of appropriate mathematical models. We review the theory for interpretation of TREC data during aging, HIV infection, and anti-retroviral treatment. Also, we show how TRECs can be used to accurately quantify thymic output in the context of thymectomy experiments. We show that mathematical models are not only useful but absolutely necessary for these analyses. As such, they should be taken as just another tool in the immunologist's arsenal.

Understanding the slow depletion of memory CD4+ T cells in HIV infection

BACKGROUND

The asymptomatic phase of HIV infection is characterised by a slow decline of peripheral blood CD4(+) T cells. Why this decline is slow is not understood. One potential explanation is that the low average rate of homeostatic proliferation or immune activation dictates the pace of a "runaway" decline of memory CD4(+) T cells, in which activation drives infection, higher viral loads, more recruitment of cells into an activated state, and further infection events. We explore this hypothesis using mathematical models.

METHODS AND FINDINGS

Using simple mathematical models of the dynamics of T cell homeostasis and proliferation, we find that this mechanism fails to explain the time scale of CD4(+) memory T cell loss. Instead it predicts the rapid attainment of a stable set point, so other mechanisms must be invoked to explain the slow decline in CD4(+) cells.

CONCLUSIONS

A runaway cycle in which elevated CD4(+) T cell activation and proliferation drive HIV production and vice versa cannot explain the pace of depletion during chronic HIV infection. We summarize some alternative mechanisms by which the CD4(+) memory T cell homeostatic set point might slowly diminish. While none are mutually exclusive, the phenomenon of viral rebound, in which interruption of antiretroviral therapy causes a rapid return to pretreatment viral load and T cell counts, supports the model of virus adaptation as a major force driving depletion.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Dynamics of CD4+ T cells in HIV-1 infection

Mathematical modeling is becoming established in the immunologist's toolbox as a method to gain insight into the dynamics of the immune response and its components. No more so than in the case of the study of human immunodeficiency virus (HIV) infection, where earlier work on the viral dynamics brought significant advances in our understanding of HIV replication and evolution. Here, I review different areas of the study of the dynamics of CD4+ T cells in the setting of HIV, where modeling played important and diverse roles in helping us understand CD4+ T-cell homeostasis and the effect of HIV infection. As the experimental techniques become more accurate and quantitative, modeling should play a more important part in both experimental design and data analysis.

Toward a multiscale model of antigen presentation in immunity

A functioning immune system and the process of antigen presentation in particular encompass events that occur at multiple length and time scales. Despite a wealth of information in the biological literature regarding each of these scales, no single representation synthesizing this information into a model of the overall immune response as it depends on antigen presentation is available. In this article, we outline an approach for integrating information over relevant biological and temporal scales to generate such a representation for major histocompatibility complex class II-mediated antigen presentation. In addition, we begin to address how such models can be used to answer questions about mechanisms of infection and new strategies for treatment and vaccines.

Genetic changes associated with distinct patterns of HIV type 1 persistence in chronically infected cell lines

Three persistently infected cell lines (H61, M61, and U61) were established by infection with an HIV-1 isolate (s61) of two T cell lines, H9 and MT-4, and the promonocytic U937-2. In H61, 35% of cells expressed viral antigens yielding low virus titers and a majority of mature particles. M61 showed viral expression in every cell but with the frequent generation of immature particles. In U61, 1% of cells displayed viral expression, which increased after cell activation, indicating a latent infection. Nucleotide sequences of the complete provirus from the persistent cell lines revealed extremely high mutation rates in accessory genes and non-coding regions from 1.1 to 2.8 x 10(-2), whereas in structural genes they ranged from 3.2 to 9.8 x 10(-3). Ten nonsynonymous mutations were shared by all persistent proviruses including five strong amino acid changes in the env gene (related to the NSI phenotype) and in vpr and tat genes; other alterations were in accessory genes and two in the USF and c-Myb motifs in LTR. Truncated vpr and vpu proteins were found specifically in H61 and in vif in M61. This comprehensive study disclosed the role of the cell on the HIV-1 persistence pattern as well as common and specific mutations in the virus.

Oxidative stress and the pathogenesis of neurodegenerative disorders

Microglia-derived inflammatory neurotoxins play a principal role in the pathogenesis of neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and HIV-associated dementia; chief among these is reactive oxygen species. The detrimental effects of oxidative stress in the brain and nervous system are primarily a result of the diminished capacity of the central nervous system to prevent ongoing oxidative damage. A spectrum of environmental cues, mitochondrial dysfunction, accumulation of aberrant misfolded proteins, inflammation, and defects in protein clearance are known to evolve and form as a result of disease progression. These factors likely affect glial function serving to accelerate the tempo of disease. Understanding the relationships between disease progression, free radical formation, neuroinflammation, and neurotoxicity is critical to elucidating disease mechanisms and the development of therapeutic modalities to combat disease processes. In an era where populations continue to age, the prevalence and incidence of age-related neurodegenerative diseases are on the rise; therefore, the need for novel therapeutic strategies that attenuate neuroinflammation and protect neurons against oxidative stress is ever more immediate.

