Reconstitution of the immune system of NSG mice by using human lymphocytes. Inoculation of NSG mice with human umbilical cord stem cells (HUSC) led to the development of human lymphocytes that reconstituted the spleen and other lymphoid organs. (A) Spleen size was reduced overall (noted as few, small white pulp lining muscular arterioles) in naïve adult NSG mice compared with immunocompetent mice. The red pulp of humanized mice contained modest numbers of hematopoietic (mainly erythrocyte) precursors. Hematoxylin and eosin stain; original magnification, 10×. (B) Spleen of an adult NSG mouse engrafted with HUSC, showing pronounced expansion of white pulp. Hematoxylin and eosin stain; original magnification, 10×. (C) Spleen of an adult NSG mouse engrafted with HUSC demonstrates that the majority of lymphocytes within the reconstituted white pulp expressed human CD3 (thus compatible with human T lymphocytes). Antihuman CD3 indirect immunoperoxidase with hematoxylin counterstain; original magnification, 10×. (D) Spleen of an adult NSG mouse engrafted with HUSC, illustrating that the expression of the human B-lymphocyte marker CD79 was limited chiefly to the periphery (marginal zones) of white pulp foci. Antihuman CD79 indirect immunoperoxidase with hematoxylin counterstain; original magnification, 10×.