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

Human T-Cell Development and Thymic Egress: An Infectious Disease Perspective

Emigration of mature naïve CD4 SP T cells from the human thymus to the periphery is not fully understood, although elucidation of the mechanisms that govern egress of T cells is crucial to understanding both basic immunology and the immune response in diseases such as HIV infection. Recent work has brought to light the requirement for sphingosine-1-phosphate (S1P) and its receptors in a variety of fields including mature naïve T-cell egress from the thymus of mice. We are examining the expression and function of this novel requisite T-cell egress receptor within the human thymus, characterizing changes observed in the expression and function of this receptor in infectious diseases. To perform this work, we use a variety of humanized murine models reviewed in this article. Future work in the field of T-cell egress, especially as it pertains to S1P receptors, should advance the fields of basic T-cell immunology and immunopathology and open new avenues for exploration into novel therapeutics.

SIV infection of rhesus macaques of Chinese origin: a suitable model for HIV infection in humans

Simian immunodeficiency virus (SIV) infection of Indian-origin rhesus macaques (RM) has been widely used as a well-established nonhuman primate (NHP) model for HIV/AIDS research. However, there have been a growing number of studies using Chinese RM to evaluate immunopathogenesis of SIV infection. In this paper, we have for the first time reviewed and discussed the major publications related to SIV or SHIV infection of Chinese RM in the past decades. We have compared the differences in the pathogenesis of SIV infection between Chinese RM and Indian RM with regard to viral infection, immunological response, and host genetic background. Given AIDS is a disease that affects humans of diverse origins, it is of importance to study animals with different geographical background. Therefore, to examine and compare results obtained from RM models of Indian and Chinese origins should lead to further validation and improvement of these animal models for HIV/AIDS research.

Different immunological phenotypes associated with preserved CD4+ T cell counts in HIV-infected controllers and viremic long term non-progressors

BACKGROUND

HIV-infected controllers control viral replication and maintain normal CD4+ T cell counts. Long Term Non-Progressors (LTNP) also maintain normal CD4+ T cell counts, but have on-going viral replication. We hypothesized that different immunological mechanisms are responsible for preserved CD4+ T cell counts in controllers and LTNP.

METHODS

25 HIV-infected controllers and 14 LTNP were included in this cross-sectional study. For comparison, 25 progressors and 34 healthy controls were included. Production and destruction of T cells were addressed by determination of T cell receptor excision circles (TREC), recent thymic emigrants, naïve cells, immune activation, senescence and apoptosis. Furthermore, telomere length was determined, and the amount of lymphoid tissue in tonsil biopsies was quantified.

RESULTS

Controllers presented with partly preserved thymic output, preserved expression of the IL-7 receptor and IL-7 receptor density, and lower levels of destruction of cells than progressors resembling HIV-negative healthy controls. In contrast, LTNP appeared much like progressors, and different from controllers in immune activation, senescence, and apoptosis. Interestingly, CD8+ RTE, TREC and telomere length were partly preserved. Finally, both controllers and LTNP displayed decreased amounts of lymphoid tissue compared to healthy controls.

CONCLUSIONS

Controllers presented with an immunological profile different from LTNP. While controllers resembled healthy controls, LTNP were similar to progressors, suggesting different immunological mechanisms to be responsible for preserved CD4+ T cell counts in LTNP and controllers. However, both controllers and LTNP presented with reduced amounts of lymphoid tissue despite preserved CD4+ T cell counts, indicating HIV to cause damage even in non-progressors.

DRAM triggers lysosomal membrane permeabilization and cell death in CD4(+) T cells infected with HIV

Productive HIV infection of CD4⁺ T cells leads to a caspase-independent cell death pathway associated with lysosomal membrane permeabilization (LMP) and cathepsin release, resulting in mitochondrial outer membrane permeabilization (MOMP). Herein, we demonstrate that HIV infection induces damage-regulated autophagy modulator (DRAM) expression in a p53-dependent manner. Knocking down the expression of DRAM and p53 genes with specific siRNAs inhibited autophagy and LMP. However, inhibition of Atg5 and Beclin genes that prevents autophagy had a minor effect on LMP and cell death. The knock down of DRAM gene inhibited cytochrome C release, MOMP and cell death. However, knocking down DRAM, we increased viral infection and production. Our study shows for the first time the involvement of DRAM in host-pathogen interactions, which may represent a mechanism of defense via the elimination of infected cells.

Lysosomes are acidic organelles capable of digesting macromolecules and regulating autophagy. In the context of host-pathogen interactions, productive viral infections are associated with lysosome membrane permeabilization (LMP) and programmed cell death (PCD). At a molecular level, the tumor suppressor protein 53 (p53), which is a key player in the detection of DNA damage, acts also as a sensor of pathogen replication. Activation of p53 has been considered to be an altruistic cell suicide mechanism that limits viral infection. Here, we provide new evidence that damage-regulated autophagy modulator (DRAM), a p53 target gene, regulates both LMP and PCD of HIV-infected CD4 T cells. Whereas the inhibition of DRAM or p53 prevents autophagy in infected cells, the inhibition of the autophagy machinery has a minor role in this context. As a consequence, the silencing of DRAM leads to increased HIV viral infection. This is the first report describing the role of DRAM in the context of host-pathogen interaction. Whereas it is to the advantage of the pathogens to preserve their hosts and thus facilitate their multiplication and dissemination, hosts have developed altruistic cellular processes to defend themself and limit the spread of the infectious agent in multicellular organisms. We propose that the ancestral DRAM protein represents a mechanism of self-defense, inducing elimination of infected cells through LMP.

