BACKGROUND: Programmed death ligand-1 (PD-L1) is a widely used biomarker for immunotherapy in melanoma. The expression of PD-L1 in melanoma cells is known to vary considerably among and within patients' tumor samples. Recent studies suggest that there may be a link between PD-L1 expression and the differentiation status of melanoma cells which is known to fluctuate in response to external stimuli and to be influenced by a multitude of regulators. Here, we systematically review which differentiation regulators affect PD-L1 expression in melanoma. METHODS: , 2026. Screening of abstracts and titles was performed independently by two reviewers. Screening of full-text articles, data extraction, and validity assessments were performed by one reviewer. Studies published in English reporting changes in PD-L1 expression associated with a known differentiation regulator in a melanocytic/melanoma model were included. Studies were assessed for risk of bias regarding imprecision in experimental reporting and model validity, using previously defined assessment tools. Synthesis of results was performed narratively. RESULTS: Out of 496 identified articles, 57 studies met the inclusion criteria. A total of 16 differentiation regulators were significantly associated with PD-L1 expression in melanoma cells. Most studies (45/57) reported a single regulator, while 12/57 reported multiple. STAT3 appeared in 20/57 studies; all other regulators were reported in eight or fewer. PD-L1 expression was positively associated with all dedifferentiation-linked regulators (9/9). Among differentiation-linked regulators, 3/4 (MITF, SOX10, IRF4) showed contrasting associations depending on the PD-L1 expression mode and study. In total, 120 human and 10 mouse melanoma cell lines were used. The A375, SK-MEL-28, and B16 cell lines were used in 20, 18, and 28 studies, respectively, suggesting lineage bias. Most studies had unclear risk of bias regarding imprecision (82%) and model validity (70%). CONCLUSIONS: PD-L1 expression is commonly affected by differentiation regulators in melanoma cells, linking high PD-L1 expression to dedifferentiated cell states. The various regulators and inducible pathways that affect PD-L1 may in turn explain the heterogeneity observed in PD-L1 expression between and within patients with melanoma. Owing to the clinical significance of PD-L1 expression as a predictive biomarker for immunotherapy response, it is crucial to better understand the impact of various regulators on its expression.
Sævarsson et al. (Wed,) studied this question.