Role of natural killer cells in control of cancer metastasis

Biochemical regulation of adenylate cyclase in murine melanoma clones with different metastatic properties

The regulation of adenylate cyclase in murine melanoma tumor cell clones with different metastatic capacities has been studied in intact cells and isolated membrane preparations. Analysis of the responses of intact cells from a number of B16 melanoma clones revealed that treatment with melanocyte-stimulating hormone (MSH) or the diterpene, forskolin, produced significantly greater accumulation of intracellular cyclic adenosine 3',5' monophosphate (cAMP) in strongly metastatic clones than in weakly metastatic tumor cell clones. In contrast, in isolated membranes from the same panel of clones, the extent of activation by forskolin but not by MSH correlated with metastatic capacity. Sodium fluoride and 5'-guanyl-beta-gamma-imidodiphosphate [Gpp(NH)p] also stimulated adenylate cyclase in isolated membranes but the extent of activation did not correlate with the metastatic behavior of the donor cells. A combination of forskolin and Gpp(NH)p proved to be a sensitive prospective indicator for identifying differences in the metastatic capabilities of individual B16 melanoma clones. Adenylate cyclase in membrane preparations from strongly metastatic B16 clones displayed synergistic activation but stimulation of the enzyme from weakly metastatic clones was less than additive. To test the generality of these findings, similar investigations were performed on B16-BL6 melanoma cells, a highly invasive subline of the B16 melanoma, and the K1735, an ultraviolet-light-induced murine melanoma arising in a different mouse strain (C3H). Consistent with their high metastatic potential, clones derived from the B16-BL6 melanoma displayed elevated levels of hormonally-stimulated adenylate cyclase, thereby confirming, for this tumor system, a close association between hormonal responsiveness and metastatic capacity. In contrast, K1735 melanoma cell clones exhibited significant interclonal variation in adenylate cyclase activity and metastatic performance, but no consistent relationship between the two traits was detected. Differences in the regulation and/or the intrinsic catalytic capacity of adenylate cyclase may account, at least in part, for the variation in hormonal responsiveness observed among B16 clones with distinct metastatic properties and suggest that cAMP-dependent molecular processes may be required for the expression of B16 melanoma experimental metastatic potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Rationale and methods for the use of nude mice to study the biology and therapy of human cancer metastasis

Human neoplasms are biologically heterogeneous. The extensive cellular diversity found in malignant neoplasms is generated by the rapid emergence of clonal subpopulations of tumor cells with different properties that include invasion, metastasis and responsiveness to treatment. Studies in rodent systems have indicated that cancer metastases can be clonal in their origin and that different metastases can originate from different progenitor cells from the primary tumor. This metastatic heterogeneity of tumor cells has many ramifications for studies of tumor biology, in general, and studies of therapy, in particular. The heterogeneous nature of metastatic human neoplasms can now be studied under defined conditions in healthy athymic nude mice. The neoplasms must be free of mouse pathogens and the mice must be kept in specific-pathogen-free conditions. Careful consideration must be given to the intimate tumor-host relationship for each tumor system studied, because the metastatic potential of human neoplasms can vary with the site of implantation into nude mice. Several methods for studying the biology of human neoplasms in the nude mouse are described as well as techniques to assure the success of these studies. The data show that the healthy young nude mouse can be a useful in vivo model for ascertaining the metastatic potential of human neoplasms, for selecting and maintaining cell variants of high metastatic potential from heterogeneous human tumors, and for studying therapeutic agents directed against metastatic cells proliferating in visceral organs.

Treatment of adenocarcinoma in the peritoneum of mice: chemoimmunotherapy with IL-2-stimulated cytotoxic lymphocytes as a model for treatment of minimal residual disease

