Kinetics of Myeloid-Derived Suppressor Cell Frequency and Function during Simian Immunodeficiency Virus Infection, Combination Antiretroviral Therapy, and Treatment Interruption

MDSC expansion during HIV infection: regulators, ART and immune reconstitution

Myeloid-derived suppressor cells (MDSCs) become expanded in different pathological conditions including human immunodeficiency virus (HIV) infection and this may worsen the disease status and accelerate disease progression. In HIV infection, MDSCs suppress anti-HIV immune responses and hamper immune reconstitution. Understanding the factors and mechanisms of MDSC expansion during HIV infection is central to understanding the pathophysiology of HIV infection. This may pave the way to developing new therapeutic targets or strategies. In this work we addressed (i) the mechanisms that regulate MDSC expansion, (ii) the impact of antiretroviral therapy (ART) on the frequency of MDSCs during HIV infection; (iii) the impact of MDSCs on immune reconstitution during successful ART; and (iv) the potential of MDSCs as a therapeutic target.

Brief Report: In cART-Treated HIV-Infected Patients, Immunologic Failure Is Associated With a High Myeloid-Derived Suppressor Cell Frequency

BACKGROUND

During HIV infection, effective combined antiretroviral therapy suppresses viral replication and restores the number of circulating CD4+ T cells. However, 15%-30% of treated patients show a discordant response to combined antiretroviral therapy. Myeloid-derived suppressor cells (MDSC) are expanded in HIV+ patients; to better understand the role of MDSC on CD4 T-cell recovery, we evaluated the frequency of MDSC in HIV+ patients under combined antiretroviral therapy and its association with immunologic response.

METHODS

We enrolled 60 HIV+ patients, including complete responders (R, n = 44), virologic nonresponders (VNR, n = 5), and immunologic nonresponders (INR, n = 11). The frequency of circulating MDSC and the percentage of activated and naïve CD4 T cells were evaluated by flow cytometry. Plasmatic cytokine levels were analyzed by automated ELISA.

RESULTS

As previously observed, polymorphonuclear MDSC (PMN-MDSC) frequency was higher in HIV+ patients compared with healthy donors. Furthermore, PMN-MDSC percentage was higher in INR than R patients, and a significant association between MDSC frequency and immunologic failure was confirmed by a receiver operator characteristic analysis. Accordingly, an inverse correlation was found between the percentages of PMN-MDSC and naïve CD4 T cells. A positive correlation was observed between PMN-MDSC frequency and the percentage of human leucocyte antigen locus DR + CD4 T cells and the plasmatic level of IL-1β and IL-8.

CONCLUSION

Our results show that a high frequency of PMN-MDSC persists in INR, possibly because of immune activation, contributing to CD4 T-cell recovery failure. These findings further highlight the detrimental role of MDSC during HIV infection, suggesting these cells as a possible new therapeutic target.

Anatomical Distribution of Myeloid-Derived Suppressor Cells During HIV Infection

In recent years, expansion of myeloid-derived suppressor cells (MDSCs) has been reported to play a detrimental role in the pathogenesis of human immunodeficiency virus (HIV) infection. Much effort has been focused to comprehend the mechanisms and factors that regulate the expansion of such unwanted immune cell populations. Of particular interest has been the mechanisms by which MDSCs could contribute to the pathogenesis of HIV infection. So far, the studies have been restricted to MDSCs in the circulatory system of HIV patients, but not in other tissue compartments. In fact, lymphatic tissues/organs are the primary sites where HIV replication and immune depletion/dysfunction occur during the course of HIV infection. Therefore, investigating the anatomical distribution of MDSCs in such compartments is essential to understand the role that they play in the pathogenesis of HIV infection. Hence, we aim to shed light on the available literature about the anatomical distribution of MDSCs during HIV infection and compare it with the distribution of MDSCs in other pathological conditions, mainly cancer.

