Immunohistochemical status of PD-L1 in thymoma and thymic carcinoma

The immune checkpoint ligand programmed cell death 1 ligand 1 (PD-L1) is expressed in various tumors and is associated with the response to anti programmed cell death 1 (PD-1)/PD-L1 drugs. We evaluated PD-L1 expression in thymomas and thymic carcinomas to determine whether PD-L1 represents a therapeutic target in unresectable thymomas or thymic carcinomas that could be amenable to antibody-based immunotherapy.

A tissue microarray (TMA) comprised of 101 thymomas and 38 thymic carcinomas samples was evaluated. After validation of the rabbit monoclonal PD-L1 antibody (clone E1L3N), the TMA was stained and the tumor PD-L1 expression score was calculated using a semiquantitive method (by multiplying the intensity [0-3] by the staining area [0-100%]).

Seventy percent of thymic carcinoma (type C) and 23% of thymoma (types A, AB, and B) samples stained positive for PD-L1 (P<.001). A WHO classification (type C vs types A, AB, and B) was significantly associated with positive PD-L1 expression (P=0.006), though multivariate analysis did not show PD-L1 positive was a significant negative factor of overall survival (hazard ratio=0.99, 95% confidence interval=0.35-2.73; P=0.987).

To the best of our knowledge, this is the largest-scale study to evaluate PD-L1 expression in thymomas and thymic carcinomas. The data suggest that the anti PD-1/PD-L1 drug could be of potential use in immunotherapy for unresectable or relapsed thymomas and thymic carcinomas.