Decreased levels of recent thymic emigrants in peripheral blood of simian immunodeficiency virus-infected macaques correlate with alterations within the thymus

Primary and secondary defects of the thymus

The thymus is the primary site of T-cell development, enabling generation, and selection of a diverse repertoire of T cells that recognize non-self, whilst remaining tolerant to self- antigens. Severe congenital disorders of thymic development (athymia) can be fatal if left untreated due to infections, and thymic tissue implantation is the only cure. While newborn screening for severe combined immune deficiency has allowed improved detection at birth of congenital athymia, thymic disorders acquired later in life are still underrecognized and assessing the quality of thymic function in such conditions remains a challenge. The thymus is sensitive to injury elicited from a variety of endogenous and exogenous factors, and its self-renewal capacity decreases with age. Secondary and age-related forms of thymic dysfunction may lead to an increased risk of infections, malignancy, and autoimmunity. Promising results have been obtained in preclinical models and clinical trials upon administration of soluble factors promoting thymic regeneration, but to date no therapy is approved for clinical use. In this review we provide a background on thymus development, function, and age-related involution. We discuss disease mechanisms, diagnostic, and therapeutic approaches for primary and secondary thymic defects.

FRugally Optimized DNA Octomer (FRODO) qPCR Measurement of HIV and SIV in Human and Nonhuman Primate Samples

Quantitative polymerase chain reactions (qPCRs) are commonly employed to enumerate genes of interest among particular biological samples. Insertion of PCR amplicons into plasmid DNA is a mainstay for creation of known quantities of target sequences to standardize qPCRs. Typically, one amplicon is inserted into one plasmid construct, and the plasmid is then amplified, purified, serially diluted, and quantified to be used to enumerate target sequences in unknown samples. As qPCR is often used to detect multiple amplicons simultaneously, individual qPCR standards are often desired to normalize one to another. Here we report a single plasmid containing eight amplicons, which can be used to quantify several different strains of simian immunodeficiency virus and human immunodeficiency virus, cell number equivalents for humans and nonhuman primates, T cell receptor excision circles, and bacterial 16S DNA. This FRugally Optimized DNA Octomer (FRODO) plasmid was created and standardized to quantify all eight PCR amplicons. © 2021 US Government. Basic Protocol 1: Total genomic DNA extraction from primary cells Basic Protocol 2: Quantitative PCR for viral, bacterial, and cell number equivalents Support Protocol: Purification, quantification, and storage of FRODO standard plasmid DNA.

Infection-Associated Thymic Atrophy

The thymus is a vital organ of the immune system that plays an essential role in thymocyte development and maturation. Thymic atrophy occurs with age (physiological thymic atrophy) or as a result of viral, bacterial, parasitic or fungal infection (pathological thymic atrophy). Thymic atrophy directly results in loss of thymocytes and/or destruction of the thymic architecture, and indirectly leads to a decrease in naïve T cells and limited T cell receptor diversity. Thus, it is important to recognize the causes and mechanisms that induce thymic atrophy. In this review, we highlight current progress in infection-associated pathogenic thymic atrophy and discuss its possible mechanisms. In addition, we discuss whether extracellular vesicles/exosomes could be potential carriers of pathogenic substances to the thymus, and potential drugs for the treatment of thymic atrophy. Having acknowledged that most current research is limited to serological aspects, we look forward to the possibility of extending future work regarding the impact of neural modulation on thymic atrophy.

Primate lentiviral Nef proteins deregulate T-cell development by multiple mechanisms

Background

A nef gene is present in all primate lentiviral genomes and is important for high viral loads and progression to AIDS in human or experimental macaque hosts of HIV or SIV, respectively. In these hosts, infection of the thymus results in a decreased output of naive T cells that may contribute to the development of immunodeficiency. We have previously shown that HIV-1 subtype B Nef proteins can block human T-cell development. However, the underlying mechanism(s) and the conservation of this Nef function between different groups of HIV and SIV remained to be determined.

Results

We investigated whether reduction of thymic output is a conserved function of highly divergent lentiviral Nef proteins including those from both types of human immunodeficiency viruses (HIV-1 and HIV-2), their direct simian counterparts (SIVcpz, SIVgor and SIVsmm, respectively), and some additional SIV strains. We found that expression of most of these nef alleles in thymocyte progenitors impaired T-cell development and reduced thymic output. For HIV-1 Nef, binding to active p21 protein (Cdc42/Rac)-activated kinase (PAK2) was a major determinant of this function. In contrast, selective disruption of PAK2 binding did not eliminate the effect on T-cell development of SIVmac239 Nef, as was shown by expressing mutants in a newly discovered PAK2 activating structural motif (PASM) constituted by residues I117, H121, T218 and Y221, as well as previously described mutants. Rather, down-modulation of cell surface CD3 was sufficient for reduced thymic output by SIVmac Nef, while other functions of SIV Nefs contributed.

