Depletion but Activation of CD56dimCD16+ NK Cells in Acute Infection with Severe Fever with Thrombocytopenia Syndrome Virus

Current insights into human pathogenic phenuiviruses and the host immune system

Phenuiviruses are a class of segmented negative-sense single-stranded RNA viruses, typically consisting of three RNA segments that encode four distinct proteins. The emergence of pathogenic phenuivirus strains, such as Rift Valley fever phlebovirus (RVFV) in sub-Saharan Africa, Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV) in East and Southeast Asia, and Heartland Virus (HRTV) in the United States has presented considerable challenges to global public health in recent years. The innate immune system plays a crucial role as the initial defense mechanism of the host against invading pathogens. In addition to continued research aimed at elucidating the epidemiological characteristics of phenuivirus, significant advancements have been made in investigating its viral virulence factors (glycoprotein, non-structural protein, and nucleoprotein) and potential host-pathogen interactions. Specifically, efforts have focused on understanding mechanisms of viral immune evasion, viral assembly and egress, and host immune networks involving immune cells, programmed cell death, inflammation, nucleic acid receptors, etc. Furthermore, a plethora of technological advancements, including metagenomics, metabolomics, single-cell transcriptomics, proteomics, gene editing, monoclonal antibodies, and vaccines, have been utilized to further our understanding of phenuivirus pathogenesis and host immune responses. Hence, this review aims to provide a comprehensive overview of the current understanding of the mechanisms of host recognition, viral immune evasion, and potential therapeutic approaches during human pathogenic phenuivirus infections focusing particularly on RVFV and SFTSV.

[Flow cytometry analysis of normal range of natural killer cells and their subsets in peripheral blood of healthy Chinese adults]

OBJECTIVE

To study the distribution characteristics of natural killer (NK) cells and their subsets in normal peripheral blood in China, and to explore their normal value and significance.

METHODS

In this study, peripheral blood was collected from 200 healthy adults. Their age range was 18-87 years. All the subjects were divided into 6 age groups: 18-30, 31-40, 41-50, 51-60, 61-70, and 71-87 years. With CD16, CD56, CD4, CD19, as surface markers, fluid cytology detection techniques were used to detect NK cells and the relative and absolute counts. SPSS 27.0 was used for systematic analysis of the data, and the measurement data were expressed as mean±standard deviations. A t test, variance analysis or rank sum test were performed to compare the differences between the age groups and the sex groups. The significance level was set at α=0.05, and P < 0.05 was considered statistically significant.

RESULTS

The range of NK B cells in the 200 healthy adult subjects was (0.46±0.24)×106/L, that of CD3-CD56+NK cells was (13.14±7.56)×106/L, that of CD56dimCD16+NK cells was (5.23±3.12)×106/L, that of CD56brightNK cells was (85.61±7.40)×106/L, and that of NK T cells was (4.16±3.34)×106/L. There were no statistically significant differences in CD3-CD56+NK cells and NK T cells with respect to age (P= 0.417, P=0.217). However, there was a decreasing trend in the number of NK B cells and CD56dimCD16+NK cells with increasing age (r=0.234, P < 0.001; r=0.099, P < 0.001), particularly after the age of 50. Conversely, CD56brightNK cells showed an increasing trend with age (r=0.143, P < 0.001).

CONCLUSION

The detection of NK cells and their subsets has significant reference value for the diagnosis, treatment, and prognosis of autoimmune diseases, infectious diseases, and tumors. This study provides a preliminary reference range for clinical detection of NK cell subsets, but further research with a larger sample size and multi-center trials are needed to confirm these findings.

TLR2/NF-кB signaling may control expansion and function of regulatory T cells in patients with severe fever with thrombocytopenia syndrome (SFTS)

