Development of breast tumors during pregnancy show a tendency for early spread of cancer to distant sites (metastasis). Researchers have uncovered the possible reason behind this mechanism.
Stamenkovic and colleagues found that the increased metastasis from tumors of several different types that they observed in pregnant mice was a result of decreased activity of immune cells known as NK cells. Furthermore, at least part of the inhibitory effect on NK cells was mediated by another group of immune cells, myeloid-derived suppressor cells.
Consistent with this, the gene expression profile of the lungs of pregnant mice (a site to which many of the tumors metastasized) was reflective of myeloid-derived suppressor cell accumulation. Of clinical interest, the majority of genes downregulated in the lungs of pregnant mice were also expressed at lower levels in samples from lung cancer patients with poor prognosis than in samples from patients with better prognosis. The authors therefore suggest that myeloid-derived suppressor cells may represent a shared mechanism of immune suppression during pregnancy and tumor growth.