CD4+ memory stem cells are infected by HIV-1 in a manner regulated in part by SAMHD1 expression

Molecular mechanisms by which the HIV-1 latent reservoir is established and therapeutic strategies for its elimination

The human immunodeficiency virus type 1 (HIV-1) reservoir, composed of cells harboring the latent, integrated virus, is not eliminated by antiretroviral therapy. It therefore represents a significant barrier to curing the infection. The biology of HIV-1 reservoirs, the mechanisms of their persistence, and effective strategies for their eradication are not entirely understood. Here, we review the molecular mechanisms by which HIV-1 reservoirs develop, the cells and compartments where the latent virus resides, and advancements in curative therapeutic strategies. We first introduce statistics and relevant data on HIV-1 infection, aspects of pathogenesis, the role of antiretroviral therapy, and the general features of the latent HIV reservoir. Then, the article is built on three main pillars: The molecular mechanisms related to latency, the different strategies for targeting the reservoir to obtain a cure, and the current progress in immunotherapy to counteract said reservoirs.

Persistence of envelopes in different CD4+ T-cell subsets in antiretroviral therapy-suppressed people with HIV

OBJECTIVES

Despite suppressive antiretroviral therapy (ART), HIV can persist in a diverse range of CD4+ T-cell subsets. Through longitudinal env sampling from people with HIV (PWH) on ART, we characterized the persistence and phenotypic properties of HIV envs over two time-points (T1 and T2).

METHODS

Longitudinal blood and lymphoid tissue samples were obtained from eight PWH on suppressive ART. Single genome amplification (SGA) was performed on env to understand the genetic diversity and degree of clonal expansions over time. A subset of envs were used to generate pseudovirus particles to assess sensitivity to autologous plasma IgG and broadly neutralizing antibodies (bNAbs).

RESULTS

Identical env sequences indicating clonal expansion persisted between T1 and T2 and within multiple T-cell subsets. At both time-points, CXCR4-tropic (X4) Envs were more prevalent in naive and central memory cells; the proportion of X4 Envs did not significantly change in each subset between T1 and T2. Autologous purified plasma IgG showed variable neutralization of Envs, with no significant difference in neutralization between R5 and X4 Envs. X4 Envs were more sensitive to neutralization with clinical bNAbs, with CD4-binding site bNAbs demonstrating high breadth and potency against Envs.

CONCLUSION

Our data suggest the viral reservoir in PWH on ART was predominantly maintained over time through proliferation and potentially differentiation of infected cells. We found the humoral immune response to Envs within the latent reservoir was variable between PWH. Finally, we identified coreceptor usage can influence bNAb sensitivity and may need to be considered for future bNAb immunotherapy approaches.

T-cell exhaustion and stemness in antitumor immunity: Characteristics, mechanisms, and implications

T cells play a crucial role in the regulation of immune response and are integral to the efficacy of cancer immunotherapy. Because immunotherapy has emerged as a promising treatment for cancer, increasing attention has been focused on the differentiation and function of T cells in immune response. In this review, we describe the research progress on T-cell exhaustion and stemness in the field of cancer immunotherapy and summarize advances in potential strategies to intervene and treat chronic infection and cancer by reversing T-cell exhaustion and maintaining and increasing T-cell stemness. Moreover, we discuss therapeutic strategies to overcome T-cell immunodeficiency in the tumor microenvironment and promote continuous breakthroughs in the anticancer activity of T cells.

Stem-like T cells and niches: Implications in human health and disease

Recently, accumulating evidence has elucidated the important role of T cells with stem-like characteristics in long-term maintenance of T cell responses and better patient outcomes after immunotherapy. The fate of TSL cells has been correlated with many physiological and pathological human processes. In this review, we described present advances demonstrating that stem-like T (TSL) cells are central players in human health and disease. We interpreted the evolutionary characteristics, mechanism and functions of TSL cells. Moreover, we discuss the import role of distinct niches and how they affect the stemness of TSL cells. Furthermore, we also outlined currently available strategies to generate TSL cells and associated affecting factors. Moreover, we summarized implication of TSL cells in therapies in two areas: stemness enhancement for vaccines, ICB, and adoptive T cell therapies, and stemness disruption for autoimmune disorders.

The reservoir of latent HIV

The persistence of latent reservoir of the human immunodeficiency virus (HIV) is currently the major challenge in curing HIV infection. After HIV infects the human body, the latent HIV is unable to be recognized by the body’s immune system. Currently, the widely adopted antiretroviral therapy (ART) is also unble to eliminate it, thus hindering the progress of HIV treatment. This review discusses the existence of latent HIV vault for HIV treatment, its formation and factors affecting its formation, cell, and tissue localization, methods for detection and removing latent reservoir, to provide a comprehensive understanding of latent HIV vault, in order to assist in the future research and play a potential role in achieving HIV treatment.

Effective Treatment of Patients Experiencing Primary, Acute HIV Infection Decreases Exhausted/Activated CD4+ T Cells and CD8+ T Memory Stem Cells

Several studies have identified main changes in T- and B-lymphocyte subsets during chronic HIV infection, but few data exist on how these subsets behave during the initial phase of HIV infection. We enrolled 22 HIV-infected patients during the acute stage of infection before the initiation of antiretroviral therapy (ART). Patients had blood samples drawn previous to ART initiation (T0), and at 2 (T1) and 12 (T2) months after ART initiation. We quantified cellular HIV-DNA content in sorted naïve and effector memory CD4 T cells and identified the main subsets of T- and B-lymphocytes using an 18-parameter flow cytometry panel. We identified correlations between the patients’ clinical and immunological data using PCA. Effective HIV treatment reduces integrated HIV DNA in effector memory T cells after 12 months (T2) of ART. The main changes in CD4+ T cells occurred at T2, with a reduction of activated memory, cytolytic and activated/exhausted stem cell memory T (T_SCM) cells. Changes were present among CD8+ T cells since T1, with a reduction of several activated subsets, including activated/exhausted T_SCM. At T2 a reduction of plasmablasts and exhausted B cells was also observed. A negative correlation was found between the total CD4+ T-cell count and IgM-negative plasmablasts. In patients initiating ART immediately following acute/early HIV infection, the fine analysis of T- and B-cell subsets has allowed us to identify and follow main modifications due to effective treatment, and to identify significant changes in CD4+ and CD8+ T memory stem cells.