We have used a transplantable murine adenocarcinoma of renal origin (Renca) introduced to the abdomen by i.p. injection of a tumor cell suspension, to study the therapeutic potential of adoptive immunotherapy and/or biological response modifiers (BRMs). This tumor model is therapeutically challenging since the tumor grows progressively resulting in extensive peritoneal carcinomatosis, with hemorrhagic ascites, metastases to abdominal lymph nodes, liver, most serous membranes, spleen, and in some animals, pulmonary metastases. Without therapy, death occurs invariably in 36 +/- 3 days. In vitro, the tumor is lysed by lymphocytes obtained from the peritoneal cavity of mice treated with human recombinant interleukin-2 (rIL-2) and by cytotoxic lymphocytes stimulated by in vitro culture with human rIL-2. Treatment of i.p. Renca with a single i.p. injection of the chemotherapeutic agent doxorubicin hydrochloride (DOX), or adoptive transfer of in vitro stimulated cytotoxic lymphocytes together with rIL-2 cured 50% and 20% of the tumor-bearing mice, respectively. In contrast, combined therapy with DOX and adoptive transfer of in vitro stimulated cytotoxic lymphocytes and rIL-2 cured the majority (90%) of tumor-bearing mice. These results suggest that administration of immunotherapy with in vitro activated cytotoxic cells together with human rIL-2 substantially enhances the effectiveness of chemotherapy.

Tumor cell metastasis

Local tissue invasion and the formation of metastatic lesions are characteristic properties of many malignant tumors. The formation of metastases is a complex process involving the passage of tumor cells from the site of the primary bulk tumor through successive connective tissue barriers, ultimately resulting in the growth of secondary tumor cell colonies in distinct target organ locations. At many stages in the metastatic process, tumor cells interact with multiple components of the extracellular matrix. Recently, the importance of basement membrane as a barrier to invasive cells has been recognized. In the course of the transition from in situ to invasive carcinoma, normal or dysplastic epithelial cells residing on a basement membrane are replaced by neoplastic cells which subsequently invade the basement membrane and enter the underlying stroma. Once in the stroma, tumor cells can then penetrate the walls of blood vessels or the lymphatic system and enter into the circulation. Circulating tumor cells next arrest in the lumina of small vessels, invade the vessel wall, and leave the circulation. These cells are now directly exposed to the extracellular matrix of a target organ where they may grow to form secondary tumors. Throughout the metastatic process tumor cells are thus in contact with, and are potentially responsive to, various components of the extracellular matrix. This review provides a survey of the recent advances in our understanding of the interactions of metastatic tumor cells with the extracellular matrix. Specifically, the role of basement membrane as a barrier to metastatic tumor cells is examined.

Restoration by recombinant interferon alpha A/D of host defense systems against tumor in immunosuppressed mice

Recombinant human interferon alpha A/D (alpha A/D) restored or augmented host defense systems against tumors in immunosuppressed mice. In normal C57BL/6 mice, inoculation of B16 melanoma F1 cells caused few pulmonary metastasis, whereas in mice pretreated with cyclophosphamide (CY) it caused a high incidence of pulmonary metastasis, leading to earlier death than in the normal mice inoculated with the same dose of the tumor. alpha A/D given after the CY treatment counteracted the deleterious effects of the CY treatment. Since such restorative activity was seen even against the subline of B16 F1 which had been made resistant to its direct antiproliferative effect, alpha A/D seems to exert its effect indirectly through host defense systems. However, this activity of alpha A/D in the mice pretreated with CY was abrogated by inoculation of anti-asialo GM1 serum but not by i-carrageenan. The CY treatment reduced NK activity, while alpha A/D given after the CY treatment restored or augmented the NK cell activity in lung cells and peripheral blood mononuclear cells, but not in spleen cells. These findings suggest that the restoration or augmentation of NK activity in the lung and/or peripheral blood might be the major factor leading to the antimetastatic activity of alpha A/D in the mice treated with CY.

Augmentation of NK activity and/or macrophage-mediated cytotoxicity in the liver by biological response modifiers including human recombinant interleukin 2

Administration of several biological response modifiers (BRMs) to mice strongly augmented natural killer (NK) activity of leukocytes isolated from the liver. This augmentation of NK activity was induced by two synthetic molecules (MVE-2 and poly ICLC), by two BRMs of bacterial origin (formalin-fixed Propionibacterium acnes: P. acnes and a streptococcal cell wall preparation designated OK-432), as well as a single injection of human recombinant interleukin-2 (hrIL 2). All of these BRMs augmented NK activity in the liver to a greater degree than in the spleen. In addition, adherent leukocytes (greater than 90% macrophages) isolated from the liver following P. acnes administration also exhibited augmented macrophage-mediated cytotoxicity. This cytotoxicity was characterized as macrophage mediated and distinguished from NK activity, on the basis of adherence purification, kinetics of cytotoxicity, and target cell selectivity. The results demonstrate that a variety of BRMs induce augmented natural immunity in the liver and suggest that such organ-associated immune responses may play an important role in the antimetastatic effects of BRMs.

