Ultrastructural alteration of cytolytic T lymphocytes following their interaction with target cells. II. Morphogenesis of secretory granules and intracellular vacuoles

In Vivo Measurement of Granzyme Proteolysis from Activated Immune Cells with PET

The biology of human granzymes remains enigmatic in part due to our inability to probe their functions outside of in vitro assays or animal models with divergent granzyme species. We hypothesize that the biology of human granzymes could be better elaborated with a translational imaging technology to reveal the contexts in which granzymes are secreted and biochemically active in vivo. Here, we advance toward this goal by engineering a Granzyme targeting Restricted Interaction Peptide specific to family member B (GRIP B) to measure secreted granzyme B (GZMB) biochemistry with positron emission tomography. A proteolytic cleavage of 64Cu-labeled GRIP B liberates a radiolabeled form of Temporin L, which sequesters the radioisotope by binding to adjacent phospholipid bilayers. Thus, at extended time points postinjection (i.e., hours, not seconds), tissue biodistribution of the radioisotope in vivo reflects relative units of the GZMB activity. As a proof of concept, we show in three syngeneic mouse cancer models that 64Cu-GRIP B detects GZMB from T cells activated with immune checkpoint inhibitors (CPI). Remarkably, the radiotracer detects the proteolysis within tumors but also in lymphoid tissue, where immune cells are activated by a systemic CPI. Control experiments with an uncleavable analogue of 64Cu-GRIP B and tumor imaging studies in germline GZMB knockout mice were applied to show that 64Cu-GRIP B is specific for GZMB proteolysis. Furthermore, we explored a potential noncytotoxic function for GZMB by applying 64Cu-GRIP B to a model of pulmonary inflammation. In summary, we demonstrate that granzyme biochemistry can be assessed in vivo using an imaging modality that can be scaled vertically into human subjects.

Distinct structural and catalytic roles for Zap70 in formation of the immunological synapse in CTL

T cell receptor (TCR) activation leads to a dramatic reorganisation of both membranes and receptors as the immunological synapse forms. Using a genetic model to rapidly inhibit Zap70 catalytic activity we examined synapse formation between cytotoxic T lymphocytes and their targets. In the absence of Zap70 catalytic activity Vav-1 activation occurs and synapse formation is arrested at a stage with actin and integrin rich interdigitations forming the interface between the two cells. The membranes at the synapse are unable to flatten to provide extended contact, and Lck does not cluster to form the central supramolecular activation cluster (cSMAC). Centrosome polarisation is initiated but aborts before reaching the synapse and the granules do not polarise. Our findings reveal distinct roles for Zap70 as a structural protein regulating integrin-mediated control of actin vs its catalytic activity that regulates TCR-mediated control of actin and membrane remodelling during formation of the immunological synapse. DOI: http://dx.doi.org/10.7554/eLife.01310.001.

Granzymes, cytotoxic granules and cell death: the early work of Dr. Jurg Tschopp

Within the powerful legacy left by Jurg Tschopp, we should not forget his early work that helped to elucidate the molecular pathways responsible for the clearance of virus-infected and transformed cells by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. Jurg's skilful biochemical approach formed a firm platform upon which the work of so many other biochemists, cell biologists and immunologists would come to rely. Jurg coined the shorthand term 'granzyme' to denote the individual members of a family of serine proteases sequestered in and secreted from the cytotoxic granules of CTL/NK cells. He was also one of the first to describe the lytic properties of purified perforin and to postulate the synergy of perforin and granzymes, which we now know to underpin target cell apoptosis. Jurg was a major protagonist in the debate that raged throughout the 1980's and early 1990's on the physiological relevance of the 'granule exocytosis' pathway. Ultimately, resolving this issue led Jurg and his colleagues to even greater and impactful discoveries in the broader field of apoptosis research. Jurg Tschopp ranks with other pioneers, particularly Gideon Berke, Chris Bleackley, Pierre Golstein, Pierre Henkart and Eckhard Podack for making seminal discoveries on our understanding of how the immune system eliminates dangerous cells.

