Insufficient natural killer cell responses against retroviruses: how to improve NK cell killing of retrovirus-infected cells

New sights of low dose IL-2: Restoration of immune homeostasis for viral infection

Viral infection poses a significant threat to human health. In addition to the damage caused by viral replication, the immune response it triggers often leads to more serious adverse consequences. After the occurrence of viral infection, in addition to the adverse consequences of infection, chronic infections can also lead to virus-related autoimmune diseases and tumours. At the same time, the immune response triggered by viral infection is complex, and dysregulated immune response may lead to the occurrence of immune pathology and macrophage activation syndrome. In addition, it may cause secondary immune suppression, especially in patients with compromised immune system, which could lead to the occurrence of secondary infections by other pathogens. This can often result in more severe clinical outcomes. Therefore, regarding the treatment of viral infections, restoring the balance of the immune system is crucial in addition to specific antiviral medications. In recent years, scientists have made an interesting finding that low dose IL-2 (ld-IL-2) could potentially have a crucial function in regulating the immune system and reducing the chances of infection, especially viral infection. Ld-IL-2 exerts immune regulatory effects in different types of viral infections by modulating CD4+ T subsets, CD8+ T cells, natural killer cells, and so on. Our review summarised the role of IL-2 or IL-2 complexes in viral infections. Ld-IL-2 may be an effective strategy for enhancing host antiviral immunity and preventing infection from becoming chronic; additionally, the appropriate use of it can help prevent excessive inflammatory response after infection. In the long term, it may reduce the occurrence of infection-related autoimmune diseases and tumours by promoting the restoration of early immune homeostasis. Furthermore, we have also summarised the application of ld-IL-2 in the context of autoimmune diseases combined with viral infections; it may be a safe and effective strategy for restoring immune homeostasis without compromising the antiviral immune response. In conclusion, focusing on the role of ld-IL-2 in viral infections may provide a new perspective for regulating immune responses following viral infections and improving prognosis.

Fatty acids are crucial to fuel NK cells upon acute retrovirus infection

Natural killer (NK) cells are cytotoxic innate immune cells, able to recognize and eliminate virus-infected as well as cancer cells. Metabolic reprogramming is crucial for their activity as they have enhanced energy and nutritional demands for their functions during an infection. Fatty acids (FAs) represent an important source of cellular energy and are essential for proliferation of immune cells. However, the precise role of FAs for NK cells activity in retrovirus infection was unknown. Here we show that activated NK cells increase the expression of the FA uptake receptor CD36 and subsequently the uptake of FAs upon acute virus infection. We found an enhanced flexibility of NK cells to utilize FAs as source of energy compare to naïve NK cells. NK cells that were able to generate energy from FAs showed an augmented target cell killing and increased expression of cytotoxic parameters. However, NK cells that were unable to generate energy from FAs exhibited a severely decreased migratory capacity. Our results demonstrate that NK cells require FAs in order to fight acute virus infection. Susceptibility to severe virus infections as it is shown for people with malnutrition may be augmented by defects in the FA processing machinery, which might be a target to therapeutically boost NK cell functions in the future.

Purtscher-like retinopathy associated with systemic lupus erythematosus treated with rituximab plus low-dose interleukin-2: A case report

Purtscher-like retinopathy, an occlusive microvasculopathy, is a rare and severe ophthalmic complication of systemic lupus erythematosus (SLE) characterized by a sudden loss of vision with retinal whitening, cotton wool spots and minimal intraretinal hemorrhage. Recovery in visual acuity is usually poor even with prompt treatment. This case showed a patient with SLE concurrent Purtscher-like retinopathy treated with rituximab and interleukin-2 (IL-2) with good prognosis. A 16-year-old female with a 2-year history of SLE was admitted because of unrelieved disease activity of SLE when treated with a high dose of corticosteroids and immunosuppressants and she further suffered from reduced visual acuity in both eyes. She was diagnosed with Purtscher-like retinopathy secondary to SLE after ocular examination. Rituximab and low-dose IL-2 for systemic treatment and intravitreal injection of anti-vascular endothelial growth factor antibody to right eye were given. The SLE disease was completely relieved with the sight recovering and no recurrence of Purtscher-like retinopathy was reported during 6-year follow-up. Purtscher-like retinopathy associated with SLE should be treated early and promptly. Rituximab should be considered in SLE patients with Purtscher-like retinopathy who have an incomplete response to initial immunosuppressive therapy and low-dose IL-2 may help induction of clinical remission.

Low-dose IL-2 therapy limits the reduction in absolute numbers of peripheral lymphocytes in systemic lupus erythematosus patients with infection

BACKGROUND

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disorder characterized by disturbed cellular and humoral immune responses. Dysregulations of immune system and immunosuppressive medications predispose SLE patients to infection. This study aims to investigate the alterations and absolute concentrations of lymphocyte subpopulations in SLE patients with different infection and their responses of low-dose IL-2 therapy.

