T-cell homeostasis, competition, and drift: AIDS as HIV-accelerated senescence of the immune repertoire

Impact of highly active antiretroviral therapy on the prevalence of oral lesions in HIV-positive patients: a systematic review and meta-analysis

The aim of this study was to determine whether highly active antiretroviral therapy (HAART) is associated with the prevalence of oral lesions in HIV-positive patients. This systematic review and meta-analysis was performed in accordance with the PRISMA guidelines (Preferred Reporting Items for Systematic Reviews and Meta-Analyses). The search was conducted in seven electronic databases (PubMed, Scopus, SciELO, LILACS, Embase, Web of Science, and OpenGrey), without restriction on publication period or language. Studies that showed the prevalence of oral lesions manifested in adult HIV-positive patients, subjected or not to HAART, were selected. The meta-analysis estimate of relative risk was calculated using the Mantel-Haenszel method and DerSimonian and Laird estimator to determine the variance between studies in the random-effects model. The meta-analysis showed significant results in favour of the group on HAART, with lower prevalence for angular cheilitis, erythematous candidiasis, oral herpes, pseudomembranous candidiasis, Kaposi sarcoma, and oral hairy leukoplakia. The prevalence of oral mucosal hyperpigmentation was higher in patients on HAART. These results suggest that the prevalence of oral lesions in HIV-positive patients is lower for those on HAART, which might occur because of the improvement in immunity provided by the therapy.

Impact of HIV-1 tropism on the emergence of non-AIDS events in HIV-infected patients receiving fully suppressive antiretroviral therapy

Objective:

The impact of HIV-1 tropism on the emergence of non-AIDS events was evaluated in a cohort of 116 antiretroviral therapy (ART) responder patients.

Methods:

The patients were followed for the emergence of hypertension, renal impairment, metabolic and bone disorders (defined as non-AIDS events) each 8 weeks at standard visits. A V3 plasma sequence genotype analysis was performed at the time of ART initiation and the geno2pheno algorithm with the results that defines the false-positive rate (FPR) was used to infer HIV tropism. The associations between the non-AIDS events and the FPR at baseline were evaluated using the χ² test for trend. A Cox-regression analysis using the counting process formulation of Andersen and Gill was performed to define whether the emergence of non-AIDS events was correlated to FPR.

Results:

The prevalence of at least one non-AIDS event resulted higher in patients with a FPR below 10% than in patients with a R5 virus (P = 0.033). Patients with a FPR below 5.0% most frequently developed non-AIDS events during ART (P = 0.01). A higher prevalence of patients with at least two AIDS events was found in the group of patients with a FPR below 5.0% with respect to the others (P < 0.001). At multivariate Cox-regression analysis, having an X4 virus and age were independently associated with a higher probability of non-AIDS event development.

Conclusion:

This study shows that an X4 virus, particularly a FPR less than 5%, is related to non-AIDS events development. Further studies are warranted to understand the mechanisms underlying this phenomenon.

Aging and HIV/AIDS: pathogenetic role of therapeutic side effects

The intersection of aging and HIV/AIDS is a looming 'epidemic within an epidemic.' This paper reviews how HIV/AIDS and its therapy cause premature aging or contribute mechanistically to HIV-associated non-AIDS illnesses (HANA). Survival with HIV/AIDS has markedly improved by therapy combinations containing nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors, and protease inhibitors (PIs) called HAART (highly active antiretroviral therapy). Because NRTIs and PIs together prevent or attenuate HIV-1 replication, and prolong life, the population of aging patients with HIV/AIDS increases accordingly. However, illnesses frequently associated with aging in the absence of HIV/AIDS appear to occur prematurely in HIV/AIDS patients. Theories that help to explain biological aging include oxidative stress (where mitochondrial oxidative injury exceeds antioxidant defense), chromosome telomere shortening with associated cellular senescence, and accumulation of lamin A precursors (a nuclear envelop protein). Each of these has the potential to be enhanced or caused by HIV/AIDS, antiretroviral therapy, or both. Antiretroviral therapy has been shown to enhance events seen in biological aging. Specifically, antiretroviral NRTIs cause mitochondrial dysfunction, oxidative stress, and mitochondrial DNA defects that resemble features of both HANA and aging. More recent clinical evidence points to telomere shortening caused by NRTI triphosphate-induced inhibition of telomerase, suggesting telomerase reverse transcriptase (TERT) inhibition as being a pathogenetic contributor to premature aging in HIV/AIDS. PIs may also have a role in premature aging in HIV/AIDS as they cause prelamin A accumulation. Overall, toxic side effects of HAART may both resemble and promote events of aging and are worthy of mechanistic studies.