Perforin: structure, function, and role in human immunopathology

The secretory granule-mediated cell death pathway is the key mechanism for elimination of virus-infected and transformed target cells by cytotoxic lymphocytes. The formation of the immunological synapse between an effector and a target cell leads to exocytic trafficking of the secretory granules and the release of their contents, which include pro-apoptotic serine proteases, granzymes, and pore-forming perforin into the synapse. There, perforin polymerizes and forms a transmembrane pore that allows the delivery of granzymes into the cytosol, where they initiate various apoptotic death pathways. Unlike relatively redundant individual granzymes, functional perforin is absolutely essential for cytotoxic lymphocyte function and immune regulation in the host. Nevertheless, perforin is still the least studied and understood cytotoxic molecule in the immune system. In this review, we discuss the current state of affairs in the perforin field: the protein's structure and function as well as its role in immune-mediated diseases.

The immunological synapse of CTL contains a secretory domain and membrane bridges

Cytotoxic T lymphocytes (CTL) rapidly destroy their targets. Here we show that although target cell death occurs within 5 min of CTL-target cell contact, an immunological synapse similar to that seen in CD4 cells rapidly forms in CTL, with a ring of adhesion proteins surrounding an inner signaling molecule domain. Lytic granule secretion occurs in a separate domain within the adhesion ring, maintaining signaling protein organization during exocytosis. Live and fixed cell studies show target cell plasma membrane markers are transferred to the CTL as the cells separate. Electron microscopy reveals continuities forming membrane bridges between the CTL and target cell membranes, suggesting a possible mechanism for this transfer.

The ultrastructure of the tubular complex in the cytoplasm of cytolytic T lymphocytes

The DNA synthesis, the cytolytic activity, and the ultrastructure of cytolytic T lymphocytes (CTL) derived from mixed mouse thymocyte culture on day 5 were studied. CTL of thymic origin were absorbed by centrifugation on the surface of target cell (TC) monolayers. At different time intervals after absorption, single tubular structures (TS) and complex of tubular structures (CTS) linked with ergastoplasmic reticulum, secretory granules, Golgi apparatus, coated vesicles, and multivesicular bodies were detected in the CTL cytoplasm. The linkage of CTS with ergastoplasmic reticulum, ribosomes, and secretory lymphocyte mechanism suggests the possibility of a secretory-receptor mechanism of TC cytolysis. The release of numerous secretory vacuoles was accompanied by the enlargement of the cytoplasmatic lymphocyte membrane surface, which seemed to cause its shedding. 'Membranosomas' were observed on CTL membrane; their role is still obscure and awaits further study.

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.

Molecular mechanisms of lymphocyte-mediated cytotoxicity and their role in immunological protection and pathogenesis in vivo

Studies with perforin-deficient mice have demonstrated that two independent mechanisms account for T cell-mediated cytotoxicity: A main pathway is mediated by the secretion of the pore-forming protein perforin by the cytotoxic T cell, whereas an alternative nonsecretory pathway relies on the interaction of the Fas ligand that is upregulated during T cell activation with the apoptosis-inducing Fas molecule on the target cell. NK cells use the former pathway exclusively. The protective role of the perforin-dependent pathway has been shown for infection with the noncytopathic lymphocytic choriomeningitis virus, for infection with Listeria monocytogenes, and for the elimination of tumor cells by T cells and NK cells. In contrast, perforin-dependent cytotoxicity is not involved in protection against the cytopathic vaccinia virus and vesicular stomatitis virus. LCMV-induced immunopathology and autoimmune diabetes have been found to require perforin-expression. A contribution of perforin-dependent cytotoxicity to the rejection of MHC class I-disparate heart grafts has also been observed. Its absence is efficiently compensated in rejection of fully allogeneic organ or skin grafts. So far, evidence for a role of Fas-dependent cytotoxicity as a T cell effector mechanism in vivo is lacking. Current data suggest that the main function of Fas may be in regulation of the immune response and apparently less at the level of an effector mechanism in host defense. Further analysis is necessary, however, to settle this point finally.

Perforin and its role in T lymphocyte-mediated cytolysis

The killing mediated by cytotoxic T lymphocytes (CTL) represents an important mechanism in the immune defence against tumors and virus infections. The lytic mechanism has been proposed to consist of a polarized secretion of granule-stored molecules, occurring on effector-target cell contact. By electron microscopy, membrane deposited, pore-like lesions are detected on the target cell membrane during cytolysis by CTL. These structures resembled strikingly pores formed during complement attack. Granules of CTL isolated by nitrogen cavitation and Percoll gradient centrifugation were shown to retain cytotoxic activity. Further purification of proteins stored in these granules led to the discovery of a membranolytic protein named perforin which was capable of polymerizing into pore-like structures. In addition to this cytolytic protein, a set of serine esterases was found as well as lysosomal enzymes and proteoglycans, whose function are not yet clearly defined. The role of perforin in the cytotoxic process is currently being explored by ablating the active gene in mice.

