The expanding B7 superfamily: Increasing complexity in costimulatory signals regulating T cell function

Upon encounter with specific antigen, naive T helper precursor (THP) cells become activated,This event is regulated not only by engagement of the T ce ll receptor (TCR) with peptide presented in the context of major histocompa tibility complex (MHC) class II molecules but by a number of costimulatory signals, CD28 engagement by B7-1 and B7-2 on resting THP cells provides a c ritical signal for initial cell cycle progression, interleukin 2 production and clonal expansion. However, largely as a consequence of the unraveling of the human genome, it is becoming clear that B7-1 and B7-2 are part of a larger family of related counter-receptors that play an essential role in r egulating the fate of primed, rather then vesting,THP cells,These molecules play an important sequential role and act, together with B7-1- and B7-2-pr imed T cells, in the acquisition of effector function and/or tolerance indu ction.