Peripheral selection rather than thymic involution explains sudden contraction in naive CD4 T-cell diversity with age

A diverse array of T cells is required for defense against pathogens. The naive CD4 T-cell repertoire reaches its peak diversity by early human adulthood and is maintained until older age. Surprisingly, around age 70, this diversity appears to plummet abruptly. A similar qualitative pattern holds for the CD4 T memory-cell population. We used mathematical models to explore different hypotheses for how such a loss of diversity might occur. The prevailing hypotheses suggest that the loss of diversity is due to a decline in emigration of cells from the thymus or a contraction in total number of cells. Our models reject these mechanisms because they yield only a gradual and minimal decline in the repertoire instead of the observed sudden and profound decrease later in life. We propose that an abrupt decline in the repertoire could be caused by the accumulation of mutations (defined here as any cell-intrinsic heritable event) that provide a short-term fitness advantage to a small number of T-cell clones (e.g., by an increased division rate or decreased death rate), with the person as a whole incurring the long-term cost of a decreased ability to fight infections.

Computational immunology meets bioinformatics: the use of prediction tools for molecular binding in the simulation of the immune system

We present a new approach to the study of the immune system that combines techniques of systems biology with information provided by data-driven prediction methods. To this end, we have extended an agent-based simulator of the immune response, C-ImmSim, such that it represents pathogens, as well as lymphocytes receptors, by means of their amino acid sequences and makes use of bioinformatics methods for T and B cell epitope prediction. This is a key step for the simulation of the immune response, because it determines immunogenicity. The binding of the epitope, which is the immunogenic part of an invading pathogen, together with activation and cooperation from T helper cells, is required to trigger an immune response in the affected host. To determine a pathogen's epitopes, we use existing prediction methods. In addition, we propose a novel method, which uses Miyazawa and Jernigan protein-protein potential measurements, for assessing molecular binding in the context of immune complexes. We benchmark the resulting model by simulating a classical immunization experiment that reproduces the development of immune memory. We also investigate the role of major histocompatibility complex (MHC) haplotype heterozygosity and homozygosity with respect to the influenza virus and show that there is an advantage to heterozygosity. Finally, we investigate the emergence of one or more dominating clones of lymphocytes in the situation of chronic exposure to the same immunogenic molecule and show that high affinity clones proliferate more than any other. These results show that the simulator produces dynamics that are stable and consistent with basic immunological knowledge. We believe that the combination of genomic information and simulation of the dynamics of the immune system, in one single tool, can offer new perspectives for a better understanding of the immune system.

Understanding the mechanisms and limitations of immune control of HIV

A large number of experimental studies have been performed over the past decade in an attempt to develop a vaccine for human immunodeficiency virus (HIV). These studies have used a variety of approaches aimed at stimulating both antibody-mediated and cell-mediated immunity. Many of these experiments have been performed in macaque models of HIV. Analysis and modeling of the results of these studies provide the opportunity to investigate the mechanisms and limitations of viral control by humoral and cell-mediated immunity. These studies suggest that CD8(+) T cells do 'too little too late' to prevent the establishment of viral infection and latency. By contrast, passively administered antibody acts extremely early to reduce the initial inoculum and slow viral growth. In both cases, reduction in peak viral load appears crucial to the maintenance of CD4(+) T cells in acute infection and for effective long-term viral control. The insights gained from studies of simian human immunodeficiency virus infection have important implications for HIV vaccination. However, important questions remain as to whether differences in pathogenesis in HIV will lead to different 'rules of engagement' for immune control of virus.

