Inhibition of proliferation and differentiation during early T cell development by anti‐transferrin receptor antibody

Proliferating cells require iron and, therefore, express the transferrin receptor (CD71) that mediates cellular iron uptake. Cycling thymocytes, which have the CD4⁻8⁻3⁻, CD4⁻8⁺3⁻, or CD4⁺8⁺3⁻ phenotypes, also express CD71. The importance of CD71‐mediated iron uptake for proliferation and maturation of thymocytes was studied using fetal thymus organ cultures at day 14 of gestation and treating them for 7 days with a CD71 monoclonal antibody (mAb). The intracellular iron deficiency caused by this treatment, inhibits both proliferation and maturation of the thymocytes. Cell recovery was reduced by 60%, but cells still expanded tenfold during the culture. Remarkably, the final maturation of αβ T cells was completely blocked as no thymocytes with low or high CD3/αβTcR expression developed. Moreover, only few cells reached the CD4⁺8⁺3⁻ stage of T cell development. CD4⁻8⁻3⁻ thymocytes, however, as well as its CD44⁻25⁺ subset developed in normal numbers, suggesting that CD44⁻25⁺ CD4⁻8⁻3⁻ cells, or their immediate progeny, were most vulnerable to CD71 mAb treatment. The development of γδ T cells, which also express CD71, was not affected in these cultures. This suggests that γδ T cells are either less iron‐dependent or possess alternative iron‐uptake mechanisms. Thus, our observations indicate that CD71 treatment, causing decreased intracellular iron levels, severely inhibits the major proliferation phase from the CD44⁻25⁺ CD4⁻8⁻3⁻ to the CD4⁺8⁺3⁻ cells, and completely abrogates the final maturation of CD4⁺8⁺3⁻ cells into αβTcR‐expressing cells. In contrast, proliferation and differentiation of the earliest thymic precursors into CD44⁻25⁺ CD4⁻8⁻3⁻ cells is not affected by CD71 treatment.