Mechanisms associated with thymocyte apoptosis induced by simian immunodeficiency virus

Acute Thymic Involution and Mechanisms for Recovery

Acute thymic involution (ATI) is usually regarded as a virulence trait. It is caused by several infectious agents (bacteria, viruses, parasites, fungi) and other factors, including stress, pregnancy, malnutrition and chemotherapy. However, the complex mechanisms that operate during ATI differ substantially from each other depending on the causative agent. For instance, a transient reduction in the size and weight of the thymus and depletion of populations of T cell subsets are hallmarks of ATI in many cases, whereas severe disruption of the anatomical structure of the organ is also associated with some factors, including fungal, parasitic and viral infections. However, growing evidence shows that ATI may be therapeutically halted or reversed. In this review, we highlight the current progress in this field with respect to numerous pathological factors and discuss the possible mechanisms. Moreover, these new observations also show that ATI can be mechanistically reversed.

Bacterial clearance reverses a skewed T-cell repertoire induced by Salmonella infection

Salmonella typhimurium invades the spleen, liver, and peripheral lymph nodes and has recently been detected in the bone marrow and thymus, resulting in a reduced thymic size and a decline in the total number of thymic cells. A specific deletion of the double-positive cell subset has been characterized, yet the export of mature T cells to the periphery remains normal. We analyzed Salmonella pathogenesis regarding thymic structure and the T-cell maturation process. We demonstrate that, despite alterations in the thymic structure, T-cell development is maintained during Salmonella infection, allowing the selection of single-positive T-cell clones expressing particular T-cell receptor beta chains (TCR-Vβ). Moreover, the treatment of infected mice with an antibiotic restored the normal thymic architecture and thymocyte subset distribution. Additionally, the frequency of TCR-Vβ usage after treatment was comparable to that in non-infected mice. However, bacteria were still recovered from the thymus after 1 month of treatment. Our data reveal that a skewed T-cell developmental process is present in the Salmonella-infected thymus that alters the TCR-Vβ usage frequency. Likewise, the post-treatment persistence of Salmonella reveals a novel function of the thymus as a potential reservoir for this infectious agent.

Modified interferon-α subtypes production and chemokine networks in the thymus during acute simian immunodeficiency virus infection, impact on thymopoiesis

OBJECTIVES

Thymus dysfunction characterizes human/simian immunodeficiency virus (SIV) infections and contributes to physiopathology. However, both the mechanisms involved in thymic dysfunction and its precise timing remain unknown. We here analyzed thymic function during acute SIV infection in rhesus macaques.

DESIGN AND METHODS

Rhesus macaques were intravenously infected with SIVmac251 and bled every 2/3 days or necropsied at different early time points postinfection. Naive T-cell counts were followed by flow cytometry and their T-cell receptor excision circle content evaluated by qPCR. Thymic chemokines were quantified by reverse transcription-qPCR and localized by in-situ hybridization in thymuses collected at necropsy. Thymic interferon alpha (IFN-α) subtype production was quantified by reverse transcription-qPCR combined to heteroduplex tracking assay. The effect of thymic IFN-α subtypes was tested on sorted triple negative thymocytes cultured on OP9-hDL1 cells.

RESULTS

A reduced intrathymic proliferation history characterizes T cells produced during the first weeks of infection. Moreover, we evidenced a profound alteration of both chemokines and IFN-α subtypes transcriptional patterns in SIV-infected thymuses. Finally, we showed that IFN-α subtypes produced in the infected thymuses inhibit thymocyte proliferation, still preserving their differentiation capacity.

CONCLUSION

Thymopoiesis is deeply impacted from the first days of SIV infection. Reduced thymocyte proliferation - a time-consuming process - together with modified chemokine networks is consistent with thymocyte differentiation speed-up. This may transiently enhance thymic output, thus increasing naive T-cell counts and diversity and the immune competence of the host. Nonetheless, long-lasting modification of thymic physiology may lead to thymic exhaustion, as observed in late primary HIV infection.

c-Jun NH2 -terminal kinase is a critical node in the death of CD4+ CD8+ thymocytes during Salmonella enterica serovar Typhimurium infection

Thymic atrophy, due to the depletion of CD4(+) CD8(+) thymocytes, is observed during infections with numerous pathogens. Several mechanisms, such as glucocorticoids and inflammatory cytokines, are known to be involved in this process; however, the roles of intracellular signaling molecules have not been investigated. In this study, the functional role of c-Jun NH2 -terminal kinase (JNK) during infection-induced thymic atrophy was addressed. The levels of phosphorylated JNK in immature CD4(+) CD8(+) thymocytes from C57BL/6 (Nramp-deficient) and 129/SvJ (Nramp-sufficient) mice were increased upon oral infection of mice with Salmonella enterica serovar Typhimurium (S. typhimurium). Furthermore, inhibition of JNK signaling, but not ERK or p38 MAPK, prevented the in vitro death of infected thymocytes. Importantly, the in vivo inhibition of JNK signaling with SP600125 protected C57BL/6 CD4(+) CD8(+) thymocytes from depletion via multiple mechanisms as follows: lower intracellular ROS, inflammatory cytokines, Bax and caspase 3 activity, increase in Bcl-xL amounts, and prevention of the loss in mitochondrial membrane potential. Notably, thymic architecture was preserved in infected mice treated with SP600125. Overall, this study identifies a novel role for JNK as a crucial regulator of the death of CD4(+) CD8(+) thymocytes during S. typhimurium infection.

