Fms-like tyrosine kinase receptor 3 (Flt3) is a membrane protein that is strongly expressed on hematopoietic stem cells (HSCs). FLT3LG (FLT3 ligand) is a growth factor that can stimulate the proliferation, differentiation, and survival of these cells when it binds to its receptor FLT3. The Flt3/Flt3L signaling axis indeed plays an important role in the production of conventional dendritic cells (cDCs) and plasmacytoid dendritic cells (pDCs). Knockout of mouse Flt3 results in a reduction of mouse dendritic cells (DCs), which increases the availability of mouse Flt3L to the human receptor after human HSCs transplantation, thereby supporting the development of human immune cells, particularly dendritic cells.

Biocytogen developed B-NDG Flt3 KO mice by knocking out the Flt3 gene in the NDG background. mFLT3 was detectable in DCs and CD117+ cells of B-NDG mice, but not in homozygous B-NDG Flt3 KO mice. To determine whether the knockout of Flt3 will affect the expression of FLT3LG. Serum were isolated from B-NDG mice and homozygous B-NDG Flt3 KO mice. Expression level of mouse FLT3L were analyzed by ELISA. Mouse FLT3L was detectable in B-NDG mice and homozygous B-NDG Flt3 KO mice. Levels of mFLT3L were substantially elevated in homozygous B-NDG Flt3 KO mice compared to B-NDG mice. To evaluate whether the knockout of Flt3 impacts the frequency of leukocyte subpopulations, Bone marrow cells were isolated from B-NDG mice and homozygous B-NDG Flt3 KO mice, Frequencies of T cells, B cells, NK cells, granulocytes, monocytes, macrophages in B-NDG Flt3 KO mice were like those in B-NDG mice. Frequencies of DCs in B-NDG Flt3 KO mice were lower than those in B-NDG mice.

In summary, the significant decrease of DCs in B-NDG Flt3 KO mice creates an opportunity for the successful reconstruction of human DCs, which could have a significant positive impact on tumor immunotherapy.