Severe fever with thrombocytopenia syndrome (SFTS) is a recently identified infectious ailment triggered by a new strain of bunyavirus. It is distinguished by elevated fatality rates, ranging from 12 % to 30 %. The mechanism underlying the development of severe illness caused by SFTS bunyavirus (SFTSV) is not yet fully understood. To evaluate the role of the TLR2 receptor pathway in regulating Treg function in the progression of SFTS disease and possible mechanisms, sequential serum samples from 29 patients with SFTS (15 mild, 14 severe cases) were examined. Flow cytometry was employed to scrutinize the phenotypic and functional characteristics of TLR2 expression on circulating CD4 T cells, CD8 T cells, and Tregs. In all admitted patients, the evaluation of correlations between the frequencies of the aforementioned cells and SFTS index (SFTSI) was conducted. For SFTS, the levels of TLR2 on CD4 T cells and Tregs were significantly heightened when compared to those in healthy subjects. Additionally, the expression of TLR2 on Tregs exhibited a positive correlation with Ki-67 expression in Tregs and the severity of disease. Additionally, compared with those in uninfected controls, the expression levels of NF-κB in Tregs were significantly increased. Collectively, Tregs may be activated and proliferate through the stimulation of the TLR2/NF-кB pathway in reaction to SFTSV infection.

Antiviral immunity of severe fever with thrombocytopenia syndrome: current understanding and implications for clinical treatment

Dabie Banda virus (DBV), a tick-borne pathogen, was first identified in China in 2009 and causes profound symptoms including fever, leukopenia, thrombocytopenia and multi-organ dysfunction, which is known as severe fever with thrombocytopenia syndrome (SFTS). In the last decade, global incidence and mortality of SFTS increased significantly, especially in East Asia. Though previous studies provide understandings of clinical and immunological characteristics of SFTS development, comprehensive insight of antiviral immunity response is still lacking. Here, we intensively discuss the antiviral immune response after DBV infection by integrating previous ex- and in-vivo studies, including innate and adaptive immune responses, anti-viral immune responses and long-term immune characters. A comprehensive overview of potential immune targets for clinical trials is provided as well. However, development of novel strategies for improving the prognosis of the disease remains on challenge. The current review may shed light on the establishment of immunological interventions for the critical disease SFTS.

Changes in Cytokine Levels in Patients with Severe Fever with Thrombocytopenia Syndrome Virus

Purpose

To characterize the cytokine profile of patients with severe fever with thrombocytopenia syndrome (SFTS) in relation to disease severity.

Patients and Methods

60 laboratory-confirmed SFTS patients and 12 healthy individuals from multi-centers in Shandong Province of China were included, and all patients were divided into fatal patients (9) and recovered patients (51) due to their final outcomes. Multiplex-microbead immunoassays were conducted to estimate levels of 27 cytokines in the sera of patients and controls.

Results

The results showed that levels of IL-2, IL-4, IL-6, IL-7, IL-8, IL-15, IL-1RA, G-CSF, GM-CSF, IFN-γ, TNF-α, basic FGF, PDGF-BB, RANTES, IP-10, MIP-1α, MIP-1β, MCP-1, and Eotaxin differed significantly among the SFTS fatal patients, recovered patients, and the healthy controls (all p<0.05). Compared to the healthy controls, the fatal patients and recovered patients had reduced levels of IL-2, IL-4, IL-7, PDGF-BB, RANTES, and Eotaxin, while the levels of PDGF-BB and RANTES were significantly lower in fatal patients compared to recovered patients. The increasing levels of IL-6, IL-8, IL-15, IL-1RA, G-CSF, GM-CSF, IFN-γ, TNF-α, basic FGF, IP-10, MIP-1α, MIP-1β, and MCP-1 were observed in fatal patients (all p<0.05), and the levels of IL-6, IP-10, MIP-1α, and MCP-1 were significantly higher than other two groups. The Spearman correlation analysis indicated a positive correlation between platelet count and PDGF-BB levels (p<0.05), while the white blood cell count had a negative correlation with MIP-1 level (p<0.05).

Conclusion

The research exhibited that the SFTS virus (SFTSV) caused an atypical manifestation of cytokines. The levels of IL-6, IP-10, MIP-1α, and MCP-1 had been observed a positive association with the severity of the illness.

The change of Siglec-9 expression in peripheral blood NK cells of SFTS patients can affect the function of NK cells

OBJECTIVES

To investigate the changes and mechanism of Siglec-9 on NK cells in peripheral blood of patients with severe fever with thrombocytopenia syndrome (SFTS).

METHODS

First, we used single-cell RNA sequencing to analyze the frequency of NK cells in Peripheral Blood Mononuclear Cells (PBMCs) of SFTS patients and healthy controls (HCs), as well as the differences in the genes on NK cells. Secondly, we analyzed the expression of Siglec-9 and other receptors on NK cells by flow cytometry. Thirdly, we analyzed the correlation between Siglec-9 on NK cells and DBV viral load in plasma.

