Phenotypic Alterations of CD8+ and CD8− CD56+ NK Cell Subsets in Moderate and Severe COVID‐19 Disease

Abstract

The COVID‐19 pandemic has created a global health challenge. Severe cases are associated with immune system dysfunction, which can lead to uncontrolled inflammation. There are no published data on the specific roles of natural killer (NK)‐cell subsets and immune checkpoint (IC) molecules in disease severity. Thirty‐five patients diagnosed with COVID‐19 and 14 healthy controls were involved in the study. From peripheral blood, CD56dim and CD56bright cell subsets were analyzed by flow cytometry for the expression of IC molecules (T‐cell immunoglobulin and ITIM domain [TIGIT], CD226, and PD‐1), activation markers (CD69), and cytotoxic potential (CD107a degranulation, granzymes, and perforin content). In COVID‐19 patients, the proportion of CD8− CD56dim cells was significantly higher than the CD8+ subset. Inhibitory receptors TIGIT and PD‐1 exhibited significantly higher relative expression in CD8+ CD56dim cells compared to their CD8− counterparts across infected groups, an effect particularly pronounced in deceased patients. Conversely, activating CD226 expression was reduced in the CD8− CD56dim subset only in severe cases. Functional assays revealed significantly elevated CD107a and CD69 expression in CD56dim cells of patients versus controls. Notably, CD8− CD56bright cells from deceased patients demonstrated enhanced CD107a expression and elevated perforin content, suggesting a shift toward hyperactivation. The preferential upregulation of inhibitory checkpoint molecules on the highly active CD8+ subset may reflect a compensatory response to immune activation, whereas the enhanced activation of CD8− NK‐cell subsets was associated with disease severity and mortality. Similarly, the heightened cytotoxic profile of CD8− CD56bright cells observed in fatal cases may reflect immune dysregulation associated with severe COVID‐19.