Zebrafish cell transplantation platform has been brought to the forefront to study the cell properties, and it gains advantages in cost-effective, high-throughput and facile imaging. Hematopoietic stem cell (HSC) transplantation (HSCT) is an important approach to evaluate the characteristics of HSC and also a treatment to cure malignant blood diseases in clinic [1]. Initially, irradiated animals were used to receive engrafted HSCs, however, irradiation not only eliminates the immune cells, but also damages the HSC niches [2]. Zebrafish transplantation platform has been greatly improved, owing to the generation of a transparent adult zebrafish, Casper, which allows for direct observation of fluorescence-labeled cells post transplantation [3]. Subsequently, through outcrossing of Casper with immunodeficient lines, several transplantation platforms in zebrafish have been established, such as prkdc^-/-/il2rga^-/-/Casper line which could achieve the engraftment of human tumor cells and high-throughput drug screening [4]. However, the survival rate of these lines are very low and homozygous mutants are infertility; meanwhile, HSCT in zebrafish is limited in what they revealed of the stage-dependent characteristics of developmental HSCs.

Recently，Feng Liu from Institute of Zoology (Chinese Academy of Sciences) and Lu Wang from Institute of Hematology and Blood Diseases Hospital (Chinese Academy of Medical Sciences & Peking Union Medical College) collaborate on a paper entitled “Generation of foxn1/Casper Mutant Zebrafish for Allograft and Xenograft of Normal and Malignant Cells” in Stem Cell Reports journal. This work introduces a new transparent immunodeficient zebrafish model, foxn1/Casper, which permits engraftment of allogenic or xenogenic cells without pre-condition.

Our previous study demonstrated that zebrafish embryos with the foxn1 deficienty displayed deficient T cell development, largely due to defective thymic epithelial cell development [5]. Based on this，researchers generated a foxn1 mutant zebrafish with zinc finger nuclease (ZFN) technique and obtained the foxn1/Casper transplantation platform by outcrossing the foxn1 mutant with Casper line. Importantly, the survival rate of foxn1/Casper mutants can reach approximately 60% (150 day-post-fertilization) under non-antibiotic conditions and the female homozygous mutant is fertile. Similar to the impaired T cell development in nude mice, foxn1 mutant zebrafish also present T cell defects and thus might be suitable for cell transplantation. Subsequently, they assessed the HSC engraftment efficiency in different recipients, and found that there is no significant difference between irradiated Casper recipients and nonconditioned foxn1/Casper recipients. However, the higher survival rate in nonconditioned foxn1/Casper mutant after HSCT indicated that the irradiation damaged the HSC niche and impaired the survival of transplanted HSC in recipients.

Then, they found that fetal HSCs exhibit higher engraftment efficiency than adult HSCs, RNA-seq data revealed that fetal HSCs harbor stronger cell-cycle activity than adult HSCs. By successful engraftment of allogeneic zebrafish or xenogenic medaka muscle cells in foxn1/Casper mutant, they proposed that foxn1/Casper mutant can serve as a great model for direct visualization of transplanted cells in vivo and xenografting in future studies. They also validated that foxn1/Casper mutant could engraft allogeneic MDS-like cells. Transplantation with foxn1/Casper mutant could achieve direct observation of the transplanted MDS-like and muscle cells in living recipients, which provides a potential platform to study therapeutic strategies for blood disease and solid tumors.

Figure 1. Zebrafish foxn1/Casper mutant cell transplantation platform and application.

Taken together, they demonstrated that the foxn1/Casper mutant is a feasible model to permit transplantation of HSCs, muscle cells, and MDS-like cells without irradiation preconditioning. Given the immunodeficiency and transparency of the foxn1/Casper mutant, this model can be further applied to study the physiological and pathological events of normal and malignant cells in vivo.

References

1. Mantel, C.R., et al., Enhancing hematopoietic stem cell transplantation efficacy by mitigating oxygen shock. Cell, 2015. 161(7): p. 1553-65.

2. Traver, D., et al., Effects of lethal irradiation in zebrafish and rescue by hematopoietic cell transplantation. Blood, 2004. 104(5): p. 1298-305.

3. White, R.M., et al., Transparent adult zebrafish as a tool for in vivo transplantation analysis. Cell Stem Cell, 2008. 2(2): p. 183-9.

4. Yan, C., et al., Visualizing Engrafted Human Cancer and Therapy Responses in Immunodeficient Zebrafish. Cell, 2019. 177(7): p. 1903-1914 e14.

5. Ma, D., et al., Foxn1 maintains thymic epithelial cells to support T-cell development via mcm2 in zebrafish. Proc Natl Acad Sci U S A, 2012. 109(51): p. 21040-5.

(Contact: Feng Liu, liuf@ioz.ac.cn)