Mechanism of specific tumor-cell lysis by alloimmune T lymphocytes: resolution and characterization of discrete steps in the cellular interaction

Cytolytic peptides induce biphasic permeability changes in mammalian cell membranes

The cytolytic peptides melittin and gramicidin S are naturally occurring agents that provide a comparative model for studies of complement, immunotoxin and cell-mediated membrane permeability. Most attempts to characterize cytolytic peptides have used model membrane systems including phospholipid vesicles or erythrocytes. Membrane vesicles permit the use of self-quenching concentrations of fluorescent permeability markers, while erythrocytes release measurable hemoglobin. Attempts at measuring early membrane permeability changes in nucleated mammalian cells have been limited. To measure the kinetics of mammalian cell membrane permeability changes induced by cytolytic peptides, we developed a 96-well fluorescence cytolysis assay using the cytoplasmic fluorescent dye calcein as the membrane permeability marker. To facilitate rapid assessment of membrane permeability, trypan blue was added to the assay solution to quench (a) released fluorescence and (b) retained intracellular fluorescence. Trypan blue also provided a complementary visual assessment of cell viability. Using this assay, a detailed kinetic analysis demonstrated permeability of the cell membranes within seconds of exposure to the cytolytic peptides. The rapid permeabilization of the cell membranes was confirmed by flow cytometry using the calcium indicator dye fluo-3. The assay also demonstrated a second slower phase of marker release over the next several hours. The fluorescence cytolysis assay was able to reliably detect the biphasic permeability changes associated with the melittin and gramicidin S peptides suggesting the potential utility of this assay in the assessment of other cytolytic agents.

T-cell stimulation through the T-cell receptor/CD3 complex regulates CD2 lateral mobility by a calcium/calmodulin-dependent mechanism

T lymphocyte activation through the T cell receptor (TCR)/CD3 complex alters the avidity of the cell surface adhesion receptor CD2 for its ligand CD58. Based on the observations that activation-associated increases in intracellular [Ca2+] ([Ca2+]i) strengthen interactions between T cells and antigen-presenting cells, and that the lateral mobility of cell surface adhesion receptors is an important regulator of cellular adhesion strength, we postulated that [Ca2+]i controls CD2 lateral mobility at the T cell surface. Human Jurkat T leukemia cells were stimulated by antibody-mediated cross-linking of the TCR/CD3 complex. CD2 was labeled with a fluorescently conjugated monoclonal antibody. Quantitative fluorescence microscopy techniques were used to measure [Ca2+]i and CD2 lateral mobility. Cross-linking of the TCR/CD3 complex caused an immediate increase in [Ca2+]i and, 10-20 min later, a decrease in the fractional mobility of CD2 from the control value of 68 +/- 1% to 45 +/- 2% (mean +/- SEM). One to two hours after cell stimulation the fractional mobility spontaneously returned to the control level. Under these and other treatment conditions, the fraction of cells with significantly elevated [Ca2+]i was highly correlated with the fraction of cells manifesting significantly reduced CD2 mobility. Pretreatment of cells with a calmodulin inhibitor or a calmodulin-dependent kinase inhibitor prevented Ca2+-mediated CD2 immobilization, and pretreatment of cells with a calcineurin phosphatase inhibitor prevented the spontaneous reversal of CD2 immobilization. These data suggest that T cell activation through the TCR/CD3 complex controls CD2 lateral mobility by a Ca2+/calmodulin-dependent mechanism, and that this mechanism may involve regulated phosphorylation and dephosphorylation of CD2 or a closely associated protein.

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.

Signaling Pathways Regulating CD44-Dependent Cytolysis in Natural Killer Cells

CD44 is a cytotoxic triggering molecule on activated, but not fresh natural killer (NK) cells. In the current study, metabolic pathways used in CD44-directed lysis (CD44DL) were examined using activated human NK cells as effectors. We found that CD44 expressed by activated NK cells was indistinguishable in isoform and molecular weight from CD44 on unactivated cells. However, de novo protein expression was required for the induction of CD44DL, suggesting that activated NK cells contain proteins not present in fresh NK cells that couple CD44 to the lytic machinery. Concanimycin A, a selective inhibitor of perforin-based cytolysis, totally blocked CD44DL, natural cytototoxicity, and antibody-dependent cell-mediated cytolysis (ADCC). Moreover, studies in which kinase inhibitors were added during the effector phase of lysis indicated that protein-tyrosine and ser/thr kinases were required for all three cytolytic activities and that protein kinase C played a nonessential role in lysis. By contrast, wortmannin totally inhibited CD44DL, but failed to block natural cytotoxicity and only partially blocked ADCC, suggesting that phosphatidylinositol 3-kinase (PI 3-kinase) is required at an early, receptor-specific stage of CD44DL. Finally, cytochalasin B enhanced CD44DL, but not ADCC, indicating that CD44DL is modulated by actin polymerization. Taken together, our data suggest that CD44 in NK cells interacts with proteins induced during interleukin-2 activation in a triggering pathway that induces perforin release, requires PI 3-kinase, and is modulated by the cytoskeleton.

Signaling Pathways Regulating CD44-Dependent Cytolysis in Natural Killer Cells

CD44 is a cytotoxic triggering molecule on activated, but not fresh natural killer (NK) cells. In the current study, metabolic pathways used in CD44-directed lysis (CD44DL) were examined using activated human NK cells as effectors. We found that CD44 expressed by activated NK cells was indistinguishable in isoform and molecular weight from CD44 on unactivated cells. However, de novo protein expression was required for the induction of CD44DL, suggesting that activated NK cells contain proteins not present in fresh NK cells that couple CD44 to the lytic machinery. Concanimycin A, a selective inhibitor of perforin-based cytolysis, totally blocked CD44DL, natural cytototoxicity, and antibody-dependent cell-mediated cytolysis (ADCC). Moreover, studies in which kinase inhibitors were added during the effector phase of lysis indicated that protein-tyrosine and ser/thr kinases were required for all three cytolytic activities and that protein kinase C played a nonessential role in lysis. By contrast, wortmannin totally inhibited CD44DL, but failed to block natural cytotoxicity and only partially blocked ADCC, suggesting that phosphatidylinositol 3-kinase (PI 3-kinase) is required at an early, receptor-specific stage of CD44DL. Finally, cytochalasin B enhanced CD44DL, but not ADCC, indicating that CD44DL is modulated by actin polymerization. Taken together, our data suggest that CD44 in NK cells interacts with proteins induced during interleukin-2 activation in a triggering pathway that induces perforin release, requires PI 3-kinase, and is modulated by the cytoskeleton.