Interferon-γ- and glucocorticoid-mediated pathways synergize to enhance death of CD4(+) CD8(+) thymocytes during Salmonella enterica serovar Typhimurium infection

Thymic atrophy is known to occur during infections; however, there is limited understanding of its causes and of the cross-talk between different pathways. This study investigates mechanisms involved in thymic atrophy during a model of oral infection by Salmonella enterica serovar Typhimurium (S. typhimurium). Significant death of CD4(+) CD8(+) thymocytes, but not of single-positive thymocytes or peripheral lymphocytes, is observed at later stages during infection with live, but not heat-killed, bacteria. The death of CD4(+) CD8(+) thymocytes is Fas-independent as shown by infection studies with lpr mice. However, apoptosis occurs with lowering of mitochondrial potential and higher caspase-3 activity. The amounts of cortisol, a glucocorticoid, and interferon-γ (IFN-γ), an inflammatory cytokine, increase upon infection. To investigate the functional roles of these molecules, studies were performed using Ifnγ(-/-) mice together with RU486, a glucocorticoid receptor antagonist. Treatment of C57BL/6 mice with RU486 does not affect colony-forming units (CFU), amounts of IFN-γ and mouse survival; however, there is partial rescue in thymocyte death. Upon infection, Ifnγ(-/-) mice display higher CFU and lower survival but more surviving thymocytes are recovered. However, there is no difference in cortisol amounts in C57BL/6 and Ifnγ(-/-) mice. Importantly, the number of CD4(+) CD8(+) thymocytes is significantly higher in Ifnγ(-/-) mice treated with RU486 along with lower caspase-3 activity and mitochondrial damage. Hence, endogenous glucocorticoid and IFN-γ-mediated pathways are parallel but synergize in an additive manner to induce death of CD4(+) CD8(+) thymocytes during S. typhimurium infection. The implications of this study for host responses during infection are discussed.

Relationships between IL-17(+) subsets, Tregs and pDCs that distinguish among SIV infected elite controllers, low, medium and high viral load rhesus macaques

Comprehensive studies of the frequencies and absolute numbers of the various cell lineages that synthesize IL-17 in the blood and corresponding gastrointestinal (GI) tissues, their correlation with CD4(+) Tregs, CD8(+) Tregs, total and IFN-α synthesizing plasmacytoid dendritic cells (pDC) relative to plasma viral load in SIV infection has been lacking. The unique availability of SIV infected rhesus macaques (RM) classified as Elite Controllers (EC), and those with Low, Intermediate and High Viral Loads (HVL) provided a unique opportunity to address this issue. Results of these studies showed that EC demonstrated a remarkable ability to reverse changes that are induced acutely by SIV in the various cell lineages. Highlights of the differences between EC and HVL RM within Gastro-intestinal tissues (GIT) was the maintenance and/or increases in the levels of IL-17 synthesizing CD4, CD8, and NK cells and pDCs associated with slight decreases in the levels of CD4(+) Tregs and IFN-α synthesizing pDCs in EC as compared with decreases in the levels of IL-17 synthesizing CD4, CD8 and NK cells associated with increases in pDCs and IFN-α synthesizing pDCs in HVL monkeys. A previously underappreciated role for CD8(+) Tregs was also noted with a moderate increase in ECs but further increases of CD8(+) Tregs with increasing VL in viremic monkeys. Positive correlations between plasma VL and decreases in the levels of Th17, Tc17, NK-17, CD4(+) Tregs and increases in the levels of CD8(+) Tregs, total and IFN-α synthesizing pDCs were also noted. This study also identified 2 additional IL-17(+) subsets in GIT as CD3(-/)CD8(+)/NKG2a(-) and CD3(+)/CD8(+)/NKG2a(+) subsets. Studies also suggest a limited role for IFN-α synthesizing pDCs in chronic immune activation despite persistent up-regulation of ISGs. Finally, elevated persistent innate immune responses appear associated with poor prognosis. These findings provide an initial foundation for markers important to follow for vaccine design.

