TGF‐β cooperates with TGF‐α to induce the self–renewal of normal erythrocytic progenitors: evidence for an autocrine mechanism

Simultaneous addition of both TGF‐α and TGF‐β induces the sustained, long‐term outgrowth of chicken erythrocytic progenitor cells, referred to as T2ECs from both chick bone marrow and 2‐day‐old chicken embryos. By analysis for differentiation antigens and gene expression, these cells were shown to represent very immature haematopoietic progenitors committed to the erythrocytic lineage. T2ECs differentiate into almost pure populations of fully mature erythrocytes within 6 days, when TGF‐α and TGF‐β are withdrawn and the cells exposed to anaemic chicken serum plus insulin. Outgrowth of these cells from various sources invariably required both TGF‐α and TGF‐β, as well as glucocorticoids. Proliferating, established T2ECs still require TGF‐α, but are independent of exogenous TGF‐β. Using a TGF‐β‐neutralizing antibody or expressing a dominant‐negative TGF‐β receptor II, we demonstrate that T2ECs generate an autocrine loop involving TGF‐β during their establishment, which is required for sustained proliferation. Using specific inhibitors, we also show that signalling via Mek‐1 is specifically required for induction and maintenance of cell proliferation driven by cooperation between the TGF‐α and ‐β receptors. These results establish a novel mechanism by which self‐renewal of erythrocytic progenitors is induced and establish avian T2ECs as a new, quasi‐optimal model system to study erythrocytic progenitors.