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

Human T cell antigens involved in cytotoxicity against allogeneic or autologous chemically modified targets. Association of the Leu 2a/T8 antigen with effector-target cell binding and of the T3/Leu 4 antigen with triggering

Monoclonal antibodies (mAb) recognizing human T cell differentiation antigens were employed to analyze the role of these antigens on T cell-mediated cytotoxicity against autologous 2,4,6-trinitrophenyl (TNP)-modified targets. The OKT3/anti-Leu 4 and anti-Leu 2a/OKT8 mAb inhibited T cell-mediated cytotoxicity against autologous or unrelated TNP-modified targets, in the absence of complement and at the effector cell level. These cytotoxic effector cells were T3+, T8+, T11+, T4-. To analyze the role of the T3/Leu 4 and Leu 2a/T8 T cell differentiation antigens in the cytolytic process, we investigated the stages of this process that were inhibited by the OKT3/anti-Leu 4 and anti-Leu 2a/OKT8 mAb. Using: (a) visual adhesions, and (b) a single cell cytotoxicity assay in agarose, we observed that the OKT3 and anti-Leu 4 mAb did not inhibit binding of effector cells to allogeneic targets or to autologous E rosette-negative TNP-modified targets, although they significantly inhibited both the proportions of target cells in conjugates that were lysed, and the 51Cr release. In contrast, the anti-Leu 2a and OKT8 mAb blocked cytotoxicity by inhibiting binding of effector cells to the allogeneic or to autologous chemically modified targets. To further analyze the stages of the cytolytic process (adhesion; programming for lysis or lethal hit; and cytotoxic cell-independent lysis) that were inhibited by these mAb, we employed the detachment and dispersion method. This method is based on the differential temperature requirements of adhesion (which occurred both at 37 degrees C or 20 degrees C) and of programming for lysis (which occurred at 37 degrees C but not at 20 degrees C). Operational adhesions were determined by the 51Cr-release assay after dispersion of effector and target cells in a 10% solution of dextran (mol. wt. 500 000). Programming for lysis was determined by the 51Cr-release assay after detachment of effector-target cell conjugates with 10 mM EDTA and dispersion in 10% dextran solution. Using this method we determined that mAb recognizing the Leu 2a/T8 antigen blocked cytotoxicity by inhibiting adhesion and binding of effector cells to target cells. These antibodies do not affect post-adhesion stages of the cytolytic process. In contrast, the OKT3 and anti-Leu 4 mAb inhibit a post-adhesion step of the cytolytic process, that occurs before irreversible events of the programming for lysis stage have taken place.(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium current in clonal cytotoxic T lymphocytes from the mouse

The electrical properties of the cell membrane of clonal cytotoxic T lymphocytes in the mouse were studied by using the whole cell variation of the patch electrode voltage-clamp technique. Outward currents were activated with an exponential time course of several milliseconds time constant when the membrane potential was made more positive than -50 to -40 mV. This current is not activated as a result of Ca2+ entry. The estimated reversal potential of the current indicates that the current is predominantly carried by K+. The activation kinetics depend only on membrane potential, not on [K+]0. The amplitude of the current decreases exponentially with time constants of several hundred milliseconds during a maintained voltage pulse, due mainly to a decrease in conductance. Recovery from inactivation roughly followed a single exponential time course with a time constant of tens of seconds; this time constant depended upon not only the membrane potential but also the amount of initial inactivation. The current is suppressed by quinidine and tetraethylammonium, their half-suppression concentrations being 23 microM and 14 mM respectively. An increase of the outward current is suggested to be associated with the lethal hit of the cytotoxic reaction.

Inhibition of activity of human NK and K cells by simple sugars: discrimination between binding and postbinding events

A variety of sugars were tested for their ability to inhibit the lytic activity of highly purified populations of human natural killer (NK) cells (large granular lymphocytes [LGL] ). Studies were also performed to determine whether inhibitory sugars were active at the level of recognition and binding to target cells (as determined by conjugate formation) or at a postbinding lytic stage. Mannose-6-PO4 and galactose-6-PO4 demonstrated strong and consistent inhibition of NK cytolysis at 50 mM concentration, while nonphosphorylated analogs were at most minimally effective. The inhibitory phosphorylated sugars did not block conjugate formation, indicating that the sugars affected some postbinding event rather than recognition of target cells by NK cells. The inhibition of NK activity by some sugars was not paralleled by inhibition of antibody-dependent cellular cytotoxicity (ADCC) activity by LGL. This suggests some divergence in the lytic mechanisms for NK and ADCC.

