Improved Homing In A Conditioning-Free Zebrafish Transplantation Model

Hematopoietic stem cell (HSC) transplantation is a treatment used to replace the defective blood system of patients with malignant and non-malignant blood disorders. However, the biology behind donor-cell engraftment and cell-niche interaction is incompletely understood. Zebrafish is an ideal model to visualize engraftment and cell-niche interaction due to its optical transparency, which allow for whole-organism imaging. We have established a novel conditioning-free transplantation model using an HSC-deficient (runx1 W84X) mutant zebrafish. The lack of definitive hematopoiesis in these mutants provides empty HSC niche and no adaptive immune system thus diminishing the risk of graft failure or rejection. Transplantation of donor marrow cells that ubiquitously express green fluorescent protein (ubi:GFP) into these HSC-deficient mutants at 2 days-post fertilization resulted in significantly higher GFP+ donor cell engraftment than in HSC-competent animals. Transplanted runx1 mutants showed improved survival by more than 3-fold with sustained long-term, serial-repopulating, multi-lineage engraftment in adults. To decipher when the differences in engraftment occurs, we employed serial imaging to visualize hematopoietic cell homing throughout zebrafish larvae. From 4-7 days post fertilization, we observed seeding of GFP+ cells into all hematopoietic tissues including the thymus and kidney marrow (equivalent to mammalian bone marrow) with HSC-deficient runx1 mutants showing better donor cell seeding compared to HSC-competent (wild type and runx1 heterozygote) animals as early as 3 days post-transplant. This finding demonstrates the importance of the availability of recipient HSC niche in the success of engraftment, and indicate the utility of the model to provide novel insight into the earliest stages of engraftment improving our understanding of HSC transplantation outcome.