Perforin binding to cells and lipid membranes determined by a simple competition assay

Perforin: an important player in immune response

Perforin is a glycoprotein responsible for pore formation in cell membranes of target cells. Perforin is able to polymerize and form a channel in target cell membrane. Many research groups focus on the role of perforin in various diseases, immune response to bacterial and viral infections, immune surveillance and immunopathology. In addition, perforin is involved in the pathogenesis of autoimmune diseases and allogeneic transplant rejection. Natural killer (NK) cells and CD8-positive T-cells are the main source of perforin. However, CD4-positive T-cells are also able to express a low amount of perforin, when classic cytotoxicity is ineffective or disturbed. Polymerized perforin molecules form channels enabling free, non-selective, passive transport of ions, water, small-molecule substances and enzymes. In consequence, the channels disrupt protective barrier of cell membrane and destroy integrity of the target cell. This review will focus on mechanisms of action and structure of perforin. Also, in this review we discuss the problem of abnormal perforin production in diseases such as: hemophagocytic lymphohistiocytosis (HLH), leukemias and lymphomas, infectious diseases and autoimmune diseases. Better understanding of the role of these molecules in health and disease will open a new field of research with possible therapeutic implications.

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.

Penetration of the salivary glands of Rhodnius domesticus Neiva & Pinto, 1923 (Hemiptera: Reduviidae) by Trypanosoma rangeli Tejera, 1920 (Protozoa: Kinetoplastida)

Penetration of the heteroxenous protozoan Trypanosoma rangeli into the salivary glands of its invertebrate host Rhodnius domesticus has been investigated here using different approaches. Electron microscopy showed that epimastigotes coming from the insect hemocoel cross the basal lamina that surrounds the salivary glands and penetrate through the gland cells cytoplasm. After reaching the gland lumen, epimastigote forms remain adhered to the gland cell microvilli by their flagella, while metacyclic trypomastigotes are found swimming free in the saliva. Analysis by flow cytometry, western blotting and hemolytic activity allowed to demonstrate the presence in T. rangeli of a hemolytic molecule with antigenic cross-reactivity with murine perforin, which could be used by the parasites to reach the salivary gland lumen. This molecule, which we named as rangelysin, has 120 kDa molecular weight, is able to induce hemolysis only in acidic pH, and is produced by both trypomastigote and epimastigote forms.

Detection of functional cell surface perforin by flow cytometry

How perforin (PFN) delivers the granzymes during cytotoxic granule mediated apoptosis remains a mystery. A major obstacle has been the inability to visualize PFN in either monomeric or polymeric form after interaction with the target cell surface. An antibody based technique is described which detects cell surface PFN on intact cells by flow cytometry. The methodology requires the presence of calcium (Ca2+) at a concentration which supports binding but not polymerization of PFN. Functionality was ensured by showing the cell surface PFN was able to deliver GrB causing caspase-3 activation and mitochondrial depolarization. The technique demonstrates a role for heparan sulfate proteoglycans in PFN binding. Further, the variable sensitivity of effector versus target cell lines to the permeabilizing effects of PFN could not be attributed to differential binding of PFN.

Activation of natural killer cells with interleukin 2 (IL-2) and IL-12 increases perforin binding and subsequent lysis of tumour cells

Natural killer (NK) cells can lyse a variety of different tumour cells by exocytosis of perforin, subsequent binding of perforin to the target cell membrane and formation of lytic pores. Some tumour cells, however, are resistant to cellular cytotoxicity. Using the NK-resistant tumour cell lines ML-2, MONOMAC-1, RPMI and L540Cy, we demonstrated that activation of NK cells with interleukin 2 (IL-2) and IL-12 resulted in significant lysis of these tumour targets. To investigate the underlying mechanisms, we isolated the cytotoxic granules from non-activated and IL-2-/IL-12-activated NK cells and compared the killing of K562 leukaemia cells (sensitive to NK cell-mediated lysis) and ML-2 leukaemia cells (resistant to NK cell-mediated lysis). In contrast to K562 cells, which were easily killed by NK-cell granules, ML-2 cells were resistant to granules from non-activated NK cells. However, granules from NK cells activated with IL-2 and IL-12 were able to induce significant tumour cell lysis. Cell death of both K562 and ML-2 cells by granules from activated NK cells was completely blocked by anti-perforin antibodies, indicating that perforin mainly accounts for the lysis induced by NK granules. Comparing granules from non-activated and IL-2-/IL-12-activated NK cells, the increased cell death of ML-2 cells was caused by an improved binding of perforin to the target cell membrane. Functional assays, however, indicated that the differences in perforin binding were not as a result of an augmented production of perforin by activated NK cells. We conclude that activation of NK cells results in an increased binding of perforin and subsequent lysis of tumour cells.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

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.