Development of an HIV reporter virus that identifies latently infected CD4+ T cells

There is no cure for HIV infection, as the virus establishes a latent reservoir, which escapes highly active antiretroviral treatments. One major obstacle is the difficulty identifying cells that harbor latent proviruses. We devised a single-round viral vector that carries a series of versatile reporter molecules that are expressed in an LTR-dependent or LTR-independent manner and make it possible to accurately distinguish productive from latent infection. Using primary human CD4+ T cells, we show that transcriptionally silent proviruses are found in more than 50% of infected cells. The latently infected cells harbor proviruses but lack evidence for multiple spliced transcripts. LTR-silent integrations occurred to variable degrees in all CD4+ T subsets examined, with CD4+ TEM and CD4+ TREG displaying the highest frequency of latent infections. This viral vector permits the interrogation of HIV latency at single-cell resolution, revealing mechanisms of latency establishment and allowing the characterization of effective latency-reversing agents.

Phenotypic and Immunometabolic Aspects on Stem Cell Memory and Resident Memory CD8+ T Cells

The immune system, smartly and surprisingly, saves the exposure of a particular pathogen in its memory and reacts to the pathogen very rapidly, preventing serious diseases.

Immunologists have long been fascinated by understanding the ability to recall and respond faster and more vigorously to a pathogen, known as “memory”.

T-cell populations can be better described by using more sophisticated techniques to define phenotype, transcriptional and epigenetic signatures and metabolic pathways (single-cell resolution), which uncovered the heterogeneity of the memory T-compartment. Phenotype, effector functions, maintenance, and metabolic pathways help identify these different subsets. Here, we examine recent developments in the characterization of the heterogeneity of the memory T cell compartment. In particular, we focus on the emerging role of CD8+ TRM and TSCM cells, providing evidence on how their immunometabolism or modulation can play a vital role in their generation and maintenance in chronic conditions such as infections or autoimmune diseases.

Heterogeneity of Latency Establishment in the Different Human CD4 + T Cell Subsets Stimulated with IL-15

HIV integrates into the host genome, creating a viral reservoir of latently infected cells that persists despite effective antiretroviral treatment. CD4-positive (CD4⁺) T cells are the main contributors to the HIV reservoir. CD4⁺ T cells are a heterogeneous population, and the mechanisms of latency establishment in the different subsets, as well as their contribution to the reservoir, are still unclear. In this study, we analyzed HIV latency establishment in different CD4⁺ T cell subsets stimulated with interleukin 15 (IL-15), a cytokine that increases both susceptibility to infection and reactivation from latency. Using a dual-reporter virus that allows discrimination between latent and productive infection at the single-cell level, we found that IL-15-treated primary human CD4⁺ T naive and CD4⁺ T stem cell memory (T_SCM) cells are less susceptible to HIV infection than CD4⁺ central memory (T_CM), effector memory (T_EM), and transitional memory (T_TM) cells but are also more likely to harbor transcriptionally silent provirus. The propensity of these subsets to harbor latent provirus compared to the more differentiated memory subsets was independent of differential expression of pTEFb components. Microscopy analysis of NF-κB suggested that CD4⁺ T naive cells express smaller amounts of nuclear NF-κB than the other subsets, partially explaining the inefficient long terminal repeat (LTR)-driven transcription. On the other hand, CD4⁺ T_SCM cells display similar levels of nuclear NF-κB to CD4⁺ T_CM, CD4⁺ T_EM, and CD4⁺ T_TM cells, indicating the availability of transcription initiation and elongation factors is not solely responsible for the inefficient HIV gene expression in the CD4⁺ T_SCM subset.

IMPORTANCE The formation of a latent reservoir is the main barrier to HIV cure. Here, we investigated how HIV latency is established in different CD4⁺ T cell subsets in the presence of IL-15, a cytokine that has been shown to efficiently induce latency reversal. We observed that, even in the presence of IL-15, the less differentiated subsets display lower levels of productive HIV infection than the more differentiated subsets. These differences were not related to different expression of pTEFb, and modest differences in NF-κB were observed for CD4⁺ T naive cells only, implying the involvement of other mechanisms. Understanding the molecular basis of latency establishment in different CD4⁺ T cell subsets might be important for tailoring specific strategies to reactivate HIV transcription in all the CD4⁺ T subsets that compose the latent reservoir.

Disruption of HIV-1 co-receptors CCR5 and CXCR4 in primary human T cells and hematopoietic stem and progenitor cells using base editing

Disruption of CCR5 or CXCR4, the main human immunodeficiency virus type 1 (HIV-1) co-receptors, has been shown to protect primary human CD4⁺ T cells from HIV-1 infection. Base editing can install targeted point mutations in cellular genomes, and can thus efficiently inactivate genes by introducing stop codons or eliminating start codons without double-stranded DNA break formation. Here, we applied base editors for individual and simultaneous disruption of both co-receptors in primary human CD4⁺ T cells. Using cytosine base editors we observed premature stop codon introduction in up to 89% of sequenced CCR5 or CXCR4 alleles. Using adenine base editors we eliminated the start codon in CCR5 in up to 95% of primary human CD4⁺ T cell and up to 88% of CD34⁺ hematopoietic stem and progenitor cell target alleles. Genome-wide specificity analysis revealed low numbers of off-target mutations that were introduced by base editing, located predominantly in intergenic or intronic regions. We show that our editing strategies prevent transduction with CCR5-tropic and CXCR4-tropic viral vectors in up to 79% and 88% of human CD4⁺ T cells, respectively. The engineered T cells maintained functionality and overall our results demonstrate the effectiveness of base-editing strategies for efficient and specific ablation of HIV co-receptors in clinically relevant cell types.

Stem cell-like memory T cells: A perspective from the dark side

Much attention has been paid to a newly discovered subset of memory T (T_M) cells-stem cell-like memory T (T_SCM) cells for their high self-renewal ability, multi-differentiation potential and long-term effector function in adoptive therapy against tumors. Despite their application in cancer therapy, an excess of T_SCM cells also contributes to the persistence of autoimmune diseases for their immune memory and HIV infection as a long-lived HIV reservoir. Signaling pathways Wnt, AMPK/mTOR and NF-κB are key determinants for T_M cell generation, maintenance and proinflammatory effect. In this review, we focus on the phenotypic and functional characteristics of T_SCM cells and discuss their role in autoimmune diseases and HIV-1 chronic infection. Also, we explore the potential mechanism and signaling pathways involved in immune memory and look into the future therapy strategies of targeting long-lived T_M cells to suppress pathogenic immune memory.

