Both stromal tumor-infiltrating lymphocytes (sTILs) and programmed death-ligand 1 (PD-L1) affect responses to immunotherapy; however, the extent of sTIL and PD-L1 expression within various metaplastic components in metaplastic breast carcinoma (MBC), which are critical for the characterization of immune microenvironments, remains unreported. We profiled sTIL infiltration and PD-L1 expression in different metaplastic components of specimens from 82 MBC patients. The overall positivity for high or intermediate (H/I) sTIL, immune cell-PD-L1 (IcPD-L1), and tumor cell-PD-L1 (TcPD-L1) was 34.1%, 47.6%, and 17.1%, respectively, but differences specific to MBC subtypes and each metaplastic component existed. Squamous cell carcinoma exhibited the highest positivity rates of sTIL(H/I) (50.0%) and IcPD-L1 (66.7%), while matrix-producing carcinoma had the lowest respective rates (14.3% and 28.6%). The positivity rates of sTIL(H/I) and IcPD-L1 were the highest in squamous component (Sq) and the lowest in chondroid component (Ch). All cases that had discordant sTIL categories between carcinoma of no special type (NST) and metaplastic components showed sTIL(H/I) positivity higher in Sq, but lower in spindled component (Sp) and Ch. While there was no pattern of higher IcPD-L1-positivity in Sp, six of the seven cases that were TcPD-L1-discordant between NST and Sp were TcPD-L1-positive in Sp, suggesting a trend for higher TcPD-L1 in Sp. The diagnostic predictability of total tumor IcPD-L1 positivity based on IcPD-L1 positivity in Sq and Ch was 95.2% and 33.3%, respectively. Multivariate analysis showed that sTIL(H/I) positivity, but not PD-L1 positivity, correlated with better survival. Our data implicate distinct immune microenvironments in different metaplastic components in MBC, which may have immunopathologic, diagnostic, and therapeutic significance.
Keywords: Metaplastic breast carcinoma; Program death-ligand 1; Tumor-infiltrating lymphocyte.