Decreased macrophage-mediated cytotoxicity in mammary-tumor-bearing mice is related to alteration of nitric-oxide production and/or release

Insights into thymic involution in tumor-bearing mice

The thymus is a central lymphoid organ critical for the development and maintenance of an effective peripheral T-cell repertoire. Most important, it provides a specialized environment for the selection of rearranged clones that will function appropriately in the adaptive immune response. Thymic involution has been observed in several model systems; including graft-versus-host disease, aging, viral infection, and tumor development, however, the precise mechanisms involved in this phenomenon remain poorly defined. Here, we review some of our results related to the studies of the cell-mediated immunity in a mammary tumor model; more specifically, those related to the tumor-induced impaired T-cell development and thymic involution. Collectively, the understanding of the mechanisms and pathways associated with the tumor-induced thymic involution is essential for the development of innovative and safe therapies to fight against the immune suppression caused by the tumor development.

Macrophage expression of hypoxia-inducible factor-1 alpha suppresses T-cell function and promotes tumor progression

T cells can inhibit tumor growth, but their function in the tumor microenvironment is often suppressed. Many solid tumors exhibit abundant macrophage infiltration and low oxygen tension, yet how hypoxic conditions may affect innate immune cells and their role in tumor progression is poorly understood. Targeted deletion of the hypoxia-responsive transcription factor hypoxia-inducible factor-1α (HIF-1α) in macrophages in a progressive murine model of breast cancer resulted in reduced tumor growth, although vascular endothelial growth factor-A levels and vascularization were unchanged. Tumor-associated macrophages can suppress tumor-infiltrating T cells by several mechanisms, and we found that hypoxia powerfully augmented macrophage-mediated T-cell suppression in vitro in a manner dependent on macrophage expression of HIF-1α. Our findings link the innate immune hypoxic response to tumor progression through induction of T-cell suppression in the tumor microenvironment.

Diminished expression of transcription factors nuclear factor kappaB and CCAAT/enhancer binding protein underlies a novel tumor evasion mechanism affecting macrophages of mammary tumor-bearing mice

Interactions between malignant tumors and the host immune system shape the course of cancer progression. The molecular basis of such interactions is the subject of immense interest. Proinflammatory cytokines produced by macrophages are critical mediators of immune responses that contribute to the control of the advancement of neoplasia. We have shown that the expressions of interleukin 12 (IL-12) and inducible nitric oxide synthase (iNOS) are decreased in macrophages from mammary tumor-bearing mice. In this study, we investigated the causes of IL-12 dysregulation and found deficient nuclear factor kappaB (NFkappaB) and CCAAT/enhancer binding protein (C/EBP) expression and function in tumor bearers' peritoneal macrophages. The constitutive expressions of NFkappaB p50, c-rel, p65, and C/EBPalpha and beta, as well as the lipopolysaccharide-induced nuclear translocation and DNA binding of NFkappaB components and C/EBPalpha and beta, are profoundly impaired in macrophages from mice bearing D1-DMBA-3 tumors. Because similar findings occur with the iNOS gene, it seems that it represents a novel mechanism by which tumor-derived factors interfere with the host immune defenses.

Persistent transgene expression following intravenous administration of a liposomal complex: role of interleukin-10-mediated immune suppression

Studies conducted in non-tumor-bearing, immunocompetent mice have shown that intravenous administration of liposome-DNA complex elicits an inflammatory response that results in a failure to sustain adequate transgene expression. In the present study, however, we investigated the effects of a cationic liposomal DOTAP:cholesterol (DOTAP:Chol)-DNA complex on cytokine production and transgene expression in both experimental lung tumor-bearing (TB) mice and non-tumor-bearing (NTB) syngeneic mice and nude mice. Intravenous injection of DOTAP:Chol-luciferase (luc) DNA complex resulted in tumor necrosis factor-alpha levels that were 50% lower and interleukin-10 levels that were 50-60% higher in TB mice than in NTB mice. Furthermore, a significant increase in luc expression (P = 0.001) that persisted for 7 days was observed in TB mice. In contrast, luc expression decreased significantly from day 1 to day 2 in NTB mice. Also, luc expression was two- to threefold higher in TB mice that were given multiple injections of DOTAP:Chol-luc complex than in mice who received a single injection. In contrast, luc expression was significantly suppressed following multiple injections in NTB mice (P = 0.01). Further analysis revealed IL-10 protein expression by the tumor cells in TB mice. Injection of anti-IL-10 antibody in TB mice resulted in a significant decrease in luc expression (P = 0.01) compared with that in mice injected with a control antibody. Based on these findings, we conclude that transgene expression persists in TB mice and is partly mediated by IL-10. Additionally, multiple injections of liposome-DNA complex can increase transgene expression in TB mice. These findings have clinical applications in the treatment of cancer.

