CD19/CD3 bsAb induced T-cell activation and cytotoxic effector functions. PBMCs from BTKi-naïve (BN, n = 13) and BTKi-treated (BTKi, n = 26) patients were cultured with either CD19/CD3 bsAb, or medium only. Markers of T-cell activation and cytotoxic potential were assessed by flow cytometry in CD4⁺ and CD8⁺ T cells. (A) Fold changes in the frequencies of T cells expressing the respective markers after 3 days of in vitro culture with CD19/CD3 vs medium alone were log² transformed, median-centered, and samples grouped by complete clustering. Heatmap depicts the fold changes on a log² scale for BN, ibrutinib (Ibr), and acalabrutinib (Aca) patient samples. (B) Pie charts show the proportion of patient samples in cluster 1 (blue) or cluster 2 (red) within BN (n = 13), Aca (n = 11), and Ibr (n = 15) groups. (C) Comparison of CLL-specific killing after 5 days of treatment between cluster 1 and cluster 2 based on CD4⁺ and CD8⁺ T-cell activation, respectively. Each dot represents 1 patient sample; BN (gray triangles), Aca (purple circles), and Ibr (green diamonds). Results are reported as median and IQR. Asterisks indicate statistical significance using Mann-Whitney U tests. **P < .01.