Nicotinamide and 3-aminobenzamide interfere with receptor-mediated transmembrane signaling in murine cytotoxic T cells: independence of Golgi reorientation from calcium mobilization and inositol phosphate generation

The two competitive inhibitors of ADP-ribosylation, nicotinamide and 3-aminobenzamide, have been reported to interfere with TNF-induced cell apoptosis, and there is evidence that they inhibit killer-induced target cell lysis as well. There are very few drugs known to specifically interfere with target apoptosis induced by killer cells. We therefore sought to explore the effects these inhibitors have on CTL-mediated cell lysis. Here we show that TcR-mediated transmembrane signaling in CTL, measured by Ca2+ mobilization and generation of inositol phosphates, is inhibited by nicotinamide. The possibility that all cell functions are suppressed by the drug is excluded by the finding that constitutive secretion of BLT serine esterase is not inhibited, whereas stimulated secretion of this enzyme is suppressed. We also show that nicotinamide does not interfere with CTL target cell binding or reorientation of the Golgi apparatus toward the target binding site. It is concluded that nicotinamide inhibits transmembrane signaling in CTL and thereby interferes with delivery of the lethal hit to targets.

Cytoskeletal function in CD8- and T cell receptor-mediated interaction of cytotoxic T lymphocytes with class I protein

Cloned allospecific cytolytic T lymphocytes (CTL) adhere to purified class I alloantigen immobilized on plastic and degranulate in response to it. Binding and degranulation are inhibited by drugs that impair cytoskeletal function. Cytochalasins D and E, which interfere with microfilament function, and colchicine, which disrupts microtubules, were used and gave qualitatively similar results. Concentrations of these drugs that inhibited degranulation in response to alloantigen did not inhibit response to immobilized anti-T cell receptor (TCR) antibody. Neither did they inhibit response when alloantigen was co-immobilized with an antibody against class I on the CTL to promote adhesion between the CTL and antigen-bearing surface. Thus, neither transmembrane signal generation via the TCR nor degranulation per se were prevented. Instead, the drugs act to prevent the initial adhesion to alloantigen. CTL binding to alloantigen depends in part on CD8-class I interaction, and adhesion via CD8 is "activated" by crosslinking the TCR with soluble anti-TCR antibody. This adhesion, too, is shown to be cytoskeleton dependent.

Differential effects of protein synthesis inhibition on CTL and targets in cell-mediated cytotoxicity

The reactions that lead to target cell lysis by cytotoxic T cells (CTL) are despite intensive investigations poorly understood. To examine the relative roles effectors and targets play in the lytic reaction, protein synthesis in either CTL or targets was inhibited before assay of lysis. We show, in agreement with previous results, that de novo protein synthesis is not necessary in either effectors or targets during the cytolytic reaction. However, activation of CTL requires protein synthesis. Activated CTL respond to protein synthesis inhibitors with a cycling of activity, a result that is interpreted to be consistent with a stimulus secretion mechanism. Treatment of targets with protein synthesis inhibitors prior to incubation with CTL leads to a very rapid and irreversible loss of lytic susceptibility. It is shown that the decrease in lysability is not due to lack of proper CTL target interaction: MHC class I antigens are expressed on drug-treated targets and these cells serve as cold targets in competitive inhibition experiments. Moreover, drug-treated targets trigger transient Ca2+ mobilization and generation of inositol phosphates in CTL. It is therefore concluded that drug-treated targets are able to trigger CTL function but lack a component that is required for their successful lysis.

