Antigen shedding vs. development of natural suppressor cells as mechanism of tumor escape in mice bearing Ehrlich tumor

A tumor-associated heparan sulfate-related glycosaminoglycan promotes the generation of functional regulatory T cells

Functional Tregs play a key role in tumor development and progression, representing a major barrier to anticancer immunity. The mechanisms by which Tregs are generated in cancer and the influence of the tumor microenvironment on these processes remain incompletely understood. Herein, by using NMR, chemoenzymatic structural assays and a plethora of in vitro and in vivo functional analyses, we demonstrate that the tumoral carbohydrate A10 (Ca10), a cell-surface carbohydrate derived from Ehrlich's tumor (ET) cells, is a heparan sulfate-related proteoglycan that enhances glycolysis and promotes the development of tolerogenic features in human DCs. Ca10-stimulated human DCs generate highly suppressive Tregs by mechanisms partially dependent on metabolic reprogramming, PD-L1, IL-10, and IDO. Ca10 also reprograms the differentiation of human monocytes into DCs with tolerogenic features. In solid ET-bearing mice, we found positive correlations between Ca10 serum levels, tumor size and splenic Treg numbers. Administration of isolated Ca10 also increases the proportion of splenic Tregs in tumor-free mice. Remarkably, we provide evidence supporting the presence of a circulating human Ca10 counterpart (Ca10H) and show, for the first time, that serum levels of Ca10H are increased in patients suffering from different cancer types compared to healthy individuals. Of note, these levels are higher in prostate cancer patients with bone metastases than in prostate cancer patients without metastases. Collectively, we reveal novel molecular mechanisms by which heparan sulfate-related structures associated with tumor cells promote the generation of functional Tregs in cancer. The discovery of this novel structural-functional relationship may open new avenues of research with important clinical implications in cancer treatment.

Immunosuppressive Factors: Role in Cancer Development and Progression

The concept of the immunological surveillance against neoplastic cells was initially proposed by Erlich in 1909 and later elaborated by Burnet. This hypothesis states that the normal function of the immune system, in particular the cell-mediated immunity, is to recognize and destroy the transformed and proliferating tumor cells. The role of cell-mediated immunity during the first steps of tumorigenesis remains controversial. However, there is certain evidence about its importance in the progression and dissemination of cancer. The frequent immunosuppressed condition of cancer patients at tumor relapse or recurrence of secondary tumors is a clinical sign supporting this hypothesis, and many studies have demonstrated a defective immune response in patients diagnosed with advanced cancer. Several mechanisms of escape from the immune surveillance have been described, including the immunoselection of tumor antigen-negative variants, the downregulation of MHC class I expression, suppressive T cells, and the elaboration of immunosuppressive cytokines and other factors. Because of the technical difficulty of isolating the very small amounts from culture supernatants or body fluids, only a few of these substances have been characterized and studied with respect to their biological activity: transforming growth factor-β (TGF-β), the protein p15E, interleukin 10 (IL-10), prostaglandin E2 (PGE2), mucins, suppressive E-receptor (SER), immunosuppressive acidic protein (IAP), and adhesion molecules. The possibility of monitoring cancer patients by testing biochemical factors related to cancer growth led to a proposal to measure a number of these factors as tumor markers. Some of them, e.g mucins, enjoy the consensus of the oncologic community, as for some indications they can help the clinician in the management of cancer patients. Except for the class of mucins, the other above-mentioned immunosuppressive factors have not found any clinical application in the laboratory routine because the information deriving from their measurement, although of speculative and scientific interest, has limited clinical value at present. Nevertheless, even if they have no impact on patient management, these substances do have a potential role to play in the study of cancer patients, and should be taken into account when developing new therapeutic strategies.

Conus vexillum venom induces oxidative stress in Ehrlich's ascites carcinoma cells: an insight into the mechanism of induction

BACKGROUND

It is estimated that venoms of marine cone snails (genus Conus) contain more than 100,000 different small peptides with a wide range of pharmacological and biological actions. Some of these peptides were developed into potential therapeutic agents and as molecular tools to understand biological functions of nervous and cardiovascular systems. In this study we examined the cytotoxic and anticancer properties of the marine vermivorous cone snail Conus vexillum (collected from Hurgada and Sharm El-Shaikh, Red Sea, Egypt) and suggest the possible mechanisms involved. The in vitro cytotoxic effects of Conus venom were assessed against Ehrlich's ascites carcinoma (EAC) cells.