Role of natural killer activity in development of spontaneous metastases in murine renal cancer

We have studied the role of natural killer activity during the growth and dissemination of a transplantable renal adenocarcinoma (Renca) of spontaneous origin in BALB/c mice. The pattern of growth of this tumor accurately mimics that of adult human renal cell carcinoma in terms of clinical stages I-IV, particularly with regard to spontaneous metastasis to lung and liver. Renca is moderately sensitive to lysis by natural killer cells from normal mice and is more efficiently lysed by natural killer cells from mice treated with the biological response modifier maleic anhydride divinyl ether, a pyran copolymer. Our studies demonstrate that selective depression of natural killer activity by administration of antiserum specific for the neutral glycosphingolipid asialo GM1 correlated with increased formation of spontaneous metastases in the lungs, liver, and lymph nodes. Conversely, augmentation of natural killer activity by the biological response modifier decreased the formation of spontaneous metastases in lungs, liver and lymph nodes. Further, the suppression of natural killer activity and subsequent increased formation of metastases were accompanied by a significantly reduced survival time, whereas the augmented natural killer activity and decreased incidence of metastases in biological response modifier-treated mice were accompanied by an increase in time of survival. These results demonstrate a significant role for natural killer cells in the control of spontaneous metastasis during growth of this murine renal cancer.

Cell surface thiols, but not intracellular glutathione, are essential for cytolysis by a cloned murine natural killer cell line

Cell surface thiols are required for a line of cloned murine natural killer lymphocytes to bind to and lyse tumor target cells. These lymphocytes neither bound to nor killed YAC-1 or G1Tc cells when the effector lymphocyte cell surface thiols were covalently coupled with the non-penetrating reagent, monobromotrimethylammoniobimane (qBBr). A limited number of thiol-bearing proteins were identified by gel electrophoresis on the cell surface using the fluorescence of the group that remains associated with the sulfur molecule. These results indicate that either one or more of these reactive proteins or different cell surface thiol-bearing molecules present at low frequencies are crucial to lymphocyte binding and killing. In contrast, we found little evidence that intracellular thiols are required for natural killer cell activity. Killing was relatively unimpaired when over 90% of lymphocyte glutathione was depleted with DL buthionine-S,R-Sulfoximine (BSO). Blocking the intracellular or the extracellular thiols of tumor targets had no effect on their ability to be lysed. Based on these data, we suggest that infrequently expressed extracellular thiols are required either for the conformation or for the disulfide crosslinking of proteins that participate in lymphocyte-mediated cytolysis.

Changes in the host natural killer cell population in mice during tumor development. 2. The mechanism of suppression of NK activity

Our earlier studies revealed that a rapid and progressive loss of splenic NK activity in mice during the development of a number of transplanted tumors as well as of spontaneous tumors was due to an inactivation of natural killer (NK) lineage cells rather than to their disappearance. The mechanism of this inactivation have now been explored in CBA/J mice receiving transplanted Ehrlich ascites tumors and in C3H/HeJ mice bearing spontaneous mammary tumors or receiving transplants of syngeneic mammary tumor lines of recent origin. A poor activation state or maturation arrest of NK lineage cells due to a low interferon level in vivo was ruled out, since the host NK activity could not be restored after administration of either an interferon inducer poly(I:C) or interferon-alpha, although such treatments enhanced the activity in tumor-free mice by four- to eightfold. Possible presence of host suppressor cells acting on the effector or preeffector stage of NK cells was explored by mixing spleen cells from tumor bearers with normal spleen cells either during the NK assay, or for a 20-hr period of in vitro short-term culture prior to the NK assay. Mixing during the NK assay led to a reduction of NK activity explicable by a simple dilution of active NK cell concentration rather a suppression of active NK cells. On the other hand, a 20-hr coculture of the mixed population at various ratios led to a complete abrogation of the NK activity, indicating that the suppressor cells acted on the preeffector stage of the NK Lineage. A further characterization of suppressor cells revealed that they were (1) contained in the adherent fraction of the spleen of tumor bearers, (2) of monocyte/macrophage morphology, (3) capable of phagocytosing latex particles, and (4) positive for surface Mac-1 antigen, as noted from a radioautographic binding of 125I-labeled monoclonal anti-Mac-1 antibody. The mechanism of the suppression was identified, at least in part, as being mediated by prostaglandin-like molecules, since the presence of indomethacin, a prostaglandin-inhibitor, during the 20-hr coculture period completely abrogated the suppression. Indomethacin exerted no direct effect on the recruitment or killer activity of NK lineage cells in vitro. NK cell suppression may be another normal immunoregulatory mechanism which alters the host-tumor balance in favor of the tumor rather than the host.

