Changes in intracellular calcium concentration and pH of target cells during the cytotoxic process: a quantitative study at the single cell level

Perforin oligomers form arcs in cellular membranes: a locus for intracellular delivery of granzymes

Perforin-mediated cytotoxicity is an essential host defense, in which defects contribute to tumor development and pathogenic disorders including autoimmunity and autoinflammation. How perforin (PFN) facilitates intracellular delivery of pro-apoptotic and inflammatory granzymes across the bilayer of targets remains unresolved. Here we show that cellular susceptibility to granzyme B (GzmB) correlates with rapid PFN-induced phosphatidylserine externalization, suggesting that pores are formed at a protein-lipid interface by incomplete membrane oligomers (or arcs). Supporting a role for these oligomers in protease delivery, an anti-PFN antibody (pf-80) suppresses necrosis but increases phosphatidylserine flip-flop and GzmB-induced apoptosis. As shown by atomic force microscopy on planar bilayers and deep-etch electron microscopy on mammalian cells, pf-80 increases the proportion of arcs which correlates with the presence of smaller electrical conductances, while large cylindrical pores decline. PFN appears to form arc structures on target membranes that serve as minimally disrupting conduits for GzmB translocation. The role of these arcs in PFN-mediated pathology warrants evaluation where they may serve as novel therapeutic targets.

Perforin rapidly induces plasma membrane phospholipid flip-flop

The cytotoxic cell granule secretory pathway is essential for host defense. This pathway is fundamentally a form of intracellular protein delivery where granule proteases (granzymes) from cytotoxic lymphocytes are thought to diffuse through barrel stave pores generated in the plasma membrane of the target cell by the pore forming protein perforin (PFN) and mediate apoptotic as well as additional biological effects. While recent electron microscopy and structural analyses indicate that recombinant PFN oligomerizes to form pores containing 20 monomers (20 nm) when applied to liposomal membranes, these pores are not observed by propidium iodide uptake in target cells. Instead, concentrations of human PFN that encourage granzyme-mediated apoptosis are associated with pore structures that unexpectedly favor phosphatidylserine flip-flop measured by Annexin-V and Lactadherin. Efforts that reduce PFN mediated Ca influx in targets did not reduce Annexin-V reactivity. Antigen specific mouse CD8 cells initiate a similar rapid flip-flop in target cells. A lipid that augments plasma membrane curvature as well as cholesterol depletion in target cells enhance flip-flop. Annexin-V staining highly correlated with apoptosis after Granzyme B (GzmB) treatment. We propose the structures that PFN oligomers form in the membrane bilayer may include arcs previously observed by electron microscopy and that these unusual structures represent an incomplete mixture of plasma membrane lipid and PFN oligomers that may act as a flexible gateway for GzmB to translocate across the bilayer to the cytosolic leaflet of target cells.

Relevance of tumor microenvironment for progression, therapy and drug development

Tumor interstitium exhibits a microenvironment that differs from corresponding normal tissues. Tumor phenotype shows, for example, an elevated intracellular pH (pHi), a lowered extracellular pH (pHe), a low oxygen concentration and low glucose levels. These differences are caused by cell biological (so called intrinsic) factors, e.g. a higher acidification rate, as well as by more systemic (extrinsic) factors, e.g. poor tumor vascularization. They represent important factors for invasiveness, immune suppression and proliferation, and they imply possibilities for diagnosis, prognosis and therapy. We have developed an experimental data-based computer model, which has simulated the potential role of metabolic effects on tumor progression. We show an experiment on cellular metabolism demonstrating the immunosuppressive impact of low pHe on peripheral blood mononuclear cells. Finally, we review important findings on the tumor microenvironment leading to possibilities for therapy which are currently evolving and which promise higher effectiveness for cancer therapy.