Persistence of an intact HIV reservoir in phenotypically naive T cells

Despite the efficacy of antiretroviral therapy (ART), HIV persists in a latent form and remains a hurdle to eradication. CD4+ T lymphocytes harbor the majority of the HIV reservoir, but the role of individual subsets remains unclear. CD4+ T cells were sorted into central, transitional, effector memory, and naive T cells. We measured HIV DNA and performed proviral sequencing of more than 1900 proviruses in 2 subjects at 2 and 9 years after ART initiation to estimate the contribution of each subset to the reservoir. Although our study was limited to 2 subjects, we obtained comparable findings with publicly available sequences. While the HIV integration levels were lower in naive compared with memory T cells, naive cells were a major contributor to the intact proviral reservoir. Notably, proviral sequences isolated from naive cells appeared to be unique, while those retrieved from effector memory cells were mainly clonal. The number of clones increased as cells differentiated from a naive to an effector memory phenotype, suggesting naive cells repopulate the effector memory reservoir as previously shown for central memory cells. Naive T cells contribute substantially to the intact HIV reservoir and represent a significant hurdle for HIV eradication.

Naive CD4+ T Cells Harbor a Large Inducible Reservoir of Latent, Replication-competent Human Immunodeficiency Virus Type 1

BACKGROUND

The latent human immunodeficiency virus type 1 (HIV-1) reservoir represents a major barrier to a cure. Based on the levels of HIV-1 DNA in naive (TN) vs resting memory CD4+ T cells, it is widely hypothesized that this reservoir resides primarily within memory cells. Here, we compared virus production from TN and central memory (TCM) CD4+ T cells isolated from HIV-1-infected individuals on suppressive therapy.

METHODS

CD4+ TN and TCM cells were purified from the blood of 7 HIV-1-infected individuals. We quantified total HIV-1 DNA in the CD4+ TN and TCM cells. Extracellular virion-associated HIV-1 RNA or viral outgrowth assays were used to assess latency reversal following treatment with anti-CD3/CD28 monoclonal antibodies (mAbs), phytohaemagglutinin/interleukin-2, phorbol 12-myristate 13-acetate/ionomycin, prostratin, panobinostat, or romidepsin.

RESULTS

HIV-1 DNA was significantly higher in TCM compared to TN cells (2179 vs 684 copies/106 cells, respectively). Following exposure to anti-CD3/CD28 mAbs, virion-associated HIV-1 RNA levels were similar between TCM and TN cells (15 135 vs 18 290 copies/mL, respectively). In 4/7 donors, virus production was higher for TN cells independent of the latency reversing agent used. Replication-competent virus was recovered from both TN and TCM cells.

CONCLUSIONS

Although the frequency of HIV-1 infection is lower in TN compared to TCM cells, as much virus is produced from the TN population after latency reversal. This finding suggests that quantifying HIV-1 DNA alone may not predict the size of the inducible latent reservoir and that TN cells may be an important reservoir of latent HIV-1.

Longitudinal analysis of subtype C envelope tropism for memory CD4+ T cell subsets over the first 3 years of untreated HIV-1 infection

Background

HIV-1 infects a wide range of CD4⁺ T cells with different phenotypic properties and differing expression levels of entry coreceptors. We sought to determine the viral tropism of subtype C (C-HIV) Envelope (Env) clones for different CD4⁺ T cell subsets and whether tropism changes during acute to chronic disease progression. HIV-1 envs were amplified from the plasma of five C-HIV infected women from three untreated time points; less than 2 months, 1-year and 3-years post-infection. Pseudoviruses were generated from Env clones, phenotyped for coreceptor usage and CD4⁺ T cell subset tropism was measured by flow cytometry.

Results

A total of 50 C-HIV envs were cloned and screened for functionality in pseudovirus infection assays. Phylogenetic and variable region characteristic analysis demonstrated evolution in envs between time points. We found 45 pseudoviruses were functional and all used CCR5 to mediate entry into NP2/CD4/CCR5 cells. In vitro infection assays showed transitional memory (TM) and effector memory (EM) CD4⁺ T cells were more frequently infected (median: 46% and 25% of total infected CD4⁺ T cells respectively) than naïve, stem cell memory, central memory and terminally differentiated cells. This was not due to these subsets contributing a higher proportion of the CD4⁺ T cell pool, rather these subsets were more susceptible to infection (median: 5.38% EM and 2.15% TM cells infected), consistent with heightened CCR5 expression on EM and TM cells. No inter- or intra-participant changes in CD4⁺ T cell subset tropism were observed across the three-time points.

Conclusions

CD4⁺ T cell subsets that express more CCR5 were more susceptible to infection with C-HIV Envs, suggesting that these may be the major cellular targets during the first 3 years of infection. Moreover, we found that viral tropism for different CD4⁺ T cell subsets in vitro did not change between Envs cloned from acute to chronic disease stages. Finally, central memory, naïve and stem cell memory CD4⁺ T cell subsets were susceptible to infection, albeit inefficiently by Envs from all time-points, suggesting that direct infection of these cells may help establish the latent reservoir early in infection.

Preferential Homing of Tumor-specific and Functional CD8+ Stem Cell-like Memory T Cells to the Bone Marrow

Supplemental Digital Content is available in the text.

The bone marrow (BM) harbors not only hematopoietic stem cells but also conventional memory T and B cells. Studies of BM-resident memory T cells have revealed the complex relationship between BM and immunologic memory. In the present study, we identified CD122^high stem cells antigen-1 (Sca-1)^high, B-cell lymphoma protein-2 (Bcl-2)^high, CD8⁺ stem cell-like memory T cells (T_SCMs) as a distinct memory T-cell subset preferentially residing in the BM, where these cells respond vigorously to blood-borne antigens. We found that the most T_SCMs favorably relocate to the BM by adhesion molecules such as vascular cell adhesion protein 1, P-selectin glycoprotein 1, and P-selectin or E-selectin. Moreover, the BM-resident T_SCMs exhibited much higher levels of antitumor activity than the spleen-resident T_SCMs. These results indicate that the BM provides an appropriate microenvironment for the survival of CD8⁺ T_SCMs, thereby broadening our knowledge of the memory maintenance of antigen-specific CD8⁺ T lymphocytes. The present findings are expected to be instructive for the development of tumor immunotherapy.

Reduce and Control: A Combinatorial Strategy for Achieving Sustained HIV Remissions in the Absence of Antiretroviral Therapy

Human immunodeficiency virus (HIV-1) indefinitely persists, despite effective antiretroviral therapy (ART), within a small pool of latently infected cells. These cells often display markers of immunologic memory and harbor both replication-competent and -incompetent proviruses at approximately a 1:100 ratio. Although complete HIV eradication is a highly desirable goal, this likely represents a bridge too far for our current and foreseeable technologies. A more tractable goal involves engineering a sustained viral remission in the absence of ART––a “functional cure.” In this setting, HIV remains detectable during remission, but the size of the reservoir is small and the residual virus is effectively controlled by an engineered immune response or other intervention. Biological precedence for such an approach is found in the post-treatment controllers (PTCs), a rare group of HIV-infected individuals who, following ART withdrawal, do not experience viral rebound. PTCs are characterized by a small reservoir, greatly reduced inflammation, and the presence of a poorly understood immune response that limits viral rebound. Our goal is to devise a safe and effective means for replicating durable post-treatment control on a global scale. This requires devising methods to reduce the size of the reservoir and to control replication of this residual virus. In the following sections, we will review many of the approaches and tools that likely will be important for implementing such a “reduce and control” strategy and for achieving a PTC-like sustained HIV remission in the absence of ART.