Conclusions

Our results indicate that primate lentiviral Nef proteins impair development of thymocyte precursors into T cells in multiple ways. The interaction of HIV-1 Nef with active PAK2 by HIV-1 seem to be most detrimental, and downregulation of CD3 by HIV-2 and most SIV Nef proteins sufficient for reduced thymic output. Since the reduction of thymic output by Nef is a conserved property of divergent lentiviruses, it is likely to be relevant for peripheral T-cell depletion in poorly adapted primate lentiviral infections.

T-Cell Signaling in HIV-1 Infection

HIV exploits the T-cell signaling network to gain access to downstream cellular components, which serves as effective tools to break the cellular barriers. Multiple host factors and their interaction with viral proteins contribute to the complexity of HIV-1 pathogenesis and disease progression. HIV-1 proteins gp120, Nef, Tat and Vpr alter the T-cell signaling pathways by activating multiple transcription factors including NF-ĸB, Sp1 and AP-1. HIV-1 evades the immune system by developing a multi-pronged strategy. Additionally, HIV-1 encoded proteins influence the apoptosis in the host cell favoring or blocking T-cell apoptosis. Thus, T-cell signaling hijacked by viral proteins accounts for both viral persistence and immune suppression during HIV-1 infection. Here, we summarize past and present studies on HIV-1 T-cell signaling with special focus on the possible role of T cells in facilitating viral infection and pathogenesis

Molecular measurement of T cell receptor excision circles

This chapter provides protocols necessary for quantifying human, mouse, and nonhuman primate signal joint T cell receptor excision circles (sjTRECs) produced during T cell receptor alpha (TCRA) gene rearrangement. These non-replicated episomal circles of DNA are generated by the recombination process used to produce antigen-specific T cell receptors. The number of sjTRECs per mg of thymus tissue or per 100,000 lysed cells has been shown to be a molecular marker of thymopoiesis and naïve T cells. This technology is beneficial to investigators interested in quantitating the level of naïve T cell production occurring under various circumstances in a variety of systems, and complements traditional phenotypic analyses of thymopoiesis. This chapter specifically describes procedures required for rapid detection and quantitation of sjTRECs in thymus tissue or isolated cells using real-time quantitative polymerase chain reaction (PCR). The sjTREC assay system comprises species-specific forward and reverse primers for amplification of a unique site on the T cell receptor δ (TCRD) sjTREC, a fluorescently labeled (FAM/ZEN/IABkFQ) species-specific real-time probe, and a species-specific sjTREC DNA plasmid standard for quantitation.

Thymic function is maintained during Salmonella-induced atrophy and recovery

Thymic atrophy is a frequent consequence of infection with bacteria, viruses, and parasites and is considered a common virulence trait between pathogens. Multiple reasons have been proposed to explain this atrophy, including premature egress of immature thymocytes, increased apoptosis, or thymic shutdown to prevent tolerance to the pathogen from developing. The severe loss in thymic cell number can reflect an equally dramatic reduction in thymic output, potentially reducing peripheral T cell numbers. In this study, we examine the relationship between systemic Salmonella infection and thymic function. During infection, naive T cell numbers in peripheral lymphoid organs increase. Nevertheless, this occurs despite a pronounced thymic atrophy caused by viable bacteria, with a peak 50-fold reduction in thymocyte numbers. Thymic atrophy is not dependent upon homeostatic feedback from peripheral T cells or on regulation of endogenous glucocorticoids, as demonstrated by infection of genetically altered mice. Once bacterial numbers fall, thymocyte numbers recover, and this is associated with increases in the proportion and proliferation of early thymic progenitors. During atrophy, thymic T cell maturation is maintained, and single-joint TCR rearrangement excision circle analysis reveals there is only a modest fall in recent CD4(+) thymic emigrants in secondary lymphoid tissues. Thus, thymic atrophy does not necessarily result in a matching dysfunctional T cell output, and thymic homeostasis can constantly adjust to systemic infection to ensure that naive T cell output is maintained.

