Developmental Expression of IL‐2‐Receptor Light Chain (CD25) in the Chicken Embryo

Thymocyte differentiation obeys the same fundamental principles in mammals as in avian species. This parallelism does not only affect the developmentally controlled acquisition of CD3, 4, 8, and TcR isotype expression, but also concerns CD25, the light chain of the interleukin‐2 receptor (IL‐2R). On chicken thymocytes, surface CD25, which is recognized by the monoclonal antibody INN Ch16, is first observed during day 11 of embryonic life, and peaks at day 14, when it is expressed by about one‐third of all lymphoid cells. CD25 is found on subsets of all ,thymocyte populations as defined by TcR α β , TcR γ δ , 2, CD4, and CD8 expression, cortical or medullary localization, and is also present on a subset of intrathymic nurse‐cell lymphocytes. These findings suggest phylogenetic conservation of the IL‐2/IL‐2R‐triggered differentiation pathway previously described for mammalian species, thus under‐lining its probable functional importance.