METHODS

A total of 333 patients with SLE without recent infection, 162 patients suffering infection, and age and sex-matched 132 healthy controls (HCs) were recruited. Of them, 54 SLE patients (including 41 non-infected group and 13 infected group) received a 5-day course of low-dose IL-2 administration at a dose of 0.5 million IU per day. Lymphocyte subpopulations were analyzed by flow cytometry.

RESULTS

Patients with SLE had lower levels of lymphocyte subpopulations in peripheral blood such as T, B, NK, CD4 + T, CD8+ T, Th1, Th2, Th17, and Treg cells, and the reduction in these cells was more obvious in patients with infection (p <.05 to p <.01). Low-dose IL-2 effectively expanded T (p <.001), B (p <.001), CD4 + T (p <.01), CD8 + T (p <.001), Th1 (p <.01), Th17 (p <.1), and Treg cells (p <.01) of SLE patients, these cells were comparable to that of HCs after the IL-2 treatment.

CONCLUSIONS

Patients with SLE had insufficiency of circulating lymphocyte subsets. This phenomenon was more obverse in those accompanying infection, suggesting the low concentration of lymphocytes may be used as indicators of high infection risk in SLE patients. Low-dose IL-2 induced expansion of Treg cells and NK cells, which may contribute to the restoration of immune homeostasis in SLE patients.

Metabolic requirements of NK cells during the acute response against retroviral infection

Natural killer (NK) cells are important early responders against viral infections. Changes in metabolism are crucial to fuel NK cell responses, and altered metabolism is linked to NK cell dysfunction in obesity and cancer. However, very little is known about the metabolic requirements of NK cells during acute retroviral infection and their importance for antiviral immunity. Here, using the Friend retrovirus mouse model, we show that following infection NK cells increase nutrient uptake, including amino acids and iron, and reprogram their metabolic machinery by increasing glycolysis and mitochondrial metabolism. Specific deletion of the amino acid transporter Slc7a5 has only discrete effects on NK cells, but iron deficiency profoundly impaires NK cell antiviral functions, leading to increased viral loads. Our study thus shows the requirement of nutrients and metabolism for the antiviral activity of NK cells, and has important implications for viral infections associated with altered iron levels such as HIV and SARS-CoV-2.

Metabolic alterations control the fate and function of immune cells in response to infections, but the function of NK cell metabolism in the context of acute viral infections is unclear. Here the authors show that acute NK cell responses to Friend retrovirus involve increased glycolysis and mitochondrial metabolism and require amino acid transport as well as iron sufficiency.

The DARC-null trait is associated with moderate modulation of NK cell profiles and unaltered cytolytic T cell profiles in black South Africans

The Duffy Antigen Receptor for Chemokines (DARC)-null trait, common among persons of African descent and associated with lower absolute neutrophil counts (ANCs), may be linked to increased risk to certain infections including HIV-1 but the underlying causes are poorly understood. We hypothesized that DARC-null-linked neutropenia may negatively impact neutrophil immunoregulatory modulation of other immune cells such as natural killer (NK) and CD8+ T cells leading to altered phenotype, functionality and homeostatic activity of these immune cells. HIV-1 uninfected (n = 20) and HIV-1 chronically infected (n = 19) participants were assessed using multi-parametric flow cytometry to determine NK and CD8+ T cell counts, phenotypic profiles, and cytokine production and degranulation. Annexin V and carboxyfluorescein succinimidyl ester (CFSE) staining were used to examine NK cell survival and NK cell and CD8+ T cell proliferation respectively. Participants were genotyped for the DARC-null polymorphism using allelic discrimination assays and ANCs were measured by full blood count. In HIV uninfected individuals, a reduction of total NK cell counts was noted in the absence of DARC and this correlated with lower ANCs. HIV uninfected DARC-null subjects displayed a less mature NK cell phenotype. However, this did not translate to differences in NK cell activation or effector functionality by DARC state. Whilst HIV-1 infected subjects displayed NK cell profiling that is typical of HIV infection, no differences were noted upon DARC stratification. Similarly, CD8+ T cells from HIV infected individuals displayed phenotypic and functional modulation that is characteristic of HIV infection, but profiling was unaffected by the DARC-null variant irrespective of HIV status. Overall, the data suggests that the DARC-null polymorphism and lower ANCs does not impede downstream cytolytic cell priming and functionality.

Efficacy and safety of low-dose IL-2 in the treatment of systemic lupus erythematosus: a randomised, double-blind, placebo-controlled trial

Objectives

Open-labelled clinical trials suggested that low-dose IL-2 might be effective in treatment of systemic lupus erythematosus (SLE). A double-blind and placebo-controlled trial is required to formally evaluate the safety and efficacy of low-dose IL-2 therapy.

