Augmentation by transferrin of IL-2-inducible killer activity and perforin production of human CD8+ T cells.

The effects of human transferrin (Tf) on lymphokine (IL-2)-activated killer (LAK) induction from blood lymphocytes of healthy donors was examined. LAK cells were induced by 6-day incubation in medium with recombinant human IL-2 of lymphocytes, and their cytotoxic activity was assessed by measuring 51Cr release from NK-resistant Daudi cells. Tf alone did not induce any LAK activity, but in combination with IL-2, it augmented LAK induction dose- and time-dependently. This augmenting effect was completely abolished by pretreatment with anti-Tf antiserum. Tf augmented the proliferative response of lymphocytes to IL-2 and their expressions of receptors for IL-2 and Tf. CD8+ T cells were isolated from purified blood lymphocytes using antibody-bound magnetic beads. Addition of Tf to cultures of CD8+ cells resulted in significant augmentation of killer cell induction and perforin (PFP) production after 4 days stimulation with IL-2. These results indicate that Tf is important in generation of IL-2-inducible killer properties and PFP activity of human CD8+ T cells.     

Suppression of lymphokine-activated killer (LAK) cell induction mediated by interleukin-4 and transforming growth factor-beta 1: effect of addition of exogenous tumour necrosis factor-alpha and interferon-gamma, and measurement of their endogenous production.

Recombinant human interleukin-4 (rhIL-4) and transforming growth factor-beta 1 (TGF-beta 1) suppressed the induction of lymphokine-activated killer (LAK) activity induced by recombinant human interleukin-2 (rhIL-2) in peripheral blood lymphocytes. DNA synthesis and the expression of the p55 alpha chain of the IL-2 receptor (Tac antigen) were also inhibited. The inhibitory effect was greatest when these factors were added during the first 48 h of a 4-day culture, with reduced cytolytic activity against both natural killer (NK) resistant and NK-sensitive tumour cell line targets. The suppressive action of both cytokines was accompanied by a reduction in tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) levels in lymphocyte culture supernatants. Recombinant human IFN-gamma (rhIFN-gamma), but not recombinant human TNF-alpha (rhTNF-alpha) was able to overcome the inhibitory effect of recombinant human interleukin-4 (rhIL-4) on LAK induction and DNA synthesis but not Tac antigen expression. However, cytotoxicity induced by rhIFN-gamma alone was also suppressed by rhIL-4 and TGF-beta 1, inferring that rhIFN-gamma-mediated abrogation of rhIL4 suppression was not simply a direct IL-2-independent effect on cytotoxicity. In addition, rhIL-4 did not increase TGF-beta production from rhIL-2-activated peripheral blood mononuclear cells, suggesting that rhIL-4 did not mediate reduction of rhIL-2 responses through the induction of TGF-beta release.     

Effect of Interleukin (IL)-12 and IL-15 on Activated Natural Killer (ANK) and Antibody-Dependent Cellular Cytotoxicity (ADCC) in HIV Infection

The ability of IL-12 and IL-15 to enhance natural killer (NK) activity and antibody-dependent cellular cytotoxicity (ADCC) of mononuclear cells (MNCs) from HIV⁺ children and their mothers was investigated. MNCs from HIV⁺ patients were deficient in NK and ADCC activity compared to control MNCs against several target cells. Overnight incubation with IL-15 or IL-12 augmented NK activity of MNCs from both patients and controls, and the combination of IL-12 and IL-15 resulted in the greatest enhancement. ADCC in HIV⁺ patients against gp120-coated CEM.NKR cells or chicken erythrocytes could also be enhanced by IL-2 or IL-15 in overnight cultures. Culturing MNCs with either IL-2 or IL-15 for 1 week increased the NK activity in patients to levels of controls treated with these cytokines. However, the response to the combination of IL-12 and IL-15 was less than that to IL-15 alone in 1-week cultures. Culturing MNCs with IL-2 and IL-15 for 1 week also increased the percentage of CD16⁺/CD56⁺ cells in both patients and controls. Thus, IL-15 can restore the deficient NK activity in patients and may be a candidate for immunomodulative therapy in HIV⁺ patients.     

