Human purified PBLs cause lethal GvHD in NOD/scid mice in a dose-dependent fashion. NOD/scid mice were conditioned with anti-NK antibodies and sublethally irradiated prior to the intraperitoneal transfer of increasing numbers of human purified PBLs. (A) After infusion, mice were followed for human chimerism in peripheral blood over time. Each symbol represents a single mouse infused with 5 × 10⁶, 10 × 10⁶, or 20 × 10⁶ PBLs. (B) Mice were also followed for weight loss over time. Each line represents a single mouse infused with 5 × 10⁶, 10 × 10⁶, or 20 × 10⁶ PBLs. Results from mice infused with cells from one representative donor of 2 are shown. All mice with severe GvHD, defined weight loss greater than 5% (solid horizontal line in panel B) concomitant with human chimerism greater than 10% (solid horizontal line in panel A) died before week 8. (C) Spleen, liver, and gut were excised from dying animals and analyzed by histopathology after staining with hematoxylin and eosin (top row). Tissue samples were simultaneously analyzed by immunohistochemistry after counterstaining with human monoclonal anti-CD3 antibodies and peroxidase-conjugated second-step reagent (bottom row). Sections from one representative of 18 analyzed animals are shown.