The pore-forming protein (perforin) of cytolytic T lymphocytes is immunologically related to the components of membrane attack complex of complement through cysteine-rich domains

Critical Review of Existing MHC I Immunopeptidome Isolation Methods

Major histocompatibility complex class I (MHC I) plays a crucial role in the development of adaptive immune response in vertebrates. MHC molecules are cell surface protein complexes loaded with short peptides and recognized by the T-cell receptors (TCR). Peptides associated with MHC are named immunopeptidome. The MHC I immunopeptidome is produced by the proteasome degradation of intracellular proteins. The knowledge of the immunopeptidome repertoire facilitates the creation of personalized antitumor or antiviral vaccines. A huge number of publications on the immunopeptidome diversity of different human and mouse biological samples-plasma, peripheral blood mononuclear cells (PBMCs), and solid tissues, including tumors-appeared in the scientific journals in the last decade. Significant immunopeptidome identification efficiency was achieved by advances in technology: the immunoprecipitation of MHC and mass spectrometry-based approaches. Researchers optimized common strategies to isolate MHC-associated peptides for individual tasks. They published many protocols with differences in the amount and type of biological sample, amount of antibodies, type and amount of insoluble support, methods of post-fractionation and purification, and approaches to LC-MS/MS identification of immunopeptidome. These parameters have a large impact on the final repertoire of isolated immunopeptidome. In this review, we summarize and compare immunopeptidome isolation techniques with an emphasis on the results obtained.

Nemertean toxin genes revealed through transcriptome sequencing

Nemerteans are one of few animal groups that have evolved the ability to utilize toxins for both defense and subduing prey, but little is known about specific nemertean toxins. In particular, no study has identified specific toxin genes even though peptide toxins are known from some nemertean species. Information about toxin genes is needed to better understand evolution of toxins across animals and possibly provide novel targets for pharmaceutical and industrial applications. We sequenced and annotated transcriptomes of two free-living and one commensal nemertean and annotated an additional six publicly available nemertean transcriptomes to identify putative toxin genes. Approximately 63–74% of predicted open reading frames in each transcriptome were annotated with gene names, and all species had similar percentages of transcripts annotated with each higher-level GO term. Every nemertean analyzed possessed genes with high sequence similarities to known animal toxins including those from stonefish, cephalopods, and sea anemones. One toxin-like gene found in all nemerteans analyzed had high sequence similarity to Plancitoxin-1, a DNase II hepatotoxin that may function well at low pH, which suggests that the acidic body walls of some nemerteans could work to enhance the efficacy of protein toxins. The highest number of toxin-like genes found in any one species was seven and the lowest was three. The diversity of toxin-like nemertean genes found here is greater than previously documented, and these animals are likely an ideal system for exploring toxin evolution and industrial applications of toxins.

An unexpected loss of domains in the conservative evolution ninth complement component in a higher teleost, Miichthys miiuy

The complement systems of fish are well developed and play an important role in the innate immune response. C9 is the ninth member of complement components, involved in creating the membrane attack complex (MAC). In the present study, a truncated C9 cDNA sequence encoding 461 amino acids was cloned and characterized in the miiuy croaker (Miichthys miiuy). Typical fish C9 molecules have five characteristic modular domains, i.e. TSP1, LDLRA, MACPF, EGF, and a second TSP domain which is absent in mammalian counterparts. While in miiuy croaker, this truncated C9 presents only TSP1, LDLRA and MACPF domains. It is the first time of finding a truncated C9 in teleost components. RT-PCR analysis detected these C9 transcripts among all tissues examined, demonstrating its constitutive expression pattern in healthy fish. The highest levels of transcripts were detected in liver of both healthy and pathogen-infected miiuy croaker. Its constitutive and inducible expression pattern of this truncated C9 in liver is similar to most complement components which belong to the type of acute-phase proteins and are in general of hepatic origin. We cannot exclude the possibility that miiuy croaker presents the typical C9 although it has not yet been found. The molecular evolutionary analysis showed that this truncated C9 of miiuy croaker had a significantly higher omega value comparing with other fish and the positive selection pressure had happened on it after its divergence with other fish.

Imaging of lytic granule exocytosis in CD8+ cytotoxic T lymphocytes reveals a modified form of full fusion

Here we imaged the exocytosis of lytic granules from human CD8(+) cytotoxic T lymphocytes using rapid total internal reflection microscopy, Lamp-1 tagged with mGFP to follow the fate of the lytic granule membrane, and granzyme A, granzyme B or serglycin tagged with mRFP to follow the fate of lytic granule cargo. Lytic granules were released by full fusion with the plasma membrane, such that the entire granule content for all three cargos visualized was released on a subsecond time scale. The behavior of GFP-Lamp-1 was, however, more complex. While it entered the plasma membrane in all cases, the extent to which it then diffused away from the site of exocytosis varied from nearly complete to highly restricted. Finally, the diffusion properties upon release of the three cargos examined put an upper limit on the size of the macromolecular complex of granzyme and serglycin that is presented to the target cell.

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.

Molecular characterization of the alpha subunit of complement component C8 (GcC8alpha) in the nurse shark (Ginglymostoma cirratum)

Target cell lysis by complement is achieved by the assembly and insertion of the membrane attack complex (MAC) composed of glycoproteins C5b through C9. The lytic activity of shark complement involves functional analogues of mammalian C8 and C9. Mammalian C8 is composed of alpha, beta, and gamma subunits. The subunit structure of shark C8 is not known. This report describes a 2341 nucleotide sequence that translates into a polypeptide of 589 amino acid residues, orthologue to mammalian C8alpha and has the same modular architecture with conserved cysteines forming the peptide bond backbone. The C8gamma-binding cysteine is conserved in the perforin-like domain. Hydrophobicity profile indicates the presence of hydrophobic residues essential for membrane insertion. It shares 41.1% and 47.4% identity with human and Xenopus C8alpha respectively. Southern blot analysis showed GcC8alpha exists as a single copy gene expressed in most tissues except the spleen with the liver being the main site of synthesis. Phylogenetic analysis places it in a clade with C8alpha orthologs and as a sister taxa to the Xenopus.