Immunology in the clinic review series: focus on type 1 diabetes and viruses: the role of viruses in type 1 diabetes: a difficult dilemma

Convincing evidence now indicates that viruses are associated with type 1 diabetes (T1D) development and progression. Human enteroviruses (HEV) have emerged as prime suspects, based on detection frequencies around clinical onset in patients and their ability to rapidly hyperglycaemia trigger in the non-obese diabetic (NOD) mouse. Whether or not HEV can truly cause islet autoimmunity or, rather, act by accelerating ongoing insulitis remains a matter of debate. In view of the disease's globally rising incidence it is hypothesized that improved hygiene standards may reduce the immune system's ability to appropriately respond to viral infections. Arguments in favour of and against viral infections as major aetiological factors in T1D will be discussed in conjunction with potential pathological scenarios. More profound insights into the intricate relationship between viruses and their autoimmunity-prone host may lead ultimately to opportunities for early intervention through immune modulation or vaccination.

Thymus atrophy and double-positive escape are common features in infectious diseases

The thymus is a primary lymphoid organ in which bone marrow-derived T-cell precursors undergo differentiation, leading to migration of positively selected thymocytes to the T-cell-dependent areas of secondary lymphoid organs. This organ can undergo atrophy, caused by several endogenous and exogenous factors such as ageing, hormone fluctuations, and infectious agents. This paper will focus on emerging data on the thymic atrophy caused by infectious agents. We present data on the dynamics of thymus lymphocytes during acute Trypanosoma cruzi infection, showing that the resulting thymus atrophy comprises the abnormal release of thymic-derived T cells and may have an impact on host immune response.

Signal joint T-cell receptor excision circle assay in miniature swine

OBJECTIVE

Assays for T cell receptor excision circles (TREC) have been utilized in human, primate, and mouse models as a measure of thymic activity, but no comparable assay has been described in artiodactyls. We describe the development of the porcine signal joint (sj) TREC assay, and provide a likely reason for previous difficulties in its identification in artiodactyls.

DESIGN AND METHODS

Utilizing the homology between the known genomic sequences in sjTREC in human and mouse, polymerase chain reaction (PCR) primers were derived for the putative porcine sjTREC. Primers from the ψJα side of the sjTREC were derived from the known porcine sequence.

RESULTS

The sjTREC in two artiodactyls, swine and sheep, was identified using forward primers from the ψJα region, and reverse primers from the putative δ-rec region. Unlike in the detection of primate TRECs, initially the use of similar primers close to the δ-rec failed to yield the sjTREC product. Marching about 800 basepairs into δ-rec, primers derived from a homology region between human and mouse led to the detection of sjTREC. Comparing sjTREC amongst the species revealed highest homology between the two artiodactyls. A quantitative PCR (QPCR) assay of porcine sjTREC was also developed.

CONCLUSION

Identification and analysis of the sjTREC sequences in two artiodactyls suggested why previous attempts at cloning the pig TREC using known sjTREC sequences were unsuccessful. The development of the porcine signal joint TREC assay should enable a more direct quantification of thymic activity in porcine models of transplant biology.

HIV infection of thymocytes inhibits IL-7 activity without altering CD127 expression

BACKGROUND

Thymic function is altered in HIV infection and characterized by dysregulation of the thymic epithelial network, reduced thymic output and ultimately an impaired naïve T-cell pool. The IL-7/IL-7 receptor (IL-7R) signalling pathway is critical for the maturation and differentiation of thymocytes. HIV infection is associated with a decrease in IL-7Rα (CD127) expression and impaired CD127 signalling in circulating CD8+ T-cells; however, little is known about the effect of HIV on CD127 expression and IL-7 activity in the thymus. Therefore, the effect of in vitro HIV infection on CD127 expression and IL-7-mediated function in thymocytes was investigated.

FINDINGS

In vitro HIV infection of thymocytes did not affect CD127 expression on either total thymocytes or on single positive CD4 or single positive CD8 subsets. However, HIV infection resulted in a decrease in the level of IL-7-induced STAT-5 phosphorylation and Bcl-2 expression in unfractionated thymocytes.

CONCLUSION

These findings indicate that HIV infection alters IL-7 responsiveness of thymocytes by a mechanism other than CD127 downregulation and potentially explain the disruption in thymopoiesis observed in HIV infection.

