Constitutive activation of Stat5 induces mast cell hyperplasia and leukemic cell infiltrates in the gastrointestinal tract of cS5^F mice. (A) Immunohistochemical analysis of stomach walls of mice that underwent transplantation with cS5^F BM cells versus control mice that received GFPv BM cells (n > 6 mice analyzed in each group and a representative example is shown). To detect neoplastic MCs, sections of the stomach mucosa and submucosa of CS5^F mice and control GFPv-mice were immunostained with an antitryptase antibody (i-vi). The presence of tyrosine^694/695 phosphorylated Stat5 (P-Y-Stat5; vii-xii) and serine⁴⁷³ phosphorylated Akt (P-S-Akt; xiii-xviii) was analyzed with specific antibodies on consecutive tissue sections (original magnification, ×400). An overview of each staining on consecutive sections is shown in the 2 panels labeled “Overview” (iii, iv, viiix, xv, xvi, xxi, xxii) for leukemia cS5^F and control GFPv mice (original magnifications, ×25 and ×100, respectively). H&E staining is shown for organotypic structure comparison (xviiii-xxiv). (B) Quantification of P-Y-Stat5, tryptase, and P-S-Akt stainings was performed on consecutive sections using HistoQuest analysis software (results shown in the scattergram plots are the mean of 4 fields of view for each staining). The cutoff values for background staining were chosen manually using the forward/backward gating tool of the HistoQuest software.