A modified 51Cr release assay for cytotoxic lymphocytes

Review of flow cytometry as a tool for cell and gene therapy

Quality control testing and analytics are critical for the development and manufacture of cell and gene therapies, and flow cytometry is a key quality control and analytical assay that is used extensively. However, the technical scope of characterization assays and safety assays must keep apace as the breadth of cell therapy products continues to expand beyond hematopoietic stem cell products into producing novel adoptive immune therapies and gene therapy products.  Flow cytometry services are uniquely positioned to support the evolving needs of cell therapy facilities, as access to flow cytometers, new antibody clones and improved fluorochrome reagents becomes more egalitarian. This report will outline the features, logistics, limitations and the current state of flow cytometry within the context of cellular therapy.

A flow cytometry-based assay to assess minute frequencies of CD8+ T cells by their cytolytic function

Limited sample size and low sensitivity of currently used functional assays challenge direct analysis of cytotoxic CD8+ T lymphocyte activity to quantify antigen-specific immunity after infection or vaccination. Our flow cytometry-based assay reproducibly detects at least three epitope-specific CD8+ T lymphocytes by their cytolytic function. As exemplified for viral epitopes restricted to the human leukocyte antigen (HLA)-A2, the HLA-A2+ human somatic cell hybrid T2 provided an about 10-fold more sensitive readout as compared to autologous B-lymphoblastoid cells or the human erythroleukemia cell line K562 transfected to express HLA-A2 when used as target cells. We named our assay VITAL-FR assay, referring to Hermans et al. (2004) and indicating the modification of using Far Red (FR) dye instead of CMTMR. Under optimal conditions the VITAL-FR assay proved 30 times more sensitive than the 51chromium-release assay to assess epitope-specific target cell lysis. The high overall sensitivity of the VITAL-FR assay basically depended on the negligible spectral overlap of the emission of a stable Far Red fluorescent reporter with the green tracer for target cell labelling. It also profited from long co-incubation of effector and target cells of up to 72, from prior in-vitro culture increasing the frequency of epitope-specific CD8+ T cells and from generic, easily accessible standardized target cells that were used with only 10(3) specific and 10(3) control target cells per individual experimental reaction. Our functional approach with the VITAL-FR assay therefore ideally suits for monitoring CD8+ T cell-mediated cytotoxicity in e.g. vaccination studies with known MHC-restricted immunogenic peptides in scientific and diagnostic applications.

Computational models in immunological methods: an historical review

The utilization of computational models in immunology dates from the birth of the science. From the description of antibody-antigen binding to the structural models of receptors, models are utilized to bring fundamental understandings of the processes together with laboratory measurements to uncover implications of these data. In this review, an historical view of the role of computational models in the immunology laboratory is presented, and short mathematical descriptions are given of fundamental assays. In addition, the range of current uses of models is explored -- especially as seen through papers which have appeared in the Journal of Immunological Methods from volume 1 (1971/1972) to volume 208 (1997). Each paper which introduced a new mathematical, statistical, or computer simulation model, or introduced an enhancement to an instrument through a model in those volumes is cited and the type of computational model noted.

Cytotoxicity of bovine leucocytes for parainfluenza type-3 virus-infected cells

The cytotoxic effect of bovine neutrophils, alveolar macrophages, monocytes and lymphocytes for parainfluenza type-3 (PI-3) virus-infected cells in 51chromium-release assays is described. Specific lysis of virus-infected target cells with PI-3 virus antibody and complement was first observed 8 h after infection coincident with the appearance of haemadsorption-positive cells. Specific lysis increased rapidly reaching a peak 18-24 h after infection. This increase was paralleled by the increase in the percentage of cells with surface haemagglutinin. Target cells were subsequently used in 51chromium-release assays between 18 and 20 h after virus infection. Antibody-independent killing of PI-3 virus-infected cells was observed with neutrophils, alveolar macrophages and lymphocytes. Levels of specific lysis up to 30% for neutrophils and 68% for alveolar macrophages were observed, although there was considerable variation in activity from animal to animal. Lymphocyte preparations showed levels of cytotoxicity up to 20% in some cases while monocytes had low killing ability. Addition of PI-3 virus-specific antibodies enhanced killing by neutrophils, monocytes and lymphocytes but inhibited killing by alveolar macrophages. Complement, particularly guinea pig complement, was cytotoxic for virus-infected but not for uninfected cells, and also considerably enhanced the cytotoxic effect of neutrophils and lymphocytes.

