Expression of a broad array of negative costimulatory molecules and Blimp-1 in T cells following priming by HIV-1 pulsed dendritic cells

Exploring the Role of PD-1 in the Autoimmune Response: Insights into Its Implication in Systemic Lupus Erythematosus

Despite advances in understanding systemic lupus erythematosus (SLE), many challenges remain in unraveling the precise mechanisms behind the disease's development and progression. Recent evidence has questioned the role of programmed cell death protein 1 (PD-1) in suppressing autoreactive CD4+ T cells during autoimmune responses. Research has investigated the potential impacts of PD-1 on various CD4+ T-cell subpopulations, including T follicular helper (Tfh) cells, circulating Tfh (cTfh) cells, and T peripheral helper (Tph) cells, all of which exhibit substantial PD-1 expression and are closely related to several autoimmune disorders, including SLE. This review highlights the complex role of PD-1 in autoimmunity and emphasizes the imperative for further research to elucidate its functions during autoreactive T-cell responses. Additionally, we address the potential of PD-1 and its ligands as possible therapeutic targets in SLE.

A T follicular helper cell origin for T regulatory type 1 cells

Chronic antigenic stimulation can trigger the differentiation of antigen-experienced CD4+ T cells into T regulatory type 1 (TR1) cells, a subset of interleukin-10-producing Treg cells that do not express FOXP3. The identities of the progenitor(s) and transcriptional regulators of this T-cell subset remain unclear. Here, we show that the peptide-major histocompatibility complex class II (pMHCII) monospecific immunoregulatory T-cell pools that arise in vivo in different genetic backgrounds in response to pMHCII-coated nanoparticles (pMHCII-NPs) are invariably comprised of oligoclonal subpools of T follicular helper (TFH) and TR1 cells with a nearly identical clonotypic composition but different functional properties and transcription factor expression profiles. Pseudotime analyses of scRNAseq data and multidimensional mass cytometry revealed progressive downregulation and upregulation of TFH and TR1 markers, respectively. Furthermore, pMHCII-NPs trigger cognate TR1 cell formation in TFH cell-transfused immunodeficient hosts, and T-cell-specific deletion of Bcl6 or Irf4 blunts both the TFH expansion and TR1 formation induced by pMHCII-NPs. In contrast, deletion of Prdm1 selectively abrogates the TFH-to-TR1 conversion. Bcl6 and Prdm1 are also necessary for anti-CD3 mAb-induced TR1 formation. Thus, TFH cells can differentiate into TR1 cells in vivo, and BLIMP1 is a gatekeeper of this cellular reprogramming event.

HIV-1 induction of tolerogenic dendritic cells is mediated by cellular interaction with suppressive T cells

HIV-1 infection gives rise to a multi-layered immune impairment in most infected individuals. The chronic presence of HIV-1 during the priming and activation of T cells by dendritic cells (DCs) promotes the expansion of suppressive T cells in a contact-dependent manner. The mechanism behind the T cell side of this HIV-induced impairment is well studied, whereas little is known about the reverse effects exerted on the DCs. Herein we assessed the phenotype and transcriptome profile of mature DCs that have been in contact with suppressive T cells. The HIV exposed DCs from cocultures between DCs and T cells resulted in a more tolerogenic phenotype with increased expression of e.g., PDL1, Gal-9, HVEM, and B7H3, mediated by interaction with T cells. Transcriptomic analysis of the DCs separated from the DC-T cell coculture revealed a type I IFN response profile as well as an activation of pathways involved in T cell exhaustion. Taken together, our data indicate that the prolonged and strong type I IFN signaling in DCs, induced by the presence of HIV during DC-T cell cross talk, could play an important role in the induction of tolerogenic DCs and suppressed immune responses seen in HIV-1 infected individuals.

LncRNA XR_001779380 Primes Epithelial Cells for IFN-γ-Mediated Gene Transcription and Facilitates Age-Dependent Intestinal Antimicrobial Defense

Interferon (IFN) signaling is key to mucosal immunity in the gastrointestinal tract, but cellular regulatory elements that determine interferon gamma (IFN-γ)-mediated antimicrobial defense in intestinal epithelial cells are not fully understood. We report here that a long noncoding RNA (lncRNA), GenBank accession no. XR_001779380, was increased in abundance in murine intestinal epithelial cells following infection by Cryptosporidium, an important opportunistic pathogen in AIDS patients and a common cause of diarrhea in young children. Expression of XR_001779380 in infected intestinal epithelial cells was triggered by TLR4/NF-κB/Cdc42 signaling and epithelial-specific transcription factor Elf3. XR_001779380 primed epithelial cells for IFN-γ-mediated gene transcription through facilitating Stat1/Swi/Snf-associated chromatin remodeling. Interactions between XR_001779380 and Prdm1, which is expressed in neonatal but not adult intestinal epithelium, attenuated Stat1/Swi/Snf-associated chromatin remodeling induced by IFN-γ, contributing to suppression of IFN-γ-mediated epithelial defense in neonatal intestine. Our data demonstrate that XR_001779380 is an important regulator in IFN-γ-mediated gene transcription and age-associated intestinal epithelial antimicrobial defense. IMPORTANCE Epithelial cells along the mucosal surface provide the front line of defense against luminal pathogen infection in the gastrointestinal tract. These epithelial cells represent an integral component of a highly regulated communication network that can transmit essential signals to cells in the underlying intestinal mucosa that, in turn, serve as targets of mucosal immune mediators. LncRNAs are recently identified long noncoding transcripts that can regulate gene transcription through their interactions with other effect molecules. In this study, we demonstrated that lncRNA XR_001779380 was upregulated in murine intestinal epithelial cells following infection by a mucosal protozoan parasite Cryptosporidium. Expression of XR_001779380 in infected cells primed host epithelial cells for IFN-γ-mediated gene transcription, relevant to age-dependent intestinal antimicrobial defense. Our data provide new mechanistic insights into how intestinal epithelial cells orchestrate intestinal mucosal defense against microbial infection.

A Comparison of Cell Activation, Exhaustion, and Expression of HIV Coreceptors and Restriction Factors in HIV-1- and HIV-2-Infected Nonprogressors

Human immunodeficiency viruses induce rare attenuated diseases due either to HIV-1 in the exceptional long-term nonprogressors (LTNPs) or to HIV-2 in West Africa. To better understand characteristics of these two disease types we performed a multiplex comparative analysis of cell activation, exhaustion, and expression of coreceptors and restriction factors in CD4 T cells susceptible to harbor those viruses. We analyzed by flow cytometry the expression of HLA-DR, PD1, CCR5, CXCR6, SAMHD1, Blimp-1, and TRIM5α on CD4 T cell subsets from 10 HIV-1⁺ LTNPs and 14 HIV-2⁺ (12 nonprogressors and 2 progressors) of the ANRS CO-15 and CO-5 cohorts, respectively, and 12 HIV^- healthy donors (HD). The V3 loop of the HIV-1 envelope from 6 HIV-1+ LTNPs was sequenced to determine the CXCR6-binding capacity. Proportions of HLA-DR⁺ and PD1+ cells were higher in memory CD4 T subsets from HIV-1 LTNPs compared with HIV-2 and HD. Similar findings were observed for CCR5+ cells although limited to central-memory CD4 T cell (TCM) and follicular helper T cell subsets, whereas all major subsets from HIV-1 LTNPs contained less CXCR6+ cells compared with HIV-2. All six V3 loop sequences from HIV-1 LTNPs contained a proline at position 326. Proportions of SAMHD1+ cells were higher in all resting CD4 T subsets from HIV-1 LTNPs compared with the other groups, whereas Blimp-1+ and Trim5α+ cells did not differ. The CD4 T cell subsets from HIV-1 LTNPs differ from those of HIV-2-infected subjects by higher levels of activation, exhaustion, and SAMHD1 expression that can reflect the distinct patterns of host/virus relationships.

SLAMF7 Signaling Reprograms T Cells toward Exhaustion in the Tumor Microenvironment

T cell exhaustion represents one of the most pervasive strategies tumors employ to circumvent the immune system. Although repetitive, cognate TCR signaling is recognized as the primary driving force behind this phenomenon, and it remains unknown what other forces drive T cell exhaustion in the tumor microenvironment (TME). In this study, we show that activation of the self-ligand SLAMF7 immune receptor on T cells induced STAT1 and STAT3 phosphorylation, expression of multiple inhibitory receptors, and transcription factors associated with T cell exhaustion. Analysis of The Cancer Genome Atlas revealed that SLAMF7 transcript levels were strongly correlated with various inhibitory receptors and that high SLAMF7 expression was indicative of poor survival in clear cell renal cell carcinoma (ccRCC). Targeted reanalysis of a CyTOF dataset, which profiled the TME in 73 ccRCC patients, revealed cell-type-specific SLAMF7 expression patterns, strong correlations between exhausted T cells and SLAMF7⁺ tumor-associated macrophages (TAMs), and a unique subset of SLAMF7^highCD38^high TAMs. These SLAMF7^highCD38^high TAMs showed the strongest correlations with exhausted T cells and were an independent prognostic factor in ccRCC. Confirmatory ex vivo coculture studies validated that SLAMF7-SLAMF7 interactions between murine TAMs and CD8⁺ T cells induce expression of multiple inhibitory receptors. Finally, mice lacking SLAMF7 show restricted growth of B16-F10 tumors, and CD8⁺ T cells from these mice express less PD-1 and TOX and exhibited an impaired ability to progress through the exhaustion developmental trajectory to terminal exhaustion. These findings suggest that SLAMF7 might play an important role in modulating T cell function in the TME.

