RAC2 expression is enriched in the aggressive subtypes of primary MCL tissues, and RAC2 suppression by ibrutinib is accompanied by reduced cell adhesion. (A) Immunohistochemical staining (400×) shows RAC2 positivity in the mantle zone and paracortex area of benign lymphoid hyperplasia in tonsils, weak scattered positivity in classic MCL tissues, and strong diffuse positivity in pleomorphic MCL. Insets show enlarged photos of individual cells, revealing a cytoplasmic staining pattern. Summary of RAC2 immunostaining in archived primary tissues is shown on the right. (B) Effect of ibrutinib 400 nM on cell adhesion in six thawed primary MCL samples from the University of Chicago (UChicago) cohort. Error bars represent mean ± SEM of 6 replicate reactions. (C) Effects of ibrutinib 400 nM on the RAC2 protein level in the primary MCL cells. For quantification, RAC2 protein levels were first normalized to the loading control glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and RAC2/GAPDH was then normalized to the DMSO control as a ratio. (D) Effect of ibrutinib 400 nM on cell adhesion in 6 thawed primary MCL samples from the MD Anderson cohort. Error bars represent mean ± SEM of 6 replicate reactions. (E) Effects of ibrutinib 400 nM on the RAC2 protein level in the MD Anderson primary MCL samples. For quantification, RAC2 protein levels were first normalized to the loading control GAPDH, and RAC2/GAPDH was then normalized to the DMSO control as a ratio. (F) Summary of the association between decreased RAC2 and decreased adhesion. Numbers represent number of cases. **P < .01.