T cells are critical for pre-metastatic niche formation through the STAT3/IL-17 axis

Pre-metastatic niches composed of mainly myeloid cells are recognized as critical for tumor metastasis. However, whether adaptive immune cells also play an important role in pre-metastatic niche formation remains to be explored.

CD4⁺ T cell accumulation in tumor-free lung tissues from mice bearing subcutaneous mouse tumors was detected by immunofluorescence/confocal microscopy. Tumor-conditioned media (TCM) from MB49-S1pr1 ^high mouse bladder tumor cells or ID8 ovarian tumor cells were administered to tumor-free mice to induce pre-metastatic niche formation. We used mice lacking functional Signal Transducer and Activator of Transcription 3 (STAT3) in T cells and Il17a^‒/‒ mice to investigate the roles of STAT3 and interleukin (IL)-17. In vivo time-course experiments were performed to assess whether CD4⁺ T cell clusters contribute to CD11b⁺ pre-metastatic clusters. CD4⁺ T cell migration and chemokine receptor expression assays were employed to identify tumor factors driving CD4⁺ T cell recruitment. A co-culture system with human MRC-5 lung fibroblasts, healthy donor-derived CD4⁺ T cells, myeloid cells, and TCM derived from human cancer cells was used to evaluate CD4⁺ T cell-driven fibroblast activation and IL-17A dependency for myeloid cell migration. Microscopic analyses were performed to confirm CD4⁺ T cell clusters in tumor-free lymph node tissues from patients with prostate cancer and postmortem lung and liver specimens from patients with ovarian cancer.

We demonstrate that CD4⁺ T cells accumulate in tumor-free lungs and promote tumor metastasis in mouse models. CD4⁺ T-cell pre-metastatic niche formation requires STAT3, which regulates Th17 CD4⁺ T cells. TCM drives IL-6-dependent CCR4/CCR6 upregulation on naive CD4⁺ T cells. CD4⁺ T cell clusters contribute to myeloid cell accumulation, and ablating STAT3 in T cells abrogates both T cell and myeloid cluster formation. IL-17 inhibition reduces myeloid lung infiltration. In human co-cultures, CD4⁺ T cells amplify TCM-induced fibroblast pre-metastatic niche-like activation and myeloid recruitment in an IL-17A-dependent manner. CD4⁺ and IL-17⁺ or p-STAT3⁺ clusters were also detected in non-metastatic tissues from patients with several cancers.

CD4⁺ T cells form pre-metastatic niches through the STAT3-IL-17 axis, contributing to myeloid cell cluster formation, in part through amplifying fibroblast pre-metastatic niche-like activation. STAT3 and IL-17 in CD4⁺ T cells therefore are important for pre-metastatic niche formation and metastasis.