Possible involvement of the T4 molecule in T cell recognition of class II HLA antigens. Evidence from studies of CTL-target cell binding

The present study examines the potential role of the T4 molecule in functional cell-cell interactions between target cells and human cytotoxic T lymphocyte (CTL) clones that are specific for HLA class II alloantigens encoded by the SB locus. There were marked differences (greater than 30-fold) between the seven SB-specific clones studied with respect to their susceptibility to inhibition by anti-T4 as well as anti-T3 antibodies. We wished to test the hypothesis that such variation among the clones would be due to differences in clonal "affinity" for antigen. To quantitate differences among the CTL clones in the tightness with which they bind target cells, the clones were analyzed using a previously published assay of susceptibility of CTL-target cell conjugates to dissociation in the presence of unlabeled targets. The results revealed that the clones that were most susceptible to inhibition by anti-T4 and anti-T3 were the weakest target cell binders, and vice versa. Anti-T4 antibody could partially induce dissociation of functional CTL-target cell conjugates in the absence of any added cold targets. For the "highest affinity" clone such anti-T4 antibody-induced dissociation could be observed at 4 degrees C but not 23 degrees C. These results indicate that the T4 molecule is functionally involved in target cell binding by CTL, and raise the possibility that although it is easiest to demonstrate the function of the T4 molecule in "low affinity" clones, that function may also be operative in the "high affinity" clones.

Human lymphocyte function-associated antigens

Three cell surface molecules, designated LFA-1, LFA-2, and LFA-3 were identified by mAbs selected for their ability to block cytolysis by an OKT4+, HLA-DR-specific CTL line. The LFA mAbs block all CTL and proliferative functions studied. In addition, anti-LFA-1 mAbs inhibit NK-mediated cytolysis. By analogy with murine LFA-1, human LFA-1 may be involved in the adhesion stage of cellular interactions. LFA-2, the SRBC receptor molecule, appears to be a T cell function-specific molecule. We have not yet established whether LFA-2 participates in antigen recognition or whether it is involved in antigen-non-specific interactions. The anti-LFA-3 mAb specifically blocks function by binding to the target cells, implying that LFA-3 may be a target ligand for an effector-specific receptor. The CTL-target interaction involves a number of steps, including antigen recognition, cell adhesion, and delivery of the lethal hit [22]. The LFA antigens show the complexity of this process at the molecular level. The anti-LFA monoclonal antibodies will be useful probes into the T cell immune response and may prove clinically relevant, both diagnostically and therapeutically.

Direct transfer of antigen-specific cytolytic activity to noncytolytic cells upon fusion with liposomes derived from cytolytic T cell clones

Murine cytolytic T lymphocytes (CTL) clones were solubilized in Nonidet P-40 detergent, and the matrix and membrane proteins separated from the nuclear constituents. These proteins, in combination with exogenous lipids and Sendai virus envelope proteins, were used to construct liposomes that were then fused with noncytolytic cloned T cell recipients. The resultant fusion products were found to be highly cytolytic and appeared to express the same specificity as the original donor clone. Further analysis showed that the liposomal transfer process was extremely efficient. Moreover, in addition to noncytolytic T cell clones, three transformed T cell lines and one B cell line were found to express specific cytolytic activity after fusion with appropriate liposomes. Inhibition experiments using monoclonal antibodies against target cell antigens, as well as analysis of the lytic reactivity pattern of the fusion products, confirmed the high degree of specificity conferred to the recipient cells. This study thus indicates that the two characteristics typically associated with CTL, namely antigen-specific recognition and cytolytic activity, can be solubilized from CTL and transferred to recipient cells that do not express these characteristics.

