SIgM levels/signaling capacity inversely correlate with signaling inhibition by ibrutinib in vitro. CLL cells taken before ibrutinib therapy start were treated with 10 µM of ibrutinib in vitro (n = 13). Red symbols represent U-CLL; blue symbols represent M-CLL; the green triangle represents a IGHV3-21 M-CLL case. (A) SIgM-mediated iCa²⁺ mobilization was measured by flow cytometry. Non treated cells (NT) were incubated with DMSO as a control. (B-C) Correlation between percent signaling inhibition by ibrutinib in vitro and sIgM MFI (B) or sIgM signaling capacity (iCa²⁺ mobilization) (C) measured prior to ibrutinib treatment. Case 495 (IGHV3-21 M-CLL) was excluded from the graphs for better visualization of the correlation. Analysis and Spearman correlation were performed on all 13 cases.