Myeloid-derived suppressor cells and the pathogenesis of human immunodeficiency virus infection

There are several mechanisms by which human immunodeficiency virus (HIV) can mediate immune dysfunction and exhaustion during the course of infection. Chronic immune activation, after HIV infection, seems to be a key driving force of such unwanted consequences, which in turn worsens the pathological status. In such cases, the immune system is programmed to initiate responses that counteract unwanted immune activation, for example through the expansion of myeloid-derived suppressor cells (MDSCs). Although the expansion of immune suppressor cells in the setting of systemic chronic immune activation, in theory, is expected to contain immune activation, HIV infection is still associated with a remarkably high level of biomarkers of immune activation. Paradoxically, the expansion of immune suppressor cells during HIV infection can suppress potent anti-viral immune responses, which in turn contribute to viral persistence and disease progression. This indicates that HIV hijacks not only immune activation but also the immune regulatory responses to its advantage. In this work, we aim to pave the way to comprehend how such unwanted expansion of MDSCs could participate in the pathology of acute/primary and chronic HIV infection in humans, as well as simian immunodeficiency virus infection in rhesus macaques, according to the available literature.

Role of myeloid-derived suppressor cells in viral respiratory infections; Hints for discovering therapeutic targets for COVID-19

The expansion of myeloid-derived suppressor cells (MDSCs), known as heterogeneous population of immature myeloid cells, is enhanced during several pathological conditions such as inflammatory or viral respiratory infections. It seems that the way MDSCs behave in infection depends on the type and the virulence mechanisms of the invader pathogen, the disease stage, and the infection-related pathology. Increasing evidence showing that in correlation with the severity of the disease, MDSCs are accumulated in COVID-19 patients, in particular in those at severe stages of the disease or ICU patients, contributing to pathogenesis of SARS-CoV2 infection. Based on the involved subsets, MDSCs delay the clearance of the virus through inhibiting T-cell proliferation and responses by employing various mechanisms such as inducing the secretion of anti-inflammatory cytokines, inducible nitric oxide synthase (iNOS)-mediated hampering of IFN-γ production, or forcing arginine shortage. While the immunosuppressive characteristic of MDSCs may help to preserve the tissue homeostasis and prevent hyperinflammation at early stages of the infection, hampering of efficient immune responses proved to exert significant pathogenic effects on severe forms of COVID-19, suggesting the targeting of MDSCs as a potential intervention to reactivate T-cell immunity and thereby prevent the infection from developing into severe stages of the disease. This review tried to compile evidence on the roles of different subsets of MDSCs during viral respiratory infections, which is far from being totally understood, and introduce the promising potential of MDSCs for developing novel diagnostic and therapeutic approaches, especially against COVID-19 disease.

MDSC in Mice and Men: Mechanisms of Immunosuppression in Cancer

Myeloid-derived suppressor cells (MDSCs) expand during pathological conditions in both humans and mice and their presence is linked to poor clinical outcomes for cancer patients. Studying MDSC immunosuppression is restricted by MDSCs’ rarity, short lifespan, heterogeneity, poor viability after freezing and the lack of MDSC-specific markers. In this review, we will compare identification and isolation strategies for human and murine MDSCs. We will also assess what direct and indirect immunosuppressive mechanisms have been attributed to MDSCs. While some immunosuppressive mechanisms are well-documented in mice, e.g., generation of ROS, direct evidence is still lacking in humans. In future, bulk or single-cell genomics could elucidate which phenotypic and functional phenotypes MDSCs adopt in particular microenvironments and help to identify potential targets for therapy.

Neutrophils in COVID-19

Strong evidence has been accumulated since the beginning of the COVID-19 pandemic that neutrophils play an important role in the pathophysiology, particularly in those with severe disease courses. While originally considered to be a rather homogeneous cell type, recent attention to neutrophils has uncovered their fascinating transcriptional and functional diversity as well as their developmental trajectories. These new findings are important to better understand the many facets of neutrophil involvement not only in COVID-19 but also many other acute or chronic inflammatory diseases, both communicable and non-communicable. Here, we highlight the observed immune deviation of neutrophils in COVID-19 and summarize several promising therapeutic attempts to precisely target neutrophils and their reactivity in patients with COVID-19.

Early systemic immune biomarkers predict bone regeneration after trauma

Severe traumatic injuries are a widespread and challenging clinical problem, and yet the factors that drive successful healing and restoration of function are still not well understood. One recently identified risk factor for poor healing outcomes is a dysregulated immune response following injury. In a preclinical model of orthopedic trauma, we demonstrate that distinct systemic immune profiles are correlated with impaired bone regeneration. Most notably, elevated blood levels of myeloid-derived suppressor cells (MDSCs) and the immunosuppressive cytokine interleukin-10 (IL-10) are negatively correlated with functional bone regeneration as early as 1 wk posttreatment. Nonlinear multivariate regression also implicated these two factors as the most influential in predictive computational models. These results support a significant relationship between early systemic immune responses to trauma and subsequent local bone regeneration and indicate that elevated circulating levels of MDSCs and IL-10 may be predictive of poor functional healing outcomes and represent novel targets for immunotherapeutic intervention.