A human leukocyte differentiation antigen family with distinct alpha-subunits and a common beta-subunit: the lymphocyte function-associated antigen (LFA-1), the C3bi complement receptor (OKM1/Mac-1), and the p150,95 molecule

The human lymphocyte function-associated antigen-1 (LFA-1), the complement receptor-associated OKM1 molecule, and a previously undescribed molecule termed p150,95, have been found to be structurally and antigenically related. Each antigen contains an alpha- and beta-subunit noncovalently associated in an alpha 1 beta 1-structure as shown by cross-linking experiments. LFA-1, OKM1, and p150,95 alpha-subunit designations and their molecular weights are alpha L = 177,000 Mr, alpha M = 165,000 Mr, and alpha X = 150,000 Mr, respectively. The beta-subunits are all = 95,000 Mr. Some MAb precipitated only LFA-1, others only OKM1, and another precipitates all three antigens. The specificity of these MAb for particular subunits was examined after subunit dissociation by high pH. MAb specific for LFA-1 or OKM1 bind to the alpha L- or alpha M-subunits, respectively, while the cross-reactive MAb binds to the beta-subunits. Coprecipitation experiments with intact alpha 1 beta 1-complexes showed anti-alpha and anti-beta MAb can precipitate the same molecules. In two-dimensional (2D) isoelectric focusing-SDS-PAGE, the alpha subunits of the three antigens are distinct, while the beta-subunits are identical. Biosynthesis experiments showed alpha L, alpha M, and alpha X are synthesized from distinct precursors, as is beta. The three antigens differ in expression on lymphocytes, granulocytes, and monocytes. During maturation of the monoblast-like U937 line, alpha M and alpha X are upregulated and alpha L is downregulated. Some MAb to the alpha subunit of OKM1 inhibited the complement receptor type three. LFA-1, OKM1, and p150,95 constitute a novel family of functionally important human leukocyte antigens that share a common beta-subunit.

The mechanism of cell-mediated cytotoxicity. III. Protease-specific inhibitors preferentially block later events in cytotoxic T lymphocyte-mediated lysis than do inhibitors of methylation or thiol-reactive agents

Highly active mouse cytotoxic T lymphocytes (CTL) generated in secondary mixed-lymphocyte responses were used to examine the manner in which adenosine derivatives, thiol-specific reagents, or protease-specific probes affected CTL-mediated lysis (CML). The adenosine deaminase inhibitor deoxycoformycin (dCF) enhanced inhibition by adenosine (AR) or by deoxyadenosine (AdR), but not by 7-deazaadenosine (tubercidin). L-Homocysteinethiolactone (L-Hcy) acted synergistically with AR, but not with AdR or tubercidin, to block CML. Thus, AR derivatives may act both by affecting cellular methylation reactions, as demonstrated by the synergism between AR and L-Hcy, and by inhibiting other events required for CML. Conditions were then established to determine whether these reagents preferentially affected either the Ca2+-independent initial stage of cytolysis or the subsequent Ca2+-dependent events. Methylation inhibitors blocked lysis most effectively if added before effector-target binding. Similarly, the nonpenetrating thiol-specific reagent quaternary ammonium monobromobimane (qBBr) was more inhibitory when added prior to the Ca2+-dependent stage. Protease inhibitors such as alpha-1-antichymotrypsin and protease substrates such as acetyltyrosine ethyl ester (ATEE) or tyrosine ethyl ester (TEE) also inhibited CML. But, in contrast to qBBr or methylation inhibitors, neither TEE nor ATEE was more effective when added prior to the initial effector-target interaction. Furthermore, TEE did not appreciably affect CTL binding to target cells at concentrations that nearly abrogated CML. Thus, the implicated protease step is unique in that it does not appear to participate in recognition or binding.

Attachment of myelin to coated pits on macrophages in experimental allergic encephalomyelitis

In experimental allergic encephalomyelitis (EAE) invading macrophages appear to destroy the antigen-containing region of the plasma membrane of the myelinating cell, leaving the cell body intact. We report here that during myelin destruction and phagocytosis, myelin lamellae are attached to coated pits on the macrophage surface. As coated pits are sites where receptor-ligand complexes concentrate prior to endocytosis, we conclude that ligand-mediated phagocytosis may explain the targeted activity of macrophages in autoimmune demyelination.

