CCL4 and COVID-19: In support of qualitative differences, single-cell differential gene expression analysis showed that cells in the ‘exhaustion’ cluster (cluster 1) from severe COVID-19 patients expressed significantly higher levels of transcripts encoding for cytotoxicity-associated molecules (granzyme B, granzyme H, granulysin, Fas ligand (5, 81)) and proinflammatory cytokines (CCL3, CCL4, CSF-2, TNF-α, LTA and LTB (5, 9, 82) (Fig. 3G-I, fig.