Enumeration of specific cytotoxic T cells

Mechanisms of immune-mediated myocyte injury

Much progress has been made in defining the mechanisms by which altered systolic and diastolic function of the heart may be produced by components of the immune system activated during allograft rejection and myocarditis and in patients with dilated cardiomyopathy. It is clear that injury of the vascular bed can occur via both humoral and cellular mediators and probably accounts for the acute alterations in ventricular compliance that occur during allograft rejection, as well as the accelerated development of graft atherosclerosis. Altered myocyte function and lysis can be produced by CTL in vitro, but the importance of this injury process in vivo remains uncertain. Other cells present in the inflammatory infiltrate can also affect myocyte function and survival. Neutrophils may cause lysis of myocytes, and cytokines produced by infiltrating macrophages and HtL may reach a sufficient concentration in the interstitial microenvironment to decrease myocyte catecholamine responsiveness and/or directly depress myocyte contractility. Humoral antibodies to myocyte cell surface antigens may cause cell damage by an antibody-dependent cytotoxic cell mechanism or by directly binding to and altering sarcolemmal receptor and/or ion channel function. Further elucidation of the extent of involvement of these different mechanisms in specific clinical settings may provide a basis for improved therapy of immune-mediated cardiac injury and dysfunction.

Kinetics of cellular cytotoxicity mediated by cloned cytotoxic T lymphocytes

Kinetic methods can provide significant information concerning the mechanism of cellular cytotoxicity reactions. Previous kinetic studies of cytotoxic T lymphocytes (Tc) have been hampered by the heterogeneity of the effector cell population tested. We therefore examined the kinetics of lysis mediated by cloned, IL 2 and antigen-dependent murine Tc cells with strong cytotoxic activity that is restricted to distinct tumor-associated antigens on P815 mastocytoma target cells. Initial velocity measurements for cytotoxicity mediated by these clones fit a simple Michaelian kinetic model. Specific activity values obtained from these initial rate measurements are compared to those obtained for polyclonal Tc preparations, NK cells, and activated killer cells. Whereas the initial rate of lytic programming mediated by these cloned cells was very rapid, the rate of cytolysis mediated by the cloned cells decreased significantly within one hour. Since this decrease was observed over a wide range of E:T ratios, it is unlikely to result from product inhibition or a significant decrease in the concentration of unlysed target cells, but may be due to a decrease in the rate of programming and/or effector cell recycling. These results indicate that a simple Michaelian kinetic model is not adequate for tumor cell cytolysis by Tc cells in vitro.

Cellular cytotoxicity assessed by the 51Cr release assay. Biological interpretation of mathematical parameters

This study deals with the interpretation of primary data of the 51Cr release assay for cellular cytotoxicity. In particular the dose-effect relationship between increasing numbers of lymphoid cells and the percentage of target cells killed has been considered. The number of target cells killed depends on the frequency and the activity of cytotoxic cells. These two parameters are often not distinguished from each other, which causes confusion and frequently results in vague descriptions of cytotoxicity data. Because in many cases not all cells in the target cell population can be lysed, we recommend the introduction of the plateau value for target cell kill. This maximum of target cell kill is a measure of the frequency of lysable target cells, but also depends on the cytotoxic cell activity. Description of the dose-effect curve by y = A(1 - e-kx) allows the simultaneous calculation of the maximum kill (A) and the frequency of cytotoxic cells (k) in the lymphoid cell population (x). Results are presented which indicate that A and k represent totally independent biological parameters the use of which permits a more objective description of cytotoxicity data.

Direct estimation of the frequency of human cytotoxic T lymphocytes and their precursors following in vitro allosensitization

Cell mediated lympholysis (CML) has been proposed as an in vitro model of the rejection process that results from transplantation of allogeneic tissue. To date, the absolute frequencies of cytotoxic T lymphocytes (CTL) and their precursors (CTL.P) have not been directly estimated in man because of technical difficulties. Through optimizing the conditions for radiometric detection of 51Cr release and the attendant improvement in CML sensitivity, direct CTL frequency estimates have been determined in peripheral blood (PBL), spleen (SPL), and lymph nodes (LNC) after in vitro allostimulation using unrelated human cells and limiting dilution assays. The mean frequency of CTL generated from PBL is 1 in 826 cells (0.121% +/- 0.101%) which, from preliminary experiments, is significantly greater than that generated from either LNC or SPL (p less than 0.05). With restimulation of primed cells on day 10, the frequency of CTL generated from PBL was increased 400%. The CTL.P frequency (0.0064% +/- 0.0050%) was approximately 5% of the corresponding CTL frequency. The CTL.P frequencies were found to be minimal estimates as both accessory "filler" cells and T cell growth factors increased the level of detection of CTL.P an average of threefold. The limiting cell dilution assay as detailed in this report should be a powerful tool for defining the cellular requirements and related factors necessary for optimal induction of a CTL response and should provide the means for determination of the immunogenetic requirements and the allospecificity of human cytotoxic lymphocytes.

An estimate of the minimal frequency of cytolytic T lymphocyte effector cells generated in allogeneic reactions

A 'micro' modification of the standard 51Cr release assay was developed. With this assay, significant cytolysis was obtained employing 100--200 51Cr-labeled target cells and fewer than 100 lymphocytes obtained from mixed leukocyte cultures. A minimal estimate as to the actual frequency of effector cells was determined for both primary (1 cell in 30) and secondary (1 cells in 8) cytolytic T lymphocyte populations.