Mesenchymal-epithelial transition reduces proliferation but increases immune evasion in tumor spheroids

The mesenchymal-epithelial transition (MET) has been implicated in secondary tumor growth during metastasis, yet the underlying mechanisms remain unclear. Here, we explore whether MET enhances tumor outgrowth by boosting proliferation, using MET-inducible mesenchymal breast cancer cells. Surprisingly, we found that crowding inhibition of proliferation persists both before and after MET, with mesenchymal cells exhibiting a proliferative edge through more effective escape from crowded cell islands. In three-dimensional culture, MET caused a reduction in proliferation, accompanied by distinct changes in proliferative signalling of focal adhesions and actomyosin. However, Src inhibition led to enhanced proliferation upon MET. Furthermore, co-culture experiments with peripheral blood mononuclear cells reveal that MET-induced spheroids demonstrate an increased ability to evade immune cell attacks. This effect is likely mediated by the confined epithelial morphology and alterations in the expression of key immunomodulatory molecules. Together, these findings suggest that MET may contribute to secondary tumor outgrowth, not by boosting proliferation, but by enhancing survival in immune-challenging environments.