Flow cytometric analysis showed that IFN-gamma, but not IL-2 expression in T cells was impaired with a high concentration of recombinant CD25 (Fig. 1f, g ), which was consistent with the previous report that soluble CD25 could inhibit the proliferation and function of T cells.13 Correlation analysis between sIL-2R concentration and percentage of different immune cell types in COVID-19 patients suggested that sIL-2R might be a negative regulatory factor for T cells, especially CD8+ T cells (Fig. 1h), but not CD4+ T cells, NK cells, and B cells (Supplementary Fig. 1f). Here, CD4 is linked to COVID-19.