The Mechanism of CD47-Dependent Killing of T Cells: Heterotrimeric Gi-Dependent Inhibition of Protein Kinase A

Abstract

CD47 has been implicated in both positive and negative regulation of T cells as well as in T cell death. To clarify the role of CD47 in T cell function, we have studied the mechanism of T cell death in response to CD47 ligands, including mAb 1F7, thrombospondin-1, and a CD47 agonist peptide derived from it. CD47−/− Jurkat T cells (JINB8) were resistant to killing by all three ligands, indicating the essential role of CD47. Primary human T cells were also killed by CD47 ligands, but only after activation with anti-CD3. CD47-mediated cell death occurred without active caspases, DNA fragmentation, or Bcl-2 degradation. Pretreatment of Jurkat and primary T cells with pertussis toxin (PTX) prevented CD47-mediated death, indicating the involvement of Giα. Pretreatment of T cells with 8-bromo cAMP, forskolin, or 3-isobutyl-1-methylxanthine prevented the CD47-mediated apoptosis, and 1F7 dramatically reduced intracellular cAMP levels, an effect reversed with PTX. H89 and protein kinase A (PKA) inhibitor peptide, a specific PKA inhibitor, prevented rescue of T cells by PTX, 8-bromo cAMP, and forskolin, indicating a direct role for one or more PKA substrates. Thus, CD47-mediated killing of activated T cells occurs by a novel pathway involving regulation of cAMP levels by heterotrimeric Giα with subsequent effects mediated by PKA.