Discoveries and developments of CXCR4-targeted HIV-1 entry inhibitors

The chemokine receptor CXCR4 is required for the entry of human immunodeficiency virus type 1 (HIV-1) into target cells and its expression correlates with more profound pathogenicity, rapid progression to acquired immunodeficiency syndrome (AIDS), and greater AIDS-related mortality. There is still no cure for AIDS and no method for preventing or eradicating HIV-1 infection. HIV-1 entry begins with the interaction of the viral envelope glycoprotein gp120 and the primary receptor CD4, and subsequently with the coreceptors, CCR5 or CXCR4, on the host cells. Blocking the interaction of HIV-1 and its coreceptors is therefore a promising strategy for developing new HIV-1 entry inhibitors. This approach has a dual benefit, as it prevents HIV-1 infection and progression while also targeting the reservoirs of HIV-1 infected, coreceptor positive macrophages and memory T cells. To date, multiple classes of CXCR4-targeted anti-HIV-1 inhibitors have been discovered and are now at different preclinical and clinical stages. In this review, we highlight the studies of CXCR4-targeted small-molecule and peptide HIV-1 entry inhibitors discovered during the last two decades and provide a reference for further potential HIV-1 exploration in the future.

Impact statement

This minireview summarized the current progress in the identification of CXCR4-targeted HIV-1-entry inhibitors based on discovery/developmental approaches. It also provided a discussion of the inhibitor structural features, antiviral activities, and pharmacological properties. Unlike other reviews on anti-HIV-1 drug development, which have generally emphasized inhibitors that target intracellular viral replication and host genomic integration, this review focused on the drug discovery approaches taken to develop viral-entry inhibitors aimed at disturbing the initial step of viral interaction with uninfected host cells and preventing the subsequent viral replication/genomic integration. This review amalgamated recently published and important work on bivalent CXCR4-targeted anti-HIV-1-entry candidates/conjugates, discussed the research challenges faced in developing drugs to prevent and eradicate HIV-1 infection, and provided a perspective on strategies that can lead to future drug discoveries. The findings and strategies summarized in this review will be of interest to investigators throughout the microbiological, pharmaceutical, and translational research communities.

HIV-1-Infected CD4+ T Cells Facilitate Latent Infection of Resting CD4+ T Cells through Cell-Cell Contact

HIV-1 is transmitted between T cells through the release of cell-free particles and through cell-cell contact. Cell-to-cell transmission is more efficient than cell-free virus transmission, mediates resistance to immune responses, and facilitates the spread of virus among T cells. However, whether HIV cell-to-cell transmission influences the establishment of HIV-1 latency has not been carefully explored. We developed an HIV-1 latency model based on the transmission of HIV-1 directly to resting CD4+ T cells by cell-cell contact. This model recapitulates the spread of HIV-1 in T-cell-dense anatomical compartments. We demonstrate that productively infected activated CD4+ T cells transmit HIV-1 to resting CD4+ T cells in a cell-contact-dependent manner. However, proviruses generated in this fashion are more difficult to induce compared to proviruses generated by cell-free infection, suggesting that cell-to-cell transmission influences the establishment and maintenance of latent infection in resting CD4+ T cells.

Expansion of Stem Cell-Like CD4+ Memory T Cells during Acute HIV-1 Infection Is Linked to Rapid Disease Progression

Acute HIV-1 infection is characterized by high viremia and massive depletion of CD4⁺ T cells throughout all tissue compartments. During this time the latent viral reservoir is established but the dynamics of memory CD4⁺ T cell subset development, their infectability and influence on disease progression during acute HIV-1 infection has not been carefully described. We therefore investigated the dynamics of CD4⁺ T cell memory populations in the RV217 (ECHO) cohort during the acute phase of infection. Interestingly, while we found only small changes in central or effector memory compartments, we observed a profound expansion of stem cell-like memory CD4⁺ T cells (SCM) (2.7-fold; P < 0.0001). Furthermore, we demonstrated that the HIV-1 integration and replication preferentially take place in highly differentiated CD4⁺ T cells such as transitional memory (TM) and effector memory (EM) CD4⁺ T cells, while naive and less mature memory cells prove to be more resistant. Despite the relatively low frequency of productively infected SCM, we suggest that their quiescent phenotype, increased susceptibility to HIV-1 integration compared to naive cells and extensive expansion make them one of the key players in establishment and persistence of the HIV-1 reservoir. Moreover, the expansion of SCM in acute HIV-1 infection was a result of Fas upregulation on the surface of naive CD4⁺ T cells. Interestingly, the upregulation of Fas receptor and expansion of SCM in acute HIV-1 infection was associated with the early viral set point and disease progression (rho = 0.47, P = 0.02, and rho = 0.42, P = 0.041, respectively). Taken together, our data demonstrate an expansion of SCM during early acute HIV-1 infection which is associated with disease outcome.IMPORTANCE Understanding the immunopathology of acute HIV-1 infection will help to develop eradication strategies. We demonstrate here that a CD4⁺ T cell memory subset expands during acute HIV-1 infection, which is associated with disease progression.

Mechanisms of HIV-1 cell-to-cell transmission and the establishment of the latent reservoir

HIV-1 spreads through both the release of cell-free particles and by cell-to-cell transmission. Mounting evidence indicates that cell-to-cell transmission is more efficient than cell-free transmission of particles and likely influences the pathogenesis of HIV-1 infection. This mode of viral transmission also influences the generation and maintenance of the latent reservoir, which represents the main obstacle for curing the infection. In this review we will discuss general cell contact-dependent mechanisms that HIV-1 utilizes for its spread and the evidence pointing to cell-to-cell transmission as a mechanism for the establishment and maintenance of latent infection.