The derivation of binding parameters from effector and target conjugate frequency data using linear and non-linear data-fitting transformations. Application of such transformations to the NK-MOLT4 and NK-K562 effector-target systems

Effector-target conjugation is described quantitatively by binding isotherms which are characterized by three parameters, the maximum effector and target conjugate frequencies, alpha max and beta max, and the dissociation constant of the conjugates formed, KD. In this paper the application of non-linear data-fitting techniques, as well as linear transformations of the binding isotherms that permit us to use standard regression analysis, has been tested to calculate estimates of these parameters in the NK-MOLT4 and NK-K562 effector-target systems. Both unweighted and weighted data were used to calculate alpha max, beta max and KD for six different donors which were used as a source of NK cells. The results obtained have shown that these regression methods are useful for revealing potential disparities between binding efficiencies in effector-target systems.

Synergistic roles of granzymes A and B in mediating target cell death by rat basophilic leukemia mast cell tumors also expressing cytolysin/perforin

We have studied the cytotoxic activity of rat basophilic leukemia (RBL) cells transfected with cDNAs for the cytotoxic T lymphocyte (CTL) granule components, cytolysin (perforin), granzyme A, and granzyme B. With red cell targets, cytolysin expression conferred potent hemolytic activity, which was not influenced by coexpression of granzymes. With tumor targets, RBL cells expressing cytolysin alone were weakly cytotoxic, but both cytolytic and nucleolytic activity were enhanced by coexpression of granzyme B. RBL cells expressing all three CTL granule components showed still higher cytotoxic activities, with apoptotic target death. Analysis of the cytotoxic activity of individual transfectant clones showed that cytolytic and nucleolytic activity correlated with granzyme expression but was independent of cytolysin expression within the range examined. A synergism between granzymes A and B was apparent when the triple transfectant was compared with RBL cells expressing cytolysin and one granzyme. These data implicate granzymes as the major mediators of tumor target damage by cytotoxic lymphocytes.

Participation of target Fas protein in apoptosis pathway induced by CD4+ Th1 and CD8+ cytotoxic T cells

The results presented here provide evidence that the presence of Fas protein in target cells is essential to permit cytotoxicity (resulting in apoptosis) mediated by cloned CD4+ Th1 cells. Using mitogen-activated B cells as targets, antigen-dependent lysis by CD4+ Th1 effectors was observed with MRL/MpJ+ but not with MRL/MpJ-lpr targets. The congenic MRL/MpJ-lpr strain is defective in Fas expression. Target cells from various lymphoid tissues of C3H.MRL-lpr mice were also resistant to the lectin-dependent cytotoxicity of Th1 effectors, whereas C3H/HeJ targets were sensitive. Moreover, a rapid DNA fragmentation prior to 51Cr release was induced only in C3H/HeJ targets. Thus, cytotoxicity induced by Th1 effectors correlates with target Fas expression. In contrast to Th1 effectors, CD8+ cytotoxic T lymphocytes (CTLs) killed C3H.MRL-lpr targets. When cytotoxicity was assayed in the presence of EGTA and MgCl2, which chelates extracellular Ca2+ [(Ca2+)ext], only C3H.MRL-lpr targets became resistant to CD8+ CTLs. This (Ca2+)ext-independent cytotoxicity of both Th1 and CD8+ effectors could be inhibited with unlabeled C3H/HeJ thymocytes or with a transfectoma carrying a murine Fas-human mu gene construct. In comparison, C3H.MRL-lpr thymocytes and the nontransfected parental cell line were poor inhibitors. Our study demonstrates that CD4+ Th1 cells and CD8+ CTLs differ in their (Ca2+)ext-dependent cytotoxicity but share a (Ca2+)ext-independent cytotoxicity that requires participation of Fas molecules for cytotoxic signal transduction leading to target apoptosis.

Binding units (BU) and the area under binding isotherms (AUI). New indices of effector-target conjugation

New methods for simplified quantitation of effector-target conjugation have been developed. The binding unit (BU) is defined as the number of target cells required to bind a specified percentage of effector cells. The number of binding units is determined from binding isotherms in which effector conjugate frequencies are measured by holding constant the number of effector cells and by varying the number of target cells. Alternately, a binding unit can be defined as the number of effector cells required to bind a specified percentage of target cells. In this case, BU is computed from binding isotherms in which target conjugate frequencies are measured at different values of effector cells by holding constant the number of target cells. Also, the area under the curve (AUI) of these isotherms is another index that can be used as an overall measure of the binding capacity in an effector-target system. The experimental values of BU and AUI determined from effector and target isotherms agree well with theoretical predictions based on our previously developed binding model (J. Immunol. Methods (1992) 155, 133-147). The relationship between BU and AUI, and procedures to determine these parameters are shown. The value of these indices to express effector-target conjugation quantitatively has been confirmed by determining the values of BU and AUI for the NK-K562 effector-target system.

Binding of alanine-substituted peptides to the MHC class I protein, Kd

Peptides eluted from the native MHC class I molecule, Kd, are generally nonamers that display a strong preference for Tyr in position 2. We investigated the molecular basis for this 'consensus motif' by synthesizing a virally derived peptide, NP 147-155, that is known to be presented by Kd on living cells, and peptide variants of NP 147-155 in which the amino acids in the different positions were sequentially replaced by Ala. All of the peptides bound to purified Kd molecules in vitro with high affinity, except for the peptide in which Tyr2 was replaced by Ala, for which the affinity for Kd decreased at least 100-fold. These results confirm the interpretation of the in vivo studies; namely, that Tyr2 is a critical anchor residue for binding to Kd.