Structure and sequence variation of the canine perforin gene

Lymphocyte-mediated cytotoxicity is essential to control viral infections, limit lymphocyte expansion and activation, and survey for malignant cells. Humans with defects in lymphocyte cytotoxicity have reduced perforin function resulting in uncontrolled lymphocyte expansion, leading to excessive histiocyte activation and a hemophagocytic disorder. Dog breeds such as Bernese mountain dogs (BMD) have a high incidence of reactive and malignant diseases affecting histiocytes. This study addressed the hypothesis that changes in the perforin gene contribute to the development of hemophagocytic histiocytic sarcoma (HHS) in BMD. Canine perforin DNA was amplified and sequenced through multiple PCR assays from healthy and diseased dogs, and the gene structure determined by rapid amplification of cDNA ends. The coding component of the gene consists of 1679bp, with two exons of 536bp and 1143bp separated by an intron of 865bp. Gene configuration and location differ from that in other species although the coding sequence is highly conserved. Three silent single nucleotide polymorphisms (SNP) were identified. Analysis of their distribution indicated a consistent genotype among 6 middle-aged to older BMD without histiocytic diseases. Among samples from 10 dogs with HHS and 10 without histiocytic diseases SNP occurred with variable frequency. It was concluded that changes in the amino acid sequence of perforin were not associated with HHS but that a constellation of SNP may characterize BMD without histiocytic disease. Investigation of more dogs is required to confirm a specific genotype. Future studies should focus on the potential contribution of reduced perforin expression and/or function to HHS in dogs.

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.

Mutual interference of HIV and natural killer cell-mediated immune response

Natural killer (NK) cells represent important early effector cells in innate immune defense as they exert their functions without prior sensitization. They participate in regulation of innate and adaptive immune responses and hematopoiesis by producing various cytokines and chemokines. In addition, NK cells lyse virally infected and malignant cells raising them to multifunctional members of the first line of defense. Unlike other lymphocytes they lack specific antigen receptors. They rather bind cells using ubiquitous molecules and communicate via a pattern of receptors specific for MHC-I molecules with their counterparts. In general, successful binding of the receptors delivers an inhibitory signal to NK cells thus sparing the target cell from lysis. In contrast, down-regulated or altered MHC-I expression as frequently observed during virus infection or on malignant cells prevents ligation of inhibitory receptors and MHC-I paralyzing inhibition and thus inducing lysis of the target cell. In human immunodeficiency virus (HIV) infection NK cells are of central importance since they can combat viral infection itself and opportunistic pathogens like fungi and protozoa that usually spread during the course of HIV infection. However, various studies have reported alterations in HIV patients affecting NK cell numbers and functions that might negatively influence course and severity of the disease. This review will focus on the mutual interference of NK cells and the HI virus.

Enforced c-REL deficiency prolongs survival of islet allografts1

BACKGROUND

The NF-kappaB/Rel family of transcription factors regulates biologic processes ranging from apoptosis to inflammation and innate immunity. Whether c-Rel, a lymphoid-predominant member of the NF-kappaB/Rel family, is essential for transplantation immunity is not known.

METHODS

We explored the role of c-Rel in the anti-allograft repertory using mice with targeted disruption of the c-Rel gene (c-Rel-/-) as recipients of H-2 mismatched islet allografts. Allogeneic DBA/2 (H-2d) islets were transplanted into the renal subcapsular space of diabetic c-Rel-/- C57BL/6 (H-2b) mice or the c-Rel +/+ C57BL/6 wild-type mice. Islet graft survival, cellular traffic into the islet grafts and their phenotype, and intragraft expression of cytokines and cytotoxic attack molecules were determined at the protein (by immunohistochemistry) and mRNA (by real-time quantitative polymerase chain reaction) levels.

RESULTS

We found superior islet graft survival in the c-Rel-/- recipients compared to c-Rel+/+ C57BL/6 recipients. Splenocytes from c-Rel-/- mice proliferated poorly compared to splenocytes from the c-Rel+/+ mice on stimulation with anti-CD3 mAbs or Con A. Peri-islet infiltration composed of T lymphocytes and macrophages was found in both c-Rel+/+ recipients and c-Rel-/- recipients, but intra-islet infiltration was observed only in c-Rel+/+ recipients. Immunohistologic and molecular studies showed impaired T helper-type 1 immunity and decreased intragraft expression of cytotoxic attack molecules perforin and granzyme B in c-Rel-/- recipients as compared to wild-type recipients.

CONCLUSIONS

Our results demonstrate that c-Rel is essential for robust rejection of islet allografts and support the idea that strategies that impair c-Rel function may be of value for constraining alloimmunity and facilitating survival of allogafts.

Macrophage damage by Leishmania amazonensis cytolysin: evidence of pore formation on cell membrane

We have previously shown that both promastigotes and amastigotes of Leishmania amazonensis contain a lytic protein that damages erythrocytes and nucleated cells, including macrophages (F. S. M. Noronha, F. J. Ramalho-Pinto, and M. F. Horta, Infect. Immun. 64:3975-3982, 1996). Using the patch-clamp technique, we show here that cell damage by parasite extracts is mediated by the formation of nonselective pores on the target membrane. This demonstrates that L. amazonensis cytolysin is a pore-forming protein (PFP), here named leishporin. We show that the diameters of the pores formed by parasite extracts are heterogeneous, varying from approximately 1.6 to >6.1 nm according to cytolysin concentration or time. We also show that pore formation involves the binding of the PFP to the target cell membrane, a temperature-independent event that is necessary but not sufficient to lyse cells. This is followed by a temperature-dependent step that triggers lysis, probably the insertion and the polymerization of protein subunits in the lipid bilayer. We provide evidence that suggests that polymerization of single subunits must occur for pore formation. We show, in addition, that L. amazonensis expresses molecules antigenically homologous to other PFPs.