Cellular Metabolism Is a Major Determinant of HIV-1 Reservoir Seeding in CD4+ T Cells and Offers an Opportunity to Tackle Infection

HIV persists in long-lived infected cells that are not affected by antiretroviral treatment. These HIV reservoirs are mainly located in CD4⁺ T cells, but their distribution is variable in the different subsets. Susceptibility to HIV-1 increases with CD4⁺ T cell differentiation. We evaluated whether the metabolic programming that supports the differentiation and function of CD4⁺ T cells affected their susceptibility to HIV-1. We found that differences in HIV-1 susceptibility between naive and more differentiated subsets were associated with the metabolic activity of the cells. Indeed, HIV-1 selectively infected CD4⁺ T cells with high oxidative phosphorylation and glycolysis, independent of their activation phenotype. Moreover, partial inhibition of glycolysis (1) impaired HIV-1 infection in vitro in all CD4⁺ T cell subsets, (2) decreased the viability of preinfected cells, and (3) precluded HIV-1 amplification in cells from HIV-infected individuals. Our results elucidate the link between cell metabolism and HIV-1 infection and identify a vulnerability in tackling HIV reservoirs.

The role of integration and clonal expansion in HIV infection: live long and prosper

Integration of viral DNA into the host genome is a central event in the replication cycle and the pathogenesis of retroviruses, including HIV. Although most cells infected with HIV are rapidly eliminated in vivo, HIV also infects long-lived cells that persist during combination antiretroviral therapy (cART). Cells with replication competent HIV proviruses form a reservoir that persists despite cART and such reservoirs are at the center of efforts to eradicate or control infection without cART. The mechanisms of persistence of these chronically infected long-lived cells is uncertain, but recent research has demonstrated that the presence of the HIV provirus has enduring effects on infected cells. Cells with integrated proviruses may persist for many years, undergo clonal expansion, and produce replication competent HIV. Even proviruses with defective genomes can produce HIV RNA and may contribute to ongoing HIV pathogenesis. New analyses of HIV infected cells suggest that over time on cART, there is a shift in the composition of the population of HIV infected cells, with the infected cells that persist over prolonged periods having proviruses integrated in genes associated with regulation of cell growth. In several cases, strong evidence indicates the presence of the provirus in specific genes may determine persistence, proliferation, or both. These data have raised the intriguing possibility that after cART is introduced, a selection process enriches for cells with proviruses integrated in genes associated with cell growth regulation. The dynamic nature of populations of cells infected with HIV during cART is not well understood, but is likely to have a profound influence on the composition of the HIV reservoir with critical consequences for HIV eradication and control strategies. As such, integration studies will shed light on understanding viral persistence and inform eradication and control strategies. Here we review the process of HIV integration, the role that integration plays in persistence, clonal expansion of the HIV reservoir, and highlight current challenges and outstanding questions for future research.

SAMHD1 Suppression of Antiviral Immune Responses

SAMHD1 is a host triphosphohydrolase that degrades intracellular deoxynucleoside triphosphates (dNTPs) to a lower level that restricts viral DNA synthesis, and thus prevents replication of diverse viruses in nondividing cells. Recent progress indicates that SAMHD1 negatively regulates antiviral innate immune responses and inflammation through interacting with various key proteins in immune signaling and DNA damage-repair pathways. SAMHD1 can also modulate antibody production in adaptive immune responses. In this review, we summarize how SAMHD1 regulates antiviral immune responses through distinct mechanisms, and discuss the implications of these new functions of SAMHD1. Furthermore, we propose important new questions and future directions that can advance functional and mechanistic studies of SAMHD1-mediated immune regulation during viral infections.

IL-15 regulates susceptibility of CD4+ T cells to HIV infection

HIV integrates into the host genome to create a persistent viral reservoir. Stimulation of CD4⁺ memory T lymphocytes with common γc-chain cytokines renders these cells more susceptible to HIV infection, making them a key component of the reservoir itself. IL-15 is up-regulated during primary HIV infection, a time when the HIV reservoir established. Therefore, we investigated the molecular and cellular impact of IL-15 on CD4⁺ T-cell infection. We found that IL-15 stimulation induces SAM domain and HD domain-containing protein 1 (SAMHD1) phosphorylation due to cell cycle entry, relieving an early block to infection. Perturbation of the pathways downstream of IL-15 receptor (IL-15R) indicated that SAMHD1 phosphorylation after IL-15 stimulation is JAK dependent. Treating CD4⁺ T cells with Ruxolitinib, an inhibitor of JAK1 and JAK2, effectively blocked IL-15-induced SAMHD1 phosphorylation and protected CD4⁺ T cells from HIV infection. Using high-resolution single-cell immune profiling using mass cytometry by TOF (CyTOF), we found that IL-15 stimulation altered the composition of CD4⁺ T-cell memory populations by increasing proliferation of memory CD4⁺ T cells, including CD4⁺ T memory stem cells (T_SCM). IL-15-stimulated CD4⁺ T_SCM, harboring phosphorylated SAMHD1, were preferentially infected. We propose that IL-15 plays a pivotal role in creating a self-renewing, persistent HIV reservoir by facilitating infection of CD4⁺ T cells with stem cell-like properties. Time-limited interventions with JAK1 inhibitors, such as Ruxolitinib, should prevent the inactivation of the endogenous restriction factor SAMHD1 and protect this long-lived CD4⁺ T-memory cell population from HIV infection.

CD4+ T Cell Subsets and Pathways to HIV Latency

Latent infection of CD4⁺ T cells is the main barrier to eradicating HIV-1 infection from infected patients. The cellular and molecular mechanisms involved in the establishment and maintenance of latent infection are directly linked to the transcriptional program of the different CD4⁺ T cell subsets targeted by the virus. In this review, we provide an overview of how T cell activation, T cell differentiation into functional subsets, and the mode of initial viral infection influence HIV proviral transcription and entry into latency.

Peering into the HIV reservoir

The main obstacle to HIV eradication is the establishment of a long-term persistent HIV reservoir. Although several therapeutic approaches have been developed to reduce and eventually eliminate the HIV reservoir, only a few have achieved promising results. A better knowledge of the mechanisms involved in the establishment and maintenance of HIV reservoir is of utmost relevance for the design of new therapeutic strategies aimed at purging it with the ultimate goal of achieving HIV eradication or alternatively a functional cure. In this regard, it is also important to take a close look into the cellular HIV reservoirs other than resting memory CD4 T-cells with key roles in reservoir maintenance that have been recently described. Unraveling the special characteristics of these HIV cellular compartments could aid us in designing new therapeutic strategies to deplete the latent HIV reservoir.