RESULTS

Compared with HCs, the frequency of NK cells in peripheral blood of SFTS patients was significantly decreased, and the activating receptors on NK cells were reduced. The expression of Siglec-9 on NK cells and the frequency of Siglec-9+NK cells decreased significantly in SFTS patients. The expression of Siglec-9 on CD16+CD56dim NK cells was negatively correlated with DBV viral load. In addition, Siglec-9+NK cells expressed higher levels of activating receptors and exhibited stronger effector functions than Siglec-9-NK cells.

CONCLUSIONS

The decreased expression of Siglec-9 on NK cells predicts NK cell dysfunction in SFTS patients, suggesting that Siglec-9 may be a potential marker for functional NK cell subsets in SFTS patients.

Recent Advances in the Study of the Immune Escape Mechanism of SFTSV and Its Therapeutic Agents

Sever fever with thrombocytopenia syndrome (SFTS) is a new infectious disease that has emerged in recent years and is widely distributed, highly contagious, and lethal, with a mortality rate of up to 30%, especially in people with immune system deficiencies and elderly patients. SFTS is an insidious, negative-stranded RNA virus that has a major public health impact worldwide. The development of a vaccine and the hunt for potent therapeutic drugs are crucial to the prevention and treatment of Bunyavirus infection because there is no particular treatment for SFTS. In this respect, investigating the mechanics of SFTS-host cell interactions is crucial for creating antiviral medications. In the present paper, we summarized the mechanism of interaction between SFTS and pattern recognition receptors, endogenous antiviral factors, inflammatory factors, and immune cells. Furthermore, we summarized the current therapeutic drugs used for SFTS treatment, aiming to provide a theoretical basis for the development of targets and drugs against SFTS.

Longitudinal analysis of immunocyte responses and inflammatory cytokine profiles in SFTSV-infected rhesus macaques

Severe fever with thrombocytopenia syndrome virus (SFTSV), an emerging bunyavirus, causes severe fever with thrombocytopenia syndrome (SFTS), with a high fatality rate of 20%-30%. At present, however, the pathogenesis of SFTSV remains largely unclear and no specific therapeutics or vaccines against its infection are currently available. Therefore, animal models that can faithfully recapitulate human disease are important to help understand and treat SFTSV infection. Here, we infected seven Chinese rhesus macaques (Macaca mulatta) with SFTSV. Virological and immunological changes were monitored over 28 days post-infection. Results showed that mild symptoms appeared in the macaques, including slight fever, thrombocytopenia, leukocytopenia, increased aspartate aminotransferase (AST) and creatine kinase (CK) in the blood. Viral replication was persistently detectable in lymphoid tissues and bone marrow even after viremia disappeared. Immunocyte detection showed that the number of T cells (mainly CD8⁺ T cells), B cells, natural killer (NK) cells, and monocytes decreased during infection. In detail, effector memory CD8⁺ T cells declined but showed increased activation, while both the number and activation of effector memory CD4⁺ T cells increased significantly. Furthermore, activated memory B cells decreased, while CD80⁺/CD86⁺ B cells and resting memory B cells (CD27⁺CD21⁺) increased significantly. Intermediate monocytes (CD14⁺CD16⁺) increased, while myeloid dendritic cells (mDCs) rather than plasmacytoid dendritic cells (pDCs) markedly declined during early infection. Cytokines, including interleukin-6 (IL-6), interferon-inducible protein-10 (IP-10), and macrophage inflammatory protein 1 (MCP-1), were substantially elevated in blood and were correlated with activated CD4⁺ T cells, B cells, CD16⁺CD56⁺ NK cells, CD14⁺CD16⁺ monocytes during infection. Thus, this study demonstrates that Chinese rhesus macaques infected with SFTSV resemble mild clinical symptoms of human SFTS and provides detailed virological and immunological parameters in macaques for understanding the pathogenesis of SFTSV infection.