Experimental exposure of Burkholderia pseudomallei crude culture filtrate upregulates PD-1 on T lymphocytes

Burkholderia pseudomallei is the causative agent for melioidosis. Because of its intracellular nature, the bacterium is capable of replicating within a plethora of eukaryotic cell lines. B. pseudomallei can remain dormant within host cells without symptoms for years, causing recrudescent infections. Here, we investigated the pathogenesis mechanism behind the suppression of T cell responses by B. pseudomallei . Peripheral blood mononuclear cells (1×10⁶ cells/well) isolated by Ficoll Paque (Sigma-Aldrich) density gradient centrifugation were incubated with optimized concentrations of bacterial crude culture filtrate antigens (CFAs) (10 ug ml^-1) and heat-killed bacteria [1 : 10 multiplicity of infection (m.o.i.)]. Following incubation, cells were investigated for surface expression of coinhibitory molecules by flow cytometry. We found that B. pseudomallei induced the upregulation of programmed death 1 (PD-1), a molecule responsible for T cell exhaustion, on T cells in vitro following exposure to crude CFAs of B. pseudomallei . This upregulation of PD-1 probably contributes to poor immune surveillance and disease pathogenesis.

Cancer and HIV-1 Infection: Patterns of Chronic Antigen Exposure

The main role of the human immune system is to eliminate cells presenting foreign antigens and abnormal patterns, while maintaining self-tolerance. However, when facing highly variable pathogens or antigens very similar to self-antigens, this system can fail in completely eliminating the anomalies, leading to the establishment of chronic pathologies. Prototypical examples of immune system defeat are cancer and Human Immunodeficiency Virus-1 (HIV-1) infection. In both conditions, the immune system is persistently exposed to antigens leading to systemic inflammation, lack of generation of long-term memory and exhaustion of effector cells. This triggers a negative feedback loop where effector cells are unable to resolve the pathology and cannot be replaced due to the lack of a pool of undifferentiated, self-renewing memory T cells. In addition, in an attempt to reduce tissue damage due to chronic inflammation, antigen presenting cells and myeloid components of the immune system activate systemic regulatory and tolerogenic programs. Beside these homologies shared between cancer and HIV-1 infection, the immune system can be shaped differently depending on the type and distribution of the eliciting antigens with ultimate consequences at the phenotypic and functional level of immune exhaustion. T cell differentiation, functionality, cytotoxic potential and proliferation reserve, immune-cell polarization, upregulation of negative regulators (immune checkpoint molecules) are indeed directly linked to the quantitative and qualitative differences in priming and recalling conditions. Better understanding of distinct mechanisms and functional consequences underlying disease-specific immune cell dysfunction will contribute to further improve and personalize immunotherapy. In the present review, we describe relevant players of immune cell exhaustion in cancer and HIV-1 infection, and enumerate the best-defined hallmarks of T cell dysfunction. Moreover, we highlight shared and divergent aspects of T cell exhaustion and T cell activation to the best of current knowledge.

ThymicPeptides Reverse Immune Exhaustion in Patients with Reactivated Human Alphaherpesvirus1 Infections

Recurrent infection with human alphaherpesvirus 1 (HHV-1) may be associated with immune exhaustion that impairs virus elimination. Thymic peptides enhance immune function and thus could overcome immune exhaustion. In this study, we investigated whether reactivation of herpes infections was associated with immune exhaustion. Moreover, we examined the impact of treatment with thymostimulin on the expression of programmed cell death protein 1 (PD-1) and its ligand (PD-L1) on T and B lymphocytes in patients suffering from recurrent HHV-1 reactivation. We also assessed the effector function of peripheral blood mononuclear cells (PBMCs) after stimulation with thymic peptides. We enrolled 50 women with reactivated HHV-1 infections and healthy volunteers. We measured the expression of various activation and exhaustion markers on the surface of PBMCs using flow cytometry. In ex vivo experiments, we measured the secretion of inflammatory cytokines by PBMCs cultured with thymostimulin. Compared with controls, patients with reactivated HHV-1 infections had increased percentages of CD3+ co-expressing CD25, an activation marker (p < 0.001). Moreover, these patients had increased percentages of CD4+ and CD8+ cells co-expressing the inhibitory markers PD-1 and PD-L1. In cultures of PBMCs from the patients, thymostimulin increased the secretion of interferon gamma (p < 0.001) and interleukin (IL)-2 (p = 0.023), but not IL-4 or IL-10.Two-month thymostimulin therapy resulted in no reactivation of HHV-1 infection during this period and the reduction of PD-1 and PD-L1 expression on the surface of T and B lymphocytes (p < 0.001). In conclusion, reactivation of herpes infection is associated with immune exhaustion, which could be reversed by treatment with thymic peptides.

HIV Interferes with the Dendritic Cell-T Cell Axis of Macrophage Activation by Shifting Mycobacterium tuberculosis-Specific CD4 T Cells into a Dysfunctional Phenotype

HIV coinfection is the greatest risk factor for transition of latent Mycobacterium tuberculosis infection into active tuberculosis (TB). Epidemiological data reveal both the reduction and the impairment of M. tuberculosis-specific CD4 T cells, although the cellular link and actual mechanisms resulting in immune impairment/suppression need further characterization. M. tuberculosis-specific CD4 T cells play a central role in development of protective immunity against TB, in which they participate in the activation of macrophages through the dendritic cell (DC)-T cell axis. Using an in vitro priming system for generating Ag-specific T cells, we explored if HIV-M. tuberculosis-infected (coinfected) human DCs can dysregulate the M. tuberculosis-specific CD4 T cell phenotype and functionality and subsequently mediate the failure to control M. tuberculosis infection in macrophages. After coculture with coinfected DCs, M. tuberculosis Ag-specific CD4 T cells lost their ability to enhance control of M. tuberculosis infection in infected macrophages. Coinfection of DCs reduced proliferation of M. tuberculosis Ag-specific CD4 T cells without affecting their viability, led to increased expression of coinhibitory factors CTLA-4, PD-1, and Blimp-1, and decreased expression of costimulatory molecules CD40L, CD28, and ICOS on the T cells. Expression of the regulatory T cell markers FOXP3 and CD25, together with the immunosuppressive cytokines TGF-β and IL-10, was also significantly increased by coinfection compared with M. tuberculosis single infection. Our data suggest a pattern in which HIV, through its effect on DCs, impairs the ability of M. tuberculosis-specific CD4 T cells to maintain a latent TB within human macrophages, which could play an early role in the subsequent development of TB.

Myeloid Dendritic Cells Induce HIV Latency in Proliferating CD4+ T Cells

HIV latency occurs predominantly in long-lived resting CD4⁺ T cells; however, latent infection also occurs in T cell subsets, including proliferating CD4⁺ T cells. We compared the establishment and maintenance of latent infection in nonproliferating and proliferating human CD4⁺ T cells cocultured with syngeneic myeloid dendritic cells (mDC). Resting CD4⁺ T cells were labeled with the proliferation dye eFluor 670 and cultured alone or with mDC, plasmacytoid dendritic cells, or monocytes in the presence of staphylococcal enterotoxin B (SEB). Cells were cultured for 24 h and infected with CCR5-tropic enhanced GFP (EGFP) reporter HIV. Five days postinfection, nonproductively infected EGFP^- CD4⁺ T cells that were either nonproliferating (eFluor 670^hi) or proliferating (eFluor 670^lo) were sorted and cultured for an additional 7 d (day 12) with IL-7 and antiretrovirals. At day 5 postinfection, sorted, nonproductively infected T cells were stimulated with anti-CD3/CD28, and induced expression of EGFP was measured to determine the frequency of latent infection. Integrated HIV in these cells was confirmed using quantitative PCR. By these criteria, latent infection was detected at day 5 and 12 in proliferating T cells cocultured with mDC and monocytes but not plasmacytoid dendritic cells, where CD4⁺ T cells at day 12 were poor. At day 5 postinfection, nonproliferating T cells expressing SEB-specific TCR Vβ-17 were enriched in latent infection compared with non-SEB-specific TCR Vβ-8.1. Together, these data show that both nonproliferating and proliferating CD4⁺ T cells can harbor latent infection during SEB-stimulated T cell proliferation and that the establishment of HIV latency in nonproliferating T cells is linked to expression of specific TCR that respond to SEB.

