Mechanisms of HIV Latency: an Emerging Picture of Complexity

Rarely HIV type 1 establishes proviral latency within the host genome, maintained with little or no viral gene expression. This state has been quantitated in peripheral blood and lymphoid tissues of HIV-infected patients, appearing in the earliest days of infection. These rare cellular reservoirs are unaffected by current antiretroviral therapy and unrecognized by the host immune response, and can regenerate disseminated viremia if therapy is interrupted. Proviral latency may be established when a newly HIV-infected cell exits the cell cycle and returns to the resting state. Rarely, direct infection of resting cells may also occur. Multiple molecular mechanisms appear to underlie the establishment and maintenance of persistent, latent HIV infection, most frequent in the resting central memory CD4+ T cell. Interrupting processes that maintain latency may allow therapeutic attack of this primary form of persistent HIV infection, but a better understanding of relevant mechanisms in vivo is needed.     

中国HIV/AIDS患者T细胞及调节性T细胞CTLA-4表达与疾病进展关系研究

目的 对中国HIV感染者T细胞及凋节性T细胞CTLA-4表达与HIV疾病进展的相关性进行研究,探讨CTLA-4在HIV感染中的作用.方法 选取58名HIV/AIDS患者(长期不进展组、无症状HIV组、AIDS组),应用流式细胞仪胞内染色技术检测T细胞及CD4+CD25+Foxp3+调节性T细胞内CTLA-4表达水平,分析其与CD4+T细胞、病毒载量、淋巴细胞活化凋亡水平的相关性.结果 长期不进展组、无症状HIV组、AIDS组CD4+T细胞内CTLA-4表达水平依次增岛(P<0.05);与CD+T细胞显著负相关(P<0.01),与CD8+T细胞活化(CD38表达)、凋亡水平(CD95表达)及CD4+T细胞凋亡水平显著相关(P<0.05),与病毒载量无显著相关性.长期不进展组、无症状HIV组、AIDS组CD8+T细胞内CTLA-4表达水平差异无统计学意义;与CD4+T细胞、病毒载量、CD4,'+>、CD8+T细胞活化及凋亡水平均无显著相关性.CD4+CD25+Foxp3+T细胞内CTLA-4表达水平长期不进展组明显低于无症状HIV组及AIDS纽(P<0.05);与CD4+T细胞显著负相关(P<0.05);与CD4+、CD8+T淋巴细胞活化(HLA-DR表达)显著相关(P<0.01).结论 中国HIV感染者CD4+T细胞及CD4+CD25+Foxp3+调节性T细胞内CTLA-4表达水平与疾病进展及免疫活化状态显著相关,参与HIV感染免疫平衡的调节.     

Peripheral CD4 loss of regulatory T cells is associated with persistent viraemia in chronic HIV infection

Chronic HIV infection is associated with T cell abnormalities and altered effector function. Regulatory T cells (Treg) are CD4+ T cells that play a critical role in regulating the immune system. The impact of regulatory T cells on HIV infection and disease progression may be highly significant. We hypothesize that chronic antigenic stimulation from a persistent, high viraemic state may promote a population of Treg that contributes to HIV-associated immune dysfunction. We evaluated the pattern of Treg in chronically infected, HIV-positive individuals over a course of 6 months. Treg are depleted at a distinct rate from that of absolute CD4 cells and loss of Treg is slower in the presence of viral suppression. In vitro depletion of CD25+ CD4+ cells resulted in increased Gag-specific CD4 and CD8 responses. A significant correlation between ex vivo measurement of Treg and Gag-specific CD4 T cell responses was observed (r=-0 x 41, P=0 x 018) with a trend observed with Gag-specific CD8 T cell responses (P=0 x 07). The impact of HIV infection on the Treg population directly complicates the measured effect of Treg on the immune dysfunction although our data support the important role of Treg on modulating the effector T cell response in chronic infection.     

