Variation in selectivity of tumor cell cytolysis by murine macrophages, macrophage-like cell lines and NK cells

Several murine tumor-cell lines were tested by isotope release assays for their susceptibility to lysis by either activated peritoneal macrophages (apMPh), macrophage-like (MPh-like) cell lines, or natural killer (NK) cells. The qualitative selectivity of tumor-cell lysis by these different effector cells was quite disparate. The rank order of target cell susceptibility to lysis by apMPh in 24 h assay was L5178Y greater than P815 approximately equal to RL male greater than YAC-1 approximately equal to MBL-2. This was seen regardless of whether peritoneal MPh (pMPh) were activated by LPS or poly I:C. Two MPh-like cell lines, PU-5R and FC-1, had a pattern of cytotoxic activity against these target cells that closely paralleled that associated with apMPh, although levels of reactivity differed quantitatively among the effector cells. In contrast, the MPh-like cell line RAW-264 expressed a qualitatively different pattern of target-cell selectivity, preferentially lysing MBL-2, which was relatively refractory to lysis by other MPh-like cell lines or apMPh in the 24 h cytolytic assay. When spontaneous or interferon (IFN)-augmented NK activity was measured against the same panel of target cells, the pattern of selectivity was qualitatively different from that observed for apMPh. The consistent rank order of susceptibility to lysis by NK cells was YAC-1 greater than RL male 1 greater than P815 approximately equal to L5178Y approximately equal to MBL-2. The characteristic patterns of target-cell selectivity for apMPh or NK cells were the same for all of the strains of mice tested. From the different selectivity patterns of apMPh and NK cells, it is concluded that lysis of target cells is not based solely on inherent sensitivity to cytolysis. Instead, selectivity of lysis is probably due to variations in expression of target-cell structures recognized by each type of effector cell, and/or in susceptibility to the lytic mechanism(s) of the various effector populations.     

In vitro and in vivo interaction between murine fibrosarcoma cells and natural killer cells

Murine fibrosarcoma cells were examined for sensitivity to killing by natural killer (NK) and natural cytotoxic lymphocytes from mouse spleens. These tumor cell lines were sensitive to killing by effector cells which were nonadherent to plastic or nylon wool, Thy-1 negative, asialo-GM1 negative, and present in the spleens of beige mice, nude mice, and A/J mice, as well as in the spleens of normal syngeneic and allogeneic control mice. This indicates that the cytotoxic effects were due to natural cytotoxic lymphocytes rather than to NK lymphocytes, T-cells, or macrophages. Although the fibrosarcoma cells were not killed in vitro by endogenous NK cells, these tumor cells were able to "cold target" compete for Yac-1 (an NK-sensitive target) killing and to bind to asialo-GM1-positive, nonadherent spleen lymphocytes in a target cell binding assay. This suggests that the fibrosarcoma cells were recognized by NK cells. In addition, these cell lines were killed in a 4-h NK cytotoxicity assay by polyinosinic-polycytidylic acid-activated effector lymphocytes. The interaction between NK cells and the murine fibrosarcoma cells may have in vivo significance. When syngeneic mice were treated with anti-asialo-GM1 serum to eliminate NK activity and then given i.v. injections of the fibrosarcoma cells, many more lung tumors developed than in control animals. The structural basis for the recognition of the murine fibrosarcoma cells by the NK effector cells is not known. However, laminin may be involved. When the fibrosarcoma cells, which have receptors for the laminin molecule, were preincubated with laminin, they were reduced in their ability to compete for the killing of Yac-1 cells by the NK effectors and had reduced capacity to bind to NK cells in a target cell binding assay.     