Programmed cell death-1 contributes to the establishment and maintenance of HIV-1 latency

OBJECTIVE

In HIV-infected individuals on antiretroviral therapy (ART), latent HIV is enriched in CD4 T cells expressing immune checkpoint molecules, in particular programmed cell death-1 (PD-1). We therefore assessed the effect of blocking PD-1 on latency, both in vitro and in vivo.

METHODS

HIV latency was established in vitro following coculture of resting CD4+ T cells with myeloid dendritic cells. Expression of PD-1 was quantified by flow cytometry, and latency assessed in sorted PD-1high and PD-1low/-nonproliferating CD4+ memory T cells. The role of PD-1 in the establishment of latency was determined by adding anti-PD-1 (pembrolizumab) to cocultures before and after infection. In addition, a single infusion of anti-PD-1 (nivolumab) was administered to an HIV-infected individual on ART with metastatic melanoma, and cell-associated HIV DNA and RNA, and plasma HIV RNA were quantified.

RESULTS

HIV latency was significantly enriched in PD-1high compared with PD-1low/- nonproliferating, CD4 memory T cells. Sorting for an additional immune checkpoint molecule, T-cell immunoglobulin domain and mucin domain-3, in combination with PD-1, further enriched for latency. Blocking PD-1 prior to HIV infection, in vitro, resulted in a modest but significant decrease in latently infected cells in all donors (n = 6). The administration of anti-PD-1 to an HIV-infected individual on ART resulted in a significant increase in cell-associated HIV RNA in CD4 T cells, without significant changes in HIV DNA or plasma HIV RNA, consistent with reversal of HIV latency.

CONCLUSION

PD-1 contributes to the establishment and maintenance of HIV latency and should be explored as a target, in combination with other immune checkpoint molecules, to reverse latency.

Recent advances on T-cell exhaustion in malaria infection

T-cell exhaustion reportedly leads to dysfunctional immune responses of antigen-specific T cells. Investigations have revealed that T cells expand into functionally defective phenotypes with poor recall/memory abilities to parasitic antigens. The exploitation of co-inhibitory pathways represent a highly viable area of translational research that has very well been utilized against certain cancerous conditions. Malaria, at times, evolve into a sustained chronic state where T cells express several co-inhibitory molecules (negative immune checkpoints) facilitating parasite escape and sub-optimal protective responses. Experimental evidence suggests that blockade of co-inhibitory molecules on T cells in malaria could result in the sustenance of protective responses together with dramatic parasite clearance. The role of several co-inhibitory molecules in malaria infection largely remain unclear, and here we discussed the potential applicability of co-inhibitory molecules in the management of malaria with a view to harness protective host responses against chronic disease and associated consequences.

Host Methyltransferases and Demethylases: Potential New Epigenetic Targets for HIV Cure Strategies and Beyond

A successful HIV cure strategy may require reversing HIV latency to purge hidden viral reservoirs or enhancing HIV latency to permanently silence HIV transcription. Epigenetic modifying agents show promise as antilatency therapeutics in vitro and ex vivo, but also affect other steps in the viral life cycle. In this review, we summarize what we know about cellular DNA and protein methyltransferases (PMTs) as well as demethylases involved in HIV infection. We describe the biology and function of DNA methyltransferases, and their controversial role in HIV infection. We further explain the biology of PMTs and their effects on lysine and arginine methylation of histone and nonhistone proteins. We end with a focus on protein demethylases, their unique modes of action and their emerging influence on HIV infection. An outlook on the use of methylation-modifying agents in investigational HIV cure strategies is provided.

Aberrant production of soluble inducible T cell co‑stimulator and soluble programmed cell death protein 1 in patients with chronic hepatitis B

Previous studies demonstrated that immune dysregulation is an important cause of hepatitis B virus (HBV)‑mediated liver damage. Co‑stimulators including programmed cell death protein 1 (PD‑1) and inducible T cell co‑stimulator (ICOS) are involved in the pathogenesis of HBV. In the present study, the serum levels of soluble (s)PD‑1 and sICOS in patients with chronic HBV infections, were investigated, and the association between sPD‑1 and sICOS levels and liver injury degree was investigated. Serum sPD‑1 and sICOS levels were increased in the HBV‑patient group particularly in the HBV external core antigen positive group. In the immune clearance group, sPD‑1 and sICOS were increased compared with the tolerance group. Furthermore, the relative mRNA expression levels were also increased in patients with HBV. However there was no correlation between sPD‑1 and sICOS levels and HBV antibodies or PD‑1/ICOS mRNA copies. The altered sPD‑1 and sICOS serum levels in the different HBV groups may reflect the dysregulation of T cell activation, and may be associated with the HBV pathological process.

Persistent infection due to a small-colony variant of Burkholderia pseudomallei leads to PD-1 upregulation on circulating immune cells and mononuclear infiltration in viscera of experimental BALB/c mice

Background

Melioidosis is a neglected tropical disease endemic across South East Asia and Northern Australia. The etiological agent, Burkholderia pseudomallei (B.pseudomallei), is a Gram-negative, rod-shaped, motile bacterium residing in the soil and muddy water across endemic regions of the tropical world. The bacterium is known to cause persistent infections by remaining latent within host cells for prolonged duration. Reactivation of the recrudescent disease often occurs in elders whose immunity wanes. Moreover, recurrence rates in melioidosis patients can be up to ~13% despite appropriate antibiotic therapy, suggestive of bacterial persistence and inefficacy of antibiotic regimens. The mechanisms behind bacterial persistence in the host remain unclear, and hence understanding host immunity during persistent B. pseudomallei infections may help designing potential immunotherapy.

Methodology/Principal findings

A persistent infection was generated using a small-colony variant (SCV) and a wild-type (WT) B. pseudomallei in BALB/c mice via intranasal administration. Infected mice that survived for >60 days were sacrificed. Lungs, livers, spleens, and peripheral blood mononuclear cells were harvested for experimental investigations. Histopathological changes of organs were observed in the infected mice, suggestive of successful establishment of persistent infections. Moreover, natural killer (NK) cell frequency was increased in SCV- and WT-infected mice. We observed programmed death-1 (PD-1) upregulation on B cells of SCV- and WT-infected mice. Interestingly, PD-1 upregulation was only observed on NK cells and monocytes of SCV-infected mice. In contrast, cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) downregulation was seen on NK cells of WT-infected mice, and on monocytes of SCV- and WT-infected mice.

Conclusions/Significance

The SCV and the WT of B. pseudomallei distinctly upregulated PD-1 expression on B cells, NK cells, and monocytes to dampen host immunity, which likely facilitates bacterial persistence. PD-1/PD-L1 pathway appears to play an important role in the persistence of B. pseudomallei in the host.

B. pseudomallei is a bacterium that causes melioidosis, a disease endemic in Southeastern Asia and Northern Australia. It is estimated that melioidosis leads to 89,000 deaths worldwide each year. Nevertheless, melioidosis continues to remain a neglected tropical disease that is not even on the list of neglected tropical diseases of the World Health Organization. Furthermore, the disease has a high mortality and recurrence rate, which can be up to 40% and 13%, respectively. It has also been well documented that B. pseudomallei causes latent/persistent infections for a prolonged period without showing apparent symptoms in the infected individual. The mechanisms that are responsible for bacterial persistence in the host remain unclear. Our results demonstrated that B. pseudomallei were able to upregulate PD-1 expression on B cells, NK cells, and/or monocytes during persistent diseases, which likely diminish optimal host immunity. The weakened host immunity in turns facilitates persistence of the bacterium. Interestingly, the SCV had a higher PD-1 expression on distinct immune cells compared to the WT, which might explain its frequent association with persistent infections. Immunotherapies by targeting PD-1/PD-L1 pathway could serve as a better treatment than the conventional antibiotic regimens, which cause a high rate of recurrence in melioidosis patients.

