Role of the thymus in pediatric HIV-1 infection

The within-host fitness of HIV-1 increases with age in ART-naïve HIV-1 subtype C infected children

As the immune system develops with age, children combat infections better. HIV-1, however, targets an activated immune system, potentially rendering children increasingly permissive to HIV-1 infection as they grow. How HIV-1 fitness changes with age in children is unknown. Here, we estimated the within-host basic reproductive ratio, R₀, a marker of viral fitness, in HIV-1 subtype C-infected children in India, aged between 84 days and 17 years. We measured serial viral load and CD4 T cell counts in 171 children who initiated first-line ART. For 25 children, regular and frequent measurements provided adequate data points for analysis using a mathematical model of viral dynamics to estimate R₀. For the rest, we used CD4 counts for approximate estimation of R₀. The viral load decline during therapy was biphasic. The mean lifespans of productively and long-lived infected cells were 1.4 and 27.8 days, respectively. The mean R₀ was 1.5 in children aged < 5 years, increased with age, and approached 6.0 at 18 years, close to 5.8 estimated previously for adults. The tolerogenic immune environment thus compromises HIV-1 fitness in young children. Early treatment initiation, when the R₀ is small, will likely improve viral control, in addition to suppressing the latent reservoir.

The Role of Immune Response in Optimal HIV Treatment Interventions

A mathematical model for the transmission dynamics of human immunodeficiency virus (HIV) within a host is developed. Our model focuses on the roles of immune response cells or cytotoxic lymphocytes (CTLs). The model includes active and inactive cytotoxic immune cells. The basic reproduction number and the global stability of the virus free equilibrium is carried out. The model is modified to include anti-retroviral treatment interventions and the controlled reproduction number is explored. Their effects on the HIV infection dynamics are investigated. Two different disease stage scenarios are assessed: early-stage and advanced-stage of the disease. Furthermore, optimal control theory is employed to enhance healthy CD4+ T cells, active cytotoxic immune cells and minimize the total cost of anti-retroviral treatment interventions. Two different anti-retroviral treatment interventions (RTI and PI) are incorporated. The results highlight the key roles of cytotoxic immune response in the HIV infection dynamics and corresponding optimal treatment strategies. It turns out that the combined control (both RTI and PI) and stronger immune response is the best intervention to maximize healthy CD4+ T cells at a minimal cost of treatments.

The dual role of autoimmune regulator in maintaining normal expression level of tissue-restricted autoantigen in the thymus: A modeling investigation

The expression level of tissue-restricted autoantigens (TSA) in the thymus is crucial for the negative selection of autoreactive T cells during central tolerance. The autoimmune regulator factor (AIRE) plays an important role in the positive regulation of these TSA in medullary thymic epithelial cells and, consequently, in the negative selection of high-avidity autoreactive T cells. Recent studies, however, revealed that thymic islet cell autoantigen (ICA69) expression level in non-obese diabetic (NOD) mice, prone to developing type 1 diabetes (T1D), is reduced due to an increase in the binding affinity of AIRE to the Ica1-promoter region, which regulates ICA69 protein synthesis. This seemed to suggest that AIRE acts as a transcriptional repressor of Ica1 gene in the thymus, causing down regulation in the expression level of ICA69. To investigate this hypothesis and the apparent dual role of AIRE in negative selection, we develop a series of mathematical models of increasing complexity describing the temporal dynamics of self-reactive T cells, AIRE-mRNA and AIRE-(in)dependent thymic TSA-associated genes. The goal is to understand how changing the binding affinity of AIRE to Ica1-promoter affects both T-cell tolerance and the dual role of the transcription factor. Using stability analysis and numerical computations, we show that the model possesses a bistable switch, consisting of healthy and autoimmune states, in the expression level of Ica1 gene with respect to AIRE binding affinity, and that it can capture the experimentally observed dual role of AIRE. We also show that the model must contain a positive feedback loop exerted by T cells on AIRE expression (e.g., via lymphotoxin released by T cells) to produce bistability. Our results suggest that the expression-level of AIRE-mRNA in the healthy state is lower than that of the autoimmune state, and that negative selection is very sensitive to parameter perturbations in T-cell avidity.

Computational study to determine when to initiate and alternate therapy in HIV infection

HIV is a widespread viral infection without cure. Drug treatment has transformed HIV disease into a treatable long-term infection. However, the appearance of mutations within the viral genome reduces the susceptibility of HIV to drugs. Therefore, a key goal is to extend the time until patients exhibit resistance to all existing drugs. Current HIV treatment guidelines seem poorly supported as practitioners have not achieved a consensus on the optimal time to initiate and to switch antiretroviral treatments. We contribute to this discussion with predictions derived from a mathematical model of HIV dynamics. Our results indicate that early therapy initiation (within 2 years postinfection) is critical to delay AIDS progression. For patients who have not received any therapy during the first 3 years postinfection, switch in response to virological failure may outperform proactive switching strategies. In case that proactive switching is opted, the switching time between therapies should not be larger than 100 days. Further clinical trials are needed to either confirm or falsify these predictions.