Peripheral blood cytotoxic gammadelta T lymphocytes from patients with human immunodeficiency virus type 1 infection and AIDS lyse uninfected CD4+ T cells, and their cytocidal potential correlates with viral load

Progression of human immunodeficiency virus type 1 (HIV-1) infection in humans is marked by declining CD4+-T-cell counts and increasing virus load (VL). Cytotoxic T lymphocytes (CTL) play an important role in the lysis of HIV-infected cells, especially during the early phase of asymptomatic infection. CTL responses in the later phase of disease progression may not be as effective since progressors with lower CD4+-T-cell counts have consistently higher VL despite having elevated CTL counts. We hypothesized that, apart from antiviral effects, some CTL might also contribute to AIDS pathogenesis by depleting CD4+ T cells and that this CTL activity may correlate with the VL in AIDS patients. Therefore, a cross-sectional study of 31 HIV-1-infected patients at various clinical stages was carried out. Purified CTL from these donors as well as HIV-seronegative controls were used as effectors against different human cell targets by using standard 51Cr release cytolytic assays. A direct correlation between VL and CTL-mediated, major histocompatibility complex (MHC)-unrestricted lysis of primary CD4+-T-cell, CEM.NKR, and K562 targets was observed. CD4+-T-cell counts and duration of infection also correlated with MHC-unrestricted cytolytic activity. Our data clearly show that gammadelta CTL are abnormally expanded in the peripheral blood of HIV-infected patients and that the Vdelta1 subset of gammadelta T cells is the main effector population responsible for this type of cytolysis. The present data suggest that gammadelta CTL can contribute to the depletion of bystander CD4+ T cells in HIV-infected patients as a parallel mechanism to HIV-associated immunopathogenesis and hence expedite AIDS progression.

Antigen-driven T-cell turnover

A mathematical model is developed to characterize the distribution of cell turnover rates within a population of T lymphocytes. Previous models of T-cell dynamics have assumed a constant uniform turnover rate; here we consider turnover in a cell pool subject to clonal proliferation in response to diverse and repeated antigenic stimulation. A basic framework is defined for T-cell proliferation in response to antigen, which explicitly describes the cell cycle during antigenic stimulation and subsequent cell division. The distribution of T-cell turnover rates is then calculated based on the history of random exposures to antigens. This distribution is found to be bimodal, with peaks in cell frequencies in the slow turnover (quiescent) and rapid turnover (activated) states. This distribution can be used to calculate the overall turnover for the cell pool, as well as individual contributions to turnover from quiescent and activated cells. The impact of heterogeneous turnover on the dynamics of CD4(+) T-cell infection by HIV is explored. We show that our model can resolve the paradox of high levels of viral replication occurring while only a small fraction of cells are infected.

The role of antigenic stimulation and cytotoxic T cell activity in regulating the long-term immunopathogenesis of HIV: mechanisms and clinical implications

This paper develops a predictive mathematical model of cell infection, host immune response and viral replication that reproduces observed long-term trends in human immunodeficiency virus (HIV) pathogenesis. Cell activation induced by repeated exposure to many different antigens is proposed as the principal mechanism of providing target cells for HIV infection and, hence, of CD4+ T cell depletion, with regulation of the overall T cell pool size causing concomitant CD8 pool increases. The model correctly predicts the cross-patient variability in disease progression, the rate of which is found to depend on the efficacy of anti-HIV cytotoxic T lymphocyte responses, overall viral pathogenicity and random effects. The model also predicts a variety of responses to anti-viral therapy, including episodic residual viral replication and discordant responses and we find that such effects can be suppressed by increasing the potency of treatment.

Evolutionary process and the ecology of human immune function

Evolutionary principles inform central design features of human immune defenses and provide key insights into this complicated host defense system. This article explores the selection pressures and adaptive responses that have elaborated the immune system over the course of evolution and discusses their implications for understanding contemporary immune development and function. Special attention is given to the challenges posed by diverse, rapidly evolving pathogens and the mammalian response to these challenges. The process of lymphocyte diversity generation and subsequent clonal selection is quintessentially Darwinian: pathogens provide selection pressure that drives differential replication of host immune cell lines, resulting in changes in genetic frequencies within an individual's population of lymphocytes. The immune system also incorporates nongenetic transgenerational processes in the transfer of antibodies from mother to offspring through the placenta and breast milk. The consequences of these observations for human development, health, and the ecology of immune function are considered throughout the life cycle. Specifically, evolutionary processes provide insight into autoimmunity, thymic function, lymphocyte development, infectious disease risk, and lactation. While much work in evolutionary medicine focuses on the discordance between evolved biology and rapidly changing cultural environments, with respect to the immune system, evolutionary processes may be most revealing when applied within individuals. Am. J. Hum. Biol. 11:705-717, 1999. Copyright 1999 Wiley-Liss, Inc.