High proportion of PD‐1‐expressing CD4+ T cells in adipose tissue constitutes an immunomodulatory microenvironment that may support HIV persistence

We and others have demonstrated that adipose tissue is a reservoir for HIV. Evaluation of the mechanisms responsible for viral persistence may lead to ways of reducing these reservoirs. Here, we evaluated the immune characteristics of adipose tissue in HIV‐infected patients receiving antiretroviral therapy (ART) and in non‐HIV‐infected patients. We notably sought to determine whether adipose tissue's intrinsic properties and/or HIV induced alteration of the tissue environment may favour viral persistence. ART‐controlled HIV infection was associated with a difference in the CD4/CD8 T‐cell ratio and an elevated proportion of Treg cells in subcutaneous adipose tissue. No changes in Th1, Th2 and Th17 cell proportions or activation markers expression on T cell (Ki‐67, HLA‐DR) could be detected, and the percentage of CD69‐expressing resident memory CD4⁺ T cells was not affected. Overall, our results indicate that adipose‐tissue‐resident CD4⁺ T cells are not extensively activated during HIV infection. PD‐1 was expressed by a high proportion of tissue‐resident memory CD4⁺ T cells in both HIV‐infected patients and non‐HIV‐infected patients. Our findings suggest that adipose tissue's intrinsic immunomodulatory properties may limit immune activation and thus may strongly contribute to viral persistence.     

补体调节蛋白CD59在HIV感染者外周血T细胞上的表达研究

目的 观察补体调节蛋白CD59在HIV感染者外周血T细胞上的表达情况,并进一步探讨高效抗逆转录病毒治疗（Highly Active Antiretroviral Therapy,HARRT）前后CD59在外周血T细胞上的表达变化及其意义.方法 收集48例确诊HIV感染者外周血,根据是否HARRT分为未治疗组（23例）及治疗组（25例）,并收集14例健康志愿者外周血,全血细胞表面染色后使用BD FACSCanto 流式仪检测CD59在外周血T细胞上的表达情况,并进一步检测CD59在CD45RO＋记忆型CD4T细胞表面的表达情况;同时用定量PCR技术检测HIV感染者的血浆病毒载量及CD4细胞绝对计数,将结果进行统计学分析并比较各组间差异.结果 与健康对照者比较,CD59在HIV感染者CD4＋T细胞上的表达水平明显增高（P＜0.05）,其中以在CD45RO＋记忆型CD4T细胞上增高为主（P＜0.05）;治疗组CD4＋T细胞上CD59的表达较未治疗组明显下降（P＜0.05）,但与健康对照组比较其表达仍处于较高水平（P＜0.05）,进一步分析发现CD59的这种变化主要发生在CD45RO＋（记忆型）CD4＋T细胞上;CD59在CD4＋T细胞上的表达变化与病毒载量及外周血CD4＋T细胞绝对计数均有一定的相关性（R2=0.2181,P=0.0247;R2=0.1586,P=0.0486）.结论 HIV感染可引起补体调节蛋白CD59在CD4＋T细胞上表达增加,HARRT可使其表达水平有所下降.这种表达增加可能与病毒免疫逃逸及CD4＋T细胞的功能抑制有关,HARRT治疗可部分逆转这种免疫紊乱现象.     

The emerging role of CD40 ligand in HIV infection

CD40 ligand (also called CD40L, CD154, or TNFSF5) is a membrane protein expressed mainly by activated CD4+ T cells, which interacts with its receptor, CD40, on a variety of cells. The crucial importance of the CD40L-CD40 system for many immune responses has been extensively described. This review focuses on the multiple roles that this system may play in HIV infection. In early HIV infection, CD40L expression contributes to the immunological control of viral replication by inducing HIV-suppressive chemokines and supporting the production of anti-HIV antibodies and cytotoxic T cells. However, by activating antigen-presenting cells, such as dendritic cells and macrophages, CD40L can also lead to increased CD4+ T cell activation, which promotes the replication of HIV in these lymphocytes. Later, with the development of AIDS, CD40L-expressing CD4+ T cells become selectively depleted, perhaps as a result of a gp120-induced signal through CD4 that down-regulates CD40L expression. This acquired CD40L deficiency may explain the similarity between the types of opportunistic infections that occur in AIDS and in congenital CD40L deficiency. Vaccines or other strategies that promote the growth of CD4+ T cells capable of expressing CD40L may help to sustain host immunity against HIV and prevent AIDS-defining opportunistic infections.     