Enhanced natural killer sensitivity with concomitant clonal selection for cells bearing homogeneously staining regions in the human melanoma cell line MeWo upon induction of differentiation with theophylline

Culture of the human melanoma cell line MeWo in the presence of 1 mM theophylline was associated with an increase in susceptibility to natural killer (NK)-mediated cytolysis. The phenomenon was detected as early as 72 hours after initiation of theophylline treatment, reaching maximum values at 3-4 weeks and remaining stable for longer than 3 months of testing, provided the cells were maintained in the presence of theophylline. The alteration in target sensitivity was selective for NK-mediated cytolysis, since other mechanisms of cell-mediated cytolysis, including antibody-dependent cell-mediated cytotoxicity and monocyte-mediated and lectin-induced cytolysis, were comparable between untreated and treated cells. The enhanced susceptibility of theophylline-treated cultures to NK lysis, as compared to NK lysis susceptibility of untreated MeWo cells, was not significantly changed by pretreatment of effector lymphocytes with interferon. Evidence for differentiation in theophylline-treated cultures was obtained. In addition, however, cytofluorometric and karyologic analysis revealed the existence of two subpopulations of differing ploidy in the MeWo line. The hypodiploid, NK-sensitive subpopulation, bearing homogeneously staining regions on two chromosomes, could be selected by growth in theophylline. Therefore, selection of subpopulations in heterogeneous tumor cell lines by chemical inducers suggests an alternative and novel mechanism for enhancement of NK sensitivity.     

Natural cytotoxicity of haemopoietic cell populations against murine lymphoid tumours.

Homozygous nude and normal mice of 3 strains, BALB/c, CBA and C57BL, were used as sources of nucleated haemopoietic "natural killer" (NK) cells. These killer cells could lyse a wide range of syngeneic and allogeneic lymphoid tumour cell lines in vitro, and it was found that cell suspensions from nude mice were always significantly more active than those from normal mice, and that the most active effector population was a polymorph-enriched peritoneal-exudate cell suspension. Eosinophils did not appear to be involved in the phenomenon, and mononuclear peritoneal-exudate cell suspensions were actually highly inhibitory. Three non-lymphoid tumours, a carcinoma, a fibrosarcoma and a mastocytoma, were totally resistant to in vitro lysis. Although all susceptible tumour cell lines were C-type virus-associated, not all of these tumours were killed by all strain sources of spleen cells, indicating a specificity of killing.     

Increased cytotoxicity against B-chronic lymphocytic leukemia by cellular manipulations: potentials for therapeutic use

B-cell chronic lymphocytic leukemia (CLL) is characterized by profound immune dysfunction and a marked resistance to apoptosis. Understanding the cellular biology of immune effector cells from CLL patients as well as leukemic target cells is essential to developing immune mediated therapeutic strategies for CLL. In this study, an immortal CLL cell line called WSU-CLL has been used to study the characteristics of B-cell CLL as a tumor target for natural killer (NK), activated natural killer, and lymphokine activated killer (LAK) cells. The WSU-CLL cells were significantly less (p<0.001) susceptible to NK cell mediated cytotoxicity compared to K562, a standard tumor target cell line. In vitro activation of effector cells with either short term, low dose IL-2 or long term, high dose IL-2 significantly increased the susceptibility of CLL cells for cell mediated killing. The addition of CD1a+/CD3-/CD4+/CD80+/CD83+ dendritic cells derived from human umbilical cord blood increased the cytotoxicity of LAK cells against WSU-CLL. There is an increased expression of Bcl-2 and decreased expression of Fas on WSU-CLL cells as determined by RT-PCR techniques indicating possible roles for these genes in exerting resistance to immune cell mediated lysis. When Bcl-2 expression was downregulated in WSU-CLL cells using gene specific antisense oligonucleotides, the susceptibility of WSU-CLL cells to the cytotoxicity of chemotherapeutic agent Fludarabine was increased. Thus, our results suggest that in vitro activation with cytokines, addition of accessory cell populations such as dendritic cells and/or manipulation of key gene expression i.e. down regulation of Bcl-2 might be potential strategies to increase the antitumor cytotoxicity against CLL cells.     

