Spontaneous and experimental models of lymph node metastasis.

Lymph node (LN) metastasis is a conserved feature across most solid organ malignancies and portends worse prognoses. Functionally, LN metastases induce systemic tumor-specific immune tolerance and may serve as a reservoir for distant metastases. Nonetheless, there are relatively few preclinical models for interrogating the biology of LN metastasis and its systemic effects at various stages of metastatic progression. We describe a method for modeling LN metastasis of melanoma tumors in mice that enables assessment of tumor and immune cell phenotypes and the functional roles of nodal involvement on distant metastasis. Our model comprises a family of transplantable syngeneic melanoma tumor cell lines evolved to exhibit enhanced LN metastatic potential, which can be used to probe cancer-immune interactions and test new therapeutics. We present both (i) a spontaneous LN metastasis model involving primary tumor implantation and assessment of LN colonization 21-28 d later and (ii) an experimental metastasis model involving implantation of primary tumors followed by direct intra-LN injections of tumor cells. Both models can be extended to assess the impact of LN metastasis on the development of distant metastases through asynchronous intravenous injections of tumors. Finally, we discuss experimental design considerations including when to use spontaneous or experimental models and troubleshooting consistent LN metastasis, making this model accessible for researchers with basic mouse survival-surgery skills. We highlight how LN metastasis models can be used to profile metastatic immune reprogramming, measure the impact of nodal metastases on distant metastases and assess novel anti-metastatic therapeutics.