| Abstract
| - Peyer's patch (PP) organogenesis proceeds through three histologically distinct steps: formation of organizing centers expressing VCAM-1 and ICAM-1 in segregated regions of the intestine at 15.5 days post-coitus (d.p.c.) (step I), accumulation of blood cells expressing different sets of surface markers to this region at 16.5-17.0 d.p.c. (step II), and entry of CD3+ and B220+ lymphocytes just before birth (step III). PP formation of both Il7ra-/- and Lta-/- mice is impaired from step I, suggesting involvement of the two molecules at the same timing in PP organogenesis. Expression of lymphotoxin (LT) α and LTβ in IL-7 receptor (IL-7R) α+ cells in the intestine indicates that defects of Il7ra-/- and Lta-/- mice are due to functional inability of IL-7Rα+ cells in the induction of PP anlage. Blocking of IL-7Rα function by a single injection of the antagonistic mAb in 15.5 d.p.c. embryos suppressed appearance of VCAM-1+ spots and expression of LTα and LTβ in the intestine, which eventually resulted in mice without PP but are otherwise normal. Intestinal IL-7Rα+ cells are lymphoid in morphology but CD3- and functional in both nu/nu and Rag2-/- mice. These results implicate IL-7Rα+ CD3- cells as the direct inducer of the organizing center of PP.
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