Recognition specificities, development, and possible biological function of natural killer cells in the mouse. I. Spleen focus forming assay for natural killer activity and analysis of lectin-like recognition structures on the surface of murine natural killer cells

A number of simple sugars have been tested and found to be effective in blocking lysis of YAC-1 tumor target cells by nonimmune murine natural killer (NK) effector cells. Using a spleen fragment culture system an assay has been developed which allows us to compare the inhibition of lysis observed in replicate culture wells prepared from cells contained in one spleen fragment (less than or equal to 1 X 10(6) cells). The inhibition pattern of any well was found to fall naturally into 1 of 25 (of the total 128 possible tested) patterns. Using this panel analysis of NK activity in individual mice of the same or different strain has been compared. Our data suggest that within any given strain the inhibition pattern of NK effector cells is quite uniform. Consistent differences are seen between strains which are interpreted in terms of a genetic control of the final expression of the NK recognition repertoire. In adult F1 hybrid individuals the pattern of recognition by NK cells is best considered a result of the codominant expression of genes contributed by each parent.

Characterization of antibody-mediated inhibition of natural killer (NK) cytotoxicity: evidence for blocking of both recognition and lethal hit stages of cytolysis

Rat antisera prepared against murine, periodate-activated alloimmune cytotoxic lymphocytes (termed RAT) have previously been shown to effectively block T-cell-mediated cytotoxicity (CMC) at the "lethal hit" stage of cytolysis (J.C. Hiserodt and B. Bonavida, J. Immunol. 126, 256, 1981). Both natural killer (NK) and cytotoxic T lymphocytes (CTL) have been shown to mediate lysis by the same pathway, namely binding of effector to target cells, programming for lysis, and killer cell-independent target cell lysis. This result suggested that the molecular mechanism of NKCMC and CTLCMC may also be similar. In this context, RAT-mediated blocking of CTL was examined for its ability to block NKCMC. The results show that (1) addition of RAT serum or IgG fractions blocked NKCMC in the absence of complement in a 4-hr 51CR-release assay, and blocking was directed at the effector cell; (2) at the single-cell level, RAT serum blocked the formation of conjugates between effector and target cells; (3) in a Ca2+-pulse experiment, in which the effectors and targets were first allowed to bind in the absence of Ca2+ for 1 hr at 37 degrees C, followed by the addition of Ca2+ to initiate the lytic event, RAT was capable of blocking cytotoxicity after conjugate formation at the Ca2+-dependent lethal hit stage of cytolysis. The similarity of results in RAT blocking experiments of both the CTL and NK systems suggests a common molecular mechanism of cytolysis.

Cytolysis by H-2-specific T killer cells. Assembly of tubular complexes on target membranes

Cloned T killer cells derived from one-way mixed lymphocyte reactions were characterized with regard to their Lyt phenotype and specificity. Two clones of Lyt-1-2+ phenotype that recognized H-2Dd were selected and examined for their cytolytic function by negative staining and thin section electron microscopy. When incubated with the H-2d target S194, they assemble two types of tubular complexes, polyperforin 1 and 2. Both structures appear to arise by polymerization of precursors that may originate in dense granules and/or Golgi of T killer cells. Polyperforins appear to be associated with vesicles that are released during the lytic reaction and transferred to target membranes as shown by immunoelectron microscopy. Since there is a close correlation between target lysis and the appearance of polyperforins on target membranes, it is suggested that polyperforins take part in the cell-mediated killing reaction. Although polyperforins are different in size and several molecular properties from complement, there are striking similarities between these circular complexes and polyperforin (C9). It is therefore possible that they belong to a closely related family of cytolytic effector molecules.

T cell-mediated cytolysis: on the strength of effector-target cell interaction

Allosensitized lymphoid cell populations contain T lymphocytes that can bind to target cells and lyse them. We asked whether there was a relationship between lymphocyte target cell-binding strength and occurrence of cytolysis. Using graded shear forces to dissociate effector-target cell conjugates, we found that (a) within an allosensitized lymphoid cell population derived from an heterogeneous mixed leukocyte culture, there were lymphocyte-target cell conjugates with binding strengths differing by a factor of at least 10(2), (b) even the minimal force required to release a significant amount of bound target cells could disrupt the plasma membranes of some tumor cells and (c) these tumor cells disrupted by shear forces were probably part of cytolysis-conducive rather than of non-cytolysis-conductive conjugates. We combined this approach with the use of cytolysis-inhibiting monoclonal antibodies (mAb), and found that antibody-induced decrease of cytolysis was correlated with a decrease in the percentage of strong or total conjugates, depending on the mAb used. When lectins were added to overcome the inhibitory effect of the mAb, reappearance of cytolytic activity correlated with reappearance of conjugates. This was especially striking using wheat germ agglutinin (WGA): the addition of WGA to irrelevant effector-target cell combinations did not lead to cytolysis; however, the addition of WGA to relevant effector-target cell combinations inhibited by mAb led to reappearance of cytolysis and of strong conjugates. Taken together, these and other results suggested that under our experimental conditions a threshold level of binding strength between effector and target cells might be important, although not sufficient, for T cell-mediated cytotoxicity. These results were not inconsistent with the involvement of mechanical factors in this process. Also, they were in line with the concept of nonantigen-specific lymphoid cell surface interacting molecules, detected by the mAb used and important for the establishment of strong, functional lymphocyte target cell interactions. Finally, they underlined the necessity of a quantitative estimate of cell-cell binding strength when investigating the effect of a given agent (e.g. a mAb) on lymphocyte target cell recognition.

