Characterizing the Immune Landscape of a Syngeneic Mouse Model with Regional Lymph Node Metastasis and Distant Lung Metastasis.

Background/Objectives: Tumor-draining lymph nodes (TDLNs) play a critical role in shaping a pre-metastatic and immunosuppressive niche that facilitates tumor dissemination. However, a robust in vivo model that recapitulates regional lymph node and distant metastasis remains elusive. We sought to establish and characterize an animal model that reliably mimics spontaneous human melanoma progression from primary tumor to regional lymph node and distant organ metastasis. Methods: B16F10-Luc2 melanoma cells were subcutaneously implanted from the hock area into the lower medial thigh of C57BL/6J mice to establish a metastatic melanoma mouse model with reliable regional lymph node metastasis and distant metastasis. Tumor growth was monitored over time. 18F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) and bioluminescence imaging (BLI) were used to track lymph node and lung metastases in vivo. Immunocytochemistry (IHC) was performed to confirm the metastatic melanoma cells in tissues. The immune landscape of metastatic lymph nodes was characterized by Cytometry by Time-Of-Flight (CyTOF). Results: This implantation strategy resulted in consistent primary tumor growth accompanied by reproducible metastasis to ipsilateral inguinal lymph nodes (IILN) and subsequent lung metastasis. Multi-modal imaging (PET/CT and BLI) enabled non-invasive monitoring of metastatic progression in vivo. Histological analyses confirmed melanoma cell infiltration in lymph nodes and lung tissues. Notably, CyTOF profiling revealed a distinct immunological remodeling within the TDLN, characterized by immunosuppressive phenotypes in the tumor-draining ipsilateral inguinal lymph node (IILN) compared with naïve nodes, indicative of a potential immunotolerant environment in tumor-bearing mice associated with metastatic progression. Conclusions: We present a reproducible and clinically relevant syngeneic melanoma mouse model that recapitulates lymphatic dissemination to regional lymph nodes and distant lung metastasis, and mimics the immunosuppressive phenotypes in the tumor-draining lymph node microenvironment seen in human melanoma patients. This model provides a valuable platform for investigating the mechanisms of lymphatic metastasis and the immunological dynamics of TDLNs, with potential applications in evaluating therapeutic strategies targeting metastatic progression.