Higher concentrations of ibrutinib and fostamatinib are required to efficiently suppress α-BCR–induced signaling in CD79B^hicells as compared with CD79B^lowcells. Granta 519 cells overexpressing CD79B were left untreated or treated with different concentrations of ibrutinib (BTK-i, 0.016-10.0 μM) or fostamatinib (SYK-i, 0.004-2.5 μM) for 1 hour, prior to α-IgM Ab stimulation for 4 minutes. The cells were stained with Abs as described in Figure 4. (A) Contour plots for CD79B expression vs p-BTK or p-PLCγ in cells treated with various concentrations of ibrutinib. (B) Same gating strategy was used to study α-BCR–induced signaling in cells with distinct expression levels of CD79B, from L1 to L6 (low-high). The effect of ibrutinib on α-IgM–induced phosphorylation is shown as heatmaps, relative to unstimulated cells. (C) Dose response for ibrutinib per CD79B expression level (mean ± SEM) (n = 3). (D) Dose response for fostamatinib per CD79B level (mean ± SEM) (n = 3).