Morphology of peripheral blood leukocytes in patients with new coronavirus infection (COVID-19)

Introduction. The rapid spread of the COVID-19 pandemic caused by SARS-CoV-2 initially indicated a significant immune system involvement, including peripheral blood leukocytes. Hematologic changes have already been described in the first cases of COVID-19; however, only few studies reported morphological cell abnormalities in peripheral blood smears. The aim of the study was to characterize the morphology of peripheral blood leukocytes in patients with COVID-19 and determine the significance of its changes in the immune system dysfunction and for the disease diagnosis. Materials and methods. We studied blood samples of 30 patients infected by SARS-CoV-2 virus (confirmed with PCR tests) who were treated in Regional Clinical Hospital No. 2 in Vladivostok. Peripheral blood buffy coat samples fixed in osmium tetroxide were embedded in epoxy with subsequent preparation of semithin and ultrathin sections. Results. To the best of our knowledge, it is the first study to show nuclear pathological changes in neutrophils and monocytes in blood buffy coat of the patients with coronavirus infection with light and electron microscopy of peripheral blood. We found cells with abnormal nuclear shape and leukocyte apoptotic degeneration absent in healthy individuals. The identified morphological leukocyte abnormalities should be considered as signs of hyporeactivity of these cells, which indirectly indicate a decrease in their bactericidal potential in COVID-19. At the same time, the secretory degranulation of neutrophils with the release of the contents of the granules into the extracellular space was poorly visualized. We observed smooth plasmalemma of leukocytes with a small number of microvilli involved in phagocytosis and numerous apoptotic leukocytes with reduced phagocytic ability. Conclusion. The detection of morphological abnormalities of circulating leukocytes in patients with COVID-19 is of diagnostic and prognostic value in this pathology. Keywords: blood, neutrophils, monocytes, morphology, COVID-19, SARS-CoV-2, karyopathological changes