A human lymphocyte-associated antigen involved in cell-mediated lympholysis

Human lymphocyte function antigen (HLFA) is a cell surface protein defined by two monoclonal antibodies MHM23 and MHM24. It is present on both B and T lymphocytes but in greater amounts on the latter. Both antibodies precipitated antigen, from radiolabeled HSB-2 cells, which ran as two chains on sodium dodecyl sulfate polyacrylamide gel electrophoresis at 180 and 94 kDa. Neither antibody inhibited binding of the other, indicating that distinct epitopes were recognized. Both antibodies were shown to inhibit HLA-restricted lysis of influenza virus-infected and Epstein-Barr virus-transformed target cells by cytotoxic T lymphocytes. Blocking occurred at the level of the effector cells and in the presence of subsaturating concentrations of antibody. Both reagents also inhibited lysis of K562 cells, mediated by natural killer cells. These blocking effects differ from the inhibitory effects of monoclonal anti-HLA ABC and anti-suppressor cytotoxic T cell antibodies which inhibit only HLA-restricted lysis when present in saturating amounts. It is concluded therefore that HLFA is likely to be involved in the nonspecific adherence or lytic functions of killer cells rather than specific antigen recognition.

A clone-specific monoclonal antibody that inhibits cytolysis of a cytolytic T cell clone

Monoclonal antibody 384.5 specifically inhibited cytolysis of P-815 target cells by cloned L3 cytotoxic T lymphocyte (CTL) effector cells. The lytic activity of other cloned CTL that have other distinct specificities was not affected. Antibody 384.5 did not inhibit the cytolytic activity of bulk populations of C57BL/6 mixed lymphocyte culture (MLC) cells. Concanavalin A-facilitated cytolysis by T cell clone L3 but not T cell clone B18 was inhibited by antibody 384.5, whereas phytohemagglutinin-facilitated cytolysis by L3 cells was not strongly inhibited. Antibody 384.5 binds specifically to L3 cells but not to several other T lymphocytes clones, or to a detectable portion of populations of primary MLC cells, normal spleen, thymus, lymph node, or bone marrow cells. In contrast, C57BL/6 anti-B10.A(5R) secondary MLC cells (genetically enriched for reactivity against the H-2Dd region gene products) contained a small population which reacted with the antibody 384.5. The determinant detected by antibody 384.5 was susceptible to trypsin treatment, and was reexpressed after overnight incubation. These results suggest that the monoclonal antibody 384.5 detects an endogenously synthesized clone-specific determinant associated with the cytolytic activity of the L3 CTL clone. These properties make antibody 384.5 an attractive candidate for an antibody that reacts with the antigen-recognition site of a cytolytic T cell antigen receptor.

Enhancement of the recognition by cytotoxic T lymphocytes (CTL) of target membrane antigens after fusion with whole cells

Plasma membrane vesicles (R4-PM) prepared from mouse lymphoma cells (RDM4,H2k) were employed to investigate requirements for recognition of target cell membranes by allogeneic cytotoxic T lymphocytes (CTL). Using immunofluorescent staining and fluorescence microscopy, the R4-PM were tested for binding to CTL and were found to bind to these effector cells in a specific manner. However, this binding was very inefficient compared to the binding of whole RDM4 cells to CTL. The R4-PM were then attached to P388D1 cells (H-2d) in the presence of wheat germ agglutinin and polyethylene glycol (PEG), both under conditions which promote membrane fusion (40% PEG) and under conditions which do not (10% PEG). About 1 cell equivalent R4-PM becomes associated per P388D1 cell in both situations. In the cytotoxicity assays that were carried out, the P388D1 cells which had R4-PM attached under fusion conditions were lysed by CTL directed against H2k in a specific manner, while the P388D1 cells which had R4-PM attached under nonfusion conditions were not lysed above background levels by these CTL. These results suggest that recognition of target cells by allogeneic CTL such that lysis occurs requires more than presentation of the alloantigens as they are expressed in plasma membrane vesicles. However, fusion of these vesicles back into living cells apparently enhances the ability of the alloantigens to be recognized.