Electron microscopic study on cell-to-cell interactions in oral lichen planus

Ultrastructural analysis of oral lichen planus was performed in 18 cases, focusing on cell-to-cell interactions. In the peripheral portion of the lesion, the most consistent findings were a widening of intercellular spaces, separation of the basement membrane (BM) from basal cells and scarce inflammatory cells. In the central portion of the lesions basal cells and BM showed severe damage and numerous inflammatory cells infiltrated into both the epithelium and subepithelial stroma. The infiltrates predominantly consisted of T-lymphocytes, a few Langerhans cells (LC) and macrophages. Most lymphocytes were large and positive for CD45RO. Dendritic LC and macrophages with cytoplasm containing abundant organelles were seen in the epithelium and subepithelial stroma, respectively. Close contacts of lymphocytes with LCs, macrophages and also with keratinocytes were sometimes observed in the central portion. In the subepithelial stroma, some lymphocytes contacted an HLA-DR+ dendritic cell, which was possibly a macrophage, forming a rosette-like arrangement. Conjugations between CD4 cells and dendritic cells (possibly LC) and also between CD8 cells and basal cells were observed in the epithelium. These T cells were large in size, and the CD8 cells which made contact with degenerated keratinocytes possessed cytoplasm containing numerous polarized organelles and a nucleus toward and contrary to the contact side, respectively. These lymphocytes expressed LFA-1 on the cell surface, and many basal cells exhibited ICAM-1. These findings indicate that T cells may receive information from LC and macrophages concerning degenerative keratinocytes, and that informed T cells attack perhaps the keratinocytes.

On the kinetics and optimal specificity of cytotoxic reactions mediated by T-lymphocyte clones

Using the chromium release assay and the single cell assay in agarose, we study the cytotoxic reaction of the MHC-restricted T lymphocyte clones P89:15 and P1:3, which recognize distinct but specific tumour antigens on the surface of syngeneic P815 mastocytoma cells. We propose a mathematical model which describes these experiments, accounts for the strongly non-Michaelian behaviour of the reaction and permits us to estimate the kinetic parameters characterizing effector-target conjugation and lethal hit delivery. The results show that the binding and lytic activity of effector cells is modulated by the number of targets bound to them. The binding of a second target by an effector having already a target bound is facilitated; on the other hand, an effector having bound two targets delivers a lethal hit more slowly than one with a single target bound. We investigate the role of these kinetic properties in the competition between the process of tumour progression due to cancer cell replication and the process of tumour regression due to T lymphocyte cytotoxic activity. For both clones, we estimate the effector-target ratio beyond which rejection prevails. This ratio is nine times larger for P1:3 than for P89:15. Furthermore, our analysis suggests that there exists an optimal specificity minimizing this ratio. Deviations from this optimum, be it in the sense of an increase or decrease of specificity, tends to stabilize the tumoural state: a situation which in the broader context of the immune response evolution and regulation can be viewed as an immune response dilemma.

Targeting of anti-tumor responses with bispecific antibodies

T cells can be induced to specifically lyse tumor cells with bispecific antibodies containing anti-T cell receptor mAbs crosslinked to anti-tumor mAbs. Such "targeted cytolysis" requires that the target cell be bound directly to the cytotoxic cell. In addition, targeted T cells mediate a second activity, the secretion of factors that can block the growth of both tumor target cells and bystander tumor cells. When given to nude mice bearing intraperitoneal human ovarian carcinoma, targeted human T cells cause the rapid removal of most tumor cells from the peritoneum, and markedly prolong the times of survival of treated mice. The efficacy of targeted T cells for treating human cancer is currently being tested in clinical trials.

Programmed cell death: concept, mechanism and control

Programmed cell death or apoptosis occurs under physiological conditions as a result of physiological effectors. It is a relatively slower process and requires active participation of the cell in the suicidal mechanism. Apoptosis is controlled by precise intrinsic genetic programme and may be induced by almost all those stimuli causing necrosis. The role played by the intensity in determining the death process and the underlying mechanism is imperfectly understood. Morphologically apoptotic cells appear as small condensed body. The chromatin is dense and fragmented, packed into compact membrane-bound bodies together with randomly distributed cell organelles. The plasma membrane loses its characteristic architecture and shows extensive blebbing. It buds off projections so that the whole cell may split into several membrane-bound apoptotic bodies. Significant chemical changes take place in the plasma membrane. This helps in recognition of the apoptotic bodies by phagocytes. At this moment it is unclear if all cells can undergo apoptosis or it is a characteristic of only some tissues which are predisposed to apoptotic death being directly under the control of hormones or growth factors. Experimental studies aimed at comparison of induction of apoptosis in cells of different origin are warranted to elucidate this point. Biochemically a pre-commitment step for induction of death programmation through macromolecular synthesis is essential for most systems. The double-stranded linker DNA between nucleosomes is cleaved at regular inter-nucleosomal sites through the action of a Ca2+, Mg(2+)-sensitive neutral endonuclease. Zinc is a potent inhibitor of the enzyme. Calcium probably plays a key controlling role in activation of the enzyme since prevention of Ca2+ increase prevents endonuclease activation. It is becoming evident that signal transduction through appropriate receptors control the Ca2+ flux in the cells. Most apoptotic cells require synthesis of RNA and proteins. Delay or abrogation of apoptosis by inhibition of macromolecular synthesis is well known. The dying cells show high mRNA levels for several enzymes. Several degradative enzymes become active. Regulatory proteins maintain control over the apoptotic cascade. At the molecular level, search has been initiated for the mammalian equivalents of the cell death (ced) gene. Activation of several specific genes is indicated. Specific expression of cell death-associated gene products (e.g. TRPM-2/SGP-2) has been reported in several unrelated apoptotic cell systems. Sequential induction of c-fos, c-myc and 70 kDa heat shock protein is reported. Studies demonstrate that certain genes must remain in a transcriptionally active demethylated state during programmed cell death. Recent evidences clearly indicate that apoptosis may be positively or negatively modulated by certain genes.(ABSTRACT TRUNCATED AT 400 WORDS)

Adhesion of activated T lymphocytes to vascular smooth muscle cells and dermal fibroblasts is mediated by beta 1- and beta 2-integrins

Several studies during recent years have demonstrated the potential for vascular smooth muscle cells (SMC) and dermal fibroblasts to participate in immune interactions such as antigen presentation and alloreactivity. The molecular interactions mediating lymphocyte adhesion to these mesenchymal cells have, however, not previously been characterized in detail. In the present study we demonstrate ICAM-1 (CD54) expression by cultured human SMC and its up-regulation by IL-1, IFN-gamma, and bacterial lipopolysaccharide. Monoclonal antibodies were used to define the molecular interactions in the adhesion of 51Cr-labelled T lymphoblasts to adherent SMC and fibroblasts. ICAM-1 appeared to mediate adhesion of T lymphocytes by binding to the beta 2-integrin CD11a/CD18 (LFA-1) expressed by the lymphoblasts. We present evidence for the involvement of at least three different mechanisms in the adhesion of activated T lymphocytes to cultured fibroblasts. It was found that beta 2-integrin-mediated interaction could only account for less than half of the binding activity. The remaining adhesion was partly mediated by beta 1-integrins, presumably via VLA-5 since an anti-VLA-5 antibody and an RGD-containing peptide blocked adhesion to the same degree. However, antibodies to beta 1-, beta 2-, and beta 3-integrin subunits added together only inhibited adhesion by approximately 50%. The residual adhesion could be blocked by inhibition of cell metabolism and was increased by stimulation of the lymphocytes with phorbol ester, suggesting involvement of other, as yet undefined, adhesion molecules. The molecular interactions between lymphocytes and mesenchymal cells demonstrated in this study may have implications in several inflammatory conditions such as vasculitis, atherosclerosis, and connective tissue diseases.