Natural cytotoxicity in humans: susceptibility of freshly isolatd tumor cells to lysis

The cytotoxic potential of blood lymphocyters from healthy donors was tested against freshly isolated lung cancer cells and the erythroleukemia K562 cell line in short-term 51Cr release assays conducted at an effector:target ratio of 50:1. Most donors exhibited significant activity against K6-562 cells. By contrast, fresh tumor cells were refractory, only 6 of 30 showing significant cytotoxicity. The low susceptibility of these tumor cells was confirmed in third-party cold inhibition assays in which they interfered minimally with killing of K562 targets under conditions in which unlabeled K562 cells efficiently blocked cytotoxicity. Cells prepared from normal lung tissue and Raji cells also failed to inhibit killing. Although in comparison to the K562 cell line freshly isolated tumor cells were resistant, their susceptibility may not be so low as to be biologically irrelevant, inasmuch as boosting of natural killing activity by interferon induced levels of cytotoxicity against both types of target cell that were unattainable by unstimulated effectors. Interferon-boosted killers were lytic for "normal" lung cells and the Raji cell line.     

Sensitivity of human carcinoma cell lines to lysis by blood natural killer cells correlating with surface expression of carcinoembryonic antigen

The HCT-8R clone of the HCT-8 human colon tumor line, which expresses increased quantities of carcinoembryonic antigen (CEA) on its surface, was discovered to have an enhanced susceptibility to lysis by natural killer (NK) cells in human peripheral blood. This increase in susceptibility to lysis by peripheral blood mononuclear cells was not explained by stimulation of interferon release by HCT-8R cells but rather was found to be attributable to an increased susceptibility of HCT-8R cells to lysis by those NK cells that bind to sheep erythrocytes (E-RFC). Cold target competition experiments and single-cell assay for cytotoxic cells suggested that the presence of surface CEA did not increase lysis of HCT-8R by facilitating "recognition" by E-RFC-type cytotoxic cells but by rendering HCT-8R cells more susceptible to the lytic mechanism of NK cells. The magnitude of expression of surface CEA by a variety of human carcinoma cell lines with a few exceptions and subclones of HCT-8 also correlated with increased susceptibility to lysis by blood mononuclear cells. The possible clinical significance of these findings was discussed.     

Differential susceptibility of metastatic lymphoma cells to natural immunity

The susceptibility of metastatic variant lymphoma cells to natural immunity was studied using a low malignant/metastatic parental RAW117-P cell line and its liver colonizing highly malignant/metastatic RAW117-H10 cell line. The metastatic variant RAW117-H10 cells express a significantly lower amount of laminin-like and fibronectin-like molecules as determined by flow cytometry using monospecific polyclonal antibodies to laminin and fibronectin. Our studies indicated that the RAW117-H10 cells are resistant to natural killer (NK) cell-mediated cytotoxicity. In vitro activation of the effector cells with interferon-gamma increased the susceptibility of these cells to NK-mediated cytotoxicity while maintaining the difference between the two cell lines. However, when recombinant interleukin-2 was used to activate the effector cells, the cytotoxicity of the lymphokine-activated effector cells to both parental low metastatic RAW117-P cells and highly metastatic RAW117-H10 cells was similar.     

