Programmed death ligand 1 (PD-L1) expression in parathyroid tumors

Introduction: PD-L1 is associated with prognosis and immunotherapeutic response in patients with malignancies. In previous studies, PD-L1 expression was detected in many endocrine tumors. However, the PD-L1 expression status in parathyroid tumors is unknown.

Methods: We included 26 parathyroid carcinoma and 37 adenoma samples, as well as the corresponding patient information. PD-L1 was stained using the FDA-approved PD-L1 IHC 22C3 pharmDx and Ventana PD-L1 (SP263) assays, and staining was assessed by the estimated percentages of positive tumor cells and immune cells, respectively.

Results: We classified the PD-L1 expression in the parathyroid tumors into four groups: (0) <1%, (1) 1-4%, (2) 5-9% and (3) ≥10% positive. With the SP263 clone, 37 (carcinoma:adenoma = 18:19) samples scored 0, 13 (carcinoma:adenoma = 4:9) scored 1, 7 (carcinoma:adenoma = 1:6) scored 2 and 6 (carcinoma:adenoma = 3:3) scored 3. However, in the series of cases using the 22C3 clone, 45 (carcinoma:adenoma = 20:25) samples scored 0, 10 (carcinoma: adenoma = 3:7) scored 1, 5 (carcinoma:adenoma = 1:4) scored 2, and 3 (carcinoma:adenoma = 2:1) scored 3. Concerning tumor-infiltrating immune cells, 57 samples were negative and six were positive with SP263, and 59 were negative and four were positive with 22C3. Moreover, PD-L1 expression was negatively correlated with the Ki-67 index and mitotic rate in parathyroid tumors depending on the different clones. However, the results indicated only moderate consistency between the SP263 and 22C3 clones in parathyroid tumors.

Conclusion: We found deficient PD-L1 expression in the majority of parathyroid tumors. However, the PD-L1 expression score in parathyroid tumors depended greatly on the antibody clone used.