Human mast cell development in hSCF Tg NSG recipient spleen. (A) Human mast cell development is enhanced in hSCF Tg NSG recipient spleens (S4-1, killed at 13 weeks; and S2-1, killed at 16 weeks). (B) Frequencies of human c-Kit⁺CD203c⁺ mast cells, CD33⁺HLA-DR⁻ granulocyte population, and CD33⁺HLA-DR⁺ APCs within total hCD45⁺hCD33⁺ myeloid cells in the spleens of hSCF Tg and non-Tg NSG recipients. Human mast cell development in the spleen was significantly greater in the hSCF Tg NSG recipients (hSCF Tg: n = 20, non-Tg NSG: n = 12, P = .0304). (C) FACS-sorted hCD45⁺CD33⁺CD117⁺CD203c⁺ human mast cells from a representative non-Tg NSG recipient spleen (N1-1, 59.3% human mast cells within hCD45⁺CD33⁺ population) and hSCF Tg NSG recipient spleen (S1-2, 85.7%; S1-3, 77.7%; and S12-3, 56.1% human mast cells within hCD45⁺CD33⁺ population) were examined by MGG staining (N1-1, killed at 21 weeks; S1-2, killed at 20 weeks; S1-3, killed at 21 weeks; and S12-3, killed at 13 weeks). (D) H&E- and anti–mast cell tryptase antibody–stained spleen sections demonstrating the presence of human mast cells in non-Tg NSG recipients and hSCF Tg NSG recipients (non-Tg NSG recipient: N11-1, 94.0% hCD45⁺; hSCF Tg NSG recipients: S11-1, 95.3%; and S9-1, 97.0% hCD45⁺; N11-1, killed at 20 weeks; S11-1, killed at 10 weeks; and S9-1, killed at 16 weeks).