Ultrastructural changes during lysis of L929 target cells by class II-restricted influenza virus-specific murine cytotoxic T-lymphocyte clones

Lysis of virus-infected L929 target cells transfected with the H-2 class II IAk gene by class II-restricted influenza virus-specific murine cytotoxic T lymphocyte (CTL) clones was studied by electron microscopy and compared with lysis of L929 cells by class I-restricted CTL clones. T lymphocytes predominantly approached the basal surface of target cells grown on a plastic dish and also approached uninfected L929 target cells, although virus maturation exhibited no polarity with respect to the cell surface site. After incubation for 30 min, the target cell nuclei began to change: chromatin became irregularly redistributed and aggregated, and the nuclei appeared swollen. Later, electron-dense and -light areas of nuclei became segregated, and the cytoplasm became disorganized with many vacuoles. The ultrastructural changes of target cells during lysis by class I- and class II-restricted CTL clones appeared to be similar. These findings and other cytotoxicity data of class I and class II CTLs are discussed.

Clonal analysis in the ultrastructure of cell-to-cell interaction between a human glioma cell line and autologous tumor-specific cytotoxic T lymphocytes

The clonal analysis in the ultrastructure of tumor-lymphocyte interaction was carried out in order to investigate the precise mechanism responsible for CTL-mediated cytolysis of tumor cells. A glioma-derived cell line (GI-1) and autologous tumor-specific cytotoxic T lymphocyte (CTL) clones were established. The CTL lines were composed of the morphologically homogeneous lymphocytes with intracytoplasmic electron-dense secretory granules. After the stimulation by GI-1, the size of the CTLs increased, and the intracytoplasmic organellas were developed. It was noted that the intracytoplasmic secretory granules markedly increased in number and size, and many of them exhibited an "immature" appearance. On the other hand, the tumor cells underwent a progressive degeneration. In contrast, the stimulation by other antigens caused only small morphological changes in the CTLs. It is suggested, therefore, that the secretory function of tumor-specific CTLs is activated by the stimulation of the specific antigen, and that soluble factors in the secretory granules in the CTLs may be closely associated with the mechanism of target cell lysis.

A new model for lethal hit delivery by cytotoxic T lymphocytes

Human cytotoxic T lymphocyte (CTL) granules contain an electron-dense core and small membrane vesicles. By immuno-electron microscopy, molecules relevant to CTL-target cell (TC) interactions have been identified on the membranes of the dense core and small vesicles within the granule. Moreover, perforin, the component implicated in the lethal hit, and serine esterases are localized within these granule substructures. In this article Peter Peters and colleagues argue that these observations necessitate a revision of the current model for lethal hit delivery. They suggest that the cytotoxic mediators exocytosed into the cleft between CTL and TC are not in soluble form, but rather are membrane-enveloped. The presence of the CD3-T-cell receptor (TCR) complex, CD8 and possibly other relevant molecules on these membranes may ensure unidirectional delivery of the lethal compounds to the TC.

Molecules relevant for T cell-target cell interaction are present in cytolytic granules of human T lymphocytes

An ultrastructural analysis of human cytotoxic T lymphocyte-target cell (CTL-TC) interaction has been undertaken to enable a better understanding of the killing mechanism. Attention was focused on granules in the CTL, which are known to contain lethal compounds. Within the membrane-delimited cytotoxic granule an electron-dense core as well as numerous membrane vesicles were identified. In CTL-TC conjugates, specific membrane interactions take place, allowing the formation of intercellular clefts into which the granule cores and internal vesicles are released. T cell surface membrane molecules known to be involved in CTL-TC interaction (T cell receptor, CD3 and CD8) are present on the membranes of the granule cores and internal vesicles, facing outward. An explanation for this localization of the membrane may be found in the fact that the granule is connected with an endocytotic pathway. Moreover, the lumen of the granule is rich in the enzyme cathepsin D, which indicates an association with a lysosomal compartment. Exocytosed vesicles and cores are seen to adhere to the plasma membrane of the TC. Although the exact contents of the granule vesicles and core remain to be identified, we suggest that specific interaction of CTL membrane molecules on the cytolytic granule components with molecules on the plasma membrane of the TC may ensure the unidirectional delivery of the lethal hit.

