Two to three weeks after tumor cell transplantation, tumor load in blood (defined as the number of CD5+CD19+ CLL cells/L) was measured, and mice were assigned to different treatment arms to achieve comparable tumor load in all groups at baseline prior to treatment. Ibrutinib (provided by Pharmacyclics LLC, an AbbVie Company) was administered in drinking water containing sterile control vehicle (1% HP--CD) at a concentration of 0.16 mg/mL, as previously described.26 (link) For PD-1/PD-L1 blockade, mice were injected i.p. with 0.2 mg of PD-1 (clone: RMP1-14), PD-L1 (clone: 10F.9G2), or rat IgG2a isotype control antibody (clone: 2A3; all from BioXcell, West Lebanon, NH, USA) every 3 days for 4 weeks.