Trapping and apoptosis of novel subsets of memory T lymphocytes expressing CCR6 in the spleen of HIV-infected patients

CCR6, a homeostatic chemokine receptor, is shown here to characterize subsets of both central and effector memory T cells that secrete high levels of IL-2 and TNF-alpha in response to polyclonal and antigen-specific stimulation. CCR6(+) T lymphocytes disappeared dramatically from the peripheral blood of HIV-infected patients as HIV disease progressed. The capacity of CD4(+)CCR6(+) to secrete multiple cytokines remained intact among HIV-infected long-term nonprogressors but was partially lost from subjects with standard disease progression. CCR6(+) T lymphocytes, regardless of their CCR7 expression, accumulated in the spleen of HIV-infected patients, where they died by apoptosis. Assessment of CCR6 expression allowed us to describe novel memory T-cell subpopulations capable of high cytokine production and provided evidence of a pathologic CCR6-dependent pathway of memory T-cell homing that may participate in the loss of memory response against infections.

Modeling regulation mechanisms in the immune system

We develop a mathematical framework for modeling regulatory mechanisms in the immune system. The model describes dynamics of key components of the immune network within two compartments: lymph node and tissue. We demonstrate using numerical simulations that our system can eliminate virus-infected cells, which are characterized by a tendency to increase without control (in absence of an immune response), while tolerating normal cells, which are characterized by a tendency to approach a stable equilibrium population. We experiment with different combinations of T cell reactivities that lead to effective systems and conclude that slightly self-reactive T cells can exist within the immune system and are controlled by regulatory cells. We observe that CD8+ T cell dynamics has two phases. In the first phase, CD8+ cells remain sequestered within the lymph node during a period of proliferation. In the second phase, the CD8+ population emigrates to the tissue and destroys its target population. We also conclude that a self-tolerant system must have a mechanism of central tolerance to ensure that self-reactive T cells are not too self-reactive. Furthermore, the effectiveness of a system depends on a balance between the reactivities of the effector and regulatory T cell populations, where the effectors are slightly more reactive than the regulatory cells.

Evolution of the human immunodeficiency virus envelope gene is dominated by purifying selection

The evolution of the human immunodeficiency virus (HIV-1) during chronic infection involves the rapid, continuous turnover of genetic diversity. However, the role of natural selection, relative to random genetic drift, in governing this process is unclear. We tested a stochastic model of genetic drift using partial envelope sequences sampled longitudinally in 28 infected children. In each case the Bayesian posterior (empirical) distribution of coalescent genealogies was estimated using Markov chain Monte Carlo methods. Posterior predictive simulation was then used to generate a null distribution of genealogies assuming neutrality, with the null and empirical distributions compared using four genealogy-based summary statistics sensitive to nonneutral evolution. Because both null and empirical distributions were generated within a coalescent framework, we were able to explicitly account for the confounding influence of demography. From the distribution of corrected P-values across patients, we conclude that empirical genealogies are more asymmetric than expected if evolution is driven by mutation and genetic drift only, with an excess of low-frequency polymorphisms in the population. This indicates that although drift may still play an important role, natural selection has a strong influence on the evolution of HIV-1 envelope. A negative relationship between effective population size and substitution rate indicates that as the efficacy of selection increases, a smaller proportion of mutations approach fixation in the population. This suggests the presence of deleterious mutations. We therefore conclude that intrahost HIV-1 evolution in envelope is dominated by purifying selection against low-frequency deleterious mutations that do not reach fixation.

Inefficient human immunodeficiency virus replication in mobile lymphocytes

Cell-to-cell viral transfer facilitates the spread of lymphotropic retroviruses such as human immunodeficiency virus (HIV) and human T-cell leukemia virus (HTLV), likely through the formation of "virological synapses" between donor and target cells. Regarding HIV replication, the importance of cell contacts has been demonstrated, but this phenomenon remains only partly characterized. In order to alter cell-to-cell HIV transmission, we have maintained cultures under continuous gentle shaking and followed viral replication in this experimental system. In lymphoid cell lines, as well as in primary lymphocytes, viral replication was dramatically reduced in shaken cultures. To document this phenomenon, we have developed an assay to assess the relative contributions of free and cell-associated virions in HIV propagation. Acutely infected donor cells were mixed with carboxyfluorescein diacetate succinimidyl ester-labeled lymphocytes as targets, and viral production was followed by measuring HIV Gag expression at different time points by flow cytometry. We report that cellular contacts drastically enhance productive viral transfer compared to what is seen with infection with free virus. Productive cell-to-cell viral transmission required fusogenic viral envelope glycoproteins on donor cells and adequate receptors on targets. Only a few syncytia were observed in this coculture system. Virus release from donor cells was unaffected when cultures were gently shaken, whereas virus transfer to recipient cells was severely impaired. Altogether, these results indicate that cell-to-cell transfer is the predominant mode of HIV spread and help to explain why this virus replicates so efficiently in lymphoid organs.

CD25+CD4+ regulatory T cells and memory T cells prevent lymphopenia-induced proliferation of naive T cells in transient states of lymphopenia

Lymphopenia has been associated with autoimmune pathology and it has been suggested that lymphopenia-induced proliferation of naive T cells may be responsible for the development of immune pathology. In this study we demonstrate that lymphopenia-induced proliferation is restricted to conditions of extreme lymphopenia, because neither naive nor memory T cells transferred into T cell-depleted hosts proliferate unless the depletion exceeds 90% of the peripheral repertoire. Memory CD4 T cells as well as regulatory CD4 T cells proved to be relatively resistant to depletion regimes, and both subsets restrict the expansion and phenotypic conversion of naive T cells by an IL-7R-dependent mechanism. It therefore seems unlikely that lymphopenia-induced proliferation of peripheral T cells causes deleterious side effects that result in immune pathology in states of partial and transient lymphopenia.

