Attraction of fetal thymocytes to CCL21-transfected NIH-3T3 cells. (A) Diagram of pMRX-CCL21-IRES-GFP and pMRX-IRES-GFP constructs. (B) Retroviruses expressing either GFP alone or CCL21 along with GFP were infected into NIH-3T3 cells. GFP⁺ cells sorted by flow cytometry were reanalyzed for GFP expression. Numbers indicate frequency of cells within marked area. (C) CCL21-GFP–infected NIH-3T3 cells were examined for the production of functional CCL21. Reaggregated CCL21-GFP–infected NIH-3T3 cells (5 × 10⁵) were examined for the attraction of adult CD4⁺CD8^– thymocytes from wild-type or CCR7-deficient mice. Means and SEs of attracted cell numbers are indicated (***P < .001). (D) Reaggregates of either CCL21-GFP–infected NIH-3T3 cells or GFP-infected NIH-3T3 cells were examined for the attraction of E14.5 fetal thymocytes. Means and SEs of the numbers of cells that moved out of the cell spot (□) and reached the cell reaggregate (▧) were determined in visualized field (*P < .05; ***P < .001).