Calculation of lytic units for the expression of cell-mediated cytotoxicity

Over the past 10 years, the lytic unit has become the most common means by which activity is expressed in cell-mediated cytotoxicity assays. The strengths and weaknesses of the lytic unit as a summary of cytotoxicity are discussed, and computational methods reviewed. The fundamental "assumption of proportional effect" which is implicit in the interpretation of lytic units is described and empirically tested. Based on extensive data from the assay of human natural killer (NK) activity against K562 targets, simplified computational methods are recommended. The proposed methods are easily explained, may be made robust to occasional erratic data, and permit a reasonable interpretation of lytic units even when the assumption of proportional effect breaks down.

The effect of the synthetic lecithin analogue, dimethyl-DL-2, 3-distearolyoxypropyl-2'hydroxylethylammonium acetate, on cytolytic T lymphocyte (CTL) activity

The effect of the synthetic lecithin analogue, dimethyl-DL-2, 3-distearolyoxypropyl-2'hydroxylethylammonium acetate, on CTL cytolytic activity was studied. The analogue significantly inhibits H-2b anti H-2d cytolytic T lymphocytes at concentrations which do not impair lymphocyte viability, protein synthesis, or RNA synthesis. At these concentrations the inhibition is reversible upon removing the analogue. Thus, the inhibition produced by analogue simply is not a result of analogue toxicity. At higher concentrations of the analogue, CTL inhibition is very pronounced; however at these higher concentrations there is evidence of non-specific toxicity of the analogue on CTL.

Improved quantification of cellular cytotoxicity reactions: determination of kinetic parameters for natural cytotoxicity by a distribution-free procedure

A distribution-free statistical procedure for estimating kinetic parameters for cellular cytotoxicity reactions is described. Estimates of cytotoxic activity obtained using this procedure are considerably more precise than percent cytotoxicity values, lytic unit values, and V max values determined by least-squares analysis of the same sets of experimental data. Application of this procedure to the study of natural cytotoxicity allows for precise quantitative comparisons of the lytic activity of different lymphocyte preparations for a given target cell line, of a single lymphocyte preparation for different target cell lines, or of a given lymphocyte and target cell combination under different experimental conditions. This procedure, which does not involve a more complex experimental protocol than other methods, should also allow for more accurate assessment of the relative cytotoxic activity of lymphocytes obtained from patient populations.

A new assay for cytotoxic lymphocytes, based on a radioautographic readout of 111 In release, suitable for rapid, semi-automated assessment of limit-dilution cultures

A new assay for cytotoxic T lymphocytes is described, of general application, but particularly suitable for rapid, semi-automated assessment of multiple microculture tests. Target cells are labelled with high efficiency and to high specific activity with the oxine chelate of 111 indium. After a 3-4 h incubation of test cells with 5 X 10(3) labelled target cells in V wells of microtitre trays, samples of the supernatant are spotted on paper (5 microliter) or transferred to soft-plastic U wells (25-50 microliter) and the 111 In release assessed by radio-autography. Overnight exposure of X-ray film with intensifying screens at -70 degrees C gives and image which is an intense dark spot for maximum release, a barely visible darkening with the low spontaneous release, and a definite positive with 10% specific lysis. The degree of film darkening, which can be quantitated by microdensitometry, shows a linear relationship with cytotoxic T lymphocyte dose up to the 40% lysis level. The labelling intensity and sensitivity can be adjusted over a wide range, allowing a single batch of the short half-life isotope to serve for 2 weeks. The 96 assays from a single tray are developed simultaneously on a single small sheet of film. Many trays can be processed together, and handling is rapid if 96-channel automatic pipettors are used. The method allows rapid visual scanning for positive and negative limit dilution cultures in cytotoxic T cell precursor frequency and specificity studies. In addition, in conjunction with an automated densitometer designed to scan microtitre trays, the method provides an efficient alternative to isotope counting in routine cytotoxic assays.