Dual mechanisms of apoptosis induction by cytotoxic lymphocytes

Cytotoxic T lymphocytes and natural killer cells together comprise the means by which the immune system detects and rids higher organisms of virus-infected or transformed cells. Although differing considerably in the way they detect foreign or mutated antigens, these cells utilize highly analogous mechanisms for inducing target cell death. Both types of effector lymphocytes utilize two principal contact-dependent cytolytic mechanisms. The first of these, the granule exocytosis mechanism, depends on the synergy of a calcium-dependent pore-forming protein, perforin, and a battery of proteases (granzymes), and it results in penetration by effector molecules into the target cell cytoplasm and nucleus. The second, which requires binding of FasL (CD95L) on the effector cell with trimeric Fas (CD95) molecules on receptive target cells, is calcium independent and functions by generating a death signal at the inner leaflet of the target cell membrane. Exciting recent developments have indicated that both cytolytic mechanisms impinge on an endogenous signaling pathway that is strongly conserved in species as diverse as helminths and humans and dictates the death or survival of all cells.

Mechanisms and management of acute renal allograft rejection

We used RT-PCR for the molecular characterization of human renal graft rejection. The studies showed that intragraft display of mRNA encoding cytotoxic attack molecule granzyme B, and immunoregulatory cytokines IL-10 or IL-2 are correlates of acute rejection, and intrarenal expression of TGF-1 mRNA, of chronic rejection. The current immunosuppressive protocol involves the use of multiple drugs, each directed at a discrete site in the T-cell activation cascade and each with distinct side effects. The immunosuppressants can be classified as inhibitors of: transcription (CsA, tacrolimus); nucleotide synthesis (azathioprine, mycophenolate mofetil, and mizoribine); growth factor signal transduction (sirolimus); and differentiation (DSG). Polyclonal antibodies and monoclonal antibodies directed at cell surface proteins are quite effective as induction therapy or anti-rejection drugs.

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.

Zanvil Alexander Cohn 1926-1993

Zanvil Alexander Cohn, an editor of this Journal since 1973, died suddenly on June 28, 1993. Cohn is best known as the father of the current era of macrophage biology. Many of his scientific accomplishments are recounted here, beginning with seminal studies on the granules of phagocytes that were performed with his close colleague and former editor of this Journal, James Hirsch. Cohn and Hirsch identified the granules as lysosomes that discharged their contents of digestive enzymes into vacuoles containing phagocytosed microbes. These findings were part of the formative era of cell biology and initiated the modern study of endocytosis and cell-mediated resistance to infection. Cohn further explored the endocytic apparatus in pioneering studies of the mouse peritoneal macrophage in culture. He described vesicular inputs from the cell surface and Golgi apparatus and documented the thoroughness of substrate digestion within lysosomal vacuoles that would only permit the egress of monosaccharides and amino acids. These discoveries created a vigorous environment for graduate students, postdoctoral fellows, and junior and visiting faculty. Some of the major findings that emerged from Cohn's collaborations included the radioiodination of the plasma membrane for studies of composition and turnover; membrane recycling during endocytosis; the origin of the mononuclear phagocyte system in situ; the discovery of the dendritic cell system of antigen-presenting cells; the macrophage as a secretory cell, including the release of proteases and large amounts of prostaglandins and leukotrienes; several defined parameters of macrophage activation, especially the ability of T cell-derived lymphokines to enhance killing of tumor cells and intracellular protozoa; the granule discharge mechanism whereby cytotoxic lymphocytes release the pore-forming protein perforin; the signaling of macrophages via myristoylated substrates of protein kinase C; and a tissue culture model in which monocytes emigrate across tight endothelial junctions. In 1983, Cohn turned to a long-standing goal of exploring host resistance directly in humans. He studied leprosy, focusing on the disease site, the parasitized macrophages of the skin. He injected recombinant lymphokines into the skin and found that these molecules elicited several cell-mediated responses. Seeing this potential to enhance host defense in patients, Cohn was extending his clinical studies to AIDS and tuberculosis. Zanvil Cohn was a consummate physician-scientist who nurtured the relationship between cell biology and infectious disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell death mechanisms and the immune system

The immune system provides good models for cell death, a phenomenon now recognized to be of fundamental importance in many fields of biology. Cell death is strikingly polymorphic: it can proceed via necrosis (as in complement-mediated cell death) or apoptosis, but the latter displays different patterns (in the receptor-mediated death of some thymocytes, in cell death mediated by TNF alpha or by cytotoxic T cells), perhaps reflecting different pathways of control of a common core mechanism. Even though there are differences in the morphological and metabolic changes associated with the different patterns of apoptosis, some recurrent sequences of events are observed in almost all dying cells. The metabolic state of a cell often seems to play a major role in determining if and how this cell will die in given external circumstances. The nature of molecules causally involved in the dying cell can now be approached in some systems.

The distribution of perforin in normal tissues

We describe the production of monoclonal antibodies to murine and human forms of the lymphocyte pore-forming protein (perforin, PFP, or cytolysin), a major granule-localized cytolytic mediator of CTL and NK cells. Antibodies were raised against both murine perforin purified from a CTL line, and human perforin expressed in bacteria as a fusion protein with the Escherichia coli TrpE protein. Antibodies raised against either immunogen inhibited the hemolytic activity of murine perforin, and thus may enable us to identify the pore-forming or self-associative domain of perforin. One mAb, MP1, was used to study the distribution of perforin in murine tissues under physiological conditions. We found that perforin was expressed in the granular metrial gland (GMG) cells of the pregnant murine uterus, but not in other tissues examined. These results further support the view that perforin is induced only in activated cytolytic lymphocytes, and raise the question whether perforin-containing GMG cells represent an effector of a maternal immune response to the fetus.

