Expression of programmed death 1 (PD-1) and its ligand (PD-L1) in thymic epithelial tumors: Impact on treatment efficacy and alteration in expression after chemotherapy.

BACKGROUNDS: To understand the clinical impact of PD-1/L1 expression in thymoma (TM) and thymic carcinoma (TC), we evaluated the frequency of PD-1/L1 expression in pre/post chemotherapy specimens and the correlation with the treatment efficacy.
METHODS: The expression of PD-1/L1 was evaluated using immunohistochemistry in patients with TM or TC treated with chemotherapy between 2000 and 2014. Using formalin-fixed, paraffin-embedded tissue samples and a PD-L1 antibody, the expression of PD-L1 in the TM and TC specimens was reported in terms of the H-score (0-300), with a score ≥1 being defined as positive. The PD-1 expression in the tumor-infiltrating immune cells was evaluated based on the intensity (0-3) of staining using a PD-1 antibody. The objective response rate, progression-free survival, and the difference in PD-1/L1 expression between the pre/post chemotherapy were evaluated.
RESULTS: Thirty patients (TM/TC 12/18) were evaluated. PD-L1 positivity were TM/TC 67%/41%. Within the PD-L1 positive/negative populations, the objective response rates were 50%/0% for TM and 14%/20% for TC. No significant differences in progression-free survival were seen according to the PD-L1 expression status. Increases in both the PD-L1 and PD-1 scores were observed after chemotherapy in six serial pre/post chemotherapy TM specimens, with a mean PD-L1 score and a median PD-1 intensity of 42/93, and 0/2.5, respectively.
CONCLUSIONS: The substantially high expression of PD-L1 and the increase in PD-L1 and PD-1 expression after chemotherapy supports anti-PD-1/L1 drugs therapy for TM and TC as well as the development of a strategy for its sequential use after chemotherapy.