Detection of human platelets in NSG mouse blood. (A) FACS distinction of human and mouse CD41⁺ platelets in a mixture of human and mouse blood cells after staining with human- and mouse-specific anti-CD41 Abs and gating on low forward and side light scattering events. (B) The minimum number of human platelets detectable in mouse blood determined from an analysis of serial dilutions of human blood cells in undiluted mouse blood (= 0.01%). (C) Representative FACS plots of human platelets produced in NSG mice transplanted with human lin⁻ CB (top panel) or lin⁻ mPB (middle panel) cells. The bottom panel shows the blood of a nontransplanted NSG mouse. (D) Linear relationship between the dose of Lin⁻ALDH⁺ CB cells transplanted and the level of circulating human platelets detected 3 weeks later. Shown are the mean ± SEM of data pooled from 7 experiments (2-5 mice per cell dose per experiment). (E) Comparison of Wright-Giemsa–stained human platelets (arrows) present in normal human blood (top panel) and isolated by FACS (as small human CD41⁺ cells) from the blood of NSG mice 3 weeks after transplantation of human CB cells (bottom panel). (F) Representative FACS profiles of circulating human and mouse platelets in NSG mice 3 weeks after transplantation of human CB (left panel) or mPB cells (middle panel), or in a fresh mixture of human and mouse blood cells (right panel). Red and gray dots are human CD41⁺ and mouse CD41⁺ platelets, respectively. SSC indicates side light scattering activity; and FSC, forward light scattering activity. (G) Half-life determinations of human and mouse platelets in NSG mice transplanted with human CB cells. Platelets were labeled by injecting the mice IV with sulfo-NHS-biotin and the changing percentage thereafter of the initial level of biotin-labeled mouse and human platelets was then determined. Each symbol type represents a single mouse.