Lymphatic tissue fibrosis is associated with reduced numbers of naive CD4+ T cells in human immunodeficiency virus type 1 infection

The organized structure of lymphatic tissues (LTs) constitutes a microenvironment referred to as a niche that plays a critical role in immune system homeostasis by promoting cellular interactions and providing access to cytokines and growth factors on which cells are dependent for survival, proliferation, and differentiation. In chronic human immunodeficiency virus type 1 (HIV-1) infection, immune activation and inflammation result in collagen deposition and disruption of this LT niche. We have previously shown that these fibrotic changes correlate with a reduction in the size of the total population of CD4+ T cells. We now show that this reduction is most substantial within the naïve CD4+ T-cell population and is in proportion to the extent of LT collagen deposition in HIV-1 infection. Thus, the previously documented depletion of naïve CD4+ T cells in LTs in HIV-1 infection may be a consequence not only of a decreased supply of thymic emigrants or chronic immune activation but also of the decreased ability of those cells to survive in a scarred LT niche. We speculate that LT collagen deposition might therefore limit repopulation of naïve CD4+ T cells with highly active antiretroviral therapy, and thus, additional treatments directed to limiting or reversing inflammatory damage to the LT niche could potentially improve immune reconstitution.

Disseminated and sustained HIV infection in CD34+ cord blood cell-transplanted Rag2-/-gamma c-/- mice

Because of species selectivity, HIV research is largely restricted to in vitro or clinical studies, both limited in their ability to rapidly assess new strategies to fight the virus. To prospectively study some aspects of HIV in vivo, immunodeficient mice, transplanted with either human peripheral blood leukocytes or human fetal tissues, have been developed. Although these are susceptible to HIV infection, xenoreactivity, and short infection spans, resource and ethical constraints, as well as biased HIV coreceptor tropic strain infection, pose substantial problems in their use. Rag2(-/-)gamma(c)(-/-) mice, transplanted as newborns with human CD34(+) cells, were recently shown to develop human B, T, and dendritic cells, constituting lymphoid organs in situ. Here we tested these mice as a model system for HIV-1 infection. HIV RNA levels peaked to up to 2 x 10(6) copies per milliliter of plasma early after infection, and viremia was observed for up to 190 days, the longest time followed. A marked relative CD4(+) T cell depletion in peripheral blood occurred in CXCR4-tropic strain-infected mice, whereas this was less pronounced in CCR5-tropic strain-infected animals. Thymus infection was almost exclusively observed in CXCR4-tropic strain-infected mice, whereas spleen and lymph node HIV infection occurred irrespective of coreceptor selectivity, consistent with respective coreceptor expression on human CD4(+) T cells. Thus, this straightforward to generate and cost-effective in vivo model closely resembles HIV infection in man and therefore should be valuable to study virus-induced pathology and to rapidly evaluate new approaches aiming to prevent or treat HIV infection.

Going wild: lessons from naturally occurring T-lymphotropic lentiviruses

Over 40 nonhuman primate (NHP) species harbor species-specific simian immunodeficiency viruses (SIVs). Similarly, more than 20 species of nondomestic felids and African hyenids demonstrate seroreactivity against feline immunodeficiency virus (FIV) antigens. While it has been challenging to study the biological implications of nonfatal infections in natural populations, epidemiologic and clinical studies performed thus far have only rarely detected increased morbidity or impaired fecundity/survival of naturally infected SIV- or FIV-seropositive versus -seronegative animals. Cross-species transmissions of these agents are rare in nature but have been used to develop experimental systems to evaluate mechanisms of pathogenicity and to develop animal models of HIV/AIDS. Given that felids and primates are substantially evolutionarily removed yet demonstrate the same pattern of apparently nonpathogenic lentiviral infections, comparison of the biological behaviors of these viruses can yield important implications for host-lentiviral adaptation which are relevant to human HIV/AIDS infection. This review therefore evaluates similarities in epidemiology, lentiviral genotyping, pathogenicity, host immune responses, and cross-species transmission of FIVs and factors associated with the establishment of lentiviral infections in new species. This comparison of consistent patterns in lentivirus biology will expose new directions for scientific inquiry for understanding the basis for virulence versus avirulence.

Viral and latent reservoir persistence in HIV-1-infected patients on therapy

Despite many years of potent antiretroviral therapy, latently infected cells and low levels of plasma virus have been found to persist in HIV-infected patients. The factors influencing this persistence and their relative contributions have not been fully elucidated and remain controversial. Here, we address these issues by developing and employing a simple, but mechanistic viral dynamics model. The model has two novel features. First, it assumes that latently infected T cells can undergo bystander proliferation without transitioning into active viral production. Second, it assumes that the rate of latent cell activation decreases with time on antiretroviral therapy due to the activation and subsequent loss of latently infected cells specific for common antigens, leaving behind cells that are successively less frequently activated. Using the model, we examined the quantitative contributions of T cell bystander proliferation, latent cell activation, and ongoing viral replication to the stability of the latent reservoir and persisting low-level viremia. Not surprisingly, proliferation of latently infected cells helped maintain the latent reservoir in spite of loss of latent infected cells through activation and death, and affected viral dynamics to an extent that depended on the magnitude of latent cell activation. In the limit of zero latent cell activation, the latent cell pool and viral load became uncoupled. However, as the activation rate increased, the plasma viral load could be maintained without depleting the latent reservoir, even in the absence of viral replication. The influence of ongoing viral replication on the latent reservoir remained insignificant for drug efficacies above the "critical efficacy" irrespective of the activation rate. However, for lower drug efficacies viral replication enabled the stable maintenance of both the latent reservoir and the virus. Our model and analysis methods provide a quantitative and qualitative framework for probing how different viral and host factors contribute to the dynamics of the latent reservoir and the virus, offering new insights into the principal determinants of their persistence.