Influence of adoptively transferred thioglycollate-elicited peritoneal macrophages on metastasis formation in mice with depressed or stimulated NK activity

The effect of thioglycollate-elicited macrophages (TG-M phi) on natural killer (NK)-cell activity and metastases formation in mice was investigated. Intravenously (i.v.) inoculated TG-M phi inhibited spleen NK activity of normal mice and abrogated polyinosinic: polycytidylic (poly I:C) induced augmentation of NK cell function. TG-M phi also inhibited the clearance of i.v.-injected radiolabeled B16 melanoma cells from the lungs of normal or poly I:C stimulated mice. Formation of experimental B16 melanoma metastases was dramatically increased in mice pretreated with TG-M phi. Administration of TG-M phi increased metastasis formation to a greater extent than anti-asialo GM1 serum, while anti-asGM1 serum was more efficient than TG-M phi in depressing spleen NK cell activity. When mice with low NK reactivity (beige mice or mice treated with anti-asialo GM1 serum) were inoculated with TG-M phi, there was a substantial additive augmenting effect on metastasis formation in the lungs. Treatment with poly I:C elevated NK-cell activity and had profound antimetastatic effects in normal but not in TG-M phi pretreated mice. The metastasis augmenting effect of TG-M phi was fully expressed in poly I:C-treated mice as well as in athymic nude mice. Inoculation of proteose peptone-elicited macrophages (PM phi), unlike TG-M phi, did not depress NK activity or augment metastasis formation in normal or poly I:C-treated mice. However, since the inhibition of NK activity in TG-M phi-treated mice was relatively weak, and a substantial additional increase in metastases was observed in NK-depressed mice after transfusion of TG-M phi, it seems unlikely that the TG-M phi-induced inhibition of NK reactivity is entirely responsible for the augmented formation of metastases. Further studies revealed that i.v. inoculation of TG-M phi, but not PM phi, induced intravascular inflammatory reactions, and damage to endothelial cells and basement membrane of the lung vasculature. These reactions may contribute to increased tumor cell extravasation and metastasis formation in mice pretreated with TG-M phi.

Cancer metastasis: experimental approaches, theoretical concepts, and impacts for treatment strategies

It has been the purpose of this article to describe recent advances in cancer metastasis research. Clinical realities and experimental approaches to the study of underlying basic mechanisms of metastasis formation were discussed. Wherever possible, results were reported which led to the development of theoretical concepts. Such results and concepts were finally evaluated in light of their possible impact for the design of new treatment strategies. Experimental findings from many diverse research fields were summarized with the help of tables, figures, and references. It was concluded that the process of metastasis is a dynamic event that can be described as a sequence of interrelated steps. Experimental results indicated that malignant cells that migrate and disseminate from the primary organ to distant sites and there eventually develop into metastases have to survive a series of potentially lethal interactions. Intimate tumor-host interactions were reported to take place all along the metastatic process. They were elucidated at the steps of angiogenesis, invasion, organ interaction, dormancy, tumor rejection, and tumor immune escape. The outcome of such tumor-host interactions seemed to depend on intrinsic properties of the tumor cells themselves as well as on the responsiveness of the host. Metastasis does not appear as a merely random process. Both clinical and experimental studies revealed that the whole process can be described more appropriately in terms of stochastic, sequential, and selective events, each of which is controlled and influenced by a number of mechanisms. With regard to therapeutic intervention, a selective event offers more possibilities than a random one because it is governed by rules that can be exploited experimentally. Various impacts from experimental studies for the design of antimetastatic cancer treatment strategies were discussed. Sequential steps of the metastatic cascade could become new therapy targets. Conventional empirically derived treatment modalities should become flanked by methods aimed more specifically at critical steps of cancer spread in order to prevent progression of the disease. This is where basic research on mechanisms could make significant contributions to therapy planning in the future. Furthermore, possible negative effects of surgery, radiotherapy, and adjuvant chemotherapy or immunotherapy that could result in enhancement of metastatic progression need to be critically evaluated to limit them as much as possible.(ABSTRACT TRUNCATED AT 400 WORDS)