New insights into Blimp-1 in T lymphocytes: a divergent regulator of cell destiny and effector function

B lymphocyte-induced maturation protein-1 (Blimp-1) serves as a master regulator of the development and function of antibody-producing B cells. Given that its function in T lymphocytes has been identified within the past decade, we review recent findings with emphasis on its role in coordinated control of gene expression during the development, differentiation, and function of T cells. Expression of Blimp-1 is mainly confined to activated T cells and is essential for the production of interleukin (IL)-10 by a subset of forkhead box (Fox)p3⁺ regulatory T cells with an effector phenotype. Blimp-1 is also required to induce cell elimination in the thymus and critically modulates peripheral T cell activation and proliferation. In addition, Blimp-1 promotes T helper (Th) 2 lineage commitment and limits Th1, Th17 and follicular helper T cell differentiation. Furthermore, Blimp-1 coordinates with other transcription factors to regulate expression of IL-2, IL-21 and IL-10 in effector T lymphocytes. In CD8⁺ T cells, Blimp-1 expression is distinct in heterogeneous populations at the stages of clonal expansion, differentiation, contraction and memory formation when they encounter antigens. Moreover, Blimp-1 plays a fundamental role in coordinating cytokine receptor signaling networks and transcriptional programs to regulate diverse aspects of the formation and function of effector and memory CD8⁺ T cells and their exhaustion. Blimp-1 also functions as a gatekeeper of T cell activation and suppression to prevent or dampen autoimmune disease, antiviral responses and antitumor immunity. In this review, we discuss the emerging roles of Blimp-1 in the complex regulation of gene networks that regulate the destiny and effector function of T cells and provide a Blimp-1-dominated transcriptional framework for T lymphocyte homeostasis.

Blimp-1 impairs T cell function via upregulation of TIGIT and PD-1 in patients with acute myeloid leukemia

Background

T cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif (ITIM) domain (TIGIT) and programmed cell death protein 1 (PD-1) are important inhibitory receptors that associate with T cell exhaustion in acute myeloid leukemia (AML). In this study, we aimed to determine the underlying transcriptional mechanisms regulating these inhibitory pathways. Specifically, we investigated the role of transcription factor B lymphocyte-induced maturation protein 1 (Blimp-1) in T cell response and transcriptional regulation of TIGIT and PD-1 in AML.

Methods

Peripheral blood samples collected from patients with AML were used in this study. Blimp-1 expression was examined by flow cytometry. The correlation of Blimp-1 expression to clinical characteristics of AML patients was analyzed. Phenotypic and functional studies of Blimp-1-expressing T cells were performed using flow cytometry-based assays. Luciferase reporter assays and ChIP assays were applied to assess direct binding and transcription activity of Blimp-1. Using siRNA to silence Blimp-1, we further elucidated the regulatory role of Blimp-1 in the TIGIT and PD-1 expression and T cell immune response.

Results

Blimp-1 expression is elevated in T cells from AML patients. Consistent with exhaustion, Blimp-1⁺ T cells upregulate multiple inhibitory receptors including PD-1 and TIGIT. In addition, they are functionally impaired manifested by low cytokine production and decreased cytotoxicity capacity. Importantly, the functional defect is reversed by inhibition of Blimp-1 via siRNA knockdown. Furthermore, Blimp-1 binds to the promoters of PD-1 and TIGIT and positively regulates their expression.

Conclusions

Our study demonstrates an important inhibitory effect of Blimp-1 on T cell response in AML; thus, targeting Blimp-1 and its regulated molecules to improve the immune response may provide effective leukemia therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-017-0486-z) contains supplementary material, which is available to authorized users.

HIV-1 and hijacking of the host immune system: the current scenario

Human immunodeficiency virus (HIV) infection is a major health burden across the world which leads to the development of acquired immune deficiency syndrome (AIDS). This review article discusses the prevalence of HIV, its major routes of transmission, natural immunity, and evasion from the host immune system. HIV is mostly prevalent in Sub-Saharan Africa and low income countries. It is mostly transmitted by sharing syringe needles, blood transfusion, and sexual routes. The host immune system is categorized into three main types; the innate, the adaptive, and the intrinsic immune system. Regarding the innate immune system against HIV, the key players are mucosal membrane, dendritic cells (DCs), complement system, interferon, and host Micro RNAs. The major components of the adaptive immune system exploited by HIV are T cells mainly CD4+ T cells and B cells. The intrinsic immune system confronted by HIV involves (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G) APOBEC3G, tripartite motif 5-α (TRIM5a), terherin, and (SAM-domain HD-domain containing protein) SAMHD1. HIV-1 efficiently interacts with the host immune system, exploits the host machinery, successfully replicates and transmits from one cell to another. Further research is required to explore evasion strategies of HIV to develop novel therapeutic approaches against HIV.

Burkholderia pseudomallei Differentially Regulates Host Innate Immune Response Genes for Intracellular Survival in Lung Epithelial Cells

Background

Burkholderia pseudomallei, the causative agent of melioidosis poses a serious threat to humankind. B. pseudomallei secretes numerous virulence proteins that alter host cell functions to escape from intracellular immune sensors. However, the events underlying disease pathogenesis are poorly understood.

Methods

We determined the ability of B. pseudomallei to invade and survive intracellularly in A549 human lung epithelial cells, and also investigated the early transcriptional responses using an Illumina HumanHT-12 v4 microarray platform, after three hours of exposure to live B. pseudomallei (BCMS) and its secreted proteins (CCMS).

Results

We found that the ability of B. pseudomallei to invade and survive intracellularly correlated with increase of multiplicity of infection and duration of contact. Activation of host carbohydrate metabolism and apoptosis as well as suppression of amino acid metabolism and innate immune responses both by live bacteria and its secreted proteins were evident. These early events might be linked to initial activation of host genes directed towards bacterial dissemination from lungs to target organs (via proposed in vivo mechanisms) or to escape potential sensing by macrophages.

Conclusion

Understanding the early responses of A549 cells toward B. pseudomallei infection provide preliminary insights into the likely pathogenesis mechanisms underlying melioidosis, and could contribute to development of novel intervention strategies to combat B. pseudomallei infections.

Burkholderia pseudomallei, the causative agent of the fatal infectious disease melioidosis, is endemic across parts of South East Asia and Northern Australia. Melioidosis poses a serious worldwide emerging infectious disease problem and bioterrorism threat. Of the key features of B. pseudomallei, is its ability to remain latent in the host causing recrudescent disease years after initial infection. Relapses are also commonly reported despite appropriate and prolonged antibiotic therapy, suggesting the bacteria’s ability to escape the host’s front-line immune defenses and to manipulate the host’s responses to sustain survival in the host. However, the likely underlying mechanisms of bacterial persistence still remain unclear. Thus, here we proposed to study the host responses towards early interaction of the cell with live B. pseudomallei and its secretory proteins, in order to understand the potential roles of innate responses against the bacteria.

Experimental Persistent Infection of BALB/c Mice with Small-Colony Variants of Burkholderia pseudomallei Leads to Concurrent Upregulation of PD-1 on T Cells and Skewed Th1 and Th17 Responses

Background

Burkholderia pseudomallei (B. pseudomallei), the causative agent of melioidosis, is a deadly pathogen endemic across parts of tropical South East Asia and Northern Australia. B. pseudomallei can remain latent within the intracellular compartment of the host cell over prolonged periods of time, and cause persistent disease leading to treatment difficulties. Understanding the immunological mechanisms behind persistent infection can result in improved treatment strategies in clinical melioidosis.

Methods

Ten-day LD₅₀ was determined for the small-colony variant (SCV) and its parental wild-type (WT) via intranasal route in experimental BALB/c mice. Persistent B. pseudomallei infection was generated by administrating sub-lethal dose of the two strains based on previously determined LD₅₀. After two months, peripheral blood mononuclear cells (PBMCs) and plasma were obtained to investigate host immune responses against persistent B. pseudomallei infection. Lungs, livers, and spleens were harvested and bacterial loads in these organs were determined.

Results

Based on the ten-day LD₅₀, the SCV was ~20-fold less virulent than the WT. The SCV caused higher bacterial loads in spleens compared to its WT counterparts with persistent B. pseudomallei infection. We found that the CD4+ T-cell frequencies were decreased, and the expressions of PD-1, but not CTLA-4 were significantly increased on the CD4+ and CD8+ T cells of these mice. Notably, persistent infection with the SCV led to significantly higher levels of PD-1 than the WT B. pseudomallei. Plasma IFN-γ, IL-6, and IL-17A levels were elevated only in SCV-infected mice. In addition, skewed plasma Th1 and Th17 responses were observed in SCV-infected mice relative to WT-infected and uninfected mice.

Conclusion

B. pseudomallei appears to upregulate the expression of PD-1 on T cells to evade host immune responses, which likely facilitates bacterial persistence in the host. SCVs cause distinct pathology and immune responses in the host as compared to WT B. pseudomallei.

