Abstract

Immunity against SARS‐CoV‐2 that is acquired by convalescent COVID‐19 patients is examined in reference to (A) the Th17 cell generation system in psoriatic epidermis and (B) a recently discovered phenomenon in which Th17 cells are converted into tissue‐resident memory T (TRM) cells with Th1 phenotype. Neutrophils that are attracted to the site of infection secrete IL‐17A, which stimulates lung epithelial cells to express CCL20. Natural Th17 (nTh17) cells are recruited to the infection site by CCL20 and expand in the presence of IL‐23. These nTh17 cells are converted to TRM cells upon encounter with SARS‐CoV‐2 and continue to exist as ex‐Th17 cells, which exert Th1‐like immunity during a memory response. G‐CSF can induce nTh17 cell accumulation at the infection site because it promotes neutrophil egress from the bone marrow. Hence, G‐CSF may be effective against COVID‐19. Administration of G‐CSF to patients infected with SARS‐CoV‐2 is worth a clinical trial.