CD38 and COVID-19: This analysis identified an increase in clusters 13 and 14 (representing populations that express HLA-DR, CD38, PD1, KI67 and CD95) as well as cluster 15 (containing Tbet+CX3CR1+ effector-like CD4 T cells) in COVID-19 patients compared with HDs and RDs (Fig. 3, I and J, and fig.