Methods

A randomised, double-blind and placebo-controlled clinical trial was designed to treat 60 patients with active SLE. These patients received either IL-2 (n=30) or placebo (n=30) with standard treatment for 12 weeks, and were followed up for additional 12 weeks. IL-2 at a dose of 1 million IU or placebo was administered subcutaneously every other day for 2 weeks and followed by a 2-week break as one treatment cycle. The primary endpoint was the SLE Responder Index-4 (SRI-4) at week 12. The secondary endpoints were other clinical responses, safety and dynamics of immune cell subsets.

Results

At week 12, the SRI-4 response rates were 55.17% and 30.00% for IL-2 and placebo, respectively (p=0.052). At week 24, the SRI-4 response rate of IL-2 group was 65.52%, compared with 36.67% of the placebo group (p=0.027). The primary endpoint was not met at week 12. Low-dose IL-2 treatment resulted in 53.85% (7/13) complete remission in patients with lupus nephritis, compared with 16.67% (2/12) in the placebo group (p=0.036). No serious infection was observed in the IL-2 group, but two in placebo group. Besides expansion of regulatory T cells, low-dose IL-2 may also sustain cellular immunity with enhanced natural killer cells.

Conclusions

Low-dose IL-2 might be effective and tolerated in treatment of SLE.

Trial registration number

ClinicalTrials.gov Registries (NCT02465580 and NCT02932137).

Friend retrovirus studies reveal complex interactions between intrinsic, innate and adaptive immunity

Approximately 4.4% of the human genome is comprised of endogenous retroviral sequences, a record of an evolutionary battle between man and retroviruses. Much of what we know about viral immunity comes from studies using mouse models. Experiments using the Friend virus (FV) model have been particularly informative in defining highly complex anti-retroviral mechanisms of the intrinsic, innate and adaptive arms of immunity. FV studies have unraveled fundamental principles about how the immune system controls both acute and chronic viral infections. They led to a more complete understanding of retroviral immunity that begins with cellular sensing, production of type I interferons, and the induction of intrinsic restriction factors. Novel mechanisms have been revealed, which demonstrate that these earliest responses affect not only virus replication, but also subsequent innate and adaptive immunity. This review on FV immunity not only surveys the complex host responses to a retroviral infection from acute infection to chronicity, but also highlights the many feedback mechanisms that regulate and counter-regulate the various arms of the immune system. In addition, the discovery of molecular mechanisms of immunity in this model have led to therapeutic interventions with implications for HIV cure and vaccine development.

Clinical Relevance of Autoantibodies against Interleukin-2 in Patients with Systemic Lupus Erythematosus

Background:

Increased serum autoantibodies against interleukin-2 (anti-IL-2 autoantibodies) were reported in patients with systemic lupus erythematosus (SLE) and in patients receiving IL-2 therapy. This study aimed to explore the clinical relevance of serum anti-IL-2 autoantibodies and the interactions between low-dose IL-2 therapy and serum anti-IL-2 autoantibodies.

Methods:

Serum samples were collected from 152 SLE patients and 100 age- and gender-matched healthy controls (HCs). Among them, 75 SLE patients were followed up for 10 weeks, and all of them were treated with corticosteroids, antimalarials, and/or immunosuppressants. Forty-six out of the 75 SLE patients received low-dose IL-2 therapy additionally. Clinical and laboratory parameters were collected at baseline and week 10. Serum anti-IL-2 autoantibodies were determined by enzyme-linked immunosorbent assay.

Results:

Compared with HCs, median levels and positive rates of serum anti-IL-2 autoantibodies were higher in SLE patients (32.58 [23.63, 45.23] arbitrary unit [AU] vs. 37.54 [27.88, 60.74] AU, P = 0.006, and 5.0% vs. 18.4%, P = 0.002, respectively). Compared to those without the corresponding disorders, serum anti-IL-2 autoantibody was increased in patients with alopecia (49.79 [36.06, 64.95] AU vs. 35.06 [25.40, 58.46] AU, P = 0.033), but it was decreased in those with lupus nephritis (31.71 [22.60, 43.25] AU vs. 44.15 [31.43, 68.52] AU, P = 0.001). Moreover, serum anti-IL-2 autoantibody was positively correlated with serum IgA (r = 0.229, P = 0.005), total IgG (r = 0.327, P < 0.001), and total IgM (r = 0.164, P = 0.050). Treatment with exogenous IL-2 was not significantly associated with serum anti-IL-2 autoantibody. In addition, no significant difference was found in serum anti-IL-2 autoantibody between responders and nonresponders to low-dose IL-2 therapy.