Human and murine cytotoxic T lymphocyte serine proteases: subsite mapping with peptide thioester substrates and inhibition of enzyme activity and cytolysis by isocoumarins

The active site structures of human Q31 granzyme A, murine granzymes (A, B, C, D, E, and F), and human granzymes (A, B, and 3) isolated from cytotoxic T lymphocytes (CTL) were studied with peptide thioester substrates, peptide chloromethyl ketone, and isocoumarin inhibitors. Human Q31, murine, and human granzyme A hydrolyzed Arg- or Lys-containing thioesters very efficiently with kcat/KM of 10(4)-10(5) M-1 s-1. Murine granzyme B was found to have Asp-ase activity and hydrolyzed Boc-Ala-Ala-Asp-SBzl with a kcat/KM value of 2.3 X 10(5) M-1 s-1. The rate was accelerated 1.4-fold when the 0.05 M NaCl in the assay was replaced with CaCl2. The preparation of granzyme B also had significant activity toward Boc-Ala-Ala-AA-SBzl substrates, where AA was Asn, Met, or Ser [kcat/KM = (4-5) X 10(4) M-1 s-1]. Murine granzymes C, D, and E did not hydrolyze any thioester substrate but contained minor contaminating activity toward Arg- or Lys-containing thioesters. Murine granzyme F had small activity toward Suc-Phe-Leu-Phe-SBzl, along with some contaminating trypsin-like activity. Human Q31 granzyme A, murine, and human granzyme A were inhibited quite efficiently by mechanism-based isocoumarin inhibitors substituted with basic groups (guanidino or isothiureidopropoxy). Although the general serine protease inhibitor 3,4-dichloroisocoumarin (DCI) inactivated these tryptases poorly, it was the best isocoumarin inhibitor for murine granzyme B (kobs/[I] = 3700-4200 M-1 s-1). Murine and human granzyme B were also inhibited by Boc-Ala-Ala-Asp-CH2Cl; however, the inhibition was less potent than that with DCI. DCI, 3-(3-amino-propoxy)-4-chloroisocoumarin, 4-chloro-3-(3-isothiureidopropoxy)isocoumarin, and 7-amino-4-chloro-3-(3-isothiureidopropoxy)isocoumarin inhibited Q31 cytotoxic T lymphocyte mediated lysis of human JY lymphoblasts (ED50 = 0.5-5.0 microM).

Channel fluctuations induced by membrane attack complex C5B-9

The assembly of complement (C) components C5b-9 in membranes results in the formation of transmembrane lesions. The C9 component has been shown to be mainly responsible for formation of the ultrastructurally visible tubules associated with C5b-9 complexes. Several studies have disputed the role of C9 polymerization in C-mediated cytolysis on the grounds that C5b-9 lyses cells in the absence of tubular formation. Here, C5b-9 complexes were reconstituted into high-impedance planar lipid bilayers and shown to form channels which are heterogenous in size. The smallest channels had unitary conductances of 15 picoSiemens (pS) in 0.1 M NaCl. The closing of these channels showed voltage-dependence at membrane potentials exceeding 40 mV. These channels were more cation-selective, with K+ ions being favored over Na+. The 15-pS channels described here are much smaller than the channels attributed previously to either C5b-9 or polymerized C9 complexes but resemble channels formed by the C9b fragment, which does not polymerize into tubules. These results indicate that C5b-9 complexes are capable of damaging membranes by forming initially small ion channels which then aggregate in the membrane to form tubular lesions with much larger conductances. Like C5b-9, C5b-8 also increased membrane permeability. However, this increase in membrane conductance could not be resolved into single channels, suggesting that C5b-8 may induce membrane leakiness by perturbing the packing of membrane lipids, whereas addition of C9 results in authentic production of ion channels.

Macrophages regulate intracellular glutathione levels of lymphocytes. Evidence for an immunoregulatory role of cysteine

Macrophages consume cystine and generate approximately equivalent amounts of acid-soluble thiol. Stimulation of macrophages with bacterial lipopolysaccharide (LPS) or tumor necrosis factor (TNF) strongly augments the amount of thiol released into the culture supernatant. Cysteine constitutes most of the acid-soluble thiol. The intracellular glutathione level and the DNA synthesis activity in mitogenically stimulated lymphocytes are strongly increased by either exogenously added cysteine, or (syngeneic) macrophages. This cysteine dependency is observed even in the presence of relatively high extracellular cystine concentration as they occur in the blood plasma. The extracellular cysteine concentration also has a strong influence on the intracellular glutathione concentration, viability, and DNA synthesis of cycling T cell clones. Moreover, the cysteine concentration in the culture medium on Day 3 and Day 4 of a 5-day allogeneic mixed lymphocyte culture (i.e., in the late phase of incubation) has a strong influence on the generation of cytotoxic T cell activity, indicating that regulatory effects of cysteine are not restricted to the early phase of the blastogenic response. The inhibitory effect of cysteine starvation on the DNA synthesis of the T cell clones and on the activation of cytotoxic T lymphocytes can be explained essentially by the depletion of intracellular glutathione, since similar effects are observed after treatment with buthionine sulfoximine (BSO), a specific inhibitor of the glutathione biosynthesis. BSO has practically no influence, however, on the N alpha-benzyloxycarbonyl Ne-t-butyloxycarbonyl-L-lysine-thiobenzyl-ester (BLT)-esterase activity and hemolytic activity of the cell lysates from cytotoxic T cells against sheep red blood cells (perforin activity). Taken together, our experiments indicate that cysteine has a regulatory role in the immune system analogous to the hormone-like lymphokines and cytokines. It is released by macrophages at a variable and regulated rate and regulates immunologically relevant functions of lymphocytes in the vicinity.

