The JJ316 antibody reacts with CD28 on virtually all T lymphocytes bearing αβT-cell receptors (TCR), on most γδTCR-bearing T cells, and on a subset of NK cells. In the thymus, CD28 expression is developmentally regulated during the maturation of αβTCR-bearing T cells. CD28 is a costimulatory receptor required for activation of T cells; its ligands include CD80 (B7-1) and CD86 (B7-2). Soluble JJ316 enhances the proliferative responses and IL-2 production by T cells activated by anti-aß-TCR mAb R73 (Cat. no. 554910). Furthermore, at high concentrations, cross-linked JJ316 mAb stimulates resting CD4+ and CD8+T cells to proliferate, secrete IL-2, express activation markers, and gain CTL activity in the absence of signaling through the TCR. In vivo administration of mAb JJ316 results in blastogenic transformation of peripheral CD4+ cells and dramatic lymphocytosis. The alternate anti-rat CD28 mAb JJ319 (Cat. no. 554993) cannot directly activate T cells, although it has in vitro costimulatory activity.