Measuring integrated HIV DNA ex vivo and in vitro provides insights about how reservoirs are formed and maintained

The identification of the most appropriate marker to measure reservoir size has been a great challenge for the HIV field. Quantitative viral outgrowth assay (QVOA), the reference standard to quantify the amount of replication-competent virus, has several limitations, as it is laborious, expensive, and unable to robustly reactivate every single integrated provirus. PCR-based assays have been developed as an easier, cheaper and less error-prone alternative to QVOA, but also have limitations. Historically, measuring integrated HIV DNA has provided insights about how reservoirs are formed and maintained. In the 1990s, measuring integrated HIV DNA was instrumental in understanding that a subset of resting CD4 T cells containing integrated HIV DNA were the major source of replication-competent virus. Follow-up studies have further characterized the phenotype of these cells containing integrated HIV DNA, as well as shown the correlation between the integration levels and clinical parameters, such as duration of infection, CD4 count and viral load. Integrated HIV DNA correlates with total HIV measures and with QVOA. The integration assay has several limitations. First, it largely overestimates the reservoir size, as both defective and replication-competent proviruses are detected. Since defective proviruses are the majority in patients on ART, it follows that the number of proviruses capable of reactivating and releasing new virions is significantly smaller than the number of integrated proviruses. Second, in patients on ART clonal expansion could theoretically lead to the preferential amplification of proviruses close to an Alu sequence though longitudinal studies have not captured this effect. Proviral sequencing combined with integration measures is probably the best estimate of reservoir size, but it is expensive, time-consuming and requires considerable bioinformatics expertise. All these reasons limit its use on a large scale. Herein, we review the utility of measuring HIV integration and suggest combining it with sequencing and total HIV measurements can provide insights that underlie reservoir maintenance.

Differentiating Immune Cell Targets in Gut-Associated Lymphoid Tissue for HIV Cure

The single greatest challenge to an HIV cure is the persistence of latently infected cells containing inducible, replication-competent proviral genomes, which constitute only a small fraction of total or infected cells in the body. Although resting CD4+ T cells in the blood are a well-known source of viral rebound, more than 90% of the body's lymphocytes reside elsewhere. Many are in gut tissue, where HIV DNA levels per million CD4+ T cells are considerably higher than in the blood. Despite the significant contribution of gut tissue to viral replication and persistence, little is known about the cell types that support persistence of HIV in the gut; importantly, T cells in the gut have phenotypic, functional, and survival properties that are distinct from T cells in other tissues. The mechanisms by which latency is established and maintained will likely depend on the location and cytokine milieu surrounding the latently infected cells in each compartment. Therefore, successful HIV cure strategies require identification and characterization of the exact cell types that support viral persistence, particularly in the gut. In this review, we describe the seeding of the latent HIV reservoir in the gut mucosa; highlight the evidence for compartmentalization and depletion of T cells; summarize the immunologic consequences of HIV infection within the gut milieu; propose how the damaged gut environment may promote the latent HIV reservoir; and explore several immune cell targets in the gut and their place on the path toward HIV cure.

CD4+ T Memory Stem Cells Correlate with Disease Progression in Chronically HIV-1-Infected Patients

Recently identified T memory stem (Tscm) cells have stem-cell-like properties, including long lifespan, self-renewal capacity, and multipotency to differentiate into other memory T cell types. In the study of simian immunodeficiency virus (SIV) infection, selective depletion of CCR5⁺CD4⁺ Tscm cells and the high proliferation rate of these cells are believed to be responsible for the pathogenesis of SIV-infected rhesus macaques. Here, we conducted a cohort study to investigate the influence of chronic human immunodeficiency virus (HIV)-1 infection on CD4⁺ Tscm cell homeostasis, and the effect of antiretroviral therapy (ART) on CD4⁺ Tscm cells. Chronic HIV-1 infection resulted in a decrease of the CD4⁺ Tscm cell proportion in HIV-1 patients. The decreased number of CD4⁺ Tscm cells in HIV-1 patients correlated positively with that of circulating CD4⁺ T cells. Further, the depletion of CD4⁺ Tscm cells was inversely correlated with an increased level of T cell immune activation during chronic HIV-1 infection. Prolonged ART recovered the CD4⁺ Tscm cells, and the dynamic change of CD4⁺ Tscm cells was in parallel with CD4⁺ T cell restoration and a decrease in the level of T cell immune activation. We propose that the abnormity of CD4⁺ Tscm cells may contribute to the pathogenesis and disease progression in HIV-1-infected individuals.

T memory stem cells in health and disease

T memory stem (T_SCM) cells are a rare subset of memory lymphocytes endowed with the stem cell-like ability to self-renew and the multipotent capacity to reconstitute the entire spectrum of memory and effector T cell subsets. Cumulative evidence in mice, nonhuman primates and humans indicates that T_SCM cells are minimally differentiated cells at the apex of the hierarchical system of memory T lymphocytes. Here we describe emerging findings demonstrating that T_SCM cells, owing to their extreme longevity and robust potential for immune reconstitution, are central players in many physiological and pathological human processes. We also discuss how T_SCM cell stemness could be leveraged therapeutically to enhance the efficacy of vaccines and adoptive T cell therapies for cancer and infectious diseases or, conversely, how it could be disrupted to treat T_SCM cell driven and sustained diseases, such as autoimmunity, adult T cell leukemia and HIV-1.

HIV signaling through CD4 and CCR5 activates Rho family GTPases that are required for optimal infection of primary CD4+ T cells

Background

HIV-1 hijacks host cell machinery to ensure successful replication, including cytoskeletal components for intracellular trafficking, nucleoproteins for pre-integration complex import, and the ESCRT pathway for assembly and budding. It is widely appreciated that cellular post-translational modifications (PTMs) regulate protein activity within cells; however, little is known about how PTMs influence HIV replication. Previously, we reported that blocking deacetylation of tubulin using histone deacetylase inhibitors promoted the kinetics and efficiency of early post-entry viral events. To uncover additional PTMs that modulate entry and early post-entry stages in HIV infection, we employed a flow cytometric approach to assess a panel of small molecule inhibitors on viral fusion and LTR promoter-driven gene expression.

Results

While viral fusion was not significantly affected, early post-entry viral events were modulated by drugs targeting multiple processes including histone deacetylation, methylation, and bromodomain inhibition. Most notably, we observed that inhibitors of the Rho GTPase family of cytoskeletal regulators—including RhoA, Cdc42, and Rho-associated kinase signaling pathways—significantly reduced viral infection. Using phosphoproteomics and a biochemical GTPase activation assay, we found that virion-induced signaling via CD4 and CCR5 activated Rho family GTPases including Rac1 and Cdc42 and led to widespread modification of GTPase signaling-associated factors.

Conclusions

Together, these data demonstrate that HIV signaling activates members of the Rho GTPase family of cytoskeletal regulators that are required for optimal HIV infection of primary CD4+ T cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-017-0328-7) contains supplementary material, which is available to authorized users.

