Abstract
Testing for PD-L1 expression by IHC is used to predict immune checkpoint blockade (ICB) benefits but has performed inconsistently in urothelial cancer clinical trials. Different approaches are used for PD-L1 IHC. We analyzed paired PD-L1 IHC data on urothelial cancer samples using the SP142 and 22C3 assays from the phase III IMvigor130 trial and found discordant findings summarized by four phenotypes: PD-L1 positive by both assays, PD-L1 positive by the SP142 assay only, PD-L1 positive by the 22C3 assay only, and PD-L1 negative by both assays double negative. PD-L1 positive by both assays and PD-L1 positive by the SP142 assay only urothelial cancers were associated with more favorable ICB outcomes and increased dendritic cell (DC) infiltration. SP142 PD-L1 staining co-localized with DC-LAMP, a DC marker, whereas 22C3 staining was more diffuse. PD-L1 positive by the 22C3 assay only urothelial cancers, associated with worse outcomes, were enriched in tumor cell (TC)-dominant PD-L1 expression. Multiplex IHC in an independent ICB-treated cohort confirmed that TC-dominant PD-L1 expression was associated with shorter survival. Using different PD-L1 assays, we uncovered that SP142 may preferentially stain PD-L1-expressing DCs, key to orchestrating antitumor immunity, whereas TC-dominant PD-L1 expression, which underlies a subset of "PD-L1-positive" specimens, is associated with poor ICB outcomes. See related Spotlight by Karunamurthy and Davar, p. 454 .