Double-positive T cells form heterotypic clusters with circulating tumor cells to foster cancer metastasis.

The immune ecosystem is central to maintaining effective defensive responses. However, it remains largely understudied how immune cells in the peripheral blood interact with circulating tumor cells (CTCs) in metastasis. Here, blood analysis of patients with advanced breast cancer revealed that over 75% of CTC-positive blood specimens contained heterotypic CTC clusters with CD45+ white blood cells (WBCs), which correlates with breast cancer subtypes, racial groups, and decreased survival. CTC-WBC clusters included overrepresented T cells and underrepresented neutrophils. Specifically, a rare subset of CD4 and CD8 double-positive T (DPT) cells was 140-fold enriched in CTC clusters versus their frequency in WBCs. DPT cells shared properties with CD4+ and CD8+ T cells but exhibited unique features of T cell exhaustion and immune suppression. Mechanistically, the integrin heterodimer α4β1, also named very late antigen 4 (VLA-4), in DPT cells and its ligand, VCAM1, in tumor cells are essential mediators of DPT-CTC clusters. Neoadjuvant administration of anti-VLA-4 neutralizing antibodies markedly blocked CTC-DPT clusters, inhibited metastasis, and extended mouse survival. These findings highlight a pivotal role of rare DPT cells in fostering cancer dissemination through CTC clustering. It lays a foundation for developing innovative biomarker-guided therapeutic strategies to prevent and target cancer metastasis.