Conclusions:

Serum anti-IL-2 autoantibody was increased and associated with disease severity in SLE. Exogenous low-dose IL-2 did not significantly induce anti-IL-2 autoantibody production.

Friend retrovirus infection induces the development of memory-like natural killer cells

Traditionally, NK cells belong to the innate immune system and eliminate virus-infected cells through their germline-encoded receptors. However, NK cells were recently reported to possess memory-like functions that were predominantly provided by hepatic NK cells. Memory properties were mainly documented in contact hypersensitivity models or during cytomegalovirus infections. However, the precise role and the physiologic importance of memory-like NK cells during retroviral infections are still under investigation. Here, we show that Friend retrovirus (FV) infection of mice induced a population of phenotypically memory-like NK cells at 28 days post infection. Upon secondary antigen encounter, these NK cells showed an increased production of the pro-inflammatory cytokines IFNγ and TNFα as well as the death ligand FasL in comparison to naïve NK cells. Furthermore, we found an augmented elimination of antigen-matched but not antigen-mismatched target cells by these memory-like NK cells. In adoptive cell transfer experiments, equal antiviral activities of splenic and hepatic memory-like NK cells during the late phase of acute FV infection were found. Our results strongly imply the existence and antiviral activity of spleen and liver memory-like NK cells in FV infection, which efficiently respond upon secondary exposure to retroviral antigens.

The Cytotoxic Activity of Natural Killer Cells Is Suppressed by IL-10+ Regulatory T Cells During Acute Retroviral Infection

Natural killer (NK) cells play a key role in host defense against cancer and viral infections. It was shown that NK cells are important for the control of acute retroviral infections, but their antiviral activity depends on multiple parameters such as viral inoculation dose, interactions with myeloid cell types and the cytokine milieu. In addition, during an ongoing retroviral infection regulatory T cells (Tregs) can suppress NK cell functions. However, the precise role of Tregs on the initial NK cell response and their immediate antiviral activity after an acute retroviral infection is still unknown. Here we show that thymus-derived Tregs suppress the proliferation, effector functions and cytotoxicity of NK cells very early during acute Friend Retrovirus (FV) infection. Tregs exhibited an activated phenotype and increased the production of the immunosuppressive cytokines IL-10 and TGF-β after FV infection of mice. Neutralization of the immunosuppressive cytokine IL-10 resulted in a significant augmentation of NK cell functions. Although the activation of dendritic cells (DCs) and macrophages as well as the IL-15 cytokine levels were increased after Treg depletion, Tregs mainly affect the NK cell activity in an IL-10-regulated pathway. In this study we demonstrate an IL-10-dependent suppression of NK cells by activated Tregs during the first days of a retroviral infection.

IL-10+ NK and TGF-β+ NK cells play negative regulatory roles in HIV infection

Background

Natural killer (NK) cells play cytotoxic roles by targeting tumor cells or virus infected cells, they also play regulatory roles by secreting cytokines and chemokines. Transforming growth factor (TGF)-β and interleukin (IL)-10 are important immunosuppressive cytokines potentially related to the immune dysregulation that occurs in the infection of human immunodeficiency virus (HIV). NK cells are an important source of TGF-β and a main early producer of IL-10 in response to viral infection. Here, we evaluated the percentages of IL-10⁺ and TGF-β⁺ NK cells in HIV-infected patients relative to healthy controls (HCs).

Methods

Study participants (n = 63) included 31 antiretroviral treatment (ART)-naïve HIV-infected patients, 17 ART-treated HIV-infected patients, and 15 HIV-negative HCs. Expression of IL-10 or TGF-β in NK cells was examined by flow cytometry, and the influences of recombinant IL-10 (rIL-10) or recombinant TGF-β (rTGF-β) on NK cell function were investigated in vitro.

Results

Compared with HCs, ART-naïve HIV-infected patients had increased percentages of IL-10⁺ (2.0% vs. 0.4%, p = 0.015) and TGF-β⁺ (4.5% vs. 2.1%, p = 0.022) NK cells, and ART-treated patients also had a higher percentage of IL-10⁺ NK cells (2.5% vs. 0.4%, p = 0.002). The percentages of IL-10⁺ and TGF-β⁺ NK cells were positively correlated (r = 0.388; p = 0.010). The results of in vitro experiments demonstrated that rIL-10 and rTGF-β inhibited NK cell CD107a expression (p = 0.037 and p = 0.024, respectively), IFN-γ secretion (p = 0.006, p = 0.016, respectively), and granzyme B release after stimulation (p = 0.014, p = 0.040, respectively).

Conclusions

Our data suggest that the percentages of IL-10⁺ or TGF-β⁺ NK cells are increased in HIV-infected patients, and that rIL-10 and/or rTGF-β can inhibit NK cell functions in vitro, providing a potential therapeutic target for strategies aimed at combating HIV infection.