Sialomucin and lytic susceptibility of rat mammary tumor ascites cells

The potential role of cell surface sialomucin in preventing natural killer (NK)-mediated lysis of tumor cell targets has been addressed by comparing the properties of 2 NK-resistant [ascites (ASC) and short-term cultured (STC)] and 2 NK-susceptible [tunicamycin-treated (TUN) and long-term cultured (LTC)] preparations of 13762 MAT-B1 rat mammary tumor cells. Both the ASC and STC cell preparations contain elevated levels of the sialomucin ASGP-1 relative to TUN and LTC preparations as determined by [3H]glucosamine labeling and by binding of peanut agglutinin. The major difference in the susceptibility to NK-mediated lysis appeared to be due to the differences in the susceptibility to lysis by lytic granules, rather than to differences in the ability to bind or trigger effector cells, since TUN and LTC cells were approximately 10-fold more sensitive to lysis by lytic granules than were ASC and STC cells. All preparations inhibited the lysis of the susceptible target YAC-1 by normal rat splenocytes, indicating an ability to bind these effector cells. Triggering of effectors, as monitored either by incorporation of 32P into phosphatidylinositol or by transmethylation of phosphatidylcholine, was similar for the positive control YAC-1, STC, TUN, and LTC, whereas ASC appeared to be defective in triggering effectors. These results suggest that tumor sialomucin blocks the final phase of lysis, but not the initial recognition of tumor cells by NK effectors.     

NK sensitivity and lung clearance of MHC-class-I-deficient cells within a heterogeneous fibrosarcoma

The present study examines clonal variations in NK sensitivity in a methylcholanthrene-induced fibrosarcoma. Previous studies of clones from this tumor have shown considerable heterogeneity in H-2 expression, and an association between deleted or low levels of class-I products and increased tumorigenicity after subcutaneous implantation in immunocompetent syngeneic mice. Here, fibrosarcoma clones with no or low expression of MHC-class-I products were found to be sensitive to NK-mediated lysis, while clones with high levels of MHC-class-I expression were relatively resistant. One H-2+ (G2) and one H-2- (B9) clone were chosen for more detailed studies. Cold-target competition assays and conjugate cytotoxicity assays in agarose showed that splenic effector cells bound equally well to the H-2+ and H-2- tumor clone, although only the latter was sensitive to NK cell lysis. Treatment with 50 U/ml of rIFN-gamma for 48 hr increased the levels of H-2 expression and made both clones more resistant to NK-mediated lysis. In vivo studies with radiolabelled tumor cells showed that cells from the H-2+ clone survived better than cells from the H-2- clone in the pulmonary capillary bed after i.v. inoculation. This difference disappeared in mice treated with anti-asialo GM1 serum, known to deplete NK cell activity.     

DNA virus-transformed hamster cell--host effector cell interactions: level of resistance to cytolysis correlated with tumorigenicity

Spontaneously cytolytic hamster spleen cells and BCG-activated hamster macrophages were used to examine susceptibilities to nonspecific effector cell-induced lysis among 13 DNA virus-transformed hamster cell lines exhibiting four different tumorigenic phenotypes. Hamster cells transformed by adenovirus type 12 (an oncogenic adenovirus serotype) or simian virus 40 (an oncogenic papovavirus) readily induced tumors in immunocompetent syngeneic hamsters and were relatively resistant to spleen-cell-induced lysis compared to cells transformed by adenovirus type 2 (a non-oncogenic adenovirus serotype) which induced tumors only in immunoincompetent hosts. Simian virus 40-transformed cells, which possess the unusual property of efficient tumor induction in allogeneic hosts, were uniquely resistant to lysis by activated macrophages. These differential patterns of susceptibility to cytolysis suggest an association between the level of transformed cell resistance to lysis by nonspecific host effector cells and the oncogenicity of the transforming virus. Furthermore, these data suggest that tumor-cell properties, other than those commonly associated with neoplastic transformation, determine the level of susceptibility or resistance to host effector cell mechanisms.     