Diagnostic Value of Peripheral Blood Lymphocytes for Primary Immune Thrombocytopenia

The purpose of this study was to investigate the absolute value of peripheral blood lymphocytes in patients with primary immune thrombocytopenia and the diagnostic effect on patients with primary immune thrombocytopenia. From January 2020 to June 2021, 76 patients with primary immune thrombocytopenia and 80 healthy check-ups admitted to our hospital were selected as study subjects and divided into a control group (80 patients, healthy check-ups) and an observation group (76 patients, primary immune thrombocytopenia), according to the health status of the organism. Early morning fasting venous blood was collected from both groups, and the absolute value of peripheral blood lymphocytes was measured and compared using a fully automated hematology analyzer to investigate the diagnostic value of absolute peripheral blood lymphocytes in primary immune thrombocytopenia. The CD3+, CD3+CD4+, CD4+/CD8+, and CD16+CD56+ assay values in the observation group were lower than those in the control group, and the CD3+CD8+, CD19+, and ALC assay values were higher than those in the control group (P < 0.05). The CD3+CD8+ detection values of newly diagnosed patients were similar to those of relapsed refractory patients (P > 0.05); CD3+, CD3+CD4+, CD4+/CD8+, and CD16+CD56+ detection values of newly diagnosed patients were lower than those of relapsed refractory patients, and CD19+ and ALC detection values were higher than those of relapsed refractory patients; CD3+, CD3+CD4+, CD4+, CD4+/CD8+, and CD16+CD56+ detection values of mild patients were lower than those of relapsed refractory patients; CD3+, CD3+CD4+, CD4+/CD8+, and CD16+CD56+ detection values were higher in mild patients than in severe patients, and CD3+CD8+, CD19+, and ALC detection values were lower than in severe patients (P < 0.05). The absolute lymphocyte values were of high diagnostic value in primary immune thrombocytopenia, with a sensitivity and specificity of 93.42% and 90.00%. The application of absolute peripheral blood lymphocyte value in the clinical diagnosis of primary immune thrombocytopenia can achieve a better detection and diagnosis effect, which has a positive impact on the early diagnosis rate and can help patients to obtain more timely, effective and targeted treatment, and is worthy of promotion.

Immune escape mechanisms of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome (SFTS), which is caused by SFTS virus (SFTSV), poses a serious threat to global public health, with high fatalities and an increasing prevalence. As effective therapies and prevention strategies are limited, there is an urgent need to elucidate the pathogenesis of SFTS. SFTSV has evolved several mechanisms to escape from host immunity. In this review, we summarize the mechanisms through which SFTSV escapes host immune responses, including the inhibition of innate immunity and evasion of adaptive immunity. Understanding the pathogenesis of SFTS will aid in the development of new strategies for the treatment of this disease.

Overview of the immunological mechanism underlying severe fever with thrombocytopenia syndrome (Review)

Severe fever with thrombocytopenia syndrome (SFTS) has been acknowledged as an emerging infectious disease that is caused by the SFTS virus (SFTSV). The main clinical features of SFTS on presentation include fever, thrombocytopenia, leukocytopenia and gastrointestinal symptoms. The mortality rate is estimated to range between 5-30% in East Asia. However, SFTSV infection is increasing on an annual basis globally and is becoming a public health problem. The transmission cycle of SFTSV remains poorly understood, which is compounded by the pathogenesis of SFTS not being fully elucidated. Since the mechanism underlying the host immune response towards SFTSV is also unclear, there are no effective vaccines or specific therapeutic agents against SFTS, with supportive care being the only realistic option. Therefore, it is now crucial to understand all aspects of the host-virus interaction following SFTSV infection, including the antiviral states and viral evasion mechanisms. In the present review, recent research progress into the possible host immune responses against SFTSV was summarized, which may be useful in designing novel therapeutics against SFTS.

The Endless Wars: Severe Fever With Thrombocytopenia Syndrome Virus, Host Immune and Genetic Factors

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging arboviral infectious disease with a high rate of lethality in susceptible humans and caused by severe fever with thrombocytopenia syndrome bunyavirus (SFTSV). Currently, neither vaccine nor specific antiviral drugs are available. In recent years, given the fact that both the number of SFTS cases and epidemic regions are increasing year by year, SFTS has become a public health problem. SFTSV can be internalized into host cells through the interaction between SFTSV glycoproteins and cell receptors and can activate the host immune system to trigger antiviral immune response. However, SFTSV has evolved multiple strategies to manipulate host factors to create an optimal environment for itself. Not to be discounted, host genetic factors may be operative also in the never-ending winning or losing wars. Therefore, the identifications of SFTSV, host immune and genetic factors, and their interactions are critical for understanding the pathogenic mechanisms of SFTSV infection. This review summarizes the updated pathogenesis of SFTS with regard to virus, host immune response, and host genetic factors to provide some novel perspectives of the prevention, treatment, as well as drug and vaccine developments.