Role of active and inactive cytotoxic immune response in human immunodeficiency virus dynamics

Objectives

Mathematical models can be helpful to understand the complex dynamics of human immunodeficiency virus infection within a host. Most of work has studied the interactions of host responses and virus in the presence of active cytotoxic immune cells, which decay to zero when there is no virus. However, recent research highlights that cytotoxic immune cells can be inactive but never be depleted.

Methods

We propose a mathematical model to investigate the human immunodeficiency virus dynamics in the presence of both active and inactive cytotoxic immune cells within a host. We explore the impact of the immune responses on the dynamics of human immunodeficiency virus infection under different disease stages.

Results

Standard mathematical and numerical analyses are presented for this new model. Specifically, the basic reproduction number is computed and local and global stability analyses are discussed.

Conclusion

Our results can give helpful insights when designing more effective drug schedules in the presence of active and inactive immune responses.

Intrathymic effect of acute pathogenic SHIV infection on T-lineage cells in newborn macaques

We intrarectally infected newborn macaques with a pathogenic simian/human immunodeficiency virus (SHIV) that induced rapid and profound CD4 (+) T cell depletion, and examined the early effects of this SHIV on the thymus. After intrarectal infection, viral loads were much higher in the thymus than in other lymphoid tissues in newborns. In contrast, no clear difference was seen in the viral loads of different tissues in adults. Histological and immunohistochemical observations showed severe thymic involution. Depletion of CD4 (+) thymocytes began in the medulla at 2 weeks post infection and spread over the whole thymus. After in vivo infection, the CD2 (+) subpopulation, which represents a relatively later stage of T cell progenitors, was selectively reduced and development of thymocytes from CD3 (-) CD4 (-) CD8 (-) cells to CD4 (+) CD8 (+) cells was impaired. These results suggest that profound and irreversible loss of CD4 (+) cells that are observed in the peripheral blood of SHIV-infected monkeys are due to destruction of the thymus and impaired thymopoiesis as a result of SHIV infection in the thymus.

Viral infection in internally structured hosts. I. Conditions for persistent infection

For a virus population within its host, two important levels of structure can be considered: multiple cell types which can be infected, and tissue types or body compartments which may be coupled via movement. We develop a model with both types of structure. Migration between compartments can create "sources" and "sinks" within the virus population, where realized viral growth rate and abundance is lowered in some compartments compared to what would be observed in isolation. Using both analytical and numerical methods, we investigate how this within-host spatial structure affects the conditions for persistent viral infection. We find that migration between compartments makes the establishment of infection more difficult than it would be in the absence of migration, implying that within-host spatial structure combined with viral movement decreases the likelihood of viral establishment. If migration is symmetrical and compartments are heterogeneous, an increase in migration rates between compartments generally makes establishment less likely. This may help to explain the tissue specificity observed for many viruses. There are, however, important exceptions to this result. These include circumstances where the virus initially invades a compartment that is unfavorable to population growth and migration is necessary to infect other parts of the host body. Stochastic aspects of viral establishment may also favor increased migration as it tends to dampen the amplitude of fluctuations in population size during the initial transient phase of establishment.

Immunopathogenesis of oropharyngeal candidiasis in human immunodeficiency virus infection

Oropharyngeal and esophageal candidiases remain significant causes of morbidity in human immunodeficiency virus (HIV)-infected patients, despite the dramatic ability of antiretroviral therapy to reconstitute immunity. Notable advances have been achieved in understanding, at the molecular level, the relationships between the progression of HIV infection, the acquisition, maintenance, and clonality of oral candidal populations, and the emergence of antifungal resistance. However, the critical immunological defects which are responsible for the onset and maintenance of mucosal candidiasis in patients with HIV infection have not been elucidated. The devastating impact of HIV infection on mucosal Langerhans' cell and CD4(+) cell populations is most probably central to the pathogenesis of mucosal candidiasis in HIV-infected patients. However, these defects may be partly compensated by preserved host defense mechanisms (calprotectin, keratinocytes, CD8(+) T cells, and phagocytes) which, individually or together, may limit Candida albicans proliferation to the superficial mucosa. The availability of CD4C/HIV transgenic mice expressing HIV-1 in immune cells has provided the opportunity to devise a novel model of mucosal candidiasis that closely mimics the clinical and pathological features of candidal infection in human HIV infection. These transgenic mice allow, for the first time, a precise cause-and-effect analysis of the immunopathogenesis of mucosal candidiasis in HIV infection under controlled conditions in a small laboratory animal.