Establishment and Reversal of HIV-1 Latency in Naive and Central Memory CD4+ T Cells In Vitro

The latent HIV-1 reservoir primarily resides in resting CD4(+) T cells which are a heterogeneous population composed of both naive (TN) and memory cells. In HIV-1-infected individuals, viral DNA has been detected in both naive and memory CD4(+) T cell subsets although the frequency of HIV-1 DNA is typically higher in memory cells, particularly in the central memory (TCM) cell subset. TN and TCM cells are distinct cell populations distinguished by many phenotypic and physiological differences. In this study, we used a primary cell model of HIV-1 latency that utilizes direct infection of highly purified TN and TCM cells to address differences in the establishment and reversal of HIV-1 latency. Consistent with what is seen in vivo, we found that HIV-1 infected TN cells less efficiently than TCM cells. However, when the infected TN cells were treated with latency-reversing agents, including anti-CD3/CD28 antibodies, phorbol myristate acetate/phytohemagglutinin, and prostratin, as much (if not more) extracellular virion-associated HIV-1 RNA was produced per infected TN cell as per infected TCM cell. There were no major differences in the genomic distribution of HIV-1 integration sites between TN and TCM cells that accounted for these observed differences. We observed decay of the latent HIV-1 cells in both T cell subsets after exposure to each of the latency-reversing agents. Collectively, these data highlight significant differences in the establishment and reversal of HIV-1 latency in TN and TCM CD4(+) T cells and suggest that each subset should be independently studied in preclinical and clinical studies.

IMPORTANCE

The latent HIV-1 reservoir is frequently described as residing within resting memory CD4(+) T cells. This is largely due to the consistent finding that memory CD4(+) T cells, specifically the central (TCM) and transitional memory compartments, harbor the highest levels of HIV-1 DNA in individuals on suppressive therapy. This has yielded little research into the contribution of CD4(+) naive T (TN) cells to the latent reservoir. In this study, we show that although TN cells harbor significantly lower levels of HIV-1 DNA, following latency reversal, they produced as many virions as did the TCM cells (if not more virions). This suggests that latently infected TN cells may be a major source of virus following treatment interruption or failure. These findings highlight the need for a better understanding of the establishment and reversal of HIV-1 latency in TN cells in evaluating therapeutic approaches to eliminate the latent reservoir.

T memory stem cells and HIV: a long-term relationship

In analogy to many tissues in which mature, terminally differentiated cells are continuously replenished by the progeny of less differentiated, long-lasting stem cells, it has been suspected that memory T lymphocytes might contain small numbers of stem cell-like cells. However, only recently have such cells been physically identified and isolated from humans, mice, and nonhuman primates. These cells, termed "T memory stem cells" (TSCM), represent approximately 2-4 % of all circulating T lymphocytes, seem to be extremely durable, and can rapidly differentiate into more mature central memory, effector memory, and effector T cells, while maintaining their own pool size through homeostatic self-renewal. Although it is becoming increasingly evident that that these cells have critical roles for T cell homeostasis and maintaining life-long cellular immunity against microbial pathogens during physiological conditions, they also seem intrinsically involved in many key aspects of HIV/SIV disease pathogenesis. Current data suggest that CD4+ TSCM cells represent a core element of the HIV-1 reservoir in patients treated with suppressive antiretroviral therapy (ART) and that relative resistance of CD4+ TSCM cells to SIV represents a distinguishing feature of non-pathogenic SIV infection in natural hosts. This article summarizes recent studies investigating the role of TSCM in HIV/SIV infection.

T cell therapies-are T memory stem cells the answer?

T memory stem cells (TSCM) are the earliest developmental stage of memory T cells, displaying stem cell-like properties and exhibiting a gene profile between naive and central memory (CM) T cells. Their long-lifespan, robust proliferative potential and self-renewal capacity has generated much research and clinical interest particularly for therapeutic use. Here, we discuss recent findings published in Science Translational Medicine by Biasco and colleagues [2015 Feb 4;7(273):273ra13], which provided evidence for the persistence of TSCM in humans for up to 12 years after infusion of genetically modified lymphocytes, and we examine the implications for the development of novel immunotherapies using TSCM.

Central memory CD4+ T cells are preferential targets of double infection by HIV-1

Background

Template switching between two distinct HIV-1 RNA genomes during reverse transcription gives rise to recombinant viruses that greatly expand the genetic diversity of HIV-1 and have adverse implications for drug resistance, immune escape, and vaccine design. Virions with two distinct genomes are produced exclusively from cells infected with two or more viruses, or ‘doubly infected’ cells. Previous studies have revealed higher than expected frequencies of doubly infected cells compared to frequencies based on chance alone, suggesting non-random enhancement of double infection.

Methods

We investigated double infection of unstimulated primary CD4+ T cells using reporter viruses carrying genes for different fluorescent proteins, EGFP and mCherry, combined with sophisticated modeling techniques based on Poisson distribution. Additionally, through the use of multiparameter flow cytometry we examined the susceptibility of naïve and memory subsets of CD4+ T cells to double infection by HIV.

Results

Using our double infection system, we confirm non-random enhancement of multiple infection events. Double infection of CD4+ T cells was not found to be a consequence of suboptimal provirus expression rescued by Tat in trans—as has been reported in cell lines—but rather due to a heterogeneous cell population in which only a fraction of primary peripheral blood CD4+ T cells are susceptible to HIV infection regardless of viral titer. Intriguingly, double infection of CD4+ T cells occurred preferentially in memory CD4+ T cells—particularly the central memory (T_CM) subset—but was not a consequence of SAMHD1-mediated restriction of HIV infection in naïve cells.

Conclusions

These findings reveal that double infection in primary CD4+ T cells is primarily a consequences of cellular heterogeneity and not rescue of suboptimal provirus expression by Tat in trans. Additionally, we report a previously unappreciated phenomenon of enhanced double infection within primary T_CM cells and suggest that these long-lived cells may serve as an archive that drive ongoing viral recombination events in vivo.

Low SAMHD1 expression following T-cell activation and proliferation renders CD4+ T cells susceptible to HIV-1

Objectives:

HIV-1 replication depends on the state of cell activation and division. It is established that SAMHD1 restricts HIV-1 infection of resting CD4⁺ T cells. The modulation of SAMHD1 expression during T-cell activation and proliferation, however, remains unclear, as well as a role for SAMHD1 during HIV-1 pathogenesis.

Methods:

SAMHD1 expression was assessed in CD4⁺ T cells after their activation and in-vitro HIV-1 infection. We performed phenotype analyzes using flow cytometry on CD4⁺ T cells from peripheral blood and lymph nodes from cohorts of HIV-1-infected individuals under antiretroviral treatment or not, and controls.

