Tumor-derived cytokines dysregulate macrophage interferon-gamma responsiveness and interferon regulatory factor-8 expression

Tumor-derived Hsp70-CD14 interaction enhances the antitumor potential of cytotoxic T cells by activating tumor-associated macrophages to express CC chemokines and CD40 costimulatory molecules

Heat shock proteins are a widely distributed group of proteins. It is constitutively expressed in almost all organisms and shows little variation throughout evolution. Previously, HSPs, particularly Hsp70, were recognized as molecular chaperones that aid in the proper three-dimensional folding of newly synthesized polypeptides in cells. Recently, researchers have focused on the potential induction of immune cells, including macrophages, antigen-specific CD8+ cytotoxic T cells, and PBMCs. It induces the expression of CC chemokines such as MIP-1α and RANTES, which are responsible for the chemotactic movement and migration of immune cells at the site of infection to neutralize foreign particles in vivo and in vitro in several cell lines but their effect on tumor-associated macrophages is still not known. These cytokines are also known to influence the movement of several immune cells, including CD8+ cytotoxic T cells, toward inflammatory sites. Therefore, the effect of tumor-derived autologous Hsp70 on the expression of MIP-lα and RANTES in tumor-associated macrophages (TAMs) was investigated. Our results indicated that Hsp70 treatment-induced MIP-lα and RANTES expression was significantly greater in TAMs than in NMOs. According to the literature, the CC chemokine shares the same receptor, CCR5, as HIV does for their action, and therefore could provide better completion to the virus for ligand binding. Furthermore, Hsp70-preactivated TAMs induced increased IL-2 and IFN-γ expression in T cells during coculture for 48 h and upregulated the antitumor immune response of the host. Therefore, the outcome of our study could be useful for developing a better approach to restricting the growth and progression of tumors.

Tumor-induced myeloid dysfunction and its implications for cancer immunotherapy

Immune function relies on an appropriate balance of the lymphoid and myeloid responses. In the case of neoplasia, this balance is readily perturbed by the dramatic expansion of immature or dysfunctional myeloid cells accompanied by a reciprocal decline in the quantity/quality of the lymphoid response. In this review, we seek to: (1) define the nature of the atypical myelopoiesis observed in cancer patients and the impact of this perturbation on clinical outcomes; (2) examine the potential mechanisms underlying these clinical manifestations; and (3) explore potential strategies to restore normal myeloid cell differentiation to improve activation of the host antitumor immune response. We posit that fundamental alterations in myeloid homeostasis triggered by the neoplastic process represent critical checkpoints that govern therapeutic efficacy, as well as offer novel cellular-based biomarkers for tracking changes in disease status or relapse.

Tumor-associated myeloid cells can be activated in vitro and in vivo to mediate antitumor effects

Tumor growth is often accompanied by the accumulation of myeloid cells in the tumors and lymphoid organs. These cells can suppress T cell immunity, thereby posing an obstacle to T cell-targeted cancer immunotherapy. In this study, we tested the possibility of activating tumor-associated myeloid cells to mediate antitumor effects. Using the peritoneal model of B16 melanoma, we show that peritoneal cells (PEC) in tumor-bearing mice (TBM) had reduced ability to secrete nitric oxide (NO) following in vitro stimulation with interferon gamma and lipopolysaccharide, as compared to PEC from control mice. This reduced function of PEC was accompanied by the influx of CD11b(+) Gr-1(+) myeloid cells to the peritoneal cavity. Nonadherent PEC were responsible for most of the NO production in TBM, whereas in naïve mice NO was mainly secreted by adherent CD11b(+) F4/80(+) macrophages. Sorted CD11b(+) Gr-1(-) monocytic and CD11b(+) Gr-1(+) granulocytic PEC from TBM had a reduced ability to secrete NO following in vitro stimulation (compared to naïve PEC), but effectively suppressed proliferation of tumor cells in vitro. In vivo, treatment of mice bearing established peritoneal B16 tumors with anti-CD40 and CpG resulted in activation of tumor-associated PEC, reduction in local tumor burden and prolongation of mouse survival. Inhibition of NO did not abrogate the antitumor effects of stimulated myeloid cells. Taken together, the results indicate that in tumor-bearing hosts, tumor-associated myeloid cells can be activated to mediate antitumor effects.

Interferon regulatory factor 8 mediates tumor-induced inhibition of antigen processing and presentation by dendritic cells

INTRODUCTION

Suppression of dendritic cells (DCs) is a crucial mechanism by which tumor cells escape immune recognition and elimination. We have recently reported that MHC class I antigen processing machinery (APM) component expression in human DCs is down-regulated by tumor-derived gangliosides. However, the molecular mechanisms underlying this abnormality were not identified. Thus, the aim of this work was to analyze the role of interferon regulatory factor 8 (IRF-8) in APM protein expression and the antigen presenting capacity of DCs developed in the tumor microenvironment.

RESULTS

We demonstrate that the expression of several MHC class I APM components, including delta, MB-1, LMP-10, ERp57, and tapasin, is significantly decreased in murine DCs generated in the presence of prostate cancer cells. APM component down-regulation was associated with decreased ability of DCs to present model antigen to antigen-specific T cells. Notable, impaired antigen-presenting activity of DCs co-cultured with tumor cells was accompanied by decreased levels of IRF-8. Transduction of DCs with the silencing RNA for the IRF-8 gene also led to reduced expression of APM components in DCs and decreased antigen presenting function.

CONCLUSION

Together, our data suggest that tumor-induced inhibition of antigen processing and presenting function of DCs is mediated by IRF-8, a member of the interferon regulatory factor family. These results provide a new molecular target for optimizing the generation of efficient DC vaccines for cancer therapy.

Immune cell-specific amplification of interferon signaling by the IRF-4/8-PU.1 complex

Both type I interferon (IFN-alpha/beta) and type II IFN (IFN-gamma) exert many functions that are restricted to immune cells. Thus, they play critical roles in innate and adaptive immunity. IFN regulatory factor-4 (IRF-4) and IRF-8 (formerly PU.1 interaction partner [Pip] and IFN consensus sequence binding domain [ICSBP], respectively) are immune cell-specific members of the IRF family that regulate the development of myeloid, lymphoid, and dendritic cells. They form a heterodimeric complex with another immune cell-specific transcription factor PU.1-Spi-1 and regulate transcription of genes in the immune system. This review describes the role of the IRF-8-PU.1 complex in modulating IFN signaling in an immune cell-specific manner. Our studies revealed that some but not all IFN-gamma-inducible genes carry an IFN-gamma activation site (GAS) element that contains a binding site for the IRF- 8-PU.1 complex. The IRF-8-PU.1 complex can take part in GAS-mediated transcription and amplify expression of IFN-gamma-responsive genes initiated by Stat1 in macrophages. Similarly, some but not all IFN-alpha/beta-responsive genes are shown to carry an IFN-stimulated response element (ISRE) that contains an IRF-8-PU.1 binding site. The participation of IRF-8-PU.1 in ISRE-mediated transcription results in the augmentation of IFN-stimulated gene factor 3 (ISGF3)-induced transcription in macrophages. Thus, GAS and ISRE elements, classically defined as universal IFN-alpha/beta and IFN-gamma response sequences, are not the same, and some harbor an embedded motif for IRF- 8-PU.1 binding that functions only in immune cells. Accordingly, the IRF-8-PU.1complex provides secondary IFN signaling pathways unique to the immune system. Collectively, the contribution of IRF-8 and PU.1 to IFN-regulated gene expression may in part account for immune cell-specific functions of IFNs.