Mechanisms of lymphocyte-mediated lysis

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells use multiple mechanisms to destroy their target cells. Pore formation resulting in osmotic lysis of the target is one mechanism; the pore-forming protein (perforin) responsible for this activity has been purified. Antigenically and functionally it resembles proteins of the membrane attack complex of complement. The other known mediators of cytotoxicity appear to be closely interrelated. Tumor necrosis factor (TNF), lymphotoxin (LT), and leukalexin are the three members of this group that have been purified, although their mechanisms of action are still unknown. CTLs fragment the DNA of target cells, as do TNF, LT, and leukalexin; this may be one of the mechanisms of action of these mediators. CTLs and NK cells do not self lyse. The basis of this phenomenon is unclear, although recent advances have shed some light on the problem.

Mechanism of target cell lysis by cytotoxic T lymphocytes (CTL) employing RSV-induced H-2 congenic and recombinant mouse tumor cells: demonstration of soluble mediators released from H-2-restricted CTL clones

The secretion and the specificity of cytotoxic mediators from H-2-restricted cytotoxic T lymphocytes (CTL) were examined using non-virus-producing target tumor cells induced by the Schmidt-Ruppin strain of Rous sarcoma virus (SR-RSV) in B10 congenic and recombinant mice. By using rat concanavalin A supernatant, two H-2-restricted CTL clones were established from cytotoxic effector cells of B10.A(5R) mice primed with SR-RSV-induced syngeneic tumor Cell-free supernatants from the H-2-restricted CTL clones cocultured with syngeneic tumor cells had selectively high cytotoxic activity for syngeneic and H-2-compatible tumor cells, but not for H-2-incompatible tumor cells. YAC-1 cells, and B10.A(5R) blasts as defined in the 5-hr 51Cr-release assay. The cytotoxic activity was detected in the cell-free supernatants from the CTL clones cocultured with the CTL-sensitive syngeneic and H-2-compatible tumor cells, but not with the CTL-insensitive tumor cells and YAC-1 cells. The cytotoxic activity of the cell-free supernatant could be adsorbed by the syngeneic tumor cells, but not by YAC-1 and L(s) cells. Thus, the H-2-restricted CTL clones against SR-RSV-induced tumor cells were capable of releasing cytotoxic mediators by coculturing with syngeneic or H-2-compatible tumor cells, and the cytotoxic mediators showed a certain H-2-restricted manner in killing the target cells. These results suggest that the lysis of RSV-induced tumor cells by H-2-restricted CTL can at least in part be mediated by cytotoxic factors.

Mechanistic, functional and immunopharmacological implications of biochemical studies of antigen receptor-triggered cytolytic T-lymphocyte activation

Biochemical events that follow the engagement of cytotoxic T lymphocytes (CTL) with an Ag-bearing target cell (TC) or triggering by the crosslinking of the Ag-receptor (TcR) by immobilized anti-TcR mAb were studied using cloned CTL and a novel CTL activation assay. The approach described here was undertaken to shed light on the molecular mechanisms of "ON", "STOP" and "OFF" signalling that allow CTL to be activated, kill TC and disengage from the target cell after delivery of the "lethal hit" and then to proceed with the destruction of the next Ag-bearing target encountered. Biochemical studies of TcR-regulated and TcR-triggered constitutive exocytosis in CTL provided a detailed description of the molecular requirements for this important phenomenon in T lymphocytes and provided an alternative CTL activation assay; this assay measures the TcR-dependent response in the absence of a TC. These studies also helped to envision CTLs screening activities as a cycle of engagements-disengagements with the TC, where every surrounding cell is treated by the CTL as a potential Ag-bearing TC. Both constitutive and regulated exocytosis in CTL are triggered through a transmembrane signalling pathway which involves protein kinase C and extracellular Ca2+ that, most likely, is translocated through Ca2+ channels. This is followed by the involvement of calmodulin (CaM)-binding proteins, e.g., calcineurin, a CaM-dependent phosphatase, which was shown to be a major CaM-binding protein in murine lymphocytes. Unexpectedly, these biochemical studies demonstrated that the granule exocytosis model of CTL-mediated cytotoxicity cannot account for the mechanism of target cell lysis by CTL, at least in in vitro conditions in the absence of extracellular Ca2+. These results indicate the existence of an extracellular Ca2+-independent, TcR-regulated CTL response and raise the possibility that second messenger(s) other than Ca2+ and/or products of phosphoinositide turnover are involved in T-cell lysis. Predominance of "non-lethal" engagements between some CTL and TC, revealed during time-lapse cinematographic studies, together with comparative studies of TcR-regulated exocytosis of granules and of constitutive exocytosis of gamma-interferon, suggested that TC destruction by CTL may not be their only or even their most important function in vivo. It is possible that CTL, triggered by Ag recognition to exocytose storage granules and to synthesize and constitutively exocytose macrophage-activating factors, in turn promote tumor destruction by macrophages.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