Effects of Plasmodium berghei on thymus: high levels of apoptosis and premature egress of CD4(+)CD8(+) thymocytes in experimentally infected mice

We have previously showed alterations in the thymus during experimental infection with Plasmodium berghei, the causative agent of Malaria. Such alterations comprised histological changes with loss of delimitation between cortical and medullar regions, a profound atrophy with depletion of CD4(+)CD8(+) double-positive (DP) thymocytes, and severe changes in the expression of cell migration-related molecules, belonging to the extracellular matrix and chemokine protein families. Taken together, these considerations prompted us to evaluate if the acute thymic atrophy observed during Plasmodium infection was correlated with increased apoptotic levels of thymocytes or with their premature emigration to the periphery. Our results confirmed that the marked reduction of the thymus weight in infected animals was accompanied by histological alterations, which included a very large number of cells showing nuclear condensation and karyorrhectic changes surrounded by histiocytes suggesting increased levels of apoptosis. This was confirmed by immunohistochemistry and flow cytometry techniques. In order to verify if an accelerated emigration of thymic cells to the peripheral lymphoid organs was also occurring we analyzed the spleen and mesenteric lymph nodes from control and infected mice. No significant differences were found in the spleen, but were seen after 14 days of infection between control and infected mice in the mesenteric lymph nodes. The main alteration was the presence of double negative (CD4(-)CD8(-)) and double positive (CD4(+)CD8(+)) cells. We concluded that both apoptosis of thymocytes and premature egress of immature cells take place during infection. Additional studies will be necessary to verify how such alterations might influence the systemic immune response to the parasite.

Clinical implications of discordant viral and immune outcomes following protease inhibitor containing antiretroviral therapy for HIV-infected children

Many HIV-infected children treated with protease inhibitors (PI) reconstitute immunity despite viral breakthrough predicting disease progression. We studied a unique cohort of PI treated children with advanced disease who demonstrated sustained CD4 T cell counts but median post therapy viral load rebounded to >4.0 log(10) copies/ml. Phylogenetic relationships between pre- and post-therapy viruses reveals significant bottlenecks for quasispecies with natural polymorphisms mapping outside of protease active site providing selective advantage for emergence. Among discordant subjects post-therapy viruses fell into two phenotypes; high viral loads (median >5.0 log(10) copies/ml) and attenuated post-therapy replication (median <4.0 log(10) copies/ml). Both groups showed similar degrees of CD4 T cell immune reconstitution and were similar to children who optimally suppressed virus to <400 copies/ml. Both high fit and low fit discordant response groups showed high reconstitution of naïve CD4 CD45RA T cells (median 388 and 357 cells/microl, respectively). Naïve T cells increases suggest virus replicating under PI selective pressure do not impair thymic output. If therapeutic options are limited, selection of therapy which allows immune reconstitution despite suboptimal viral control may be beneficial. This novel paradigm for virus/host interactions may lead to therapeutic approaches to attenuate viral pathogenesis.

Thymic alterations in Plasmodium berghei-infected mice

The primary function of the thymus is to develop immature T-cells into cells that further in the periphery will be able to carry out immune functions. The Literature has shown that thymus can be a target for many pathogens and severe structural alterations take place in this organ during infectious diseases. Here, we investigated if thymus is also a target organ during experimental malaria infection by analyzing the presence of parasites inside the organ and histological alterations in thymuses from Plasmodium berghei NK65-infected BALB/c. After 14 days of infection, parasites were found inside the thymus that presented a profound atrophy with total loss of its architecture. We propose that the presence of parasites in the thymus induces histological modifications that alter the microenvironment, impairing by consequence the successful T cell development. Additional studies are currently being developed in our laboratory to verify if such thymic alterations can influence the systemic immune response to the parasite.

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.

Fusion-induced apoptosis contributes to thymocyte depletion by a pathogenic human immunodeficiency virus type 1 envelope in the human thymus

The mechanisms of CD4(+) T-cell depletion during human immunodeficiency virus type 1 (HIV-1) infection remain incompletely characterized. Of particular importance is how CD4(+) T cells are depleted within the lymphoid organs, including the lymph nodes and thymus. Herein we characterize the pathogenic mechanisms of an envelope from a rapid progressor (R3A Env) in the NL4-3 backbone (NL4-R3A) which is able to efficiently replicate and deplete CD4(+) thymocytes in the human fetal-thymus organ culture (HF-TOC). We demonstrate that uninterrupted replication is required for continual thymocyte depletion. During depletion, NL4-R3A induces an increase in thymocytes which uptake 7AAD, a marker of cell death, and which express active caspase-3, a marker of apoptosis. While 7AAD uptake is observed predominantly in uninfected thymocytes (p24(-)), active caspase-3 is expressed in both infected (p24(+)) and uninfected thymocytes (p24(-)). When added to HF-TOC with ongoing infection, the protease inhibitor saquinavir efficiently suppresses NL4-R3A replication. In contrast, the fusion inhibitors T20 and C34 allow for sustained HIV-1 production. Interestingly, T20 and C34 effectively prevent thymocyte depletion in spite of this sustained replication. Apoptosis of both p24(-) and p24(+) thymocytes appears to be envelope fusion dependent, as T20, but not saquinavir, is capable of reducing thymocyte apoptosis. Together, our data support a model whereby pathogenic envelope-dependent fusion contributes to thymocyte depletion in HIV-1-infected thymus, correlated with induction of apoptosis in both p24(+) and p24(-) thymocytes.

