Repressibility of disease in vivo with doxycycline. Five weeks after the start of doxycycline treatment, mice were killed, and spleens and bones were taken for histologic analysis, flow cytometry, and tissue culture. (A) Flow cytometry was performed on single-cell suspensions of spleen tissue. Cells were stained with anti-hCD2 and either anti–c-Kit (top row) or anti–GR-1 (bottom row). Singly transgenic (left column) and doubly transgenic animals were analyzed. A substantial population of c-Kit⁺, GR-1⁺ cells, characteristic of mast cells, is present in doubly transgenic mice not treated with doxycycline (middle column), whereas a similar population is absent in mice treated with doxycycline (right column). Representative results are shown, with percentages of positive cells indicated in each quadrant. (B) Histologic analysis showing (i) granulocyte and (ii) mast cell infiltrates in the bone marrow of untreated mice and (iii-iv) reduced infiltration in the bone marrow of treated animals. A single sternal vertebra is shown in each panel. Original magnification, × 10.