RESULTS

Conus venom treatment resulted in concentration-dependent cytotoxicity as indicated by a lactate dehydrogenase leakage assay. Apoptotic effects were measured in vivo by measuring levels of reactive oxygen species and oxidative defense agents in albino mice injected with EAC cells. Conus venom (1.25 mg/kg) induced a significant increase (p < 0.05) in several oxidative stress biomarkers (lipid peroxidation, protein carbonyl content and reactive nitrogen intermediates) of EAC cells after 3, 6, 9 and 12 hours of venom injection. Conus venom significantly reduced (p < 0.05) the activities of oxidative defense enzymes (catalase and superoxide dismutase) as well as the total antioxidant capacity of EAC cells, as evidenced by lowered levels of reduced glutathione.

CONCLUSIONS

These results demonstrate the cytotoxic potential of C. vexillum venom by inducing oxidative stress mediated mechanisms in tumor cells and suggest that the venom contains novel molecules with potential anticancer activity.

Euphorbia tirucalli L. modulates myelopoiesis and enhances the resistance of tumour-bearing mice

Myelosuppression concomitant with increased numbers of spleen CFU-GM was observed in tumour-bearing mice. Treatment of these animals with Euphorbia tirucalli L. (ET) (125, 250 and 500 mg/kg) stimulated marrow myelopoiesis and reduced spleen colony formation, with no differences observed between the effects of the three doses. The changes produced by the tumour in total and differential marrow cell counts were restored by the treatment with ET. Prostaglandin E2 (PGE2) levels, which were dramatically increased in tumour bearers, was also abrogated by the treatment with the plant extract. Euphorbia tirucalli L. significantly enhanced survival and concurrently reduced tumour growth in the peritoneal cavity. We propose that the modulatory effect of Euphorbia tirucalli L. on myelopoietic response and on the levels of PGE2 may be related to its antitumour activity as a possible mechanism for the regulation of granulocyte and macrophage production and expression of functional activities.

Ehrlich Ascites Tumor as a Tool in the Development of Compounds with Immunomodulatory Properties

In previous works, we have demonstrated that the myeloprotective properties of several natural and synthetic compounds are partly responsible for their antitumor activity in the Ehrlich ascites tumor (EAT) model. In this work, we present information that may be useful to the study of pharmacological and toxicological properties of compounds that affect the hematological compartment. Clonogenic studies in EAT-inoculated mice demonstrated a rapid decrease in bone marrow CFU-GM, whereas a progressive increase in splenic CFU-GM and cellularity was observed, followed by splenomegaly. Bone marrow cellularity declined on the third day after tumor challenge, returning to normal values thereafter. Serum from EAT-bearing mice produced detectable colony-stimulating activity in vitro. Similar results were observed with the conditioned medium from Ehrlich tumor cell cultures, but not with the cell-free Ehrlich tumor ascitic fluid. Tumor inoculation also resulted in a more striking depletion in the number of non-adherent cells in long-term bone marrow cell cultures (LTBMCs) with no bone marrow stroma formation. We speculate that the physiological alterations induced by the EAT growth can be used to assess the ability of compounds to modulate the hematopoietic response.

Cyclophosphamide Induces the Development of Early Myeloid Cells Suppressing Tumor Cell Growth by a Nitric Oxide-Dependent Mechanism

Adoptive immunotherapy with cyclophosphamide (Cy) increases the host resistance against tumor growth. The precise mechanism(s) by which this therapy enhances tumor suppression is unclear. Cy induces the development of early myeloid cells that may be strongly antiproliferative through NO production. These cells are similar to the natural suppressor cells found in normal bone marrow with a potential antitumor effect. Here we have addressed whether the development of NO-producing cells may be involved in this tumor resistance in Cy-treated mice. The results show a synergism between Cy treatment and tumor-specific lymphocytes transferred systemically (i.v.) or locally (Winn's assay) that results in a strong tumor suppression. Inhibition of NO production by N(G)-monomethyl-L-arginine at the site of tumor inoculation results in a loss of the protection achieved by the combined therapy. Cy-treated mice develop splenic early myeloid (CD11b, Gr-1, CD31 (ER-MP12), ER-MP20, ER-MP54) cells producing large amounts of NO upon T cell-derived signals (IFN-gamma plus CD40 ligation) able to inhibit tumor cell growth in vitro. Early myeloid cells (ER-MP54(+)) and cells expressing inducible NO synthase are increased at the site of tumor challenge in mice treated with the combined therapy, but not in those treated with Cy or immune cell transfer alone. Thus, Cy induces the expansion of early myeloid cells, inhibiting tumor cell growth by a mechanism involving NO. Both the recruitment and the activation of these myeloid cells at the site of tumor challenge appear to be dependent on the presence of tumor-specific lymphocytes.

