22Greenwald RJ, Freeman GH, Sharpe AH. The B7 family revisited. Annu Rev Immunol 2004;23:515-48. 23Freeman FJ, Long AJ, Iwai Y, et al. Engagement of the PD-1 immunohibitory receptor by a novel B7 family member l eads to negative regu lation of lymphocyte activation. J Exp Med 2000;192(7):1027-34.