Alteration in lymphocyte recognition repertoire during aging. II. Changes in the expressed T-cell receptor repertoire in aged mice and the persistence of that change after transplantation to a new differentiative environment

Changes in the T-lymphocyte alloreceptor repertoire associated with aging by exploring the frequency of cytotoxic T-lymphocyte precursors (CTLp) available for activation by various major histocompatibility complex (MHC) haplotypes in mice of different ages have been investigated. There was no consistent pattern of change in CTLp frequencies. Thus, for instance, while the frequency of responder C57B1/6 CTLp for ATH alloantigen decreased with age, the frequency for C3H alloantigen increased. There was no significant change in the overall frequency of splenic CTLp (assessed irrespective of antigen specificity). No evidence was found that CTL produced by activated CTLp of aged mice were less specific in their lytic capacity that CTL produced by CTLp of young mice. However, by assaying responder CTLp cultures at limiting dilution we obtained evidence that the "burst size" (mean lytic capacity per responder well assayed at limiting dilution) was diminished with age of the donor of the CTLp pool. Furthermore, we obtained evidence that the apparent affinity of CTL for their target antigen was consistently decreased when those effector cells were derived from a pool of CTLp of aged mice. All of these changes reflected in mature T cells derived from aged mice were already apparent in the bone marrow stem cell pool of aged individuals and were not due to environmental influences alone, as assessed by the phenotype of T cells derived from young or old bone marrow stem cells transplanted to young or aged recipient mice. A final study has examined evidence for more subtle changes in the T-cell alloreceptor repertoire, reflecting heterogeneity in young or aged mice in the recognition repertoire associated with a given antigenic specificity. By preparing F1 anti (parent anti-F1)-suppressor cells directed against CTL from young parental mice (a, b, c), or aged parental mice (x, y, z), we have explored the heterogeneity in the anti-C3H alloreceptor repertoire in individual young or aged C57B1/6 mice. Suppression by immunized F1 animals was assessed in tissue culture (inhibition of mixed lymphocyte culture (MLC) responses) or in vivo (inhibition of lethal GvHD induced by inoculation of parental lymphocytes into sublethally irradiated F1 hybrid mice). Irrespective of the assay system used, the data suggests that the receptor repertoire of aged T lymphocytes uses recognition structures different from those of young individuals, and that there is less individual-to-individual variation in the receptor repertoire of aged mice than in young mice.

Studies on the induction and expression of T-cell-mediated immunity. XIII. Membrane-associated antigens of cytotoxic T lymphocytes involved in cytotoxicity

An xenogeneic rat anti-mouse T-cell serum, designated RAT*, has been shown to block the cytolytic activity of cytotoxic T lymphocytes (CTL) at a postbinding step. RAT* serum or the IgG fraction was extensively absorbed with the target cell, P815, a DBA mastocytoma, and used with or without further absorption to immunoprecipitate specific molecules from radiolabeled membrane extracts of CTL derived from either in vivo-allosensitized mice or from cytotoxic clones maintained in in vitro cultures. Cell surface sialic acid residues were labeled by oxidation with sodium periodate (NaIO4) and reduction with tritiated sodium borohydride ([3H]NaBH4). Alternatively, cell surface proteins were labeled with 125I by lactoperoxidase-catalyzed iodination. Nonidet P-40 (NP-40)-solubilized radiolabeled membranes were then immunoprecipitated with RAT* serum and analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). Three membrane-associated molecules of 95,000, 140,000 and 180,000 Mr were found by such analysis. The sensitivity of these three molecules to trypsinization and their susceptibility to labeling with [3H]NaBH4 suggested that they are glycoproteins. Moreover, when RAT* serum or the IgG fraction was absorbed with various cell types, its ability to immunoprecipitate the three molecules correlated with its ability to block cytolysis. Adsorption of RAT* serum with CTL, but not with nonimmune thymocytes, significantly reduced the ability of RAT* serum to inhibit cytotoxicity and to immunoprecipitate the 95k, 140k, and 180k molecules. Thus, these findings suggest that one or more of these cell surface molecules of CTL may be involved in the cytolytic process.