Culturing of HIV-1-specific cytotoxic T lymphocytes with interleukin-7 and interleukin-15

The ability to study HIV-1-specific cytotoxic T cell (CTL) clones in models in vitro or to expand them for immunotherapeutic use is limited by the technical difficulty of propagating these cells. The factors that determine the survival and proliferation of the cells are incompletely understood and could include cytokines provided from feeder cells or serum. We therefore investigated the effects of adding two cytokines reported to have effects on T cell proliferation and function, interleukin (IL)-7 and IL-15. Four HIV-1-specific clones derived from infected persons were cultured under standard conditions with IL-2 compared to IL-7 or IL-15 alone or in combination with IL-2. Proliferation and survival, as reflected by cell numbers after stimulation, were poorly supported by IL-7 or IL-15 alone, and these cytokines appeared to provide no additional benefit when added to IL-2. Similarly, these cytokines alone did not support the functional status of these cells as measured by chromium release assays with peptide-pulsed target cells. Addition of IL-7 or IL-15 to IL-2 did not augment function of the cells. These data suggest that supplementing CTL cultures with these cytokines does not provide improvement of cell growth or function.     

Enhancing effect of natural killer cell stimulatory factor (NKSF/interleukin-12) on cell-mediated cytotoxicity against tumor-derived and virus-infected cells

Natural killer cell stimulatory factor (NKSF) or interleukin-12 (IL-12) is a heterodimeric cytokine with pleiomorphic effects on T and NK cells, including induction of lymphokine production, mitogenesis, and enhancement of spontaneous cytotoxic activity. Similarly to IL-2, NKSF/IL-12 enhances NK cell-mediated cytotoxicity within a few hours and independently from induced proliferation. This effect is independent from other induced cytokines, because it is not prevented by antibodies neutralizing interferon (IFN)-α, IFN-β IFN-γ, IL-2 or tumor necrosis factor (TNF)-α and, unlike the induction of IFN-γ production by peripheral blood lymphocytes, it does not require HLA class II-positive accessory cells. Enhanced cytotoxicity is accompanied by morphologic changes in NK cells, including a significant increase in the number of cytoplasmic granules. In addition to the previously described ability to enhance the cytotoxic activity of NK cells against tumor-derived target cells, NKSF/IL-12 is also a potent stimulator of cytotoxicity against virus-infected cells, either fibroblasts acutely infected with herpes viruses or T cell lines chronically infected with human immunodeficiency virus-1. NK cell-mediated antibody-dependent cytotoxicity or anti-CD16 antibody-redirected lysis is not significantly enhanced by NKSF/IL-12. However, the ability of resting peripheral blood T cells to mediate anti-CD3 antibody-redirected lysis is enhanced by 18-h incubation with NKSF/IL-12, indicating that this lymphokine can modulate the cytotoxic capability of both NK and T cells.     

Lysis of pulmonary fibroblasts by lymphokine (IL-2)-activated killer cells—a mechanism affecting the human lung microenvironment?

In this study we investigated whether IL-2-activated killer cells may bind and exert lytic activity against non-transformed lung fibroblasts. We demonstrated that human lymphokine-activated killer (LAK) cells generated in vitro following incubation with recombinant IL-2 of either peripheral blood mononuclear cells (PB-LAK) or lymphocytes obtained from bronchoalveolar lavage (BAL-LAK), but not resting cells, can lyse normal lung fibroblasts obtained from transbronchial lung biopsies in a 4-h ⁵¹Cr release assay. Both autologous and allogeneic fibroblasts were consistently lysed by LAK cells, thus suggesting that the phenomenon we observed is not MHC-restricted. Since fibroblasts can bind IL-2 through specific receptors, we evaluated whether long-term culture with rIL-2 could modulate the susceptibility to lysis of target cells. Our data showed that autologous fibroblasts were more resistant to lysis than allogeneic fibroblasts when they were cultured with rIL-2. Since LAK cells have been demonstrated to release a series of different immunomodulatory cytokines, we evaluated the effect of short-term incubation of fibroblasts with different factors, including IL-1, IL-2, IL-3, IL-4, IL-6, tumour necrosis factor-alpha (TNF-α), and interferon-gamma (IFN-γ), on the binding and the lysis mediated by LAK cells. These cytokines were not directly cytotoxic on fibroblasts. Only IFN-γ was found to have a significant protective effect against the lysis. Our data support the concept that a self-directed cytotoxicity against pulmonary fibroblasts is generated during lymphocyte activation with rIL-2.     

