Cross-talk with Ca2+ Influx Does Not Underlie the Role of Extracellular Signal-regulated Kinases in Cytotoxic T Lymphocyte Lytic Granule Exocytosis

Regulation of CD8 T-cell signaling, metabolism, and cytotoxic activity by extracellular lysophosphatidic acid

Lysophosphatidic acid (LPA) is an endogenous bioactive lipid that is produced extracellularly and signals to cells via cognate LPA receptors, which are G-protein coupled receptors (GPCRs). Mature lymphocytes in mice and humans express three LPA receptors, LPA2 , LPA5, and LPA6 , and work from our group has determined that LPA5 signaling by T lymphocytes inhibits specific antigen-receptor signaling pathways that ultimately impair lymphocyte activation, proliferation, and function. In this review, we discuss previous and ongoing work characterizing the ability of an LPA-LPA5 axis to serve as a peripheral immunological tolerance mechanism that restrains adaptive immunity but is subverted during settings of chronic inflammation. Specifically, LPA-LPA5 signaling is found to regulate effector cytotoxic CD8 T cells by (at least) two mechanisms: (i) regulating the actin-microtubule cytoskeleton in a manner that impairs immunological synapse formation between an effector CD8 T cell and antigen-specific target cell, thus directly impairing cytotoxic activity, and (ii) shifting T-cell metabolism to depend on fatty-acid oxidation for mitochondrial respiration and reducing metabolic efficiency. The in vivo outcome of LPA5 inhibitory activity impairs CD8 T-cell killing and tumor immunity in mouse models providing impetus to consider LPA5 antagonism for the treatment of malignancies and chronic infections.

Role of alpha7 nicotinic acetylcholine receptor in calcium signaling induced by prion protein interaction with stress-inducible protein 1

The prion protein (PrP(C)) is a conserved glycosylphosphatidylinositol-anchored cell surface protein expressed by neurons and other cells. Stress-inducible protein 1 (STI1) binds PrP(C) extracellularly, and this activated signaling complex promotes neuronal differentiation and neuroprotection via the extracellular signal-regulated kinase 1 and 2 (ERK1/2) and cAMP-dependent protein kinase 1 (PKA) pathways. However, the mechanism by which the PrP(C)-STI1 interaction transduces extracellular signals to the intracellular environment is unknown. We found that in hippocampal neurons, STI1-PrP(C) engagement induces an increase in intracellular Ca(2+) levels. This effect was not detected in PrP(C)-null neurons or wild-type neurons treated with an STI1 mutant unable to bind PrP(C). Using a best candidate approach to test for potential channels involved in Ca(2+) influx evoked by STI1-PrP(C), we found that α-bungarotoxin, a specific inhibitor for α7 nicotinic acetylcholine receptor (α7nAChR), was able to block PrP(C)-STI1-mediated signaling, neuroprotection, and neuritogenesis. Importantly, when α7nAChR was transfected into HEK 293 cells, it formed a functional complex with PrP(C) and allowed reconstitution of signaling by PrP(C)-STI1 interaction. These results indicate that STI1 can interact with the PrP(C)·α7nAChR complex to promote signaling and provide a novel potential target for modulation of the effects of prion protein in neurodegenerative diseases.

Calcium influx and signaling in cytotoxic T-lymphocyte lytic granule exocytosis

Cytotoxic T lymphocytes (CTLs) kill targets by releasing cytotoxic agents from lytic granules. Killing is a multi-step process. The CTL adheres to a target, allowing its T-cell receptors to recognize antigen. This triggers a signal transduction cascade that leads to the polarization of the microtubule cytoskeleton and granules towards the target, followed by exocytosis that occurs specifically at the site of contact. As with cytokine production by helper T cells (Th cells), target cell killing is absolutely dependent on Ca2+ influx, which is involved in regulating both reorientation and release. Current evidence suggests that Ca2+ influx in CTLs, as in Th cells, occurs via depletion-activated channels. The molecules that couple increases in Ca2+ to reorientation are unknown. The Ca2+/calmodulin-dependent phosphatase calcineurin, which plays a critical role in cytokine production by Th cells, is also involved in lytic granule exocytosis, although the relevant substrates remain to be identified and calcineurin activation is only one Ca2+-dependent step involved. There are thus striking similarities and important differences between Ca2+ signals in Th cells and CTLs, illustrating how cells can use similar signal transduction pathways to generate different functional outcomes.

No specific subcellular localization of protein kinase C is required for cytotoxic T cell granule exocytosis

Cytotoxic T cells kill virus-infected cells and tumor cells by releasing lytic granules that contain cell-killing contents. Exocytosis requires calcium influx and protein kinase C (PKC) activation. Here, we extend our previous finding regarding the lack of isoform specificity of PKCs in the granule release step, showing that mutant constitutively active PKCdelta can substitute for phorbol esters and support exocytosis. PKCdelta, a novel PKC isoform, was recently shown to play a role in lytic granule reorientation. Surprisingly, however, our results suggested that mutant PKCdelta did not localize to the plasma membrane (PM). To test directly whether PKC has to be in the PM to drive exocytosis, we generated mutants of various PKC isoforms that were tethered either to the outer mitochondrial membrane or to the PM. Tethered mutant PKCdeltas were able to promote exocytosis as effectively as the untethered version. The substrates of PKCs involved in lytic granule exocytosis are currently unknown, but subcellular localization is believed to be a critical factor in determining PKC accessibility to substrates. That there is no requirement for specific PKC localization in lytic granule exocytosis may have important implications for the identity of PKC substrates.

