Enforced expression of PTPN22 in B104 cells blocks anti-IgM–induced apoptosis. (A) B104 cells were transfected with control (CTRL) or PTPN22 (PTP) mRNA, left in culture for 3 hours to achieve maximal PTPN22 protein expression, and then cultured for an additional 22 hours with or without soluble anti-IgM. The percentage of viable cells was determined by annexin V/PI staining and flow cytometry. One representative experiment of 6 is shown. Rectangle represents viable cells. Levels of PTPN22 protein 3 hours after transfection of PTPN22 mRNA are shown in the bottom panel. A CLL sample with high PTPN22 expression was loaded on the same gel for comparison. (B) Graph represents summary of data from the 6 independent experiments. Significance of difference in leukemic cell viability was evaluated with the paired t test.