A challenging task of the immune system is to fight cancer cells. However, a variety of human cancers educate immune cells to become tumor supportive. This is exemplified for tumor-associated macrophages (TAMs), which are polarized towards an antiinflammatory and cancer promoting phenotype. Mechanistic explanations, how cancer cells influence the macrophage phenotype are urgently needed to address potential anti-cancer strategies along this line. One potential immune modulating compound, sphingosine-1-phosphate (S1P), was recently highlighted in both tumor growth and immune modulation. Using a xenograft model in nude mice, we demonstrate a supportive role of sphingosine kinase 2 (SphK2), one of the S1P-producing enzymes for tumor progression. The growth of SphK2-deficient MCF-7 breast tumor xenografts was markedly delayed when compared with controls. Infiltration of macrophages in SphK2-deficient and control tumors was comparable. However, TAMs from SphK2-deficient tumors displayed a pronounced anti-tumor phenotype, showing an increased expression of pro-inflammatory markers/mediators such as NO, TNF-a, IL-12 and MHCII and a low expression of anti-inflammatory IL-10 and CD206. These data suggest a role for S1P, generated by SphK2, in early tumor development by affecting macrophage polarization. ' 2009 UICC

Summary Intrinsic immunosuppression is a major obstacle for a successful cancer therapy. Mechanisms how immunosuppression is induced and regulated in humans are ill-defined. A micro-environmental component that might prevent anti-tumor immunity is the presence of dying tumor cells, which is abundant following conventional cancer ablation methods such as chemo- or radiotherapy. Shedding of apoptotic debris and/or secretion of factors to the tumor bed or draining lymph nodes thus might have a profound impact on professional phagocytes such as DC and subsequent priming of lymphocytes. In this study, we exposed human DC to supernatants of living, apoptotic or necrotic human breast cancer cells and co-cultured them with autologous T cells. Priming with apoptotic debris prevented DC from establishing cytotoxicity towards living human tumor cells by inducing a regulatory T cell population, defined by co-expression of CD39 and CD69. Immunosuppression via Treg was transferable and required the release of sphingosine-1-phosphate (S1P) from apoptotic cells, acting via S1P receptor 4 on DC to induce IL-27 secretion. We propose that CD69-expression on CD39+ Treg enables them to interact with CD73-expressing CD8+ T cells to generate adenosine, thereby suppressing cytotoxicity. These findings aid the understanding how dying tumor cells limit anti-tumor immunity.

Macrophages are a group of heterogeneous cells of the innate immune system that are crucial to the initiation, progression, and resolution of inflammation. Moreover, they control tissue homeostasis in healthy tissue and command a broad sensory arsenal to detect disturbances in tissue integrity. Macrophages possess a remarkable functional plasticity to respond to irregularities and to initiate programs that allow overcoming them in order to return back to normal. Thus, macrophages kill malignant or transformed cells, rearrange extracellular matrix, take up and recycle cellular as well as molecular debris, initiate cellular growth cascades, and favor directed migration of cells. As an example, apoptotic death of bystander cells is sensed by macrophages, initiating functional responses that support all hallmarks of cancer. In this chapter, we describe how tumor cell apoptosis hijacks tumor-associated macrophages to promote tumor growth. We propose that tumor therapy should not only kill malignant cells but also target the interaction of the host with apoptotic cancer cells, as this might be efficient to limit the protumor action of apoptotic cells and boost the antitumor potential of macrophages. Leaving the apoptotic cell/macrophage interaction untouched might also limit the benefit of conventional tumor cell apoptosis-focused therapy since surviving tumor cells might receive overwhelming support by the wound healing response that apoptotic tumor cells will trigger in local macrophages, thereby enhancing tumor recurrence.

Restoration of one of the major physiological functions of the body's immune response, the rejection of malignant cells, is a promising yet challenging task for cancer therapy. Prinicipally, immunotherapeutic approaches make use of cells of the adaptive immune system, since antigen-based tumor rejection might be the most specific approach. However, other immune cell populations, such as tumor-associated macrophages (TAMs), contribute significantly to protumor mechanisms elicited by a distorted immune response. In this review, we summarize the current knowledge about the pathology of TAMs and discuss potential therapeutic approaches to overcome TAM-mediated tumor promotion. Hereby, we focus on TAM phenotypes that were observed in the clinically relevant stages of cancer progression. The function of macrophages and other inflammatory cells in the onset of cancer has been discussed elsewhere.

Natural Killer T cells (NKT cells) are emerging as critical regulators of pro- and anti-tumor immunity, both at baseline and in therapeutic settings. While type I NKT cells can promote anti-tumor immunity, their activity in the tumor microenvironment may be limited by negative regulators such as inhibitory immune checkpoints. We observed dominant expression of B- and T-lymphocyte attenuator (BTLA) on type I NKT cells in polyoma middle T oncogene-driven (PyMT) murine autochthonous mammary tumors. Other immune checkpoint receptors, such as programmed cell death 1 (PD-1) were equally distributed among T cell populations. Interference with BTLA using neutralizing antibodies limited tumor growth and pulmonary metastasis in the PyMT model in a therapeutic setting, correlating with an increase in type I NKT cells and expression of cytotoxic marker genes. While therapeutic application of an anti-PD-1 antibody increased the number of CD8+ cytotoxic T cells and elevated IL-12 expression, tumor control was not established. Expression of ZBTB16, the lineage-determining transcription factor of type I NKT cells, was correlated with a favorable patient prognosis in the METABRIC dataset, and BTLA levels were instrumental to further distinguish prognosis in patents with high ZBTB16 expression. Taken together, these data support a role of BTLA on type I NKT cells in limiting anti-tumor immunity.