A T. cruzi-secreted protein immunologically related to the complement component C9: evidence for membrane pore-forming activity at low pH

Protozoan parasite T. cruzi invades cells within acidic vacuoles, but shortly afterward escapes into the cytosol. Exit from the phagosome is blocked by raising the pH of acidic compartments, suggesting that a previously described acid-active hemolysin secreted by T. cruzi might be involved in the membrane disruption process. Here we show that T. cruzi supernatants are cytotoxic for nucleated cells at pH 5.5 and contain a protein reactive with antibodies against reduced and alkylated human C9 (the ninth component of complement). The C9 cross-reactive protein (TC-TOX) copurified with the cytolytic activity, and the active fractions induced conductance steps characteristic of transmembrane ion channels in planar phospholipid bilayers. Immunocytochemical studies using antibodies against purified TC-TOX showed that the protein was localized to the luminal space of parasite-containing phagosomes. We postulate that TC-TOX, when secreted into the acidic environment of the phagosome, forms pores in the membrane, which contribute to its disruption.

In vivo expression of perforin by natural killer cells during a viral infection. Studies on uveitis produced by herpes simplex virus type I

A potent cytolytic pore-forming protein (PFP, perforin, or cytolysin) is associated with the cytoplasmic granules of cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. The role of PFP/perforin in cytolytic reactions carried out in vivo is still unclear. Here, the authors performed immunohistochemical analysis using antibodies monospecific for perforin and made use of a murine uveitis model produced by intracameral inoculation of herpes simplex virus I (HSV-I). The main cell infiltrate found in the anterior segment of virus-inoculated eyes consisted of Thy-1+/asialo GM1+/CD8-/CD4- cells, presumably representing NK cells. Perforin staining was detected mainly in cells bearing this phenotype. Perforin was only detected in cells displaying the large granular lymphocyte morphology. A small number of perforin-positive cells (less than 5%) colabeled as CD8+, indicating that these cells could have belonged to the CTL lineage. These observations show for the first time the presence of perforin-containing NK cells in tissues of animals undergoing acute viral infections.

Anti-idiotypic antibodies derived against C8, C9 and perforin bind homologous restriction factor

A pore-forming protein (PFP/perforin/cytolysin), stored in the cytoplasmic granules of cytolytic lymphocytes, lyses a variety of target cells but not the cytolytic lymphocytes. In the complement (C) system, a C8-binding protein (C8bp) or homologous restriction factor (HRF) has been described that protects cells against lysis mediated by homologous C. C8bp/HRF is known to bind to C8 and C9 and has also been suggested to protect lymphocytes against perforin-mediated lysis. Here, using an anti-idiotypic antibody approach, several polyclonal antisera were raised against IgGs that are specific for mouse perforin, and human C8 and C9. These anti-idiotypic antisera were shown to react against an overlapping epitope(s) on C8bp/HRF as indicated by the following evidence: (i) all three types of antisera reacted against partially purified C8bp/HRF and against a 65 kDa protein band in cell lysates; reactivity was only observed against disulfide-reduced antigens; (ii) the three antibodies react with a protein band in normal erythrocytes (E) but not with type III E of patients with paroxysmal nocturnal hemoglobinuria or with a mutant B lymphoblastoid cell line, both of which cell types are known to be deficient in C8bp/HRF; and (iii) the three antibodies compete with each other for binding to C8bp/HRF. Type III E and the C8bp/HRF-deficient mutant lymphoblastoid cell line, however, are as susceptible to perforin-mediated lysis as type I E and wild-type lymphoblastoid cell line, respectively, indicating that C8bp/HRF does not play a role in protecting cells against perforin-mediated lysis. These paradoxical findings suggest that perforin may share with C8 and C9 the same domain(s) that bind to C8bp/HRF and yet, unlike C8 and C9, perforin is not inactivated by this type of putative interaction. Since C8 and C9 are now readily available, the anti-idiotypic approach described here provides a convenient protocol for production of antisera specific for C8bp/HRF.

The homologous restriction factor is immunologically related to complement components C8 and C9 and to lymphocyte pore-forming protein perforin through cysteine-rich domains

The 65 kDa C8-binding protein or homologous restriction factor (C8bp/HRF) protects cells from complement (C)-mediated lysis by binding to C8 and abrogating lytic channel formation. Human C8bp/HRF is shown here to be immunologically related to human C8 and C9 and to murine lymphocyte poreforming protein (PFP, perforin). Polyclonal antibodies raised against purified C8, C9 and perforin react with C8bp/HRF. The antigenic epitopes shared by these four proteins are limited to cysteine-rich or disultide bridge-masked domains. Only complement proteins or perforin that have been disulfide-reduced elicit the production of cross-reactive antibodies when used as immunogens. Analogously, only C8bp/HRF that has been disulfide-reduced reacts with these antibodies. These results suggest that C8bp/HRF may belong to the complement/perforin supergene family. The function of homologous domains shared by these four proteins remains to be elucidated.

