Negative cooperativity exhibited by the lytic amino-terminal domain of human perforin: implications for perforin-mediated cell lysis

Human NK cell lytic granules and regulation of their exocytosis

Natural killer (NK) cells form a subset of lymphocytes that play a key role in immuno-surveillance and host defense against cancer and viral infections. They recognize stressed cells through a variety of germline-encoded activating cell surface receptors and utilize their cytotoxic ability to eliminate abnormal cells. Killing of target cells is a complex, multi-stage process that concludes in the directed secretion of lytic granules, containing perforin and granzymes, at the immunological synapse. Upon delivery to a target cell, perforin mediates generation of pores in membranes of target cells, allowing granzymes to access target cell cytoplasm and induce apoptosis. Therefore, lytic granules of NK cells are indispensable for normal NK cell cytolytic function. Indeed, defects in lytic granule secretion lead or are related to serious and often fatal diseases, such as familial hemophagocytic lymphohistiocytosis (FHL) type 2–5 or Griscelli syndrome type 2. A number of reports highlight the role of several proteins involved in lytic granule release and NK cell-mediated killing of tumor cells. This review focuses on lytic granules of human NK cells and the advancements in understanding the mechanisms controlling their exocytosis.

Expression and bioactivity of recombinant segments of human perforin

The aim of the study was to prepare an active recombinant human perforin by comparing 5 candidate segments of human perforin. Full-length perforin, MAC1 (28-349 aa), MAC2 (166-369 aa), C-100, and N-60 of human perforin were selected as candidate active segments and designated, respectively, HP1, HP2, HP3, HP4, and HP5. The target genes were amplified by PCR and the products were individually subcloned into pGEM-T. The genes for HP1, HP2, HP3, and HP5 were subcloned into pET-DsbA, whereas pET-41a (+) was used as the expression vector of HP4. The fusion proteins were expressed in Escherichia coli BL21pLysS(DE3) and purified using nickel nitrilotriacetic acid (NTA) agarose affinity chromatography. The hemolysis microassay was used as an activity assay of fusion protein. From this study, we obtained the recombinant plasmids pGEM-T-HP1, -HP2, -HP3, -HP4 and -HP5, consisting of 1600, 960, 600, 300bp, and 180, respectively. From these recombinant plasmids, expression plasmids were successfully constructed and expressed in E. coli BL21pLysS(DE3). The resultant fusion proteins, affinity purified using Ni-NTA, were approximately 80, 58, 45, 44, and 30 kDa, respectively. The recombinant proteins were assayed for activity on hemolysis. HP2 and HP5 were the only recombinant proteins that were active in hemolysis, and the hemolytic function was concentration dependent. These results demonstrate that active recombinant forms of perforin can be synthesized in a prokaryote model. The recombinant N-60 and MAC1 (28-349 aa) of human perforin have the function of forming pores. Our study provides the experimental basis for further investigation on the application of perforin.

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.

Infective, neoplastic, and homeostatic sequelae of the loss of perforin function in humans

Perforin, a pore-forming protein toxin synthesized and stored in the cytoplasmic vesicles of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells, is secreted when these effector lymphocytes encounter virus-infected or neoplastic cells. Perforin is encoded by a single-copy gene and is critical for immune homeostasis and defense of the organism against intracellular sepsis. A complete deficiency of perforin expression in either mice or humans is associated with a syndrome of immune insufficiency and severely deregulated lymphoid homeostasis. Humans who inherit inactivating mutations of perforin or defects in various parts of the cellular machinery that delivers perforin to the target cell suffer from familial hemophagocytic lymphohistiocytosis (FHL), a fatal condition necessitating bone marrow transplantation, usually in infancy. In mice, a high incidence of spontaneous B cell lymphoma has also been noted as the animals age. Across human populations, a number of polymorphisms that result in measurable, but suboptimal CTL activity have been noted, and some of these predispose to attenuated FHL or susceptibility to infectious disease, but in many cases, to no discernible disease predisposition. This chapter discusses the significance of human perforin polymorphisms, particularly those associated with diseases other than FHL, and recent advances in our understanding of perforin biology and function.

