Development of B cells, white blood cells that make antibodies, follows a progression of stages: common lymphoid progenitors, pre-pro-B cells, pro-B cells, pre-B cells, immature B cells, and then more mature and specialized B cells. By the time the development hits the pro-B stage, the cell is fated to stay a B cell rather than another type of cell. But researchers from the School of Veterinary Medicine and Perelman School of Medicine have found that knockout of YY1in pro-B cells impairs this lineage commitment, enabling unusual plasticity in blood cell formation. YY1 is a ubiquitous transcription factor that is capable of both activation and repression functions and plays significant roles in cell proliferation and replication, DNA repair, and the development of embryos. They found that YY1 knockout pro-B cells can generate T lineage cells -- which help B cells produce antibodies -- in vitro and in a mouse model.

 

"The data has come out better than my wildest fantasy," says senior author Michael Atchison, professor of biomedical sciences at Penn Vet. He envisions the ability to temporarily knock down YY1, push cells in another direction, and then remove the block so now there is a new lineage. "Although B cell lineage plasticity has been observed following knockout of several lineage-specific transcription factors," the authors write, "YY1 is unique in being a ubiquitous transcription factor expressed in all cell types suggesting a potentially universal mechanism of lineage commitment." Atchison says the basis for this paper began nearly a decade ago, with a former postdoctoral researcher who, looking at RNA sequencing data, had the idea that YY1 knockout pro-B cells could turn into T cells. The researchers eliminated YY1 with Mb1-driven CRE. CRE is a protein that can delete DNA and Mb1 is a promoter that drives expression of CRE in pro-B cells. The sequencing data showed that as YY1 knockout pro-B cells developed in culture, 85% identified as monocytic or dendritic cells, which are involved in the presentation of antigens to B and T cells to mount an immune response, while only 3% identified as T cells.

 

"A very useful, insightful, and unexpected finding was that while we were pushing these B cells to T cells, in the appropriate T cell environment, we observed that during this transition (midway) they were making a lot of other cell types," says co-author Sarah Naiyer, a research associate in immunology at Penn. She says, "Because we showed that Notch, a receptor in a highly conserved cell signaling pathway, was involved in the process, this could mean that it's the gradient of Notch signaling that is required to make these other cell types during the early phase while also inhibiting their differentiation during later stages, pushing them to T cells." Looking forward, the mountain of data the team amassed "can be mined for a long time to look at what's happening when you knock out YY1," Atchison says. "We've looked at a small handful of genes, but there's 20,000 genes in the genome, and there's a lot we can do just with the data we have."