Most if not all metastatic tumours are infiltrated by macrophages. This type of white blood cell is an important component of the immune system in healthy people, but also promotes inflammation in tumour tissues and stimulates cancer cell proliferation and therefore metastasis. These cells are known to differentiate from monocytes, another type of white blood cell that acts as a "reserve" immune cell population. Jeffrey Pollard and his co-workers at the Albert Einstein College of Medicine, New York, USA, working with collaborators from Ortho Biotech Oncology R&D in Radnor, Pennsylvania, USA, have now investigated the origin of tumour-associated macrophages by measuring the trafficking of monocytes to tumours in a mouse breast cancer model (findings published in Nature). 

Monocytes can be classified as resident or inflammatory, only the latter type being associated with tumour metastasis. Pollard and his co-workers transferred each type of mouse monocyte separately into transgenic mice bearing Polyoma Middle T (PyMT) mammary tumours with spontaneous pulmonary metastases. Eighteen hours after cell transfer, they found resident monocytes to be located predominantly in the primary tumours and inflammatory monocytes in the metastases. Within a few days, many inflammatory monocytes had differentiated into a type of macrophage that is known to be associated with tumour metastasis. Similar results were observed when sorted human monocytes were transferred into a mouse model bearing metastatic human tumour xenografts.

Inflammatory but not resident monocytes are known to express CCR2, the protein product of which is the receptor for CCL2, a chemokine expressed in both metastatic tumour tissue and its micro-environment. The researchers neutralised CCL2 in the mouse metastases using an antibody and showed that this reduced the recruitment of inflammatory monocytes into those tumours. This result could similarly be reproduced in human tumour xenografts using an antibody against human CCL2. They also showed that anti-CCL2 antibodies reduced the number of metastasic nodules in the mouse lungs, whereas antibodies against another mouse CCR2 ligand, CCL12, had no such effect.

The researchers then used intact-lung imaging of tumour cells expressing cyan fluorescent protein to show that anti-CCL2 antibodies significantly reduced the number of inflammatory macrophages interacting directly with tumour cells. Blocking CCL2-CCR2 signalling also inhibited the process known as extravasation in which tumour cells leave the capillaries and enter organs. Analysis and comparison of the transcriptomes of resident and inflammatory monocytes showed that vascular endothelial growth factor A (VegfA) was highly expressed only in inflammatory cells, and further tests showed that bone-marrow derived macrophages lacking VegfA were unable to promote tumour cell migration.

Taken together, these results indicated that CCL2 secretion and CCL2-CCR2 signalling recruit inflammatory macrophages to metastatic tumour sites and that these cells differentiate into macrophages and promote further tumour-cell extravasation and metastasis. Clinical data has shown both expression of this cytokine and macrophage infiltration to be associated with poor prognosis in human breast cancer. Pollard and his co-workers conclude that this signalling pathway may represent a novel therapeutic target for the treatment of metastatic breast cancer.

Reference

Qian, B-Z., Li, J., Zhang, H., Kitamura, T., Zhang, J., Campion, L.R., Kaiser, E.A., Snyder, L.A. and Pollard, J.W, CCL2 recruits inflammatory monocytes to facilitate breast-tumour metastasis, Nature, published online ahead of print 8 June 2011, doi: 10.1038/nature10138