IFN-Î³ inhibits the growth and differentiation of erythroid precursor cells and mediates hemopoietic suppression through mechanisms that are not completely understood. We found that treatment of human erythroid precursor cells with IFN-Î³ up-regulates the expression of multiple members of the TNF family, including TRAIL and the recently characterized protein TWEAK. TWEAK and its receptor fibroblast growth factor-inducible 14 (Fn14) were expressed by purified erythroblasts at all the stages of maturation. Exposure to recombinant TWEAK or agonist anti-Fn14 Abs was able to inhibit erythroid cell growth and differentiation through caspase activation. Because other members of the TNF family such as TRAIL and CD95 ligand (CD95L) are known to interfere with erythroblast growth and differentiation, we investigated the role of different TNF/TNFR family proteins as potential effectors of IFN-Î³ in the immature hemopoietic compartment. Treatment of erythroid precursor cells with agents that blocked either TRAIL, CD95L, or TWEAK activity was partially able to revert the effect of IFN-Î³ on erythroid proliferation and differentiation. However, the simultaneous inhibition of TRAIL, TWEAK, and CD95L resulted in a complete abrogation of IFN-Î³ inhibitory effects, indicating the requirement of different receptor-mediated signals in IFN-Î³-mediated hemopoietic suppression. These results establish a new role for TWEAK and its receptor in normal and IFN-Î³-mediated regulation of hematopoiesis and show that the effects of IFN-Î³ on immature erythroid cells depend on multiple interactions between TNF family members and their receptors. Copyright Â© 2005 by The American Association of Immunologists, Inc.