IN VITRO CYTOTOXIC EFFECT OF LYMPHOID CELLS FROM MICE IMMUNIZED WITH ALLOGENEIC TISSUE

DCs at the center of help: Origins and evolution of the three-cell-type hypothesis

Last year was the 10th anniversary of Ralph Steinman's Nobel Prize awarded for his discovery of dendritic cells (DCs), while next year brings the 50th anniversary of that discovery. Current models of anti-viral and anti-tumor immunity rest solidly on Steinman's discovery of DCs, but also rely on two seemingly unrelated phenomena, also reported in the mid-1970s: the discoveries of "help" for cytolytic T cell responses by Cantor and Boyse in 1974 and "cross-priming" by Bevan in 1976. Decades of subsequent work, controversy, and conceptual changes have gradually merged these three discoveries into current models of cell-mediated immunity against viruses and tumors.

A calcium optimum for cytotoxic T lymphocyte and natural killer cell cytotoxicity

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells are required for host defense. They destroy malignant target cells like cancer cells. Among metal cations, Ca²⁺ plays a prescinded role for CTL and NK cytotoxicity as it is the only cation used as ubiquitous second messenger. Measuring intracellular Ca²⁺ concentrations [Ca²⁺]_int in single cells has greatly changed our understanding of Ca²⁺ signaling. Yet, comparing the role of Ca²⁺ in the pre-[Ca²⁺]_int and [Ca²⁺]_int measurement era reveals that even in the pre-[Ca²⁺]_int measurement era (before 1980), the functions of Ca²⁺ and some other metal cations for the cytotoxic immune response were well established. It was even shown that Ca²⁺ influx across the plasma membrane but not Ca²⁺ release from intracellular sources is relevant for lymphocyte cytotoxicity and that very little Ca²⁺ is needed for efficient lymphocyte cytotoxicity against cancer cells. In the [Ca²⁺]_int measurement era after 1980, many of the important findings were better and more quantitatively refined and in addition the molecules important for Ca²⁺ transport were defined. The unexpected finding that there is a Ca²⁺ optimum of CTL and NK cell cytotoxicity deserves some attention and may be important for anti-cancer therapy.

In vivo evidence for extracellular DNA trap formation

Extracellular DNA trap formation is a cellular function of neutrophils, eosinophils, and basophils that facilitates the immobilization and killing of invading microorganisms in the extracellular milieu. To form extracellular traps, granulocytes release a scaffold consisting of mitochondrial DNA in association with granule proteins. As we understand more about the molecular mechanism for the formation of extracellular DNA traps, the in vivo function of this phenomenon under pathological conditions remains an enigma. In this article, we critically review the literature to summarize the evidence for extracellular DNA trap formation under in vivo conditions. Extracellular DNA traps have not only been detected in infectious diseases but also in chronic inflammatory diseases, as well as in cancer. While on the one hand, extracellular DNA traps clearly exhibit an important function in host defense, it appears that they can also contribute to the maintenance of inflammation and metastasis, suggesting that they may represent an interesting drug target for such pathological conditions.

Untangling "NETosis" from NETs

Neutrophil extracellular trap (NET) formation is a cellular function of neutrophils that facilitates the immobilization and killing of invading microorganisms in the extracellular milieu. To form NETs, neutrophils release a DNA scaffold consisting of mitochondrial DNA binding granule proteins. This process does not depend on cell death, but requires glycolytic ATP production for rearrangements in the microtubule network and F-actin. Such cytoskeletal rearrangements are essential for both mitochondrial DNA release and degranulation. However, the formation of NETs has also been described as a distinct form of programed, necrotic cell death, a process designated "NETosis." Necrotic cell death of neutrophils is associated with the permeabilization of both plasma and nuclear membranes resulting in a kind of extracellular cloud of nuclear DNA. The molecular mechanisms eliciting necrotic neutrophil death have been investigated and appear to be different from those responsible for NET formation following mitochondrial DNA release. Here, we discriminate between the mechanisms responsible for the release of mitochondrial versus nuclear DNA and address their respective functions. Our aim is to clarify existing differences of opinion in the fields of NET formation and neutrophil death.