Effect of IFN-gamma treatment and in vivo passage of murine tumor cell lines on their sensitivity to lymphokine-activated killer (LAK) cell lysis in vitro; association with H-2 expression on the target cells

Interferon-gamma (IFN-gamma) treatment or in vivo passage of the murine YAC-1 lymphoma resulted in reduced sensitivity to in vitro lysis by syngeneic murine spleen cells cultured in rIL-2 (LAK-cells). IFN-gamma treatment also rendered the murine B16 melanoma less sensitive to lysis by syngeneic LAK cells, whereas in vivo passage did not alter LAK sensitivity. The reduction in sensitivity to lysis correlated with enhanced expression of cell surface H-2 on the target cells. The possible role of H-2 was studied with a beta 2-microglobulin-deficient, and thus H-2-deficient, variant of the YAC-1 lymphoma. This variant line remained H-2 negative even after IFN-gamma treatment or in vivo passage, and was highly sensitive to LAK-cell-mediated lysis, even after IFN-gamma treatment or in vivo passage. The present results are discussed in relation to IFN-gamma and in vivo induced modulation of MHC class-1 molecules on target cells and the possible consequences for interaction with activated as well as "natural" effector cells.     

Adriamycin-induced resistance to natural killer (NK)-mediated cytotoxicity

Studies were done to determine the susceptibility of a colon carcinoma cell line, LoVo, to natural killer (NK) mediated lysis after exposure to the tumor cells to Adriamycin (ADR) (doxorubicin; Adria Laboratories, Columbus, OH). LoVo cells were exposed to ADR (0.4 micrograms/ml) for various time intervals and then tested for sensitivity to lysis by NK cells from the peripheral blood of normal donors in a sodium chromate (Cr51) release cytotoxicity assay. Exposure of tumor targets to ADR induced a resistance to NK-mediated lysis. Susceptibility to lysis decreased progressively to approximately 60% of control levels after 72 hours of ADR exposure. The induction of resistance was dependent on ADR dose, but did not magnify with doses greater than 0.4 micrograms/ml. When target cells were allowed to recover from ADR in fresh medium for 48 hours, complete reversal of the ADR effect was seen. The effect of interleukin-2 (IL-2) stimulation on the NK lysis of LoVo also was tested. IL-2-stimulated effector cells demonstrated enhanced cytotoxicity to LoVo targets and were able to overcome the ADR-induced resistance to lysis. The mechanism of resistance does not appear to be related to the altered binding of effectors to chemotherapy-treated targets, as suggested by single cell assay results.     

Susceptibility of NB-I neuroblastoma cells to tumoricidal activity of monocytes activated by gamma-interferon

The purpose of this study was to examine the susceptibility of NB-I human neuroblastoma cells to direct cellular cytotoxicity mediated by peripheral blood monocytes from pediatric cancer patients receiving chemotherapy. Nonactivated monocytes from patients showed spontaneous cytotoxicity to NB-I neuroblastoma cells (37 +/- 18%) but only marginal cytotoxicity to A375 melanoma cells (21 +/- 14%) at the effector:target cell ratio of 20:1. This spontaneous cytotoxicity to NB-I cells was observed only after greater than 24 h of cocultivation and was proportional to the effector:target cell ratio. Activation of monocytes by recombinant human interferon gamma (rIFN) (1 x 10(4) U/ml) consistently and strongly enhanced their tumoricidal activity to NB-I cells (87 +/- 6%) and this tumoricidal activity was even superior to that observed against A375 cells, which are known to be extremely sensitive to lysis by activated monocytes. In contrast, activation of monocytes by lipopolysaccharide (LPS, 1 microgram/ml) had no effect on monocyte-mediated lysis of NB-I cells, while A375 cells were equally lysed by rIFN- and LPS-activated monocytes, thus suggesting that different mechanisms are involved in the monocyte-mediated lysis of A375 melanoma and NB-I neuroblastoma cells. Susceptibility of the neuroblastoma cell line to monocyte-mediated cytotoxicity has not been reported so far and our results may have some clinical implication if this observation can be extended to other neuroblastoma cell lines as well.     

