Faculty Bibliography

Interferon gamma (IFN-gamma) responsiveness in certain cells depends on the state of cellular differentiation or activation. Here an in vitro developmental system was used to show that IFN-gamma produced during generation of the CD4+ T helper cell type 1 (TH1) subset extinguishes expression of the IFN-gamma receptor beta subunit, resulting in TH1 cells that are unresponsive to IFN-gamma. This beta chain loss also occurred in IFN-gamma-treated TH2 cells and thus represents a specific response of CD4+ T cells to IFN-gamma rather than a TH1-specific differentiation event. These results define a mechanism of cellular desensitization where a cytokine down-regulates expression of a receptor subunit required primarily for signaling and not ligand binding

Signaling by interferon gamma (IFN-gamma) requires two structurally related cell surface proteins: a ligand-binding polypeptide, known as the IFN-gamma receptor (IFN-gamma R), and an accessory factor. However, it is not known whether IFN-gamma forms a ternary complex with the IFN-gamma R and accessory factor to initiate signaling. Here we demonstrate complex formation between IFN-gamma and the two proteins, both in solution and at the cell surface. We observe complexes containing ligand, two molecules of IFN-gamma R (designated the IFN-gamma R alpha chain), and one or two molecules of accessory factor (designated the IFN-gamma R beta chain). Transfected cells expressing both IFN-gamma R chains bind IFN-gamma with higher affinity than do cells expressing alpha chain alone. Anti-beta-chain antibodies prevent the beta chain from participating in the ligand-receptor complex, reduce the affinity for IFN-gamma, and block signaling. Soluble alpha- or beta-chain extracellular domains also inhibit function. These results demonstrate that IFN-gamma signals via a high-affinity multisubunit complex that contains two types of receptor chain and suggest a potential approach to inhibiting specific actions of IFN-gamma by blocking the association of receptor subunits

Bcl-2, a proto-oncogene that can block apoptosis, was found to be expressed throughout the thymic medulla, but in only scattered cells in the thymic cortex. In order to determine the precise distribution of Bcl-2 protein during thymocyte development, we utilized mAb specific for either mouse or human Bcl-2. Thymocyte subpopulations were assessed using three-color flow cytometry and a saponin-permeabilization method. Staining of adult mouse and human thymocytes was comparable, with 20 to 35% of cells expressing Bcl-2. Bcl-2 was expressed in nearly all CD4+ and CD8+, and CD3hi cells, but in only 5 to 10% of CD4+8+ cells. The CD4-8- population was more variable, with 25 to 40% of human cells and 65 to 80% of murine cells expressing Bcl-2. In sorted adult murine CD4-8- cells, the very immature Pgp-1+/IL-2R alpha- subset had a high percentage of Bcl-2+ cells. Bcl-2 expression was also examined during murine fetal development. At fetal day 15.5 to 16.5, 60 to 70% of total thymocytes expressed Bcl-2. By fetal day 17.5, overall Bcl-2 expression fell to adult levels of 20 to 30%. Bcl-2 was present in peripheral T cells from lymph node, spleen, and peripheral blood at uniformly high levels. In vitro stimulation with anti-CD3 or anti-TCR antibodies increased Bcl-2 expression in total thymocyte cultures, but could not induce Bcl-2 expression in CD4+8+ cells, even with the addition of a variety of cytokines. These data suggest that early double negative thymocytes express Bcl-2 but lose Bcl-2 with differentiation to the double positive stage. Thymocytes regain Bcl-2 during selection to a single positive state and retain Bcl-2 in the periphery