The IKK-neutralizing compound Bay11 kills supereffector CD8 T cells by altering caspase-dependent activation-induced cell death

Antigen with dual costimulation through CD137 and CD134 induces powerful CD8 T cell responses. These effector T cells are endowed with an intrinsic survival program resulting in their accumulation in vivo, but the signaling components required for survival are unknown. We tested a cadre of pathway inhibitors and found one preclinical compound, Bay11-7082 (Bay11), which prevented survival. Even the gammac cytokine family members IL-2, -4, -7, and -15 could not block death, nor could pretreatment with IL-7. We found that dual costimulation caused loading of phosphorylated IkappaBalpha (p-IkappaBalpha) and high basal levels of NF-kappaB activity in the effector CD8 T cells. Bay11 trumped both events by reducing the presence of p-IkappaBalpha and ensuing NF-kappaB activity. Not all pathways were impacted to this degree, however, as mitogen-mediated ERK phosphorylation was evident during NF-kappaB inhibition. Nonetheless, Bay11 blocked TCR-stimulated cytokine synthesis by rapidly accentuating activation-induced cell death through elicitation of a caspase-independent pathway. Thus, in effector CD8 T cells, Bay11 forces a dominant caspase-independent death signal that cannot be overcome by an intrinsic survival program nor by survival-inducing cytokines. Therefore, Bay11 may be a useful tool to deliberately kill death-resistant effector T cells for therapeutic benefit.

ERK activation is only one role of PKC in TCR-independent cytotoxic T cell granule exocytosis

Cytotoxic T cells (CTLs) kill target cells by releasing lytic agents via regulated exocytosis. Three signals are known to be required for exocytosis: an increase in intracellular Ca(2+), activation of protein kinase C (PKC) and activation of extracellular signal regulated signal kinase (ERK). ERK activation required for exocytosis depends on activity of PKC. The simplest possibility is that the sole effect of PKC required for exocytosis is ERK activation. Testing this requires dissociating ERK and PKC activation. We did this using TCR-independent stimulation of TALL-104 human leukemic CTLs. When cells are stimulated with thapsigargin and PMA, agents that increase intracellular Ca(2+) and activate PKC, respectively, PKC-dependent ERK activation is required for lytic granule exocytosis. Expressing a constitutively active mutant MAP kinase kinase activates ERK independent of PKC. However, activating ERK without PKC does not support lytic granule exocytosis, indicating that there are multiple effects of PKC required for granule exocytosis.

Protein kinase Cdelta regulates antigen receptor-induced lytic granule polarization in mouse CD8+ CTL

Lytic granule exocytosis is the major pathway used by CD8+ CTL to kill virally infected and tumor cells. Despite the obvious importance of this pathway in adaptive T cell immunity, the molecular identity of enzymes involved in the regulation of this process is poorly characterized. One signal known to be critical for the regulation of granule exocytosis-mediated cytotoxicity in CD8+ T cells is Ag receptor-induced activation of protein kinase C (PKC). However, it is not known which step of the process is regulated by PKC. In addition, it has not been determined to date which of the PKC family members is required for the regulation of lytic granule exocytosis. By combination of pharmacological inhibitors and use of mice with targeted gene deletions, we show that PKCdelta is required for granule exocytosis-mediated lytic function in mouse CD8+ T cells. Our studies demonstrate that PKCdelta is required for lytic granule exocytosis, but is dispensable for activation, cytokine production, and expression of cytolytic molecules in response to TCR stimulation. Importantly, defective lytic function in PKCdelta-deficient cytotoxic lymphocytes is reversed by ectopic expression of PKCdelta. Finally, we show that PKCdelta is not involved in target cell-induced reorientation of the microtubule-organizing center, but is required for the subsequent exocytosis step, i.e., lytic granule polarization. Thus, our studies identify PKCdelta as a novel and selective regulator of Ag receptor-induced lytic granule polarization in mouse CD8+ T cells.

