The mechanism of T-cell-mediated lysis

Model for population distributions of lymphocyte-target cell conjugates

A quantitative model for the population distributions of the different types of conjugates formed between cytotoxic T lymphocytes and target cells has been developed. The comparison of the theoretical predictions with data of the literature reveals that the transit populations among the different types of conjugates depends on the lymphocyte-to-target ratio, R, and two constants, k and k1. These constants (where k greater than k1) govern, respectively, the transit populations among conjugates of the type LTi (LTn----LTn-1----...LT), and among LjT conjugates (LT----L2T----...----LmT). We have found that high ratios are necessary to obtain conjugates where multiple T lymphocytes are bound to one target cell, and that under these conditions the predominant conjugate, LjT, varies according to j = 1 + k1R. Conversely, for low values of R the predominant population is of the type LTi, where i also shows a linear dependence on R. Our model explains also why the conjugate LT is normally the predominant population under the experimental conditions reported in the literature. A discussion of the influence exerted by the population distributions of lymphocyte-target cell conjugates on the kinetic of the lytic process for these kinds of effector-target systems has also been made.

Effect of conjugate size on the kinetics of cell-mediated cytotoxicity at the population level

We developed a model for the kinetics of target cell lysis by cytotoxic T-lymphocytes which accounts for most facts observed at the population level. In contrast to previous models, the following facts: conjugate frequency of cytotoxic T-lymphocytes bound to target cell, dependence of this frequency on the lymphocyte-to-target ratio (R), variation of R with time as target cells are destroyed, and population distributions of the different types of conjugates formed between lymphocytes and target cells, which are involved in the kinetics of these kinds of effector-target systems have been contemplated in the model. The relationship with effector-kinetic analogy models for the lytic process has been discussed. Predictions of the model have been explored and compared with experimental observations about target cell lysis reported in the literature.

Suppression of T-lymphocyte cytotoxicity following exposure to 60-Hz sinusoidal electric fields

A significant 25% inhibition (P less than .005) of allogeneic cytotoxicity of the target cell MPC-11 by the murine cytotoxic T-lymphocyte line CTLL-1 was observed when the 4-h cytotoxicity assay was conducted immediately following a 48-h pre-exposure of the effector lymphocytes to a 10-mV/cm (rms) 60-Hz sinusoidal electric field. At 1.0 mV/cm a significant 19% inhibition (P less than .0005) was seen. At 0.1 mV/cm a nonsignificant 7% inhibition of cytotoxicity was noted. When the 4-h cytotoxicity assay was conducted in the presence of the field using previously unexposed effector lymphocytes, cytotoxicity was not significantly reduced. Cell proliferation in the presence of interleukin-2 was unaffected by the field. These data suggest a dose response and threshold (between 0.1 and 1.0 mV/cm) for inhibition of cytotoxicity in clonal T-lymphocytes by exposure to a 60-Hz sinusoidal electric field. These results suggest mechanisms by which 60-Hz electric fields could affect the function of cells of the immune system.

Lymphokine-activated cell-associated antigen involved in broad-reactive killer cell-mediated cytotoxicity

Spleen cells from rats which had been hyperimmunized with mouse lymphokine-activated killer (LAK) cells, were fused with the mouse myeloma cell line, P3 X 63 Ag8.653. Antibodies secreted by 1500 cultures were selected by their blocking effect on LAK cell-mediated cytotoxicity in the absence of complement. Two monoclonal antibodies (KBA4 and KBA6) greatly inhibited the cytotoxic activity of LAK cells, which were induced from mouse spleen cells by culture with recombinant human interleukin 2 (r-IL-2). These antibodies also blocked the cytotoxic activity of natural killer (NK) cells, but activated macrophages (A-M phi) were only slightly sensitive to them. However, no effect of the antibodies on the cytotoxic activity of cytotoxic T lymphocytes (CTL) was detected. These data suggest that the specific antigen, lymphokine-activated cell-associated (LAA) antigen, defined by these monoclonal antibodies may be associated with the recognition mechanisms of broad-reactive killer (BRK) cell-mediated cytotoxicity. The observation that low levels of LAA antigen are distributed in all lymphoid cells and that it was significantly enhanced by treatment of the cells with r-IL-2 suggests that the antigen may be involved in lymphocyte-activation mechanisms. We also found that the LAA antigen consists of two distinct polypeptides with Mr of 180,000 and 95,000 Da, which are similar to that of LFA 1 antigen. However, the biological characteristics of LAA antigen did not coincide with those of LFA 1. Therefore, KBA MAb may recognize a carbohydrate epitope distinct from that of LFA 1.

