Cytotoxic Granule Exocytosis Research Articles

β1 Integrin Cross-Linking Inhibits CD16-Induced Phospholipase D and Secretory Phospholipase A2 Activity and Granule Exocytosis in Human NK cells: Role of Phospholipase D in CD16-Triggered Degranulation

AbstractRecent data indicate that integrin-generated signals can modulate different receptor-stimulated cell functions in both a positive (costimulation) and a negative (inhibition) fashion. Here we investigated the ability of β1 integrins, namely α4β1 and α5β1 fibronectin receptors, to modulate CD16-triggered phospholipase activation in human NK cells. β1 integrin simultaneous cross-linking selectively inhibited CD16-induced phospholipase D (PLD) activation, without affecting either phosphatidylinositol-phospholipase C or cytosolic phospholipase A2 (PLA2) enzymatic activity. CD16-induced secretory PLA2 (sPLA2) protein release as well as its enzymatic activity in both cell-associated and soluble forms were also found to be inhibited upon β1 integrin coengagement. The similar effects exerted by specific PLD pharmacological inhibitors (2,3-diphosphoglycerate, ethanol) suggest that in our experimental system, sPLA2 secretion and activation are under the control of a PLD-dependent pathway. By using pharmacological inhibitors (2,3-diphosphoglycerate, wortmannin, ethanol) we also demonstrated that PLD activation is an important step in the CD16-triggered signaling cascade that leads to NK cytotoxic granule exocytosis. Consistent with these findings, fibronectin receptor engagement, by either mAbs or natural ligands, resulted in a selective inhibition of CD16-triggered, but not of PMA/ionomycin-induced, degranulation that was reversed by the exogenous addition of purified PLD from Streptomyces chromofuscus.

CD45 Regulates Apoptosis Induced by Extracellular Adenosine Triphosphate and Cytotoxic T Lymphocytes

Several lines of evidence implicate protein tyrosine phosphatases (PTP) in the regulation of apoptotic cell death. We have evaluated the role of CD45, the major PTP of hematopoietic cells, in apoptosis induced by extracellular ATP (ATPe) and cytotoxic T lymphocytes (CTL). We observed that two CD45−clones obtained by mutagenesis of the Fas−cell line L1210, exhibit a higher susceptibility to apoptosis induced by ATPe, which was also evident in Ca2+-free conditions, when compared to the parental cell line or CD45+variants. The CD45−cells were also more susceptible to death mediated by an alloreactive CTL clone. When the cytotoxic assay was performed in the presence of EGTA, a Ca2+chelator, which prevents cytotoxic granule exocytosis and perforin polymerization on target cell membranes, only the CD45−target cells were killed by the CTL clone. These results suggest that a cytotoxic pathway other than the secretory or Fas-dependent pathways was responsible for the enhanced susceptibility of CD45−cells to death, and therefore provide further evidence for the role of ATPeas a possible mediator of Ca2+-independent target cell destruction by CTL.

Protein tyrosine kinase p56-Lck regulates lymphocyte function-associated 1 adhesion molecule expression, granule exocytosis, and cytolytic effector function in a cloned T cell.

Elevated levels of p56-Lck kinase activity were achieved in an interleukin 2 (IL-2)-dependent cloned cytolytic T cell CTLL-2 through gene transfer approaches. CTLL-2-Lck cells remained dependent on IL-2 for growth and survival in culture but exhibited markedly elevated, IL-2-independent cytolytic activity against a variety of tumor targets. This immune cell effector function was similar to the non-major histocompatibility complex-restricted cytolytic activity previously described for lymphokine activated killer (LAK) cells, and involved a cytolytic mechanism that was independent of protein synthesis in either the T cells or the tumor targets. Characterization of CTLL-2-Lck cells revealed markedly elevated levels of both the alpha (CD11a) and beta (CD18) chains of the cell adhesion molecule lymphocyte function-associated 1 (LFA-1) and increased binding of these T cells to a recombinant protein representing the extracellular domain of the LFA-ligand, intercellular adhesion molecule 1 (ICAM-1). Antibodies to CD11a partially abrogated cytolytic killing of tumor target cells by CTLL-2-Lck cells, suggesting that the upregulation in LFA protein levels potentially accounts at least in part for the phenotype of these T cells. Gene transfer-mediated elevations in p56-Lck kinase activity in an IL-3-dependent myeloid cell clone 32D.3 also resulted in increased LFA-1 expression, demonstrating that the findings are not unique to CTLL-2 cells. In addition to upregulation of LFA-1 expression, CTLL-Lck cells also exhibited more efficient exocytosis of cytotoxic granules upon activation with Ca(2+)-ionophore and phorbol ester, relative to control transfected and untransfected CTLL-2 cells. The findings functionally link the Lck kinase to a T cell effector pathway involved in cell-mediated cytotoxicity.

1,25(OH) 2D 3 increases cytotoxicity and exocytosis in lymphokine-activated killer cells

The effect of 1,25-dihydroxyvitamin D 3 on lymphokine-activated killer (LAK) cells activity was studied. Treatment of LAK cells with 1,25-dihydroxyvitamin D 3 for 24 h increased their cytotoxic activity without affecting cell proliferation. This effect was dose-dependent, detectable already at 10 −11 M attaining 44 ± 7% increase at 10 −8 M. 1,25-dihydroxyvitamin D 3 increased LAK cell content of the cytotoxic granule granzyme A by 21%. Secretion of granzyme A by LAK cells was triggered by 12- O-tetradecanoylphorbol 13-acetate and the calcium ionophore A23187. 1,25-dihydroxyvitamin D 3 decreased the lag preceding secretion, increased the rate constant of exocytosis and the fraction of granzyme A cell content secreted. The potentiation of exocytosis was more pronounced at suboptimal calcium ionophore concentration suggesting that 1,25-dihydroxyvitamin D 3 affects a calcium-dependent process. Since exocytosis of cytotoxic granules is a pivotal event in the killing of tumor cells by LAK cells, it is plausible that the enhancement of this process underlies the stimulation of LAK cell cytotoxic activity by 1,25-dihydroxyvitamin D 3.