Therapy of peritoneal murine cancer with biological response modifiers

We have used a murine renal adenocarcinoma of spontaneous origin (Renca) inplanted in the peritoneal cavity to study the therapeutic potential of biological response modifiers (BRMs) used alone or in conjunction with chemotherapy. This tumor model is therapeutically challenging since following intraperitoneal (i.p.) injection, the tumor grows progressively with hemorrhagic ascites, abdominal metastases to lymph nodes, liver, spleen, most serous membranes, and, in some animals, metastases to extra-abdominal sites (lungs). In the absence of therapy, death invariably occurs within 36 +/- 2 days. The tumor is efficiently lysed in 4 hours by peritoneal cells isolated from mice treated with BRMs. Both MVE-2 and rIL-2 significantly increased the survival time of tumor-bearing mice, but only treatment with MVE-2 led to definite cures of i.p. Renca. A single i.p. injection of MVE-2 cured 20% of the tumor-bearing mice, while repeated i.p. administration of this drug at 12 day intervals cured 70% of i.p. Renca-bearing mice. Combined therapy with doxorubicin hydrochloride and a single dose of MVE-2 cured 90% of tumor-bearing animals. The superior therapeutic efficiency of MVE-2 compared to that of the rIL-2 may be due to its ability, after i.p. inoculation, to generate and maintain high levels of cytotoxic effector cell activity for an elevated period of time within the peritoneal cell population. Additionally, MVE-2 augments effector cell activity in the liver, lungs, spleen, and blood and may therefore more efficiently interfere with metastasis formation in those compartments. The additive effects of MVE-2 and the chemotherapeutic agent suggest that more effective therapy may be achieved by the combination of immunotherapy with BRMs with chemotherapeutic drugs.

Stimulatory effect of Propionibacterium granulosum KP-45, glucan and pyran copolymer on the activity of natural killer (NK) cells in murine lungs

Propionibacterium granulosum KP-45, glucan and pyran copolymer stimulated the elimination of 75Selenomethionine-labelled 3LL tumor cells from murine lungs, as measured 4 hr after intravenous injection of these cells into 16- to 25-week-old B6DF1 mice. This effect was most pronounced 4 to 6 days following intravenous administration of the above biological response modifiers and disappeared 6 to 8 days later. Intraperitoneal injection of all three agents produced only insignificant stimulation results. Spontaneous clearance of 3LL cells from lungs of 8-week-old B6DF1 mice was significantly more effective than in animals over 16 weeks old. Cyclophosphamide suppressed the elimination of tumor cells from lungs in both young and older mice and neutralized the stimulatory effect of P. granulosum KP-45 and glucan. The results suggest that the effector cells responsible for the clearance of radiolabelled 3LL cells from lungs of B6DF1 mice are at least similar to natural killer (NK) lymphocytes.