The thymus is a common target organ in infectious diseases

Infectious disease immunology has largely focused on the effector immune response, changes in the blood and peripheral lymphoid organs of infected individuals, and vaccine development. Studies of the thymus in infected individuals have been neglected, although this is progressively changing. The thymus is a primary lymphoid organ, able to generate mature T cells that eventually colonize secondary lymphoid organs, and is therefore essential for peripheral T cell renewal. Recent data show that normal thymocyte development and export can be altered as a result of an infectious disease. One common feature is the severe atrophy of the infected organ, mainly due to the apoptosis-related depletion of immature CD4⁺CD8⁺ thymocytes. Additionally, thymocyte proliferation is frequently diminished. The microenvironmental compartment of the thymus is also affected, particularly in acute infectious diseases, with a densification of the epithelial network and an increase in the deposition of extracellular matrix. In the murine model of Chagas disease, intrathymic chemokine production is also enhanced, and thymocytes from Trypanosoma cruzi-infected mice exhibit greater numbers of cell migration-related receptors for chemokines and extracellular matrix, as well as increased migratory responses to the corresponding ligands. This profile is correlated with the appearance of potentially autoreactive thymus-derived immature CD4⁺CD8⁺ T cells in peripheral organs of infected animals. A variety of infectious agents—including viruses, protozoa, and fungi—invade the thymus, raising the hypothesis of the generation of central immunological tolerance for at least some of the infectious agent-derived antigens. It seems clear that the thymus is targeted in a variety of infections, and that such targeting may have consequences on the behavior of peripheral T lymphocytes. In this context, thymus-centered immunotherapeutic approaches potentially represent a new tool for the treatment of severe infectious diseases.

Apoptosis in SIV infection

Pathogenic human immunodeficiency virus (HIV)/Simian immunodeficiency virus (SIV) infection is associated with increased T-cell apoptosis. In marked contrast to HIV infection in humans and SIV infection in macaques, the SIV infection of natural host species is typically nonpathogenic despite high levels of viral replication. In these nonpathogenic primate models, no observation of T-cell apoptosis was observed, suggesting that either SIV is less capable of directly inducing apoptosis in natural hosts (likely as a result of coevolution/coadaptation with the host) or, alternatively, that the indirect T-cell apoptosis plays the key role in determining the HIV-associated T-cell depletion and progression to acquired immune deficiency syndrome (AIDS). Understanding the molecular and cellular mechanisms responsible for the disease-free equilibrium in natural hosts for SIV infection, including those determining the absence of high levels of T-cell apoptosis, is likely to provide important clues regarding the mechanisms of AIDS pathogenesis in humans.

Impaired T-cell differentiation in the thymus at the early stages of acute pathogenic chimeric simian-human immunodeficiency virus (SHIV) infection in contrast to less pathogenic SHIV infection

One of the mechanisms by which HIV infection induces the depletion of CD4+ T cells has been suggested to be impairment of T-cell development in the thymus, although there is no direct evidence that this occurs. To examine this possibility, we compared T-cell maturation in the intrathymic progenitors between macaques infected with an acute pathogenic chimeric simian-human immunodeficiency virus (SHIV), which causes profound and irreversible CD4+ T-cell depletion, and macaques infected with a less pathogenic SHIV, which causes only a transient CD4+ T-cell decline. Within 27 days post-inoculation (dpi), the two virus infections caused similar increases in plasma viral loads and similar decreases in CD4+ T-cell counts. However, in the thymus, the acute pathogenic SHIV resulted in increased thymic involution, atrophy and the depletion of immature T cells including CD4(+)CD8(+) double-positive (DP) cells, whereas the less pathogenic SHIV did not have these effects. Ex vivo differentiation of CD3(-)CD4(-)CD8(-) triple-negative (TN) intrathymic progenitors to DP cells was assessed by a monkey-mouse xenogenic fetal thymus organ culture system. Differentiation was impaired in the TN intrathymic progenitors of the acute pathogenic SHIV-infected monkeys, while differentiation was not impaired in the TN intrathymic progenitors of the less pathogenic SHIV-infected monkeys. These differences suggest that dysfunction of thymic maturation makes an important contribution to the irreversible depletion of circulating CD4+ T cells in vivo.

