Biological characterization of guinea pig lymphotoxin production

5-Hydroxytryptamine releasing activity in culture supernatants of guinea pig lymphocytes

5-Hydroxytryptamine (5-HT) is not only involved in anaphylactic reactions but also in delayed-type hypersensitivity (DTH) reactions. Supernatants of cultivated guinea pig spleen lymphocytes were investigated for possessing 5-HT releasing activity. Both fractionated supernatants of mixed lymphocytes unspecifically stimulated by Concanavalin A (Con A) and fractionated supernatants of non-stimulated mixed lymphocytes decrease the 5-HT content of rabbit thrombocytes. Fractions of a molecular weight smaller than 12,500 daltons are more effective than fractions of a higher molecular weight range. The 5-HT releasing activity of the supernatants is discussed in comparison to a cell migration stimulating activity.

Normal levels of lymphotoxin secretion by freshly isolated and refrigerated human peripheral blood lymphocytes

Lymphocytes obtained by Ficoll-Hypaque density gradient centrifugation of leukocytes from 40 blood bank donors were incubated at 10(6) cells/ml for 24 h with 5 micrograms phytohemagglutinin/ml serum-free RPMI 1640 medium and secreted 34 +/- 6 U lymphotoxin/ml and 52 +/- 29 U interferon/ml. Addition of 5% fetal bovine serum (FBS) or acid-citrate-dextrose treated autologous plasma (AP) augmented lymphotoxin secretion 2-fold and interferon secretion 6-8-fold. Lymphotoxin alone was secreted in 39% of serum free cultures. Both lymphotoxin and interferon were secreted in the presence of FBS or AP. Lymphocytes refrigerated at 4 degrees C for 18 h secreted upon serum-free culture similar levels of lymphotoxin as fresh cells but cultures secreting lymphotoxin alone increased from 39 to 77%. Lymphotoxin secretion by refrigerated lymphocytes was similar in the presence or absence of FBS or AP. Interferon secretion by refrigerated lymphocytes in the presence of FBS or AP, however, increased 30-40-fold approaching levels produced by freshly isolated cells and indicating that the modulatory effects of refrigeration, FBS, and AP affected interferon not lymphotoxin secretion. The levels of lymphotoxin and interferon secreted by lymphocyte populations producing both lymphokines, furthermore, varied independently of one another providing additional evidence for the dissociability of their secretion yet possible dependence of interferon upon lymphotoxin secretion. The ability to reliably quantitate lymphotoxin secretion using refrigerated lymphocytes permits transport of lymphocytes to laboratories performing quantitative bioassay of the lymphokine. This should facilitate further evaluation of the presence and role of lymphotoxin in immunopathological and other disease states.

Lymphotoxin amplification of tumor growth inhibition is specific for natural killer cells but not for macrophages

Lymphotoxin augments the susceptibility of tumorigenic guinea-pig cells to natural killer (NK) cell cytolysis in vitro but does not directly stimulate either NK cell or macrophage cytolytic action. The question whether lymphotoxin enhances the susceptibility of tumorigenic guinea-pig cells to cytolysis or other means of growth inhibition in vivo by syngeneic NK cells or macrophages was, therefore, examined using a modified tumor cell neutralization (Winn) assay. Mineral oil-, thioglycollate- or casein-induced peritoneal leukocytes, but not the macrophages isolated from the elicited leukocytes obtained from nonimmunized strain 2/N guinea-pigs, effected enhanced cytolysis of lymphotoxin-treated guinea-pig benzo (a)pyrene-induced 104CI tumor cells in vitro. Neither guinea-pig splenic NK cells nor oil-induced peritoneal macrophages alone inhibited the growth of 104CI cells as tumors in vivo when admixed with 104CI cells and injected into guinea-pigs. However, when the 104CI cells were treated with lymphotoxin before addition of effector cells, NK cells but not macrophages significantly reduced tumor growth in vivo. Therefore, the ability of lymphotoxin to increase the sensitivity of tumor cells to destruction mediated by natural leukocytes is specific for NK cells as compared to macrophages. This form of lymphokine amplification of natural leukocyte cytotoxicity may be one mechanism by which natural and acquired immunity serves or fails to prevent cancer and should be an important consideration in therapeutic approaches to eradicate or control tumor growth.

Comparative effectiveness of lymphotoxin anticarcinogenic and tumor cell growth-inhibitory activities

Prevention of ultraviolet radiation- or chemical carcinogen-induced morphologic transformation and inhibition of tumor-producing transformed cell growth by lymphotoxin and by normal spleen leukocytes were quantitatively compared to define the antineoplastic activity spectra of these natural immune mediators. When Syrian golden hamster embryo cells seeded for colony formation in culture dishes were treated simultaneously with carcinogen and lymphotoxin, the number of morphologically transformed cell colonies was irreversibly reduced by 50% in the presence of 6 units of lymphotoxin/ml. Lymphotoxin inhibition of tumor cell growth, however, was reversible and 50% reduction in tumor cell growth in three transformed lines required 124, 330, and 477 units/ml. Thus, the anticarcinogenic activity of lymphotoxin can be 20-fold or more greater than its tumor growth-inhibitory activity. Similarly spleen leukocytes also were more effective as an anticarcinogen than as an inhibitor of tumor cell growth, consistent with previous observations that naturally occurring spleen leukocyte antineoplastic activity may result from lymphotoxin secretion.