Enhancement of Antibody-Dependent Cellular Cytotoxicity of Neonatal Cells by Interleukin-2 (IL-2) and IL-12

Newborn infants are more susceptible to infections due in part to deficiencies in the cytotoxic functions of their lymphocytes. We investigated the ability of interleukin-2 (IL-2) and IL-12 to enhance the cytotoxicity of neonatal (cord blood) and adult mononuclear cells (MNCs) in both natural killer (NK) cell and antibody-dependent cellular cytotoxicity (ADCC) assays. The cytotoxic activity of cord blood MNCs was less than 50% that of adult MNCs in most assays prior to exposure to cytokines. Incubation with IL-2 (100 U/ml) or IL-12 (1 ng/ml) for 18 h increased the NK cell activity (using K562 target cells) of both cord blood and adult MNCs, and the combination of IL-2 and IL-12 increased cord blood cytotoxicity threefold, making the cytotoxicity of cord blood cells equivalent to that of adult cells treated with the same cytokines. In ADCC assays with chicken erythrocyte targets, the combination of IL-2 and IL-12 increased the cytotoxicities of both cord blood and adult MNCs, with greater enhancement again seen with cord blood cells. In assays with NK cell-resistant CEM cells coated with human immunodeficiency virus (HIV) gp120 antigen in the presence of hyperimmune anti-HIV immunoglobulin, ADCC of cord blood MNCs was about 50% that of adult MNCs; ADCC of cord blood MNCs increased two- to threefold with the addition of IL-2 and IL-12, whereas ADCC of adult MNCs did not increase. Incubation of cord blood cells, but not adult cells, with IL-2 or IL-12 for 1 week increased the percentage of CD16⁺/CD56⁺ cells two- to fivefold and enhanced ADCC activity. Thus, IL-2 and IL-12 greatly enhance both the NK cell and ADCC activities of neonatal MNCs and increase the number of NK cells in longer-term culture.     

IL-15 in human visceral leishmaniasis caused by Leishmania infantum

Interleukin (IL)-15 is a recently discovered cytokine with the ability to stimulate the proliferation activity of Th1 and/or Th2 lymphocytes. Here, we investigated the involvement of IL-15 in the immune response to Leishmania infantum infection by studying patients with visceral leishmaniasis (VL). We found that IL-15 is produced by leishmanial antigen (LAg)-stimulated peripheral blood mononuclear cells (PBMC) from active VL patients at a significantly higher level than those produced by cells from healed VL subjects or healthy controls. A significant increase in IL-15 serum blood levels was also observed in acute VL patients compared with healed ones. Furthermore, recombinant IL-15 had an appreciable effect in vitro in reducing IL-4 and increasing the production of IL-12 in response to LAg, but it was ineffective in altering the production of interferon-gamma (IFN-gamma). The production of endogenous IL-15 in acute VL patients appeared to be insufficient to activate both IFN-gamma and IL-12, as attested by the absence of modification of these two cytokines by neutralization experiments in the presence of anti-IL-15 monoclonal antibodies (MoAB). On the contrary, the neutralization of IL-15 increased IL-4 production. Together, these results indicate that endogenous IL-15 plays a role in the suppression of Th2-type cytokines, even though it does not enhance the production of Th1 cytokines in acute VL patients. Since IL-15, in the presence of anti-IL-4 MoAb, caused a further increase in IL-12 production and led to a significant production of IFN-gamma, one of its indirect effects on Th1 cell activation could be due to the latter's effect on Th2 cytokines such as IL-4. Therefore, our observations indicate that there is a potential for IL-15 to augment the T-cell response to human intracellular pathogens.     

IL-4-producing NK T cells are biased towards IFN-γ production by IL-12. Influence of the microenvironment on the functional capacities of NK T cells