Epidermal keratinocytes of bullous pemphigoid express intercellular adhesion molecule-1 (ICAM-1)

Intercellular adhesion molecule-1 (ICAM-1) is the ligand for lymphocyte function associated antigen-1 (LFA-1), mediating the adhesion of lymphocytes to vascular endothelium. Keratinocytes are known to express ICAM-1 in some inflammatory dermatoses. Using an indirect immunofluorescence method, we examined the patterns of ICAM-1 and LFA-1 staining in bullous pemphigoid (BP) lesions and compared them to pemphigus vulgaris (PV) cases. ICAM-1 was expressed on the cell surface and in the cytoplasm of epidermal keratinocytes at the sites of erythematous and bullous lesions of BP. LFA-1 molecules were expressed on T cells at the basement membrane zone. In addition, HLA-DR-positive keratinocytes were observed in the basal layer. ICAM-1 was not, however, expressed on epidermal keratinocytes in uninvolved skin from BP patients, PV or normal control skin. It is known that ICAM-1 is expressed on keratinocytes at the site of lymphoid infiltration in cutaneous dermatoses and that LFA-1-positive T cells can bind to interferon gamma-induced ICAM-1-positive keratinocytes. Our results suggest that cellular immunity involving ICAM-1 and LFA-1 may play a part in the pathogenesis of bullous pemphigoid.

Increased expression of intercellular adhesion molecule 1 on bile ducts in primary biliary cirrhosis and primary sclerosing cholangitis

It has been suggested that immunological mechanisms involving lymphocyte-mediated damage are important in the characteristic bile-duct damage that occurs in primary biliary cirrhosis and primary sclerosing cholangitis. Because adhesion is necessary for the interaction of lymphocytes with their target structures, we have studied the expression of intercellular adhesion molecule 1, a ligand for the leukocyte adhesion receptor lymphocyte function-associated antigen 1 in the liver of patients with primary biliary cirrhosis and primary sclerosing cholangitis. Strong expression of intercellular adhesion molecule 1 was seen on interlobular bile ducts and proliferating bile ductules in both conditions. In primary biliary cirrhosis, medium-sized ducts, which are spared by the disease, were negative. Minimal bile-duct staining was seen in conditions in which bile-duct damage is not a major feature, such as nonbiliary cirrhosis and acute liver diseases. In patients with cirrhosis from any cause, strong expression of intercellular adhesion molecule 1 was detected on the periseptal hepatocytes adjacent to new connective tissue. The intensity of immunohistochemical staining was recorded using a semiquantitative visual scoring system that was subsequently validated quantitatively by confocal laser scanning microscopy. The expression/induction of intercellular adhesion molecule 1 on bile ducts may be important in the pathogenesis of bile-duct damage in primary biliary cirrhosis and primary sclerosing cholangitis and is further evidence to support an immune pathogenesis in these two conditions. Furthermore, the induction of intercellular adhesion molecule 1 on hepatocytes may be an important factor in the liver-cell damage and fibrosis that occur during the development of cirrhosis.

Cytotoxic T lymphocyte (CTL)-mediated cytolysis proceeds in the absence of Na+/H+ antiport activity: regulation of cytosolic pH by the Na+/H+ antiport in a cloned CTL

Cytotoxic T lymphocyte (CTL)-mediated cytolysis of specifically bound target cells (TC) is thought to be triggered by cross-linking the T-cell antigen receptor (TcR). Biochemical events associated with TcR cross-linking include increased intracellular calcium levels [Ca2+]i, hydrolysis of phosphatidylinositol (PI), and an increase in intracellular pH [pH]i. Whereas CTL-mediated cytolysis of some TC is calcium-dependent, and PI hydrolysis is speculated to trigger the CTL lethal hit via activation of PKC, little is known about changes in [pH]i relating to activation of the lethal hit stage. We report regulation of [pH]i in a cloned CTL by the electroneutral Na+/H+ antiport during activation with PMA and specific antigen-bearing TC. Furthermore, using 5-(N-methyl-N-isobutyl) amiloride (MIBA), a potent antiport inhibitor, we demonstrate that Na+/H+ exchange is not required for activation of CTL cytolytic activity.

T-cell-mediated cytotoxicity

There are two competing theories to explain the mechanism(s) by which cytolytic T lymphocytes kill target cells: granule exocytosis of a pore-forming protein, and contact-induced internal disintegration. Accumulated evidence supports alternative pathways in lymphocytoxicity, possibly reflecting distinct effector functions expressed by different killer cells and cells at different stages of activation.

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

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

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.

T cell targeting in cancer therapy

Targeting of immune cells by bispecific antibodies has proven a powerful tool for the investigation of cellular cytotoxicity, lymphocyte activation and induction of cytokine production, as well as to represent an innovative form of immunotherapy for the treatment of cancer. The hallmark of this approach is the use of the specificity of monoclonal antibodies to join target and immune cells by virtue of the dual specificity of bispecific antibodies for the two entities. More precisely the bispecific antibody has two different binding sites, which are capable of recognizing tumor associated antigens on the one hand and lymphocyte activation sites on the other. This process of crosslinking results in the activation of the lymphocyte and triggering of its lytic machinery, as well as lymphokine production. A major advantage of this therapeutic modality is, that use is made of the normal cellular immune defence system and therefore is only associated with minor toxicity. The distinct lymphocyte populations, which can be used for adoptive immunotherapy and the various bispecific antibody preparations, as well as the chimeric immunoglobulin/T cell receptor construction are the major topics of this review.

