Inhibition of anti-tumor immunity by melanoma cell-derived Activin-A depends on STING (scRNA-Seq)

The transforming growth factor-β (TGF-ꞵ) family member activin A (hereafter Activin-A) is overexpressed in many cancer types, often correlating with cancer-associated cachexia and poor prognosis. Activin-A secretion by melanoma cells indirectly impedes CD8+ T cell-mediated anti-tumor immunity and promotes resistance to immunotherapies, even though Activin-A can be proinflammatory in other contexts. To identify underlying mechanisms, we here analyzed the effect of Activin-A on syngeneic grafts of Braf mutant YUMM3.3 mouse melanoma cells and on their microenvironment using single-cell RNA sequencing. We found that the Activin-A-induced immune evasion was accompanied by a proinflammatory interferon signature across multiple cell types, and that the associated increase in tumor growth depended at least in part on pernicious STING activity within the melanoma cells. Besides corroborating a role for proinflammatory signals in facilitating immune evasion, our results suggest that STING holds considerable potential as a therapeutic target to mitigate tumor-promoting Activin-A signaling at least in melanoma. To assess activin-induced changes in melanoma and in their TME, YUMM3.3 mouse melanoma grafts expressing lentiviral INHBA (βA) or empty control lentivirus (Ctrl) were analyzed by single-cell RNA sequencing (scRNA-seq) using the 10x Genomics platform. The YUMM3.3-Ctrl and -βA melanoma cells were transduced with a lentiviral green fluorescent protein (GFP) expression vector to distinguish stromal and cancerous cells. Tumors were digested and single cells were separated into GFPhi tumor cells and GFPint/low stromal cells and then mixed at a 1:9 ratio to enrich the library for stromal cells. Triplicate Ctrl and ꞵA samples consisting of each of two tumors were collected on day 13 post-injection when tumor sizes were not yet significantly enlarged by Activin-A.