Tumor growth changes the contribution of granulocyte-macrophage colony-stimulating factor during macrophage-mediated suppression of allorecognition

Paclitaxel Enhances Macrophage IL-12 Production in Tumor-Bearing Hosts Through Nitric Oxide

Tumor-induced macrophages (Mφs) mediate immunosuppression, in part, through increased production of factors that suppress T cell responsiveness and underproduction of positive regulatory cytokines. Pretreatment of tumor-bearing host (TBH) Mφs with the anticancer agent paclitaxel (Taxol) partially reverses tumor-induced Mφ suppressor activity, suggesting that paclitaxel may restore TBH Mφ production of proimmune factors. Because paclitaxel demonstrates LPS-mimetic capabilities and increased production of the LPS-induced immunostimulatory cytokine IL-12 could account for enhanced T cell responsiveness, we investigated whether paclitaxel induces Mφ IL-12 production. Tumor growth significantly down-regulated Mφ IL-12 p70 production through selective dysregulation of IL-12 p40 expression. LPS stimulation failed to overcome tumor-induced dysregulation of p40 expression. In contrast, paclitaxel significantly enhanced both normal host and TBH Mφ IL-12 p70 production in vitro, although TBH Mφ IL-12 production was lower than that of similarly treated normal host Mφs. Paclitaxel enhanced p40 expression in a dose-dependent manner. Through reconstituted Mφ IL-12 expression, paclitaxel pretreatment relieved tumor-induced Mφ suppression of T cell alloreactivity. Blocking Mφ NO suppressed paclitaxel’s ability to induce IL-12 production. This suggests that paclitaxel-induced activities may involve a NO-mediated autocrine induction pathway. Collectively, these data demonstrate that paclitaxel restores IL-12 production in the TBH and ascribe a novel immunotherapeutic component to the pleiotropic activities of NO. Through its capacity to induce IL-12 production, paclitaxel may contribute to the correction of tumor-induced immune dysfunction.

Treating tumor-bearing mice with vitamin D3 diminishes tumor-induced myelopoiesis and associated immunosuppression, and reduces tumor metastasis and recurrence

Metastatic Lewis lung carcinoma (LLC-LN7) tumors that secrete granulocyte/macrophage-colony-stimulating factor (GM-CSF) stimulate myelopoiesis and induce bone marrow-derived immunosuppressor cells that are homologous to granulocyte/macrophage progenitor cells. In vitro treatment of the LLC-LN7 cells with 1 alpha,25-dihydroxyvitamin D3 reduced tumor cell production of suppressor-inducing activity, although suppressor-inducing activity could be restored by reconstituting the tumor supernatants with recombinant GM-CSF. Treatment of mice having LLC-LN7 tumors with vitamin D3 reduced tumor production of GM-CSF and the frequency of myeloid progenitor cells. This was associated with a reduction in immunosuppressor activity and an increase in T cell function. Vitamin D3 treatment of mice having palpable tumors transiently retarded tumor growth, but caused a prominent reduction in tumor metastasis. Treating mice with vitamin D3 after tumor excision resulted in a reduction in the tumor-induced myelopoietic stimulation and associated immunosuppressive activity, and enhanced T cell function. These mice had a markedly reduced incidence of tumor recurrence. The results of this study suggest that vitamin D3 treatment of mice with GM-CSF-secreting tumors can interrupt the myelopoiesis-associated immunosuppressor cascade and, in turn, reduce tumor metastasis and recurrence.

Tumor-induced macrophage tumor necrosis factor-alpha production suppresses autoreactive T cell proliferation

T cells can react to self-cells bearing the syngenic major histocompatibility complex class II molecule Ia. Decreased autoreactive T cell responses are associated with cancer. Tumor growth causes syngeneic macrophages (M phi) to suppress autoreactive T cell proliferation by decreasing M phi Ia expression and increasing M phi production of the suppressor molecule prostaglandin E2 (PGE2). Because M phi produce tumor necrosis factor-alpha (TNF-alpha) during cancer, and TNF-alpha stimulates M phi PGE2 synthesis, we determined if TNF-alpha mediates tumor-induced suppression of autoreactive T cell proliferation stimulated by syngeneic M phi. We showed that tumor growth increases TNF-alpha production because tumor-bearing host (TBH) M phi synthesized more TNF-alpha than normal host (NH) M phi when cultured with lipopolysaccharide. Exogenous TNF-alpha increased NH CD4+ autoreactive T cell proliferation stimulated by syngeneic NH M phi but not by TBH M phi. When endogenous TNF-alpha activity was neutralized by anti-TNF-alpha antibody addition, T cell proliferation decreased when stimulated by NH M phi but increased when stimulated by TBH M phi. Kinetic studies showed that TNF-alpha affected M phi-stimulated T cell proliferation during the first few hours (4h) of the 96 h culture time. Indomethacin-treatment allowed TNF-alpha to increase T cell proliferation stimulated by TBH M phi. A PGE2-specific enzyme-linked immunosorbent assay showed that TBH M phi T cell cultures contained significantly more PGE2 than those containing NH M phi, and that exogenous TNF-alpha increased PGE2 production in TBH M phi cultures more than in NH M phi cultures. Short-term (4h) pretreatment of M phi with TNF-alpha increased T cell proliferation stimulated by NH, but not TBH, M phi. However, long-term (16 h) TNF-alpha pretreatment reversed TBH M phi-mediated suppression, suggesting that early suppressor molecular production inhibits synthesis or activity of TNF-alpha-induced stimulatory monokines. Although TNF-alpha is known to increase T cell proliferation, these results show that the tumor-induced increase in M phi TNF-alpha synthesis suppress autoreactive T cell proliferation, which is mediated by PGE2 production.