ICAM- melanoma cells are relatively resistant to CD3-mediated T-cell lysis

The primary activation pathway of T cells is via the T-cell receptor (TCR)/CD3 complex, which is functionally interrelated with various accessory molecules. We examined the contribution of the lymphocyte-function-associated antigen-I/intercellular adhesion molecule 1 (LFA-1/ICAM-1) interaction to CD3/TCR-mediated lysis by cytotoxic T lymphocytes (CTL). We used ICAM-I-or+ tumor cell lines as target cells and anti-CD3- or anti-LFA-1 containing hetero-cross-linked monoclonal antibody (MAb) to bridge CTL and target cells and simultaneously to activate CTL. The ICAM-1- melanoma-derived cell line IgR39 was relatively resistant to CD3-mediated lysis by both TCR alpha beta + and TCR gamma delta + CTL, when compared with ICAM-1+ cell lines. Induction of ICAM-1 on the membrane of IgR39 cells by tumor necrosis factor (TNF) rendered these cells more susceptible to CD3-mediated lysis. Anti-ICAM-1 MAb inhibited this TNF-enhanced susceptibility to lysis, directly demonstrating that the induction of ICAM-1 was critical in the TNF-induced increase in susceptibility to lysis of IgR39 cells. CTL formed less efficient conjugates with the ICAM-1- cells as compared to ICAM-1+ cells. Both spontaneous and CD3-induced conjugate formation as well as CD3-mediated lysis of ICAM-1- tumor cells by CTL were enhanced by the addition of anti-LFA-1 containing hetero-cross-linked MAb, thereby mimicking the LFA-1/ICAM-1 interaction between CTL and target cells. Soluble anti-CD18 MAb inhibited CD3-mediated lysis of ICAM-1- target cells by CTL without affecting their conjugate formation. Anti-LFA-1 MAb added after conjugate formation still inhibited lysis of both ICAM-1+or- tumor cells. Taken together, these findings suggest that the LFA-1/ICAM-1 interaction co-activates CD3/TCR-mediated lysis by CTL through both an enhanced CTL-target cell binding and the delivery of post-conjugate costimulatory signals.

T lymphocyte aggregation with immobilized anti-TCR-antibodies is dependent upon energy and microfilament assembly

An assay has been developed to quantitate the binding of beads coated with anti-T cell receptor (TCR) monoclonal antibodies (MoAb) to T lymphocytes. The Ab used were a hamster MoAb, 145.2C11 (2C11), directed against the epsilon chain of the CD3 complex of the murine TCR, and a murine MoAb, F23.1, directed against the V beta 8-encoded determinant of the alpha/beta heterodimer of the TCR. Ab were adsorbed onto polystyrene beads and the beads labeled with [125I]bovine serum albumin [( 125I]BSA). The labeled, Ab-coated beads were mixed at 4 degrees C with murine, cloned T-helper (Th) cells and contact between beads and cells was promoted by centrifugation. The mixtures were incubated at 37 degrees C for 10-20 min, and unbound beads were separated from cell-bound beads by Percoll gradient centrifugation. Beads coated with anti-TCR Ab formed stable conjugates with Th cells; an average of 6-10 2C11 Ab-coated beads/cell, or 10-15 F23.1 Ab-coated beads/cell was measured under optimal conditions. Beads coated with control Ab (hamster or mouse IgG) did not appreciably bind to the cells. Conjugation with 2C11 Ab-coated beads could be prevented by coating the cells with soluble 2C11 Ab, but not with soluble F23.1 Ab. Blocking the CD3 epsilon chain with soluble 2C11 Ab also reduced conjugate formation with F23.1 Ab-coated beads, suggesting a steric hindrance phenomenon. The extent of conjugation depended on the density of immobilized Ab. Maximum conjugation was observed when 100 micrograms of 2C11 Ab was used to coat 10(6) beads; higher Ab amounts did not further increase binding. Increasing the bead to cell ratio in the mixture increased binding, reaching optimal binding at 300:1, irrespectively of the amount of Ab adsorbed onto the beads. Stable binding of anti-TCR Ab-coated beads to T cells was temperature and energy dependent. It was prevented when glucose was removed from the medium and the glycolysis inhibitor, 2-deoxy-D-glucose was added, or when cells were treated with sodium azide. Conjugate formation was prevented by pretreatment of the cells with cytochalasins, indicating that microfilament assembly was essential. Microtubules were not involved, as vinca alkaloids were without effect. This novel assay system provides a simple means of studying aspects of TCR function including its physical and metabolic regulation.

Roles of adhesion molecules in T-cell recognition: fundamental similarities between four integrins on resting human T cells (LFA-1, VLA-4, VLA-5, VLA-6) in expression, binding, and costimulation

This review summarizes our recent work on expression and function of 4 integrins on resting human CD4+ T cells. Three themes are highlighted: multiplicity of molecular pathways of adhesion, regulation of adhesion, and costimulation by adhesion molecules. Four distinct receptor/ligand interactions have been elucidated: LFA-1/ICAM-1, VLA-5/fibronection, VLA-4/fibronectin, and VLA-6/laminin. Our studies indicate fundamental similarities in function and regulation of these four receptor/ligand interactions: 1) acute activation of the T cell (by CD3/TCR crosslinking or by PMA) induces rapid but transient integrin binding function; and 2) higher expression of each integrin on memory T cells compared to naive T cells results in greater binding of memory cells to each ligand. The identification of T-cell integrins (VLA-4, VLA-5, VLA-6) which interact with ECM components directs attention to the potential importance of T-cell interactions with ECM components which either may be immobilized in ECM or which may act as molecular bridges between cells. The existence of multiple adhesion pathways, of multiple ligands for a single receptor (such as LFA-1/ICAM-1 and LFA-1/ICAM-2), multiple receptors for a single ligand (such as VLA-4/FN and VLA-5/FN), and regulation of ligand expression (ICAM-1) provides opportunities for co-operativity, rebundancy and diversity which the T cell utilizes to exquisitely regulate its adhesive interactions. The thesis that adhesion molecules can be multifunctional receptors that also influence signalling is demonstrated by our findings that each of these integrin receptor/ligand interactions is capable of providing a potent costimulatory signal to CD3-mediated T-cell activation. The importance of interactions of T-cell integrins with their cell surface and ECM ligands is discussed with respect to T-cell migration, differentiation and recognition. Analysis of the precise mechanisms by which T cells regulate and exploit these multiple receptor/ligand interactions and the resulting functional consequences of those interactions will be exciting areas of future research.

