There and back again: PD-L1 Positivity as a Biomarker for Immune Checkpoint Blockade in Urothelial Carcinoma.

Biomarkers of responsiveness to immune checkpoint blockade (ICB) are heavily sought given the breadth and depth of the use of ICB in cancer. PD-L1 expression was among the first biomarkers identified, but multiple factors have precluded more widespread use. In this issue, Galsky and colleagues utilize two separate PD-L1 assays to study urothelial carcinoma specimens and observe that SP142 (relative to 22C3) preferentially stains dendritic cells. These observations may help reconcile the discordant performance of the two PD-L1 assays in ICB-treated urothelial carcinoma while underscoring the role of dendritic cells in orchestrating ICB response. See related article by Galsky et al., p. 476 .