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Letters to the Editor
function in B1 progenitors.8 GSEA of STAT5-induced genes antagonized by Ikaros in pre-B2 cells revealed their enrichment among the upregulated genes in Ikaros deficient B1 cells, suggesting that Ikaros also antagonizes STAT5 gene activation in B1 progenitors. Thus, Ikaros appears to affect similar biological pathways in B1 and B2 progenitors.
That Ikaros deficiency correlates with increased STAT5-activated gene expression suggests that Ikaros may act as a tumor suppressor in B1 cells. We transduced BM WT and cKO B1 progenitors to express the BCR- ABL1 oncoprotein, or an empty vector (pMITo), carrying a TdTomato (TdTo) reporter. Transduction efficiency was assessed and BCR-ABL1 expression was confirmed by western blot (Online Supplementary Figure S3A and B). TdTo+ cells were injected into NSG mice and the health status was monitored over time. All mice that received cKO B1 BCR-ABL1+ cells developed BCP-ALL, as charac- terized by BM failure (anemia, thrombocytopenia, splenomegaly) (Online Supplementary Figure S3C), which was sometimes associated with hepatomegaly and neu- rological impairment. Blast cell (CD19+TdTo+) infiltra- tion was observed in the BM and spleen (Figure 3A). In contrast, only 25% of mice transplanted with WT B1 BCR-ABL1+ cells developed BCP-ALL, with slower kinet- ics and fewer symptoms (Figure 3B). We also analyzed
symptom-free mice that received WT B1 BCR-ABL1+ or cKO B1 pMITo+ cells, but did not detect CD19+TdTo+ cells in the BM or spleen (Figure 3A). The fact that not all mice that received WT B1 BCR-ABL1+ cells developed leukemia, as previously described, may be due to the limiting numbers of cells injected here.9 Our results therefore indicated that Ikaros limits the ability of BM B1 progenitors to develop BCR-ABL1-induced BCP-ALL.
In order to determine if Ikaros also functions as a tumor suppressor in fetal-derived B1 cells, we trans- duced WT and cKO FL B1 progenitors (from E17-E18 organs) to express BCR-ABL1 and injected them into NSG mice. Fetal B1 progenitors gave rise to BCP-ALL similar to BM B1 cells. Ikaros deficiency was associated with a higher tumor burden and decreased survival (Figure 3B). These results demonstrated that Ikaros reduces BCP-ALL development in B1 progenitors.
Lastly, we asked if Ikaros loss is implicated in human B1-like BCP-ALL. Because human B1 cells are ill-defined, we first compared a published set of genes, differentially expressed between human B1- and non-B1-like BCP- ALL, with our available human BCP-ALL RNA-seq dataset, but this did not identify clear subgroups among samples.10 We then separated a collection of 370 pedi- atric and 102 adult BCP-ALL samples into B1-like and non-B1-like groups according to Vh usage, and further
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Figure 3. Continued on following page.
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haematologica | 2022; 107(1)