The importance of advanced cytometry in defining new immune cell types and functions relevant for the immunopathogenesis of HIV infection

: In the last years, novel, exciting immunological findings of interest for HIV research and treatment were identified thanks to different cytometric approaches. The analysis of the phenotypes and functionality of cells belonging to the immune system could clarify their role in the immunopathogenesis of HIV infection, and to elaborate key concepts, relevant in the treatment of this disease. Important discoveries have been made concerning cells that are important for protective immunity like lymphocytes that display polyfunctionality, resident memory T cells, innate lymphoid cells, to mention a few. The complex phenotype of myeloid-derived suppressor cells has been investigated, and relevant changes have been reported during chronic and primary HIV infection, in correlation with changes in CD4 T-cell number, T-cell activation, and with advanced disease stage. The search for markers of HIV persistence present in latently infected cells, namely those molecules that are important for a functional or sterilizing cure, evidenced the role of follicular helper T cells, and opened a discussion on the meaning and use of different surface molecules not only in identifying such cells, but also in designing new strategies. Finally, advanced technologies based upon the simultaneous detection of HIV-RNA and proteins at the single cell level, as well as those based upon spectral cytometry or mass cytometry are now finding new actors and depicting a new scenario in the immunopathogenesis of the infection, that will allow to better design innovative therapies based upon novel drugs and vaccines.

Persistent lentivirus infection induces early myeloid suppressor cells expansion to subvert protective memory CD8 T cell response✰,✰✰

Background

Memory CD8⁺T cell responses play an essential role in protection against persistent infection. However, HIV-1 evades vaccine-induced memory CD8⁺T cell response by mechanisms that are not fully understood.

Methods

We analyzed the temporal dynamics of CD8⁺T cell recall activity and function during EcoHIV infection in a potent PD1-based vaccine immunized immunocompetent mice.

Findings

Upon intraperitoneal EcoHIV infection, high levels of HIV-1 GAG-specific CD8⁺T lymphocytes recall response reduced EcoHIV-infected cells significantly. However, this protective effect diminished quickly after seven days, followed by a rapid reduction of GAG-specific CD8⁺T cell number and activity, and viral persistence. Mechanistically, EcoHIV activated dendritic cells (DCs) and myeloid cells. Myeloid cells were infected and rapidly expanded, exhibiting elevated PD-L1/-L2 expression and T cell suppressive function before day 7, and were resistant to CD8⁺T cell-mediated apoptosis. Depletion of myeloid-derived suppressor cells (MDSCs) reduced EcoHIV infection and boosted T cell responses.

Interpretation

This study provides an overview of the temporal interplay of persistent virus, DCs, MDSCs and antigen-specific CD8⁺T cells during acute infection. We identify MDSCs as critical gatekeepers that restrain antiviral T cell memory responses, and highlight MDSCs as an important target for developing effective vaccines against chronic human infections.

Funding

Hong Kong Research Grant Council (T11–709/18-N, HKU5/CRF/13G), General Research Fund (17122915 and 17114114), Hong Kong Health and Medical Research Fund (11100752, 14130582, 16150662), Grant RGC-ANR A-HKU709/14, the San-Ming Project of Medicine (SZSM201512029), University Development Fund of the University of Hong Kong and Li Ka Shing Faculty of Medicine Matching Fund to HKU AIDS Institute.

Paradoxes in the Phenotype, Frequency and Roles of Myeloid-Derived Suppressor Cells During HIV Infection

Myeloid-derived suppressor cells (MDSCs) are heterogeneous groups of pathologically activated myeloid cells with potent immunosuppressive function. Due to their role in negatively regulating the immune system, MDSCs have been strongly correlated with disease progression during HIV. However, findings vary considerably between studies. The dominant phenotype of MDSC subsets during HIV is not well ascertained. Moreover, there is no clear understanding on the clinical significance of MDSCs during HIV infection. The existing evidences showed the double-sided roles of MDSCs in HIV. On the one hand, MDSCs are linked to deleterious effects during HIV infection as they inhibit proliferation of protective T cell response. On the other hand, the immunosuppressive abilities of MDSCs were shown to be beneficial in curbing the damaging effects of persistent immune activation associated with chronic HIV infection. Therefore, this review aimed to describe the differences in the existing literatures pertaining to the phenotype, frequency and roles of MDSCs during HIV infection.