Differential susceptibility of type III erythrocytes of paroxysmal nocturnal hemoglobinuria to lysis mediated by complement and perforin

Previous reports have suggested that a 65 kDa membrane protein, termed homologous restriction factor (HRF), in addition to protecting erythrocytes (E) against lysis by homologous complement (C), may also be involved in protecting cytolytic lymphocytes against lysis mediated by a pore-forming protein (PFP/perforin), one of their own lytic mediators. Here, we used HRF-deficient type III E of patients with paroxysmal nocturnal hemoglobinuria (PNH) to study their susceptibility to lysis mediated by homologous C and perforin, and compared it with lysis of HRF-bearing control or PNH type I E. We show that type III E of PNH patients are indeed more susceptible to lysis mediated by homologous C than control or type I E, but they are as susceptible to perforin-mediated lysis as type I E. In addition, all human E (type I or III) tested here are equally susceptible to lysis mediated by either human (homologous) or murine (heterologous) perforin. By immunoblot analysis, we confirm that type III E, in contrast to type I E, were deficient in the 65 kDa HRF. These results support the notion that homologous species restriction is seen in the C- but not in the lymphocyte perforin-system and argue against an active participation of HRF in protecting cells from perforin-mediated lysis.

Generation and partial characterization of melanoma sublines resistant to lymphokine activated killer (LAK) cells. Relevance to doxorubicin resistance

To see whether a tumor cell population may contain cells resistant to lymphokine-activated killer (LAK) lymphocytes, cells from a LAK-sensitive melanoma line (Me 665/2) were co-cultured with LAKs. Three sublines were obtained after 1, 2 or 3 immunoselection cycles. Immunoselected (IS) sublines show reduced proliferation, decreased reactivity to the monoclonal antibody (MAb) R24 and appeared morphologically more differentiated in comparison with the parental Me 665/2 line. A progressively reduced sensitivity to LAKs was observed in IS sublines with a more than 8-fold reduction in LAK susceptibility. A reduced complement (C)-mediated lysis was also observed in IS sublines. Since we have previously shown that LAK sensitivity of melanoma cells may be associated with Doxorubicin (Dx) resistance, the sensitivity to Dx was tested in these lines. An augmented sensitivity to Dx was noted in IS sublines as compared with Me 665/2. The differences in LAK susceptibility between the IS sublines and the parental Me 665/2 line remained stable for 2 weeks but declined and disappeared thereafter. These results indicate that (1) a LAK-sensitive tumor line may contain a subpopulation of cells which are significantly less lysed by LAKs; (2) a correlation between LAK sensitivity and susceptibility to C-mediated lysis is also present; and (3) increased sensitivity to Dx is evident in the IS sublines.

The structure of the mouse lymphocyte pore-forming protein perforin

Purified murine lymphocyte pore-forming protein (PFP or perforin) was partially sequenced. Oligonucleotides synthesized on the basis of this sequence information were used to screen a murine cytotoxic T lymphocyte (CTL) cDNA library. Seven clones were obtained, two of which were sequenced, providing full-length sequence information on PFP. Murine PFP (534 a.a.) is 68% identical to human PFP. Hydropathic analysis revealed a predominantly hydrophilic protein with some hydrophobic domains, including a region (a.a. 191-251) that could contain putative membrane-spanning domains. PFP is approx. 20% identical to human C7, C8 and C9 within a region encompassing 270 a.a., confirming previous immunological cross-reactivity studies. Northern blot analysis showed that expression of PFP but not of a serine esterase transcript is enhanced in a CTL line by antigen receptor-stimulation. Southern blot analysis of mouse genomic DNA indicated that PFP is encoded as a single-copy gene with the coding region contained within 10 kilobases of genomic DNA.

Susceptibility of human and murine drug-resistant tumor cells to the lytic activity of rIL2-activated lymphocytes (LAK)

This article surveys the available data on the sensitivity of drug-resistant tumor cells to recombinant interleukin 2 (rIL2)-activated lymphocytes (LAK). In our own study, three different experimental systems were used: 1. in vitro treatment of tumor cells with an anticancer drug followed by the use of surviving cells as targets of LAK; 2. use of pairs of drug-resistant and drug-sensitive cell sublines; 3. analysis of several tumor clones obtained from the same tumor. The antitumor activity of LAK was evaluated both by the 51Cr release and the human tumor clonogenic assay (HTCA). In all the experimental systems used, drug-resistant tumor cells were found to be significantly lysed by LAK, with a consistent trend towards a higher susceptibility than their drug-sensitive counterparts. A positive correlation between the sensitivity to LAK and the ID50 for doxorubicin (Dx) was found in 44 melanoma clones analyzed, suggesting that spontaneously drug-resistant clones have a higher sensitivity to LAK than the drug-sensitive clones. Drug-resistant cells were also more sensitive to antibody and complement-mediated lysis, whereas the higher lysis of drug-resistant tumor cells exerted by LAK was maintained in a lectin dependent cytotoxicity assay. These data offer a rationale for combining chemotherapy with adoptive immunotherapy in the treatment of cancer. Moreover, studying the reasons for the higher LAK sensitivity of drug-resistant tumor cells may provide insights into the mechanisms by which tumor cells can resist LAK action.

Resistance of cytolytic lymphocytes to perforin-mediated killing. Lack of correlation with complement-associated homologous species restriction

CTL and NK cells resist self-mediated killing and lysis by their own pore-forming protein (PFP; perforin). Perforin, like C, lyses RBC. Efficient C-mediated lysis of RBC occurs when both C and RBC are from different species (homologous species restriction). A protective surface protein (C8-binding protein, homologous restriction factor) has been reported to mediate both homologous species restriction in C-dependent cytolysis and protection of some target cells against perforin-induced lysis. We show here that perforin, unlike C, lyses target cells across a variety of species, including the homologous one, while the same target cell populations resist the attack by homologous C. Perforin-containing extracts of CTL and LAK/NK cells from three species (rat, mouse, and human) and purified mouse perforin were tested against RBC from 10 different species, several nucleated target cell lines, and one primary cell population (thymocytes). While resisting lysis by homologous C, most of these cell types were lysed effectively by perforin without any homologous restriction pattern. CTL and NK cells, like other nucleated targets, are resistant to lysis by homologous but not heterologous C; however, these cell types are resistant to both homologous and heterologous perforin. Together, our results suggest that the protective mechanisms associated with C- and perforin-mediated lysis are distinct.