The role of virus-induced regulatory T cells in immunopathology

In recent years, regulatory T cells have received increased attention for their role in immune responses to microbial infections. The list of microbial pathogens associated with regulatory T cell responses is growing rapidly and includes bacteria, viruses, parasites, and fungi. As the biology of regulatory T cells is revealed, we are discovering that their induction during infection is a normal aspect of immunity, necessary to limit collateral damage from inflammatory responses and aggressive immunological effectors. Thus, these cells play a critical role in maintaining the delicate balance between preventing immunopathology and allowing the immune response to clear infections. While generally successful, there are notable exceptions where regulatory T cell-mediated suppression appears to be responsible for allowing certain viruses to establish and maintain a persistent state. In this review, we will discuss our current understanding of what virus-induced regulatory T cells are, how they are induced, and what mechanisms they use to suppress immunity. The complex role of Tregs in regulating immunity to viral infections, and the consequences their activity has on disease is illustrated by a review of specific viral infections including hepatitis C virus and human immunodeficiency virus.

Feasibility of Weekly HIV Drug Delivery to Enhance Drug Localization in Lymphoid Tissues Based on Pharmacokinetic Models of Lipid-Associated Indinavir

PURPOSE

Compare the simulated pharmacokinetics of lipid-associated and soluble indinavir (IDV) to determine the potential for greater control of virus replication in the lymphoid tissues.

METHODS

Two-compartment mathematical models were developed to simulate the human pharmacokinetics of soluble and lipid-associated forms of IDV in the central compartment and the lymphoid tissue. The lipid-associated IDV model was then used to determine the minimum dosing schedule needed to attain central or lymph drug concentrations comparable to the soluble form.

RESULTS

Association of IDV to lipid nanoparticles has a favorable half-life and tissue distribution and allows comparable minimum drug concentration in the lymph (where the majority of viral replication occurs) to be achieved with a dosing schedule of every 95.5 h (approximately 4 days).

CONCLUSIONS

Presuming pharmacodynamics of lipid-associated IDV are similar to soluble IDV, estimations based on the proposed kinetic model suggest the novel delivery system could have a tremendous impact on the current standard of HIV treatment, particularly for therapy targeted to clear virus sanctuaries in lymphoid tissues. With less frequent and more effective dosing, lipid-associated indinavir delivery as an adjunct to conventional antiviral therapy could lead to better suppression of viral replication, increased immunological benefit, and fewer treatment failures.

HIV vaccines: can mucosal CD4 T cells be protected?

PURPOSE OF REVIEW

The aim of this article is to understand the significance of protecting the mucosal tissue compartment during acute HIV infection, and to describe the current efforts towards this goal.

RECENT FINDINGS

The mucosa is the primary route of HIV transmission, and serves as a major site for viral dissemination leading to a massive destruction of the memory CD4 T cell compartment. This destruction is mediated as a consequence of direct viral infection and occurs in all the tissues of the body suggesting that once infection explodes out of the mucosal tissues memory CD4 T cells at all other sites are very rapidly infected and destroyed.

SUMMARY

The enrichment of highly susceptible CD4 targets in mucosal tissues suggests that the immune system will need to be in a state of high alert to contain infection once HIV crosses the mucosal barrier. This will require the generation and maintenance of strong vaccine-induced neutralizing antibodies and CD8 T cell responses in mucosal tissues. Given the challenges of inducing neutralizing antibodies, current efforts are focused on developing a T cell based vaccine that can contain the spread of HIV infection. Developing a T cell based vaccine is hampered by the lack of any predictive correlates of protection. In the absence of such correlates, protection can be measured by the extent to which mucosal CD4 T cells are preserved. Preservation of mucosal CD4 T cells will have a significant impact on disease course and long-term outcome.

HIV infection: first battle decides the war

The traditional view of HIV-1 infection characterized by the slow decline of CD4+ T cells has radically changed in light of recent observations in rhesus macaques and humans of rapid and extensive infection and removal of memory CD4+ T cells in mucosal tissues within the first three weeks of infection. This initial strike to the immune system seems to be the distinguishing feature of HIV-1 pathogenesis and its extent sets the overall course of the ensuing infection. Qualitatively different mechanisms of CD4+ T-cell depletion prevail during the acute, chronic and advanced phases of infection depending on the availability of the target-cell population and competence of the immune system. The elimination of CD4+ T cells in mucosal lymphoid tissues results in the absence of important regulatory and effector functions that these cells normally perform in controlling immune responses to environmental antigens and pathogens. Ablation of acute HIV-1 viremia limits the initial damage to the CD4+ T-cell compartment and helps to establish a state of equilibrium between the replicating virus, the availability of the target-cell population and the immune control characteristic of long-term non-progression.

