Attempt to Prove the Existence of Abnormal B Lymphocyte As Myeloma-Initiating Cells from B Cell-Derived Induced Pluripotent Stem Cells

We previously established normal B cell-derived induced pluripotent stem cells (BiPSCs; BiPSC13 and MIB2-6). BiPSCs are known to maintain VDJ rearrangement of the IgH gene, and they can be induced by the tet-off system to express activation-induced cytidine deaminase (AID; BiPSC13-AID and MIB2-6-AID) and differentiate into hematopoietic progenitor cells (HPCs) (Scientific Rep, 2017). Using these BiPSCs, we attempted to prove the existence of abnormal B cells, which are thought to be myeloma-initiating cells, originating from mature B cells transformed by reprogramming. We speculated that BiPSCs could develop into myeloma-initiating cells that undergo chromosomal translocation or gain genetic abnormalities during redifferentiation into mature B cells. First, using the comet assay, we confirmed the DNA-damaging effect of AID in BiPSCs-AID. Secondly, we differentiated BiPSC13-AID into CD34⁺/CD38^-/CD43^-/CD45^- cells by co-culture with stromal cells (mouse embryo cell line: 10T1/2), and we subsequently transplanted the cells into the bone marrow (BM) of immunodeficient NRG mice. The presence of CD34⁺ cells was still observed in mouse BM 4 months after transplantation; however, no differentiation into B cells was detected. Next, using the CRISPR/Cas9 system, we attempted to make BiPSCs with chromosomal translocation t(11;14); we succeeded in establishing a 293T cell line with t(11;14), then confirmed t(11;14) in MIB2-6-AID, and the clone is now being established. Furthermore, we established BiPSC13-Pax5, which can be induced by the tet-off system to express Pax5, and we then differentiated BiPSC13-Pax5 into CD34⁺/Pax5⁺/CD38^-/CD43^-/CD45^- cells by co-culture with stromal cells (10T1/2). We expect that the HPCs or hematopoietic stem cells (HSCs) derived from BiPSCs will further differentiate into B cells due to the expression of Pax5 in the BM of NRG mouse. We also established BiPSC13-AID-p53^-/-, in which p53 was deleted using the CRISPR/Cas9 system, and the cells differentiated into HPCs. Interestingly, we detected some CD43⁺/CD45⁺ cells among CD34⁺/CD38^- cells after the co-culture of BiPSC13-AID with aorta-gonad-mesonephros-derived stromal cell (AGM-S3) instead of 10T1/2. Therefore, AGM-S3 may promote the differentiation of BiPSCs into cells that are more similar to HSCs. These CD34⁺ cells differentiated from BiPSCs will be transplanted into the BM of NRG mouse.