Linking dynamical and population genetic models of persistent viral infection

This article develops a theoretical framework to link dynamical and population genetic models of persistent viral infection. This linkage is useful because, while the dynamical and population genetic theories have developed independently, the biological processes they describe are completely interrelated. Parameters of the dynamical models are important determinants of evolutionary processes such as natural selection and genetic drift. We develop analytical methods, based on coupled differential equations and Markov chain theory, to predict the accumulation of genetic diversity within the viral population as a function of dynamical parameters. These methods are first applied to the standard model of viral dynamics and then generalized to consider the infection of multiple host cell types by the viral population. Each cell type is characterized by specific parameter values. Inclusion of multiple cell types increases the likelihood of persistent infection and can increase the amount of genetic diversity within the viral population. However, the overall rate of gene sequence evolution may actually be reduced.

The effects of different HIV type 1 strains on human thymic function

Studies of HIV-1-infected humans indicate that the thymus can be infected by HIV-1. In some of these patients, there is a significant CD4(+) T cell decline and a faster disease progression. This phenomenon is more evident in pediatric patients who depend heavily on their thymus for generation of new T cells. We hypothesize that HIV-1 causes T cell regenerative failure within the thymus, which has a profound impact on disease progression. Building on our established human thymopoiesis model, we include dynamic interactions between different HIV-1 strains (R5 and X4) and thymocytes. Our results predict that thymic infection with different HIV-1 strains induces thymic dysfunction to varying degrees, contributing to differences in disease progression as observed in both HIV-1-infected children and adults. Thymic infection in children is more severe than in adults, particularly during X4 infection. This outcome is likely due to both a higher viral load and a more active thymus in pediatric patients. Our results also indicate that a viral strain switch from R5 to X4 induces further deterioration in thymopoiesis. We predict that both viral and host factors play key roles in controlling thymic infection, including strain virulence and health status of the thymus.

Reevaluation of T cell receptor excision circles as a measure of human recent thymic emigrants

The human thymus exports newly generated T cells to the periphery. As no markers have been identified for these recent thymic emigrants (RTE), it is presently impossible to measure human thymic output. T cell receptor excision circles (TREC) have been recently used to assess thymic output during both health and disease. Using a mathematical model, we quantify age-dependent changes both in the number of RTE generated per day and in TREC concentration during an 80-year lifespan. Through analyses, we demonstrate that RTE and peripheral T cell division have the same potential to affect TREC concentration at any age in healthy people. T cell death also influences TREC concentration, but to a lesser extent. During aging, our results indicate that thymic involution primarily induces an age-dependent decline in TREC concentrations within both CD4(+) and CD8(+) T cell populations. We further apply this model for studying TREC concentration during HIV-1 infection. Our analyses reveal that a decrease in thymic output is the major contributor to the decline in TREC concentration within CD4(+) T cells, whereas both increased peripheral T cell division and decreased thymic output induce the decline in TREC concentration within CD8(+) T cells. Therefore, we suggest that T cell turnover should be examined together with TREC concentration as a measure of RTE. If peripheral T cell division remains relatively unchanged, then TREC concentration indeed reflects thymic output.

T cell signaling mechanisms that regulate HIV-1 infection

The ability of human immunodeficiency virus type-1 (HIV-1) to establish a persistent infection is critically dependent on the cellular signals that regulate HIV-1 replication within target cells. The balance between numerous host factors that either enhance or suppress viral infection determines the clinical outcome. Perturbation of the steady-state level of viral replication can significantly influence the course and the speed at which the infection develops into clinical disease. Activation signals delivered to T cells by cytokines and antigen-presenting cells (APC), are key modulators of viral replication. Our laboratory seeks to decipher how HIV-1 exploits T cell signaling mechanisms and host factors that regulate viral replication. Elucidation of the molecular mechanisms by which cellular signals regulate the HIV-1 life cycle within target cells will significantly advance our understanding of host-virus interactions.

Thymic size on chest radiograph and rapid disease progression in human immunodeficiency virus 1-infected children

BACKGROUND

Early infection of the thymus, an organ central to the ontogeny of the immune system, has been proposed as a cause of rapid progression in pediatric HIV disease.

OBJECTIVE

To test the hypothesis that small thymic volume is associated with rapid disease progression in HIV-infected children.

DESIGN

Three pediatric radiologists established criteria for rating the size of the thymic profile on chest radiographs. All available baseline chest radiographs were reviewed in a random sequence, with radiologists blinded to study subjects' clinical status. A consensus was reached on whether the thymus was normal or small for age.

SETTING

A prospective multicenter study of the natural history of HIV-1 infection in children, the Pediatric Pulmonary and Cardiovascular Complications of Vertically Transmitted Human Immunodeficiency Virus Infection (P2C2) Study.