Opposing roles of blood myeloid and plasmacytoid dendritic cells in HIV-1 infection of T cells: transmission facilitation versus replication inhibition

CD11c(+) myeloid dendritic cells (MDCs) and CD11c(-) CD123(+) plasmacytoid DCs (PDCs) have been identified as main human DC subsets. MDCs are professional antigen-presenting cells for T cells, and include Langerhans cells, dermal DCs, and interstitial DCs. They have been associated with HIV-1 capture and sexual transmission, whereas PDCs play an important role in the innate immune responses to different types of viruses, including HIV-1. To compare the influence of MDCs and PDCs on HIV-1 infection of T cells, we isolated donor-matched MDCs and PDCs from peripheral blood, activated them by adding different maturation-inducing compounds, and cocultured them with T cells and HIV-1. We found that MDCs enhance HIV-1 infection through capture of the virus and subsequent transmission to T cells, and that differently matured MDC subsets have different HIV-1 transmission efficiencies. These differences were not due to soluble factors, viral capture differences, or the expression of integrins ICAM-1, -2, -3, or LFA-1. In contrast, regardless of their state of maturation, PDCs inhibit HIV-1 replication in T cells through the secretion of IFNalpha and an additional, unidentified small molecule. This study shows that the 2 main types of DCs have opposing roles in HIV-1 infection of T cells.

Quantification of Feline immunodeficiency virus (FIVpco) in peripheral blood mononuclear cells, lymph nodes and plasma of naturally infected cougars

Infection of domestic cats with Feline immunodeficiency virus (FIV) results in a fatal immunodeficiency disease, similar to Human immunodeficiency virus 1 (HIV-1) in humans. Elevated plasma viral loads in domestic cats are correlated to decreased survival time and disease progression. However, FIV is also maintained as an apathogenic infection in other members of the family Felidae including cougars, Puma concolour (FIVpco). It is not known whether the lack of disease in cougars is a result of diminished virus replication. A real-time PCR assay was developed to quantify both FIVpco proviral and plasma viral loads in naturally infected cougars. Proviral loads quantified from peripheral blood mononuclear cells (PBMC) ranged from 2.90 x 10(1) to 6.72 x 10(4) copies per 10(6) cells. Plasma viral loads ranged from 2.30 x 10(3) to 2.81 x 10(6) RNA copies ml(-1). These data indicate that FIVpco viral loads are comparable to viral loads observed in endemic and epidemic lentivirus infections. Thus, the lack of disease in cougars is not due to low levels of virus replication. Moreover, significant differences observed among cougar PBMC proviral loads correlated to viral lineage and cougar age (P=0.014), which suggests that separate life strategies exist within FIVpco lineages. This is the first study to demonstrate that an interaction of lentivirus lineage and host age significantly effect proviral loads.

Density-dependent decay in HIV-1 dynamics

The decay of HIV-1-infected cell populations after treatment with antiretroviral therapy has been measured using simple exponential decay models. These models are unlikely to be realistic over periods longer than a few months, however, because the population dynamics of HIV are complex. We considered an alternate model developed by Perelson and colleagues that extends the standard model for biphasic viral load decline and allows for nonlinear log decay of infected cell populations. Using data from 6 children on highly active antiretroviral therapy (HAART) and a single parameter in the new model, the assumption of log linear decay of infected cell populations is tested. Our analysis indicates that the short-lived and long-lived infected cell populations do not decay according to a simple exponential model. Furthermore, the resulting estimates of time to eradication of infected cell compartments are dramatically longer than those previously reported (eg, decades vs. years for long-lived infected cell populations and years vs. weeks for short-lived infected cell populations). Furthermore, estimates of the second-phase decay rates are significantly different than 0 for most children when obtained using the Perelson biphasic decay model. In contrast, this rate is not significantly different than 0 when the density-dependent decay model is used for parameter estimation and inference. Thus, the density-dependent decay model but not the simple exponential decay model is consistent with recent data showing that even under consistent HAART-mediated suppression of viral replication, decay rates of infected cell reservoirs decay little over several years. This suggests that conclusions about long-term viral dynamics of HIV infection based on simple exponential decay models should be carefully re-evaluated.

Endogenous factors enhance HIV infection of tissue naive CD4 T cells by stimulating high molecular mass APOBEC3G complex formation

Human immunodeficiency virus (HIV) can infect resting CD4 T cells residing in lymphoid tissues but not those circulating in peripheral blood. The molecular mechanisms producing this difference remain unknown. We explored the potential role of the tissue microenvironment and its influence on the action of the antiviral factor APOBEC3G (A3G) in regulating permissivity to HIV infection. We found that endogenous IL-2 and -15 play a key role in rendering resident naive CD4 T cells susceptible to HIV infection. Infection of memory CD4 T cells also requires endogenous soluble factors, but not IL-2 or -15. A3G is found in a high molecular mass complex in HIV infection–permissive, tissue-resident naive CD4 T cells but resides in a low molecular mass form in nonpermissive, blood-derived naive CD4 T cells. Upon treatment with endogenous soluble factors, these cells become permissive for HIV infection, as low molecular mass A3G is induced to assemble into high molecular mass complexes. These findings suggest that in lymphoid tissues, endogenous soluble factors, likely including IL-2 and -15 and others, stimulate the formation of high molecular mass A3G complexes in tissue-resident naive CD4 T cells, thereby relieving the potent postentry restriction block for HIV infection conferred by low molecular mass A3G.

T-lymphocyte subset distribution in human spleen

BACKGROUND

When analyzing human cellular immune responses, most focus is placed on the peripheral blood (PB) and, to a lesser extent, the lymph nodes. To date the spleen has not been analyzed with regard to its role in adaptive cellular immunity and more notably not with respect to T-cell immune responses.

MATERIALS AND METHODS

We analyzed the splenic lymphocyte compartment in comparison with the PB lymphocyte compartment regarding the number of NK cells, B cells, CD4(+), CD8(+) T cells and CMV-specific CD8(+) T cells. Furthermore, we analyzed the distribution of naive, memory and effector subsets of CD4(+) and CD8(+) T cells in these compartments.