A novel cytotoxic T lymphocyte activation assay. Optimized conditions for antigen receptor triggered granule enzyme secretion

A method is described for the quantitative studies of cytotoxic T lymphocyte (CTL) activation. This functional assay is based on the measurements of secreted granule-associated enzymatic activity (BLT esterase (BLT-E) ) after incubation of CTL with activating stimuli. Immobilized mAb against CTL's antigen receptor (anti-TcR mAb), concanavalin A or a combination of PMA and ionophore A23187, were able to trigger the secretion of enzyme in the absence of target cells. Soluble anti-TcR mAb alone did not activate CTL, but using their conjugate with immobilized rabbit anti-mouse Ig antibody (RAMIg) TcR-mediated secretion of BLT-E was detected. Use of non-ionic detergents Nonidet P-40 or Triton X-100 (0.0125-0.2%) did not affect measurements of BLT-E activity. The efficiency of CTL exocytosis triggering by anti-TcR mAb which were immobilized on the surface of different plasticware is compared and conditions for studies of small and large numbers of CTL are described. The intensity of CTL response varies markedly with changes in buffer system, culture medium, additions of proteins. The optimal conditions for TcR complex triggered activation of murine CTL are described. Intensity of secretion can be easily manipulated by changing the surface density of immobilized anti-TcR mAb, thereby providing the possibility to screen inhibiting or activating agents (drugs or mAb) at selected sub-optimal levels of CTL activation. The potential for the use of described assay in screening of hybridoma supernatants for the presence of activating or inhibitory mAb against CTL's surface proteins is discussed. Since BLT-E secretion reflects exocytosis of granules from CTL, the conditions described here could be used for the detection of secretion of other markers of granules in future modifications of granule exocytosis assay.

Antigen receptor-regulated exocytosis in cytotoxic T lymphocytes

We demonstrate here that T cell receptor for antigen (TCR)-triggered exocytosis in cytotoxic T lymphocytes (CTL) is not constitutive and is regulated through crosslinking of the TCR by antigen or monoclonal anti-TCR antibodies. Morphological and biochemical data using three different biochemical markers of granules and Percoll gradient fractionation analysis are presented, suggesting that TCR-triggered exocytosis is accompanied by the loss of granules from CTL and appearance of intragranular proteins and enzymatic activities in the incubation medium. The strict requirement for crosslinking of the TCR in exocytosis triggering could be bypassed by protein kinase C activators (phorbol esters or bryostatin I and II) acting in synergy with Ca2+ ionophores. It is shown that external Ca2+ is obligatory for both the TCR-triggered and for the PMA/A23187-triggered exocytosis, since Ca2+ chelators and divalent cations that compete with Ca2+ for A23187 can inhibit exocytosis of granules. These data suggest that Ca2+ from intracellular stores is not sufficient to support exocytosis in CTL. Ca2+ channel blockers and calmodulin antagonists significantly inhibited TCR-triggered exocytosis without affecting the basal level of secretion. The described results are consistent with a model in which exocytosis of granules in CTL is triggered by the crosslinking of TCR, transmembrane protein kinase C activation, and external Ca2+ translocation through CTL plasma membrane Ca2+ channels and modulation of activity of Ca2+, calmodulin-dependent enzymes, and cytoskeletal proteins.

Quantitative ultrastructure of cytolytic lymphocytes mediating allograft rejection in the mouse. I. Cellular alterations in T lymphocytes during specific target cell lysis