Spontaneous human lymphocyte-mediated cytotoxicity against tumor target cells. IX. The quantitation of natural killer cell activity

On analysis of in vitro assays of human natural killer (NK) cell function the inadequacy of commonly used methods of expressing lytic activity was apparent. A comparison was made of the data obtained using modifications of two equations-the simple exponential fit and the von Krogh equations. Both of these equations were found to satisfy the following essential criteria for use in these assays. First, the majority of the results obtained in the chromium-release assay could be used in data reduction; second, the resultant "dose-response" curve was reduced to linearity; and third, a single numerical expression was obtained which was directly proportional to the cytotoxic activity. Of the two methods the more conventional exponential fit was found to be the simpler to use. The closeness of fit of the experimentally derived data to the ideal curves did not support the possibility that normal lymphocyte preparations contain suppressor cells capable of inhibiting NK activity. Data have also been presented showing that NK-sensitive targets could be categorized with respect to their susceptibility by comparing the slopes of the target cell survival curves obtained using the exponential fit equation. These observations are relevant to the accurate assessment of NK activity in patient populations and to the determination of the effects of disease and its treatment on this activity.

Significance of cytotoxic lymphocytes after various immunizing procedures in a virus-induced non-producer syngeneic system: correlation between in vitro and in vivo lytic activity

An originally virus-induced, non-producer tumour system has been studied in relation to humoral and cellular cytotoxic responses to transplantation and other immunization techniques. In all experimental groups cytotoxic lymphocytes (CTL) were observed either directly or after mixed culture of lymphocytes and tumour cells (MLTC). Except for C'-dependent cytotoxic antibodies in mice immunized by irradiated cells, no antibody-mediated cytotoxicity was observed. In 2 protocols (transplantation and immunization by mitomycin-treated cells) CTL in vitro were not protective. In a third protocol (immunization by irradiated cells) CTL afforded partial protection and other factors appeared to be involved. The best in vivo protection was induced by immunization consequent on early surgical removal of a small number of transplanted tumour cells. This study provides lines of evidence for the effectiveness of protection supplied by CTL in well-defined conditions. Comparison with other modes of immunization indicated that these conditions were related to the quantity and to the characteristics of antigen involved.

Susceptibility of adherent versus suspension target cells derived from adherent tissue culture lines to cell-mediated cytotoxicity in rapid 51Cr-release assays

Preparation of target cells from tissue culture lines which grow adherent to tissue culture vessels is often desirable for tests of cell-mediated cytotoxicity (CMC). In the present study we used cells derived from adherent tissue culture lines to compare the merits of suspension vs. adherent target cells in short-term 51Cr-release assays. Cytotoxic activity of murine spleen cells sensitized in vitro against allogeneic spleen cells or syngeneic sarcoma cells was tested with fibroblast or sarcoma target cells. In parallel tests, aliquots of tissue culture lines were detached and used as either suspension or adherent target cells in CMC assays, matching the concentrations of suspension and adherent target cells. In both allogeneic and syngeneic combinations adherent target cells released less 51Cr spontaneously and were more susceptible to CMC than their suspension counterparts.

Spontaneous 51Cr release by isolated rat hepatocytes: an indicator of membrane damage

Radiochromium uptake and release by isolated rat hepatocytes in suspension was monitored under continuous-labeling conditions. Cell protein remained unchanged during the absorption phase, whereas the release of 51Cr correlated well with the loss of cell viability and release of cytoplasmic protein. The results suggest that under equilibrium conditions, 51Cr release represents an efflux of label from damaged or dying preparations and not an elution of radioisotope from intact cells.