Models of immune memory: on the role of cross-reactive stimulation, competition, and homeostasis in maintaining immune memory

There has been much debate on the contribution of processes such as the persistence of antigens, cross-reactive stimulation, homeostasis, competition between different lineages of lymphocytes, and the rate of cell turnover on the duration of immune memory and the maintenance of the immune repertoire. We use simple mathematical models to investigate the contributions of these various processes to the longevity of immune memory (defined as the rate of decline of the population of antigen-specific memory cells). The models we develop incorporate a large repertoire of immune cells, each lineage having distinct antigenic specificities, and describe the dynamics of the individual lineages and total population of cells. Our results suggest that, if homeostatic control regulates the total population of memory cells, then, for a wide range of parameters, immune memory will be long-lived in the absence of persistent antigen (T1/2 > 1 year). We also show that the longevity of memory in this situation will be insensitive to the relative rates of cross-reactive stimulation, the rate of turnover of immune cells, and the functional form of the term for the maintenance of homeostasis.

Anti-HIV type 1 memory cytotoxic T lymphocyte responses associated with changes in CD4+ T cell numbers in progression of HIV type 1 infection

We investigated memory cytotoxic T lymphocyte (CTLm) responses to HIV-1 as a determinant of HIV-1 disease progression, in relation to plasma HIV-1 load and T lymphocyte numbers in a longitudinal study of 14 homosexual men with incident HIV-1 infection. Study participants were selected who exhibited failure of T cell homeostasis, i.e., a downward inflection in CD3+ T cells that occurs in >75% of persons 1.5 to 2.5 years before development of AIDS, and compared with participants who developed low CD4+ T cell counts associated with possible T cell homeostasis failure, a subject who progressed rapidly to AIDS without well-defined T cell inflection, and subjects who had long-term preservation of T cell homeostasis (nonprogressors). High CTLm responses against Gag, but not Pol or Env, soon after seroconversion were associated with a slower loss of CD4+ T cells 1-4 years after seroconversion. Anti-Env CTLm responses decreased in most subjects around the time that T cell homeostasis failed. Plasma HIV-1 RNA increased exponentially (1.59-fold per year) over the 5 years preceding failure of T cell homeostasis, and there was a shift from a non-syncytium-inducing/CCR5 coreceptor phenotype of HIV-1 to a syncytium-inducing/CXCR4 phenotype, regardless of high or increasing levels of anti-HIV-1 CTLm during this time. These observations suggest that decreases in CTLm and increasing virus load are independent factors contributing to HIV-1 disease progression.

Early regeneration of thymic progenitors in rhesus macaques infected with simian immunodeficiency virus

The thymus plays a critical role in the maturation and production of T lymphocytes and is a target of infection by human immunodeficiency virus (HIV) and the related simian immunodeficiency virus (SIV). Using the SIV/macaque model of AIDS, we examined the early effects of SIV on the thymus. We found that thymic infection by SIV resulted in increased apoptosis 7-14 d after infection, followed by depletion of thymocyte progenitors by day 21. A marked rebound in thymocyte progenitors occurred by day 50 and was accompanied by increased levels of cell proliferation in the thymus. Our results demonstrate a marked increase in thymic progenitor activity very early in the course of SIV infection, long before marked declines in peripheral CD4(+) T cell counts.

Changes in CD8 and CD4 lymphocyte subsets, T cell proliferation responses and non-survival in the very old: the Swedish longitudinal OCTO-immune study

Results from a previous longitudinal study indicated that a combination of high CD8 and low CD4 percentages and poor T cell proliferation in peripheral blood lymphocytes was associated with higher mortality in a subgroup of a sample of very old Swedish individuals. The present study examined whether those results could be confirmed at a subsequent 2-year time interval by investigating if additional individuals from the same original sample had developed the immune profile associated with higher mortality. Subgroups were formed by cluster analysis and similar to our previous results, this follow-up study identified a subgroup of subjects (n = 18) with an immune profile which again included high CD8, low CD4 percentages and poor mitogen response and was associated with higher mortality. Over the 2-year period 12 additional individuals: (1) Developed this immune profile; and (2) Could be identified by changes in their CD4:CD8 ratios which progressively decreased over the study period. These results confirm our original study and indicate that in this very old sample, over a subsequent 2 year period, additional individuals moved into the cluster at risk for higher mortality.