RESULTS

The spleen contains proportionally more B cells and less CD4(+) and CD8(+) T cells than PB. The percentage of CD8(+) T cells is greater in the spleen, leading to an inverse CD4/CD8 ratio. Both splenic CD4(+) and CD8(+) T-cell populations show a greater number of activated cells, and splenic CD8(+) T cells show a more differentiated cytotoxic CD27(-)CD45RA(+) memory phenotype.

CONCLUSIONS

Our findings show that the distribution of the different lymphocyte subsets is markedly different between the spleen and the PB, thus inferring an important and distinct role for the spleen in CD4(+) and CD8(+) T-cell activation.

Two types of cytotoxic lymphocyte regulation explain kinetics of immune response to human immunodeficiency virus

The organization of the cytotoxic T lymphocyte (CTL) response at organismal level is poorly understood. We propose a mathematical model describing the interaction between HIV and its host that explains 20 quantitative observations made in HIV-infected individuals and simian immunodeficiency virus-infected monkeys, including acute infection and response to various antiretroviral therapy regimens. The model is built on two modes of CTL activation: direct activation by infected cells and indirect activation by CD4 helper cells activated by small amounts of virus. Effective infection of helper cells by virus leads to a stable chronic infection at high virus load. We assume that CTLs control virus by killing infected cells. We explain the lack of correlation between the CTL number and the virus decay rate in therapy and predict that individuals with a high virus load can be switched to a low-viremia state that will maintain stability after therapy, but the switch requires fine adjustment of therapy regimen based on the model and individual parameters.

Perturbations of cell cycle control in T cells contribute to the different outcomes of simian immunodeficiency virus infection in rhesus macaques and sooty mangabeys

In contrast to human immunodeficiency virus (HIV) infection of humans and experimental simian immunodeficiency virus (SIV) infection of rhesus macaques (RMs), SIV infection of sooty mangabeys (SMs), a natural host African monkey species, is typically nonpathogenic and associated with preservation of CD4+ T-cell counts despite chronic high levels of viral replication. In previous studies, we have shown that the lack of SIV disease progression in SMs is related to lower levels of immune activation and bystander T-cell apoptosis compared to those of pathogenic HIV/SIV infection (G. Silvestri, D. Sodora, R. Koup, M. Paiardini, S. O'Neil, H. M. McClure, S. I. Staprans, and M. B. Feinberg, Immunity 18:441-452, 2003; G. Silvestri, A. Fedanov, S. Germon, N. Kozyr, W. J. Kaiser, D. A. Garber, H. M. McClure, M. B. Feinberg, and S. I. Staprans, J. Virol. 79:4043-4054, 2005). In HIV-infected patients, increased T-cell susceptibility to apoptosis is associated with a complex cell cycle dysregulation (CCD) that involves increased activation of the cyclin B/p34-cdc2 complex and abnormal nucleolar structure with dysregulation of nucleolin turnover. Here we report that CCD is also present during pathogenic SIV infection of RMs, and its extent correlates with the level of immune activation and T-cell apoptosis. In marked contrast, naturally SIV-infected SMs show normal regulation of cell cycle control (i.e., normal intracellular levels of cyclin B and preserved nucleolin turnover) and a low propensity to apoptosis in both peripheral blood- and lymph node-derived T cells. The absence of significant CCD in the AIDS-free, non-immune-activated SMs despite high levels of viral replication indicates that CCD is a marker of disease progression during lentiviral infection and supports the hypothesis that the preservation of cell cycle control may help to confer the disease-resistant phenotype of SIV-infected SMs.

The majority of human immunodeficiency virus type 1 particles present within splenic germinal centres are produced locally

In most stages of human immunodeficiency virus (HIV) infection, cell-free viral particles can be detected in germinal centres (GCs) that are principally retained, in the form of immune complexes, on the surface of follicular dendritic cells (FDCs). The source of this virus remains unknown, although it is agreed that the FDCs themselves are not infected productively. By sequencing HIV viral DNA, genomic RNA and spliced mRNA isolated from individual splenic white pulps, it was shown here that the majority of HIV-1 viral particles are produced locally within the supporting lymphoid structure and do not result from trapping of circulating viruses or immune complexes. These findings underline the exquisite spatial organization of HIV-1 replication in vivo, suggesting a local origin for viruses trapped in splenic GCs.

On the intra-host dynamics of HIV-1 infections

An extension of a previously proposed theory for the pathogenesis of AIDS is presented and analyzed using a mathematical modelling approach. This theory is based on the observation that human immunodeficiency virus type 1 (HIV-1) predominantly infects and replicates in (CD4+)-T cells, and that the infection process within an infected individual is characterized by ongoing generation and selection of HIV variants with increasing reproductive capacity. This evolutionary process is considered to be the reason for the gradual loss of immunocompetence and the final destruction of the immune system observed in most patients. The extension presented here incorporates the effect of the permanently increasing susceptibility of (CD4+)-T cell clones, as a result of the evolutionary process. The presented model reproduces and possibly explains a wide variety of findings about the HIV infection process. Numerical results indicate that the effect of the initial dose is minimal, and restricted to the primary phase of infection. According to the model predictions the impact of the HIV evolutionary speed is crucial for the progression to disease. An important progression determinant is the initial infection rate, being a component of the viral reproductive capacity. An influential role in disease progression seems to be played by the initial (CD4+)-T cell count.