Some requirements for a linear cell dose response in vitro assay for the T-cell progenitors of cytotoxic lymphocytes

To develop a precise assay for the T-cells progenitors of cytotoxic lymphocytes (CL-progenitors), lymphoid were cultured under optimal conditions in Marbrook vessels with mitomycin-treated allogeneic stimulator cells, and the total level of CL produced 5 days later estimated by a modified 51Cr release assay. Conditions were adjusted so an arithmetically linear cell dose response relationship was obtained. Three aspects of the cell dose response curve required attention. (1) At low responding cell inputs a macrophage-like cell became limiting (despite the presence of allogeneic macrophages in the stimulating cell population), leading to a lag in the response. This limitation was overcome by adding a low level of irradiated syngeneic macrophages, or by using irradiated syngeneic spleen 'filler' cells. (2) The slope of the resultant linear dose response region could be reduced if desired by changing from cellophane dialysis membranes to 0.1 mu pore size nuclepore membranes, suggesting a stimulatory role for some higher molecular weight soluble factor produced in the cultures. (3) At higher responding cell inputs a marked and extensive plateau was obtained. CL developing early in the response appeared to be destroying the allogeneic stimulator cells causing the response to be self-limiting. This problem was overcome by using a responding cell concentration lower than commonly employed. Assays using mixed leukocyte cultures in the lag or plateau regions could give misleading vlaues for CL-progenitor activity. It is suggested that some examples of apparent synergism in CL generation may have resulted from these effects, rather than T-cell helper T-cell progenitor interactions.

The importance of the ratio between effector and target cells for detection of serum blocking of tumor lymphocytolysis

Sera from tumor-bearong guinea pigs were tested for their capacity to influence the magnitude of lymphocytolysis of tumor cells in vitro as measured by 51Cr release. Thirty-three sera were tested in 124 experiments in which the effector to target cell ratio (E:T) was 200:1. Significant blocking of cell-mediated lysis occurred in 62% of the experiments, whereas significant potentiation occurred in 7%. Six sera were found which showed significant blocking in some experiments and significant potentiation in other experiments. One of these sera was further evaluated by varying E:T, using lymphocytes from three immune donors. For each of the three populations of immune lymphocytes, significant blocking was demostrable only at one of three E:T tested. Blocking was observed only with an E:T of 200:1 in two cases. In a third case, blocking was observed at an E:T of 50:1, whereas an E:T of 200:1 gave rise to significant potentiation. The effect of these sera on lymphocytolysis in vitro is clearly dependent upon the E:T ratio.

Inhibition of the growth of lymphoid tumours in syngeneic athymic (nude) mice

A comparison of the growth of a number of BALB/c tumours in BALB/c, BALB/c.nu/ + (heterozygous nude) and BALB/c.nu (homozygous) nude mice has shown that a majority of BALB/c lymphoid tumours grow at a significantly reduced rate in BALB/c.nu mice. The three non-lymphoid BALB/c tumours (carcinomas and sarcomas) tested and a minority of lymphoid tumours, however, grew as well, or better, in the BALB/c.nu mice. Furthermore a correlation, without exception to date, has emerged from a comparison between the susceptibility of a tumour cell line to in vitro lysis by spleen cells from syngeneic nude mice and its growth in vivo in syngeneic nude mice. Only those lymphoid tumour cell lines lysed in vitro show the reduced growth rate in syngeneic nude as compared to syngeneic normal mice. Tumour cell lines, both lymphoid and non-lymphoid, which are resistant to in vitro lysis do not show the reduced in vivo growth rate in syngeneic nude mice. Both the in vivo and in vitro effect can be partially abolished by irradiating the nude mice prior to use, suggesting that a radiation-sensitive non-T-cell surveillance of lymphoid tumours is operating in nude mice. While the identity of the antigens is still unknown, the specificity of the phenomena suggest that C-type RNA oncogenic viral determined antigens are involved.