A quantitative ultrastructural analysis of cytolytic T lymphocytes (CTL) is presented which allows both the distinction of these cells from normal T lymphocytes and permits the demonstration of ultrastructural alterations of putative CTL following interaction with target cells (TC). Alloreactive CTL were generated in C57BL/10 mice receiving intraperitoneal fibroblastic allografts and target-binding splenic lymphocytes (TBSL) were concentrated by specific immunoadsorption on fibroblast monolayers. TBSL were subjected to ultrastructural quantification either at the onset of TC interaction or following 30 or 60 min incubation at 37 degrees C. By means of simple stereological relationships it was shown that, in comparison with normal, non-cytolytic splenic T lymphocytes, TBSL were slightly larger cells, displaying around 60% more cytoplasm, a similarly-sized nucleus and approximately triple the volume of Golgi apparatus. During the first 30 min of interaction with TC, the target binding surface of the TBSL plasma membrane decreased in area. This change was accompanied by a polarization of the TBSL towards the target. Incubation of lymphocytes with TC for a further 30 min resulted in a general polarization of lymphocytic cellular constituents away from the TC. These results were only attainable by objective quantitative analysis and are discussed in relation to possible mechanisms of CTL-mediated lysis.

Cloned mouse cells with natural killer function and cloned suppressor T cells express ultrastructural and biochemical features not shared by cloned inducer T cells

We have examined the morphology, cytochemistry, and biochemistry of mouse leukocyte subsets by analyzing cloned leukocyte populations specialized to perform different immunologic functions. Cloned cells expressing high-affinity plasma membrane receptors for IgE and mediating natural killer (NK) lysis and cloned antigen-specific suppressor T cells contained prominent osmiophilic cytoplasmic granules similar by ultrastructure to those of mouse basophils. Both clones also incorporated 35SO4 into granule-associated sulfated glycosaminoglycans, expressed a characteristic ultrastructural pattern of nonspecific esterase activity, incorporated exogenous [3H]5-hydroxytryptamine, and contained cytoplasmic deposits of particulate glycogen. By contrast, cloned inducer T cells lacked cytoplasmic granules and glycogen, incorporated neither 35SO4 nor [3H]5-hydroxytryptamine, and differed from the other clones in pattern of nonspecific esterase activity. These findings establish that certain cloned cells with NK activity and cloned suppressor T cells express morphologic and biochemical characteristics heretofore associated with basophilic granulocytes. However, these clones differ in surface glycoprotein expression and immunologic function, and the full extent of the similarities and differences among these populations and basophils remains to be determined.

Spatial relationships of microtubule-organizing centers and the contact area of cytotoxic T lymphocytes and target cells

Specific binding (conjugation) of cytotoxic T lymphocytes (CTL) to target cells (TC) is the first step in a multistage process ultimately resulting in dissolution of the TC and recycling of the CTL. We examined the position of the microtubule organizing center (MTOC) of immune CTL bound to specific TC. Immunofluorescence labeling of freshly prepared CTL-TC conjugates with tubulin antibodies indicated that the MTOC in essentially all conjugated CTL but not in the conjugated TC were oriented towards the intercellular contact site. This finding was corroborated by electron microscopy examination of CTL-TC conjugates fixed either immediately after conjugation or during the lytic process. Antibody-induced caps of membrane antigens of CTL such as H-2 and Thy 1, did not show a similar relationship to the MTOC. Incubation of CTL-TC conjugates, 10-15 min at room temperature, resulted in an apparent deterioration of the microtubular system of conjugated CTL. It is proposed that the CTL plasma membrane proximal to the MTOC is particularly active in forming stable intercellular contacts, resulting in CTL-TC conjugation, and that subsequent modulation of the microtubular system of the CTL may be related to the cytolytic response and to detachment of the effector cell.

Direct analysis of individual killer T cells: susceptibility of target cells to lysis and secretion of hydrolytic enzymes by CTL

Direct identification, enumeration and biological characterization of cytolytic T lymphocytes (CTL) complementing the chromium release test had allowed us to propose a mechanism for T-cell mediated cytotoxicity (T-CMC). One CTL without accessory cells is able to lyse a specific target. Binding which allows cell contact (E-T doublets), should activate effector CTLs and render target cells susceptible to lysis. Secretion of hydrolytic enzymes localized at the CTL-target junction accounts for the lethal hit given by CTLs to susceptible targets. This hypothesis that the resulting cell lesion is identified with an alteration of the lipid bilayer membrane had been confirmed by single killer cell study. Furthermore, we have proposed that phospholipase enzymes are involved in the membrane alterations, since susceptible targets bound to effector cells (conjugates) were lysed in a much higher yield in the presence of phospholipase. Membrane cell lesion determined by hydrolytic enzyme (phospholipase) could represent a basic general mechanism for other cellular or molecular mediated processes.