Melioidosis is an endemic tropical disease in South East Asia and Northern Australia, which is caused by Burkholderia pseudomallei, an environmental bacterium found in the soils of paddy fields and muddy waters across these regions. The bacterium is known to reside within the host cell for prolonged periods of time and is capable of causing long-lasting disease. Recurrent disease is common even with appropriate antibiotic treatments. The mechanisms behind the persistence of B. pseudomallei in the host are still unclear. We investigated the host cell-mediated immune responses against persistent B. pseudomallei infection in BALB/c mice. We found a reduced CD4+ T-cell frequency in mice with persistent B. pseudomallei infection, suggestive of the key role of these cells in experimental melioidosis. Moreover, we also observed significant upregulation of PD-1 on both CD4+ and CD8+ T cells in mice with persistent B. pseudomallei infection, possibly indicating that the T cells were undergoing exhaustion. Based on our results, we postulated that B. pseudomallei is able to impair host immune responses, likely by facilitating the depletion of CD4+ T cells and upregulation of PD-1 on T cells, which potentially facilitates bacterial persistence in the host. Targeting T-cell responses could be an approach to develop vaccines or therapeutics against persistent B. pseudomallei infection.

Impaired NK Cell Activation and Chemotaxis toward Dendritic Cells Exposed to Complement-Opsonized HIV-1

Mucosa resident dendritic cells (DCs) may represent one of the first immune cells that HIV-1 encounters during sexual transmission. The virions in body fluids can be opsonized with complement factors because of HIV-mediated triggering of the complement cascade, and this appears to influence numerous aspects of the immune defense targeting the virus. One key attribute of host defense is the ability to attract immune cells to the site of infection. In this study, we investigated whether the opsonization of HIV with complement (C-HIV) or a mixture of complement and Abs (CI-HIV) affected the cytokine and chemokine responses generated by DCs, as well as their ability to attract other immune cells. We found that the expression levels of CXCL8, CXCL10, CCL3, and CCL17 were lowered after exposure to either C-HIV or CI-HIV relative to free HIV (F-HIV). DCs exposed to F-HIV induced higher cell migration, consisting mainly of NK cells, compared with opsonized virus, and the chemotaxis of NK cells was dependent on CCL3 and CXCL10. NK cell exposure to supernatants derived from HIV-exposed DCs showed that F-HIV induced phenotypic activation (e.g., increased levels of TIM3, CD69, and CD25) and effector function (e.g., production of IFNγ and killing of target cells) in NK cells, whereas C-HIV and CI-HIV did not. The impairment of NK cell recruitment by DCs exposed to complement-opsonized HIV and the lack of NK activation may contribute to the failure of innate immune responses to control HIV at the site of initial mucosa infection.

Transcriptional regulation and T cell exhaustion

PURPOSE OF REVIEW

This review highlights the control of transcriptional networks, including induction of inhibitory receptors, by T cell-specific transcription factors in exhausted T cells that accumulate in chronic viral infections including HIV.

RECENT FINDINGS

Transcriptional profiling has established distinct molecular phenotypes for exhausted CD4 and CD8 T cells in chronic viral infection models. There exists a subset of transcription factors associated with exhaustion, notably Blimp-1, basic leucine zipper transcription factor, ATF-like and Helios. Epigenetic phenomena are likely important in regulating gene expression networks during exhaustion as illustrated by programmed death 1 promoter methylation patterns.

SUMMARY

Following chronic viral infections, CD4 and CD8 T cells defined functionally and phenotypically as exhausted have distinct transcriptional profiles. These studies have identified a core set of transcription factors that have been implicated in promoting exhaustion. However, no single factor appears to be an exhaustion determining factor, suggesting that T cell exhaustion reflects a combinatorial mechanism with multiple transcription factors interacting to influence the development of functionally exhausted T cells as well as different T effector populations.

Blimp-1, an intrinsic factor that represses HIV-1 proviral transcription in memory CD4+ T cells

CD4(+) T cell subsets differentially support HIV-1 replication. For example, quiescent CD4(+) memory T cells are susceptible to HIV-1 infection but do not support robust HIV-1 transcription and have been implicated as the primary reservoir of latent HIV-1. T cell transcription factors that regulate maturation potentially limit HIV-1 transcription and mediate the establishment and maintenance of HIV-1 latency. We report that B lymphocyte-induced maturation protein-1 (Blimp-1), a critical regulator of B and T cell differentiation, is highly expressed in memory CD4(+) T cells compared with naive CD4(+) T cells and represses basal and Tat-mediated HIV-1 transcription. Blimp-1 binds an IFN-stimulated response element within HIV-1 provirus, and it is displaced following T cell activation. Reduction of Blimp-1 in infected primary T cells including CD4(+) memory T cells increases RNA polymerase II processivity, histone acetylation, and baseline HIV-1 transcription. Therefore, the transcriptional repressor, Blimp-1, is an intrinsic factor that predisposes CD4(+) memory T cells to latent HIV-1 infection.

Elevated expression of LAG-3, but not PD-1, is associated with impaired iNKT cytokine production during chronic HIV-1 infection and treatment

Background

LAG-3 is a potent negative regulator of the immune response but its impact in HIV infection in poorly understood. Unlike exhaustion markers such as PD-1, Tim-3, 2B4 and CD160, LAG-3 is poorly expressed on bulk and antigen-specific T cells during chronic HIV infection and its expression on innate lymphocyte subsets is not well understood. The aim of this study was to assess LAG-3 expression and association with cellular dysfunction on T cells, NK cells and iNKT cells among a cohort of healthy and HIV-infected female sex workers in Nairobi, Kenya.

Results

Ex vivo LAG-3 expression was measured by multiparametric flow cytometry, and plasma cytokine/chemokine concentrations measured by bead array. Although LAG-3 expression on bulk T cells was significantly increased among HIV-infected women, the proportion of cells expressing the marker was extremely low. In contrast, LAG-3 was more highly expressed on NK and iNKT cells and was not reduced among women treated with ART. To assess the functional impact of LAG-3 on iNKT cells, iNKT cytokine production was measured in response to lipid (αGalCer) and PMA/Io stimulation by both flow cytometry and cytokine bead array. iNKT cytokine production is profoundly altered by both HIV infection and treatment, and LAG-3, but not PD-1, expression is associated with a reduction in iNKT IFNγ production.

Conclusions

LAG-3 does not appear to mediate T cell exhaustion in this African population, but is instead expressed on innate lymphocyte subsets including iNKT cells. HIV infection alters iNKT cytokine production patterns and LAG-3 expression is uniquely associated with iNKT dysfunction. The continued expression of LAG-3 during treatment suggests it may contribute to the lack of innate immune reconstitution commonly observed during ART.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0142-z) contains supplementary material, which is available to authorized users.

Role of PD-1 co-inhibitory pathway in HIV infection and potential therapeutic options

Virus-specific CD8⁺ T cells play an important role in controlling viral infections including human immunodeficiency virus (HIV) infection. However, during chronic HIV infection, virus-specific CD8⁺ T cells undergo functional exhaustion, lose effector functions and fail to control viral infection. HIV-specific CD8 T cells expressing high levels of co-inhibitory molecule programmed death-1 (PD-1) during the chronic infection and are characterized by lower proliferation, cytokine production, and cytotoxic abilities. Although, antiretroviral therapy has resulted in dramatic decline in HIV replication, there is no effective treatment currently available to eradicate viral reservoirs or restore virus-specific T or B-cell functions that may complement ART in order to eliminate the virus. In recent years, studies in mice and non-human primate models of HIV infection demonstrated the functional exhaustion of virus-specific T and B cells could be reversed by blockade of interaction between PD-1 and its cognate ligands (PD-L1 and PD-L2). In this review, we discuss recent advances in our understanding of PD-1 pathway in HIV/SIV infection and discuss the beneficial effects of PD-1 blockade during chronic HIV/SIV infection and its potential role as immunotherapy for HIV/AIDS.

Soluble T cell immunoglobulin mucin domain 3 is shed from CD8+ T cells by the sheddase ADAM10, is increased in plasma during untreated HIV infection, and correlates with HIV disease progression

Chronic HIV infection results in a loss of HIV-specific CD8(+) T cell effector function, termed "exhaustion," which is mediated, in part, by the membrane coinhibitory receptor T cell immunoglobulin mucin domain-3 (Tim-3). Like many other receptors, a soluble form of this protein has been described in human blood plasma. However, soluble Tim-3 (sTim-3) is poorly characterized, and its role in HIV disease is unknown. Here, we show that Tim-3 is shed from the surface of responding CD8(+) T cells by the matrix metalloproteinase ADAM10, producing a soluble form of the coinhibitory receptor. Despite previous reports in the mouse model, no alternatively spliced, soluble form of Tim-3 was observed in humans. Shed sTim-3 was found in human plasma and was significantly elevated during early and chronic untreated HIV infection, but it was not found differentially modulated in highly active antiretroviral therapy (HAART)-treated HIV-infected subjects or in elite controllers compared to HIV-uninfected subjects. Plasma sTim-3 levels were positively correlated with HIV load and negatively correlated with CD4 counts. Thus, plasma sTim-3 shedding correlated with HIV disease progression. Despite these correlations, we found that shedding Tim-3 did not improve the function of CD8(+) T cells in terms of gamma interferon production or prevent their apoptosis through galectin-9. Further characterization studies of sTim-3 function are needed to understand the contribution of sTim-3 in HIV disease pathogenesis, with implications for novel therapeutic interventions.