Heterogeneity in a spontaneous mouse lung carcinoma: selection and characterisation of stable metastatic variants

The development and characterisation of a new epithelial model for the experimental investigation of metastasis is described. A tissue culture cell line CMT64 was established from a spontaneous alveolar lung carcinoma of a 17 month old female C57BL/ICRF at mouse (Franks et al., 1976). Subcutaneous inoculation of cells produces local tumours which give rise to a small number of lung metastases within three weeks. Four different tissue culture sublines CMT167, 170, 175 and 181 with increased metastatic ability were selected from pooled lung metastases by culture, mouse inoculation and reselection from lung metastases through four culture/inoculation cycles. These sublines are themselves heterogeneous and clones derived from them display marked differences in metastatic behaviour. Both CMT64 and its sublines have remained relatively stable in morphology and behavior since their origin, are fairly well differentiated, produce basal lamina even in metastases, and metastasise rapidly and preferentially to the lung after subcutaneous and intravenous inoculation in both syngeneic C57 and Nu/Nu mice (Franks & Layton, 1984). The expression of the metastatic potential of these cells is strongly influenced by the age and immune status of the host. The CMT64 system is a particularly useful model for experimental metastasis studies.

Neoplastic cells as targets of spontaneously cytotoxic lymphocytes: studies with natural killer-like cell lines

Native natural killer (NK) cells comprise a heterogeneous family of lymphocytes distributed among several organs, which display spontaneous cytotoxic reactions directed against a broad range of tumor targets. In these studies, murine cell lines have been established in vitro following the selective expansion of bone marrow- and spleen-derived killer progenitors in culture medium supplemented with interleukin-2. Several clones of independent origin have been characterized in order to determine the extent of their phenotypic and functional diversity. With few exceptions most of them were found to be highly effective in lysing a variety of tumor cell lines, to share common cell surface alloantigens, lectin-binding receptors, and cytochemical markers. The presence of prominent azurophilic cytoplasmic granules is the most characteristic ultrastructural feature of these cells. In attempting to elucidate the nature of membrane components specifically recognized by NK cells we compared several isogenic tumor cell variants selected on the basis of their differential NK susceptibility, immunogenicity, metastatic potential or resistance to cytotoxic plant lectins. Sialylated glycoconjugates exposed on the external face of the tumor cell membrane appear to be essential determinants in the interaction between NK cells and their targets. Permanent cell lines retaining most of the functional attributes of endogenous NK cells may prove instrumental in understanding their role during tumor progression.

Natural cytotoxicity in lymphatic metastasis. I. In vitro studies using the rat tumor BSp73 and its variants

Serial transplantation of primary BSp73 ascites cells to a subcutaneous (SC) site gave rise to the appearance of two solid variants differing in their capacity to metastasize via the lymphatics. Tissue cultures derived from variant AS (nonmetastasizing) showed epithelioid morphology, while cultures derived from variant ASML (metastatic) showed spherical morphology. Upon cloning, both variants proved to be operationally homogeneous. Susceptibility of cultured BSp73 cells to NK and macrophage-mediated cytotoxicity was closely correlated to morphology, inasmuch as epithelioid cells were susceptible, while spherical cells were resistant, to lysis. With stimulated effector cells a general increase in cytotoxicity was observed, but epithelioid cells still showed a higher susceptibility level. Resistance of ASML-type cells to natural cytotoxicity was not due to the lack of recognition structures or to a general increase in the mechanical stability of spherical cells. This was concluded from cold target inhibition and from hypotonic shock treatment experiments, respectively.

Tumor growth rate varies with age in lethargic mutant BALB/cGnDu mice

BALB/cGnDu lethargic mutant mice suffer from an age-related temporary defect in their cell-mediated immune response which is "spontaneously" corrected in animals 7 weeks of age or older. Thus, mutants of different ages (3 to 4 weeks old and 7 to 9 weeks old) were used to compare tumor incidence, tumor growth rate, and host survival time of Harding-Passey (HP) melanoma, mKSA, and GI110(BK)B6D2Tu tumors. Normal littermates were used as controls. Only the HP tumor was successfully transplanted in all recipients. In the 3- to 4-week-old lethargic mutants, the HP tumor had an increased growth rate and decreased the mean lifespan of the mice, when compared to normal littermates, but only one mKSA and no GI110(BK)B6D2Tu tumors proved transplantable. In contrast, lethargic mutants 7 to 9 weeks old injected with the HP tumor survived longer than their normal littermates, and they did not accept either of the other tumors tested. These results corroborate the notion that lethargic mutant mice have a partially impaired anti-tumor cell-mediated immune response at 3 to 4 weeks of age, but that their anti-tumor response is enhanced at the time of their "spontaneous" correction of the immune deficiency. The need for future studies on the possible use of this model to study various human immunological and adrenal disorders is discussed.