CD4+ CD25+调节性T细胞在HIV/AIDS患者中的作用及其与HIV病毒载量的相关性研究

目的 研究HIV感染者／AIDS患者外周血CD4^＋ CD25^＋ 调节性T细胞（CD4^＋ CD25^＋ regulatory Tcell，Treg）频率、功能及其临床意义。方法 选择31例HIV感染者／AIDS患者和30例健康对照者，采用流式细胞仪检测各组外周血Treg的表型和频率。采取MACS磁珠分选CD4^＋CD25^＋T细胞，利用[^3H]胸腺嘧啶掺入法检测CD4^＋ CD25^＋T细胞在特异性HIV抗原刺激下对CD4^＋ CD25-T细胞的增殖影响。结果HIV／AIDS患者组与正常对照组相比较，外周血CD4^＋ CD25^＋ T细胞频率在统计学上差异无统计学意义。与正常对照组比较，HIV感染者外周血CD4^＋ CD25^＋ T细胞频率升高，差异有统计学意义（P〈0.01）；与正常对照组比较，AIDS患者者外周血CD4^＋ CD25^＋ T细胞频率降低，差异有统计学意义（P〈0．0001）。HIV RNA病毒载量与患者外周血CD4^＋ CD25^＋ T细胞数量呈正相关性（P〈0．01）。CD4^＋ CD25^＋ T细胞具有抑制HIV特异性的CD4^＋ CD25^- T细胞的增殖作用。结论HIV感染者／AIDS患者的细胞免疫功能紊乱，CD4^＋ CD25^＋ T细胞能抑制HIV感染者／AIDS患者的HIV特异性细胞免疫反应，促进HIV病毒复制，与形成持续HIV感染有关。     

Mechanisms of HIV persistence in HIV reservoirs

The establishment and maintenance of HIV reservoirs that lead to persistent viremia in patients on antiretroviral drugs remains the greatest challenge of the highly active antiretroviral therapy era. Cellular reservoirs include resting memory CD4+ T lymphocytes, implicated as the major HIV reservoir, having a half‐life of approximately 44 months while this is less than 6 hours for HIV in plasma. In some individuals, persistent viremia consists of invariant HIV clones not detected in circulating resting CD4+ T lymphocytes suggesting other possible sources of residual viremia. Some anatomical reservoirs that may harbor such cells include the brain and the central nervous system, the gastrointestinal tract and the gut‐associated lymphoid tissue and other lymphoid organs, and the genital tract. The presence of immune cells and other HIV susceptible cells, occurring in differing compositions in anatomical reservoirs, coupled with variable and poor drug penetration that results in suboptimal drug concentrations in some sites, are all likely factors that fuel the continued low‐level replication and persistent viremia during treatment. Latently, HIV‐infected CD4+ T cells harboring replication‐competent virus, HIV cell‐to‐cell spread, and HIV‐infected T cell homeostatic proliferation due to chronic immune activation represent further drivers of this persistent HIV viremia during highly active antiretroviral therapy.     

Systemic activation of the immune system in HIV infection: The role of the immune complexes (hypothesis)

Currently, immune activation is proven to be the basis for the HIV infection pathogenesis and a strong predictor of the disease progression. Among the causes of systemic immune activation the virus and its products, related infectious agents, pro-inflammatory cytokines, and regulatory CD4+ T cells’ decrease are considered. Recently microbial translocation (bacterial products yield into the bloodstream as a result of the gastrointestinal tract mucosal barrier integrity damage) became the most popular hypothesis. Previously, we have found an association between immune complexes present in the bloodstream of HIV infected patients and the T cell activation. On this basis, we propose a significantly modified hypothesis of immune activation in HIV infection. It is based on the immune complexes’ participation in the immunocompetent cells’ activation. Immune complexes are continuously formed in the chronic phase of the infection. Together with TLR-ligands (viral antigens, bacterial products coming from the damaged gut) present in the bloodstream they interact with macrophages. As a result macrophages are transformed into the type II activated forms. These macrophages block IL-12 production and start synthesizing IL-10. High level of this cytokine slows down the development of the full-scale Th1-response. The anti-viral reactions are shifted towards the serogenesis. Newly synthesized antibodies’ binding to viral antigens leads to continuous formation of the immune complexes capable of interacting with antigen-presenting cells.     