Acidic pH inhibits non-MHC-restricted killer cell functions

Immunotherapeutic strategies in advanced stages of solid tumors have generally met with little success. Various mechanisms have been discussed permitting the escape of tumor cells from an effective antitumoral immune response. Solid tumors are known to develop regions with acidic interstitial pH. In a recent study performed in the human system, we were able to demonstrate that non-MHC-restricted cytotoxicity is inhibited by an acidic microenvironment. To get more insight into the mechanisms leading to this reduced cytotoxic activity, we have now investigated the influence of an acidic extracellular pH (pH(e)) on the killing process in detail. Unstimulated PBMC and LAK cells were used as effector cells. Both populations are able to kill tumor cells in a MHC-independent manner via perforin/granzymes or TNFalpha, whereas only IL-2-activated cells can use the killing pathway via Fas/FasL. We studied the influence of a declining pH(e) on the different killing pathways against TNFalpha-sensitive and -resistant, as well as Fas-positive and -negative, target cells. Experiments in the absence of extracellular Ca(2+) were used to discriminate the Ca(2+)-dependent perforin-mediated killing. Here we show that the release of perforin/granzyme-containing granules, the secretion of TNFalpha, and also the cytotoxic action of Fas/FasL interaction or of membrane-bound TNFalpha were considerably inhibited by declining pH(e). Furthermore, the secretion of the activating cytokine IFNgamma, as well as the release of the down-regulating cytokines IL-10 and TGF-beta(1), was strictly influenced by surrounding pH. As a pH(e) of 5.8 resulted in a nearly complete loss of cytotoxic effector cell functions without affecting their viability, we investigated the influence of pH(e) on basic cellular functions, e.g. , mitochondrial activity and regulation of intracellular pH. We found an increasing inhibition of both functions with declining pH(e). Therefore, an acidic pH(e) obviously impairs fundamental cellular regulation, which finally prevents the killing process. In summary, our data show a strict pH(e) dependence of various killer cell functions. Thus, an acidic microenvironment within solid tumors may contribute to the observed immunosuppression in vivo, compromising antitumoral defense and immunotherapy in general, respectively.

Osteogenesis from cultured chick periostea has a specific requirement for chloride

Bone development, like embryonic development in general, depends on a particular internal electrical milieu. Ions are the carriers of currents that maintain this internal environment. In embryonic bone, chloride is a major carrier of such current. To explore the role chloride plays in embryonic bone development we performed several ion-removal experiments, using the chick periosteal osteogenesis (CPO) system as our model. We found that if chloride is reduced in the medium and replaced with a nontoxic anion, alkaline phosphatase (ALP) activity does not rise, nor does osteogenic development occur. However, acid phosphatase (AP) activity is not affected by level of chloride. Experiments using metabolic inhibitors showed that explants cultured in low chloride medium remain viable. Dose-response studies revealed that the response of ALP activity to chloride concentration is sigmoidal, with a [Cl-]0.5 of 45.9 mM. Reciprocal transfers of explants between complete and low chloride medium show that the rise in ALP activity depends on the length of time explants are cultured with chloride. In contrast, such transfer experiments show that osteogenesis requires chloride only during days 2-3 of culture.

Oxidized low-density lipoproteins affect natural killer cell activity by impairing cytoskeleton function and altering the cytokine network

Several lines of evidence indicate that oxidative imbalance can play an important role in determining an impairment of natural killer (NK) cell activity in a variety of human diseases. Because a specific role for oxidized low-density lipoproteins (LDL) as pro-oxidizing agents has been envisaged, we tested the activity of oxidized LDL (ox-LDL) on NK cell-mediated cytotoxicity, cytokine release, and membrane molecule modulation. Native LDL served as control. Treatment with ox-LDL at noncytotoxic concentrations (0.2 mg/ml) during the NK/target cell (TC) interaction markedly reduced NK cytotoxic activity against U937 tumor cells. This inhibitory activity was also noticed when NK cells were pretreated with ox-LDL. Scanning electron microscopy examination of NK-target cell conjugates failed to reveal any morphological cell damage. In addition, the number of conjugates and the expression of some adhesion molecules (CD11a, CD11b, CD18, CD2, and CD62L) were not modified by ox-LDL. These observations argued against a possible interference of ox-LDL with the binding process leading to the formation of NK/TC conjugates. By contrast, immunocytochemical analyses of cytoskeleton components of NK cells exposed to ox-LDL showed a partial depolymerization and a derangement of the microtubular apparatus. These alterations were accompanied by an evident decrease in their intracellular reduced glutathione content. Owing to the important role played by the microtubular network during the killing process, it is possible to infer that a cytoskeleton alteration underlies the inhibitory activity of ox-LDL on NK cell function. In addition, exposure of mitogen-stimulated peripheral blood mononuclear cells to ox-LDL markedly reduced specific mRNA transcription and release of cytokines relevant for NK cell activity (such as tumor necrosis factor-alpha, interferon gamma, and interleukin 12). These data suggest that the impairment of NK cell activity by ox-LDL likely reflects the concomitant dysregulation of some essential mechanisms of NK cell function.