B7x: A widely expressed B7 family member that inhibits T cell activation.

B7 family proteins provide costimulatory signals that regulate T cell responses. Here we report the third set of B7 family-related T cell inhibitory molecules with the identification of a homolog of the B7 family, B7x. It is expressed in immune cells, nonlymphoid tissues, and some tumor cell lines. BTx inhibits cell-cycle progression, proliferation, and cytokine production of both CD4[sup +] and CD8[sup +] T cells. BTx binds a receptor that is expressed on activated, but not resting T cells that is distinct from known CD28 family members. Its receptor may be a recently identified inhibitory molecule, B and T lymphocyte attenuator. These studies identify a costimulatory pathway that may have a unique function in down-regulation of tissue-specific autoimmunity and antitumor responses. [ABSTRACT FROM AUTHOR]