PATIENTS

Fifty-eight HIV-infected children and 38 control children (uninfected but born to HIV-infected women) for whom chest radiographs in the first year of life were available.

MAIN OUTCOME MEASURE

Rapid progression of HIV disease, defined as CDC Clinical Category C (severely symptomatic) or Immunologic Category 3 (severe immunosuppression) by 1 year of age.

RESULTS

The mean age at the time of chest radiography was 3.5 months. Ten (17%) HIV-infected children had reduced thymic profile size, whereas no controls did (P = 0.006). Of the 58 (59%) HIV-infected children 34 were classified as rapid progressors, and 9 (26%) of them had reduced thymus size, compared with 1 (4%) of the non-rapid progressor children [odds ratio, 8.28; 95% confidence interval (CI), 1.0, 70.5; P = 0.035]. Baseline mean CD4+ count was 1642 (95% CI 1322 to 2009) cells/microl for those with normal thymus and 740 (95% CI 380 to 1275) cells/microl for those with reduced thymus (P = 0.007).

CONCLUSION

Early thymic involution is associated with rapidly progressive disease in HIV-infected children.

A model to predict cell-mediated immune regulatory mechanisms during human infection with Mycobacterium tuberculosis

A key issue for the study of tuberculosis infection (TB) is to understand why individuals infected with Mycobacterium tuberculosis experience different clinical outcomes. Elaborating the immune mechanisms that determine whether an infected individual will suffer active TB or latent infection can aid in developing treatment and prevention strategies. To better understand the dynamics of M. tuberculosis infection and immunity, we have developed a virtual human model that qualitatively and quantitatively characterizes the cellular and cytokine control network operational during TB infection. Using this model, we identify key regulatory elements in the host response. In particular, factors affecting cell functions, such as macrophage activation and bactericidal capabilities, and effector T cell functions such as cytotoxicity and cytokine production can each be determinative. The model indicates, however, that even if latency is achieved, it may come at the expense of tissue damage if the response is not properly regulated. A balance in Th1 and Th2 immune responses governed by IFN-gamma, IL-10, and IL-4 facilitate this down-regulation. These results are further explored through virtual deletion and depletion experiments.

Model of HIV-1 disease progression based on virus-induced lymph node homing and homing-induced apoptosis of CD4+ lymphocytes

Several proposed theories have described the progression of HIV infection. Even so, no concrete evidence supports any as comprehensive, including, for example, why the CD4+ T-cell counts fall from 1000/mm3 of blood to roughly 100/mm3 over an average 10-year period, whereas concomitant viral loads are relatively constant, increasing by several orders of magnitude in late-stage disease. Here, we develop and validate a theoretical model that altered lymphocyte circulation patterns between the lymph system and blood due to HIV-induced enhanced lymph-node homing and subsequent apoptosis of resting CD4+ T cells can explain many aspects of HIV-1 disease progression. These results lead to a recalculation of the CD4+ lymphocyte dynamics during highly active antiretroviral therapy, and also suggest new targets for therapy.

Primary HIV infection of infants: the effects of somatic growth on lymphocyte and virus dynamics

Acute HIV infection is characterized by the appearance of high concentrations of virus in the peripheral blood. In adults, this high-level viremia spontaneously abates after several weeks. In contrast, after perinatal infection of infants, blood virus levels remain high for many months, during which the concentration of circulating CD4+ lymphocytes remains well above normal values for adults. Here we suggest an explanation for these differences, based on developmental factors including somatic growth and immunological ontogeny. Flow cytometric analysis revealed that at birth the thymus contains elevated levels of mature T lymphocytes, compared to the thymus after 3 months of age. A mathematical model is proposed incorporating immunological and virological data from longitudinally evaluated infants who acquired infection at the time of birth. This model explains the pattern of high-level viremia in infants as resulting from the replication of HIV within the progressively expanding lymphoid cell mass.

Use of coreceptors other than CCR5 by non-syncytium-inducing adult and pediatric isolates of human immunodeficiency virus type 1 is rare in vitro

We have tested a panel of pediatric and adult human immunodeficiency virus type 1 (HIV-1) primary isolates for the ability to employ the following proteins as coreceptors during viral entry: CCR1, CCR2b, CCR3, CCR4, CCR5, CCR8, CXCR4, Bonzo, BOB, GPR1, V28, US28, and APJ. Most non-syncytium-inducing isolates could utilize only CCR5. All syncytium-inducing viruses used CXCR4, some also employed V28, and one (DH123) used CCR8 and APJ as well. A longitudinal series of HIV-1 subtype B isolates from an infected infant and its mother utilized Bonzo efficiently, as well as CCR5. The maternal isolates, which were syncytium inducing, also used CXCR4, CCR8, V28, and APJ.