Thirty Years with HIV Infection-Nonprogression Is Still Puzzling: Lessons to Be Learned from Controllers and Long-Term Nonprogressors

In the early days of the HIV epidemic, it was observed that a minority of the infected patients did not progress to AIDS or death and maintained stable CD4+ cell counts. As the technique for measuring viral load became available it was evident that some of these nonprogressors in addition to preserved CD4+ cell counts had very low or even undetectable viral replication. They were therefore termed controllers, while those with viral replication were termed long-term nonprogressors (LTNPs). Genetics and virology play a role in nonprogression, but does not provide a full explanation. Therefore, host differences in the immunological response have been proposed. Moreover, the immunological response can be divided into an immune homeostasis resistant to HIV and an immune response leading to viral control. Thus, non-progression in LTNP and controllers may be due to different immunological mechanisms. Understanding the lack of disease progression and the different interactions between HIV and the immune system could ideally teach us how to develop a functional cure for HIV infection. Here we review immunological features of controllers and LTNP, highlighting differences and clinical implications.

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.

Transient nature of long-term nonprogression and broad virus-specific proliferative T-cell responses with sustained thymic output in HIV-1 controllers

BACKGROUND

HIV-1(+) individuals who, without therapy, conserve cellular anti-HIV-1 responses, present with high, stable CD4(+) T-cell numbers, and control viral replication, facilitate analysis of atypical viro-immunopathology. In the absence of universal definition, immune function in such HIV controllers remains an indication of non-progression.

METHODOLOGY/PRINCIPAL FINDINGS

CD4 T-cell responses to a number of HIV-1 proteins and peptide pools were assessed by IFN-gamma ELISpot and lymphoproliferative assays in HIV controllers and chronic progressors. Thymic output was assessed by sjTRECs levels. Follow-up of 41 HIV-1(+) individuals originally identified as "Long-term non-progressors" in 1996 according to clinical criteria, and longitudinal analysis of two HIV controllers over 22 years, was also performed. HIV controllers exhibited substantial IFN-gamma producing and proliferative HIV-1-specific CD4 T-cell responses to both recombinant proteins and peptide pools of Tat, Rev, Nef, Gag and Env, demonstrating functional processing and presentation. Conversely, HIV-specific T-cell responses were limited to IFN-gamma production in chronic progressors. Additionally, thymic output was approximately 19 fold higher in HIV controllers than in age-matched chronic progressors. Follow-up of 41 HIV-1(+) patients identified as LTNP in 1996 revealed the transitory characteristics of this status. IFN-gamma production and proliferative T-cell function also declines in 2 HIV controllers over 22 years.

CONCLUSIONS

Although increased thymic output and anti-HIV-1 T-cell responses are observed in HIV controllers compared to chronic progressors, the nature of nonprogressor/controller status appears to be transitory.

Association of progressive CD4(+) T cell decline in SIV infection with the induction of autoreactive antibodies

The progressive decline of CD4(+) T cells is a hallmark of disease progression in human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infection. Whereas the acute phase of the infection is dominated by virus-mediated depletion of memory CD4(+) T cells, chronic infection is often associated with a progressive decline of total CD4(+) T cells, including the naïve subset. The mechanism of this second phase of CD4(+) T cell loss is unclear and may include immune activation-induced cell death, immune-mediated destruction, and regenerative or homeostatic failure. We studied patterns of CD4(+) T cell subset depletion in blood and tissues in a group of 20 rhesus macaques inoculated with derivatives of the pathogenic SIVsmE543-3 or SIVmac239. Phenotypic analysis of CD4(+) T cells demonstrated two patterns of CD4(+) T cell depletion, primarily affecting either naïve or memory CD4(+) T cells. Progressive decline of total CD4(+) T cells was observed only in macaques with naïve CD4(+) T cell depletion (ND), though the depletion of memory CD4(+) T cells was profound in macaques with memory CD4(+) T cell depletion (MD). ND macaques exhibited lower viral load and higher SIV-specific antibody responses and greater B cell activation than MD macaques. Depletion of naïve CD4(+) T cells was associated with plasma antibodies autoreactive with CD4(+) T cells, increasing numbers of IgG-coated CD4(+) T cells, and increased incidence of autoreactive antibodies to platelets (GPIIIa), dsDNA, and phospholipid (aPL). Consistent with a biological role of these antibodies, these latter antibodies were accompanied by clinical features associated with autoimmune disorders, thrombocytopenia, and catastrophic thrombotic events. More importantly for AIDS pathogenesis, the level of autoreactive antibodies significantly correlated with the extent of naïve CD4(+) T cell depletion. These results suggest an important role of autoreactive antibodies in the CD4(+) T cell decline observed during progression to AIDS.