Trypanosoma cruzi: resistance to the pore forming protein of cytotoxic lymphocytes--perforin

The pore-forming protein perforin is one of the main effector molecules which cytotoxic lymphocytes utilize to kill their targets both in vivo and in vitro. Natural killer cells and cytotoxic T lymphocytes play an important role in host defense against a number of intracellular microorganisms such as virus and protozoan, but the exact way they help control infection is unknown. On the other hand, many microorganisms have evolved successful escape strategies to avoid immune-cell-mediated attack. It is thus necessary to investigate the direct interaction of infectious microorganisms with the lytic machinery of cytotoxic lymphocytes and other cells. In the present work we report the effect of perforin on both a protozoan, Trypanosoma cruzi, and the infected host cell. Epimastigote, amastigote, and trypomastigote forms of T. cruzi, as well as infected macrophages, were assayed for their susceptibility to perforin based on three different criteria. T. cruzi in all three differentiation stages were resistant to purified perforin at doses up to 100-fold larger than that sufficient to kill susceptible tumor cells. No morphological change was observed under electron microscopy. Survival rates and infectivities of the treated parasites in vitro were similar to those of control parasites. Moreover, the measurement of calcium influx using Fura-2 to assess membrane damage revealed that T. cruzi resist perforin attack by avoiding transmembrane pore formation. Resistance to perforin was not transferred to host cells since infected macrophages could be easily destroyed by perforin while intracellular amastigotes remained intact.

Perforin and lymphocyte-mediated cytolysis

We have discussed in the previous sections the recent progress made toward elucidating the regulatory mechanism of perforin gene transcription and the domain structure of the perforin molecule. It appears that the expression of perforin is, at least partially, controlled at the transcription level through the interaction between killer cell-specific cis- and trans- acting factors. One of such cognate pairs, NF-P motif (an EBS-homologous motif) and NF-P2 (a killer cell-specific DNA-binding protein), has been described. The regulatory mechanism of gene transcription, however, is likely to involve multiple factors which act in a coordinated fashion to bring about the most efficient expression of perforin limited strictly to activated killer lymphocytes. Through studies using synthetic peptides and recombinant perforins, it has been suggested that the N-terminal region of the perforin molecule is an important, though not the only, domain responsible for the lytic activity. Further studies are warranted to elucidate the role(s) of other potential amphiphilic structures located in the central portion of the perforin molecule in the overall pore-forming activity. The molecular basis underlying the resistance of killer lymphocytes to perforin-mediated lysis still remains an open question. Preliminary results, however, suggest that the surface protein(s) restricted to killer cells may account for their self-protection against perforin. Based on recent studies using perforin-deficient mice, the involvement of perforin in lymphocyte-mediated cytolysis both in vivo and in vitro has been confirmed. Two functional roles, a direct (lytic) and an indirect (endocytosis enhancer; conduit), both of which may contribute critically to the cell-killing event can be attributed to perforin. The fact that lymphocytes may also employ perforin-independent killing mechanism(s), e.g. Fas-dependent pathway, is beyond the scope of this review. There is, nevertheless, no doubt that these alternative cytolytic mechanisms may also play important roles in immune effector and/or regulatory responses associated with killer lymphocytes. Obviously, we are still a long way from concluding on the functional relevance of each individual cytolytic mechanism seen in different physiopathological situations. Suffice it to say, however, that a wealth of information on lymphocyte-mediated killing has already emerged through the multidisciplinary efforts conducted in our and other laboratories that promise to further dissect this complicated event in the years to come.

Killing of cells by perforin. Resistance to killing is not due to diminished binding of perforin to the cell membrane

Different cell types vary widely in their susceptibility to killing by the pore-forming cytolytic molecule perforin. In particular, the cells responsible for synthesis of perforin, i.e. cytotoxic T lymphocytes (CTL) and natural killer (NK) cells, are very resistant to cytolysis by this molecule. It has previously been suggested that resistance is due, at least in part, to diminished binding of perforin to these cells. The purpose of the present study was to compare binding of perforin to sensitive and resistant cell types. To this end, perforin was biosynthetically labelled prior to purification. The purified labelled protein was then utilized to obtain a direct measure of the amount of perforin bound to cells during attack. Resistant cells (CTL, neutrophils) bound at least as much perforin as did sensitive cells (K562, HL60 etc.), indicating that resistance to perforin involves mechanisms operating after binding of the lytic molecule.

Plasma membrane-associated proteins with the ability to partially inhibit perforin-mediated lysis

Cytolytic lymphocytes have previously been reported to be resistant to the lytic effects of perforin. In this work, plasma membrane proteins from a CTL cell line were fractionated by HPLC, and the eluted fractions were collected based on their ability to inhibit perforin-mediated hemolysis. Three proteins with inhibitory activity were thus purified, the serine esterase MCSP-3/granzyme F and the histones H2B and H3. A commercial source of H2B was able to potently inhibit perforin-mediated lysis, and it was confirmed by FACS analysis that H2B is in fact present on the surface of cytolytic cells. However, H2B was also found on the surface of perforin-susceptible tumor cell lines, indicating that the histones may partially inhibit perforin-mediated lysis in vitro, but that they do not represent the factor conferring specific resistance on cytolytic lymphocytes. The origin of the surface histones and the possible role of the surface MCSP-3/granzyme F are discussed.