Bacteria-immune system interactions. VIII. Modulation of human cytotoxic reactions by crude lipoteichoic acid extracts of Bacillus globigii

Crude lipoteichoic acid (LTA)1 extracts were prepared from Bacillus globigii (Bg-LTA). The addition of Bg-LTA to mixed lymphocyte reactions (MLR) produced a dose dependent inhibition of specific cytotoxic reactions and of tritiated thymidine (3H-TdR) incorporation. Much less Bg-LTA was needed to inhibit the initiation (addition at day 0) than the effector phase (addition at day 6) of the cytotoxic reactions. None of the LTA quantities used had a toxic effect on the lymphocytes. LTA pretreated stimulator cells also had a modulatory effect on the cytotoxic reactions. Stimulators pretreated with very low or very high LTA concentrations had no effect; however, pretreatment with intermediate concentrations induced a drastic inhibition of cytotoxicity. LTA pretreated stimulator cells did not significantly affect 3H-TdR incorporation. Suppression of the cytotoxic reaction could also be obtained by LTA-pretreatment of the effector cells.

Lymphoid cell surface interaction structures detected using cytolysis-inhibiting monoclonal antibodies

We screened monoclonal antibodies obtained by xenogeneic immunization for their capacity to inhibit T cell-mediated cytolysis. These antibodies fell into two classes according to the cell structures they recognized, of 30-35 K and 94-180 K apparent molecular weight, respectively. The main features of these structures and of their interaction with the corresponding antibodies were reviewed. The inhibition of cytolysis by these antibodies was shown to occur mainly at the effector cell level, at the recognition stage of cytolysis, and to depend on the nature of target cells, effector cells, and link between these cells. T cell functions other than cytolysis were also inhibited by some of these antibodies. We considered various possible mechanisms to account for the inhibition of cytolysis by these mAb. We favor an hypothesis based on inhibition by these mAb of lymphoid cell surface interaction structures. This hypothesis was discussed within the general framework of cell interaction structures in immunological and non-immunological experimental systems.

Three distinct antigens associated with human T-lymphocyte-mediated cytolysis: LFA-1, LFA-2, and LFA-3

Monoclonal antibodies were prepared to anti-HLA-DR cytolytic T lymphocytes (CTLs) and screened for inhibition of CTL-mediated killing. Binding of monoclonal antibodies to four types of molecules, LFA-1, LFA-2, LFA-3, and HLA-DR, inhibited killing, suggesting that these molecules participate in the CTL-target cell interaction. The antigens were characterized by immunoprecipitation, crosslinking, NaDodSO4/polyacrylamide gel electrophoresis, and immunofluorescence flow cytometry. The LFA-1 antigen contains alpha and beta polypeptide chains of Mr 177,000 and 95,000 that are noncovalently associated in an alpha 1 beta 1 structure. It is present on both B and T lymphocytes and marks subpopulations that differ in quantitative expression. Human LFA-1 appears to be the homologue of mouse LFA-1. Human LFA-2 is of Mr 49,000 with a minor component of Mr 36,000. It is expressed on CTL lines but not on a B-cell line and in peripheral blood preferentially on T lymphocytes. Human LFA-3 is of Mr 60,000 and is expressed on both B and T lymphocytes.

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.

Selective inhibition of human T cell cytotoxicity at levels of target recognition or initiation of lysis by monoclonal OKT3 and Leu-2a antibodies

Out of a panel of seven monoclonal antibodies with affinity for human lymphoid cells, three were shown to prevent cytotoxic T cell activity, whereas none affected natural killer cell activity when applied without complement. Anti-OKT3 and anti-Leu-2a, with affinity for all T cells and the cytotoxic/suppressive subset, respectively were both shown to inhibit T killing by their interaction with the effector cell. For anti-OKT3, the inhibition remained after free antibody was washed away. Anti-Leu-2a, in contrast, induced a rapidly reversible inhibition. Using a single cell assay, anti-OKT3 was shown to reduce the lytic ability without affecting target cell binding, whereas anti-Leu-2a prevented the effectors from binding target cells.

