Plasticity in tumor-promoting inflammation: impairment of macrophage recruitment evokes a compensatory neutrophil response

Corrupt policemen: inflammatory cells promote tumor angiogenesis

PURPOSE OF REVIEW

The last decade has seen a paradigm change in that tumor stroma contributes to malignant tumor progression in a manner comparable to genetic and epigenetic changes within cancer cells. This review summarizes recent novel insights into how inflammatory conditions stimulate the formation and expansion of blood and lymphatic vessels within tumors and, thus, allow tumors to grow, to gain invasive capabilities, and to finally seed metastasis in distant organs.

RECENT FINDINGS

Different cancer types have a highly defined microenvironment, which is composed of cancer-associated fibroblasts, blood vessel and lymphatic endothelial cells, pericytes, and a heterogeneous infiltrate of cells of the immune system. In addition to the local stimulation of tumor angiogenesis and tumor lymphangiogenesis, cytokines released by the primary tumor and by the immune cell infiltrate also instruct bone marrow-derived cells to colonize distant organs and to prepare these sites for future metastasis.

SUMMARY

Inflammatory reactions coinciding with carcinogenesis can be visualized by the presence of specific bone marrow-derived, inflammatory cells in patients' peripheral blood. Recent findings suggest that such inflammatory fingerprints may better define the inflammatory nature of the primary malignancy and, thus, allow the design of therapeutic strategies targeting the protumorigenic immune cell stroma compartment.

Tumor-associated macrophages: effectors of angiogenesis and tumor progression

Tumor-associated macrophages (TAMs) are a prominent inflammatory cell population in many tumor types residing in both perivascular and avascular, hypoxic regions of these tissues. Analysis of TAMs in human tumor biopsies has shown that they express a variety of tumor-promoting factors and evidence from transgenic murine tumor models has provided unequivocal evidence for the importance of these cells in driving angiogenesis, lymphangiogenesis, immunosuppression, and metastasis. This review will summarize the mechanisms by which monocytes are recruited into tumors, their myriad, tumor-promoting functions within tumors, and the influence of the tumor microenvironment in driving these activities. We also discuss recent attempts to both target/destroy TAMs and exploit them as delivery vehicles for anti-cancer gene therapy.

Combined IL-8 and TGF-beta blockade efficiently prevents neutrophil infiltrates into an A549-cell tumor

Neutrophil infiltrates into tumors have been reported in certain circumstances to reduce tumor growth and in other circumstances to augment tumor growth, particularly by facilitating metastasis. Neutrophil chemotaxis can be facilitated by both interleukin-8 (IL-8) and transforming growth factor-beta (TGF-beta). However, the combined effects of these two cytokines on neutrophil tumor infiltrates is unknown, and we considered the possibility that studying the combined effects might resolve apparent contradictions with regard to neutrophil effects on tumor development. First, we determined that a simultaneous IL-8 and TGF-beta blockade is far more efficient at eliminating the neutrophil infiltrate from an A549 derived tumor than is blockade of either cytokine alone. Blockade of IL-8 alone, led to smaller tumors, consistent with the known inhibitory role of TGF-beta on A549 cell proliferation. Blockade of TGF-beta alone rescued the tumor growth but led to reduced metastasis volume. Surprisingly, blockade of both cytokines rescued both tumor volume and metastasis, underscoring the difficulty of understanding the effects of complete tumor cytokine elaboration profiles by isolating the effects of only one cytokine.

The role of myeloid cells in the promotion of tumour angiogenesis

The use of various transgenic mouse models and analysis of human tumour biopsies has shown that bone marrow-derived myeloid cells, such as macrophages, neutrophils, eosinophils, mast cells and dendritic cells, have an important role in regulating the formation and maintenance of blood vessels in tumours. In this Review the evidence for each of these cell types driving tumour angiogenesis is outlined, along with the mechanisms regulating their recruitment and activation by the tumour microenvironment. We also discuss the therapeutic implications of recent findings that specific myeloid cell populations modulate the responses of tumours to agents such as chemotherapy and some anti-angiogenic therapies.