Nonselective destruction of murine neoplastic cells by syngeneic tumoricidal macrophages

The purpose of these studies was to select in vitro tumor cells that were resistant to macrophage-mediated lysis. Seven different heterogeneous murine neoplasms (four fibrosarcomas, a melanoma, a rhabdomyosarcoma, and an osteogenic sarcoma) and one cloned line of a fibrosarcoma were incubated in vitro with syngeneic tumoricidal macrophages. Surviving tumor cells were recovered and expanded to undergo subsequent interaction with tumoricidal macrophages. After six sequential interactions, all cell lines were examined for their susceptibility to lysis mediated by murine peritoneal exudate macrophages activated with liposomes containing muramyl tripeptide phosphatidylethanolamine. In all eight systems, no significant differences were detected between the parent tumor cells and cells that survived the sequential interactions. Neither macrophage infiltration into s.c. tumors nor the experimental or spontaneous metastatic potentials of the parental tumors differed from the lines established by cells surviving macrophage-mediated lysis. Collectively, the data suggest that tumor cell destruction by activated macrophages is nonselective and does not lead to the development of resistant tumor cells nor to cells with altered metastatic properties.     

Enhanced susceptibility of adriamycin-treated human renal cell carcinoma cells to lysis by peripheral blood lymphocytes and tumor infiltrating lymphocytes

Previous studies indicated that the anticancer agent adriamycin (ADR) could induce activation of cytotoxic T lymphocytes (CTL) and natural killer cells. In this study, we investigated the effect of ADR on the susceptibility of human renal cell carcinoma (RCC) cells to lysis by peripheral blood lymphocytes (PBL) and tumor infiltrating lymphocytes (TIL). Treatment of human RCC cell line ACHN and freshly derived RCC cells with ADR at 1 microg/ml or more for 3 h significantly enhanced their susceptibility to lysis by PBL (P<0.05). This ADR-induced enhancement of susceptibility of RCC cells to lysis by PBL was also observed when freshly derived TIL were used as effector cells (P<0.05). ADR up-regulated the expression of leukocyte function-associated antigen-3 (LFA-3) and intercellular adhesion molecule-1 (ICAM-1), which are critical in the binding and killing of CTL against cancer cells. Of the five fresh RCC cell cultures treated with ADR, LFA-3 was increased in all and ICAM-1 was increased in three of them, respectively (P<0.05). Up-regulation of LFA-3 and ICAM-1 was also observed in ACHN cells treated with two derivatives of ADR, epirubicin and pirarubicin. ADR further significantly increased the bindings of PBL to RCC cells (P<0.05). These findings suggest that treatment of RCC patients with low doses of ADR may sensitize the RCC cells to killing by PBL and TIL and may be a novel immunotherapeutic modality for the treatment of drug-resistant and/or immune-resistant RCC. The inducing of LFA-3 and ICAM-1 by ADR may be involved in the enhancement of susceptibility of PBL and TIL-mediated cytolysis in human RCC cells.     

Induction of target antigens and conversion to susceptible phenotype of NK-cell-resistant lymphoid cell line

Two autologous Herpesvirus papio producer lymphoid cell lines and one autologous non-producer line were compared for susceptibility to natural killer (NK) cell-mediated lysis. The non-producer cell line, 26CB-1, was more resistant to NK cell killing compared to one viral producer counterpart 13CB-1, but equally resistant when compared to another, 8CB-1. Treatment with chemical agents that affect differentiation or activate the viral cycle, including n-butyrate, luDR, 5-azacytidine and tunicamycin, increased the susceptibility to killing of the non-producer line but had less effect on the 13CB-1 producer line. The increase in susceptibility was due to induction of new target antigens: activated 26CB-1 cells were more effective at inhibiting NK-cell-mediated lysis and were bound by more NK cells than untreated control cells. The expression of NK target structures may be related to the differentiated state rather than to the viral production status of target cells.     