Results:

We show that SAMHD1 expression decreased during CD4⁺ T-cell proliferation in association with an increased susceptibility to in-vitro HIV-1 infection. Additionally, circulating memory CD4⁺ T cells are enriched in cells with low levels of SAMHD1. These SAMHD1^low cells are highly differentiated, exhibit a large proportion of Ki67⁺ cycling cells and are enriched in T-helper 17 cells. Importantly, memory SAMHD1^low cells were depleted from peripheral blood of HIV-infected individuals. We also found that follicular helper T cells present in secondary lymphoid organs lacked the expression of SAMHD1, which was accompanied by a higher susceptibility to HIV-1 infection in vitro.

Conclusion:

We demonstrate that SAMHD1 expression is decreased during CD4⁺ T-cell activation and proliferation. Also, CD4⁺ T-cell subsets known to be more susceptible to HIV-1 infection, for example, T-helper 17 and follicular helper T cells, display lower levels of SAMHD1. These results pin point a role for SAMHD1 expression in HIV-1 infection and the concomitant depletion of CD4⁺ T cells.

Interleukin 7 up-regulates CD95 protein on CD4+ T cells by affecting mRNA alternative splicing: priming for a synergistic effect on HIV-1 reservoir maintenance

Interleukin-7 (IL-7) has been used as an immunoregulatory and latency-reversing agent in human immunodeficiency virus type 1 (HIV-1) infection. Although IL-7 can restore circulating CD4(+) T cell counts in HIV-1-infected patients, the anti-apoptotic and proliferative effects of IL-7 appear to benefit survival and expansion of HIV-1-latently infected memory CD4(+) T lymphocytes. IL-7 has been shown to elevate CD95 on CD4(+) T cells in HIV-1-infected individuals and prime CD4(+) T lymphocytes to CD95-mediated proliferative or apoptotic signals. Here we observed that through increasing microRNA-124, IL-7 down-regulates the splicing regulator polypyrimidine tract binding protein (PTB), leading to inclusion of the transmembrane domain-encoding exon 6 of CD95 mRNA and, subsequently, elevation of CD95 on memory CD4(+) T cells. Moreover, IL-7 up-regulates cellular FLICE-like inhibitory protein (c-FLIP) and stimulates c-Jun N-terminal kinase (JNK) phosphorylation, which switches CD95 signaling to survival mode in memory CD4(+) T lymphocytes. As a result, co-stimulation through IL-7/IL-7R and FasL/CD95 signal pathways augments IL-7-mediated survival and expansion of HIV-1-latently infected memory CD4(+) T lymphocytes. Collectively, we have demonstrated a novel mechanism for IL-7-mediated maintenance of HIV-1 reservoir.

The CD8⁺ memory stem T cell (T(SCM)) subset is associated with improved prognosis in chronic HIV-1 infection

Memory stem T cells (T(SCM)) constitute a long-lived, self-renewing lymphocyte population essential for the maintenance of functional immunity. The hallmarks of HIV-1 pathogenesis are CD4(+) T cell depletion and abnormal cellular activation. We investigated the impact of HIV-1 infection on the T(SCM) compartment, as well as any protective role these cells may have in disease progression, by characterizing this subset in a cohort of 113 subjects with various degrees of viral control on and off highly active antiretroviral therapy (HAART). We observed that the frequency of CD8(+) T(SCM) was decreased in all individuals with chronic, untreated HIV-1 infection and that HAART had a restorative effect on this subset. In contrast, natural controllers of HIV-1 had the highest absolute number of CD4(+) T(SCM) cells among all of the infected groups. The frequency of CD4(+) T(SCM) predicted higher CD8(+) T(SCM) frequencies, consistent with a role for the CD4(+) subset in helping to maintain CD8(+) memory T cells. In addition, T(SCM) appeared to be progenitors for effector T cells (TEM), as these two compartments were inversely correlated. Increased frequencies of CD8(+) T(SCM) predicted lower viral loads, higher CD4(+) counts, and less CD8(+) T cell activation. Finally, we found that T(SCM) express the mucosal homing integrin α4β7 and can be identified in gut-associated lymphoid tissue (GALT). The frequency of mucosal CD4(+) T(SCM) was inversely correlated with that in the blood, potentially reflecting the ability of these self-renewing cells to migrate to a crucial site of ongoing viral replication and CD4(+) T cell depletion.

IMPORTANCE

HIV-1 infection leads to profound impairment of the immune system. T(SCM) constitute a recently identified lymphocyte subset with stem cell-like qualities, including the ability to generate other memory T cell subtypes, and are therefore likely to play an important role in controlling viral infection. We investigated the relationship between the size of the CD8(+) T(SCM) compartment and HIV-1 disease progression in a cohort of chronically infected individuals. Our results suggest that HAART restores a normal frequency of CD8(+) T(SCM) and that the natural preservation of this subset in the setting of untreated HIV-1 infection is associated with improved viral control and immunity. Therefore, the CD8(+) T(SCM) population may represent a correlate of protection in chronic HIV-1 infection that is directly relevant to the design of T cell-based vaccines, adoptive immunotherapy approaches, or the pharmacologic induction of T(SCM).

Targeting HIV latency: resting memory T cells, hematopoietic progenitor cells and future directions

Current therapy for HIV effectively suppresses viral replication and prolongs life, but the infection persists due, at least in part, to latent infection of long-lived cells. One favored strategy toward a cure targets latent virus in resting memory CD4(+) T cells by stimulating viral production. However, the existence of an additional reservoir in bone marrow hematopoietic progenitor cells has been detected in some treated HIV-infected people. This review describes approaches investigators have used to reactivate latent proviral genomes in resting CD4(+) T cells and hematopoietic progenitor cells. In addition, the authors review approaches for clearance of these reservoirs along with other important topics related to HIV eradication.

Stem memory T cells (TSCM)-their role in cancer and HIV immunotherapies

Stem memory T cells (TSCM) have been described in mice, non-human primates and in humans, constituting approximately 2-4% of the total CD4(+) and CD8(+) T-cell population in the periphery. TSCM represent the earliest and long-lasting developmental stage of memory T cells, displaying stem cell-like properties, and exhibiting a gene profile between naïve and central memory T cells. Their self-renewal capacity and long-term survival has sparked interest in the cancer and human immunodeficiency virus (HIV) fields. How and when the formation of TSCM occurs during the immune response to pathogens and the therapeutic potential of these cells are currently being investigated. This review will explore the potential role of TSCM to be used as, or targeted by, immunotherapies and vaccines for treatment of cancer and HIV.