Analysis of peripheral lymphocyte subsets and T-cell exhaustion in SARS-CoV-2 Infection

: Immune responses against Coronavirus (SARS-CoV-2) may be highly complex. It has been suggested that T-cell fatigue develops due to continuous stimulation of T-cells by SARS-CoV-2 in Coronavirus disease-2019 (COVID-19). It was aimed to assess peripheral lymphocyte subsets and T-cell exhaustion in various clinical courses of the disease in patients diagnosed with COVID-19. This study included 150 patients who were assigned into the "mild-to-moderate disease" group, or "severe disease" group based on their clinical and laboratory characteristics. Peripheral lymphocyte subsets and T-cell exhaustion markers [programmed cell death protein 1 (PD-1) and T-cell immunoglobulin and mucin-domain containing-3 (Tim-3)] were determined in the peripheral blood using flow cytometry. Mean (±SD) age was 53.3 ± 14.5 years, and female to male ratio was 55/95. In the mild-to-moderate disease (MMD) group, 55 patients had pneumonia and 20 patients had COVID-19 without pneumonia. In the severe disease (SD) group, 43 patients had severe pneumoniae and 32 patients were in critical condition. Lymphocyte counts were less than 1.0 x 109/L in 69.3% of the patients in the SD group, and the difference between the MMD group and SD group was statistically significant (p= 0.001). Total T cells, CD4+ and CD8+ T-cell counts were significantly lower in the SD group vs. MMD group (p< 0.001, p< 0.001, p< 0.001, respectively). PD-1 expression by CD8+ and CD4 T+ cells was higher (p= 0.042, p= 0.029, respectively) and Tim-3 expression from CD4 T+ cells was lower (p= 0.000) in the SD group vs. MMD group. Serum IFN-γ levels were not statistically different in the MMD and SD groups (p= 0.2). T-cell counts may be significantly reduced along with an increased expression of the T-cell exhaustion marker PD-1 in severe COVID-19, but Tim-3 expression was not increased in our study patients.