Subcapsular sinus macrophages promote melanoma metastasis to the sentinel lymph nodes via an IL-1α-STAT3 axis.

During melanoma metastasis, tumor cells originating in the skin migrate via lymphatic vessels to the sentinel lymph nodes (sLNs). This process facilitates tumor cell spread across the body. Here, we characterized the innate inflammatory response to melanoma in the metastatic microenvironment of the sLNs. We found that macrophages located in the subcapsular sinus (SS) produced pro-tumoral IL-1α after recognition of tumoral antigens. Moreover, we confirmed that the elimination of LN macrophages or the administration of an IL-1α-specific blocking antibody reduced metastatic spread. To understand the mechanism of action of IL-1α in the context of the sLN microenvironment, we applied single-cell RNA sequencing to microdissected metastases obtained from animals treated with the IL-1α-specific blocking antibody. Amongst the different pathways affected, we identified STAT3 as one of the main targets of IL-1α signaling in metastatic tumor cells. Moreover, we found that the antitumoral effect of the anti-IL-1α was not mediated by lymphocytes because Il1r1 knockout mice did not show significant differences in metastasis growth. Finally, we found a synergistic anti-metastatic effect of the combination of IL-1α blockade and STAT3 inhibition with stattic, highlighting a new immunotherapy approach to preventing melanoma metastasis.