A Novel Method for Assessment of Natural Killer Cell Cytotoxicity Using Image Cytometry

Natural killer (NK) cells belong to the innate arm of the immune system and though activated NK cells can modulate immune responses through the secretion of cytokines, their primary effector function is through target cell lysis. Accordingly, cytotoxicity assays are central to studying NK cell function. The ⁵¹Chromium release assay, is the “gold standard” for cytotoxicity assay, however, due to concerns over toxicity associated with the use and disposal of radioactive compounds there is a significant interest in non-radioactive methods. We have previously used the calcein release assay as a non-radioactive alternative for studying NK cell cytotoxicity. In this study, we show that the calcein release assay varies in its dynamic range for different tumor targets, and that the entrapped calcein could remain unreleased within apoptotic bodies of lysed tumor targets or incompletely released resulting in underestimation of percent specific lysis. To overcome these limitations, we developed a novel cytotoxicity assay using the Cellometer Vision Image Cytometer and compared this method to standard calcein release assay for measuring NK cell cytotoxicity. Using tumor lines K562, 721.221, and Jurkat, we demonstrate here that image cytometry shows significantly higher percent specific lysis of the target cells compared to the standard calcein release assay within the same experimental setup. Image cytometry is able to accurately analyze live target cells by excluding dimmer cells and smaller apoptotic bodies from viable target cell counts. The image cytometry-based cytotoxicity assay is a simple, direct and sensitive method and is an appealing option for routine cytotoxicity assay.

QUANTITATIVE STUDIES ON THE BEHAVIOR OF SENSITIZED LYMPHOCYTES IN VITRO : I. RELATIONSHIP OF THE DEGREE OF DESTRUCTION OF HOMOLOGOUS TARGET CELLS TO THE NUMBER OF LYMPHOCYTES AND TO THE TIME OF CONTACT IN CULTURE AND CONSIDERATION OF THE EFFECTS OF ISOIMMUNE SERUM

When lymphoid cells, derived from rats immunized with respect to homologous skin, were cultured with target cells originally of donor origin, cytocidal and cytostatic activities of the attacking lymphocytes became evident. By application of a sensitive and reproducible quantitative assay system, various aspects of the mechanism of this destructive interaction between target cells and lymphocytes were examined with the following results. 1. The degree of survival of target cells was inversely related to the number of sensitized lymphocytes. Graphic plots of the data indicated that this relationship was an exponential one similar to "single-hit" inactivation phenomena. One interpretation which could be placed on these results is that a single lymphocyte, if immunologically active, was sufficient to destroy or at least have a detectably adverse effect on one target cell. Furthermore, from such a model it could be computed that, of the lymphocytes derived from an immunized animal, approximately 1 to 2 per cent of the cells were immunologically active; i.e., capable of demonstrable destructive activities against homologous target cells in vitro. 2. Morphological studies on the effect of sensitized lymphoid cells on homologous target cells, aftervarious lengths of time in culture, showed that by 7 hours of incubation, the attacking lymphocytes firmly adhered to the target cells. The cytotoxic effect of these lymphocytes generally occurred after the 20th hour. Quantitative studies supported this conclusion; the latent period, i.e., the time required for detectable degrees of target cell destruction to occur, was approximately 20 hours. 3. A consequence of the incubation of target cells with normal lymphoid cells or even with small numbers of sensitized lymphoid cells was an increase in the rate of division of the target cells. As might be expected, this was reflected in a shorter doubling time of these cells. 4. Extracts prepared from sonically disrupted sensitized lymphocytes proved to be no more deleterious to target cells than similar preparations from normal lymphoid cells. Furthermore, no evidence could be obtained that sensitized lymphoid cells, separated from target cells by a Millipore membrane, were cytocidally effective. These data indicated that if a cell-bound substance is involved in the destruction of homologous cells, either it is not toxic by itself, or it cannot be detached from the sensitized cells. In any case, close apposition of the lymphocytes to the target cells is apparently required for the destruction of the latter in vitro. 5. Serum obtained from immunized animals, if heat-inactivated, did not adversely affect homologous target cells; if employed fresh, slight degrees of toxicity resulted. Specific isoimmune sera did not impart any detectable degrees of immunological reactivity upon otherwise normal lymphoid cells. Immune sera, even in high concentrations, did not augment the effect of sensitized lymphoid cells upon homologous target cells; rather a slight inhibitory effect of these sera was detected. 6. Attempts to detect the presence of complement activity, which might have been provided by the lymphoid cells in culture, were unsuccessful. On the basis of these results, it was suggested that the destruction of homologous target cells by sensitized lymphoid cells occurs as a two step process. First, the attacking lymphocytes attach to their targets via a non-toxic cell-bound substance having an immunologic specificity, and then, destruction of the target cells follows the result of some process dependent on the metabolic activity of the attacking lymphoid cells.