The primary structure of the lymphocyte pore-forming protein perforin: partial amino acid sequencing and determination of isoelectric point

The murine lymphocyte pore-forming protein (PFP) was purified to apparent homogeneity by successive steps of liquid chromatography. Monospecific antibodies were raised against purified PFP that detect only one protein band in murine CTL lines. 25% of the primary sequence of PFP (134 amino acids) was determined by amino terminal analysis of the purified protein and of some of its enzymatic cleavage products. These primary sequences were identical to sequences deduced by cDNA cloning. By isoelectric focusing, PFP was found to have a pI of 6.4. On the chromatofocusing column Mono P, however, PFP was found to elute at pH 4.7. This suggests a tertiary structure for monomeric PFP that is enriched in surface acidic amino acids.

Homology of perforin to the ninth component of complement (C9)

Perforin is one of the cytolytic factors present in the cytoplasmic granules of mouse cytotoxic T lymphocytes and natural killer cells. We have determined the sequence of the N-terminal amino acids of perforin purified from a mouse natural killer cell line, and, by using oligonucleotide probes corresponding to the amino acid residues, we have identified a complementary DNA encoding perforin from the cDNA library of a mouse cytotoxic T lymphocyte clone. As predicted from the functional similarities between perforin and the ninth component of the serum cytolytic system, complement (C9) (refs 4-8), the deduced primary structure of perforin has homology with C9 at their respective functionally conserved regions. We find that perforin is only expressed in killer cell lines, and not in helper T lymphocytes or other tumour cells tested. Thus we have provided direct molecular evidence that a killer-cell-specific protein evolutionally linked to C9 is involved in cell-mediated cytolysis.

Automated fluorometric assay for T cell cytotoxicity

A fluorometric assay avoiding the use of radioactivity has been developed for detecting cytotoxic T lymphocytes (Tc cells). The method involves labelling targets with Hoechst dye no. 33342 (H33342) which becomes brightly fluorescent on binding to DNA. Lysis of target cells by Tc cells is quantified by measuring the release of fluorescent H33342 into the supernatant of culture wells. The fluorescence is measured using an automated Microfluor reader which allows results to be obtained rapidly. The assay has been used to detect alloreactive Tc cells and H-2 restricted Tc cells against influenza virus in a short-term 6 h assay using P815 and L929 as targets with comparable results to those obtained with 51Cr labelling. In contrast, lymphocyte blasts were found to be less sensitive in 6 h fluorometric assays when compared with the 51Cr assay. In long-term overnight assays (possible because of the low spontaneous release of H33342 from targets) lymphocyte blasts gave high specific lysis and some anti-self reactivity. The cause of the anti-self reactivity may reflect fundamental differences between the H33342 and 51Cr release assays.

Granzymes, a family of serine proteases released from granules of cytolytic T lymphocytes upon T cell receptor stimulation

The cytolytic potential of T effector cells appears to be intimately related to the presence of proteins stored in specialized cytoplasmic granules. A striking biological property of isolated granules is their lytic activity for a variety of target cells in a nonrestricted manner. Proteins contained within these granules of CTLs are specifically released upon target cell recognition. We have isolated and characterized six granule-associated proteins in two murine CTL lines in addition to the pore-forming and target membrane-disrupting perforin. Six full length cDNA clones have been identified in a CTL-specific cDNA expression library which code for the granule-associated serine esterases, designated as granzymes A to F. Granzymes A and B represent the genuine proteins encoded by the H factor/CTLA-3 cDNA and the CTLA-1/CCPI cDNA, respectively. The covalent amino acid structures of all six granzymes show the hallmarks for serine proteases and are highly related to that of rat mast cell protease I and II and cathepsin G, which have been found in granules of mast cells and neutrophilic granulocytes, respectively. The primary translation products are processed by removal of a hydrophobic signal peptide and a two residue-long propeptide at the amino-terminus. Immuno-electron microscopy shows that granzymes and perforin are stored together within secretory granules of CTLs. Simultaneous release of at least two of these granzymes has been observed during degranulation of a murine CTL line by anti-T3 antibodies. The biological role, particularly the proteolytic events elicited by granzyme A and other granzymes in the context of target cell recognition, are not known at present. It is unlikely that they form a proteolytic activation cascade together with pore-forming proteins analogous to the complement system. The strictly regulated secretion of granzymes and the lack of measurable enzymatic activity in the case of granzymes B, C, E and F towards a variety of synthetic substrates suggest a highly specific function for each of them.

Perforin-dependent and -independent pathways of cytotoxicity mediated by lymphocytes

There is little doubt at the present time that both perforin-dependent and -independent pathways are important in mediating the cytotoxicity associated with lymphocytes. The cell distribution of perforin, initially thought to include both CTL and NK cells, now must be viewed with caution because all previous biochemical studies on CTL have been conducted with cell lines propagated in long-term cultures in the presence of T cell growth factors (IL-2 and perhaps some still undefined factors). Under these conditions, CTL are known to assume a broader, NK-like specificity in target cell killing and may thus differ significantly from primary CTL generated in the body. Accordingly, perforin does not seem to be present in primary CTL activated directly through mixed lymphocyte reactions. It remains to be shown how primary CTL lyse target cells in vivo. Initial studies conducted in several laboratories have already provided some clues. It now seems that even in cultured, perforin-containing CTL, the perforin pathway is not an obligatory mechanism required for target cell killing. Other pathways, possibly involving TNF/lymphotoxin-like molecules, may play a direct role in this type of cytotoxicity. Other still unidentified factors now also need to be sought, including membrane polypeptides that may develop cytotoxicity directly upon cell contact and binding. Although from the studies reviewed here it is clear now that perforin has a more limited role in cell killing than originally proposed, it is still intriguing that it should share structural and functional homologies with complement proteins, drawing paradoxical analogies between two systems (the cellular and the humoral immune systems) which have evolved to become specialized to carry out separate immunological tasks. The cloning of the genes for perforin and for all the C proteins that comprise the MAC should reveal important information on how these genes originated and then diverged during evolution. The cellular distribution of other granule products, such as serine esterases, also must be viewed with caution. A serine esterase activity was initially thought to be CTL-specific. This information stimulated an intensive research activity in many laboratories that resulted in both the purification of a serine esterase family and the cloning of several serine esterase transcripts. It is becoming clear from recent evidence that this group of enzymes is not truly CTL-specific and therefore would not be expected to develop any function rendered absolutely necessary for cytolysis.(ABSTRACT TRUNCATED AT 400 WORDS)

