T cell-mediated cytotoxicity: discrimination between antigen recognition, lethal hit and cytolysis phase

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.

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.

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

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

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.

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.

Influenza virus-specific T cell-mediated cytotoxicity: integration of the virus antigen into the target cell membrane is essential for target cell formation

This study deals with the requirements for target cell recognition by influenza A virus-specific cytotoxic T lymphocytes (CTL). H-2-identical cells were incubated with infectious or UV light-inactivated influenza A virus expressing either cleaved or uncleaved hemagglutinin (HA). Thereafter, the treated cells were tested in a 4-h 51Cr assay for susceptibility to CTL-mediated cytolysis. Regardless whether the influenza virus was infectious, virions expressing cleaved HA were efficient in target cell formation. In contrast, cells incubated with either active or UV-inactivated virions expressing uncleaved HA were not lysed by virus-specific CTL. Yet, after mere trypsin-mediated cleavage of the HA of cell-absorbed viroins, strong cytolysis could be observed. On the other hand, solubilization of the envelope lipid bilayer by ethylether abolished the capacity of the remaining HA to induce target cell formation. The results clearly suggest that mere absorption of virions to the membrane of cells, which is performed by virus with uncleaved HA, is insufficient for target cell formation. For this, both cleaved HA and an intact envelope appear to be crucial. We conclude that fusion of the virion into the cell membrane is essential for target cell formation.

Tumor metastases and cell-mediated immunity in a model system in DBA/2 mice. IV. Antigenic differences between a metastasizing variant and the parental tumor line revealed by cytotoxic T lymphocytes

The syngeneic cytotoxic T-cell response against a metastasizing murine lymphoma variant was investigated and compared with the response against the non-metastasizing parental tumor line Eb. Anti-tumor cytotoxicity was not detectable in a 4-h 51Cr release assay in spleens taken directly from tumor-bearing animals (primary CMC). After restimulation in vitro (secondary CMC) however, high anti-tumor cytotoxic activity was detected. This activity was mediated by immune T lymphocytes as shown by its sensitivity to treatment with anti-Thy 1.2 serum and complement. Ten cells of the metastasizing tumor ESb, inoculated subcutaneously, were sufficient to raise a local tumor and metastases and to induce cytotoxic T memory cells in the spleens. In contrast, about 104 cells were required to raise a local tumor and to induce splenic cytotoxic T memory cells, when the parental tumor Eb was tested. The specificity studies of the anti-tumor cytotoxic activity demonstrated that cytotoxic T cells could distinguish unrelated, chemically induced syngeneic tumors and also recognize antigenic differences between the parental tumor Eb and its variant ESb. Eb and ESb tumor cells were recognized as carrying distinct antigens at the responder cell level, the stimulator cell level and the target cell level. The in vivo significance of these findings is discussed.

Reversible changes in cytolytic T lymphocyte function induced by hyperthermia

The ability of cytolytic T lymphocytes (CTL) generated in vitro in mixed leucocyte cultures (MLC) to "repair" functional damage induced by moderate hyperthermia has been investigated. Repair of CTL function was assessed in a split dose system in which MLC cells were exposed to a total of 20 min at 43 degrees C delivered either as a single treatment or as two 10 min fractions separated by an interval at 37 degrees C. Significant repair was observed within 15 min at 37 degrees C in the split-dose protocol and maximum repair (corresponding to a 30-100-fold increase in lytic activity) was seen after 1.5-2 h. Repair was both temperature and energy dependent and was apparently related to a reversible lesion in the cytolytic mechanism itself. The effect of a number of metabolic inhibitors (and other agents) on repair was determined, but the precise nature of the heat-induced lesion could not be identified.

Cell membrane-mediated cytolysis by membranes from noncytolytic cells

Cytolysis was obtained with plasma membrane fractions prepared not only from cytolytic T cells, but also from nonimmune lymphoid cells and, moreover, from cell lines of nonlymphoid origin displaying no lytic ability. Furthermore, no relationship exists between the genetic specificty of T cell-mediated cytolysis and the activity of membrane preparations. If anything, it appears that certain cell types may be unusually sensitive to membrane-mediated cytolysis. Our observations argue against a mechanistic relationship between T cell-mediated and membrane-mediated cytolysis.

