The ability of insulin-like growth factor-1 (IGF-1) to regulate surface expression of the interferon-gamma receptor 2 (IFN-gamma R2) transducing chain and activation of IFN-gamma-induced signal transducer and activator of transcription-1 (STAT-1) in human T cells was analyzed. We show that, especially in the absence of serum (which contains IGF-1), IGF-1 down-regulated surface expression of the IFN-gamma R2 chain and inhibited both IFN-gamma-dependent STAT-1 activation and apoptosis in T-cell lines ST4, Jurkat, and Molt-4. IFN-gamma R2 down-regulation resulted from its enhanced internalization since IGF-1 completely restored the uptake of anti-IFN-gamma R2 monoclonal antibody (mAb) in serum-deprived T-cell lines. When the interaction between IGF-1 and its receptor was blocked by anti-IGF-1R mAb, enhancement of IFN-gamma R2 surface expression, STAT-1 activation, and reinstatement of IFN-gamma-induced apoptosis were observed. Enhanced expression of IFN-gamma R2 was also observed in phytohemagglutinin (PHA)-activated T lymphoblasts cultured in the presence of anti-IGF-1R mAb, whereas IGF-1 or anti-IGF-1R mAb did not modify the high IFN-gamma R2 expression in B and myeloid cell lines. Both IGF-1 and anti-IGF-1R mAb did not modify the constitutive expression of IFN-gamma R2 mRNA in T cells as well as the high IFN-gamma R1 binding chain surface expression in T, B, and myeloid cells. These data indicate that IGF-1 plays a critical role in the desensitization of IFN-gamma/STAT-1 signaling in T lymphocytes by delivering a signal for IFN-gamma R2 internalization.