IMPORTANCE

Despite the overall success of HAART in slowing the progression to AIDS in HIV-infected subjects, chronic immune activation and T cell exhaustion contribute to the eventual deterioration of the immune system. Understanding these processes will aid in the development of interventions and therapeutics to be used in combination with HAART to slow or reverse this deterioration. Here, we show that a soluble form of T cell exhaustion associated coinhibitory molecule 3, sTim-3, is shed from the surface of T cells. Furthermore, sTim-3 is elevated in the plasma of treatment-naive subjects with acute or chronic HIV infection and is associated with markers of disease progression. This is the first study to characterize sTim-3 in human plasma, its source, and mechanism of production. While it is still unclear whether sTim-3 contributes to HIV pathogenesis, sTim-3 may represent a new correlate of HIV disease progression.

Peptide-MHC multimer-based monitoring of CD8 T-cells in HIV-1 infection and AIDS vaccine development

The use of MHC multimers allows precise and direct detecting and analyzing of antigen-specific T-cell populations and provides new opportunities to characterize T-cell responses in humans and animals. MHC-multimers enable us to enumerate specific T-cells targeting to viral, tumor and vaccine antigens with exceptional sensitivity and specificity. In the field of HIV/SIV immunology, this technique provides valuable information about the frequencies of HIV- and SIV-specific CD8(+) cytotoxic T lymphocytes (CTLs) in different tissues and sites of infection, AIDS progression, and pathogenesis. Peptide-MHC multimer technology remains a very sensitive tool in detecting virus-specific T -cells for evaluation of the immunogenicity of vaccines against HIV-1 in preclinical trials. Moreover, it helps to understand how immune responses are formed following vaccination in the dynamics from priming point until T-cell memory is matured. Here we review a diversity of peptide-MHC class I multimer applications for fundamental immunological studies in different aspects of HIV/SIV infection and vaccine development.

Triggering through Toll-like receptor 2 limits chronically stimulated T-helper type 1 cells from undergoing exhaustion

Chronic infections result in T-cell exhaustion, a state of functional unresponsiveness. To control the infection, it is important to salvage the exhausted T cells. In this study, we delivered signals through Toll-like receptor 2 (TLR-2) to reinvigorate functionality in chronically activated T-helper type 1 (Th1) cells. This process significantly augmented the expression of T-bet, interferon γ, interleukin 2, and the antiapoptotic molecule Bcl-2, whereas it dampened the display of the exhaustion markers programmed death receptor 1 (PD-1) and lymphocyte activation gene 3 (Lag-3). Additionally, TLR-2 signaling bolstered the ability of chronically stimulated Th1 cells to activate B cells. Finally, the results were substantiated by observing reduced lung pathology upon administration of TLR-2 agonist in the chronic infection model of tuberculosis. These data demonstrated the importance of TLR-2 in rescuing chronically activated Th1 cells from undergoing exhaustion. This study will pave a way for targeting TLR-2 in developing therapeutic strategies to treat chronic diseases involving loss of Th1 cell function.

HIV-1 Tat protein induces PD-L1 (B7-H1) expression on dendritic cells through tumor necrosis factor alpha- and toll-like receptor 4-mediated mechanisms

Chronic human immunodeficiency virus type 1 (HIV-1) infection is associated with induction of T-cell coinhibitory pathways. However, the mechanisms by which HIV-1 induces upregulation of coinhibitory molecules remain to be fully elucidated. The aim of the present study was to determine whether and how HIV-1 Tat protein, an immunosuppressive viral factor, induces the PD-1/PD-L1 coinhibitory pathway on human dendritic cells (DCs). We found that treatment of DCs with whole HIV-1 Tat protein significantly upregulated the level of expression of PD-L1. This PD-L1 upregulation was observed in monocyte-derived dendritic cells (MoDCs) obtained from either uninfected or HIV-1-infected patients as well as in primary myeloid DCs from HIV-negative donors. In contrast, no effect on the expression of PD-L2 or PD-1 molecules was detected. The induction of PD-L1 on MoDCs by HIV-1 Tat (i) occurred in dose- and time-dependent manners, (ii) was mediated by the N-terminal 1-45 fragment of Tat, (iii) did not require direct cell-cell contact but appeared rather to be mediated by soluble factor(s), (iv) was abrogated following neutralization of tumor necrosis factor alpha (TNF-α) or blocking of Toll-like receptor 4 (TLR4), (v) was absent in TLR4-knockoout (KO) mice but could be restored following incubation with Tat-conditioned medium from wild-type DCs, (vi) impaired the capacity of MoDCs to functionally stimulate T cells, and (vii) was not reversed functionally following PD-1/PD-L1 pathway blockade, suggesting the implication of other Tat-mediated coinhibitory pathways. Our results demonstrate that HIV-1 Tat protein upregulates PD-L1 expression on MoDCs through TNF-α- and TLR4-mediated mechanisms, functionally compromising the ability of DCs to stimulate T cells. The findings offer a novel potential molecular target for the development of an anti-HIV-1 treatment.

IMPORTANCE

The objective of this study was to investigate the effect of human immunodeficiency virus type 1 (HIV-1) Tat on the PD-1/PD-L1 coinhibitory pathway on human monocyte-derived dendritic cells (MoDCs). We found that treatment of MoDCs from either healthy or HIV-1-infected patients with HIV-1 Tat protein stimulated the expression of PD-L1. We demonstrate that this stimulation was mediated through an indirect mechanism, involving tumor necrosis factor alpha (TNF-α) and Toll-like receptor 4 (TLR4) pathways, and resulted in compromised ability of Tat-treated MoDCs to functionally stimulate T-cell proliferation.

C/EBPα is dispensable for the ontogeny of PD-1+ CD4+ memory T cells but restricts their expansion in an age-dependent manner

Ageing and cancer is often associated with altered T cell distributions and this phenomenon has been suggested to be the main driver in the development of immunosenescence. Memory phenotype PD-1+ CD4+ T cells accumulate with age and during leukemic development, and they might account for the attenuated T cell response in elderly or diseased individuals. The transcription factor C/EBPα has been suggested to be responsible for the accumulation as well as for the senescent features of these cells including impaired TCR signaling and decreased proliferation. Thus modulating the activity of C/EBPα could potentially target PD-1+ CD4+ T cells and consequently, impede the development of immunosenescence. To exploit this possibility we tested the importance of C/EBPα for the development of age-dependent PD-1+ CD4+ T cells as well as its role in the accumulation of PD-1+ CD4+ T cells during leukemic progression. In contrast to earlier suggestions, we find that loss of C/EBPα expression in the lymphoid compartment led to an increase of PD-1+ CD4+ T cells specifically in old mice, suggesting that C/EBPα repress the accumulation of these cells in elderly by inhibiting their proliferation. Furthermore, C/EBPα-deficiency in the lymphoid compartment had no effect on leukemic development and did not affect the accumulation of PD-1+ CD4+ T cells. Thus, in addition to contradict earlier suggestions of a role for C/EBPα in immunosenescence, these findings efficiently discard the potential of using C/EBPα as a target for the alleviation of ageing/cancer-associated immunosenescence.

HIV-Mycobacterium tuberculosis co-infection: a 'danger-couple model' of disease pathogenesis

Tuberculosis (TB) and human immunodeficiency virus (HIV) infection interfere and impact the pathogenesis phenomena of each other. Owing to atypical clinical presentations and diagnostic complications, HIV/TB co-infection continues to be a menace for healthcare providers. Although the increased access to highly active antiretroviral therapy (HAART) has led to a reduction in HIV-associated opportunistic infections and mortality, the concurrent management of HIV/TB co-infection remains a challenge owing to adverse effects, complex drug interactions, overlapping toxicities and tuberculosis -associated immune reconstitution inflammatory syndrome. Several hypotheses have been put forward for the exacerbation of tuberculosis by HIV and vice versa supported by immunological studies. Discussion on the mechanisms produced by infectious cofactors with impact on disease pathology could shed light on how to design potential interventions that could decelerate disease progression. With no vaccine for HIV and lack of an effective vaccine for tuberculosis, it is essential to design strategies against HIV-TB co-infection.

Immune control of HIV-1 reservoirs

PURPOSE OF REVIEW

To discuss the recent major advances in the understanding of how host immune defenses contribute to HIV reservoir control.

