Roles of CCL21, CCL25, and CXCL12 in fetal thymus colonization. (A) Specific neutralization of CCL21-, CCL25-, and CXCL12-mediated chemotaxis by anti-CCL21, anti-CCL25, and anti-CXCL12 antibodies. Antibody specific for CCL21 (20 μg/mL), CCL25 (10μg/mL), or CXCL12 (20 μg/mL) was added to transwell chemotactic culture of E14.5 fetal thymocytes with CCL21, CCL25, or CXCL12 (20 nM). Shown are means and SEs of percentage inhibition of chemotaxis in 2 to 3 independent experiments. (B-C) Means and SEs of the numbers of E14.5 fetal thymocytes (B) or TER119^–CD45⁺FcR^low E14.5 fetal liver cells (C) that reached dGuo-treated B6 fetal thymus lobe in the microscopically visualized field were determined in the absence or presence of indicated antibodies at the concentration used in the experiments in panel A. Asterisks denote significant reduction of fetal thymus attraction (*P < .05; **P < .01; ***P < .001). NS indicates not significant. (D) Means and SEs of the numbers of E14.5 fetal thymocytes that reached dGuo-treated fetal thymus lobe from +/plt or plt/plt mice were determined in the absence or presence of anti-CCL25 antibody (*P < .05; **P < .01). (E) E14.5 fetal thymocytes from CCR7-deficient mice were examined for the movement to dGuo-treated B6 fetal thymus lobe (**P < .01). (F) Colonization of hematopoietic cells in E11.5 thymus of plt/plt mice and CCR7-deficient mice. Frozen sections of indicated fetal mice were stained for CD45 (green) and pan-cytokeratin (red). (G-H) Cellularity of thymocytes in plt/plt mice (G) and CCR7-deficient mice (H) at indicated gestational ages. P4 and P5 indicate postnatal ages 4 and 5 days, respectively. Means and SEs of the numbers of thymocytes per mouse are shown (*P < 0.05; NS, not significant).