Residual persistence of cytotoxicity lymphocytes and regulatory T cells in patients with severe coronavirus disease 2019 over a 1‐year recovery process

AIM: To clarify the immune cellular changes in critically ill patients recovering from coronavirus disease 2019 (COVID‐19). METHODS: The immune response of peripheral blood mononuclear cells from patients with severe COVID‐19 in different stages of recovery (3, 6, and 12 months from hospitalization) was evaluated by single‐cell mass cytometry. Immunological changes in patients were compared with those in age‐matched healthy donors. RESULTS: Three patients with severe COVID‐19 were compared with four healthy donors. In the patients, there was an increase in the cell density of CD4‐ and CD8‐positive T lymphocytes, and B cells, over the course of the recovery period. CD4‐ and CD8‐positive T lymphocytes expressing T‐bet and granzyme B (Gzm B) in patients were abundant during all recovery periods. The level of regulatory T cells remained high throughout the year. The levels of natural killer (NK) cells in patients were higher than in those in the healthy donors, and the frequency of CD16(+) NK cells expressing Gzm B increased throughout the year. CONCLUSION: Patients recovering from severe COVID‐19 showed persistence of cytotoxic lymphocytes, NK cells, and regulatory T cells throughout the posthospitalization year of recovery.