Abstract PO1-24-11: Distinct patterns of MHC class I, PD-L1 expression, and T-cell infiltration in different subtypes of ductal carcinoma in situ of the breast

Abstract Background: Ductal carcinoma in situ (DCIS) of the breast encompasses various subtypes with distinct behaviors. Immune cell infiltration within the tumor microenvironment is thought to influence DCIS progression. Tumor-infiltrating lymphocytes (TILs) serve as a surrogate marker for the adaptive immune response. The interaction between programmed death 1 (PD-1) and its ligand PD-L1 suppresses effector T-cell function. Major histocompatibility complex (MHC) class I molecules play a critical role in presenting tumor antigens to cytotoxic T cells. Despite the significance of PD-L1 and MHC class I in immune evasion, limited data exist on their correlation with T-cell infiltration in DCIS subtypes. Methods: Using immunohistochemistry, we examined MHC class I and PD-L1 expression, along with CD3+ and CD8+ T lymphocytes, in 131 DCIS cases through tissue microarrays. DCIS subtypes included hormone receptor-positive (HR+)/human epidermal growth factor 2-negative (HER2-), HR+/HER2+, HR-/HER2+, and triple-negative (TN) subtypes. Results: Among the 128 interpretable cases, the distribution was as follows: HR+/HER2- (50.8%), HR+/HER2+ (14.1%), HR-/HER2+ (26.5%), and TN (8.6%). HER2+ subtypes (HR+/HER2+ and HR-/HER2+) exhibited higher stromal TILs, CD3+ T cells, and CD8+ T cells compared to HR+/HER2- subtype. MHC class I loss was observed in 16.4% (21/128) of cases, with the highest frequency in the HR+/HER2- subtype (29.2%), followed by HR-/HER2+ (5.9%), HR+/HER2+ (0%), and TN (0%) subtypes. DCIS cases lacking MHC class I expression displayed lower stromal CD8+ T cell infiltration than those with intact MHC class I. PD-L1 expression in immune cells (≥1%) was detected in 18.8% (24/128) of cases and correlated with increased stromal TILs, CD3+ T cells, and CD8+ T cells. PD-L1 expression was more prevalent in HER2+ subtypes than in HR+/HER2- subtype. PD-L1-positive DCIS cases mostly retained MHC class I expression (95.8%), except for one case (4.2%) in the HR-/HER2+ subtype. The highest numbers of stromal CD3+ and CD8+ T cells were observed in DCIS cases with intact MHC class I and positive PD-L1 expression. The correlation between MHC class I and PD-L1 expression and infiltration of CD3+ and CD8+ T cells was maintained in HR+/HER2- and HR-/HER2+ subtypes. Notably, tumoral CD3+ and CD8+ T cells showed no significant association with MHC class I or PD-L1 expression. Furthermore, MHC class I and PD-L1 expression did not predict tumor recurrence. Conclusion: We observed distinct patterns of MHC class I and PD-L1 expression and T-cell infiltration among DCIS subtypes. Enhancing our understanding of MHC class I, PD-L1, and immune subsets, including CD8+ T cells, may contribute to the development of immune modulating therapies for DCIS, particularly in the HER2+ subtype. Citation Format: Nah Ihm Kim, Min Ho Park, Ji Shin Lee. Distinct patterns of MHC class I, PD-L1 expression, and T-cell infiltration in different subtypes of ductal carcinoma in situ of the breast [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 PO1-24-11.