Astroglia Induce Cytotoxic Effects on Brain Tumors via a Nitric Oxide-Dependent Pathway Both in Vitro and in Vivo

OBJECTIVE

In the central nervous system, astroglia produce nitric oxide (NO) in response to cytokines. We investigated whether cytokine stimulation of astroglia could inhibit brain tumor cell growth in vitro and prolong survival in vivo via an NO-dependent pathway.

METHODS

Astroglia cultures were stimulated with the cytokines lipopolysaccharide and interferon-gamma and subsequently seeded with tumor cell lines. Wild-type mice and inducible NO synthase-knockout mice received in vivo cytokine stimulation followed by B16F10 murine melanoma challenge.

RESULTS

Our in vitro studies demonstrate that astroglia stimulated to produce NO by the addition of cytokines dose-dependently inhibit the growth of one primary rat brain tumor cell line (9L) and three primary human brain tumor cell lines (H80, U87, and U373). This inhibition of tumor cell growth is also observed in metastatic cell lines (B16F10 melanoma, Lewis lung carcinoma, and CT26 colon). Cultured astrocytes from inducible NO synthase-knockout mice, which are incapable of induction of NO, are without the enhanced tumoricidal effect. Furthermore, when C57BL/6 mice are primed to produce NO through stereotactic intracranial administration of lipopolysaccharide plus interferon-gamma and subsequently challenged with B16F10 murine melanoma, survival is significantly prolonged, with a median survival of 26 days versus 16 days in the control group (P < 0.001). The addition of an NO synthase inhibitor (N(G)-nitro-l-arginine methyl ester) decreases this beneficial effect (median survival, 21 d).

CONCLUSION

These findings suggest that NO may have an important role as a defense mechanism molecule against brain tumors; stimulation or modification of this mechanism may represent a new approach to the treatment of primary and metastatic brain tumors.

Role of reactive nitrogen intermediates and protein nitration during immune response against a rat histiocytoma

The ability of macrophages to produce reactive nitrogen species, particularly nitric oxide (NO) is correlated with an enhanced microbicidal or tumoricidal activity during pathogenic or tumoral invasion, respectively. NO reacts in water with oxygen and its reactive intermediates to yield, among others, nitrite and nitrate, which are relatively, stable anions. In this study, we show the varying concentrations of nitrite and nitrate present in different body fluids during AK-5 tumor growth and regression in Wistar rats. We have followed the tumor progression profile and the corresponding levels of nitrite and nitrate present in three major body compartments: the tumor mass; the serum which is the intermediary site; and the peritoneal compartment which is the priming ground for the macrophages. We are thus able to show that the status of the tumor has a direct correlation with macrophage activation and motility to different sites in the body. We also demonstrate after in vitro coculture, that the levels of nitrite and nitrate secreted by the macrophages correlate with their cytocidal capacity.

Does [meso-1,2-bis(2,6-dichloro-4-hydroxyphenyl)ethylenediamine]- dichloro-platinum(II) act as an immune response modifier? Part III: Progressively growing MXT-M-3,2 breast cancer stimulates the proliferation of phagocytes in B6D2F1 mice

MXT-M-3,2 breast cancer implanted into female B6D2F1 mice accelerates the growth of an identical second tumor. This process is accompanied by a significant increase of the granulocyte and monocyte numbers in the blood and of the granulocyte and macrophage numbers in the spleen. A significant positive correlation of strong intensity was found between the tumor weight on the one hand and the number of the granulocytes and macrophages on the other hand. The tumor-dependent promotion of the myelopoiesis is explained with a secretion of hematopoietic growth factors, e.g. of the granulocyte-macrophage-stimulating growth factor (GM-CSF), by the breast cancer cells.