Adjuvant effect of Pluchea quitoc extract on the resistance of tumor-bearing mice by modulation of the host hematopoietic response

Progressive tumor growth is regularly accompanied by changes in the cellular constituents of the immune system. Evidence suggests that soluble factors generated during tumor growth can affect the amount of granulocyte-macrophage progenitors. In vitro colony growth of progenitor cells may be an early indicator of the cellular changes associated with tumor growth. Pluchea quitoc has been previously found to modulate the hematopoietic response during bacterial infection. This study was designed to investigate the effects of P. quitoc on the growth and differentiation of bone marrow granulocyte-macrophage progenitor cells (CFU-GM) in Ehrlich ascites tumor-bearing mice. In contrast to the myelosuppression developed in the tumor-bearing animals, treatment with P. quitoc ethanolic extract (250, 500 or 1000 mg/kg) for 3 consecutive days after tumor challenge reversibly stimulated myelopoiesis, restoring the number of CFU-GM to normal. This same dose-schedule also increased colony formation in normal mice as compared to controls. In addi tion, P. quitoc significantly enhanced survival of tumor-bearing mice. These results suggest an immunoregulatory role for P. quitoc in counteracting the tumor-induced myelopoietic suppression as well as usefulness as adjuvant treatment of cancer.

Ehrlich ascites tumour unbalances splenic cell populations and reduces responsiveness of T cells to Staphylococcus aureus enterotoxin B stimulation

Tumours must avoid host immune response to survive and proliferate; to achieve this purpose, tumours interact with cells of the immune system by means of tumour secreted factors. The alterations of splenic cell populations in mice bearing the Ehrlich ascites tumour have been studied. A rapid and acute response was observed, characterized by a decrease in both CD4 and CD8 T cells, and a transient increase in the number of B cells, which peaked 2 days after tumour inoculation. An increase in macrophage population and in the homing antigen CD18 was also detected. In vitro incubations of splenic cells with the Staphylococcus aureus enterotoxin B (SEB) showed that tumour induces a state of reduced responsiveness to stimulation of T cells, mainly affecting CD8 T cells, and a diminished IFN-gamma expression.

Ehrlich tumor stimulates extramedullar hematopoiesis in mice without secreting identifiable colony-stimulating factors and without engagement of host T cells

Tumor growth is associated with neutrophilia, thrombocytosis, and extramedullar hematopoiesis. The mechanism(s) accounting for these phenomena is unclear, although granulocyte-macrophage colony-stimulating factor (GM-CSF) and/or granulocyte colony-stimulating factor (G-CSF) released by tumor cells have been involved. We studied whether CSF released by Ehrlich tumor (ET) may play a role. A comparative study was performed with two cell variants (ET and ET/0) growing in euthymic, nude, and SCID mice. Extramedullar hematopoiesis was assessed in the spleen by scoring organ enlargement, wheat germ agglutinin ve+ cells, and interleukin 3-dependent granulocyte-macrophage colony-forming unit (GM-CFU). Both cell lines showed the same cytokine profile by reverse transcriptase polymerase chain reaction, including GM-CSF, G-CSF, and macrophage colony-stimulating factor (M-CSF); yet, only ET cells produced detectable colony-stimulating activity in vitro, mainly due to GM-CSF. No differences in tumorigenicity were noted between ET and ET/0 cells inoculated to normal or immunodeficient mice. An increase in extramedullar hematopoiesis, accompanied by neutrophilia and thrombocytosis, was associated with tumor progression irrespective of the cell line. A strong correlation was obtained between the increase in splenic GM-CFU and tumor mass (r = 0.96, p < 0.0001) that was independent on the tumor cell line, strain of mice, or stage of tumor development. The results point against CSF released by tumor cells and/or reactive host T cells as the only factors involved in the extramedullar hematopoiesis in this tumor model. The remarkable correlation between splenic GM-CFU and the tumor mass still suggests that a factor(s) of tumor origin may play a critical role.

Early tumor effect on splenic Th lymphocytes in mice

Tumors are characterized by their ability to avoid the host immune system. Ehrlich ascites tumor cells were used to investigate the early alterations of the host immune system after tumor inoculation. The results show that frequencies of splenic Th lymphocytes were drastically reduced during tumor growth, reaching a minimum only two days after tumor inoculation. The frequency of splenic CD4+ lymphocytes expressing IFN-gamma was significantly increased, although the total number was unchanged, suggesting that there was no net induction of Th1-type response. Splenic macrophages were increased, in both frequency and cell number, after four days of tumor growth. The same pattern was observed when mice were inoculated with cell free ascitic fluid. TGF-beta precursors were detected in tumor cells as well as in ascitic fluid. The data suggest that tumor actively interacts with host immune system by means of tumor cell secreted factors.