Therapeutic immunization for HIV

Vaccines have entered into human clinical trials against infectious diseases and as therapies against cancer. The HIV virus establishes a latent infection at a very early stage and the T cell memory of the infected patient is rapidly destroyed. However, results of immunotherapy after DNA and protein immunization show that vaccine-induced immune responses might be present for a long period of time. Patients subjected to therapeutic immunization appear to do well, and to have a small immunological advantage, which, however, will have to be improved. The vaccine therapy should start early, while adequate reservoirs of appropriate T helper cells are available and still inducible. The DNA vaccines induce a relatively long-lived immunological memory, and gene-based immunization is effective in inducing cytotoxic CD8⁺ T cells and CD4+ helper cells. Protein vaccines, on the other hand, primarily give T cell help. It thus appears that DNA and protein approaches to HIV immunization complement each other. A surprisingly broad reactivity to peptides from different subtypes of HIV was identified in individuals infected with several subtypes of HIV.     

Can antiretroviral therapy ever be stopped?

A pool of latently infected CD4+ T cells is established within the first few weeks of HIV infection. Because these memory T cells are inactive, the viral DNA integrated into their chromosomes remains invisible to immune surveillance and to antiretrovirals. Research shows that this reservoir of infected memory T cells does not decay in a clinically meaningful time frame--that is, within 60 years--in patients being treated with potent combination antiretrovirals. Even a hypothetical regimen that prevents any new infection of cells would not hasten the decay of this latent reservoir. Treatments that activate this reservoir have been studied in patients with suppressed viremia, but such interventions are highly toxic and have not succeeded so far. Studying rare individuals who manage to control activation of these latent cells may provide important clues to long-term control of HIV infection.     

HIV I infection of dendritic cells

Dendritic cells (DC) from human peripheral blood are susceptible to productive and probably to latent infection with HIV-I. Infection of DC also occurs in vivo since in HIV-seropositive individuals Langerhans' cells of the skin and DC from peripheral blood, (in preparation) are infected. In peripheral blood 3-25% of DC, identified as large, low-density cells lacking monocyte markers, are infected as judged by in situ hybridization with an HIV probe. This contrasts with the lower proportion (< 0.2%) of other cells infected. DC exposed to HIV in vitro or in vivo fail to present other antigens or mitogens to stimulate T cells. This functional defect in infected DC is not blocked by the presence of soluble CD4 antigen and occurs in the absence of T cell infection suggesting a block at the level of the antigen-presenting cell itself. Infection, depletion and dysfunction of DC in HIV seropositive patients is already present in asymptomatic individuals and this precedes the appearance of T cell defects. We speculate that loss of functional DC may be a fundamental defect leading to a block in recruitment of resting T cells into immune responses. In contrast to the HIV-induced impairment of antigen presentation by DC, these cells were potent stimulators of responses to the HIV antigens themselves. Normal DC infected with HIV in vitro stimulated primary proliferative and cytotoxic T cell responses (in preparation). These were produced in cells from individuals expressing a range of different MHC types but the cytotoxic cells, once produced, killed autologous but not allogeneic, infected T cell blasts. Primary response to viral peptides can also be produced suggesting that this system may be useful for identifying immunogenic epitopes of HIV using cells from sero-negative, non-immunocompromised individuals.     

Benign monoclonal expansion of CD8+ lymphocytes in HIV infection

BACKGROUND: A transient expansion of the CD8+ T cell pool normally occurs in the early phase of HIV infection. Persistent expansion of this pool is observed in two related settings: diffuse infiltrative lymphocytosis syndrome (DILS) and HIV associated CD8+ lymphocytosis syndrome. AIM: To investigate a group of HIV infected patients with CD8+ lymphocytosis syndrome with particular emphasis on whether monoclonality was present. METHODS: A group of 18 patients with HIV-1 infection and persistent circulating CD8+ lymphocytosis was compared with 21 HIV positive controls. Serum samples were tested for antinuclear antibodies, antibodies to extractable nuclear antigens, immunoglobulin levels, paraproteins, human T lymphotropic virus type 1 (HTLV-1), Epstein-Barr virus, and cytomegalovirus serology. Lymphocyte phenotyping and HLA-DR typing was performed, and T cell receptor (TCR) gene rearrangement studies used to identify monoclonal populations of T cells. CD4+ and CD8+ subsets of peripheral blood lymphocytes were purified to determine whether CD8+ populations inhibited HIV replication in autologous CD4+ cells. RESULTS: A subgroup of patients with HIV-1 infection was found to have expanded populations of CD8+ T cell large granular lymphocytes persisting for 6 to 30 months. The consensus immunophenotype was CD4- CD8+ DRhigh CD11a+ CD11c+ CD16- CD28+/- CD56- CD57+, consistent with typical T cell large granular lymphocytes expressing cellular activation markers. Despite the finding of monoclonal TCR gene usage in five of 18 patients, there is evidence that the CD8+ expansions are reactive populations capable of mediating non-cytotoxic inhibition of HIV replication. CONCLUSIONS: A subgroup of HIV positive patients has CD8+ lymphocytosis, but despite the frequent occurrence of monoclonal TCR gene usage there is evidence that this represents an immune response to viral infection rather than a malignant disorder.     

