The curious phenomenon of dual-positive circulating cells: Longtime overlooked tumor cells

The presence in the blood of patients with solid tumors of circulating cells expressing both epithelial and leukocyte markers (dual-positive cells, DPcells), has often been reported, though it has never been investigated in detail. A recent study suggested that DPcells are hybrid cells derived from the fusion of tumor cells with macrophages. Such fusion hybrids acquire macrophage-associated features endowing them with accelerated growth, increased motility, enhanced invasion activity and thus, a higher efficiency in metastasis formation. However, no direct evidence proving the tumor origin of circulating DPcells was provided in patients. Here we contribute a review of literature data on DPcells and on the hybrid theory with the aim of putting the current evidence both in a biological and clinical perspective and to generate new hypotheses on the mechanisms underlying tumor progression. To add further biological and clinical context to our literature review, we also report some preliminary data from our laboratory on the identification of DPcells in several solid tumors and confirmation of their malignant genotype, thus classifying them as DP-CTCs.