The Mcl-1 transgene promotes immortalization of monocytic and mast cell lines. (A and B) Establishment of cell lines from splenocytes and bone marrow cells from Mcl-1 transgenic mice. Cells from a transgenic and a nontransgenic mouse were cultured in long-term culture medium (containing IL-3 as described in Materials and Methods) and assayed at approximately weekly intervals for the total number of viable cells derived from the original culture, which contained a total of 5 × 10⁶ cells. (C) Light microscopic view of a monocytic cell line (stained with Wright’s Giemsa, original magnification × 630). Histochemical staining showed this line to be strongly positive for alpha-naphthyl butyrate esterase activity, weakly positive for chloroacetate esterase activity, and negative for myeloperoxidase activity. (D) Light microscopic view of a mast cell line (stained with Wright’s Giemsa, original magnification × 800). (E) Electron microscopic view of a mast cell line (original magnification × 8,300) showing characteristics typical of immature mast cells, including large, often lobulated, nuclei and cytoplasmic granules containing a central dense core and a mixture of particles and vesicles.57 63 (F) Characterization of mast cell lines. Cell surface markers and histochemical staining properties were assayed for the indicated three cell lines. The percentage of c-kit⁺ cells was high in all cell lines (99%), as was the percentage of Sca1⁺ cells (89% for 4Q6BM, 90% for 3Y10BM, and 97% for 4Q6SP). Negligible percentages of cells expressed CD11b, Thy 1.2, CD3, CD5, or B220. Histochemical staining for chloroacetate esterase activity showed the following: 4Q6BM exhibited staining in the majority of cells (varying intensity in different cells); 3Y10BM contained a mixture of negative and weakly positive cells; and 4Q6SP contained mostly negative cells, with a minority of cells exhibiting weakly positive staining. The three cell lines were negative for myeloperoxidase and alpha-naphthyl butyrate esterase activities.