Effacing of the T Cell Compartment by Cardiac Transplantation in Infancy

For cardiac transplantation in infants, T cells are depleted and the thymus is removed. These manipulations should cause profound defects in the T cell compartment. To test this concept, 20 subjects who underwent cardiac transplantation in infancy and healthy age-matched subjects were studied. The number of T cells in the blood was nearly normal in all subjects 1-10 years after surgery. However, newly generated T cells were undetectable in 10 recipients and 10-fold less than controls in 10, suggesting absence of thymic function. TCRbeta chain diversity, measured by a novel technique, was approximately 100-fold lower than controls. T cell function, deduced from levels of human herpesvirus 7 and response to hepatitis B immunization, were notably impaired. Yet cardiac transplant recipients were generally free of opportunistic infections. Our findings demonstrate a novel approach to measuring lymphocyte diversity and suggest that understanding how these subjects resist infection could yield important insights into immune fitness.

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.

The clinical benefits of tenofovir for simian immunodeficiency virus-infected macaques are larger than predicted by its effects on standard viral and immunologic parameters

Previous studies have demonstrated that tenofovir (9-[2-(phosphonomethoxy)propyl]adenine; PMPA) treatment is usually very effective in suppressing viremia in macaques infected with simian immunodeficiency virus (SIV). The present study focuses on a subset of infant macaques that were chronically infected with highly virulent SIVmac251, and for which prolonged tenofovir treatment failed to significantly suppress viral RNA levels in plasma despite the presence of tenofovirsusceptible virus at the onset of therapy. While untreated animals with similarly high viremia developed fatal immunodeficiency within 3-6 months, these tenofovir-treated animals had significantly improved survival (up to 3.5 years). This clinical benefit occurred even in animals for which tenofovir had little or no effect on CD4 and CD8 lymphocyte counts and antibody responses to SIV and test antigens. Thus, the clinical benefits of tenofovir were larger than predicted by plasma viral RNA levels and other routine laboratory parameters.

Timely triggering of homeostatic mechanisms involved in the regulation of T-cell levels in SIVsm-infected sooty mangabeys

Sooty mangabeys, the natural host of simian immunodeficiency virus (SIVsm), generally avoid progressive depletion of CD4+ T cells and opportunistic infections associated with infection of humans (HIV) and macaques (SIVmac). The means by which the SIVsm-infected mangabeys maintain CD4+ T-cell levels despite high rates of viral replication is unknown. One cytokine that has a key role in the regulation of T-cell levels is interleukin-7 (IL-7). Here, the longitudinal assessment of 6 SIVsm-infected mangabeys identified an early increase in plasma IL-7 levels at weeks 1 to 5 after infection. This IL-7 increase correlated with an early decline in CD4+ T-cell levels (decline of 492-1171 cells/microL) accompanying acute viremia. Elevated IL-7 levels were followed by increased T-cell proliferation (Ki67) and maintenance of lower but stable (more than 500 cells/microL) CD4+ T-cell levels in each mangabey through 37 weeks of infection. These data contrast with our earlier studies in SIVmac-infected macaques, in which the IL-7 increase was delayed until 20 to 40 weeks after infection, just before the onset of simian AIDS. Taken together, these data suggest that timely triggering of IL-7 is important for stabilizing healthy T-cell levels in mangabeys and that timely administration of exogenous IL-7 may show benefit during pathogenic SIVmac and HIV infection.

Disease progression in macaques with low SIV replication levels: on the relevance of TREC counts

BACKGROUND

An attenuated immunodeficiency virus has been long considered innocuous. Nevertheless, converging data suggest that low levels of viral replication can still provoke AIDS. Pathogenesis of these attenuated infections is not understood.

OBJECTIVES

To determine the pathogenicity of a long-term attenuated infection and to delineate T-cell dynamics during such an infection.

METHODS

This is a cross-sectional study of 12 rhesus macaques infected with SIV Delta nef for 8 years. We evaluated apoptosis (annexin V), activation (HLA-DR, Ki67), and newly generated T cells (TCR excision circle: TREC).

