Fig 1: mPTCL has a T‐follicular helper cell‐like phenotype and associates with activated B‐cells AmPTCL splenocytes from passage 3 were transplanted into CD45.1 congenic C57bl6/J mice. Host mice were sacrificed at welfare endpoint between days 31 and 36 post‐transplantation and spleens analyzed by flow cytometry. CD45.2+ lymphoma cells were gated and compared with C57bl6/J CD4+ T‐cells (n = 4). TCRvβ8+CD8− T‐cells are gated for BCL6 analysis (n = 5; shown against isotype control in dashed line). T‐test P = 0.0021 (ICOS), P = 0.00005723 (CD40L), P = 0.0008 (BCL6), P = 0.00000067 (PD1). Bars represent SEM.BImmunohistochemistry for BCL6 in a C57bl/6J versus mPTCL spleen (rp = red pulp). The magnified images at right represent the regions enclosed in dashed boxes. In the images from C57bl/6J mice (top subpanels), a normal germinal center with positive nuclear BCL6 staining is shown by an asterix and magnified in the inset. Data are representative of 8 mice.CExpression levels of MHC‐II on CD45+CD19+B220+ host B‐cells from tissues collected from C57bl/6J (n = 3) and mPTCL‐bearing mice (n = 5). Example dot plot of B‐cell gating in an mPTCL spleen is shown in the subpanel at left. T‐test results were corrected for multiple comparisons by the Holm–Sidak method. Adjusted P = 0.002323 (spleen), P = 0.001133 (bone marrow), P = 0.013809 (liver), P = 0.604980 (mesenteric lymph node, MLN), P = 0.762589 (thymus), P = 0.762589 (axillary lymph node, Ax LN). Bars represent SEM.DExpression levels of CD86 and FAS on splenic B‐cells from mice in panel C. T‐test P = 0.0004 (CD86), P = 0.0004 (FAS). Results were confirmed in three independent experiments. Bars represent SEM.EA schematic showing how a distinct B‐cell lymphoma (mBCL) was derived from lineage “d” of passaged lymphomas. Lineages “a”, “b”, and “c” resulted in only T‐cell lymphomas (mPTCL). H&E stain of a P7 spleen (representative of 5 mice) infiltrated with mBCL and compared with a wildtype C57bl/6J spleen (inset, scale = 200 µm).FProliferation as assessed by Ki67 stain levels in T‐cells (TCRβ+/PAX−) versus B‐cells (TCRβ‐/PAX+) from P5a (mPTCL) versus P5d (mBCL) spleens. Data are representative of five lymphomas.G, H(G) Activation marker expression on CD19+B220+ B‐cells and (H) on bystander CD4+ T‐cells from P6d mBCL and wildtype spleens (n = 3). CD86 on B‐cells/mBCL: P = 0.0041. On T‐cells: P = 0.0024. Results are representative of three independent in vivo experiments. MFI=mean fluorescence intensity. **P < 0.01, ***P < 0.001.