The role of the human major histocompatibility complex in cytotoxic T-cell responses to virus-infected cells

The human major histocompatibility complex (HLA) has been demonstrated to play two roles in the generation and expression of cytotoxic T-lymphocyte responses to virus-infected cells: (1) cytotoxic T cells can only recognize viral antigens in conjunction with antigens encoded by HLA-A and -B genes; and (2) HLA-linked genes may control the capacity to generate T-cell responses to a given virus or to virus in conjunction with particular self HLA-A and -B antigens. Analysis of T-cell responses generated in vivo to Epstein-Barr virus suggests that human T cells may recognize virus in conjunction with antigens other than the class I HLA polymorphic specificities.

The molecular basis for cytolytic T lymphocyte function: analysis with blocking monoclonal antibodies

During the past decade the mechanism of CTL-mediated killing has been resolved into 3 steps, and its cation requirements, and general nature have been well defined. However, biochemical understanding of the CTL-target interaction has made little progress. Recently, we have developed a monoclonal antibody (MAb) which blocks killing by binding to a previously undescribed molecule on the CTL membrane, a molecule which we therefore have termed lymphocyte function-associated antigen one (LFA-1). LFA-1 and Lyt-2,3 are the only presently identified sites for such blocking; antibodies to over a dozen other molecules expressed on the CTL do not block killing. Present evidence suggests that LFA-1 is crucial in the adhesive interaction of T cells with other cells (e.g., targets, macrophages, perhaps B cells) The continuing search for blocking MAbs provides a systematic way to link specific molecules with CTL function.

Soluble cytotoxic factors and the mechanism of NK cell mediated cytotoxicity

Soluble cytotoxic factors from mouse spleen cells have been shown to selectively lyse NK sensitive target cells. Lysis of target cells is assessed by trypan blue uptake or 51Cr--release assay in a 16-48 hour assay. The possible role of such natural killer cytotoxic factors (NKCF) in the mechanism of natural killer cell-mediated cytotoxicity (NKCMC) has been examined. Several lines of evidence are presented which indicate that there exists a strong correlation between lysis by NKCF and lysis in NKCMC. For instance, (1) NKCF are generated following stimulation of mouse spleen cells with NK sensitive targets; (2) Lysis of NKCF is selective for NK sensitive targets and is species specific; (3) Mice with poor NK activity, such as Bg/Bg mice, produce poor NKCF: (4) There is concomittant inhibition of NKCMC and NKCF activities by blocking RAT serum; and (5) Several known characteristics of the mechanism of NKCMC are shown to be shared in the NKCF system. Based on these findings, we propose a model for NKCMC in which lysis by NK effector cells is the result of multiple steps, namely target binding to an NK effector cell, activation of the lytic mechanism, and involvement of NKCF to mediate lysis. Accordingly, for targets to be NK sensitive, they ought to be able to interact and bind with NK effectors, activate the NK cells, bind NKCF, and be sensitive to the NKCF lytic activity.

A fluorescence study on the mobility of surface antigens of untreated tumor cells and of tumor cells undergoing cell-mediated lysis

Heterologous (rabbit) antibodies were raised against murine P-815 mastocytoma cells of DBA/2 origin. Antisera and IgG preparations were highly cytotoxic, whereas Fab fragments thereof lost all activity. Fab fragments also showed a much lower avidity than IgG, both for tumor and normal DBA/2 and C57 spleen cells as measured by the release of iodinated Fab and IgG. Both preparations bound specifically to P-815 cells since they were capable of inhibiting T cell-mediated target cell lysis. The binding of IgG and monovalent Fab fragments was studied by fluorescence. Rhodamine-coupled IgG bound homogeneously in the cold and quickly formed patches upon warming but did not form caps even after prolonged incubation at 37 degrees C. Rhodamine-coupled Fab fragments also bound homogeneously. Their distribution was unaltered after incubation at 37 degrees C even when tumor cells formed uropod-like tails. Fab fragments, however, could be induced to cap with a second and third antibody layer. P-815 cells labeled with rhodamine-coupled Fab fragments were incubated with cytolytic T cells (CTL). The conjugates formed between CTL and fluorescent target cells were observed. No gross redistribution of surface antigens on target cells was observed even at late stages of the lytic process. CTL, therefore, do not seem to operate via a redistribution of surface antigens.