ANTAGONISTIC EFFECTS OF HUMORAL ISOANTIBODIES ON THE IN VITRO CYTOTOXICITY OF IMMUNE LYMPHOID CELLS

The ability of specifically immunized lymphoid cells to kill H-2 incompatible target tumor cells in tissue culture was shown to depend on the source of the lymphoid tissue (spleen versus lymph nodes). Marked cytotoxic effects were obtained with regional lymph node cells 7 to 10 days after primary immunization, whereas spleen cells from the same animals had little or no effect. Hyperimmunization did not decrease the cytotoxic efficiency of lymph node cells. Experiments were performed to test the possibility that the weak effect of spleen cells is a result of humoral antibody production, antagonizing the cell-bound immunity. Humoral antibodies were cytotoxic in vitro in the presence of complement only. Their effect was manifested after 2 hours, whereas immune lymph node cells did not require complement and cytotoxicity was not expressed until 24 to 48 hours' incubation. Tumor cell cultures treated with specific humoral antibodies in the absence of complement became resistant to the cytotoxic effect of subsequently added immune lymph node cells, while no such protection was seen when normal serum was added. Thus, humoral antibodies led to an "efferent" inhibition of cell-bound immunity in vitro, in analogy with previous results in vivo.

A program to facilitate cytotoxicity assay data reduction

A report of a program which performs the initial computational reduction of the raw data from a cytotoxicity assay and outputs the reduced data as an image of the arrangement of the assay(s) upon the microwell plate. The program accepts the raw data either as manual or diskette file input.

An enzymatic assay for the detection of natural cytotoxicity

The pronase-enhanced release of 14C-labeled DNA was used to assay cytotoxicity of murine natural killer cells. YAC-1 lymphoma cells were labeled with [14C]thymidine and incubated with increasing numbers of spleen cells from nude mice. At harvest, nuclease-free pronase was used to digest damaged target cells. Increases in cytotoxicity of 5--39% were obtained using optimal conditions. The reaction could be successfully inhibited with unlabeled YAC-1 cells. The degree of cytotoxicity was similar to that detected by 51Cr release, but a longer period of incubation was required.

A simplified isotope release assay for cell-mediated cytotoxicity against anchorage dependent target cells

An assay for cell-mediated cytotoxicity has been developed in which anchorage-dependent target cells are cultured on small plastic beads in suspension. Confluent target cells on the beads are handled by methods appropriate to suspension-grown cells and labelled with chromium-51, iodine-125 and [3H]proline. Fetal human lung fibroblasts and HeLa cells were used as targets in model experiments measuring human natural killer cell activity. In 20 h experiments, chromium-51 was the most suitable isotope. In 40 h experiments, [3H]proline release assay was superior to chromium-51 and iodine-125 assays. The bead cytotoxicity assay offers a rapid and simple isotope release technique for anchorage dependent cells because no trypsinization and re-seeding of target cells is needed.

Cellular immunity in mammary cancer patients as measured by the leukocyte migration test (LMT). A follow-up study

In 58 cases of mammary cancer treated by surgery the leukocyte migration test (LMT) has been applied to the study of cellular immunity using homogenate from autochthonous and homologous tumors as antigens. A positive test, i.e. inhibition of migration by antigen, was observed in 52 patients from 1 day up to 40 days after surgery. Six patients were negative. There was extensive immunological cross-reactivity among mammary cancer in that a positive test was also obtained with homogenate from homologous mammary tumors. Generally, antigens from other tumors did not react. The test was also negative when the leukocytes were derived from healthy persons. Sera from mammary cancer patients abrogated the inhibitory effect of tumor antigen. The disappearance of positivity about 40 days after surgery and its reappearance during the metastatic process indicates that the positive LMT is associated with progressive disease.

Mixed lymphocyte cultures produce effector cells: model in vitro for allograft rejection

Mouse peripheral lymphocytes sensitized in vitro by culturing with allogeneic lymphocytes produced immunospecific destruction of target cells, as measured by release of chromium-51. Thus the sensitizing and effector phases of the cell-bound immune response can both be studied in an in vitro system.

Cytotoxicity by nonimmune allogeneic lymphoid cells. Specific suppression by antibody treatment of the lymphoid cells

Nonimmune lymphoid cells were capable of causing cytotoxicity of H-2 incompatible mouse tumor cells in vitro in the presence of PHA, whereas syngeneic cells were not. Semisyngeneic and X-irradiated (1500-3000 R) F(1) hybrid lymphoid cells were cytotoxic for target cells derived from one of the parental strains. In addition, parental nonimmune and X-irradiated lymphoid cells damaged hybrid target cells. It was concluded that one component of cytotoxicity was not related to an induction of a primary immune response in vitro, since F(1) hybrid cells are not capable of reacting immunologically against parental type target cells. It seemed probable that cytotoxicity was caused by target cell confrontation with antigenically and/or structurally incompatible lymphoid cells. This conclusion was strengthened by the demonstration that isoantibodies produced in the target strain and directed against the allogeneic lymphoid cells specifically suppressed cytotoxicity. Isoantibodies reacting against some but not all of the antigenic determinants of the lymphoid cells differentiating them from the target cells did not suppress cytotoxicity. The specific suppression of cytotoxicity by specific isoantibodies against the lymphoid cells support the allogeneic inhibition concept.