We aimed to compare the immunohistochemical expression of PD-1, PD-L1 and CTLA-4 of pregnancy-related breast cancer (PRBC) and early onset non-PRBC (YWBC), and their prognosis prediction potential was correlated to that of conventional clinicopathological factors.
Twenty-one PRBC cases were paired with 21 YWBC in this matched case-control study. Immune-checkpoint markers (ICM) were evaluated with immunohistochemistry (IHC) on whole slides using the following antibodies: PD-1 (NAT-105), PD-L1 (28-8) and CTLA-4 (F-8). IHC score was defined as the percentage of positive cells, assessed separately among tumor cells, intratumoral lymphocytes and peritumoral lymphocytes.
The optimal threshold of PD-L1 expression of tumor cells occurred at 10% for overall survival (OS, AUC = 0.847, p = 0.009), and at 1% for disease-free survival (DFS, AUC = 0.795, p = 0.010). For PD-L1 expression on intratumoral lymphocytes, the optimal cut-off was 1% (AUC = 0.763, p = 0.048). Considering PD-1, PD-L1 and CTLA-4 expression, no significant difference occurred between PRBC and YWBC (p > 0.05 for all comparisons). PD-1, PD-L1 expressed on peritumoral lymphocytes and CTLA-4 failed, but PD-L1 expressed on tumor cells and on intratumoral lymphocytes was suitable to distinguish patient cohorts with different OS and DFS (p ≤ 0.011 for all comparisons). Higher PD-L1 expression was associated with poor prognosis. PD-L1 expressed on tumor cells represented an independent association with OS (p = 0.023) and DFS (p = 0.032).
Our results suggest that PRBC and YWBC do not differ in the expression of PD-1, PD-L1 and CTLA-4. However, our findings emphasize the relevance of PD-L1 expression in early-onset breast cancer, as an independent negative predictor of prognosis.