Conservation of total T-cell counts during HIV infection: alternative hypotheses and implications

While CD4+ T-cell counts in the blood of HIV-infected individuals gradually decrease, there is a parallel increase in the number of blood CD8+ T cells such that the total number of T cells remains essentially constant for several years (1). The basis and significance of this phenomenon are not known. Based on a statistical analysis of longitudinal T-cell counts from the Transfusion Safety Study (TSS) database and on theoretical considerations, we evaluate several alternative models, including versions of the "blind homeostasis" (BH) hypothesis (1-3). At issue is the nature of the homeostatic regulation of lymphocytes and its apparent failure in HIV infection. The most plausible explanation for the conservation of total blood T-cell numbers while subset ratios change is that CD4+ and CD8+ T cells compete for a limited access to the blood compartment. Such interaction between the subsets implies, in particular, that changes in the number of CD4+ T cells occurring in other tissues cannot be reliably inferred from those observed in the blood. We reiterate propositions made earlier (4) that much of the apparent "depletion" of CD4+ lymphocytes during the asymptomatic phase of HIV infection may be attributed to redistribution between the tissues and the blood compartment.

Immune reconstitution in HIV infection

Progressive immune deficiency arising during HIV disease reflects the continual degradation and the ultimate deletion of immune specificites defined by the CD4(+) T lymphocyte repertoire. Recent evidence suggests that improvements in the immune function of patients with HIV who receive therapy primarily reflects the expansion of CD4(+) T lymphocyte populations present before therapy commenced. These observations have implications for clinical management, therapeutic strategies, and future research.

Virus dynamics and drug therapy

The recent development of potent antiviral drugs not only has raised hopes for effective treatment of infections with HIV or the hepatitis B virus, but also has led to important quantitative insights into viral dynamics in vivo. Interpretation of the experimental data depends upon mathematical models that describe the nonlinear interaction between virus and host cell populations. Here we discuss the emerging understanding of virus population dynamics, the role of the immune system in limiting virus abundance, the dynamics of viral drug resistance, and the question of whether virus infection can be eliminated from individual patients by drug treatment.

Determination of antigen-specific memory/effector CD4+ T cell frequencies by flow cytometry: evidence for a novel, antigen-specific homeostatic mechanism in HIV-associated immunodeficiency

The highly regulated secretion of effector cytokines by CD4+ T cells plays a critical role in immune protection against pathogens such as cytomegalovirus. Here, we directly compare the frequency and functional characteristics of cytomegalovirus-specific CD4+ memory/effector T cells in normal and HIV+ subjects using a novel, highly efficient multiparameter flow cytometric assay that detects the rapid intracellular accumulation of cytokine(s) after short-term (6 h) in vitro antigen stimulation. Responses in this assay correlate precisely with independent measures of sensitization history (e.g., seroreactivity), and allow the simultaneous assessment of multiple cytokines in single effector T cells. Healthy HIV- individuals manifested an average of 0.71, 0.72, 0.38, and 0.06% CD4+ T cells responding to cytomegalovirus with gamma-IFN, TNF-alpha, IL-2, and IL-4 production, respectively, with the simultaneous production of gamma-IFN, TNF-alpha, and IL-2 being the most common effector phenotype. Significantly, overall cytomegalovirus-specific CD4+ effector frequencies were markedly higher among 40% of HIV+ subjects (2.7-8.0%), and demonstrated a predominately polarized gamma-IFN+/TNF-alpha+/IL-2-/IL-4- phenotype. In contrast, CD4+ effector frequencies for heterologous, nonubiquitous viruses such as the mumps virus were low or absent in the HIV+ group. These data suggest the existence of homeostatic mechanisms in HIV disease that selectively preserve memory T cell populations reactive with ubiquitous pathogens such as cytomegalovirus-likely at the expense of T cell memory to more sporadically encountered infectious agents.