Modulation of potassium channels in human T lymphocytes: effects of temperature

1. The predominant channels found in lymphocytes with patch-clamp whole-cell recordings are voltage-gated K+ channels. Several lines of evidence suggest that these channels are involved in lymphocyte function. Most lymphocyte functions are temperature sensitive and have not been correlated with electrophysiology at different temperatures. We have examined the effect of temperature on the voltage-dependent K+ channel in normal human T lymphocytes. Both macroscopic current and single-channel events were studied with whole-cell recordings at temperatures from 5 to 42 degrees C. 2. Peak conductance, activation rate, inactivation rate and rate of recovery from inactivation all increased progressively as the temperature increased. The effect of temperature on channel opening processes was greater at low temperatures. In contrast, the inactivation process was most sensitive to temperature changes above room temperature. Arrhenius plots of conductance and kinetic parameters were curvilinear with no obvious break-points. 3. The increase in whole-cell conductance at 37 degrees C was due to both an increase in the single-channel conductance and in the probability that each channel is open at any time. 4. K+ currents were fitted by Hodgkin-Huxley equations with n4j kinetics providing the best description of the currents at all temperatures tested. 5. Steady-state activation- and inactivation-voltage curves shifted in opposite directions with warming, resulting in a greater area of overlap of the curves ('window' current). The increase in resting K+ channel activity predicted by a greater window current was confirmed with single-channel measurements. 6. The present study has shown that the behaviour of K+ channels in human T lymphocytes is temperature dependent.

Metabolic inhibitors distinguish cytolytic activity of CD4 and CD8 clones

The effect of various metabolic inhibitors on the expression of cytolytic activity of CD4 (TH1) and CD8 (CTL) clones was studied. The cytolytic activity of CD4 clones, but not CD8 clones, was sensitive to the RNA synthesis inhibitor actinomycin D and the protein synthesis inhibitor cycloheximide. Conversely, cholera toxin (CT) inhibited cytolytic activity of CD8, but not CD4 clones. Both mitomycin C, a DNA synthesis inhibitor, and cyclosporin A (CsA) failed to inhibit the cytolytic activity of either CD4 or CD8 clones. Although pretreatment with CsA or CT did not inhibit the cytolytic activity of CD4 clones, lymphokine (interleukin 2, IL2, interferon-gamma, IFN-gamma, and tumor necrosis factor, TNF) production was strongly inhibited. Similarly, pretreatment of a CD8 clone with actinomycin D or CsA inhibited lymphokine production without affecting cytolytic activity. The production of mRNA for TNF and IFN-gamma by concanavalin A-activated CD4 clones was also inhibited by CsA and CT. Moreover, perforin-specific mRNA was not detected in activated CD4 clones. Collectively, these observations demonstrated that de novo synthesis of RNA and protein is required for expression of cytolytic activity of CD4 clones, yet production of TNF, INF-gamma, IL 2 and perforin is not involved. In contrast, the cytolytic machinery of CD8 clones is present prior to activation and is quickly expressed following activation even when de novo synthesis of RNA, protein and lymphokines is blocked.

T-cell receptor cross-linking transiently stimulates adhesiveness through LFA-1

Effective interaction between T cells and their targets requires that recognition of specific antigen be coordinated with increased cell-cell adhesion. We show that antigen-receptor cross-linking increases the strength of the adhesion mechanism between lymphocyte function-associated molecule-1 (LFA-1) and intercellular adhesion molecules (ICAMs), with intracellular signals transmitted from the T-cell antigen receptor to the LFA-1 adhesion molecule. The increase in avidity is rapid and transient, providing a dynamic mechanism for antigen-specific regulation of lymphocyte adhesion and de-adhesion.

Interplay between the TCR/CD3 complex and CD4 or CD8 in the activation of cytotoxic T lymphocytes

Interactions between CTL and target cells occur in the absence of specific antigen recognition and precede subsequent interaction of the TCR with its specific antigen. This antigen-independent adhesion progresses through two different pathways, one involving the interaction of CD2 with LFA-3 on the target cell, the second the interaction of LFA-1 with ICAM-1. Such antigen-independent adhesions are critical for the activation of T cells via the TCR. Also, CD4 and CD8 can serve as adhesion molecules by binding to monomorphic determinants expressed on class II and class I MHC antigens, respectively, on the target cells, but compared to LFA-1 and CD2 antigens their contribution to conjugate formation is minor. CD4 and CD8 are required for effective T-cell activation in situations where the intrinsic affinity of the TCR or antigen expression is low, suggesting that CD4 and CD8 enhance the avidity of T cells for target cells by binding to class II and class I antigen, respectively. However, CD4 and CD8 are also involved in post-binding events that lead to CTL activation and subsequent lysis of the target cells. On the other hand, blocking of anti-TCR/CD3 mAb-induced CTL reactivity by anti-CD4/CD8 mAbs does not necessarily involve an interference with the binding of CD4 and CD8 to their respective ligands and it has been proposed that the TCR and CD4 or CD8 form functional complexes that are required for optimal T-cell activation. It is still unclear whether blocking by anti-CD4/CD8 mAbs is based on the prevention of complex formation of the TCR with CD4 or CD8, since formation of such complexes has yet to be demonstrated. The alternative hypothesis, that anti-CD4/anti-CD8 mAbs can directly confer negative regulatory signals to the CTL is not supported by our studies with antibody-directed lysis mediated by a CD4+, CD8+ CTL clone. Anti-CD4/CD8 mAbs can also inhibit T-cell cytotoxicity induced by other T-cell surface activation antigens such as CD2 or Tp103. In these situations, the triggering may involve signals transferred via CD3 requiring functional CD3/CD8 or CD3/CD4 complexes. Although most studies investigating the sequence of events leading to T-cell activation are carried out with CTL, preliminary data indicate that the same mechanisms described here for CTL activation are probably also valid for the interactions of T-helper cells with APC or B cells.(ABSTRACT TRUNCATED AT 400 WORDS)

CD4 expression and function in HLA class II-specific T cells

The CD4 molecule is currently the object of intense interest and investigation both because of its role in normal T-cell function, and because of its role in HIV infection. Our studies of CD4 expression and function on class II-specific T cells indicate that: 1) CD4 expression is generally a marker of T cells that are committed to class II antigen recognition, even when they have cytotoxic function; 2) CD4 is typically involved in the proliferative and cytotoxic function of such cells, which we hypothesized to be due to interaction with a nonpolymorphic determinant on class II; 3) the importance of CD4 to the recognition is most prominent in T-cell interactions with low "avidity"; and 4) although CD4 may play a role in adhesion per se, the majority of its function seems to be in a post-adhesion phase of cytotoxic T-cell recognition.