NK T cells are an unusual T lymphocyte subset capable of promptly producing several cytokines after stimulation, in particular IL-4, thus suggesting their influence in Th2 lineage commitment. In this study we demonstrate that, according to the cytokines present in the micro environment, NK T lymphocytes can preferentially produce either IL-4 or IFN-γ. In agreement with our previous reports showing that their IL-4-producing capacity is strikingly dependent on IL-7, CD4⁻ CD8⁻ TCRα β⁺ NK T lymphocytes, obtained after expansion with IL-1 plus granulocyte-macrophage colony-stimulating factor, produced almost undetectable amounts of IL-4 or IFN-γ in response to TCR/CD3 cross-linking. However, the capacity of these T cells to produce IFN-γ is strikingly enhanced when IL-12 is added either during their expansion or the anti-CD3 stimulation, while IL-4 secretion is always absent. A similar effect of IL-12 on IFN-γ production was observed when NK T lymphocytes were obtained after expansion with IL-7. It is noteworthy that whatever cytokines are used for their expansion, IL-12 stimulation, in the absence of TCR/CD3 cross-linking, promotes consistent IFN-γ secretion by NK T cells without detectable IL-4 production. Experiments in vivo demonstrated a significant up-regulation of the capacity of NK T cells to produce IFN-γ after anti-CD3 mAb injection when mice were previously treated with IL-12. In conclusion, we provide evidence that the functional capacities of NK T cells, which ultimately will determine their physiological roles, are strikingly dependent on the cytokines present in their microenvironment.     

IL-4 acts as a homeostatic regulator of IL-2-induced TNF and IFN-gamma.

Interleukin-4 (IL-4) is a cytokine secreted by interleukin-2 (IL-2)-activated lymphocytes. IL-2-stimulated lymphocytes also secrete two cytokines, tumour necrosis factor (TNF) and gamma-interferon (IFN-gamma), which contribute to effector function and which may themselves recruit fresh, cytokine-secreting effector cells. We have now investigated whether the IL-4 induced is able to homeostatically regulate secretion of the TNF and IFN-gamma. Peripheral blood mononuclear cells or lymphocytes from normal donors and from patients with neoplastic disease were cultured in the presence of IL-2 alone, IL-4 alone or with both cytokines. IL-2 induced high levels of TNF and IFN-gamma secretion in both groups. The addition of recombinant IL-4 to these IL-2-stimulated cultures lead to significant inhibition of IFN-gamma and TNF production. IFN-gamma secretion was reduced by 50-99% in normal donors and by between 11% and 99% in patients (P less than 0.001). TNF levels induced by IL-2 were similarly reduced by IL-4 both in normal donors (P less than 0.003) and in patients (P less than 0.01). These inhibitory effects were produced by IL-4 at doses of IL-2 attainable in vivo. Inhibition appears to represent a homeostatic regulatory mechanism which may limit recruitment of fresh activated killer (AK) cells. When endogenous IL-4 activity in IL-2-activated lymphocytes was blocked by anti-IL-4 antibody, significantly higher levels of IFN-gamma and TNF were secreted (P less than 0.05). Since both TNF and IFN-gamma may contribute to the anti-neoplastic action of IL-2, manipulating the level of IL-4 activity in vivo could augment the benefits of IL-2 immunotherapy.     

Immune stimulatory and anti-tumour properties of haemin

IL-2 induces tumour regression in some patients with metastatic disease, but the dose of IL-2 is limited by severe toxicity. Agents that increase the expression of IL-2 receptors in the effector cells could be used to improve the effectiveness of IL-2 in mediating its anti-tumour effect. We have reported that haemin increased the expression of IL-2 receptors in human peripheral blood mononuclear cells (PBMC) and synergized with IL-2 in the induction of mitogenicity, cytotoxicity and cytokine production. We now report on haemin-induced immune stimulation and tumour regression in mice. Haemin-induced mitogenicity in mouse splenocytes was potentiated up to twofold by IL-2. The combination of haemin and IL-2 was also effective in inducing cytotoxicity for natural killer (NK)-resistant target cells. Maximal induction of cytotoxicity was attained at an optimal concentration of haemin of 10 μm. Higher concentrations were less effective. Splenocytes isolated from mice that had been treated in vivo with haemin and IL-2 incorporated twice the amount of ³H-thymidine compared with splenocytes from mice treated with either haemin or IL-2 alone. Cytotoxicity of splenocytes for NK-rcsistant target cells was not increased following in vivo administration of haemin and IL-2 when fresh splenocytes were tested. Cytotoxicity was enhanced, however, up to five-fold following 48 h in vitro incubation with IL-2. Administration of haemin and IL-2 resulted in a significant decrease (40%) of established hepatic metastases in mice. Either IL-2 or haemin alone at the dose used were ineffective. The anti-tumour effect of haemin and IL-2 was enhanced (63% decrease in metastases) by administration of the thiol compound, N-acetylcysteine. Since haemin can safely be administered to patients, it may represent a new class of biologic response modifiers that could enhance IL-2-mediated anti-tumour effects.     