Differential effects of transforming growth factor-beta 1 on IgA vs. IgG2b production by lipopolysaccharide-stimulated lymph node B cells: a comparative study with spleen B cells

Transforming growth factor-beta 1 (TGF-beta 1) is a multifunctional cytokine that promotes IgA/IgG2b switching and secretion. Here, we show a differential effect of TGF-beta 1 on Ig production by lipopolysaccharide-stimulated spleen and lymph node (LN) B cells. Exogenous TGF-beta 1 increased IgA production in B cell cultures and IgG2b production by spleen B cells. In contrast, IgG2b was suppressed by TGF-beta 1 in cultures of LN B cells, although endogenous TFG-beta was required for IgG2b production in LN B cell cultures. The suppressor properties of exogenous TGF-beta 1 (0.5 ng/ml) on IgG2b production by LN B cells were also seen when testing IgG1 or IgG2a induced by interleukin-4 or interferon-gama, respectively. These differences between B cells from each lymphoid tissue appeared to be related to a different TGF-beta antiproliferative effect, since proliferation of LN B cells was extremely sensitive to TFG-beta 1 and IgG2b production was more sensitive than IgA to the TFG-beta-mediated suppression. However, by counteracting the antiproliferative effect of TGF-beta 1 with a CD40 agonistic mAb (IC10), the IgG2b response by LN B cells was still lacking. IC10 was nevertheless inhibitory for IgG2b production in most cases, while increasing secretion of IgA in the very same cultures. Taken together, the results suggest that functional differences between spleen and LN B cells do exit, at least with regard to the immunomodulating properties of TGF-beta on both proliferation and Ig production. Moreover, functional differences exist between cells committed for IgA and IgG2b regarding their sensitivity to the antiproliferative activity of TGF-beta 1 and the effect of CD40-derived signals on Ig secretion.

Levels of circulating intercellular adhesion molecule-1 in patients with metastatic cancer of the prostate and benign prostatic hyperplasia

Current reports suggest a role for intercellular adhesion molecule-1 (ICAM-1) in the progression of malignancy. The availability of a new antibody makes it possible to measure circulating ICAM-1 (cICAM-1) in human body fluids including serum; this might help in monitoring tumour burden and in providing additional prognostic information. In this study, serum levels of cICAM-1 were measured by an ELISA assay in patients with benign prostatic hyperplasia (BPH; n = 20) and metastatic cancer of the prostate (CaP; n = 25). Serum ICAM-1 concentrations were also measured in a group of healthy men (n = 8). The mean +/- S.E.M. cICAM-1 level for BPH was 339.52 +/- 15.30 ng/ml compared with 263.55 +/- 18.54 ng/ml for CaP. Even though the difference between the two groups was significant (P < 0.005), there was a marked overlap between the individual values in both groups, thus minimising the prognostic value of these measurements in prostate cancer. Endocrine therapy had no notable effect on the serum levels of cICAM-1. The mean +/- S.E.M. cICAM-1 concentrations in serum from a younger group of healthy volunteers was 204.1 +/- 10.38 ng/ml, and this value was significantly lower than that measured in serum from either BPH or CaP. We also undertook some immunohistochemical studies to examine the distribution of ICAM-1 in prostate tissue. We observed focal epithelial cell membrane staining which was exceedingly patchy in both the BPH and cancer specimens. On the basis of these studies, we suggest that cICAM-1 levels do not provide additional information on patients with metastatic CaP.

Immunosuppressive effect of shedding intercellular adhesion molecule 1 antigen on cell-mediated cytotoxicity against tumor cells

We have examined whether shedding intercellular adhesion molecule-1 (ICAM-1) antigen from cultured tumors is able to inhibit the leukocyte function-associated antigen-1 (LFA-1)/ICAM-1 interaction between cytotoxic effector cells and ICAM-1+ target tumor cells. The cytotoxic activity of lymphokine-activated killer (LAK) cells incubated with spent media from ICAM-1+ tumor cells, especially interferon-gamma-stimulated tumor cells, was significantly decreased as compared with that of LAK cells treated with fresh culture medium without ICAM-1 antigen. Treatment of LAK cells with spent media from ICAM-1- tumor cells did not cause a significant decrease of the cytolytic activity towards ICAM-1+ tumor cells. These findings suggest that shedding of ICAM-1 antigen could be involved in binding of LFA-1 to LAK cells, resulting in reduced cytolytic activity.