Impact of analytical treatment interruption on the central nervous system in a simian-HIV model

OBJECTIVE(S)

Analytical treatment interruption (ATI) studies are often used to evaluate potential HIV cure strategies. This study was conducted to determine the impact of ATI on simian-HIV (SHIV) infection in the central nervous system.

DESIGN

Animal study.

METHODS

Nine rhesus macaques were inoculated with SHIV-1157ipd3N4. Antiretroviral therapy (ART) was administered from week 2 to 18. At week 18, four animals were euthanized (no-ATI-group) and five underwent ATI (ATI-group) and were euthanized at 12 weeks post viral rebound. Plasma and cerebrospinal fluid (CSF) SHIV-RNA, markers of inflammation and brain CD3+, CD68+/CD163+ and RNA+ cells were measured.

RESULTS

All nine animals were SHIV-infected, with median pre-ART plasma and CSF SHIV-RNA of 6.2 and 3.6 log10copies/ml. Plasma and CSF IL-15, monocyte chemoattractant protein-1, IFN-γ-induced protein-10 and neopterin increased postinfection. ART initiation was associated with rapid and complete suppression of plasma viremia and reductions in plasma and CSF IL-15, IFN-γ-induced protein-10, neopterin and CSF monocyte chemoattractant protein-1. Median time to plasma viral rebound was 21 days post-ATI. At 12 weeks postrebound, CSF SHIV-RNA was undetectable and no increases in plasma and CSF markers of inflammation were found. Higher numbers of CD3+ and CD68+/CD163+ cells were seen in the brains of 3/5 and 1/5 animals, respectively, in the ATI-group when compared with no-ATI-group. SHIV-RNA+ cells were not identified in the brain in either group post-ATI.

CONCLUSION

ATI in macaques that initiated ART during early SHIV-1157ipd3N4 infection was associated with mild, localized T-cell infiltrate in the brain without detectable SHIV-RNA in the brain or CSF, or elevation in CSF soluble markers of inflammation.

MDSCs in infectious diseases: regulation, roles, and readjustment

Many pathogens, ranging from viruses to multicellular parasites, promote expansion of MDSCs, which are myeloid cells that exhibit immunosuppressive features. The roles of MDSCs in infection depend on the class and virulence mechanisms of the pathogen, the stage of the disease, and the pathology associated with the infection. This work compiles evidence supported by functional assays on the roles of different subsets of MDSCs in acute and chronic infections, including pathogen-associated malignancies, and discusses strategies to modulate MDSC dynamics to benefit the host.

Highlights from the Fourth Biennial Strategies for an HIV Cure Meeting, 10-12 October 2018, Bethesda, MD, USA

The National Institute of Allergy and Infectious Diseases (NIAID) organised the Strategies for an HIV Cure 2018 meeting focused on research to develop innovative strategies for eradicating or achieving long-term remission of HIV infection. The purpose was to bring together researchers studying HIV persistence and cure strategies, including the six National Institutes of Health (NIH)-funded Martin Delaney Collaboratories for HIV Cure Research (MDCs), as well as industry and community partners, to share scientific results and stimulate active discussion among all stakeholders about the merits of various approaches under investigation. These discussions were intended to stimulate new collaborations and ideas for future research. The meeting covered a comprehensive range of topics spanning basic and translational research, drug discovery and development, and clinical research. Aside from the oral presentations described here, the meeting also included 130 poster presentations. Each of the three days of presentations is available for viewing via the NIH VideoCast website at: https://videocast.nih.gov/PastEvents.asp.