Treatment with anti-FasL antibody preserves memory lymphocytes and virus-specific cellular immunity in macaques challenged with simian immunodeficiency virus

Immune deficiency viruses such as SIV in macaques or HIV-1 in human beings have evolved mechanisms to defeat host immunity that also impact the efficacy of vaccines. A key factor for vaccine protection is whether immune responses elicited by prior immunization remain at levels sufficient to limit disease progression once a host is exposed to the pathogen. One potential mechanism for escaping pre-existing immunity is to trigger death among antigen-activated cells. We tested whether FasL/CD178 is involved in destroying preexisting immunity. Rhesus macaques were immunized with recombinant vesicular stomatitis virus vaccine expressing SIV Gag to elicit cellular immune responses, then treated with antibody that neutralizes FasL and challenged with intravenous SIVmac251. Compared with animals injected with control antibody, anti-FasL-treated macaques had superior preservation of central memory CD4(+) and CD8(+) cells and decreased regulatory T cells in the blood. The CD4(+) and CD8(+) lymphocytes from treated animals responded better to SIV Gag compared with controls, evidenced by higher cell-mediated immune responses to viral antigens for at least 17 weeks after SIV challenge. Anti-FasL treatment during the initial stages of acute SIV infection preserved the T-cell compartment and sustained cell-mediated immunity to SIV.

Mechanism and history of evolution of symbiotic HIV strains into lethal pandemic strains: the key event may have been a 1927 trial of pamaquine in Leopoldville (Kinshasa), Congo

In previous papers, I have rejected both the zoonosis and the serial transfer hypotheses of the origin and evolution of the current lethal pandemic strains of HIV. The hypothesis that fits the critical observations is that all the human and nonhuman primate species in central Africa (an area of hyper-endemic malaria) have shared (through inter-species transfers) a "primate T-cell retrovirus" (PTRV), which has adapted to each host species. This retrovirus is believed to assist primate T-cells attack the liver stage of the malaria infection. Each geographic region has a dominant primate host and a characteristic virus. Starting in 1955 and continuing into the late 1970s, chloroquine was provided by the WHO and used for prophylaxis against malaria. Chloroquine has a number of biochemical activities but two of the most important are blocking transcription of cellular genes and proviruses activated by NF-kappaB and blocking the glycosylation of surface proteins on viruses and cells. Concurrent with the development of resistance of the malaria parasite to chloroquine, HIV strains were quickly selected, which have enhanced transcription rates (by inclusion of multiple kappaB binding sites in their long terminal repeats by recombination) and enhanced infectivity (fusogenicity) (most likely by mutations in multiple viral genes that regulate glycosylation of Env). There also may have been mutations that enhanced activation of NF-kappaB in the host cell. These changes in the retrovirus genome were not manifest in effects of the HIV strains as long as the hosts were under the influence of chloroquine. But, when the virus infects people who are not protected by chloroquine, the virus multiplies more rapidly and is more communicable. Fortunately, most of these strains (i.e., HIV-2 groups, and HIV-1 O and HIV-1 N) self-regulate (i.e., infected cells kill infected cells) well enough that viral loads remain subdued and bystander cells of the immune system are not excessively attrited. In the case of HIV-1 group M, however, there is more going on. Following the work of Korber et al. on the phylogenetics of HIV-1 groups M, I reach the conclusion that the major subgroups giving rise to the worldwide pandemic, were founded in a 1927 clinical trial of pamaquine (plasmoquine) in Leopoldville (Kinshasa). This drug is much more toxic that chloroquine and appears to have strongly selected for resistance to apoptosis in infected cells, which allows these subgroups to attrite bystander cells leading to AIDS.

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.

Human immunodeficiency virus persistence and production in T-cell development

Human immunodeficiency virus type 1 (HIV-1) replication depends on CD4 and coreceptor expression as well as host factors associated with the activation state of the cell. To determine the impact of the activation stage of thymocytes on the HIV-1 life cycle, we investigated R5 and X4 HIV-1 entry, reverse transcription, and expression in discrete thymocyte subsets at different stages of T-cell development. Early after infection, preferential entry and replication of R5 HIV-1 were predominantly detected in mature CD3(+/hi) CD27(+) thymocytes. Thus, R5 HIV-1 targets the stage of development where thymocytes acquire functional responsiveness, which has important implications for HIV pathogenesis. In contrast, X4 HIV-1 expression and replication were primarily found in immature CD3(-/+/low) CD27(-) CD69(-) thymocytes. HIV-1 proviral burden and virus expression in thymocyte subsets correlated with the expression of the highest levels of the respective coreceptor. R5 and X4 HIV-1 entered and completed reverse transcription in all subsets tested, indicating that the activation state of thymocytes and coreceptor expression are sufficient to support full reverse transcription throughout development. Although R5 HIV-1 is expressed mainly in mature CD3(+/hi) CD27(+) thymocytes, 5.3% of HIV-1-infected immature thymocytes express R5 HIV-1, indicating that potentially latent viral DNA can be established early in T-cell development.