Early block in maturation is associated with thymic involution in mammary tumor-bearing mice

We previously reported that mice implanted with mammary tumors show a progressive thymic involution that parallels the growth of the tumor. The involution is associated with a severe depletion of CD4+8+ thymocytes. We have investigated three possible mechanisms leading to this thymic atrophy: 1) increased apoptosis, 2) decreased proliferation, and 3) disruption of normal thymic maturation. The levels of thymic apoptosis were determined by propidium iodide and annexin V staining. A statistically significant, but minor, increase in thymic apoptosis in tumor-bearing mice was detected with propidium iodide and annexin V staining. The levels of proliferation were assessed by in vivo labeling with 5'-bromo-2'-deoxyuridine (BrdU). The percentages of total thymocytes labeled 1 day following BrdU injection were similar in control and tumor-bearing mice. Moreover, the percentages of CD4-8- thymocytes that incorporated BrdU during a short term pulse (5 h) of BrdU were similar. Lastly, thymic maturation was evaluated by examining CD44 and CD25 expression among CD4-8- thymocytes. The percentage of CD44+ cells increased, while the percentage of CD25+ cells decreased among CD4-8- thymocytes from tumor-bearing vs control animals. Together, these findings suggest that the thymic hypocellularity seen in mammary tumor bearers is not due to a decreased level of proliferation, but, rather, to an arrest at an early stage of thymic differentiation along with a moderate increase in apoptosis.

An immunomodulatory polysaccharide-rich substance from the fruit juice of Morinda citrifolia (noni) with antitumour activity

The fruit juice of Morinda citrifolia (noni) contains a polysaccharide-rich substance (noni-ppt) with antitumour activity in the Lewis lung (LLC) peritoneal carcinomatosis model. Therapeutic administration of noni-ppt significantly enhanced the duration of survival of inbred syngeneic LLC tumour bearing mice. It did not exert significant cytotoxic effects in an adapted culture of LLC cells, LLC1, but could activate peritoneal exudate cells (PEC) to impart profound toxicity when co-cultured with the tumour cells. This suggested the possibility that noni-ppt may suppress tumour growth through activation of the host immune system. Concomitant treatment with the immunosuppressive agent, 2-chloroadenosine (C1-Ade) or cyclosporin (cys-A) diminished its activity, thereby substantiating an immunomodulatory mechanism. Noni-ppt was also capable of stimulating the release of several mediators from murine effector cells, including tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), IL-10, IL-12 p70, interferon-gamma (IFN-gamma) and nitric oxide (NO), but had no effect on IL-2 and suppressed IL-4 release. Improved survival time and curative effects occurred when noni-ppt was combined with sub-optimal doses of the standard chemotherapeutic agents, adriamycin (Adria), cisplatin (CDDP), 5-fluorouracil (5-FU), and vincristine (VCR), suggesting important clinical applications of noni-ppt as a supplemental agent in cancer treatment.

Induction of nitric oxide production by the peritoneal macrophages after intraperitoneal or subcutaneous transplantation of AK-5 tumor

We have investigated the role of host macrophages in the activation of effector functions in addition to their direct role in the induction of tumor cell death. Peritoneal macrophages from s.c. tumor-bearing animals were in an activated state and induced NO-mediated apoptotic death in AK-5 tumor cells. They also showed overexpression of inducible nitric oxide synthase both at the transcript and at the enzyme levels. However, macrophages from the i.p. tumor-bearing animals showed suppression of activation especially when the tumor load was high, suggesting thereby tumor-induced suppression of macrophage activation in i.p. tumor-transplanted hosts. Suppression of macrophage activation was also observed upon coculture of peritoneal macrophages with tumor cells in vitro. These studies suggest an important role played by the host peritoneal macrophages in the regression of AK-5 tumor. They also indicate suppression of macrophage activation by the tumor cells.

