Intact lymphatic drainage and the drainage-promoting activity of IL-7 depend on IL-7Rα expression in stromal cells. BM chimeras between WT and IL-7Rα^−/− mice were generated and directly analyzed or subjected to IL-7/anti-IL-7 complex treatment. (A-B) Analysis of BM chimeras. (A) The LN cellularity was reduced in BM chimeras reconstituted with IL-7Rα^−/− BM. Bars represent pooled data from 2 inguinal and 2 brachial LNs per mouse. (B) To evaluate lymphatic drainage function, Evans blue dye was intradermally injected into the ears of BM chimeras and the dye content remaining in the ear was extracted and quantified 16 hours later. Quantification detected increased Evans blue levels in the ears of BM chimeras with IL-7Rα^−/− stromal cells, indicative of defective lymphatic drainage. (C-D) Treatment of BM chimeras with IL-7/anti-IL-7 complexes. (C) IL-7/anti-IL-7 complex treatment led to a significant increase in the splenic cellularity of chimeras reconstituted with WT but not with IL-7Rα^−/− BM. (D) Evans blue drainage experiments revealed that treatment of BM chimeras with IL-7/anti-IL-7 complexes only enhanced lymphatic drainage in chimeras with functional IL-7Rα in stromal cells (WT → WT; IL-7Rα^−/− → WT). Complex treatment of chimeras exclusively expressing IL-7Rα in hematopoietic cells but not in stromal cells (WT → IL-7Rα^−/−) did not improve drainage. Pooled data from 1-2 experiments involving 4-6 mice/group are shown. *P < .05; **P < .01. n.s., not significant.