CD19 and systemic lupus erythematosus: Compared to CD19hi and CD19int, the CD19lo B cells were less responsive to activation by B cell receptor cross-linking as assessed by Syk phosphorylation [16], complementing previous findings showing that low CD19 expression (in association with low CD21) in SLE was associated with B-cell intrinsic impairment of TLR9 responses [40].