T-CELL AND NEURONAL APOPTOSIS IN HIV INFECTION: IMPLICATIONS FOR THERAPEUTIC INTERVENTION

The pathogenesis of HIV infection involves the selective loss of CD4 + T cells contributing to immune deficiency. Although loss of T cells leading to immune dysfunction in HIV infection is mediated in part by viral infection, there is a much larger effect on noninfected T cells undergoing apoptosis in response to activation stimuli. In the subset of patients with HIV dementia complex, neuronal injury, loss, and apoptosis are observed. Viral proteins, gp120 and Tat, exhibit proapoptotic activities when applied to T cell and neuronal cultures by direct and indirect mechanisms. The pathways leading to cell death involve the activation of one or more death receptor pathways (i.e., TNF-α, Fas, and TRAIL receptors), chemokine receptor signaling, cytokine dysregulation, caspase activation, calcium mobilization, and loss of mitochondrial membrane potential. In this review, the mechanisms involved in T-cell and neuronal apoptosis, as well as antiapoptotic pathways potentially amenable to therapeutic application, are discussed.     

T-cell and neuronal apoptosis in HIV infection: implications for therapeutic intervention

The pathogenesis of HIV infection involves the selective loss of CD4+ T cells contributing to immune deficiency. Although loss of T cells leading to immune dysfunction in HIV infection is mediated in part by viral infection, there is a much larger effect on noninfected T cells undergoing apoptosis in response to activation stimuli. In the subset of patients with HIV dementia complex, neuronal injury, loss, and apoptosis are observed. Viral proteins, gp120 and Tat, exhibit proapoptotic activities when applied to T cell and neuronal cultures by direct and indirect mechanisms. The pathways leading to cell death involve the activation of one or more death receptor pathways (i.e., TNF-alpha, Fas, and TRAIL receptors), chemokine receptor signaling, cytokine dysregulation, caspase activation, calcium mobilization, and loss of mitochondrial membrane potential. In this review, the mechanisms involved in T-cell and neuronal apoptosis, as well as antiapoptotic pathways potentially amenable to therapeutic application, are discussed.     

A novel approach to HIV therapy: highly active antiretroviral therapy and autologous hematopoietic cell transplantation

Highly active antiretroviral therapy dramatically decreases in vivo viral replication to levels below the level of clinical detection, but does not eradicate HIV-1 infection on the basis of persistent low-level or cryptic viral replication and latent provirus in resting CD4+ T lymphocytes. Immune activation therapy has begun to be used in attempts to increase the turnover rate of the latent virus reservoir through activation of infected cells that comprise this reservoir, in order to promote cell death and accelerate virus clearance. Recent reports have not demonstrated complete virus ablation. Autologous hematopoietic cell transplantation now appears as a safe, feasible, and reasonable approach for Aids-related lymphoma in patients who meet criteria for transplantation. The hypothesis is based on the possibility that hematopoietic stem cells from a HIV-positive patient could be collected before the patient becomes infected with HIV. Then, the proposed treatment consists of the following assumptions. HAART keeps viral replication below the level of detection, limiting the infection to latent provirus in resting CD4+ T lymphocytes. It is presumed here that myeloablative conditioning regimen can lead to the killing of all the cells that, in theory, harbour the virus. The following transplantation of the autologous hematopoietic stem cells, collected before HIV infection, would allow the complete recovery. The hypothesis is to be tested on a suitable animal model. After the collection of hematopoietic stem cells, the animal is experimentally infected with the immunodeficiency virus. HAART is given after plasma viral RNA becomes detectable. By myeloablative conditioning regimen all the cells harbouring the virus are supposed to be killed. Then, as the viral load is kept undetectable by HAART, the animal undergoes autologous hematopoietic stem cell transplantation. Antiretroviral therapy is interrupted a month after engraftment has taken place. Although hematopoietic stem cells from man before infection with HIV are unlikely to be available, a successful test on the animal would suggest a new approach which could allow the cure of HIV in future.     

