In vivo anti-PD-L1 treatment effectively controls CLL development after AT. Three-month-old C57BL/6 healthy hWT mice transplanted with 4 × 10⁷ CLL cells from leukemic Eµ-TCL1 mice were randomized to treatment with 10 mg/kg rat immunoglobulin G2b (IgG2b) aPD-L1 (n = 15) or rat IgG2bκ isotype antibody (n = 10) administered intraperitoneally every 3 days starting 1 day after AT and were euthanized 31 days later. (A) Differences in spleen sizes between isotype and anti-PD-L1–treated mice. (B) Compared with isotype-treated controls, median spleen weights (g) of aPD-L1–treated mice were significantly reduced. (C-D) Single-cell suspensions of CLL-affected organs were analyzed by flow cytometry, and cells were gated on total viable 4,6 diamidino-2-phenylindole (DAPI) –negative single CD45⁺ cells. Tumor load, defined as percentage of CD5⁺CD19⁺ cells of total CD45⁺ hematopoietic cells, in (C) spleen and (D) PB, BM, and PC was compared between isotype- and aPD-L1–treated mice. (E) Tumor load distribution in spleen and PB depicted for individual mice, suggesting more effective disease control in secondary lymphoid organs than in PB. All graphs depict mean ± SD. *P < .05; ***P < .0001.