The phenotypic changes of γδ T cells in COVID‐19 patients

Abstract A novel pneumonia‐associated respiratory syndrome named coronavirus disease‐2019 (COVID‐19), which was caused by SARS‐CoV‐2，broke out in Wuhan, China, in the end of 2019. Unfortunately, there is no specific antiviral agent or vaccine available to treat SARS‐CoV‐2 infections. The information regarding the immunological characteristics in COVID‐19 patients remains limited. Here, we collected the blood samples from 18 healthy donors (HD) and 38 COVID‐19 patients to analyze changes on γδ T cell population. In comparison with HD, the γδ T cell percentage decreased, while the activation marker CD25 expression increased in response to SARS‐CoV‐2 infection. Interestingly, the CD4 expression was upregulated in γδ T cells reflecting the occurrence of a specific effector cell population, which may serve as a biomarker for the assessment of SARS‐CoV‐2 infection.


| INTRODUC TI ON
A severe pneumonia-associated respiratory syndrome spread rapidly in Wuhan, China, at the end of 2019. A novel coronavirus, officially named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was identified as the cause of emerging cases of severe pneumonia. 1,2,3 Officially named by WHO, the coronavirus infection disease-19 (COVID- 19) outbreak was listed as a public health emergency of international concern. The virus has so far caused 81 896 confirmed cases and 3287 deaths in China according to WHO by 8 April 2020. COVID-19 has rapidly spread in more than 180 countries worldwide, including Italy, Iran, Japan and the United States.
SARS-CoV-2 is an enveloped positive-sense RNA virus, which belongs to the family of coronaviruses including SARS-CoV and MERS-CoV. 4,5 Currently, there is no specific antiviral agent or vaccine available to treat SARS-CoV-2 infections. Clinical treatments for COVID-19 patients are primarily supportive and symptomatic treatments. There are several existing antiviral agents that can be repurposed to develop effective interventions against this novel coronavirus. 6 However, toxicology studies and clinical trials are required for potential uses in the clinic. According to the pathological reports for COVID-19, SARS-CoV-2 mainly caused inflammatory responses in the lungs. 7 Several studies showed that COVID-19 patients developed lymphopenia and rising pro-inflammatory cytokines in severe cases. 8,9 Inflammation can be triggered when innate and adaptive immune cells detect SARS-CoV-2 infection. Innate T cells can provide a first line of defence against pathogens. However, how innate T cells respond to SARS-COV-2 infection remains unclear.
Among innate immune cells, γδ T cells proliferate rapidly and respond to pathogens by inducing apoptosis, mediating antigen presentation and immune regulation. 10 In healthy adult humans, γδ T cells represent 1%-10% of total circulating lymphocytes, predominately displaying the CD4 and CD8 double-negative phenotype. 11 However, in some cases, a fraction of γδ T cells can express either CD4 or CD8. [12][13][14] The γδ T cells in Itk-and Id3-deficient mice exhibited an increase in CD4 and CD44 expression, as well as cytokine production (IL-4, IFNγ or IL-17), indicating an enhanced effector function in the context of infection or disease occurrence. 15,16 γδ T cells do not recognize classical peptide antigens, their TCRs are non-MHC restricted, and they can respond to pathogen-associated molecular patterns and produce cytokines in the absence of TCR ligands. 17 Furthermore, γδ T cells can defend against viral infection by secreting IFNγ and upregulating the expression of NKG2D, perforin, granzyme B and FasL, etc 18,19 In many infections, the number of γδ T cells increases both locally and systemically a few days post-infection. A study found that the ratio of γδ T cells among total lymphocytes in the lungs significantly increased in mice infected with influenza A (H1N1) virus 3 days following infection. 19 This observation suggests that γδ T cells play an important role in the host immune response. During acute HIV infection, previous studies showed that the expression of the activation marker, CD25, is significantly increased on γδ T cells, 14

| Patients
The study included 18 healthy controls and 40 patients from February 18 to March 4. In the HD group, the median age is 39.06 ± 4.26 years, with equal numbers of male and female subjects. The median age of the patient group was 45.08 ± 4.06 years, including 23 male patients (60.53%) and 15 female patients (39.47%). The 38 patients enrolled were all confirmed to have SARS-CoV-2 infection using PCR tests on throat swab specimens.
All patients were categorized as mild by clinical manifestations.

| Statistical analysis
Student's t test was performed for two group analysis using GraphPad Prism 7.0 software. * and ** stands for P < .05 and P < .01, respectively.

ACK N OWLED G EM ENT
This work was supported in part by grants from a COVID-19 special project from Xi'an Jiaotong University Foundation (xzy032020002). We thank all the doctors, nurses, public health workers and patients for their contribution against SARS-CoV-2 infection.

CO N FLI C T O F I NTE R E S T S
We declare no competing interests.  The data sets used and analysed in the current study are available from the corresponding author upon reasonable request.