Differential regulation of interleukins IL-13 and IL-15 by ovarian steroids, TNF-alpha and TGF-beta in human endometrial epithelial and stromal cells

Based on the endometrial spatial and temporal expression of interleukins (ILs) IL-13 and IL-15 during the normal menstrual cycle, we hypothesized that ovarian steroids and non-steroidal factors regulate their expression in a cell-specific manner. To test this hypothesis and determine IL-13/IL-15 actions, we used endometrial epithelial (EEC) and stromal (ESC) cells isolated and cultured under defined conditions. We confirmed the expression of IL-13 and IL-15 in these cells and further demonstrated that 17beta estradiol (E2), medroxyprogesterone acetate (MPA) and their combination differentially regulated their mRNA expression and protein production in a time- and cell-specific manner (P < 0.05). We also showed that tumour necrosis factor-alpha (TNF-alpha; 10 and 25 ng/ml) and transforming growth factor-beta (TGF-beta; 1 and 5 ng/ml), cytokines with inflammatory and immune regulatory functions in a cell- and dose-dependent manner regulate the expression of IL-13 and IL-15 (P < 0.05). Functionally, IL-13 and IL-15 1-100 ng/ml displayed a limited mitogenic activity towards EEC and ESC; however, they regulated the expression of TNF receptor type 1 (TNFR) mRNA and soluble protein in a cell-specific manner (P < 0.05). We conclude that ovarian steroids, TNF-alpha and TGF-beta act as key regulators of endometrial IL-13 and IL-15 expression which act locally regulating TNFR expression in a cell-specific manner. Based on these findings, we conclude that IL-13/IL-15, either alone or through their interactions with other cytokines, influence the outcome of endometrial inflammatory/immune responses during the normal menstrual cycle, and due to their altered expression may extend these processes in dysfunctional bleeding and endometriosis.     

Up-regulation of induction of lymphokine (IL-2)-activated killer (LAK) cell activity by FK-565 and cisplatin

The role of cisplatin and FK-565 in up-regulation of lymphokine-activated killer (LAK) cell induction by IL-2 was examined. Treatment of blood mononuclear cells (MNC) of healthy donors with cisplatin or FK-565 in the presence of IL-2 resulted in a significant increase in LAK activity against natural killer (NK)-resistant Daudi cells as assessed by the 4 h 51Cr release assay. Blood MNC treated with cisplatin alone was not cytotoxic to Daudi cells. However, MNC treated with FK-565 showed some cytotoxicity against Daudi cells. Addition of cisplatin to IL-2-stimulated MNC did not increase proliferation but did enhance cytotoxicity. FK-565 together with IL-2 increased both proliferation and cytotoxicity of blood MNC. These data suggest the potential of cisplatin and FK-565 in LAK adoptive immunotherapy for cancer treatment.     

IL-2、IL-4和IL-7体外诱导人外周血淋巴因子激活的杀伤细胞效应的比较

比较了淋巴因子ＩＬ－２、ＩＬ－４和ＩＬ－７在体外诱导健康人外周血淋巴因子激活的杀伤 （ＬＡＫ）细胞活性的效应。结果表明，ＩＬ－７可单独，亦可与ＩＬ－２、ＩＬ－４协同诱导ＬＡＫ细胞，而且ＩＬ－７ 诱导ＬＡＫ细胞的效应不被抗ＩＬ－２、抗ＩＬ－４所抑制。ＩＬ－７单独诱导的ＬＡＫ细胞活性高峰迟于ＩＬ－ ２或ＩＬ－４所诱导的活性高峰，且与增殖反应曲线一致。抗ＣＤ８抗体明显抑制ＩＬ－７诱导ＬＡＫ细胞 的效应，而ＩＬ－７诱导ＬＡＫ细胞的效应不能被抗ＮＫＨ－１所抑制。提示：ＩＬ－７ 激活ＬＡＫ细胞的效应 机制不依赖ＩＬ－２和ＩＬ－４，并很可能成为肿瘤过继治疗中的重要淋巴因子。     

The role of gamma/delta T-cell receptor-positive cells in pregnancy: part II.