[Establishment of 14-Color Panel for Determining Leukocyte Subsets in Human Peripheral Blood with Flow Cytometry]

OBJECTIVE

To establish a 14-color flow cytometry protocol for the examination of leukocyte subsets in human peripheral blood.

METHODS

We used cell membrane surface antibodies CD45, CD3, CD4, CD8, CD19, CD56, CD16, CD14, CD25, CD127, HLA-DR, CD123, CD11c and nucleus staining dye DAPI to establish a 14-color flow cytometry assay to determine the major cell subsets in human peripheral blood. We collected peripheral blood specimens from healthy volunteers to test for antibody titers and optimal photomultiplier tube (PMT) voltage, and to conduct single-color staining and fluorescence minus one control staining. After determining the test method and test conditions, the peripheral blood samples of 18 healthy volunteers were analyzed.

RESULTS

According to the cell classification and staining index, optimal antibody mass concentrations selected were as follows: CD25 and CD127 at 8.0 μg/mL, CD45, CD3, CD14 and CD123 at 4.0 μg/mL, CD8, CD19, CD56, CD16, HLA-DR and CD11c at 2.0 μg/mL, CD4 at 1.0 μg/mL and DAPI at 0.1 μg/mL. The detection voltages for CD45, CD3, CD4, CD8, CD19, CD56, CD16, CD14, CD25, CD127, HLA-DR, CD123, CD11c and DAPI were 450 V, 410 V, 400 V, 550 V, 405 V, 500 V, 520 V, 550 V, 550 V, 400 V, 450 V, 400 V, 580 V, and 300 V, respectively. The appropriate fluorescence compensation was determined by single-color staining and fluorescence minus one controls. The 14-color flow cytometry panel was established to analyze the main subsets of leukocytes in human peripheral blood, and peripheral blood samples from 18 healthy adults were examined, obtaining the percentages of each subset of peripheral blood leukocytes and the immunophenotypes of the main subsets.

CONCLUSION

We established a 14-color panel for determining leukocyte subsets in human peripheral blood by flow cytometry, which produced stable and reliable results and was easy to operate.

Low-frequency, exhausted immune status of CD56dim NK cells and disordered inflammatory cytokine secretion of CD56bright NK cells associated with progression of severe HFMD, especially in EV71-infected patients

BACKGROUND

The roles of CD56^bright and CD56^dim natural killer (NK) subsets in the viral clearance and inflammatory processes of hand, foot, and mouth disease (HFMD) remain undefined.

METHODS

A total of 39 HCs and 55 patients were enrolled to analyze peripheral CD56^bright and CD56^dim NK cells according to cell number, surface receptors, cytotoxic activities, and cytokine production. The plasma concentrations of IL-2, IL-6, IL-10, IFN-γ, TNF-α,and MCP-1 were detected using ELSA.

RESULTS

Peripheral blood NK cells was significantly lower in severe patients than in HCs due to the dramatic loss of CD56^dim NK cells with no changes in the cell count of CD56^bright NK cells. For mild patients, decreased NKp46 expression coincided with enhanced cytolysis (CD107a, GNLY, and GrB) in CD56^dim NK cells and decreased NKG2A expression with enhanced IL-10 production in CD56^bright NK cells. In contrast, severe patients showed the dominant expression of NKG2A and decreased expression of NKG2D accompanied by cytotoxic dysfunction in CD56^dim NK cells. Imbalanced receptor expression coincided with the increased concentrations of TNF-α in CD56^bright NK cells. Moreover, EV71+ patients showed significantly decreased counts of CD56^dim NK cells with cytolysis dysfunction, displayed cytokine hypersecretion in CD56^bright NK cells, while the EV71- patients displayed significantly higher plasma cytokine concentrations. The changes in the immune function of NK subsets and their subpopulations were closely related to clinical inflammatory parameters.

CONCLUSIONS

Low-frequency, exhausted immune status of CD56^dim NK cells and disordered inflammatory cytokine secretion of CD56^bright NK cells were associated with the progression of severe HFMD, especially in EV71-infected patients. This promoted the severity of inflammatory disorders, leading to enhanced disease pathogenesis.