Requirement for non-T cells in the generation of cytotoxic T lymphocytes in vitro. I. Use of nude mice as source of non-T cells

The ability of small numbers of LN cells to produce cytotoxic lymphocytes on in vitro culture with allogeneic stimulator cells is greatly augmented by the addition of spleen cells from athymic nude mice. The possibility that the synergism is a result of improved culture conditions or a "feeder effect" is excluded. All cytotoxic cells found in these cultures are shown to be T cells and to arise from precursors contained in the LN-cell component. The nude spleen cell component appears to be providing a required non-T cell which has been lost from the LN component through dilution. Synergism between the two components can occur whether they are syngeneic or allogeneic provided that both can recognize the same alloantigens in the stimulator population.

Participation of the H-2 antigens of tumor cells in their lysis by syngeneic T cells

Cytotoxic T lymphocytes were generated in vitro against H-2 compatible or syngeneic tumor cells. In vitro cytotoxic activity was inhibited by specific anti-H2 sera, suggesting that H-2 antigens are involved in cell lysis. Two observations directly demonstrated the participation of the H-2 antigens on the tumor cells in their lysis by H-2-compatible T cells. First, coating of the H-2 antigens on the target tumor cell reduced the number of cells lysed on subsequent exposure to cytotoxic T cells. Second, when cytotoxic T cells were activated against an H-2 compatible tumor and assayed against an H-2-incompatible tumor, anti-H-2 serum that could bind to the target cell, but not to the cytotoxic lymphocyte, inhibited lysis. H-2 antigens were also shown to be present on the cytotoxic lymphocytes. Specific antisera reacting with these H-2 antigens, but not those of the target cell, failed to inhibit lysis when small numbers of effector cells were assayed against H-2-incompatible target cells or when effector cells of F1-hybrid origin and bearing two H-2 haplotypes were assayed against a tumor cell of one of the parental strains. These findings suggest that it is the H-2 antigens on the tumor cell and not those on the cytotoxic lymphocytes that are important in cell-mediated lysis of H-2-compatible tumor cells.

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.

Functional interactions of viral and histocompatibility antigens at tumor cell surfaces

Several lines of evidence are presented to suggest that histocompatibility antigens can be physically associated on the cell surface with viral antigens and possibly other foreign antigens. The lysis of the murine tumor cells EL4 and P388 by syngeneic cytotoxic lymphocytes was inhibited by antisera directed against the H-2 antigens on the tumor cells, consistent with the hypothesis that H-2 antigens are part of the target of the cytotoxic lymphocytes. Moreover, it was found that patching and capping of the H-2 antigens on EL4 cells resulted in the co-patching and co-capping of viral antigens as detected by antisera against Rauscher leukemia virus. Capping of H-2 antigens also resulted in co-capping of determinants detected by an antiserum to the viral protein gp69/71. On the basis of these and other observations, we propose the hypothesis that the H-2 molecules serve as adaptors that combine with viral antigens on the cell surface to form hybrid antigens containing elements of self (H-2) and non-self (virus). The adaptor-antigen complex may then be recognized by a subclass of thymus-derived (T) lymphocytes that possesses a repertoire of receptors directed against hybirds of foreign and H-2 antigens. This raises the possibility that other products of the major histocompatibility complex may have analogous functions.

In vitro induction of tumour-specific immunity. I. Development of optimal conditions for induction and assay of cytotoxic lymphocytes

A microculture system for in vitro induction of tumour-specific immunity with syngeneic spleen cells and plasma-cell tumours is described. The system generates cytoxic lymphocytes assessed by their ability to lyse 51Cr-labelled target cells in a microcytotoxicity assay. Both the inductive and effector phases are subject to many variables in material and methodology and the optimal conditions are defined in this study. Of particular importance are the responder- to stimulator-cell ratios, the presence of 2-mercaptoethanol, the day of harvest, the method of target-cell 51Cr labelling, and preincubation of the cytotoxic lymphocytes prior to assay.