Increased neutrophil apoptosis in chronically SIV-infected macaques

Polymorphonuclear neutrophils (PMN) from chronically HIV-infected individuals have been reported to be more prone to die. However, although non-human primates models have been extensively used for improving our knowledge on T cell immunity, the impact of SIV-infection on PMN, in relationships with disease severity, has never been assessed. In our study, we demonstrate that PMN from Rhesus macaques (RMs) of Chinese origin chronically infected with the virulent strain SIVmac251 display increased susceptibility to undergo apoptosis as compared to PMN from RMs infected with the non-pathogenic SIVΔnef strain. PMN apoptosis was significantly increased in RMs progressing faster to AIDS as compared to non-progressors RMs. Furthermore, the percentage of apoptotic cells correlated with PMN activation state reflected by increased CD11b expression and reactive oxygen species production. Interestingly, whereas inflammatory cytokines IL-8 and IL-1β prevent in vitro PMN death, the levels of those cytokines were low in RMs progressing towards AIDS. Altogether, increased PMN death during SIV infection is a new pathogenic effect associated with AIDS progression, adding to the long list of markers associated with disruption of defense against infection.

Death of CD4+ T cells from lymph nodes during primary SIVmac251 infection predicts the rate of AIDS progression

Immunological and virological events that occur during the earliest stages of SIV infection are now considered to have a major impact on subsequent disease progression. In the present study, we demonstrate a clear correlation between progression to AIDS and the rate of in vitro CD4+ (but not CD8+) T cell death in lymph nodes. The dying CD4+ T cells were effector memory T cells, which are critical for the immune response to pathogens. However, there was no correlation between the rate of the viral replication within lymph nodes and the extent of Fas ligand-mediated death, despite the increased sensitivity of CD4+ T cells to death in response to recombinant human Fas ligand. CD4+ T cell death was caspase and apoptosis-inducing factor independent but was clearly associated with mitochondrion damage. Interestingly, higher expression levels of the active form of Bak, a proapoptotic molecule involved in mitochondrial membrane permeabilization, were observed in SIV-infected macaques progressing more rapidly to AIDS. Finally, we demonstrated that the strain of SIV we used requires CCR5 and BOB/GRP15 molecules as coreceptors and caused death of unstimulated noncycling primary CD4+ T cells. Altogether, these results demonstrate that CD4+ T cell death occurring early after SIV infection is a crucial determinant of progression to AIDS and that it is mediated by the intrinsic death pathway.

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.

Long lasting control and lack of pathogenicity of the attenuated Rev-independent SIV in rhesus macaques

A cohort of 22 rhesus macaques of Indian origin infected as neonates, juveniles, or adults by Rev-independent strains of SIV was monitored over several years. After the initial acute phase, virus replication was controlled and plasma virus loads were persistently below the threshold of the assay. The animals were monitored for up to 7.6 years after infection for viral loads, cellular and humoral immune responses, hematological changes, and overall health and no signs of immune dysfunction or AIDS were observed. This study represents several years of additional observation compared to the previously published results, and indicates that the Rev-independent SIV clones tested do not cause AIDS-like progressive disease within 7.6 years from infection. All the animals showed persistent humoral and cellular SIV-specific immune responses, consistent with chronic infection. Different Rev-independent SIV strains showed similar properties and lack of pathogenicity. Multicolor flow cytometric analysis demonstrated preservation of the Central Memory subset of T cells in the attenuated SIV-infected animals. This study demonstrates a potent, long-lasting control of the Rev-independent attenuated SIV in macaques independent of the age at virus exposure.

IL-7 induces immunological improvement in SIV-infected rhesus macaques under antiviral therapy

Despite efficient antiretroviral therapy (ART), CD4+ T cell counts often remain low in HIV-1-infected patients. This has led to IL-7, a crucial cytokine involved in both thymopoiesis and peripheral T cell homeostasis, being suggested as an additional therapeutic strategy. We investigated whether recombinant simian IL-7-treatment enhanced the T cell renewal initiated by ART in rhesus macaques chronically infected with SIVmac251. Six macaques in the early chronic phase of SIV infection received antiretroviral treatment. Four macaques also received a 3-wk course of IL-7 injections. Viral load was unaffected by IL-7 treatment. IL-7 treatment increased the number of circulating CD4+ and CD8+ memory T cells expressing activation (HLA-DR+, CD25+) and proliferation (Ki-67+) markers. It also increased naive (CD45RAbrightCD62L+) T cell counts by peripheral proliferation and enhanced de novo thymic production. The studied parameters returned to pretreatment values by day 29 after the initiation of treatment, concomitantly to the appearance of anti-IL-7 neutralizing Abs, supporting the need for a nonimmunogenic molecule for human treatment. Thus, IL-7, which increases T cell memory and de novo renewal of naive T cells may have additional benefits in HIV-infected patients receiving ART.