Abstract PO3-13-05: PD-L1 and PD-L2 Protein Expression is Frequently Discordant in Breast Cancer

Abstract Background: In neoadjuvant setting, PD-1 inhibitors have shown significantly higher pathological responses in patients (pts) with early-stage triple-negative breast cancer (TNBC) irrespective of PD-L1 status. However, durable responses are less common in estrogen receptor-positive (ER+) pts. Most breast cancer (BC) clinical trials of PD-1 inhibitors have thus far focused on PD-L1 expression for patient eligibility. The alternative PD-1 ligand, PD-L2, with reported ~4-fold greater affinity for PD-1, has been largely understudied. We recently reported that high cancer cell protein levels of PD-L2 in pts with treatment-naïve ER+ BC was an independent predictor of shorter progression-free survival. PD-L2 expression was significantly high in ER- group as well, however, given the low numbers of ER- pts, the study was not powered enough to determine the correlation of PD-L2 with PFS in the ER- subgroup. These findings suggest that PD-L2 has an important role in BC and combined PD-L1/PD-L2 status may help improve BC pt selection for PD-1 inhibitors. We therefore initiated efforts to determine baseline expression patterns of PD-L1 and PD-L2 in BC. Methods: PD-L1 and PD-L2 protein levels in cancer cells and tumor-infiltrating immune cells were prospectively analyzed by immunohistochemistry (IHC) using validated antibodies in diagnostic core biopsies of 31 consecutive pts diagnosed with localized or locoregional ER+/HER2- BC or TNBC. Percent positivity of PD-L1 and PD-L2 in cancer cells and immune cells was determined by our breast pathologist. Detectable PD-L1 or PD-L2 expression in ≥1% of cancer cells or stromal immune cells was considered positive. Spearman correlation and Wilcoxon paired analyses were used to measure and correlate PD-L1 and PD-L2 protein expression in cancer and immune cells. Results: There was no significant correlation between PD-L1 and PD-L2 expression, neither across all cases (N=31), nor within ER+ (N=22) or TNBC (N=9) cases. However, PD-L1 and PD-L2 expression patterns in BC differed in several ways. PD-L1-positivity in immune cells was higher than in cancer cells (median=5.0% vs. 0.0%; p=0.001), whereas PD-L2-positivity was higher in cancer cells than in immune cells (median=30% vs. 5.0%; p 0.001). PD-L1 positivity in cancer cells and immune cells were positively correlated in TNBC (rho=0.69, p=0.04) but not in ER+ BC. Conversely, PD-L2 positivity in cancer cells and immune cells were positively correlated in ER+ BC (rho=0.68, p=0.001) but not in TNBC. TNBC diverged from ER+ BC by displaying higher PD-L1 positivity in immune cells (median=20.0% vs. 1.0%; p=0.004). By the conventional cut point for positivity of ≥1% for PD-L1 and PD-L2 in cancer cells or immune cells, all TNBC tumors were PD-L1-positive (9/9), with 8 also being PD-L2-positive. Of the 22 ER+ cases, 16 were PD-L2-positive, of which only 9 were also PD-L1-positive. Table 1. shows cross tabulation data for PD-L1 and PD-L2 status by BC subtype. Conclusions: PD-L1 and PD-L2 proteins show divergent expression and are not correlated in BC. Discordant PD-L2 and PD-L1 expression may be more common in ER+ BC than in TNBC. This finding justifies efforts to explore PD-L2 as a complementary marker to PD-L1 for improved prediction of response to PD-1 inhibitors, which may benefit patients with aggressive ER+ BC that are eligible for chemotherapy. Table 1. PD-L1 vs. PD-L2 Status by Breast Cancer Subtype Citation Format: Lubna Chaudhary, Julie Jorns, Yunguang Sun, Sailaja Kamaraju, Yee Chung Cheng, Amanda Kong, Tina Yen, Caitlin Patten, Chandler Cortina, Inna Chervoneva, Christopher Chitambar, Hallgeir Rui. PD-L1 and PD-L2 Protein Expression is Frequently Discordant in Breast Cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-13-05.