A modified short-term cytotoxicity test: assessment of natural cell-mediated cytotoxicity in whole blood

Using whole blood from normal subjects, we have observed natural killing of K562 cells in a 4 h 51Cr-release assay comparable with that shown by separated PBMC and whole blood depleted of serum components. Separated plasma was not toxic towards K562 targets, and failed to potentiate the level of PBMC cytotoxicity through ADCC. The presence of red blood cells did not influence natural killing. The natural cytotoxicity of whole blood was augmented by interferon and depressed by prostaglandins E1 and E2. Studies with appropriate control blood fractions show that cytotoxicity tests with whole blood provide results reflecting natural cell-mediated cytotoxicity.

Natural killer cells: artifact to reality: an odyssey in biology

The biology of the natural killer cell system is being investigated by many different laboratories using multiple approaches. The rationale for these investigations is the experimental evidence that NK cells play some role in inhibiting tumor growth and metastasis, convey some protective immunity and may be operative in control of differentiation from fetal life to adulthood. Thus, a survey of the literature reveals a multiplicity of studies examining many of these potential roles for NK cells. This review will attempt to summarize some of the findings critical to an understanding of the role NK cells play in immunophysiology and in immune reactions to various diseases. Even after ten years of study, the whole system of 'natural' reactivity remains difficult to define. The evidence available would indicate that the NK system while incompletely understood, may be manipulated in favor of the host when threatened by infectious or neoplastic disorders.

Rat macrophages inhibit natural killer (NK) cell activity against adherent growing target cells

Peritoneal as well as peripheral blood, but not spleen macrophages inhibit natural killer (NK) cell activity directed against adherent growing target cells. Inhibition was observed with macrophages (Mø): spleen cell ratios as low as 0.01:1. It was independent from the activation status of Mø and was not MHC restricted. With respect to the mechanism, cell contact was essential. A cold target mechanism, reflected by some binding, but not killing, of NK cells by Mø was of minor influence. Spontaneously by Mø released soluble mediators could be excluded, nor was any soluble mediator released upon contact of Mø with spleen cells, but NK activity could be partly inhibited by supernatants from cocultures of Mø with adherent growing target cells.

Experimental systems for analysis of the malignant phenotype

Identification of the cellular and subcellular alterations responsible for the metastatic behavior of malignant tumor cells and development of reliable screening programs for detecting new therapeutic agents for improved treatment of metastatic disease both depend crucially on the availability of experimental systems that can serve as relevant models of human cancer. Recent advances in our understanding of the pathogenesis of cancer metastasis have raised serious doubts about the usefulness of many of the experimental approaches that have long been used in the study of metastasis. Recent findings showing that metastases are caused by specific subpopulations of metastatic tumor cells, and that not all cells in a malignant primary tumor possess metastatic properties, are of profound importance for experimental efforts to understand the mechanism of metastatic phenotype among cells from the same tumor means that the traditional, and widely used, approach of analyzing primary tumors and cultured cell lines containing multiple, phenotypically heterogeneous, subpopulations of cells may provide little or no insight into the properties of the metastatic subpopulations, particularly if they represent only a minor fraction of the entire population. Similarly, the practice of screening potential therapeutic modalities for their ability to reduce the mass and/or growth rate of a primary tumor may be inadequate in predicting the responsiveness of metastatic lesions. Solution of these problems requires that new methods must be devised to isolate and characterize the specific subpopulations of tumor cells endowed with metastatic potential. In addition, knowledge of how the extraordinary phenotypic diversity found in tumor cell subpopulations from the same tumor is generated and how subpopulation diversity is regulated during progressive growth of both the primary tumor and its metastases are of fundamental importance if we are to design meaningful experimental systems for studying the metastatic process. This article reviews our current understanding of these complex issues and their implications for the experimental analysis of the malignant phenotype. The merits and shortcomings of different experimental systems are discussed in detail together with the identification of areas in which new experimental strategies and models are now needed.'