Expression of Programmed Cell Death 1 (PD-1) as a Marker of T-Cell Exhaustion and Its Correlation with Interleukin-10 Serum Level in Patients with COVID-19

Coronavirus disease 2019 (COVID-19), which is a major global concern, is characterized by a progressive disease pattern involving diverse host immune responses. Programmed cell death marker-1(PD-1) expression, a critical checkpoint for T cell exhaustion, can be modulated by interleukin-10, which also mediates apoptotic T cell cytopenia. We aimed to measure the level of PD-1 expression and to investigate its correlation with IL-10 serum levels in modulating T cell effector function, correlating the results with the level of severity of the disease. This study involved 40 patients with COVID-19 and 20 healthy controls. Using flow cytometry, the expression of PD-1 was determined on CD8+ T lymphocytes and CD4+ T lymphocytes. ELISA was used to determine the levels of IL-10 in the serum. We found a remarkable decrease in T cell counts with functionally exhausted surviving T cells in the patient groups, especially in patients with severe disease. PD-1 expression increased significantly in CD4+, CD8+, and total T cells, showing a higher expression in CD8+ T cells. The patient groups had significantly higher serum IL-10 levels than the control group. The ROC analysis demonstrated the predictive role of IL-10 levels in disease severity (65% sensitivity, 80% specificity, and AUC = 0.806). IL-10 serum levels and PD-1 expression in total T cells were positively correlated, suggesting that IL-10 participates in T cell exhaustion.