RECENT FINDINGS

Immune control of HIV-1 reservoirs is a two-step process: viral replication activation from latent reservoirs followed by elimination of virus-expressing cells by the host. Environmental factors, such as pro-inflammatory type-I interferon, chemokines or cytokines, can facilitate HIV-1 replication, confer dormancy in CD4 cells or confer resistance to cytopathogenic effects of cytotoxic CD8 T cells. Therefore, they constitute a double-edged sword for immune control of HIV reservoirs. Concomitantly, adaptive immunity takes advantage of CD4 T-cell homeostatic mechanisms and can expose HIV-1 antigen-expressing cells to HIV-specific cytotoxic CD8 T cells, and limit virus spreading. These highly interconnected phenomena can lead to quasi-equilibrium between the HIV-1 reservoirs and host immune control that can serve as a model for the 'shock and kill' immune-based therapeutic strategies in play in the course of finding an HIV cure.

SUMMARY

Immune control of HIV reservoirs in CD4 T cells involves modulation of both HIV-1 latency and the continuous reseeding of the reservoir offering conceptual models that may advance HIV cure strategies.

T Cell Transcription Factors and Their Impact on HIV Expression

By targeting CD4⁺ effector T cells, HIV has a dramatic impact on the depletion, expansion and function of the different polarized T cell subsets. The maturation of T cell lineages is in part driven by intrinsic transcription factors that potentially influence how efficiently HIV replicates. In this review, we explore whether transcription factors that are required for polarizing T cells influence HIV replication. In particular, we examine provirus transcription as well as the establishment and maintenance of HIV latency. Furthermore, it is suggested these factors may provide novel cell-specific therapeutic strategies for targeting the HIV latent reservoir.

Programmed death-1 is a marker for abnormal distribution of naive/memory T cell subsets in HIV-1 infection

Chronic activation of T cells is a hallmark of HIV-1 infection and plays an important role in disease progression. We previously showed that the engagement of the inhibitory receptor programmed death (PD)-1 on HIV-1-specific CD4(+) and CD8(+) T cells leads to their functional exhaustion in vitro. However, little is known about the impact of PD-1 expression on the turnover and maturation status of T cells during the course of the disease. In this study, we show that PD-1 is upregulated on all T cell subsets, including naive, central memory, and transitional memory T cells in HIV-1-infected subjects. PD-1 is expressed at similar levels on most CD4(+) T cells during the acute and the chronic phase of disease and identifies cells that have recently entered the cell cycle. In contrast, PD-1 expression is dramatically increased in CD8(+) T cells during the transition from acute to chronic infection, and this is associated with reduced levels of cell proliferation. The failure to downregulate expression of PD-1 in most T cells during chronic HIV-1 infection is associated with persistent alterations in the distribution of T cell subsets and is associated with impaired responses to IL-7. Our findings identify PD-1 as a marker for aberrant distribution of T cell subsets in HIV-1 infection.

Control of HIV infection: escape from the shadow of Blimp-1

Prior murine studies have demonstrated the pivotal role that Blimp-1 has in the exhausted phenotype of T lymphocytes in chronic viral infection. In this issue of the European Journal of Immunology, Seddiki et al. [Eur. J. Immunol. 2013. 43: 510-520] demonstrate the applicability of this research to HIV infection. The authors do so by demonstrating differences in Blimp-1 expression between T lymphocytes isolated from patients with chronic active HIV versus those from long-term nonprogressors and showing that this is matched by differences in the cells' capacity to produce IL-2 and the level of expression of the inhibitory receptor PD-1. The data presented here suggest that this may relate to differential regulation of Blimp-1 by the micro RNA, mIR-9. These findings complement current murine work and fit squarely within the research priorities, as outlined by the International AIDS Society, for determining a cure for AIDS.

Complement opsonization of HIV-1 results in a different intracellular processing pattern and enhanced MHC class I presentation by dendritic cells

Induction of optimal HIV-1-specific T-cell responses, which can contribute to controlling viral infection in vivo, depends on antigen processing and presentation processes occurring in DCs. Opsonization can influence the routing of antigen processing and pathways used for presentation. We studied antigen proteolysis and the role of endocytic receptors in MHC class I (MHCI) and II (MHCII) presentation of antigens derived from HIV-1 in human monocyte-derived immature DCs (IDCs) and mature DCs, comparing free and complement opsonized HIV-1 particles. Opsonization of virions promoted MHCI presentation by DCs, indicating that complement opsonization routes more virions toward the MHCI presentation pathway. Blockade of macrophage mannose receptor (MMR) and β7-integrin enhanced MHCI and MHCII presentation by IDCs and mature DCs, whereas the block of complement receptor 3 decreased MHCI and MHCII presentation. In addition, we found that IDC and MDC proteolytic activities were modulated by HIV-1 exposure; complement-opsonized HIV-1 induced an increased proteasome activity in IDCs. Taken together, these findings indicate that endocytic receptors such as MMR, complement receptor 3, and β7-integrin can promote or disfavor antigen presentation probably by routing HIV-1 into different endosomal compartments with distinct efficiencies for degradation of viral antigens and MHCI and MHCII presentation, and that HIV-1 affects the antigen-processing machinery.

PD-1 coinhibitory signals: the link between pathogenesis and protection

In the majority of HIV-1 infected individuals, the adaptive immune response drives virus escape resulting in persistent viremia and a lack of immune-mediated control. The expression of negative regulatory molecules such as PD-1 during chronic HIV infection provides a useful marker to differentiate functional memory T cell subsets and the frequency of T cells with an exhausted phenotype. In addition, cell-based measurements of virus persistence equate with activation markers and the frequency of CD4 T cells expressing PD-1. High-level expression of PD-1 and its ligands PD-L1 and PD-L2 are found on hematopoietic and non-hematopoietic cells, and are upregulated by chronic antigen stimulation, Type 1 and Type II interferons (IFNs), and homeostatic cytokines. In HIV infected subjects, PD-1 levels on CD4 and CD8 T cells continue to remain high following combination anti-retroviral therapy (cART). System biology approaches have begun to elucidate signal transduction pathways regulated by PD-1 expression in CD4 and CD8 T cell subsets that become dysfunctional through chronic TCR activation and PD-1 signaling. In this review, we summarize our current understanding of transcriptional signatures and signal transduction pathways associated with immune exhaustion with a focus on recent work in our laboratory characterizing the role of PD-1 in T cell dysfunction and HIV pathogenesis. We also highlight the therapeutic potential of blocking PD-1-PD-L1 and other immune checkpoints for activating potent cellular immune responses against chronic viral infections and cancer.

Molecular signatures of T-cell inhibition in HIV-1 infection

Cellular immune responses play a crucial role in the control of viral replication in HIV-infected individuals. However, the virus succeeds in exploiting the immune system to its advantage and therefore, the host ultimately fails to control the virus leading to development of terminal AIDS. The virus adopts numerous evasion mechanisms to hijack the host immune system. We and others recently described the expression of inhibitory molecules on T cells as a contributing factor for suboptimal T-cell responses in HIV infection both in vitro and in vivo. The expression of these molecules that negatively impacts the normal functions of the host immune armory and the underlying signaling pathways associated with their enhanced expression need to be discussed. Targets to restrain the expression of these molecular markers of immune inhibition is likely to contribute to development of therapeutic interventions that augment the functionality of host immune cells leading to improved immune control of HIV infection. In this review, we focus on the functions of inhibitory molecules that are expressed or secreted following HIV infection such as BTLA, CTLA-4, CD160, IDO, KLRG1, LAG-3, LILRB1, PD-1, TRAIL, TIM-3, and regulatory cytokines, and highlight their significance in immune inhibition. We also highlight the ensemble of transcriptional factors such as BATF, BLIMP-1/PRDM1, FoxP3, DTX1 and molecular pathways that facilitate the recruitment and differentiation of suppressor T cells in response to HIV infection.

p38 Mitogen-activated protein kinase/signal transducer and activator of transcription-3 pathway signaling regulates expression of inhibitory molecules in T cells activated by HIV-1-exposed dendritic cells

Human immunodeficiency virus type 1 (HIV-1) infection enhances the expression of inhibitory molecules on T cells, leading to T-cell impairment. The signaling pathways underlying the regulation of inhibitory molecules and subsequent onset of T-cell impairment remain elusive. We showed that both autologous and allogeneic T cells exposed to HIV-pulsed dendritic cells (DCs) upregulated cytotoxic T-lymphocyte antigen (CTLA-4), tumor-necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), lymphocyte-activation gene-3 (LAG3), T-cell immunoglobulin mucin-3 (TIM-3), CD160 and certain suppression-associated transcription factors, such as B-lymphocyte induced maturation protein-1 (BLIMP-1), deltex homolog 1 protein (DTX1) and forkhead box P3 (FOXP3), leading to T-cell suppression. This induction was regulated by p38 mitogen-activated protein kinase/signal transducer and activator of transcription-3 (P38MAPK/STAT3) pathways, because their blockade significantly abrogated expression of all the inhibitory molecules studied and a subsequent recovery in T-cell proliferation. Neither interleukin-6 (IL-6) nor IL-10 nor growth factors known to activate STAT3 signaling events were responsible for STAT3 activation. Involvement of the P38MAPK/STAT3 pathways was evident because these proteins had a higher level of phosphorylation in the HIV-1-primed cells. Furthermore, blockade of viral CD4 binding and fusion significantly reduced the negative effects DCs imposed on primed T cells. In conclusion, HIV-1 interaction with DCs modulated their functionality, causing them to trigger the activation of the P38MAPK/STAT3 pathway in T cells, which was responsible for the upregulation of inhibitory molecules.

