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Simple Summary: Melanoma patients with high neutrophil counts often show impaired clinical response and poor prognosis, indicating that neutrophils can support melanoma progression. The precise mechanism responsible for this correlation, especially in the context of targeted therapy, still requires clarification. We show that peripheral blood neutrophils of patients with advanced melanoma are characterized by lower CD16 surface expression compared to healthy donors, which has been reported to be associated with tumor promotion. We provide evidence that melanoma cells under dual-targeted therapy can be protected in vitro by neutrophils from both patients and healthy donors. In addition, this protective effect is dependent on cell–cell contact, as well as on culture conditions, and is observed under nonadherence. Unraveling the mechanism, the interference with the protease activity of neutrophils reduced protection. Understanding the complex interaction of neutrophils and melanoma cells might aid in discovering methods to prevent the tumor-promoting effects by neutrophils in patients. Elevated levels of peripheral blood and tumor tissue neutrophils are associated with poorer clinical response and therapy resistance in melanoma. The underlying mechanism and the role of neutrophils in targeted therapy is still not fully understood. Serum samples of patients with advanced melanoma were collected and neutrophil-associated serum markers were measured and correlated with response to targeted therapy. Blood neutrophils from healthy donors and patients with advanced melanoma were isolated, and their phenotypes, as well as their in vitro functions, were compared. In vitro functional tests were conducted through nonadherent cocultures with melanoma cells. Protection of melanoma cell lines by neutrophils was assessed under MAPK inhibition. Blood neutrophils from advanced melanoma patients exhibited lower CD16 expression compared to healthy donors. In vitro, both healthy-donor- and patient-derived neutrophils prevented melanoma cell apoptosis upon dual MAPK inhibition. The effect depended on cell–cell contact and melanoma cell susceptibility to treatment. Interference with protease activity of neutrophils prevented melanoma cell protection during treatment in cocultures. The negative correlation between neutrophils and melanoma outcomes seems to be linked to a protumoral function of neutrophils. In vitro, neutrophils exert a direct protective effect on melanoma cells during dual MAPK inhibition. This study further hints at a crucial role of neutrophil-related protease activity in protection. [ABSTRACT FROM AUTHOR]

Objective: Cellular heterogeneity is regarded as a major factor affecting treatment response and resistance in malignant melanoma. Recent developments in single-cell sequencing technology have provided deeper insights into these mechanisms. Methods: Here, we analyzed a BRAFV600E-mutant melanoma cell line by single-cell RNA-seq under various conditions: cells sensitive to BRAF inhibition with BRAF inhibitor vemurafenib and cells resistant to BRAF inhibition with vemurafenib alone or vemurafenib in combination with the MEK1/2 inhibitors cobimetinib or trametinib. Dimensionality reduction by t-distributed stochastic neighbor embedding and self-organizing maps identified distinct trajectories of resistance development clearly separating the 4 treatment conditions in cell and gene state space. Results: Trajectories associated with resistance to single-agent treatment involved cell cycle, extracellular matrix, and de-differentiation programs. In contrast, shifts detected in double-resistant cells primarily affected translation and mitogen-activated protein kinase pathway reactivation, with a small subpopulation showing markers of pluripotency. These findings were validated in pseudotime analyses and RNA velocity measurements. Conclusions: The single-cell transcriptomic analyses reported here employed a spectrum of bioinformatics methods to identify mechanisms of melanoma resistance to single- and double-agent treatments. This study deepens our understanding of treatmentinduced cellular reprogramming and plasticity in melanoma cells and identifies targets of potential relevance to the management of treatment resistance. [ABSTRACT FROM AUTHOR]