Calcineurin activation is only one calcium-dependent step in cytotoxic T lymphocyte granule exocytosis

We have tested the idea that calcineurin, a calcium-dependent phosphatase that is critical for activating cytokine gene expression in helper T cells, plays a role in lytic granule exocytosis in cytotoxic T lymphocytes (CTLs). We used TALL-104 human leukemic CTLs as a model. Our results confirm an earlier report (Dutz, J. P., Fruman, D. A., Burakoff, S. J., and Bierer, B. E. (1993) J. Immunol. 150, 2591-2598) that immunosuppressive drugs inhibit exocytosis in CTLs stimulated either via the T cell receptor (TCR) or via TCR-independent soluble agents. Of the two recently reported alternate targets of immunosuppressive drugs (Matsuda, S., Shibasaki, F., Takehana, K., Mori, H., Nishida, E., and Koyasu, S. (2000) EMBO Rep. 1, 428-434 and Matsuda, S., and Koyasu, S. (2000) Immunopharmacology 47, 119-125), JNK is not required for lytic granule exocytosis, but we were not able to exclude a role for P38. Exocytosis could be inhibited by expressing GFP fused to a C-terminal fragment of CAIN (cabin 1), but not by expressing VIVIT-GFP. Finally, expressing either full-length or truncated constitutively active mutant calcineurin A enhanced lytic granule exocytosis. However, the mutant calcineurin was unable to support exocytosis when cells were stimulated in the absence of Ca2+ influx. Taken together, our results support the idea that activation of calcineurin is required for lytic granule exocytosis but suggest that it is not the sole Ca2+-dependent step.

Ca2+ influx through CRAC channels activates cytosolic phospholipase A2, leukotriene C4 secretion, and expression of c-fos through ERK-dependent and -independent pathways in mast cells

Cytosolic phospholipase A2 (cPLA2) is a Ca2+-dependent enzyme that mediates agonist-dependent arachidonic acid release in most cell types. Arachidonic acid can then be metabolized by the 5-lipoxygenase enzyme to generate the proinflammatory signal leukotriene C4 (LTC4). Here we report that Ca2+ entry through store-operated CRAC (Ca2+ release-activated Ca2+) channels activates the extracellular signal-regulated kinases (ERKs), members of the mitogen-activated protein kinase family, within minutes and this is necessary for stimulation of cPLA2. Ca2+ entry activates ERK indirectly, via recruitment of Ca2+-dependent protein kinase C alpha and betaI. Ca2+ influx also promotes translocation of cytosolic 5-lipoxygenase to the nuclear membrane, a key step in the activation of this enzyme. Translocation is dependent on ERK activation. A role for gene activation is shown by the finding that CRAC channel opening results in increased transcription and translation of c-fos. Inhibition of ERK activation failed to prevent c-fos expression. Our results show that CRAC channel activation elicits short-term effects through the co-coordinated regulation of two metabolic pathways (cPLA2 and 5-lipoxygenase), which results in the generation of both intra- and intercellular messengers within minutes, as well as longer term changes involving gene activation. These short-term effects are mediated via ERK, whereas, paradoxically, c-fos expression is not. Ca2+ influx through CRAC channels can therefore activate different signaling pathways at the same time, culminating in a range of temporally diverse responses.

Protein kinase C activity is required for cytotoxic T cell lytic granule exocytosis, but the theta isoform does not play a preferential role

CTLs kill virus-infected, tumor, and transplanted targets via secretion of lytic agents including perforin and granzymes. Knowledge of the signals controlling this important process remains vague. We have tested the idea that protein kinase C (PKC)theta, a member of the novel PKC (nPKC) family, which has been shown to play a preferential role in critical Th cell functions, plays a similar, preferential role in CTL lytic granule exocytosis using T acute lymphoblastic leukemia-104 (TALL-104) human leukemic CTLs as a model. We provide evidence consistent with the idea that PKC activity is important for the degranulation step of lytic granule exocytosis, as opposed to upstream events. In contrast with previous work, our results with pharmacological agents suggest that conventional PKCs (cPKCs) and nPKCs may participate. Our results suggest that stimulation with soluble agents that bypass the TCR and trigger granule exocytosis activates PKCalpha and PKCtheta, which can both accumulate at the site of contact with a target cell, although PKCtheta did so more often. Finally, using a novel assay that detects granule exocytosis specifically in transfected, viable cells, we find that overexpression of constitutively active mutants of PKCalpha or PKCtheta can synergize with increases in intracellular [Ca(2+)] to promote granule exocytosis. Taken together, our results lend support for the idea that PKCtheta does not play a preferential role in CTL granule exocytosis.

Exocytic responses of single leukaemic human cytotoxic T lymphocytes stimulated by agents that bypass the T cell receptor

Cytotoxic T lymphocytes kill targets via secretion of lytic agents including perforin and granzymes. Recently, new methods have been developed to monitor cytotoxic T lymphocyte degranulation. These include detecting the appearance of lysosome-associated membrane protein on the cell's surface, and monitoring decreases in cellular perforin content. We have combined these methods with microscopy and flow cytometry to provide the first analysis of how single cytotoxic T cells degranulate. We used TALL-104 human leukaemic cytotoxic T cells as a model system, and stimulated them with thapsigargin and PMA, soluble agents that mimic the two major signalling pathways activated by T cell receptor cross-linking. Our results indicate that essentially every TALL-104 cell responds to maximal stimulation by releasing about half of its lytic granule complement. This reflects complete release of the contents of half the cell's granules, rather than partial release of the contents of all of the granules. Sub-maximal stimulation reduces both the fraction of cells that respond and the magnitude of single cell responses. We find that individual cells respond to maximal stimulation with a variable latency, and provide evidence that, once it starts, degranulation is a slow process taking tens of minutes.