Can immunotherapy be useful as a "functional cure" for infection with Human Immunodeficiency Virus-1?

Immunotherapy aims to assist the natural immune system in achieving control over viral infection. Various immunotherapy formats have been evaluated in either therapy-naive or therapy-experienced HIV-infected patients over the last 20 years. These formats included non-antigen specific strategies such as cytokines that stimulate immunity or suppress the viral replication, as well as antibodies that block negative regulatory pathways. A number of HIV-specific therapeutic vaccinations have also been proposed, using in vivo injection of inactivated virus, plasmid DNA encoding HIV antigens, or recombinant viral vectors containing HIV genes. A specific format of therapeutic vaccines consists of ex vivo loading of autologous dendritic cells with one of the above mentioned antigenic formats or mRNA encoding HIV antigens.This review provides an extensive overview of the background and rationale of these different therapeutic attempts and discusses the results of trials in the SIV macaque model and in patients. To date success has been limited, which could be explained by insufficient quality or strength of the induced immune responses, incomplete coverage of HIV variability and/or inappropriate immune activation, with ensuing increased susceptibility of target cells.Future attempts at therapeutic vaccination should ideally be performed under the protection of highly active antiretroviral drugs in patients with a recovered immune system. Risks for immune escape should be limited by a better coverage of the HIV variability, using either conserved or mosaic sequences. Appropriate molecular adjuvants should be included to enhance the quality and strength of the responses, without inducing inappropriate immune activation. Finally, to achieve a long-lasting effect on viral control (i.e. a "functional cure") it is likely that these immune interventions should be combined with anti-latency drugs and/or gene therapy.

Loss of CD127 & increased immunosenescence of T cell subsets in HIV infected individuals

Background & objectives:

HIV infection is characterized by a perturbation in T cell homeostasis, leading to alteration in T cell subsets. In addition to alteration in differentiation, HIV infection also leads to change in T cell survival and regenerative capacity, as suggested by differential expression of CD127 and CD57. We evaluated the expression patterns of CD127 and CD57 on CD4 and CD8 effector, memory and naïve T cell subsets in HIV-infected and uninfected individuals.

Methods:

We characterized T cell subsets based on expression of these markers, and compared their expression pattern in HIV infected subjects and uninfected controls. We further assessed therapy generated changes in these subsets and expression of CD127 and CD57 on them.

Results:

There was a generalized decrease in naïve CD4 and CD8 T cells in HIV infected subjects. These changes in T cell subset distribution were related to antigen load. CD127 expression was significantly reduced in T cells from HIV infected subject. In association to this, HIV infected subjects had higher percentage of T cell subsets expressing CD57. Increased CD57 and reduced CD127 expression correlated with plasma viraemia and CD8 T cell activation state. Incomplete restoration of T cell subset proportions was observed, despite suppression of viral replication and increase in CD4 T cell counts. Further, the improvement was more pronounced in CD127 expression.

Interpretation & conclusions:

HIV infected subjects have reduced T cell regenerative capacity along with increased senescence, highlighting decreased proliferation and effector activities.

Human immunodeficiency virus type 1 Nef in human monocyte-like cell line THP-1 expands treg cells via toll-like receptor 2

CD4(+) CD25(+) regulatory T cells (Tregs) represent a unique T-cell lineage that is endowed with the ability to actively suppress immune responses in order to inhibit pathogenic damage resulting from over activation of the immune system. In human immunodeficiency virus-1 (HIV-1) infection, suppression of the immune response by Tregs appears to play an opposing role that promotes chronic viral infection. Treg expansion is known as a marker of the severity of HIV infection and as a potential prognostic marker of disease progression. HIV-1 Nef is one of the earliest expressed viral regulatory genes whose expression may play an important role in regulating Treg cells. We established a THP-1 cell line stably expressing HIV-1 Nef and showed that Nef protein was a potent factor for increasing Treg numbers in vitro. We further found that TLR2 plays a critical role in the increase in Treg cells induced by Nef using TLR2-specific siRNA. Our results suggest new strategies for therapeutic and preventive interventions of HIV infection.

Myeloid dendritic cells isolated from tissues of SIV-infected Rhesus macaques promote the induction of regulatory T cells

OBJECTIVE

To determine whether the ability of primary myeloid dendritic cells (mDCs) to induce regulatory T cells (Treg) is affected by chronic simian immunodeficiency virus (SIV) infection.

DESIGN

Modulation of dendritic cell activity with the aim of influencing Treg frequency may lead to new treatment options for HIV and strategies for vaccine development.

METHODS

Eleven chronically infected SIV(+) Rhesus macaques were compared with four uninfected animals. Immature and mature mDCs were isolated from mesenteric lymph nodes and spleen by cell sorting and cultured with purified autologous non-Treg (CD4(+)CD25(-) T cells). CD25 and FOXP3 up-regulation was used to assess Treg induction.

RESULTS

The frequency of splenic mDC and plasmacytoid dendritic cell was lower in infected animals than in uninfected animals; their frequency in the mesenteric lymph nodes was not significantly altered, but the percentage of mature mDCs was increased in the mesenteric lymph nodes of infected animals. Mature splenic or mesenteric mDCs from infected animals were significantly more efficient at inducing Treg than mDCs from uninfected animals. Mature mDCs from infected macaques induced more conversion than immature mDCs. Splenic mDCs were as efficient as mesenteric mDCs in this context and CD103 expression by mDCs did not appear to influence the level of conversion.

CONCLUSIONS

Tissue mDCs from SIV-infected animals exhibit an enhanced capability to induce Treg and may contribute to the accumulation of Treg in lymphoid tissues during progressive infection. The activation status of dendritic cell impacts this process but the capacity to induce Treg was not restricted to mucosal dendritic cells in infected animals.

Dendritic cells at the interface of innate and adaptive immunity to HIV-1

PURPOSE OF REVIEW

This review summarizes recent findings on how HIV-1 infection affects dendritic cells in their ability to elicit innate and adaptive immune responses.

RECENT FINDINGS

The phenomenon describing a reduction of dendritic cell numbers in the blood of HIV-1-infected individuals has been expanded on in recent studies demonstrating that dendritic cells decline very early in primary infection and that there is a mobilization of semi-mature dendritic cells to lymph nodes. Recent data suggest that dendritic cells in lymph nodes are more prone to apoptosis, which correlates with disease progression. In addition, plasmacytoid dendritic cells isolated from blood showed a semi-mature phenotype after HIV-1 exposure, which coincided with persistent IFN-α secretion. Emerging data show that semi-mature dendritic cells induce regulatory T cells and suppress effector function. There may therefore be mechanisms by which HIV-1 affects dendritic cell immune stimulation and, in doing so, interferes with the elicitation of anti-HIV-1 responses.

SUMMARY

Understanding how dendritic cells are functionally altered during HIV-1 infection is crucial for the development of new immune-therapy strategies including approaches to target dendritic cells with antigen in vivo or ex vivo to induce efficient adaptive anti-HIV immunity.

Innate immunity in acute HIV-1 infection

PURPOSE OF REVIEW

Acute HIV-1 infection (AHI) is composed of the eclipse phase, during which the transmitted virus struggles to avoid eradication and achieve amplification/spread; the expansion phase when virus disseminates and undergoes exponential replication associated with extensive CD4⁺ T-cell destruction; and the containment phase when set-point levels of viremia and immune activation are established. The importance of interactions between HIV-1 and innate responses in determining events throughout AHI is increasingly recognized, and is reviewed here.

RECENT FINDINGS

During the eclipse phase, HIV-1 subverts dendritic cell functions to promote its replication at mucosal sites and employs multiple strategies to minimize control by type 1 interferons. Systemic virus dissemination is associated with widespread activation of innate responses which fuels HIV-1 replication. To minimize the protective effects of innate responses, HIV-1 resists control by natural killer cells and may impair innate regulation of adaptive responses. Innate responses remain chronically activated after HIV-1 containment which is thought to drive HIV-1 pathogenesis.

SUMMARY

Innate responses are pivotal determinants of events at all stages of AHI. Increased understanding of mechanisms involved in innate control of HIV-1 and pathways regulating innate activation during HIV-1 infection could facilitate development of novel approaches to combating this infection.