Primary tumours modulate innate immune signalling to create pre-metastatic vascular hyperpermeability foci

In mouse models of lung metastasis, before the appearance of significant metastases, localized changes in vascular permeability have been observed, which appear to set the stage for tumour growth. However, it is unclear whether this is also true in human patients. Here, we show that MD-2, a coreceptor for Toll-like receptor 4 that has a key role in the innate immune response, triggers the formation of regions of hyperpermeability in mice by upregulating C-C chemokine receptor type 2 (CCR2) expression. The CCR2-CCL2 system induces the abundant secretion of permeability factors such as serum amyloid A3 and S100A8. Disruption of MD-2 or CCR2 abrogates the formation of hyperpermeable regions, resulting in reduced tumour cell homing. Furthermore, fibrinogen, which is processed during permeability-mediated coagulation, is also localized in areas of elevated CCR2 expression in tumour-bearing human lungs. Our findings raise the possibility that CCR2 upregulation might represent a marker for regions of increased susceptibility to metastatic homing in lung cancer.