Rapid dissemination of a pathogenic simian/human immunodeficiency virus to systemic organs and active replication in lymphoid tissues following intrarectal infection

A better understanding of virological events during the early phase of human immunodeficiency virus 1 (HIV-1) infection is important for development of effective antiviral vaccines. In this study, by using quantitative PCR and an infectious plaque assay, virus distribution and replication were examined in various internal organs of rhesus macaques for almost 1 month after intrarectal inoculation of a pathogenic simian immunodeficiency virus/HIV chimeric virus (SHIV-C2/1-KS661c). At 3 days post-inoculation (p.i.), proviral DNA was detected in the rectum, thymus and axillary lymph node. In lymphoid tissues, infectious virus was first detected at 6 days p.i. and a high level of proviral DNA and infectious virus were both detected at 13 days p.i. By 27 days p.i., levels of infectious virus decreased dramatically, although proviral DNA load remained unaltered. In the intestinal tract, levels of infectious virus detected were much lower than in lymphoid tissues, whereas proviral DNA was detected at the same level as in lymphoid tissues throughout the infection. In the thymus and jejunum, CD4CD8 double-positive T cells were depleted earlier than CD4 single-positive cells. These results show that the virus spread quickly to systemic tissues after mucosal transmission. Thereafter, infectious virus was actively produced in the lymphoid tissues, but levels decreased significantly after the peak of viraemia. In contrast, in the intestinal tract, infectious virus was produced at low levels from the beginning of infection. Moreover, virus pathogenesis differed in CD4 single-positive and CD4CD8 double-positive T cells.

Alterations in mesenteric lymph node T cell phenotype and cytokine secretion are associated with changes in thymocyte phenotype after LP-BM5 retrovirus infection

In this study, mouse MLN cells and thymocytes from advanced stages of LP-BM5 retrovirus infection were studied. A decrease in the percentage of IL-7(+) cells and an increase in the percentage of IL-16(+) cells in the MLN indicated that secretion of these cytokines was also altered after LP-BM5 infection. The percentage of MLN T cells expressing IL-7 receptors was significantly reduced, while the percentage of MLN T cells expressing TNFR-p75 and of B cells expressing TNFR-p55 increased. Simultaneous analysis of surface markers and cytokine secretion was done in an attempt to understand whether the deregulation of IFN-gamma secretion could be ascribed to a defined cell phenotype, concluding that all T cell subsets studied increased IFN-gamma secretion after retrovirus infection. Finally, thymocyte phenotype was further analyzed trying to correlate changes in thymocyte phenotype with MLN cell phenotype. The results indicated that the increase in single positive either CD4(+)CD8(-) or CD4(-)CD8(+) cells was due to accumulation of both immature (CD3(-)) and mature (CD3(+)) single positive thymocytes. Moreover, single positive mature thymocytes presented a phenotype similar to the phenotype previously seen on MLN T cells. In summary, we can conclude that LP-BM5 uses the immune system to reach the thymus where it interferes with the generation of functionally mature T cells, favoring the development of T cells with an abnormal phenotype. These new T cells are activated to secrete several cytokines that in turn will favor retrovirus replication and inhibit any attempt of the immune system to control infection.

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.

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.

The Nef-mediated AIDS-like disease of CD4C/human immunodeficiency virus transgenic mice is associated with increased Fas/FasL expression on T cells and T-cell death but is not prevented in Fas-, FasL-, tumor necrosis factor receptor 1-, or interleukin-1beta-converting enzyme-deficient or Bcl2-expressing transgenic mice

CD4(+)- and CD8(+)-T-cell death is a frequent immunological dysfunction associated with the development of human AIDS. We studied a murine model of AIDS, the CD4C/HIV transgenic (Tg) mouse model, to assess the importance of the apoptotic pathway in human immunodeficiency virus type 1 (HIV-1) pathogenesis. In these Tg mice, Nef is the major determinant of the disease and is expressed in immature and mature CD4(+) T cells and in cells of the macrophage/myeloid lineage. We report here a novel AIDS-like phenotype: enhanced death, most likely by apoptosis (as assessed by 7-aminoactinomycin D and annexin V/propidium iodide staining), of Tg thymic and peripheral CD4(+) and CD8(+) T cells. The Tg CD4(+) and CD8(+) T cells were also more susceptible to cell death after activation in vitro in mixed lymph node (LN) cultures. However, activation-induced cell death was not higher in Tg than in non-Tg-purified CD4(+) T cells. In addition, expression of Fas and FasL, assessed by flow cytometry, was increased in CD4(+) and CD8(+) T cells from Tg mice compared to that of non-Tg littermates. Despite the enhanced expression of Fas and FasL on Tg CD4(+) and CD8(+) T cells, Fas (lpr/lpr) and FasL (gld/gld) mutant CD4C/HIV Tg mice developed an AIDS-like disease indistinguishable from lpr/+ and gld/+ CD4C/HIV Tg mice, including loss of CD4(+) T cells. Similarly, CD4C/HIV Tg mice homozygous for mutations of two other genes implicated in cell death (interleukin-1beta-converting enzyme [ICE], tumor necrosis factor receptor 1 [TNFR-1]) developed similar AIDS-like disease as their respective heterozygous controls. Moreover, the double-Tg mice from a cross between the Bcl2/Wehi25 and CD4C/HIV Tg mice showed no major protection against disease. These results represent genetic evidence for the dispensable role of Fas, FasL, ICE, and TNFR-1 on the development of both T-cell loss and organ disease of these Tg mice. They also provide compelling evidence on the lack of protection by Bcl2 against Tg CD4(+)-T-cell death. In view of the high resemblance between numerous phenotypes observed in the CD4C/HIV Tg mice and in human AIDS, our findings are likely to be relevant for the human disease.

