TRANSFERRIN RECEPTOR EXPRESSION AS A MARKER OF IMMATURE CYCLING THYMOCYTES IN THE MOUSE

Dividing cells require iron and, therefore, express the transferrin re ceptor (CD71) on the cell surface to enable internalization of transfe rrin-bound iron. Since early T cell development is marked by intense p roliferation, we questioned whether CD71 might serve as a marker of im mature T cells. Therefore, we analyzed the expression of CD71 on fetal , neonatal, and adult thymocytes in correlation with cell size, cell c ycle status, and expression of CD3, CD4, CD8, alpha beta TcR, and gamm a delta TcR. Phenotypic analysis showed that only the large, immature CD4(-)8(-)3(-), CD4(-)8(+)3(-), and CD4(+)8(+)3(-) cells in fetal, neo natal, and adult thymus expressed CD71. In addition, DNA analysis show ed that all CD71(+) large adult thymocytes were cycling. Downregulatio n of CD71 occurs when proliferation ceases, i.e., within the CD4(+)8()3(-) thymocyte subpopulation. The gradual changes in size and CD71 ex pression suggest a sequential development within this CD4(+)8(+)3(-) s ubpopulation from large CD71(+) via small CD71(+) to small CD71(-) cel ls. As a consequence, CD71 expression is downregulated in adult T cell development as well as in ontogeny, before the alpha beta TcR appears on the cell surface of the thymocyte. Together, our findings show tha t CD71 is a marker of immature, proliferating T cells. (C) 1994 Academ ic Press, Inc.