Treatment Intensification in HIV-Infected Patients Is Associated With Reduced Frequencies of Regulatory T Cells

In untreated HIV infection, the efficacy of T cell responses decreases over the disease course, resulting in disease progression. The reasons for this development are not completely understood. However, immunosuppressive cells are supposedly crucially involved. Treatment strategies to avoid the induction of these cells preserve immune functions and are therefore the object of intense research efforts. In this study, we assessed the effect of treatment intensification [=5-drug antiretroviral therapy (ART)] on the development of suppressive cell subsets. The New Era (NE) study recruited patients with primary HIV infection (PHI) or chronically HIV-infected patients with conventional ART (CHI) and applied an intensified 5-drug regimen containing maraviroc and raltegravir for several years. We compared the frequencies of the immune suppressive cells, namely, the myeloid-derived suppressor cells (MDSCs), regulatory B cells (Bregs), and regulatory T cells (Tregs), of the treatment intensification patients to the control groups, especially to the patients with conventional 3-drug ART, and analyzed the Gag/Nef-specific CD8 T cell responses. There were no differences between PHI and CHI in the NE population (p > 0.11) for any of the studied cell types. Polymorphonuclear myeloid-derived suppressor cell (PMN-MDSC), monocytic myeloid-derived suppressor cell (M-MDSC), and the Breg frequencies were comparable to those of patients with a 3-drug ART. However, the Treg levels were significantly lower in the NE patients than those in 3ART-treated individuals and other control groups (p ≤ 0.0033). The Gag/Nef-specific CD8 T cell response was broader (p = 0.0134) with a higher magnitude (p = 0.026) in the NE population than that in the patients with conventional ART. However, we did not find a correlation between the frequency of the immune suppressive cells and the interferon-gamma⁺ CD8 T cell response. In the treatment intensification subjects, the frequencies of the immune suppressive cells were comparable or lower than those of the conventional ART-treated subjects, with surprisingly broad HIV-specific CD8 T cell responses, suggesting a preservation of immune function with the applied treatment regimen. Interestingly, these effects were seen in both treatment intensification subpopulations and were not attributed to the start of treatment in primary infection.

Monocytic Myeloid-Derived Suppressor Cells in Chronic Infections

Heterogeneous populations of myeloid regulatory cells (MRC), including monocytes, macrophages, dendritic cells, and neutrophils, are found in cancer and infectious diseases. The inflammatory environment in solid tumors as well as infectious foci with persistent pathogens promotes the development and recruitment of MRC. These cells help to resolve inflammation and establish host immune homeostasis by restricting T lymphocyte function, inducing regulatory T cells and releasing immune suppressive cytokines and enzyme products. Monocytic MRC, also termed monocytic myeloid-derived suppressor cells (M-MDSC), are bona fide phagocytes, capable of pathogen internalization and persistence, while exerting localized suppressive activity. Here, we summarize molecular pathways controlling M-MDSC genesis and functions in microbial-induced non-resolved inflammation and immunopathology. We focus on the roles of M-MDSC in infections, including opportunistic extracellular bacteria and fungi as well as persistent intracellular pathogens, such as mycobacteria and certain viruses. Better understanding of M-MDSC biology in chronic infections and their role in antimicrobial immunity, will advance development of novel, more effective and broad-range anti-infective therapies.

Immunomodulatory effects of myeloid-derived suppressor cells in diseases: Role in cancer and infections

Myeloid-derived suppressor cells (MDSCs) are heterogeneous cells capable of abrogating T and B cells responses and have been identified in numerous cancers. As with other regulatory cell populations, they aim to maintain balance between host-defence-associated inflammation and ensuing tissue pathology. MDSC accumulation and/or activation involve several growth factors and cytokines including Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) and Interleukin (IL)-6 and suppression has been linked to receptors such as IL-4Rα. Other immune pathways, such as Toll-like receptors (TLRs) have also been shown to interfere in MDSC activity adding to the complexity in clarifying their pathways. Monocytic- (Mo-MDSCs) and polymorphonuclear- (PMN-MDSCs) cells are two subsets of MDSCs that have been well characterized and have been shown to function through different mechanisms although both appear to require nitric oxide. In human and murine model settings, MDSCs have been shown to have inhibitory effects on T cell responses during bacterial, parasitic and viral pathologies and an increase of MDSC numbers has been associated with pathological conditions. Interestingly, the environment impacts on MDSC activity and regulatory T cells (Tregs), mast cells and a few cells that may help MDSC in order to regulate immune responses. Since the majority of pioneering data on MDSCs has stemmed from research on malignancies, this review will summarize MDSC biology and function in cancer and highlight current knowledge about these cells during infectious pathologies as well.