AIP1, a Water-Soluble Fraction from Artemisia iwayomogi, Suppresses Thymocyte Apoptosis in Vitro and Down-Regulates the Expression of Fas Gene

The AIP1 fraction, a small water-soluble fraction purified from Artemisia iwayomogi, was shown to increase antibody production and suppress transplanted tumor cell growth in mice. In order to understand its immuno-modulating activity, we have examined the effect of the AIP1 on mouse thymocytes in vitro. Treatment of mouse thymocytes in culture with the fraction resulted in the suppression of the cell death and the extension of the cell survival. A mouse gene array provided a profile of gene expression change showing the pattern of up- and down-regulated genes by the AIP1 treatment, suggesting that the Fas/FasL-dependent apoptosis pathway might be modulated by the fraction.

Induction of simian AIDS in infant rhesus macaques infected with CCR5- or CXCR4-utilizing simian-human immunodeficiency viruses is associated with distinct lesions of the thymus

Newborn rhesus macaques were infected with two chimeric simian-human immunodeficiency virus (SHIV) strains which contain unique human immunodeficiency virus type 1 (HIV-1) env genes and exhibit distinct phenotypes. Infection with either the CCR5-specific SHIV(SF162P3) or the CXCR4-utilizing SHIV(SF33A) resulted in clinical manifestations consistent with simian AIDS. Most prominent in this study was the detection of severe thymic involution in all SHIV(SF33A)-infected infants, which is very similar to HIV-1-induced thymic dysfunction in children who exhibit a rapid pattern of disease progression. In contrast, SHIV(SF162P3) induced only a minor disruption in thymic morphology. Consistent with the distribution of the coreceptors CXCR4 and CCR5 within the thymus, the expression of SHIV(SF162P3) was restricted to the thymic medulla, whereas SHIV(SF33A) was preferentially detected in the cortex. This dichotomy of tissue tropism is similar to the differential tropism of HIV-1 isolates observed in the reconstituted human thymus in SCID-hu mice. Accordingly, our results show that the SHIV-monkey model can be used for the molecular dissection of cell and tissue tropisms controlled by the HIV-1 env gene and for the analysis of mechanisms of viral immunopathogenesis in AIDS. Furthermore, these findings could help explain the rapid progression of disease observed in some HIV-1-infected children.

Caspase-dependent and -independent T-cell death pathways in pathogenic simian immunodeficiency virus infection: relationship to disease progression

Studies of human immunodeficiency virus (HIV) and nonhuman primate models of pathogenic and nonpathogenic simian immunodeficiency virus (SIV) infections have suggested that enhanced ex vivo CD4 T-cell death is a feature of pathogenic infection in vivo. However, the relative contributions of the extrinsic and intrinsic pathways to programmed T-cell death in SIV infection have not been studied. We report here that the spontaneous death rate of CD4+ T cells from pathogenic SIVmac251-infected rhesus macaques ex vivo is correlated with CD4 T-cell depletion and plasma viral load in vivo. CD4+ T cells from SIVmac251-infected macaques showed upregulation of the death ligand (CD95L) and of the proapoptotic proteins Bim and Bak, but not of Bax. Both CD4+ and CD8+ T cells from SIVmac251-infected macaques underwent caspase-dependent death following CD95 ligation. The spontaneous death of CD4+ and CD8+ T cells was not prevented by a decoy CD95 receptor or by a broad-spectrum caspase inhibitor (zVAD-fmk), suggesting that this form of cell death is independent of CD95/CD95L interaction and caspase activation. IL-2 and IL-15 prevented the spontaneous death of CD4+ and CD8+ T cells, whereas IL-10 prevented only CD8 T-cell death and IL-7 had no effect on T-cell death. Our results indicate that caspase-dependent and caspase-independent pathways are involved in the death of T cells in pathogenic SIVmac251-infected primates.