Histamine regulates the generation of human cytolytic T lymphocytes

Human cytolytic T lymphocytes (CTL) were generated in the presence and absence of histamine in order to define the role of this autacoid in immune regulation. Histamine (10(-8)-10(-4) M) suppressed the generation of class I specific CTL but, at 10(-4) M, actually increased class II specific cytolysis. Histamine acted at the level of CTL generation; histamine was not present in the cytolytic assay. When histamine was added to the cytolytic assay with CTL grown without histamine, the lytic ability of the effector cells was similar to that of controls. Histamine-induced suppression of class I specific cytolysis was blocked by continuous culture with the H2 antagonist ranitidine but not with the H1 antagonist pyrilamine. These data suggest that suppression was mediated by the H2 receptor. Continuous culture with histamine had no effect on T cell proliferation or the expression of cell surface molecules. Histamine-induced suppression of class I specific cytolysis was reversed by the addition of PHA to the cytotoxicity assay, showing that the cytolytic machinery was intact. These data provide evidence that histamine is involved in regulation of cytolytic T cells.

Intercellular adhesion molecule 1 is induced on isolated endocrine islet cells by cytokines but not by reovirus infection

The selective destruction of the pancreatic islet beta cells in type 1 diabetes mellitus is thought to be mediated by a cellular autoimmune process, possibly triggered by virus infection in genetically susceptible individuals. Because of the potentially important role of cell-cell adhesion in the immune response, we investigated whether cytokine products of mononuclear cells, or virus infection, induced the expression of intercellular adhesion molecule 1 (ICAM-1) on human endocrine islet cells. By flow cytofluorimetry, control islet cells did not express detectable ICAM-1. However, after a 72-hr exposure of islets to interferon gamma (IFN-gamma) and/or tumor necrosis factor alpha (TNF-alpha) (each at 250 units/ml), ICAM-1 was induced on greater than 85% of islet cells. IFN-gamma was 50% more potent than TNF-alpha; together, their effects were additive. Class I major histocompatibility complex (MHC) protein expression, detected on control islet cells, was also stimulated by IFN-gamma and/or TNF-alpha. In contrast, infection with reovirus type 3 did not induce ICAM-1 on islet cells, although it stimulated the expression of class I MHC proteins. By double-label indirect immunofluorescence microscopy, ICAM-1 expression was identified on both beta (insulin-secreting) and delta (somatostatin-secreting) islet cells. Monoclonal antibody to ICAM-1 precipitated protein of Mr 97,000 from [35S]methionine-labeled islets exposed to IFN-gamma and TNF-alpha, but not from control islets. RNA blot analysis revealed a major species of 3.3 kilobases and a minor species of 2.2 kilobases induced in islets exposed to the cytokines. These findings have implications for the molecular mechanisms of beta-cell destruction in type 1 diabetes, in that expression of ICAM-1 by beta cells may facilitate adhesion of antigen-targeted immune cells.

Different class I antigen oligosaccharides on a murine tumor and a lectin-resistant variant are not responsible for the differential recognition of the tumors by CTL

Previous studies have shown that whereas a highly malignant mouse cell line termed MDAY-D2 (d haplotype) does not elicit a detectable response by cytotoxic T lymphocytes (CTL) in DBA/2 mice, strong anti-tumor CTL are generated against a wheat-germ-agglutinin-resistant variant, designated MDW3. Additional evidence suggests these anti-MDW3 CTL may not be a consequence of a unique antigenic determinant on the variant cells. Because MDW3 cells are expected to differ from MDAY-D2 cells in their surface carbohydrate structures (due to their lectin resistance) and Class I major histocompatibility molecules play a crucial role in CTL-mediated responses, we speculated that the Asn-linked oligosaccharides present on Class I molecules of MDAY-D2 and MDW3 might be different and could potentially influence recognition analyses and Con A-Sepharose affinity chromatography clearly demonstrated that the oligosaccharides isolated from the H-2Dd molecule of MDAY-D2 cells are larger and more highly branched than those of the MDW3 variant. Taken together with the finding that anti-MDW3 CTL are restricted by H-2Dd, these results suggested that the larger H-2Dd oligosaccharides on MDAY-D2 cells could potentially mask or perturb determinants required for recognition by these CTL. To test this postulate, the surface Class I oligosaccharides of both MDAY-D2 and MDW3 cells were converted to simpler hybrid structures by treatment with the oligosaccharide processing inhibitor, swainsonine. However, no effect was observed on the lysis or binding of either MDAY-D2 or MDW3 cells by anti-MDW3 CTL. Thus, the results do not support the possibility that the larger H-2Dd oligosaccharides on MDAY-D2 cells are, in themselves, responsible for the poor recognition of the parent tumor by anti-MDW3 CTL. Our data do indicate, however, that CTL target binding and effector functions are not dependent on the fine structure of complex Asn-linked carbohydrates present on Class I molecules and possibly on other, accessory molecules at the target cell surface, since MDW3 cells maintained their sensitivity to lysis by CTL following swainsonine treatment.

Kinetics and characterization of intercellular adhesion molecule-1 (ICAM-1) expression on keratinocytes in various inflammatory skin lesions and malignant cutaneous lymphomas

The kinetics of expression of the intercellular adhesion molecule-1 (ICAM-1) were studied on keratinocytes in skin biopsy specimens of sensitive persons in whom the haptens were applied in a standardized format for allergic contact dermatitis testing. There was no ICAM-1 expressed on keratinocytes of normal skin; ICAM-1 was induced as early as 4 hours after the application of the patch in some subjects. By 48 hours after the application of the patch, all specimens contained ICAM-1-positive keratinocytes. This was concurrent with a heavy mononuclear cell dermal infiltrate and maximum clinical manifestations. Expression of human lymphocyte antigen (HLA)-DR or other inducible surface proteins on keratinocytes under these conditions was much less frequent. When specimens from primary irritant dermatitis were used, only 1 of 14 cases had keratinocytes expressing ICAM-1 at 48 hours, the time of maximum clinical manifestation. Among benign inflammatory lesions, most cases resembled the allergic patch test specimens in that ICAM-1 was expressed to a large degree on keratinocytes. Again, the expression of HLA-DR was variable. Malignant skin lesions, on the other hand, were much less consistent and generally lower in terms of ICAM-1 expression on keratinocytes. Furthermore, in contrast to the benign cutaneous conditions, some malignant skin lesions contained keratinocytes that expressed class II antigens or other inducible surface proteins in the absence of ICAM-1. These data suggest that ICAM-1 plays a role in the specific immune response by facilitating either antigen presentation or lymphocytic infiltration.

