B7 costimulates proliferation of CD4-8+ T lymphocytes but is not required for the deletion of immature CD4+8+ thymocytes.

In addition to TCR-derived signals, costimulatory signals derived from stimulation of the CD28 molecule by its natural ligand, B7, have been shown to be required for CD4+8- T cell activation. We investigate the ability of B7 to provide costimulatory signals necessary to drive proliferation and differentiation of virgin CD4-8+ T-cells that express a transgenic TCR specific for the male (H-Y) Ag presented by H-2Db class I MHC molecules. Virgin male-specific CD4-8+ T cells can be activated either with B7 transfected chinese hamster ovary (CHO) cells and T3.70, a mAb specific for the transgenic TCR-alpha chain that is associated with male-reactivity, or by male dendritic cells (DC). Activated CD4-8+ T cells proliferated in the absence of exogenously added IL-2. IL-2 activity was detected in supernatants of CD4-8+T3.70+ cells that were stimulated with T3.70 and B7+CHO cells. The response of CD4-8+T3.70+ cells to T3.70/B7+CHO or to male DC stimulation were inhibited by CTLA4Ig, a fusion protein comprising the extracellular portion of CTLA4 and human IgG C gamma 1. It has been previously shown that CTLA4Ig binds B7 with high affinity. Staining with CTLA4Ig revealed that DC express about 50 times more B7 than CD4-8+ T cells. CTLA4Ig also specifically blocked the proliferation of male-reactive cells in vivo. We have also used an in vitro deletion assay whereby immature CD4+8+ thymocytes expressing the transgenic male-specific TCR are deleted by overnight incubation with either immobilized T3.70 or male DC to investigate the participation of the CD28/B7 pathway in the negative selection of immature thymocytes. Staining with B7Ig established that both immature murine CD4+8+ and mature CD4-8+ thymocytes express a high level of CD28. However, despite the high expression of CD28 on CD4+8+ thymocytes, it was found that deletion of CD4+8+ thymocytes expressing the male-specific TCR by the T3.70 mAb was not inhibited by B7+CHO cells. Furthermore, the deletion of these thymocytes by DC also was not inhibited by CTLA4Ig. These findings provide evidence that although signaling through CD28 can costimulate a primary anti-male response in mature CD4-8+ T cells, the CD28/B7 pathway does not appear to participate in the negative selection of immature CD4+8+ thymocytes.