Gross-virus-induced lymphoma in the rat. V. Natural cytotoxic cells are non-T cells

The cytotoxic cells from the spleens of normal rats, which lyse Gross-virus-induced lymphoma target cells in a short-term 51Cr release test, are predominantly small to medium-sized cells sedimenting at 4-5 mm/h as shown by velocity sedimentation analysis. Their cytotoxic activity is relatively resistant to gamma-radiation, 50% surviving 1,000 rads and 30% remaining after 5,000 rads, and to heat, since 20 min incubation at 48-51 degrees C is required for its abolition. In these properties the natural killer (NK) cells are very similar to the cytotoxic T cells from tumour-immune rats, and they share in addition a requirement for Ca++ ions for cytolysis. They differ in that they are non-T cells as defined by their resistance to anti-T-cell antiserum and complement, and by their presence in T-cell-deprived rats. They lack detectable surface Ig, Fc receptors and phagocytic or adherence properties and belong therefore to that small proportion of lymphoid cells lacking the surface markers of T or B lymphocytes, as do the comparable NK cells of mice. Cytotoxicity appears to involve an autonomous, papain-sensitive recognition structure on the surface of NK cells, rather than acquired cytophilic antibody operating through an antibody-dependent mechanism.

Enhanced in vitro immunogenicity of H-2 antigens in the presence of la antigens

In a primary and secondary MLC the in vitro immunogenicity of allogeneic PHA induced blast cells (which lack i region coded determinants) was compared to that of LPS imduced blast cells. Unlike LPS induced blast lymphocytes (stimulator cells) which induced high cytotoxic activity, PHA induced blast cells were found to be poor stimulator cells in a primary MLC. Yet in a secondary MLC both types of stimulator cells induced cytotoxic activity equally well. Using one type of responder cells the relative immunogenicity of various stimulator cells incompatible on either the H-2K and H-2D region or on the i region, or on the complete H-2 complex, was compared. The magnitude of cytotoxic response induced in a strain combination differing at the complete H-2 complex exceeded by far the sum of separate responses obtained against the H-2D region, H-2K region and the i region coded determinants respectively. These results suggest that the presence of i region coded determinants on allogeneic stimulator cells enhance the in vitro immunogenicity of H-2K and H-2D region coded transplanatation antigens.

The lethal hit stage of mouse T and non-T cell-mediated cytolysis: differences in cation requirements and characterization of an analytical "cation pulse" method

We investigated in detail the cation requirements of two different systems of mouse cell-mediated cytolysis in vitro, at their recognition, post-recognition hit and target cell disintegration stages. In T cell-mediated cytolysis, respectively Mg++ or Ca++, Ca++, and no cations, were required. In non-T cell-mediated hemolysis, respectively no cations, Mg++, and no cations, were required. Two main conclusions can be drawn. First, the cation requirements are different from one system to the other especially at the post-recognition hit stage, which strongly suggests the existence of differences between both systems as to the actual mechanism of lysis. Second, the cation requirements are different within each system from one step to another. This formed the basis of a step-by-step analysis of the lytic process, leading to the characterization of a cation pulse method for the experimental isolation and further study of the post-recognition hit stage of cell-mediated cytolysis.

Energy requirements of the binding and lytic steps of T lymphocyte-mediated cytolysis of leukemic cells in vitro

The process of T lymphocyte-mediated destruction of target cells in vitro is temperature dependent and is suppressed by inhibitors of energy metabolism, but the energy requirements for the conjugation and lytic steps of the process are different. The conjugation step of lymphocytes and target cells is prevented by metabolic inhibitors and is dependent on temperature, increasing in the range from 2 degrees C to 22 degrees C. However, after lymphocyte-target cell conjugates are formed, they are stable at low temperatures (0 degree C) and in the presence of metabolic inhibitors. Thus, the formation but not the maintenance of lymphocyte-target cell conjugates is an energy-dependent process. In contrast to the conjugation step, which is influenced both by metabolic inhibitors and by temperature, the lytic step, which occurs only between 22 degrees C and 37 degrees C, is not affected by inhibitors of energy metabolism. The sigmoidal behavior of lysis as a function of temperature, showing a sharp inflection around 20 degrees C, can be explained in terms of the alteration of the viscosity of the membrane matrix of the target cells rather than as a general metabolic effect on the killer and target cells.

T cell proliferation in the mixed lymphocyte culture does not necessarily result in the generation of cytotoxic T effector cells

It was tested whether T lymphocytes, when stimulated in vitro by M locus-coded lymphocyte activating determinants (LAD), are able to mediate cytotoxic effector functions. The assay for cytotoxicity included both the use of purified appropriate target cells (i.e. purified lipopolysaccharide blasts) as well as the use of phytohemagglutinin dependent cytolysis as a model for detecting cytotoxic T lymphocytes (CTL). Although strong proliferative responses were obtained in the mixed lymphocyte culture, the T cell blast generated did not display any detectable cytotoxic effector function. Thus, it is concluded that LAD, at least in the M locus-dependent system, do have the capacity to induce T cell proliferation but do not induce CTL.