The molecular basis of target cell killing by human lymphocytes and of killer cell self-protection

The cytolytic protein (C9RP) of human cytotoxic lymphocytes was isolated from large granular lymphocytes (LGL) and anti-CD3 activated cytotoxic T cells (CTL). It is immunochemically related to the channel-forming proteins of complement. Whereas LGL constitutively contain C9RP, peripheral resting CTL do not. C9RP synthesis is induced, however, in CD8+ cells upon stimulation of the T cell antigen receptor-CD3 structure. Comparison of cellular cytotoxicity and C9RP content at various times during anti-CD3 activation of CTL yielded a coefficient of correlation, r = 0.92. Isolated C9RP (Mr approximately 70,000) readily lysed a large variety of metabolically active cells tested. Certain monoclonal antibodies to C9RP inhibited target cell killing by LGL or activated CD8+ lymphocytes. Homologous restriction factor (HRF) is a normal membrane protein of blood cells that inhibits transmembrane channel formation by the membrane attack complex of complement. It has recently been found that isolated HRF (Mr approximately 65,000), bound to sheep erythrocytes, inhibited their lysis mediated by the antibody-dependent cellular cytotoxicity reaction or by isolated C9RP. Further, stimulation of resting peripheral lymphocytes with anti-CD3 resulted in increased expression of cell surface HRF. Acquisition of HRF expression conferred upon CTL relative resistance to lysis by C9RP. A soluble form of HRF (Mr approximately 65,000) was isolated from the cytoplasmic granules of LGL, which also contain C9RP, and shown to inhibit cytotoxicity of LGL and CTL. It is conceivable that HRF is opertive in self-protection of cytotoxic lymphocytes.

Serine esterase and hemolytic activity in human cloned cytotoxic T lymphocytes

Target cell lysis by most murine cytotoxic T lymphocytes appears to be mediated by a complement (C9)-like protein called perforin, contained in high-density cytoplasmic granules. These granules also contain high levels of serine esterase activity, which may also play a role in cytolysis. Analysis of 17 cloned human cytotoxic T lymphocytes revealed the presence of serine esterase that is very similar to its murine counterpart in substrate and inhibitor specificities, pH optimum, and molecular mass; dot blot hybridization with synthetic oligonucleotides corresponding to the active sites of two known murine CTL esterases suggests homology to the murine enzyme HF. However, serine esterase was present at only approximately 10% of the level found in murine CTLs, and was not secreted during CTL-target cell interaction; moreover, hemolytic activity could not be detected in any of the seven cell lines tested. The results suggest that the human CTLs examined here kill their target cells by a mechanism different from that used by most cloned murine CTLs.

Resistance of cloned cytotoxic T lymphocytes to cell-mediated cytotoxicity

Cloned CTLs show an unusually high resistance to lysis by effector CTLs. Several cloned CTL lines in our laboratories are absolutely refractory to lysis by other cloned CTLs, either (a) directly, (b) in the presence of lectin, or (c) by PMA-induced CTLs. They can be lysed to some extent by primary CTL, although they are less than 5% as sensitive as target cells normally used to assay primary CTL lytic activity. Lysis of cloned CTLs by primary CTL effector cells is not enhanced by the presence of lectin, and cloned T cells are also highly resistant to lysis by primary lymphokine-activated killer cells. Cloned CTLs are highly resistant to lysis by isolated CTL granules that contain the membranolytic pore-forming protein (PFP or perforin), while non-CTL targets are highly susceptible to granule-mediated killing, indicating that cloned CTLs resist lysis not only at the intact effector cell level but also when soluble effector proteins are used. This resistance mechanism may explain how CTLs kill but spare themselves from being killed during the cytolytic event.

Inhibition of antibody-dependent lymphocyte cytotoxicity by homologous restriction factor incorporated into target cell membranes

The 65 kD homologous restriction factor (HRF) was isolated from normal human erythrocytes (E) by immunoadsorption using rabbit anti-human HRF. The protein was radiolabeled and incorporated into the membrane of sheep erythrocytes (Es). Es bearing HRF exhibited a markedly reduced susceptibility to reactive lysis by C5b-9. Es-HRF with 1,000-3,000 HRF molecules per cell and sensitized with rabbit IgG anti-Es also were less susceptible to lysis by human large granular lymphocytes (LGL) than untreated Es sensitized with IgG antibody. Similarly, human E of a patient with paroxysmal nocturnal hemoglobinuria (PNH), lacking HRF and sensitized with IgG antibody underwent lysis by human LGL. Lysis was abrogated by incorporation of isolated human HRF. Incorporation of human decay-accelerating factor (DAF) into sensitized Es had no effect on antibody-dependent, cell-mediated cytotoxicity. Furthermore, lysis of Es by the isolated cytolytic C9-related protein (C9RP) of human cytotoxic lymphocytes could be inhibited by cell bound human HRF. These results suggest that HRF inhibits channel formation not only by C5b-9, but also by cytotoxic lymphocytes.