Interaction between protein kinase C-dependent and G protein-dependent pathways in the regulation of natural killer cell granule exocytosis.

The binding of natural killer (NK) cells to either susceptible tumor cells or antibody-coated targets results in rapid activation of phospholipase C (PLC) in NK cells. PLC activation generates inositol-1,4,5-trisphosphate and sn-1,2-diacylglycerol as second messengers, which, in turn, increase intracellular free calcium concentrations ([Ca2+]i) and protein kinase C (PKC) activity, respectively. These proximal signals initiate a cascade of as yet undefined biochemical events, leading eventually to the exocytosis of preformed cytotoxic granules. To investigate the signal transduction pathways involved in granule exocytosis, we utilized streptolysin-O-permeabilized human NK cells as our experimental model. Our initial studies indicated that the separate activation of either PKC (using the phorbol ester, PMA) or G protein-dependent pathways (using guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S)) stimulated granule exocytosis in a time-, concentration-, and Ca(2+)-dependent manner. PMA-stimulated exocytosis was inhibited by staurosporine or a PKC pseudosubstrate antagonist peptide, but was not affected by GDP. In contrast, GTP gamma S-stimulated exocytosis was effectively inhibited by GDP, but not by staurosporine or the PKC pseudosubstrate antagonist. These observations suggest that NK cell exocytosis can be stimulated by at least two separate pathways; one involving PKC and the other involving a G protein. However, co-stimulation with PMA and GTP gamma S synergistically enhanced exocytosis, suggesting that even though the two exocytotic pathways were biochemically distinct, cross-talk between the two pathways may potently influence the exocytotic process. These results define a regulatory role for PKC- and G protein-dependent pathways during granule exocytosis from NK cells.

Studies of the Mechanism of Natural Killer (NK) Degranulation and Cytotoxicity

Exocytosis of cytotoxic granule contents towards bound target cells is thought to be of central importance in natural killer (NK) cell-mediated killing. Although cellular cytotoxicity involving degranulation is thought to be calcium-dependent, the biochemical mechanisms that mediate this granule mobilization are unknown. Inositol-1,4,5-tris-phosphate (IP3), which acts to elevate internal calcium levels, and 1,2-diacylglycerol (DAG), which activates protein kinase C, are potent second messengers that have been shown to synergistically mediate secretion in other cell types. Production of these products of inositol phospholipid metabolism has previously been demonstrated in a rat NK cell line RNK upon exposure to susceptible tumor targets. We therefore investigated the role of IP3 and DAG in NK-mediated cytotoxicity, specifically at the level of degranulation. Pretreatment of RNK cells with neomycin, a drug that interferes with the hydrolysis of inositol phospholipids and thus inhibits the formation of second messengers, inhibited RNK cytotoxicity against a susceptible tumor target and also inhibited RNK production of DAG in response to a similar target. Natural killing exhibited by normal rat nylon wool-nonadherent splenocytes was also inhibited by neomycin. Phorbol-12-myristate, 13-acetate (PMA), a phorbol ester that acts like DAG to activate protein kinase C, markedly enhanced lysis of a susceptible target cell by RNK. We evaluated whether modulation of lysis by these drugs was associated with effects on RNK degranulation by assaying the release of a granule-specific serine esterase (BLTE) in response to PMA and the calcium ionophore A23187. These agents synergized to promote the release of BLTE, and the extent of release was dependent on the concentrations of both agents. D2O and cytochalasin B, which enhance secretion in other cells, both enhanced BLTE release from RNK cells, indicating that we were detecting BLTE released via granule secretion and not due to nonspecific causes such as cell lysis. Our findings lead us to propose that NK cells form IP3 and DAG in response to susceptible target cells and that a major function of these second messengers is to mediate the exocytosis of cytotoxic granules towards the bound target cells.

Interaction of lymphokine-activated killer cells with susceptible targets does not induce second messenger generation and cytolytic granule exocytosis.

CTL activation by specific targets leads to a rapid rise of inositol phosphates (InsPs) and of cytoplasmic-free Ca2+ concentration ([Ca2+]i). While these events are considered necessary to trigger granule secretion, Ca2+-independent cytolytic mechanisms have been recently proposed in addition or as an alternative to the classical Ca2+-dependent exocytosis model. We observed that lymphokine-activated killer (LAK) cells, obtained after stimulation with supraoptimal concentrations of IL-2 in short- or long-term cultures, kill susceptible targets in the absence of a [Ca2+]i rise and InsP3 formation. Moreover, LAK cell-mediated lysis was not associated with an increase in cytotoxic granule exocytosis, as evaluated by BLT-esterase release into the culture supernatant. Furthermore, using an antigen-specific CTL clone, which acquires LAK-like activity when cultured in medium containing high IL-2 doses, second messenger generation and cytolytic granule content secretion were not detected during lysis of unrelated target cells, while killing of specific targets triggered both these processes. These findings suggest that two lytic pathways may coexist in the same effector cells: a second messenger-dependent pathway involving degranulation, which is activated after TCR interaction with specific targets, and another pathway, independent of any known second messenger generation, responsible for unrelated target cell lysis.