CD152 also known as CTLA-4 or Ly-56 is a 33 kDa member of the immunoglobulin superfamily It is expressed on activated T and B lymphocytes CD152 exhibits significant homology to CD28 in amino acid sequence structural configuration and genomic organization with both receptors sharing ligands from the B7 family (B7-1 and B7-2) While CD28 delivers co-stimulatory signals during T cell activation CTLA-4 functions as a critical negative regulator of cell-mediated immune responses CD152 is implicated in the induction and maintenance of immune tolerance the development of protective immunity and the modulation of thymocyte homeostasis