PROBLEM: We have previously demonstrated a significantly increased ratio of gamma/delta T-cell receptor (TCR)-positive progesterone receptor(PR)-positive cells in the peripheral blood of healthy pregnant women compared to that of recurrent aborters or non-pregnant individuals. Treatment of pregnancy lymphocytes with a pan anti-gamma/delta TCR antibody inhibits progesterone-induced blocking factor (PIBF) production, increases natural killer (NK) activity, and alters the cytokine profile. The present study was aimed at investigating the role of the different gamma/delta subpopulations in these phenomena. METHOD OF STUDY: Peripheral blood lymphocytes from healthy pregnant women were incubated with either anti-gamma1.4 and delta1, or anti-gamma9 and delta2 antibodies. The effect of these treatments on PR induction and interleukin (IL)-10 and IL-12 expression were tested by immunocytochemistry. NK activity of anti-gamma/delta treated lymphocytes was also determined. RESULTS: In peripheral blood of healthy pregnant women, the most frequently occurring chain combination was gamma1.4/delta1, whereas in recurrent aborters, the gamma9/delta2 combination was predominant. Treatment of normal pregnancy lymphocytes with a mixture of gamma1.4 and delta1 antibodies resulted in a significantly reduced NK activity and increased PR and IL-10 expression, whereas treatment with a mixture of gamma9 and delta2 antibodies significantly reduced IL-10 production and slightly increased IL-12 production and NK activity. These data suggest the presence of two functionally distinct subpopulations in the peripheral blood of pregnant women.     

Mistletoe Lectin I-Induced Effects on Human Cytotoxic Lymphocytes. I. Synergism with Il-2 in the Induction of Enhanced Lak Cytotoxicity

This report demonstrates that in vitro activation of human cells with the β-galactoside-specific lectin from mistletoe (ML-I) or interleukin-2 (IL-2) results in different patterns of activation and function of cytotoxic cells. It is now well established that natural killer (NK) and lymphokine-activated killer (LAK) cytotoxicity is mainly mediated by resting (NK) and IL-2-activated (LAK) CD56-positive (+) cells respectively. Culture of peripheral blood lymphocytes (PBL) for 3 days with ML-I led to expansion and activation of T cells which demonstrated NK-and LAK-like cytotoxicity. T lymphocyte subset analysis revealed that in total PBL, ML-I preferentially stimulated and expanded CD8+ T cells which mediated the cytotoxic effect. Incubation of highly purified CD8+ T cells alone with ML-I did not lead to induction of cytotoxicity, which required the presence of both CD4+ and CD 14+ (monocytes) cells, suggesting that ML-I does not exert a direct effect on CD8+ T cells. Activation of PBL with both ML-I and IL-2 resulted in simultaneous induction of T and CD56+ cell-mediated NK and LAK cytotoxicity. These data suggest that treatment with ML-I and DL-2 might provide an approach to induce maximum cytotoxicity against tumors and to recruit both T and NK cells for tumor therapy.     

Role of IFN-alpha/beta and IL-12 in the activation of natural killer cells and interferon-gamma production during experimental infection with Trypanosoma cruzi

Control of Trypanosoma cruzi infection depends largely upon the production of interferon (IFN)-gamma. During experimental infection this cytokine is produced early, mainly by natural killer (NK) cells and later by T cells. As NK cells have been reported to participate in defence against T. cruzi, it is of importance to study the regulation of NK cell functions during infection with the parasite. Several innate cytokines regulate NK cell activity, among them being interferon (IFN)-alpha and IFN-beta (type 1 IFNs) and interleukin (IL)-12, which have all been reported to be involved in protection against T. cruzi. The role of these cytokines in regulation of NK cell functions and disease outcome were studied by infection of mutant mice lacking the IFN-alpha/beta receptor (IFNalpha/betaR-/-) or IL-12 (IL-12-/-) with T. cruzi. IFNalpha/betaR-/- mice were unable to activate the cytotoxic response but produced IFN-gamma, and were not more susceptible than controls. IL-12-/- mice were extremely susceptible and failed to produce T cell-derived IFN-gamma and nitric oxide (NO), although NK cytotoxicity was induced. The results indicate that IL-12 protects against T. cruzi by initiating T cell-mediated production of IFN-gamma, but that endogenous IFN-alpha/beta and NK cell cytotoxicity are not of major importance in defence.