The Induction of Nitric Oxide by Interleukin-12 and Tumor Necrosis Factor- in Human Natural Killer Cells: Relationship With the Regulation of Lytic Activity

We have investigated the interleukin-12 (IL-12) and tumor necrosis factor- (TNF)-induced regulation of human natural killer (NK) cell function and their relationship with nitric oxide (NO) generation. We demonstrate that both cytokines were efficient to trigger the transcription of the inducible nitric oxide synthase (iNOS) mRNA, as detected by reverse transcriptase-polymerase chain reaction (RT-PCR). Western blot analysis and intracytoplasmic fluorescence showed that iNOS protein was also induced by both cytokines. However, our data indicate that NO does not play a significant role in the effector phase of the cytotoxic activity mediated by NK-stimulated cells, inasmuch as the lytic activity was not affected in the presence of specific NO synthase inhibitors. When aminoguanidine (AMG), an inhibitor of iNOS, was added during the afferent phase of NK stimulation with IL-12 and TNF, a subsequent increase in the lytic potential of the effector cells towards the NK-sensitive target cells (K562) and lymphokine-activated killer (LAK) target cells (Daudi) was observed. Conversely, the addition of chemical NO donors during the afferent step resulted in a dose-dependent inhibition of the NK and LAK cytotoxicity. Our data suggest that the enhancement of NK-cell cytotoxic activity resulting from iNOS inhibition may be correlated, at least in part, to an increase in interferon-γ production and granzyme B expression.

© 1998 by The American Society of Hematology.

The Induction of Nitric Oxide by Interleukin-12 and Tumor Necrosis Factor- in Human Natural Killer Cells: Relationship With the Regulation of Lytic Activity

We have investigated the interleukin-12 (IL-12) and tumor necrosis factor- (TNF)-induced regulation of human natural killer (NK) cell function and their relationship with nitric oxide (NO) generation. We demonstrate that both cytokines were efficient to trigger the transcription of the inducible nitric oxide synthase (iNOS) mRNA, as detected by reverse transcriptase-polymerase chain reaction (RT-PCR). Western blot analysis and intracytoplasmic fluorescence showed that iNOS protein was also induced by both cytokines. However, our data indicate that NO does not play a significant role in the effector phase of the cytotoxic activity mediated by NK-stimulated cells, inasmuch as the lytic activity was not affected in the presence of specific NO synthase inhibitors. When aminoguanidine (AMG), an inhibitor of iNOS, was added during the afferent phase of NK stimulation with IL-12 and TNF, a subsequent increase in the lytic potential of the effector cells towards the NK-sensitive target cells (K562) and lymphokine-activated killer (LAK) target cells (Daudi) was observed. Conversely, the addition of chemical NO donors during the afferent step resulted in a dose-dependent inhibition of the NK and LAK cytotoxicity. Our data suggest that the enhancement of NK-cell cytotoxic activity resulting from iNOS inhibition may be correlated, at least in part, to an increase in interferon-γ production and granzyme B expression.

© 1998 by The American Society of Hematology.

The altered tumoricidal capacity of macrophages isolated from tumor-bearing mice is related to reduce expression of the inducible nitric oxide synthase gene

Nitric oxide (NO) is a major effector molecule in the destruction of tumor cells by activated macrophages. However, in many cases, developing neoplasms appear to be capable of impairing steps in the complex process leading to NO production as a means of avoiding immune destruction. After activation with lipopolysaccharide (LPS), peritoneal-elicited macrophages (PEM) from mice bearing mammary tumors display alterations in their ability to lyse tumor cells due to reduced production of NO. In contrast, when these same cells are stimulated with LPS in combination with interferon gamma (IFN-gamma), they are able to produce NO and lyse targets at normal levels. Since tumor-associated macrophages are intimately associated with the cells of the developing tumor, their ability to produce NO and lyse tumor targets is likely to be more relevant to controlling tumor growth. This population of macrophages exhibited a more profound inability to produce NO and lyse targets and, unlike the PEM, was not able to upregulate these functions even when treated with combinations of LPS and IFN-gamma. Northern and Western blots revealed that inducible nitric oxide synthase (iNOS) mRNA and protein levels correlated directly with the ability of each macrophage population to produce NO, and the levels of these macromolecules were altered sufficiently in tumor bearers' macrophages to account for the diminished NO production described. These results indicate that a spatial gradient of suppression of macrophage cytolytic activity and iNOS expression exists in mammary tumor-bearing mice, whereby macrophages from within the tumor exhibit a more pronounced suppression than the more distally located PEM. This suppression may be due to proximity of the macrophages to the developing tumor, macrophage maturational state, or both.