Vaccinia virus proteins on the plasma membranes of infected cells. II. Expression of viral antigens and killing of infected cells by vaccinia virus-specific cytotoxic T cells

Evidence is presented that virion-derived antigens as well as viral antigens expressed on cell surfaces after infection, may participate in the formation of "target-antigen complexes" (TACs) which render vaccinia virus-infected cells susceptible to recognition and killing by syngeneic, vaccinia virus-specific cytotoxic T cells (VV-CTLs). By employing L cells infected with trypsin-treated and untreated virions, evidence was obtained that proteins with molecular weights of 32K and 37K may be among the virion-derived antigens which participate in TAC formation. Following virus infection, a sequential expression of virus-specified antigens on the plasma membrane of infected cells could be detected. At 1 hr p.i., polypeptides with molecular weights of 48K-50K and 36K-37K were present on infected cell surfaces; by 2 hr p.i., polypeptides with molecular weights of 48K-50K, 42K-44K, 36K-37K, 29K-30K, and 16K-17K were detected on plasma membranes. As measured by in vitro, 51Cr-release assays, vaccinia virus-infected L cells were completely susceptible to lysis by VV-CTLs (greater than or equal to 50% measured specific lysis) when (a) "early" but not late viral functions were expressed as measured with virus-infected cells which had been treated with hydroxyurea (5 X 10(-3) M) to block DNA replication or (b) when active protein synthesis was allowed to proceed for 90 min postadsorption and the infected cells were then treated with cycloheximide (100 micrograms/ml) to block further protein synthesis. Under these experimental conditions, polypeptides with molecular weights of 58K, 48K-50K, 42K, 36K-37K, 34K, 32K-33K, 27K-29K, and 16K-17K were expressed on the plasma membranes of vaccinia virus-infected cells but not uninfected cells. Whether each of the virion-derived and (or) virus-encoded polypeptides can associate with Class I, major histocompatibility antigens on the surfaces of virus-infected cells to form a primary or cross-reacting TAC recognized by VV-CTLs remains to be investigated.

Seminal plasma abrogates the postcoital T cell response to spermatozoal histocompatibility antigens

The T-cell responses to spermatozoal histocompatibility antigens were studied in the uterine regional lymph nodes of female rats by a specific cytoadhesion assay. The antigenic challenge was introduced either by artificial insemination with syngeneic or allogeneic epididymal spermatozoa or coitus. The uterine regional T-cell response to allogeneic spermatozoa following artificial insemination was measureable after 2 days and continued undiminished until day 5. In contrast, the measurable response to allogeneic coital challenge peaked on day 3 and disappeared by days 4-5. Since the difference between the two stimuli involved the presence of seminal plasma, these results indicate a possible immunoregulatory (suppressive) effect of seminal plasma on the cellular immune reaction of the female to the male alloantigens after mating.

Comparison of the effects of quercetin with those of other flavonoids on the generation and effector function of cytotoxic T lymphocytes

In previous studies (Schwartz et al., 1982) we showed that the naturally occurring plant flavonoid quercetin can inhibit both the in vitro generation and effector function of alloantigen specific cytotoxic T lymphocytes (CTL). In the present studies, several additional flavonoids of different chemical classes were tested similarly to determine whether structure-function relationships exist. We have found that some other flavonoids, e.g. apigenin , fisetin , hesperetin and chalcone also can inhibit both CTL generation and effector function, with the effective concentration varying with the specific flavonoid tested. On the other hand, flavonoids such as rutin, naringin and catechin were inactive in both systems. Taxifolin ( dihydroquercetin ) differed from all the other flavonoids in that it was a relatively active inhibitor of CTL generation, but was essentially unable to inhibit CTL effector function. The presence of a double bond at position C-2-3 in the flavone and flavonol aglycones, a keto group at C-4, B ring hydroxylation and/or a free hydroxyl group at C-3 may be associated with activity. We also show that the effects of some, but not all, of the flavonoids active in our systems can be blocked by Cu2+ ions. Therefore, chelation of divalent cations such as Cu2+ cannot explain the function of all flavonoids in these systems.

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

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