IL-7 Stimulates T Cell Renewal Without Increasing Viral Replication in Simian Immunodeficiency Virus-Infected Macaques

The main failure of antiretroviral therapy is the lack of restoration of HIV-specific CD4(+) T cells. IL-7, which has been shown to be a crucial cytokine for thymopoiesis, has been envisaged as an additive therapeutic strategy. However, in vitro studies suggest that IL-7 might sustain HIV replication in thymocytes and T lymphocytes. Therefore, in the present study, we evaluated the effect of IL-7 on both T cell renewal and viral load in SIVmac-infected young macaques in the absence of antiretroviral therapy. This evaluation was conducted during the asymptomatic phase in view of a potential treatment of HIV patients. We show that IL-7 induces both a central renewal and a peripheral expansion of T lymphocytes associated with cell activation. No alarming modulation of the other hemopoietic cells was observed. No increase in the viral load was shown in blood or lymph nodes. These data strengthen the rationale for the use of IL-7 as an efficient immunotherapy in AIDS.

Morphine suppresses lymphocyte apoptosis by blocking p53-mediated death signaling

Opiates such as morphine or heroin may promote cell apoptosis and cause dysfunction of immune cells. In simian immunodeficiency virus (SIV)-infected lymphocytic cells, however, morphine may protect the cells from apoptotic lysis and allow the virus to continue to replicate. To further explore this apparently antithetical effect of opiates, we evaluated in the present study the effects of morphine on human lymphocytic CEM x174 cells induced to undergo apoptosis in the presence of actinomycin D. It was found that induction of apoptosis (characterized by DNA laddering) by actinomycin D was accompanied by a stimulation of the expression of active (phosphorylated) form of p53. Pretreatment of the cells with 10nM morphine caused a transient, naloxone-reversible suppression of the appearance of activated p53 and the generation of DNA laddering. Parallel evaluation of the growth of CEM x174 indicated that morphine treatment delays the inception of cell death triggered by actinomycin D. Inasmuch as Bcl-2 suppresses while Bax accelerates apoptosis, treatment of cells with morphine reduced the expression of Bax and enhanced the expression of Bcl-2. Taken together, morphine, through binding at the opioid receptor, may protect lymphocytic cells from apoptotic lysis if cell death is initiated by apoptosis-inducing agents such as human immunodeficiency virus (HIV), SIV or actinomycin D.

HIV-1 replication and pathogenesis in the human thymus

How HIV replicates and causes destruction of the thymus, and how to restore thymic function, are among the most important questions of HIV-1 pathogenesis and therapy in adult as well as pediatric patients. The thymus appears to function, albeit at reduced levels, throughout the life of adults, to respond to T cell depletion induced by HIV and to be suppressed by HIV. In this review, we summarize recent findings concerning HIV replication and pathogenesis in the human thymus, focusing on mechanistic insights gleaned from studies in the SCID-hu Thy/Liv mouse and human fetal-thymus organ culture (HF-TOC) models. First, we discuss HIV viral determinants and host factors involved in the replication of HIV in the thymus. Second, we consider evidence that both viral factors and host factors contribute to HIV-induced thymocyte depletion. We thus propose that multiple mechanisms, including depletion and suppression of progenitor cells, paracrine and direct lytic depletion of thymocytes, and altered thymocyte selection are involved in HIV-induced pathology in the thymus. With the SCID-hu Thy/Liv mouse and HF-TOC models, it will be important in the coming years to further clarify the virological, cell biological, and immunological mechanisms of HIV replication and pathogenesis in human thymus, and to correlate their significance in HIV disease progression.

Activation of the signal transducer and activator of transcription 1 signaling pathway in thymocytes from HIV-1-infected human thymus

OBJECTIVE

To identify HIV-induced host factors in the severe combined immunodeficient (SCID)-hu Thy/Liv mouse that may contribute to HIV pathogenesis in the thymus.

DESIGN

To identify genes specifically altered by HIV-1 infection using the cDNA microarray assay, SCID-hu Thy/Liv organs derived from the same donors were used. Therefore, no genetic variations existed between HIV and mock-infected samples. In addition, the 12-14 day post-infection timepoint was chosen because no significant thymocyte depletion was detected in HIV-infected Thy/Liv organs, so mRNA from the same cell types could be compared.

METHODS

Using SCID-hu Thy/Liv mice constructed from the same donor tissues, we analysed the expression of 9183 host genes in response to HIV infection with cDNA microarrays. Expression of selected genes with more than threefold induction was confirmed by measuring RNA (reverse transcriptase-polymerase chain reaction; RT-PCR) and proteins.

RESULTS

HIV-1 (JD or NL4-3) infection of the SCID-hu Thy/Liv mouse led to more than threefold induction of 19 genes, 12 of which were IFN-inducible and six were unknown EST clones. We confirmed induction by RT-PCR and protein blots. Both signal transducer and activator of transcription (STAT)1 and STAT2 proteins were induced, and STAT1 was also activated by phosphorylation at the Tyr701 and Ser727 sites in human thymus infected with HIV-JD or NL4-3. Treatment of human fetal thymus organ culture or human thymocytes with recombinant HIV-1 gp120 proteins also led to induction or activation of STAT1.

CONCLUSION

HIV-1 infection of the thymus led to activation of the STAT1 signaling pathway in thymocytes, which may contribute to HIV-1 pathogenesis in the thymus.

