Jak3 mutations cooperate with Spi1/Spib-deficiency to confer proliferation advantage to B cells in vivo. (A) Mb1-CreΔPB bone marrow cells grow indefinitely in the presence of IL-7. Bone marrow cells were extracted from 6- to 10-week-old WT, ΔB, and ΔPB mice and cultured in IL-7 conditioned media. The number of viable cells/mL (y-axis) was determined every 4 days for 5 passages. Statistics were performed using 2-way ANOVA with Tukey's posttest: **P ≤ .01; ***P ≤ .001; ****P ≤ .0001. (B) Transplantation experiment timeline. (C) Initial infection frequencies for MSCV-empty or MSCV T844M vectors. Histograms show the frequency of GFP⁺ cells 48 hours after spin-infection. (D) Survival curve showing days elapsed after transplantation and the percentage of survival of NSG mice transplanted with MSCV empty or MSCV T844M-infected BM cells. Mice transplanted with MSCV empty BM cells did not show signs of illness at any point. The experiment was terminated at day 47. Significance was P ≤ .03, using the Gehan-Breslow-Wilcoxon test. (E) Number of cells isolated from bone marrow of NSG mice transplanted with bone marrow cells infected with the indicated vectors. (F) Number of cells isolated from spleen of NSG mice transplanted with bone marrow cells infected with the indicated vectors. (G-H) Increased frequencies of CD19⁺ cells in BM and spleen of NSG mice transplanted with MSCV T844M-infected BM cells. Bar graphs show the percentage of CD19⁺ cells in the bone marrow and spleen of NSG mice transplanted with MSCV empty or MSCV T844M BM cells. Representative pseudo-color plots (right) show the gating strategy and the representative proportions of CD19⁺ cells in the different group of mice. (I-J) High GFP⁺ cell frequency in BM and spleen of NSG mice transplanted with MSCV T844M-infected bone marrow cells. Bar graphs indicate the percentage of GFP⁺ cells (gated on CD19⁺ cells) within the bone marrow or spleen of NSG mice transplanted with MSCV empty or MSCV T844M. Representative histograms show the percentage of GFP⁺ cells in the spleen and bone marrow of NSG mice transplanted with MSCV T884M (upper) or MSCV empty (lower). For E-J, significance was determined using unpaired Student t test: *P ≤ .05; ***P ≤ .001. NS, not significant.