Cytotoxic T Lymphocytes from Cathepsin B-deficient Mice Survive Normally in Vitro and in Vivo after Encountering and Killing Target Cells

The lysosomal protease cathepsin B has been proposed to protect cytotoxic T lymphocytes from the membrane-disruptive effects of perforin secreted during the execution phase of target cell death. Accordingly, cathepsin B that translocates to the lymphocyte surface upon degranulation has been postulated to cleave and inactivate perforin molecules that diffuse back to the killer cell. We have found that recombinant perforin is cleaved inefficiently by cathepsin B and shows no significant reduction in its lytic activity following co-incubation. Furthermore, purified CD8+ cytotoxic T lymphocytes of cathepsin B-null gene-targeted mice were able to induce normal death of target cells both in vitro and in vivo and to survive the encounter with target cells as efficiently as cathepsin B-expressing killer cells. We conclude that cathepsin B is not essential for protection of cytotoxic lymphocytes from the toxic effects of their secreted perforin.

Evidence for perforin-like activity associated with earthworm leukocytes

Earthworm (Eisenia fetida) coelomic fluid contains several leukocytes (coelomocytes): basophils, acidophils and neutrophils as well as chloragocytes. Small coelomocytes and coelomocyte lysate are cytotoxic for the tumor cell target K562. The expression of a lytic factor was investigated by immunocytochemistry using light and transmission electron microscopy. A rat-anti-mouse-perforin-mAb labeled mainly small coelomocytes (nearly 20%) as visualized by light microscopy. TEM analysis using immunogold showed a homogenous labeling in the cytoplasm of small coelomocytes. The highest number of immunogold particles was estimated in coelomocytes with many small cytoplasmic granules. Coelomocytes with large lysosomal granules were also labeled but less intensely. No antibody binding was observed for chloragocytes either in light or electron microscopy. This suggests that the perforin-like activity is associated with only one cell type and that chloragocytes are responsible for other lytic activities. MALDI-MS revealed calreticulin usually associated with perforin in mammalian cells that mediate lysis (e.g. NK, CTL). Together, results strongly suggest the presence of putative perforin in earthworms. This in turn supports the hypothesis that perforin is a conserved component important in immune defense during evolution.

Solution structure of a synthetic lytic peptide: the perforin amino terminus

BACKGROUND

Killer lymphocytes secrete perforin, a 67 kDa protein that initiates T-cell cytolysis following aggregation and pore formation in target membranes. The resulting pores cause a breakdown of the transmembrane osmotic gradient and allow other cytolytic mediators to enter the target cell and initiate apoptosis. The cytolytic domain resides within the first 34 residues of the amino terminus of perforin, with residues 1-19 being sufficient for cytolytic activity.

RESULTS

The solution structure of a 22-residue synthetic peptide (P22), corresponding to the amino terminus of human perforin, has been determined using high resolution nuclear magnetic resonance spectroscopy in the presence and absence of perdeuterated detergent (SDS) micelles. In aqueous solution, P22 exists mainly in a random conformation. However, it adopts a hook-like structure at the carboxyl terminus in the presence of SDS micelles when the positively charged residues cluster to form a turn that provides a binding surface to the negatively charged sulfate headgroups.

CONCLUSIONS

The strong electrostatic interaction between the cationic region of the P22 peptide and the lipid headgroups probably weakens the membrane, facilitating insertion of the relatively neutral/hydrophobic stretch of P22, and is representative of the initial step of the lytic pathway. The structural model described here is probably relevant to understanding the mechanisms of other cationic antimicrobial peptides.

Dependence of the lytic activity of the N-terminal domain of human perforin on membrane lipid composition--implications for T-cell self-preservation

The kinetics and thermodynamics of pore formation by the 22-residue N-terminal domain of human perforin-(1-22)-peptide have been studied for a variety of model phospholipid membranes. Peptide binding and aggregation, and cell lysis were monitored through the change in the fluorescence of Trp, or vesicle-encapsulated carboxyfluorescein, respectively. Peptide binding was analyzed in terms of a model that incorporates non-ideal interactions and aggregation in a membrane bilayer. The minimum number of peptide monomers required to form an active pore averaged from four to six, according to the lipid composition of the vesicle. This combined kinetic and thermodynamic approach has provided quantitative information that allows a direct comparison of the binding behavior of the perforin-(1-22)-peptide in different lipid vesicles and affords molecular insight on the factors controlling pore formation. Pore formation is most favorable in thinner membranes with low melting temperatures. No significant difference in activity is observed for different zwitterionic headgroups. Rather, the gel state of the lipid chain, which diminishes the incorporation and aggregation of the perforin-(1-22)-peptide shows the strongest influence. This effect is observed in both the thermodynamic (incorporation isotherm) and kinetic (carboxyfluorescein release) studies. Insertion and aggregation are more facile in membranes with less densely packed lipids. The dependence of pore-forming activity on lipid composition provides important clues to understanding the self-protection mechanism employed by cytotoxic T lymphocytes (CTL) against perforin-mediated lysis.