Lymphotoxin enhances the susceptibility of neoplastic and preneoplastic cells to natural killer cell mediated destruction

Treatment of neoplastic guinea-pig cells for 1-14 days with 300 units of guinea-pig lymphotoxin/ml medium enhanced 2- to 5-fold their susceptibility to natural killer (NK) cell-mediated cytotoxicity as measured by 4 h 51Cr and 72 h [3H]-thymidine-release assays. The NK susceptibility of human neoplastic and guinea-pig non-neoplastic cells refractory to guinea-pig lymphotoxin growth-inhibitory activity was unchanged after lymphotoxin treatment. Lymphotoxin preparations were free of detectable interferon and the enhancing activity copurified with lymphotoxin during diafiltration and isoelectric focusing. Treatment of NK cells with lymphotoxin preparations also augmented NK-mediated lysis but that activity did not copurify with lymphotoxin. As with neoplastic cells, lymphotoxin-sensitive preneoplastic cells previously treated with ultraviolet light, a chemical carcinogen, or carcinogen and the tumor-promoting agent tetradecanoylphorbol-acetate also exhibited lymphotoxin enhancement to NK-cell destruction. Thus, the immunologic hormone lymphotoxin enhances the susceptibility of lymphotoxin-sensitive cells to NK-cell destruction, indicating that these two natural immune mediators, lymphotoxin and NK cells, have the potential in concert to regulate carcinogenesis and neoplastic growth.

Lymphotoxin cytotoxicity, a combination of cytolytic and cytostatic cellular responses

Guinea pig lymphotoxin inhibits the growth of mouse alpha L929 and guinea pig 104C1 tumor cells with lethal doses (LD 50's) of 0.4 and 200 units lymphotoxin/ml, respectively, in a colony inhibition assay. Refeeding with lymphotoxin-free medium is followed by resumption of 104C1 but not alpha L929 cell growth. This suggests that growth inhibition of alpha L929 is primarily due to cytolytic mechanisms, while that of 104C1 cells is due to cytostatic mechanisms. This is confirmed by radionuclide (3H, 51Cr, and 75Se) release assays with LD 50's of 1.0, 1.9, and 2.4 units lymphotoxin/ml, respectively, for alpha L929 cells, whereas as many as 100 units lymphotoxin/ml produce no radionuclide release from 104C1 cells. The L cell variant L929M is 10-fold more resistant to lymphotoxin colony inhibitory activity and 40-300-fold more resistant to cytolytic lymphotoxin activity as measured by the three radionuclide release assays than are alpha L929 cells. L929 and 2071 L cell variants are more resistant as a result of smaller cytolytic and cytostatic responses and some tumor cells, such as one strain of L1210 mouse leukemia cells, exhibit no detectable cytolytic or cytostatic responses to 100 units of guinea pig lymphotoxin. These observations demonstrate that the divergent susceptibilities of different cells to lymphotoxin result in part from constitutive variations in cellular cytolytic and reversible cytostatic responses to lymphotoxin.

Species specificity of guinea pig and human lymphotoxin colony inhibitory activity

The species specificity of lymphotoxin colony inhibitory activity was measured by in vitro colony growth of neoplastic and nonneoplastic cells in the presence of lymphotoxin from the same or from a xenogeneic species. Growth of guinea pig, rat, and mouse neoplastically transformed cells was inhibited by guinea pig lymphotoxin. Colony formation of benzo(a)pyrene transformed 104C1 guinea pig cells was inhibited 45% with lymphotoxin at 42 units/ml medium; nontransformed 104 and 118 guinea pigs cell were not inhibited. Alpha L929, a spontaneously transformed mouse cell, was completely inhibited. Dimethylbenz(a)anthracene (DMBA) 3T3-D and R4-2 mouse transformed cells were inhibited 40 and 100%, respectively, but parental nontransformed 3T3 cells were not inhibited. DMBA transformed R-2 and R-6 rat cells were inhibited 20 and 40%, respectively, but secondary nontransformed fetal rat cells were unaffected. In contrast, normal human xeroderma pigmentosum, Lesch-Nyhan and 1220 fibroblast cells were inhibited 50-60%. No inhibition of HeLa or RPMI 2650 human tumor cells occurred. HeLa and RPMI 2650 cells, however, were inhibited 30 and 60%, respectively, by human lymphotoxin, but normal human and tumorigenic guinea pig cells were unaffected. Thus, guinea pig and human lymphotoxin exhibit species specific and differential cytotoxicity mechanisms for normal and tumor cells.

Lymphotoxin: an anticarcinogenic lymphokine as measured by inhibition of chemical carcinogen or ultraviolet-irradiation-induced transformation of Syrian hamster cells

Syrian-hamster-derived lymphotoxin inhibits benzo(a)pyrene and ultraviolet carcinogen-induced in vitro morphologic transformation of diploid secondary passage hamster embryo cells. When lymphotoxin is added up to 4 days post carcinogen, the reduction in the frequency of morphologic transformation 6 days after carcinogen treatment varies directly with lymphotoxin concentration. The frequency of transformation does not change following re-feeding with lymphotoxin-free medium and incubation for 6 additional days. Furthermore, transformation by ultraviolet is prevented when cells are pulsed with lymphotoxin for as few as 6 hours after irradiation. Thus, lymphotoxin has the potential to irreversibly inhibit the first recognizable morphologic step of carcinogenesis.