Nf1 homozygosity results in thymic hyperplasia.

(A) Size comparison between WT and Nf1^−/− thymi. RAG2^−/− mice that received equal numbers of WT orNf1^−/− fetal liver or bone marrow hematopoietic stem and progenitor cells and were killed 2 to 3 months after transplantation. Shown is a representative WT andNf1^−/− thymus from a total of 16 analyzed. Original magnification, × 10. (B) Quantitative comparison of thymic cellularity in RAG2^−/− mice that received WT and Nf1^−/− fetal liver or bone marrow cells. Single cell suspensions from WT andNf1^−/− thymi were prepared and counted. Bars represent the absolute numbers of thymocytes (± SEM) from 13 different mice. * P < .001. (C) Ras activity of freshly isolated WT (lanes 1,2) and Nf1^−/− (lanes 3,4) thymocytes. GTP-bound Ras levels were determined by incubating cell lysates with Raf-1 RBD conjugated to agarose beads and by immunoblotting with an anti-Ras antibody. Immunoblots and quantitative densitometry for Ras-GTP levels and Western blot for total Ras from 2 separate WT and Nf1^−/− mice are shown. (D) Cell cycle analysis of WT and Nf1^−/−thymocytes shown in (A). Single cell suspensions from WT andNf1^−/− thymi were stained with propidium iodide and analyzed by flow cytometry, and the percentage of cells from different phases of cell cycle is shown.