Tumor growth alters macrophage responsiveness to macrophage colony-stimulating factor during reactivity against allogeneic and syngeneic MHC class II molecules

Tumor-induced changes in macrophage (M phi)2 accessory activities significantly suppress T-cell recognition of allogeneic and syngeneic major histocompatibility complex (MHC) class II molecules. Because these changes are often associated with altered responses to stimulatory and inhibitory cytokines, we investigated the possibility that tumor growth alters the contribution of a macrophage regulatory cytokine, macrophage colony-stimulating factor (M-CSF), during reactivity against allogeneic and syngeneic MHC class II molecules. T-cell reactivity against allogeneic MHC class II molecules was significantly suppressed by tumor-bearing host (TBH) M phi in the presence of M-CSF. M-CSF-induced suppression was independent of TBH M phi prostaglandin E2 (PGE2) synthesis. T-cell reactivity against syngeneic MHC class II molecules increased in the presence of M-CSF when normal host (NH) M phi served as the source of syngeneic molecules. However, T-cell reactivity against syngeneic MHC class II molecules in the presence of M-CSF did not change when TBH M phi served as stimulator/accessory cells. Although T-cell reactivity against NH syngeneic MHC class II molecules was additively increased by M-CSF and indomethacin (a PGE2 synthesis inhibitor) treatment, reactivity against TBH syngeneic MHC class II molecules increased solely through PGE2 synthesis inhibition. Admixtures of both NH and TBH M phi in the absence or presence of M-CSF suggest that tumor-induced suppression was not strictly due to decreased expression of MHC class II molecules. Collectively, these data suggest that TBH M phi are partly suppressive through altered responsiveness to M-CSF.

Tumor-induced modulation of macrophage class II MHC molecule mRNA expression

Class II MHC protein expression in macrophages (M phi) is reduced during tumor growth. Because regulation of class II MHC proteins occurs during transcription, tumor growth may suppress class II MHC protein expression by suppressing mRNA. The decrease in class II mRNA may result from (i) a decrease in M phi responsiveness to an inducing agent, such as interferon-gamma (IFN-gamma), or (ii) an increase in M phi sensitivity to suppressing agents, such as prostaglandin E2 (PGE2). To determine how tumors induce suppression of class II mRNA, M phi were cultured in the presence of IFN-gamma with or without other factors, and Northern blot analyses were performed. Unstimulated normal host (NH) or tumor-bearing host (TBH) M phi do not express detectable class II mRNA. The addition of IFN-gamma induces class II mRNA expression in NH and TBH M phi, but class II mRNA expression is significantly lower in TBH M phi. Kinetic studies suggested that NH M phi class II mRNA is induced faster and in greater amounts than TBH M phi class II mRNA. There is a decrease in M phi class II mRNA stability during tumor growth that may account for the decreased induction by IFN-gamma. Lipopolysaccharide (LPS) suppresses class II mRNA induction in both NH and TBH IFN-gamma-treated M phi, but TBH M phi are more sensitive to its suppression. PGE2 and tumor-necrosis factor-alpha (TNF-alpha), two factors produced by LPS-stimulated M phi, were tested for their ability to modulate class II mRNA expression in NH and TBH IFN-gamma-treated M phi. PGE2 suppressed class II mRNA expression in both NH and TBH M phi. The addition of TNF-alpha to IFN-gamma-treated M phi suppressed class II mRNA in NH M phi but, surprisingly, had an additive effect on IFN-gamma-induced class II mRNA expression. TNF-alpha did not induce class II mRNA expression in TBH M phi in the absence of IFN-gamma. The cause of the reduced class II mRNA expression during tumor growth is a decreased response to IFN-gamma and an increased sensitivity to PGE2. This change may cause the observed suppression mediated by TBH M phi.