Alefacept, an immunomodulatory recombinant fusion protein composed of the first extracellular domain of LFA-3 fused to the human IgG1 hinge, C H 2, and C H 3 domains, has recently been shown in phase II and III clinical trials to safely reduce disease expression in patients with chronic plaque psoriasis. Alefacept modulates the function of and selectively induces apoptosis of CD2+ human memory-effector T cells in vivo. We have sought to gain further understanding of the mechanisms of action that influence the biological activity of alefacept and may contribute to its efficacy and patient responsiveness. Specifically evaluated is the ability of alefacept to activate intracellular signals mediated via CD2 and/or FcγRIII (CD16). Experimentation using isoforms of alefacept engineered to have amino acid substitutions in the IgG1 C H 2 domain that impact FcγR binding indicate that alefacept mediates cognate interactions between cells expressing human CD2 and CD16 to activate cells, eg, increase extracellular signal-regulated kinase phosphorylation, up-regulate cell surface expression of the activation marker CD25, and induce release of granzyme B. In the systems used, this signaling is shown to require binding to CD2 and CD16 and be mediated through CD16, but not CD2. Experimentation using human CD2-transgenic mice and isoforms of alefacept confirmed the requirement for FcγR binding for detection of the pharmacological effects of alefacept in vivo. Thus alefacept acts as an effector molecule, mediating cognate interactions to activate FcγR+ cells (eg, NK cells) to induce apoptosis of sensitive CD2+ target cells.