Positive regulation of CXCR4 expression and signaling by interleukin-7 in CD4+ mature thymocytes correlates with their capacity to favor human immunodeficiency X4 virus replication

The emergence of X4 human immunodeficiency virus type 1 (HIV-1) variants in infected individuals is associated with poor prognosis. One of the possible causes of this emergence might be the selection of X4 variants in some specific tissue compartment. We demonstrate that the thymic microenvironment favors the replication of X4 variants by positively modulating the expression and signaling of CXCR4 in mature CD4(+) CD8(-) CD3(+) thymocytes. Here, we show that the interaction of thymic epithelial cells (TEC) with these thymocytes in culture induces an upregulation of CXCR4 expression. The cytokine secreted by TEC, interleukin-7 (IL-7), increases cell surface expression of CXCR4 and efficiently overcomes the downregulation induced by SDF-1 alpha, also produced by TEC. IL-7 also potentiates CXCR4 signaling, leading to actin polymerization, a process necessary for virus entry. In contrast, in intermediate CD4(+) CD8(-) CD3(-) thymocytes, the other subpopulation known to allow virus replication, TEC or IL-7 has little or no effect on CXCR4 expression and signaling. CCR5 is expressed at similarly low levels in the two thymocyte subpopulations, and neither its expression nor its signaling was modified by the cytokines tested. This positive regulation of CXCR4 by IL-7 in mature CD4(+) thymocytes correlates with their high capacity to favor X4 virus replication compared with intermediate thymocytes or peripheral blood mononuclear cells. Indeed, we observed an enrichment of X4 viruses after replication in thymocytes initially infected with a mixture of X4 (NL4-3) and R5 (NLAD8) HIV strains and after the emergence of X4 variants from an R5 primary isolate during culture in mature thymocytes.

Resting CD4+ T lymphocytes but not thymocytes provide a latent viral reservoir in a simian immunodeficiency virus-Macaca nemestrina model of human immunodeficiency virus type 1-infected patients on highly active antiretroviral therapy

Despite suppression of viremia in patients on highly active antiretroviral therapy (HAART), human immunodeficiency virus type 1 persists in a latent reservoir in the resting memory CD4(+) T lymphocytes and possibly in other reservoirs. To better understand the mechanisms of viral persistence, we established a simian immunodeficiency virus (SIV)-macaque model to mimic the clinical situation of patients on suppressive HAART and developed assays to detect latently infected cells in the SIV-macaque system. In this model, treatment of SIV-infected pig-tailed macaques (Macaca nemestrina) with the combination of 9-R-(2-phosphonomethoxypropyl)adenine (PMPA; tenofovir) and beta-2',3'-dideoxy-3'-thia-5-fluorocytidine (FTC) suppressed the levels of plasma virus to below the limit of detection (100 copies of viral RNA per ml). In treated animals, levels of viremia remained close to or below the limit of detection for up to 6 months except for an isolated "blip" of detectable viremia in each animal. Latent virus was measured in blood, spleen, lymph nodes, and thymus by several different methods. Replication-competent virus was recovered after activation of a 99.5% pure population of resting CD4(+) T lymphocytes from a lymph node of a treated animal. Integrated SIV DNA was detected in resting CD4(+) T cells from spleen, peripheral blood, and various lymph nodes including those draining the gut, the head, and the limbs. In contrast to the wide distribution of latently infected cells in peripheral lymphoid tissues, neither replication-competent virus nor integrated SIV DNA was detected in thymocytes, suggesting that thymocytes are not a major reservoir for virus in pig-tailed macaques. The results provide the first evidence for a latent viral reservoir for SIV in macaques and the most extensive survey of the distribution of latently infected cells in the host.

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

The thymus is responsible for de novo production of CD4(+) and CD8(+) T cells and therefore is essential for T-cell renewal. The goal of this study was to assess the impact of simian immunodeficiency virus (SIV) infection on the production of T cells by the thymus. Levels of recent thymic emigrants within the peripheral blood were assessed through quantification of macaque T-cell receptor excision circles (TREC). Comparison of SIV-infected macaques (n = 15) to uninfected macaques (n = 23) revealed stable or increased TREC levels at 20 to 34 weeks postinfection. Further assessment of SIV-infected macaques (n = 4) determined that TREC levels decreased between 24 and 48 weeks postinfection. Through the assessment of longitudinal time points in three additional SIVmac239-infected macaques, the SIV infection was divided into two distinct phases. During phase 1 (16 to 30 weeks), TREC levels remained stable or increased within both the CD4 and CD8 T-cell populations. During phase 2 (after 16 to 30 weeks), TREC levels declined in both T-cell populations. As has been described for human immunodeficiency virus (HIV)-infected patients, this decline in TREC levels did at times correlate with an increased level of T-cell proliferation (Ki67(+) cells). However, not all TREC decreases could be attributed to increased T-cell proliferation. Further evidence for thymic dysfunction was observed directly in a SIVmac239-infected macaque that succumbed to simian AIDS at 65 weeks postinfection. The thymus of this macaque contained an increased number of memory/effector CD8(+) T cells and an increased level of apoptotic cells. In summary, reduced levels of TREC can be observed beginning at 16 to 30 weeks post-SIV infection and correlate with changes indicative of dysfunction within the thymic tissue. SIV infection of macaques will be a useful model system to elucidate the mechanisms responsible for the thymic dysfunction observed in HIV-infected patients.

