Associations of Immune Checkpoint Predictive Biomarkers (MHC-I and MHC-II) with Clinical and Molecular Features in a Diverse Breast Cancer Cohort.

Purpose: Immunotherapy (IO) in triple-negative breast cancer (TNBC) has improved survival outcomes, with promising improvements in pCR rates among early high-risk hormone receptor (HR)+/HER2- breast cancers. However, biomarkers are needed to select patients likely to benefit from IO. MHC-I and tumor-specific MHC-II (tsMHC-II) expression are candidate biomarkers for PD-(L)1 checkpoint inhibition but existing data from clinical trials included limited racial/ethnic diversity.

Experimental Design: We performed multiplexed immunofluorescence assays in the Carolina Breast Cancer Study (CBCS; n = 1,628, 48% Black, 52% non-Black). Intrinsic subtype and P53 mutant-like status were identified using RNA-based multigene assays. We ranked participants based on tumoral MHC-I intensity (top 33% categorized as "MHC-Ihigh") and MHC-II+ (≥5% of tumor cells as tsMHC-II+). MHC-I/II were evaluated in association with clinicopathological features by race.

Results: Black participants had higher frequency of TNBC (25% vs. 12.5%, P ≤ 0.001) and basal-like (30% vs. 14%, P ≤ 0.001) tumors overall, and higher frequency of basal-like (11% vs. 5.5%, P = 0.002) and TP53 mutant tumors (26% vs. 17%, P = 0.002) among HR+/HER2-. The frequency of tsMHC-II+ was higher in HR+/HER2- Black participants (7.9% vs. 4.9%, P = 0.04). Black participants also had higher frequency of MHC-Ihigh (38.7% vs. 28.2%, P < 0.001), which was significant among HR+/HER2- (28.2% vs. 22.1%, P = 0.02).

Conclusions: In this diverse study population, MHC-I and MHC-II tumor cell expression were more highly expressed in HR+/HER2- tumors from Black women, underscoring the importance of diverse and equitable enrollment in future IO trials.