RESULTS

Infection with SIV Delta nef induced pathological CD4 T-cell depletion after 8 years of infection. Virus replication and CD8 T-cell activation positively correlated with the rate of disease progression. The frequency of TREC within CD8+CD45RA+ cells increased in SIV Delta nef-infected animals compared to age-matched non-infected controls. Moreover, in the cohort of infected animals, TREC+CD45RA+CD4+ T-cell counts correlated strongly with non-progression to AIDS. The animal with the lowest rate of disease progression exhibited a 115-fold increase in TREC+CD45RA+CD4+ T-cell counts compared to age-matched non-infected controls. In contrast, the animal showing the fastest rate of progression to AIDS displayed 600-fold lower TREC+CD45RA+CD4+ T-cell counts compared to age-matched non-infected controls.

CONCLUSIONS

Our results suggest that the thymus plays a major role in the pathogenesis of an attenuated SIV infection and that a sustained thymic output could maintain CD4 T-cell homeostasis in the context of low viral loads.

T-cell receptor excision circles (TREC) in SHIV 89.6p and SIVmac251 models of HIV-1 infection

T-cell receptor excision circles (TREC) may be a useful surrogate marker in HIV-1 infection for evaluating the likelihood of continued clinical stability and/or the response to therapeutics, including vaccines. Analysis of TREC in SHIV and SIV models of HIV-1 infection may provide additional information concerning the utility of TREC as a marker. We measured TREC in peripheral blood mononuclear cells (PBMC) from rhesus macaques in SHIV89.6p (n = 20) and SIVmac251 (n = 11) models of HIV-1 infection. TREC were also evaluated in tissues in the SIVmac251 model at end-point. In the SHIV89.6p model, TREC in PBMC were significantly lower at 12 weeks postinfection compared to preinfection levels. The decrease in TREC correlated with the decline in CD4+ T cells (r(s) = 0.496; P = 0.026), which in turn correlated inversely with serum viral loads at end-point (r(s) = -0.517; P = 0.019). Macaques that controlled SHIV89.6p infection to some degree (n = 6) had higher TREC at study end-point (P = 0.017). In the SIVmac251 model, TREC in PBMC were significantly reduced after 17 months of infection (P = 0.012) despite receiving highly active antiretroviral therapy (HAART) consisting of didanosine (ddI) and (R)-9-(2-phosphonylmethoxypropyl)-adenine (PMPA) when not cycling off therapy during scheduled treatment interruptions (STI). However, macaques that received continuous hydroxyurea (HU) in addition to the HAART regimen had higher end-point TREC compared to the non-HU group (P = 0.041), and the reduction in TREC observed at end-point within the HU group was not significant. In the SIVmac251 model, TREC correlated with the percentage of CD4+ T cells (r(s) = 0.426; P = 0.048) and CD4+CD28+ T cells (r(s) = 0.624; P = 0.002), and inversely with CD8+ T cells (r(s) = -0.622; P = 0.002), CD8+CD28- T cells (r(s) = -0.516; P = 0.014), and serum viral loads (r(s) = -0.627; P = 0.039). High levels of TREC were observed in the thymus, levels comparable to PBMC were seen in the lymph node, and low but detectable levels of TREC were present in bone marrow. The use of correlates of TREC as covariates in ANCOVA revealed that the decline in TREC in the SHIV 89.6p model reflected the decline in the percentage of CD4+ T-cells due to viral cytopathogenicity. In the SIVmac251 model, the decline in TREC was related to increased immune activation and proliferation due to viral replication, as reflected by decreases in percentages of CD4+CD28+ T cells and increases in CD8+ and CD8+CD28- T cells.

Elevated interleukin-7 levels not sufficient to maintain T-cell homeostasis during simian immunodeficiency virus-induced disease progression

Elevated levels of interleukin 7 (IL-7) have been correlated with various T-cell depletion conditions, including HIV infection, and suggested as an indicator of HIV disease progression (AIDS and death). Here, the assessment of pathogenic simian immunodeficiency virus (SIVmac239) infection in rhesus macaques demonstrated a clear association between a significant elevation in IL-7 levels and disease progression. In 5 macaques that progressed to simian AIDS and death, elevated IL-7 levels were unable to restore T-cell homeostasis. In contrast, increased IL-7 levels were followed by relatively high and stable T-cell numbers in the SIV-infected macaques with a slow-progressing phenotype. Further, studies in sooty mangabeys that do not progress to simian AIDS and that maintain stable T-cell numbers despite high levels of viral replication support the importance of IL-7 and T-cell homeostasis in disease progression. These data suggest that during pathogenic SIV infection with high viral replication, elevated IL-7 levels are unable to recover T-cell homeostasis, thereby leading to disease progression. The utility of IL-7 as a potential immunotherapeutic agent to improve HIV/SIV-related T-cell depletion may therefore depend on controlling the pathogenic effects of viral replication prior to the administration of IL-7.