Coreceptor usage of primary HIV type 1 isolates obtained from different lymph node subsets

Biological characteristics of virus quantitatively rescued from different cell types present in lymph nodes of HIV-1-infected individuals in various stages of their disease were determined, not including patients with AIDS defining illness. Viruses were obtained by cocultivation with donor monocyte-derived macrophages and T-lymphocytes and their biological phenotype compared to viruses obtained from the peripheral blood mononuclear cells of the same patient. The biological phenotype was determined on established cell lines (U937-2, CEM, and MT-2) and on the U87.CD4 coreceptor indicator cell lines and variable region 3 (V3) of the envelope was subjected to direct sequencing. All isolates obtained from lymph node subsets used CCR5 as coreceptor. Furthermore, these viruses were also sensitive to inhibition by beta-chemokines as analyzed for viruses of one patient. All 12 V3 regions showed a unique sequence indicating compartmentalization within each patient. The biological phenotype of CCR5-dependent (R5) HIV-1 isolates obtained from PBMC resembles the phenotype of viruses isolated from different lymph node cell subsets.

HIV-1 gp120 induces NFAT nuclear translocation in resting CD4+ T-cells

The replication of human immunodeficiency virus (HIV) in CD4+ T-cells is strongly dependent upon the state of activation of infected cells. Infection of sub-optimally activated cells is believed to play a critical role in both the transmission of virus and the persistence of CD4+ T-cell reservoirs. There is accumulating evidence that HIV can modulate signal-transduction pathways in a manner that may facilitate replication in such cells. We previously demonstrated that HIV gp120 induces virus replication in resting CD4+ T cells isolated from HIV-infected individuals. Here, we show that in resting CD4+ T-cells, gp120 activates NFATs and induces their translocation into the nucleus. The HIV LTR encodes NFAT recognition sites, and NFATs may play a critical role in promoting viral replication in sub-optimally activated cells. These observations provide insight into a potential mechanism by which HIV is able to establish infection in resting cells, which may have implications for both transmission of HIV and the persistence of viral reservoirs.

Distinct chemokine triggers and in vivo migratory paths of fluorescein dye-labeled T Lymphocytes in acutely simian immunodeficiency virus SIVmac251-infected and uninfected macaques

To define the possible impact of T-lymphocyte trafficking parameters on simian immunodeficiency virus (SIV) pathogenesis, we examined migratory profiles of carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled T lymphocytes in acutely SIVmac251-infected and uninfected macaques within 48 h after autologous transfer. Despite significant upregulation of homeostatic chemokine CCL19/macrophage inflammatory protein 3beta and proinflammatory chemokine CXCL9/monokine induced by gamma interferon in secondary lymphoid tissue in SIV infection, no differences in CFSE+ T-lymphocyte frequencies or cell compartmentalization in lymph nodes were identified between animal groups. By contrast, a higher frequency of CFSE+ T lymphocytes in the small intestine was detected in acute SIV infection. This result correlated with increased numbers of gut CD4 T lymphocytes expressing chemokine receptors CCR9, CCR7, and CXCR3 and high levels of their respective chemokine ligands in the small intestine. The changes in trafficking parameters in SIV-infected macaques occurred concomitantly with acute gut CD4 T-lymphocyte depletion. Here, we present the first in vivo T-lymphocyte trafficking study in SIV infection and a novel approach to delineate T-lymphocyte recruitment into tissues in the nonhuman primate animal model for AIDS. Such studies are likely to provide unique insights into T-lymphocyte sequestration in distinct tissue compartments and possible mechanisms of CD4 T-lymphocyte depletion and immune dysfunction in simian AIDS.

Prediction of the pharmacokinetics of succinylated human serum albumin in man from in vivo disposition data in animals and in vitro liver slice incubations

Suc-HSA is a potent HIV-inhibitor with possible application in man. To facilitate the assessment of dosing regimens for future phase I clinical studies, we predicted the pharmacokinetic properties of Suc-HSA in man. Slices prepared from rat, monkey and human liver were incubated with succinylated albumin, and the maximum uptake rate V(m) and Michaelis-Menten constant K(m) were calculated. The pharmacokinetics after multiple doses of Suc-HSA were studied in rats. The pharmacokinetic parameters of Suc-HSA in man were predicted from the results and data from literature, using pharmacokinetic modeling and interspecies scaling techniques, and potential intravenous dose regimens for HIV treatment in man were calculated. On the basis of in vitro uptake studies in rat, monkey and human liver slices and in vivo disposition data in monkey (data from earlier study) and rat, we predicted the following parameters for liver uptake in humans: V(m) 82.5 microg h(-1) kg(-1) and K(m) 0.228 microg ml(-1). The predicted steady-state concentration after daily intravenous bolus doses of 1 mg kg(-1) is between 4 and 30 microg ml(-1), i.e. well above the IC50 of about 0.4 microg ml(-1). Additional loading doses of 8 mg kg(-1) in total are needed to reach steady-state within a few days.

Naturally SIV-infected sooty mangabeys: are we closer to understanding why they do not develop AIDS?

Simian immunodeficiency viruses (SIV) infection of sooty mangabey (SM) monkeys (Cercocebus atys), a natural host species, does not induce CD4+ T cell depletion and acquired immunodeficiency syndrome (AIDS) despite chronic high levels of virus replication. In contrast, SIV infection of non-natural host species, such as rhesus macaques (RM), induces a disease that closely resembles AIDS in humans. The mechanisms underlying the lack of disease progression in SIV-infected SMs are incompletely understood, but certainly reflect a complex evolutionary adaptation whereby the host immune system is not significantly damaged by the highly replicating virus. It is now widely recognized that a better understanding of these mechanisms may provide clues to the pathogenesis of immunodeficiency in HIV-infected humans. In this article I discuss five different hypotheses that may account for the non-pathogenic course of infection in SIV-infected SMs and briefly review the available data supporting each of these hypotheses.