Rapid and irreversible CD4+ T-cell depletion induced by the highly pathogenic simian/human immunodeficiency virus SHIV(DH12R) is systemic and synchronous

Highly pathogenic simian/human immunodeficiency virus chimeric viruses are known to induce a rapid, irreversible depletion of CD4+ T lymphocytes in the peripheral blood of acutely infected macaque monkeys. To more fully assess the systemic effects of this primary virus infection, specimens were collected serially between days 3 and 21 postinfection from variety of lymphoid tissues (lymph nodes, thymus, and spleen) and gastrointestinal tract and examined by DNA and RNA PCR, in situ hybridization, and immunohistochemical assays. In addition, the lymphoid tissues were evaluated by fluorescence-activated cell sorting. Virus infection was initially detected by DNA PCR on day 3 postinfection in lymph node samples and peaked on day 10 in the T-lymphocyte-rich areas of this tissue. CD4+ T-cell levels remained stable through day 10 in several lymphoid tissue specimens examined but fell precipitously between days 10 and 21. In situ terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) assays revealed the accumulation of apoptotic cells during the second week of infection in both lymph nodes and thymus, which colocalized, to a large extent, to sites of both virus replication and CD4+ T-lymphocyte loss.

Expression of peripherin in the brain of macaques (Macaca mulatta and Macaca fascicularis) occurs in astrocytes rather than neurones and is associated with encephalitis

Peripherin is a member of the type III intermediate filament family, expressed in neurones of the peripheral nervous system of many species and in a discrete subpopulation of neurones of the central nervous system (CNS) during early development in rodents. Previous studies on rats have shown that peripherin immunoreactivity increased significantly in cell bodies of spinal motor neurones following axonal injury. Our study examined the expression of peripherin in the cerebrum of normal macaques (Macaca mulatta and Macaca fascicularis) and those with encephalitis of viral (simian immunodeficiency virus and simian virus 40) or autoimmune (experimental allergic encephalomyelitis) aetiology. Immunohistochemistry, immunoelectronmicroscopy, immunofluorescence and confocal microscopy were performed on tissue sections using antibodies against cell-specific markers and peripherin. Peripherin-positive cells were absent in the cerebrum of normal macaques of all ages examined, whereas animals with encephalitis had peripherin-positive cells associated with inflammatory infiltrates. Further evaluation revealed that these peripherin-positive cells were not neurones, but were predominantly astrocytes expressing glial fibrillary acidic protein. Our study suggests that peripherin is not neurone-specific in the CNS of macaques; peripherin is expressed in astrocytes of animals with encephalitis.

Interleukin-7 and infection itself by human immunodeficiency virus 1 favor virus persistence in mature CD4(+)CD8(-)CD3(+) thymocytes through sustained induction of Bcl-2

The sequence of events and the mechanisms leading to the destruction of the thymus during human immunodeficiency virus (HIV) infection are still poorly characterized. Investigated here are the survival capacity on HIV-1 infection of the mature single-positive CD4(+)CD8(-)CD3(+) (SP CD4(+)) and the intermediate CD4(+) CD8(-)CD3(-) thymocytes previously shown to be able to replicate the virus in the thymic microenvironment. It is demonstrated that the mature SP CD4(+) thymocytes exhibit a high survival capacity despite the production of a high yield of viruses. Interleukin-7, reported to be a crucial cofactor of tumor necrosis factor (TNF) to promote HIV replication, is shown here to counteract the apoptotic activity of TNF. Resistance to apoptosis of SP CD4(+) cells is conferred by a high expression of the IL-7 receptor (IL-7R) associated with the capacity of IL-7 to permanently up-regulate Bcl-2. In addition, this high Bcl-2 level is further enhanced by infection itself. In contrast, intermediate thymocytes, which replicate the virus at a lower level, are more sensitive to apoptosis, and their differentiation into double-positive CD4(+)CD8(+)CD3(-) (DP CD3(-)) cells strongly increases their death rate on infection. This sensitivity is related to a lower expression of IL-7R and Bcl-2 in intermediate thymocytes, which further decreases at the DP CD3(-) stage. In addition, a decreased level of Bcl-2 is observed in this subset during infection. Altogether these data suggest that in vivo, HIV infection might create a persistent virus reservoir within the SP CD4(+) thymocytes, whereas the later infection of intermediate cells might lead to thymopoiesis failure.