The B7 adhesion molecule is expressed on activated human T cells: Functional involvement in T‐T cell interactions

The B cell antigen B7 delivers a strong co‐stimulatory signal for the activation of T cells by binding to its ligands CD28 and CTLA4. Here we demonstrate the surface expression of the B7 molecule on activated human T cells in vitro and under certain conditions in vivo and its functional importance in T‐T cell interactions. B7 was detected by flow cytometry on antigen‐specific CD4+ and allospecific CD8+ cloned T cells from different donors with anti‐B7 monoclonal antibody (mAb) or a soluble CTLA4‐Cγ1 chimera molecule and by reverse transcription‐polymerase chain reactions. The expression of B7 was up‐regulated following restimulation of the T cell clones and peaked after 7‐9 days. Moreover, we show that the B7 molecule on T cells is functionally involved in T‐T cell interactions: mAb to CD28 and the CTLA4‐Ig fusion protein could inhibit the proliferation of specific T cell clones in response to T cells as antigen‐presenting cells (APC) or the proliferation of peripheral blood mononuclear cells in a primary allostimulation with activated T cells as stimulator cells. Finally, we found that B7 can be expressed on freshly isolated circulating T cells since in a preliminary study with a limited number of patients, B7 was present on a subset of CD3+ cells. B7 was expressed on activated T cells (CD4+ and CD8+) of certain human immunodeficiency virus (HIV)‐infected individuals (0.5–20% B7+CD8+ cells) or some patients with autoimmune diseases whereas CD3+ cells of healthy individuals did not express B7. The coexpression of major histocompatibility complex class II molecules and B7 may be relevant for the capacity of activated T cells to function as APC. The expression of B7 on T cells in vivo in autoimmune diseases and in HIV infection may be important for a better understanding of these diseases.