ISG15 and COVID-19: Moreover, cytotoxic memory CD8+ T cells from patients with COVID-19 were characterized by the expression of interferon-response genes ISG15, MX1 and others, which is in agreement with a recent report that the interferon response is a feature separating severe acute respiratory syndrome coronavirus 2 pneumonia from other viral and non-viral pneumonias57,58 (Fig. 5e, Supplementary Note 2 and Supplementary Fig. 26).