Abstract PS4-19: Evaluation of tumor-specific MHC-II expression as a biomarker
Authors: Ericsson Paula Ines Gonzalez , Balko Justin M , Sanchez Violeta , Sanders Melinda 
Published: February 2021

Abstract
Recently MHC-II protein expression levels in breast cancer tumor cells were shown to predict response to the PD-1 inhibitor pembrolizumab on a neoadjuvant clinical trial for ER+ and TNBC (ISPY-2). To better understand this biomarker and avoid post-market issues, we aimed to investigate possible pitfalls of MHC-II/HLA-DR quantification, prevalence of HLA-DR expression in breast cancer (BC) cells and its association with clinical data in a cohort of 372 BC patents. We assessed tumor and non-tumor HLA-DR expression by multiplex fluorescence immunohistochemistry on 8 TMAs containing 372 surgical BC samples, comprised of 239 ER+ and 123 TNBCs. Patient characteristics are shown in table 1. PanCK was used to define the tumor area and HLA-DR quantification was performed by training an object classifier on QuPath. Upon visual examination, 23 samples showed patchy panCK expression. We excluded normal breast epithelium and in situ components, which showed varying degrees of HLA-DR. 44.4% of breast cancers expressed HLA-DR, with 8.3% showing high tumor expression (HLA-DRHI; ≥20% of tumor cells). TNBC showed higher prevalence of HLA-DRHI cases (35% HLADR+, 14.6% HLA-DRHI, mean tumor HLA-DR expression (% of tumor cells) 10.3%, range 0-84%) than ER+ BC (51% HLADR+, 5.4% HLA-DRHI, mean 7.4%, range 0-66.5%). In some cases, HLA-DR and panCK expression were mutually exclusive. In other cases the lack of panCK expression highlighted the presence of stromal cells within the tumor area that were not evident on H&E. Tumoral MHC-II expression correlated with the presence of TILs for ER+ BC (r=0.18, p=0.0158) and TNBC (r=0.20, p= 0.0450), specifically CD3 (r=0.49 p&lt;0.0001) and CD4 (r=0.48 p&lt;0.0001), and to a lesser extent CD8 (r=0.28, p= 0.013) in a subset of TNBCs on which these stains where performed (n=81). We found no correlation between tumoral MHC-II expression and clinical characteristics (ER and PR within ER+ BC, age, presence of lymph node metastasis, stage, neoadjuvant treatment) in ER+ or TNBC. High tumor HLA-DR expression correlated with poorer recurrence-free survival (RFS) in ER+ BC (p=0.038) but showed no correlation in TNBC. Cox proportional hazard model including clinical data determined that only stage was an independent predictor of survival in ER+. Non-tumoral HLA-DR expression was associated with better RFS (p=0.007) and overall survival (p=0.01) in TNBC; however, only stage was an independent predictor in multivariant analyses. Tumor MHC-II expression in BC has been reported to range from 22% to 48.5% in TNBC or unselected BC, and high tumoral MHC-II expression was reported to be 6.9% in unselected BC and 36.3% in TNBC. Specific rates of HLA-DR expression in ER+ breast cancer have not been extensively investigated. Here we report a similar overall prevalence, but lower number of tumors presenting high HLA-DR expression and an association with poorer survival in ER+ BC. HLA-DR IHC is a robust assay that can be easily used to identify MCH-II high expressing tumors and scoring percentage of tumoral cells should be more reproducible than PD-L1 scoring on immune cells when assessed by a trained pathologist.
Table 1: Patient characteristicsTNBCER+Total number of cases239123Median age48,563Treatmenttreatment naïve26%45%neoadjuvant chemotherapy74%4%pre-surgical letrozole-51%Positive lymph node58%14%StageI27%82%II20%27%III67%8%IV0%1%
Citation Format: Paula Ines Gonzalez Ericsson, Justin M Balko, Violeta Sanchez, Melinda Sanders. Evaluation of tumor-specific MHC-II expression as a biomarker [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS4-19.