The lymphocyte pore-forming protein perforin is associated with granules by a pH-dependent mechanism

Perforin: more than just a pore-forming protein

Cellular apoptosis induced by T cells is mainly mediated by two pathways. One, granule exocytosis utilizes perforin/granzymes. The other involves signaling through death receptors of the TNF-alpha R super-family, especially FasL. Perforin plays a central role in apoptosis induced by granzymes. However, the mechanisms of perforin-mediated cytotoxicity are still not elucidated completely. Perforin is not only a pore-forming protein, but also performs multiple biological functions or perforin performs one biological function (cytolysis), but has multiple biological implications in the cellular immune responses, including regulation of proliferation of CD8+ CTLs.

Flow cytometric detection of perforin upregulation in human CD8 T cells

Perforin and granzymes work synergistically to induce apoptosis in target cells recognized by cytotoxic T lymphocytes. While perforin is readily detectable by flow cytometry in resting CD8 T cells, upregulation of perforin in activated cells is thought to require proliferation. However, perforin undergoes numerous conformational changes during its maturation, which may affect the ability of conventional antibodies to recognize newly synthesized perforin. Polychromatic flow cytometry was used to detect perforin and cytokine production following stimulation of ex vivo human CD8 T cells. Two different anti-perforin antibodies, clones B-D48 and deltaG9, were used to discriminate various forms of perforin after cellular activation. We provide evidence for the rapid upregulation of perforin protein, which may contribute to the ability of CD8 T cells to kill multiple targets over time. The deltaG9 clone recognizes the granule-associated conformation of perforin, while the B-D48 clone is able to detect perforin in multiple forms. Finally, we show there is variability in the ability of CD8 T cells to upregulate perforin. Human CD8 T cells are capable of new perforin production immediately following activation. This work defines a novel flow cytometric procedure that can be used to more completely assess the cytotoxic capacity of human CD8 T cells.

Perforin-mediated target-cell death and immune homeostasis

The granule exocytosis pathway of cytotoxic lymphocytes is crucial for immune surveillance and homeostasis. The trafficking of granule components, including the membrane-disruptive protein perforin, to the immunological synapse leads to the delivery of granule proteases (granzymes) into the target cell and its destruction through apoptosis. Several independent molecular abnormalities associated with defects of either granule trafficking or perforin function can cause cytotoxic lymphocyte dysfunction. In humans, inherited perforin mutations result in severe immune dysregulation that manifests as familial haemophagocytic lymphohistiocytosis. This Review describes recent progress in defining the structure, function, biochemistry and cell biology of perforin.

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.

L is for lytic granules: lysosomes that kill

CTL are important cells in the immune system which are able to recognise and directly destroy virally infected, tumorigenic or foreign cells. The proteins which mediate this destruction are packaged into specialised secretory granules, termed lytic granules, which are secreted in response to target cell recognition. Curiously these specialised secretory granules also contain all the lysosomal hydrolases, and in CTL the lytic granules serve two separate functions: as a lysosome within the cell, and as a secretory granule when a target cell is recognised. These "secretory lysosomes", which serve important roles in both protein degradation within the cells as well as regulated secretion of proteins from the cells, are also found in other cell types, all of which are derived from the hemopoietic lineage. This observation raises the possibility that cells of the hemopoietic lineage possess specialised sorting and secretory mechanisms which allow the lysosomes to be used as secretory organelles. Studies on Chediak Higashi syndrome support this idea, since in this naturally occurring genetic mutation, cells with secretory lysosomes are unable to secrete their granules while other conventional secretory cells are able to do so. Further studies on the mechanisms which regulate secretion of lytic proteins from CTL should identify the proteins involved in this unusual secretory pathway. Some aspects of the differences between conventional and "secretory" lysosomes remain unresolved. How the biogenesis of the secretory lysosome differs from that of a conventional secretory granule is unclear. While conventional secretory cells sort proteins destined for the granule by a selective condensation in the TGN, the secretory lysosomes seem to use a combination of lysosomal and other sorting signals. Our preliminary studies suggest that haemopoietic cells possess specialised sorting mechanisms which allow the correct sorting of the secreted products to the lysosome, and that these signals are different from those found in conventional secretory (e.g. neurosecretory) cells. This finding and the observation that fibroblast lysosomes can undergo calcium-mediated exocytosis suggests that the unusual secretory system found in haemopoietic cells may be a result of specialised sorting mechanisms in these cells. In this case the Chediak lesion may turn out to be a sorting defect.

Potential of microsensor-based feedback bioactuators for biophysical cancer treatment

Solid tumors usually exhibit a poorly organized vascularization and a deviant energy metabolism which result in an acidic pH and large hypoxic areas in the tumor microenvironment. A lot of cell biological data support the hypothesis that such physico-chemical conditions are promoters of the microevolution of malignant cells, inhibitors of the immune response, and co-factors for tumor cell invasion. Our experimental in vitro analyses and computer simulations indicate that the efficiency of immunotherapies and classical methods for cancer treatment might be improved if a physico-chemical microenvironment could be restored which reflects that found in normal tissue. In order to monitor and manipulate the tumor microenvironment, we suggest utilizing silicon-based feedback bioactuators which are controlled by on-line microsensors. These miniaturized bioactuators play the role of 'pH clamps' and can be implanted directly at the sites of inoperable tumors and metastases where they can reconstitute normal physico-chemical conditions. The drug application scheme can be precisely controlled by an integrated microprocessor. The paper summarizes the current state of development of such microsensor-based feedback bioactuators and outlines their potential for biophysical cancer treatment.

Influence of whole-body hyperthermia on natural cytotoxicity of liver blood-borne sinusoidal cells

Rat liver sinusoidal cytotoxic cells were examined after exposure to in vivo and in vitro hyperthermia at 40-41 degrees C. Whole-body hyperthermia lasting for 4 h caused a decrease in the cytotoxic activity of liver sinusoidal washout cells against YAC-1 and K562 cells. Surprisingly, the percentage of washout cells with morphology of LGL (large granular lymphocyte) increased both in the liver washout and in portal blood compared to control normothermic animals. The proportions of phenotypically characterized cell subpopulations isolated from liver sinusoids did not change. Elimination of i.v. injected 125I-labelled K562 cells was decreased during hyperthermia. In vitro incubation of liver sinusoidal cytotoxic cells for 3 h at 41 degrees C decreased their cytotoxic activity by affecting the process of effector-target cell binding. However, once the effector-target cell conjugates were formed, the cytotoxic process proceeded as in normothermic conditions. These data suggest that inhibition of liver sinusoidal cytotoxic cell activity after hyperthermia may be a result of deficient target cell recognition by the effector cell.