It is evident from the data presented in this Review that primary tumours and their metastatic off-shoots are complex ecologies consisting of numerous cell types. Many of these are derived from the bone marrow but there are also abundant resident cells. In analysing these tumours, by necessity, experiments are usually focused upon a single cell type or a single gene product within a cell. However, it is naive to think that individual cell types function in isolation in a complex system. Thus, a major area for advances in understanding the role of the microenvironment must incorporate a systems biology approach in order to model these complex interactions and their evolution over time.
It is also apparent that cancer is a systemic disease with the tumour affecting multiple systems in the host that can result in spread of cancer cells and enhancement of metastasis. It is important to realize that this usually involves mobilization of BMDCs, which home to the tumour sites and dramatically alter their ecology, generally but not always in favour of the tumour. In addition, these changes modify immune responses in the tumour-bearing animals, often with a profound immunosuppression against new antigens expressed in the tumour cells themselves. It remains obscure how the mobilization of BMDCs occurs, how they are trafficked from bone marrow into the blood and back as well as to different tissues, and what regulates the final differentiation and function of BMDCs at these sites. Several molecules have been shown to be important, including SDF1, VEGFA, KIT ligand and osteopontin as discussed above, but the major points of regulation still remain to be elucidated. Nevertheless this is an area of research that needs to be expanded, along with detailed phenotyping of the cells that BMDCs differentiate into, as this may be the key to controlling metastatic spread. These approaches will be significantly enhanced by new imaging techniques that can track cells and their interactions in vivo
in both humans and mice. These include intravital imaging using multi-photon microscopy of cells fluorescently labelled either intrinsically or by barcodes of identifying antibodies55
; nanoparticles that attach to the cell surface through tags or other innovative means; enhanced magnetic resonance imaging techniques using targeted nanoiron probes; positron emission tomography; and spectroscopic methods coupled with magnetic resonance imaging or microscopy131
to individually identify cell types based on intrinsic fluorescence in vivo
— a technique that might even be applicable to human tumours.
The goal of all these experiments is to create sufficient biological insight to reverse the tumour-enhancing effects of the microenvironment with the ideal of recreating a suppressive microenvironment that can fully revert the malignant phenotype to normal (or at least a controlled phenotype), as was found in the pioneering experiments of reprogramming malignant tumour cells in normal environments that are described above. This raises the issue of whether stromal alterations are always reversible, or whether there is a point after which these changes cannot be reversed. These questions apply both to modifications conferred on the stroma by the cancer cells and to those conferred on cancer cells by the stroma. Although it is probably optimistic to think that all changes can be fully reversed, various strategies have been used to target stromal cells, particularly in the primary tumour132,133
, several of which have therapeutic efficacy. A wide range of agents are already available, at least in animal models, to block the functions of stromal cells discussed in this Review, including EGFR and CSF1R antagonists, VEGFA and Vegfr inhibitors, TNFα inhibitors, S100 antibodies, protease inhibitors, anticoagulants and chemokine inhibitors, including CXCR4 antagonists. It is likely that these will need to be used in combination to attack the metastatic microenvironment in its diversity and to undermine its robustness. Moreover, it is unlikely that any of these strategies will work alone without the incorporation of a direct attack on the tumour cell itself.
An emerging concept in anticancer therapy involves the mobilization of several types of BMDC following treatment with traditional chemotherapeutics or targeted therapies, which may contribute either to a lack of response or acquired drug resistance. For example, EPCs are rapidly mobilized through SDF1 and VEGFR2 signalling in response to certain chemotherapies, including the taxanes and 5-fluorouracil. Resistance to these drugs can be overcome in part through anti-VEGFR2 blocking antibodies, or by genetic ablation of EPCs in ID1 and ID3-deficient mice134,135
. BMDCs can also confer an inherent refractoriness to therapy, as in the case of MDSCs, which are recruited to certain tumour models in response to anti-VEGFA treatment136
. However, when the anti-VEGFA antibody was combined with an anti-GR1 antibody, tumour growth was effectively reduced. These examples indicate that it will be important to inhibit mobilized BMDCs alongside drugs that target cancer cells, and this synergy may in fact explain some of the therapeutic efficacy observed in certain combination trials in mice and humans to date.
Among the stromal cell types important for metastatic dissemination and outgrowth, drugs that target endothelial cells are the most clinically advanced. Several Vegf antagonists have been approved by the US Food and Drug Administration137
that increase survival in patients with metastatic breast cancer138
and metastatic colorectal cancer139
when combined with chemotherapy. The translational promise of this first targeted therapy against the tumour microenvironment, however, is somewhat tempered by emerging instances of resistance to anti-angiogenic therapy140
, and examples of stromal cell targeting in animal models that unexpectedly resulted in increased invasion48
. These data reinforce the importance of fully understanding the intricacy of cellular interactions in the tumour microenvironment, using approaches discussed in this Review, in order to isolate the cancer cells from their multiple support networks and effectively destroy them.