Liu, Yi

570 Background: Major histocompatibility complex class I (MHC I) plays a critical role in immune surveillance by binding peptides derived from intracellular proteins and presenting them on the cell surface for recognition by CD8+ T cells. Loss or downregulation of MHC I expression has been identified as a key mechanism of immune evasion in cancers. Here, we evaluated MHC I expression and outcomes in all subtypes of breast cancer (BC). Methods: 9,038 BC samples were analyzed via NGS (592-gene panel, NextSeq; WES/WTS, NovaSeq; Caris Life Sciences, Phoenix, AZ), including triple-negative BC (TNBC) 3,038, HER2-positive (HER2+) 1,082, and hormone receptor-positive (HR+HER2-) 4,918. Immune cell fractions were estimated using WTS deconvolution (Quantiseq). MHC I ( HLA-A / HLA-B / HLA-C )-high (H) and -low (L) were classified by RNA expression above or below the 25th percentile. Real-world overall survival (OS) was derived from insurance claims and calculated from tissue collection to last contact using Kaplan-Meier. NanoString IO360 was performed in 114 samples from the FinXX trial (NCT00114816). Statistical significance was assessed using chi-square, Mann-Whitney U, ANOVA, and Cox regression with multiple comparison adjustments (q<.05). Results: TNBC had higher expression of HLA-A and HLA-B (median TPM: 169 and 191) compared to HER2+ (146.6 and 170, q<0.05) and HR+HER2- (141.7 and 157.5, q<0.05). However, there was no significant difference in HLA-C expression across 3 BC subtypes. In TNBC, MHC I-H tumors had higher frequencies of PD-L1 positivity (66.2% vs. 13.1%) as well as higher infiltration of B cells (4.5% vs. 3.2%), M1 macrophages (5% vs. 1.5%), M2 macrophages (4% vs. 2.1%), Tregs (2.8% vs. 0.8%), CD8 + T cells (1.8% vs. 0%), dendritic cells (3.2% vs. 2.8%), higher T-cell inflamed score (137 vs. -144), and IFN g score (0.02 vs. -0.49) compared to MHC I-L TNBC (all q<.05). MHC I-H TNBC was associated with significant improvement in median OS (30.1 vs. 15.2 months, HR 0.55, 95% CI 0.46-0.65, p<0.0001). However, this survival difference was not observed in patients with MHC I-H vs. MHC I-L in HER2+ (HR 1.04, 95% CI 0.74-1.47, p = 0.81) and HR+HER2- (HR 0.87, 95% CI 0.75-1.02, p= 0.09) BC subtypes. We further validated the MHC I expression in the FinXX trial. Similarly, patients with MHC I-H had significant improvement in recurrence-free survival (HR 0.27, 95%CI 0.11-0.66, p = 0.002) and OS (HR 0.23, 95% CI 0.09-0.57, p = 0.0005) compared to MHC I-L. Conclusions: Our findings demonstrate that higher MHC I expression is associated with higher immune infiltration and improved outcomes in TNBC but not in HER2+ or HR+HER2- BC subtypes. These results suggest that MHC I expression plays a critical role in the tumor microenvironment of TNBC. Future studies are needed to evaluate the prognostic value and potential therapeutic target of MHC I in TNBC. Support: Breast Cancer Research Foundation, Bankhead Coley, W81XWH-18-1-0564. Clinical trial information: NCT00114816 .