In various pathological conditions, cellular immunity plays an important role in immune responses. Among immune cells, T lymphocytes pdomote cellular and humoral responses as well as innate immunity. Therefore, careful investigation of these cells has a significant impact on accurate knowledge in COVID-19 disease pathogenesis. In current research, the frequency and function of various T lymphocytes involved in immune responses examined in SARS‐CoV-2 patients with various disease severity compared to normal subjects. In order to make an accurate comparison among patients with various disease severity, this study was performed on asymptomatic recovered cases (n=20), ICU hospitalized patients (n=30), non-ICU hospitalized patients (n=30), and normal subjects (n=20). To precisely evaluate T cells activity following purification, their cytokine secretion activity was examined. Similarly, immediately after purification of Treg cells, their inhibitory activity on T cells was investigated. The results showed that COVID-19 patients with severe disease (ICU hospitalized patients) not only had a remarkable increase in Th1 and Th17 but also a considerable decrease in Th2 and Treg cells. More importantly, as the IL-17 and IFN-γ secretion was sharply increased in severe disease, the secretion of IL-10 and IL-4 was decreased. Furthermore, the inhibitory activity of Treg cells was reduced in severe disease patients in comparison to other groups. In severe COVID-19 disease, current findings indicate when the inflammatory arm of cellular immunity is significantly increased, a considerable reduction in anti-inflammatory and regulatory arm occurred.

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