A monoclonal antibody to LFA-3, the CD2 ligand, specifically immobilizes major histocompatibility complex proteins

T cells are activated when the antigen-specific T cell receptor recognizes antigen in association with major histocompatibility complex (MHC) proteins. The T cell surface protein CD2 (T11, LFA-2, the T erythrocyte receptor) and its target or stimulator cell ligand, lymphocyte function-associated antigen-3 (LFA-3), are also involved in T cell adhesion and activation. The molecular mechanisms by which the CD2/LFA-3 interaction affects T cell adhesion and activation are unclear. The CD2/LFA-3 interaction may be modeled by the interaction between LFA-3 and anti-LFA-3 monoclonal antibody (mAb). We used the fluorescence photobleaching recovery technique to investigate the effect of anti-LFA-3 mAb on the lateral mobility of MHC proteins in plasma membranes of JY, a human Epstein-Barr virus-transformed B cell line. Anti-LFA-3 mAb induced immobilization of class I MHC proteins labeled with bivalent but not monovalent fluorescein-conjugated W6/32 mAb. Anti-LFA-3 mAb also caused immobilization of class II MHC proteins labeled with bivalent fluoresceinated LB3.1 mAb. In contrast, anti-LFA-3 mAb did not affect the mobilities of either a B cell membrane protein labeled with bivalent fluoresceinated anti-CD45 (human leukocyte antigen) mAb or a membrane lipid analogue. Unlike anti-LFA-3 mAb, anti-LFA-1 mAb did not affect class I MHC protein mobility. These results suggest that CD2 binding to LFA-3 may trigger a physiological response in which target cell MHC proteins, cross-linked by receptors on the T cell surface, are immobilized at and thereby localized to the T cell-target cell interface.

Intercellular adhesion molecule-I (ICAM-I) expression correlated to inflammation

The presence of intercellular adhesion molecule-I (ICAM-I) on keratinocytes of psoriatic skin lesions before and during 8-methoxapsoralen and UVA light (PUVA) treatment was studied. ICAM-I was expressed on the keratinocytes in biopsies of the skin lesions of five patients with psoriasis. The patients who responded to PUVA treatment had a concurrent reduction of ICAM-I expression on the keratinocytes with a reduction of the number of cells in the mononuclear cellular infiltrate and a lessening of the severity of the disease. Patients who went into remission during therapy and then relapsed showed an increase in ICAM-I expression on keratinocytes with an increase in the number of cells in the mononuclear cell infiltrate and an increase in the severity of the disease. HLA-DR expression on keratinocytes was variable during treatment and showed no strong correlation with disease severity.

Kinetic analysis of lymphokine-activated killer (LAK) cells: lytic parameters and determination of LAK cell frequency

Kinetic analysis was used to define lytic events in murine lymphokine-activated killer (LAK) cell-mediated tumour cell lysis. The maximum rate of target cell lysis (Vmax) and Km (target cell number resulting in 1/2 Vmax) were determined. Single cell lytic assays demonstrated that only LAK effector cells bound to target cells (i.e. non-lytic, bystander lymphocytes did not influence the determination of kinetic parameters) in contrast to natural killer (NK) cell lysis. This finding allowed for LAK cell frequency determinations where Km approximates the concentration of lytic LAK effector cells within a given number of lymphocytes. Frequencies determined in this manner were not significantly different from those obtained using the more cumbersome single cell lytic assay. Furthermore, frequencies determined for the same lymphocyte population against four different NK-resistant tumour targets, that varied in their sensitivity to LAK cell lysis, were not significantly different. In addition, LAK cell lytic programming of target cells was found to be the rate limiting lytic event. This study provides a means of determining reliable estimates of LAK cell frequencies within a lymphocyte population, which will be useful in studies evaluating LAK cytolytic mechanisms and the effects of drugs, biological response modifiers, or disease states on LAK cell lytic activity.

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.

The cytolytic T lymphocyte and its mode of action

While the binding step of cytolytic T lymphocyte (CTL) target cell interaction resulting in conjugate formation is a well-characterized event, there seems to be more than one mechanism whereby lymphocytes kill the target. In recent years, infliction of complement (C)-like "holes" (I.D. 10-20 nm) on the target cell membrane, believed to be produced by the Ca2+-dependent lytic protein(s) perforin/cytolysin of secretory lytic granule origin has been proposed to be the mechanism of lymphocytotoxicity. More recent evidence, however, suggests that Ca2+-dependent exocytosis of lytic granules (where detectable) is not involved in lymphocyte-mediated cytolysis. Furthermore, neither formation of C-like "holes" in targets exposed to CTL, nor higher-than-background levels of lytic granules, perforin or BLT-esterases, have been detected in highly potent, peritoneal exudate CTL (PEL) derived directly from the animal or in cytocidal PEL-hybridomas. Hence exocytosis of perforin and formation of the above pores may apply to certain effector cells, particularly those grown in vitro in IL-2, but not to in vivo primed CTL such as PEL. On the other hand, work from this laboratory with Ca2+ probes has shown that lysis induced by CTL such as PEL-not involving lytic granules, perforin or formation of the above "holes"-is preceded by a marked prelytic elevation of cytosolic Ca2+ in the target. CTL-induced target cell membrane perturbation--a direct result of receptor-mediated effector-to-target interaction or through a membrane-bound or secreted effector component(s)--may be responsible for triggering the prelytic influx of Ca2+ from external sources, or its mobilization from internal stores in the target. We propose that CTL-induced, persistent elevation of cytosolic Ca2+, above a critical level, rather than formation of 10-20 nm pores, is responsible for the catastrophic prelytic events observed in the target, such as bleb formation, metabolic exhaustion and DNA degradation, ultimately leading to lysis.