Diagnostic Value of Natural Killer Cells, CD56+ CD16+ Natural Killer Cells, , and Lactate Dehydrogenase in Severe/Critical COVID-19: A Prospective Longitudinal Study According to the Severe/Critical COVID-19 Definitions.

Innate immunity, as the first line of defense of our immune system, plays a crucial role in defending against SARS-CoV-2 infection and also its immunopathogenesis. We aim to investigate the immune status of natural killer (NK) cells, natural killer T (NKT) cells, and gene expression in COVID-19 patient blood samples. The immunophenotype of NK cell subsets and NKT cells was detected by flow cytometry and the expression of gene assessed by reverse transcriptase real-time polymerase chain reaction in 44 COVID-19 patients and 20 healthy individuals. The percentage of most of NK cell subpopulation and NKT cells was significantly decreased in COVID-19 patients. The percentage of CD56dim CD16- NK cell subsets, and gene expression increased. The percentage of total NK cells, CD56+ CD16+ NK cells, and gene expression had acceptable sensitivity and specificity for assisting diagnosis of severe/critical COVID-19. O saturation% and lactate dehydrogenase levels showed valuable diagnostic value to identify critical cases. The declined NK and NKT cells in COVID-19 patients and enhanced gene expression were associated with disease severity. Total NK cells, CD56+ CD16+ NK cells, and gene expression might be used as meaningful indicators for assisting diagnosis of severe/critical COVID-19.