Role of incidental and/or cured intestinal parasitic infections on profile of CD4+ and CD8+ T cell subsets and activation status in HIV-1 infected and uninfected adult Ethiopians

Intestinal parasitic infections have been suggested to cause persistent immune activation leading to an unbalanced immune state. Such a state has been proposed to be a major factor in the pathogenesis of AIDS in an African context. The present study investigated the effect of incidental parasitic infection and treatment on the profile of T cell differentiation and activation markers on CD4+ and CD8+ T cells from HIV-1 infected and uninfected adult Ethiopians. Cryopreserved PBMCs from 64 subjects (41 HIV-negative and 23 HIV-positive) with follow-up visits at 6-monthly intervals were used to compare the effect of incidental intestinal parasites and their treatment upon T cell subset profiles and activation status. The samples were stained with antibodies to various T cell differentiation and activation markers allowing naive, memory, effector, memory/effector, activated and resting CD4+ and CD8+ T cell subsets to be quantified by triple-colour FACScan. Incidental intestinal parasitic infections resulted in a significant increase in memory CD4+ T cell numbers both in HIV-negative and HIV-positive subjects (P < 0.05). There was also a significant increase in the percentage of CD8+ HLA-DR+ T cells (P < 0.05) in HIV-positive subjects co-infected with parasites. In HIV-negative subjects, a significant decline in activated cells and a significant increase in resting CD8+ T cells (P < 0.05) was observed after treatment for parasites. These data suggest that intestinal parasitic infections could result in the alteration of T cell subset counts and also in the up-regulation of T cell activation markers in peripheral blood. Treatment of parasitic infections showed a tendency to reduce the activation suggesting that, together with other community based intervention strategies, such treatment could be used to down-regulate immune activation and hence protect the host from being easily attacked by HIV.     

HIV感染后胃黏膜CD4+、CD8+T细胞数量分布及CD38表达的改变

目的 研究不同感染阶段HIV感染者胃黏膜中CD4+和CD8+T细胞数量和分布情况,及免疫细胞激活状态的改变.方法 有胃黏膜组织活检标本的HIV感染者42例,其中有明确临床分期诊断的36例,另选非HIV感染者胃黏膜组织活检标本10例为对照,利用免疫组织化学方法检测研究对象胃黏膜活检组织中CD4、CD8及CD38的表达,以图像分析系统分析其差异.结果 (1)AIDS患者胃黏膜中CD4+T细胞与对照组和无症状者相比,均显著减少(P(0.01),无症状者多数淋巴滤泡中仍有一定量CD4+T细胞,间质中多呈不均匀聚集分布,统计学与正常对照组差异无统计学意义(P>0.05);(2)HIV感染者胃黏膜内CD8+T细胞普遍噬腺体和黏膜上皮,部分病例胃黏膜内CD8+T细胞局部过度增生,与对照组相比,CD8+T细胞在感染者胃黏膜中显著增多(P<0.01),无症状者与AIDS患者CD8+T细胞差异无统计学意义(P>0.05);(3)在HIV感染者胃黏膜内,CD38阳性细胞主要分布于黏膜表层至浅1/3～2/3层,与对照组相比,HIV感染者胃黏膜中CD38表达增强(P<0.01),无症状者与MDS患者CD38表达差异无统计学意义(P>0.05).结论 HIV感染者胃黏膜中CD4+T细胞的数量和分布与疾病进展有密切联系,HIV对CD4+T细胞的感染和破坏导致黏膜免疫系统功能异常,可能是胃黏膜内CD8+T细胞数量增加,CD38表达增强的重要原因之一.