HIV-1 dynamics at different time scales under antiretroviral therapy

We exploit a model that considers three compartments: blood plasma (BP), lymphoid tissue-interstitial spaces (LT-IS), and follicular dendritic cells (FDC), for the HIV-1 dynamics under the application of highly active antiretroviral therapy (HAART) which allowed us to unravel distinct viral dynamics occurring in short- (2 days), middle- (21 days), and long-term (183 days) time scales. The different time scales are determined by the viral clearance rate, the ratio of productively infected CD4(+) T cells to chronically infected cells, and the dissociation rate of HIV-1 complexes from FDC. This generates a scenario in which, after an initial transient stage, the viral BP dynamics decouples and becomes governed by the lymphoid tissue (LT) dynamics; in a later stage, a new decoupling occurs in which the LT-IS dynamics is slaved to that of the FDC dynamics. We observed an initial increase in the viremia after HAART in a patient who did not receive protease inhibitors (PI). By means of the above-mentioned model we were able to highlight the relevant parameters which need to be estimated at three different time scales after HAART.

Current situation in the development of a preventive HIV vaccine

The uncontrolled progression of the aids epidemic has made the development of an efficacious human immunodeficiency virus (HIV) vaccine a major objective of scientific research. No effective preventive vaccine against HIV is currently available and sterilizing immunity has not yet been achieved in animal models. This review analyses the major challenges in developing an aids vaccine, in particular the mechanisms involved in viral escape from the immune response, and summarizes the results obtained with the different prototypes of therapeutic and preventive vaccines. Finally, social, economic and healthcare aspects of research into HIV vaccines and current controversies regarding the development of clinical trials are discussed.

Situación actual en el desarrollo de una vacuna preventiva frente al VIH

El avance de la epidemia de sida ha convertido la obtención de una vacuna eficaz frente al virus de la inmunodeficiencia humana (VIH) como un objetivo científico prioritario. En el momento actual no disponemos de una vacuna preventiva frente a la infección por el VIH y en ningún modelo animal se ha conseguido la protección frente a la infección. En esta revisión se analizan las dificultades existentes en el desarrollo de una vacuna contra el sida, en especial los mecanismos de escape viral a la respuesta inmunitaria y se describen los prototipos de vacunas preventivas y terapéuticas en desarrollo y los resultados obtenidos. Por otra parte se sitúa esta investigación en el contexto sanitario, económico y social de la pandemia de sida y se analizan las polémicas actualmente planteadas en el desarrollo de ensayos clínicos con los diferentes tipos de vacunas.

Massive infection and loss of memory CD4+ T cells in multiple tissues during acute SIV infection

It has recently been established that both acute human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections are accompanied by a dramatic and selective loss of memory CD4+ T cells predominantly from the mucosal surfaces. The mechanism underlying this depletion of memory CD4+ T cells (that is, T-helper cells specific to previously encountered pathogens) has not been defined. Using highly sensitive, quantitative polymerase chain reaction together with precise sorting of different subsets of CD4+ T cells in various tissues, we show that this loss is explained by a massive infection of memory CD4+ T cells by the virus. Specifically, 30-60% of CD4+ memory T cells throughout the body are infected by SIV at the peak of infection, and most of these infected cells disappear within four days. Furthermore, our data demonstrate that the depletion of memory CD4+ T cells occurs to a similar extent in all tissues. As a consequence, over one-half of all memory CD4+ T cells in SIV-infected macaques are destroyed directly by viral infection during the acute phase-an insult that certainly heralds subsequent immunodeficiency. Our findings point to the importance of reducing the cell-associated viral load during acute infection through therapeutic or vaccination strategies.

Correlation between viral RNA levels but not immune responses in plasma and tissues of macaques with long-standing SIVmac251 infection

Plasma virus in human immunodeficiency virus type 1/simian immunodeficiency virus (HIV-1/SIV) infection most likely results from the combination of viruses produced in different tissues. As immunological pressure may be higher in effector sites than secondary lymphoid tissues, we investigated quantitative and qualitative changes in viral RNA in blood and tissues of 10 Mamu-A*01-positive SIV-infected macaques in parallel with the frequency of CD8+ T cells recognizing the dominant Gag181-189 CM9 epitope. The plasma virus level in these macaques directly correlated with the viral RNA levels in lymph nodes, spleen, lungs, colon, and jejunum. In contrast, the frequency of the Gag181-189 CM9 tetramer did not correlate with SIV RNA levels in any compartment. We investigated the presence of viral immune escape in RNA from several tissues. The complete substitution of wild-type genotype with viral immune-escape variant within the Gag181-189 CM9 epitope was associated with low tetramer response in all tissues and blood of two macaques. In one macaque, the replacement of wild type with an immune-escape mutant was asynchronous. While the mutant virus was prevalent in blood and effector tissues (lungs, jejunum, and colon), secondary lymphoid organs such as spleen and lymph nodes still retained 80% and 40%, respectively, of the wild-type virus. These results may imply that there are differences in the immunological pressure exerted by cytotoxic T lymphocytes (CTLs) in tissue compartments of SIVmac251-infected macaques.