The impact of highly active antiretroviral therapy by the oral route on the CD8 subset in monkeys infected chronically with SHIV 89.6P

The objective of this study was to assess the impact of highly active antiretroviral therapy (HAART) by an oral route on the peripheral blood CD8 subset in the monkeys infected persistently with a pathogenic strain, SHIV(89.6P). Two rhesus macaques were inoculated intravenously with SHIV(89.6P), then treated with the combination of AZT, 3TC and Lopinavir/Ritonavir (LPV/RTV) as recommended in humans by the oral route with confectionery continued for 28 days. In one of two chronically infected macaques, MM260, the viral load was maintained in the range of 10(4)-10(5) copies/ml before HAART. The plasma viral load and proviral DNA decreased dramatically during the treatment, and cessation of this therapy the viral load rebounded to the pre-treatment level but the proviral DNA rebound was delayed. The other monkey, MM242, had low viral loads (1.2x10(3)-<5x10(2) copies/ml) both before and after HAART. CD4(+) and CD8(+) T cell counts and proviral DNA level were not significantly changed after the treatment. The percentages of CD8(+)CD45RA(-)Ki67(+)cells increased during (MM260) or after (MM242) HAART and the subset was maintained at a high percentage until 18 weeks post HAART in MM242. These findings suggest that this primate model might serve an important role in testing the virological and immunological efficacy of novel therapeutic strategies combined with HAART.

Intrinsic and extrinsic pathways signaling during HIV-1 mediated cell death

Infection with human immunodeficiency virus (HIV) is characterized by the gradual depletion of CD4+ T lymphocytes. The incorporation of the concept of apoptosis as a rationale to explain progressive T cell depletion has led to growing research in this field during the last 10 years. In parallel, the biochemical pathways implicated in programmed cell death have been extensively studied. Thus, the influence of mitochondrial control in the two major apoptotic pathways-the extrinsic and intrinsic pathways-is now well admitted. In this review, we summarized our current knowledge of the different pathways involved in the death of T cells in the course of HIV infection.

A study of the immunology, virology, and safety of prednisone in HIV-1-infected subjects with CD4 cell counts of 200 to 700 mm(-3)

Adult Clinical Trials Group Study 349 examined the immunology, virology, and safety of 40 mg/d prednisone as an adjunct to antiretroviral therapy in 24 HIV-infected subjects with >200 CD4+ T cells/mm in a randomized placebo-controlled trial. After 8 weeks, median lymphocyte and CD4+ cell numbers increased >40% above baseline values (p =.08). No effect was observed on markers of cell activation or apoptosis, although the proportion of CD28+ CD8+ T cells increased (p =.006). Prednisone inhibited monocyte TNFalpha production without affecting T-cell responses to antigens or mitogens. Two subjects assigned to prednisone were subsequently found to have asymptomatic osteonecrosis of the hip. Many questions remain regarding the role of activation-induced sequestration and apoptosis as causes of progressive CD4+ T-cell loss in AIDS. The potential role of corticosteroids as tools to examine this question will be limited by concerns regarding their toxicity; however, further studies of other agents to limit cellular activation in AIDS are warranted.

Nonpathogenic SIV infection of sooty mangabeys is characterized by limited bystander immunopathology despite chronic high-level viremia

HIV-infected humans and SIV-infected rhesus macaques who remain healthy despite long-term infection exhibit exceptionally low levels of virus replication and active antiviral cellular immune responses. In contrast, sooty mangabey monkeys that represent natural hosts for SIV infection do not develop AIDS despite high levels of virus replication and limited antiviral CD8(+) T cell responses. We report here that SIV-infected mangabeys maintain preserved T lymphocyte populations and regenerative capacity and manifest far lower levels of aberrant immune activation and apoptosis than are seen in pathogenic SIV and HIV infections. These data suggest that direct consequences of virus replication alone cannot account for progressive CD4(+) T cell depletion leading to AIDS. Rather, attenuated immune activation enables SIV-infected mangabeys to avoid the bystander damage seen in pathogenic infections and protects them from developing AIDS.