Expression and role of phosphatidylcholine‐specific phospholipase C in human NK and T lymphocyte subsets

Abstract

We recently reported evidence of phosphatidylcholine-specific phospholipase C (PC-PLC) involvement in NK cell-mediated cytotoxicity and in lytic granule exocytosis. In the present study, different subpopulations of human PBL were investigated in relation to PC-PLC enzyme expression. While a substantial intracellular amount of PC-PLC was detected in all lymphoid subsets, expression of this enzyme on the outer membrane surface reached high levels only in NK cells, was present at low levels in B lymphocytes and in some TCR gamma/delta T cells and was practically absent in CD4 + and CD8 + T lymphocytes. Moreover, in NK cells two different subpopulations were identified, CD56 dim PC-PLC bright and CD56 bright PC-PLC low/-cells, corresponding to distinct subsets with cytolytic and immunoregulatory functions, respectively. Interestingly, the PC-PLC expression level on the NK membrane surface correlated closely with that of the CD16 receptor, suggesting a possible relationship between enzyme externalization and NK cell maturation. In summary, our results suggest that a high PC-PLC expression on the cell membrane surface of PBL is a peculiarity of NK cytolytic cells, in which the enzyme is apparently involved in the ability of this subset to lyse sensitive target cells.

Functional role of phosphatidylcholine‐specific phospholipase C in regulating CD16 membrane expression in natural killer cells

“…We also demonstrated that, by inhibiting PC-PLC, D609 blocks the cytolytic pathway triggered by CD16, but not that triggered by NKp46 receptor. Moreover, the formation of effector/ target conjugates is not affected neither by the D609-induced down-modulation of PC-PLC from the cell surface, as suggested in this work, nor by masking the enzyme using its specific Ab, as demonstrated in a previous study [9]. This suggests that PC-PLC inhibition by D609 has specific effects on CD16 expression without inducing down-regulation of adhesion molecules, such as CD56, on the membrane surface.…”

“…We also demonstrate that the engaged CD16 receptor translocates into the cytoplasmic compartments through a passive cellular process independent of ATP availability. We recently reported a correlation between PC-PLC and CD16 on the plasma membrane of NK cells, suggesting that an increasing expression of both proteins may be related to NK cell maturation [9].…”

“…2F, lanes 3-5) and in the detergent-soluble fractions (lanes 7-11); when PC-PLC was inhibited by D609, CD16 was no longer detectable in the lipid rafts, although it remained unaffected in the detergent-soluble fractions. Notably, PC-PLC was also detected in the cholesterolrich membrane domains (lanes 4-6), even though in a smaller quantity than that found in the detergentsoluble fractions (lanes [8][9][10][11]. After cell incubation with D609, PC-PLC totally disappeared from the lipid rafts and strongly decreased in the membrane detergentsoluble fractions.…”

“…Our recent study shows that the expression of CD16 molecules is correlated with that of the PC cycle enzyme phospholipase C (PC-PLC) on the outer membrane surface of resting NK cells [9]. PC-PLC mediates the production of phosphocholine (PCho) and non-phosphatidylinositol 4,5-bisphosphate (PIP 2 )-derived diacylglycerol (DG) in a number of receptor-stimulated cells, including lymphocytes [10].…”

“…However, no PC-PLC isoforms have yet been cloned from mammalian sources, nor has the mechanism by which the enzyme is involved in cellular response been elucidated. Most early studies were devoted to isolating and cloning prokaryotic PC-PLC enzymes [11], while today there is direct evidence demonstrating the expression of PC-PLC isoforms in mammalian cells [9,[12][13][14][15]. In particular, we have already reported that an endogenous PC-PLC undergoes a cytokine-dependent distribution between cytoplasmic compartments and the outer membrane surface of human NK cells and that the PC-PLC inhibitor D609 reduces the enzyme fraction expressed on the surface of NK cells, with the formation of clusters of the cytoplasmic fraction within the Golgi region [14].…”