Murine non-lymphoid tumors are lysed by a combination of NK and NC cells

Natural cell-mediated cytotoxicity (NCMC) against a variety of tumor targets is mediated by a heterogeneous group of effector cells with the natural killer (NK) and natural cytotoxic (NC) cells being the predominant prototypes in mice. This report shows that non-lymphoid tumor targets, mostly derived from chemically induced fibrosarcomas, are susceptible to either (I) NK-mediated lysis with all the activity being the function of a poly-IC augmentable Qa-5+ effector cell; (2) NC-mediated lysis with all activity being the function of a Qa-5—cell not augmented by poly-IC; and (3) a combination of NK-and NC-mediated lysis with activity being the function of both Qa-5+ and Qa-5- cells, the NK (Qa-5+) augmented by poly-IC. These studies further support the view that murine NC and NK cells are distinct and collectively make up the NCMC system, and also that the previous association of NK cells with lymphoid tumor lysis and NC cells with nonlymphoid tumor lysis is not a valid one.     

The role of human melanoma cell ICAM-1 expression on lymphokine activated killer cell-mediated lysis, and the effect of retinoic acid.

Intercellular adhesion molecule (ICAM-1) exists as a membrane-associated form (mICAM-1) on the surface of tumour cells as well as a soluble form (sICAM-1). This study analyses the ability of all-trans retinoic acid (RA) to alter both sICAM and mICAM-1 expression in C8161 and Hs294T human melanoma cell lines and investigates the involvement of ICAM-1 in the interaction between tumour and lymphokine-activated killer (LAK) cells using the Cr-51 release assay. Our data showed that 4-day pretreatment of the tumour cells with 10(-7) M RA and 10(-6) M RA induced an increase in lysis of both cell lines and also increased mICAM-1 expression without having any effect on sICAM-1 levels. Addition of blocking ICAM-1 antibody (10 microg ml(-1)) to the C8161 cells at an effector:tumour cell ratio of 40:1 caused a 2.3-fold reduction in lysis of tumour cells and a 3-fold reduction in lysis of RA-treated cells. Blocking ICAM-1 antibody at optimum concentrations of 5 microg ml(-1) reduced lysis 1.8-fold in control Hs294T cells and 1.3-fold in RA-treated cells. Blocking the HLA-ABC complex had no effect on lysis. The more highly metastatic C8161 cells were found to secrete 4-fold greater levels of sICAM-1 than the poorly metastatic Hs294T cells and addition of sICAM-1 to the assay failed to affect lysis of either cell line but did induce a 2-fold decrease in lysis of RA-treated C8161 cells. Collectively, these data provide further evidence for ICAM-1 involvement in the tumour/LAK cell response and indicates that the RA-induced increase in mICAM-1 levels are partly responsible for the increase in susceptibility of the tumour cells. sICAM-1 appears to be unimportant in evasion of the tumour cells from LAK cell lysis, but may play a role in evasion of RA-treated C8161 cells.     

In-vitro resistance of cloned human glioma cells to natural killer activity of allogeneic peripheral lymphocytes.

Cells from an established culture of a human astrocytoma were incubated with normal allogeneic peripheral lymphocytes (PBL) in order to study the natural killer (NK) sensitivity of the in vitro propagated cell line. A proportion of cells in culture formed halos, into which lymphocytes did not penetrate. These cells were successfully cloned and showed a decreased susceptibility to NK cytolysis compared with the parent line. Both cell lines could be transplanted into athymic nude mice. The cloned NK-resistant cells underwent a frequent spontaneous regression in nu/nu mice, despite the fact that when used as targets for nu/nu NK cells in vitro they were only moderately susceptible. Phase-contrast microscopy of the mass-cultured cells co-cultivated with lymphocytes suggested that their morphology and ability to form inpenetrable translucent halos might influence their susceptibility to NK lysis. Experiments performed on this assumption revealed that quiescent and halo forming tumour cells were not the primary targets for NK lysis. Cells in mass culture, although tumorigenic, were thus heterogeneous in respect of susceptibility to NK attack. These findings might be relevant to the mechanism of immune escape and tumour heterogeneity in respect of spontaneous cell-mediated lysis.