Physiological function of phospholipase D2 in anti-tumor immunity: regulation of CD8(+) T lymphocyte proliferation

Two major phospholipase D (PLD) isozymes in mammals, PLD1 and PLD2, hydrolyze the membrane phospholipid phosphatidylcholine to choline and the lipid messenger phosphatidic acid. Although their roles in cancer cells have been well studied, their functions in tumor microenvironment have not yet been clarified. Here, we demonstrate that PLD2 in cytotoxic CD8(+) T cells plays a crucial role in anti-tumor immunity by regulating their cell proliferation. We found that growth of tumors formed by subcutaneously transplanted cancer cells is enhanced in Pld2-knockout mice. Interestingly, this phenotype was found to be at least in part attributable to the ablation of Pld2 from bone marrow cells. The number of CD8(+) T cells, which induce cancer cell death, significantly decreased in the tumor produced in Pld2-knockout mice. In addition, CD3/CD28-stimulated proliferation of primary cultured splenic CD8(+) T cells is markedly suppressed by Pld2 ablation. Finally, CD3/CD28-dependent activation of Erk1/2 and Ras is inhibited in Pld2-deleted CD8(+) T cells. Collectively, these results indicate that PLD2 in CD8(+) T cells plays a key role in their proliferation through activation of the Ras/Erk signaling pathway, thereby regulating anti-tumor immunity.