Abstract P4-09-07: Characterization of Immune Contexture in HR+/HER2- and Triple Negative Breast Cancer in a Real-world Cohort

Background: Prior studies in breast cancer (BC) have demonstrated variability in the immune microenvironment of BCs across subtypes, with higher levels of immune infiltration observed in triple-negative (TN) BC. Clinical trials of immunotherapy in BC have largely targeted patients with TNBC, however many of these patients do not respond to treatment, suggesting heterogeneity across TN tumors. Further, immune infiltrated hormone receptor–positive, HER2-negative (HR+/HER2-) tumors have been observed. Patients with these tumors may benefit from immune checkpoint blockade (ICB). Identification of patients with immunogenic tumors would be valuable for future patient selection and stratification in clinical trials. In this study, we conducted comprehensive molecular profiling of a set of commercially procured BC samples to characterize the tumor microenvironment (TME) of HR+/HER2- and TN tumors and to identify patient or tumor features associated with immune infiltration. Methods: 163 surgically resected BC samples were used for this study (93 HR+/HER2-, 70 TN). Clinically validated immunohistochemistry (IHC) assays were used to evaluate the expression of two immune checkpoint (IC) molecules, PD-L1 (by VENTANA PD-L1 (SP142) Assay and Agilent PD-L1 IHC 28-8 pharmDx) and LAG3 (antibody clone 17B4). The presence of CD8+ cells was measured by IHC (Dako/Agilent clone C8/144B) and used to derive the spatial location and density (topology) of CD8+ cells within the TME. RNA sequencing (RNA-seq) was used to construct gene expression signatures representative of immune cell types. Furthermore, RNAseq data was used to identify expressed somatic mutations and to calculate the total expressed tumor mutational burden (‘eTMB’). Results: As previously reported, the prevalence of PD-L1 + tumors was higher in TN vs HR+/HER2− BC, however close to half (44.1% SP142 IC> 1%) of HR+/HER2- tumors showed some degree of PD-L1 expression. Similar patterns were observed for LAG3 IHC expression (32.3% and 64.7% of HR+ and TN tumors respectively with >1% expression). Concordant expression of the two markers (SP142 IC> 1% and LAG3 >=1% or < 1% and < 1%) was observed in approximately 80% of tumors in both subtypes. Less than 10% of tumors in both subtypes had high LAG3 (>1%) and low PD-L1(< 1%) expression. Most (82.4%) HR+/HER2- tumors in the dataset were deficient in CD8+ cells in both stroma and tumor parenchyma, however a subset (12.9%) were identified as having an ‘excluded’ CD8 pattern, and four (4.7%) were classified as ‘inflamed’. The frequency of both the excluded and inflamed states was slightly higher among the TN tumors (20.6% and 15.9% respectively). Expression of PD-L1+ occurred more frequently among tumors with higher CD8 expression. Among both TN and HR+ tumors, most ‘inflamed’ tumors were grade 3. We also examined a range of gene expression signatures representative of various immune cell subpopulations, their distribution within each subtype, and relationships with other molecular data. For example, expression of a CXCL13+ T cell signature, a cell type that has been shown to be predictive of response to anti-PD-L1 + chemo combo therapy in TNBC, was significantly higher in TN vs HR+ tumors, however, a large subset of HR+ tumors had levels of expression of the signature above the median observed in TN tumors. In addition, we noted that eTMB, while low overall, was positively correlated with certain immune-related expression signatures across HR+/HER2− samples, including signatures previously associated with improved response to ICB, as well as PD-L1 positivity. Conclusions: We observed heterogeneity in the immune profile of the real-world HR+ and TN tumors in this cohort. A subset of tumors in both subtypes expressed markers or signatures previously reported to be associated with response to ICB. (1) Cancer Cell. 2021 Dec 13;39(12):1578-1593.e8. Citation Format: Aparna Chhibber, Lloye M. Dillon, John B. Wojcik, Vishantie Dostal, George Lee, Fayaz Seifuddin, Scott Ely, Mark D. Stern, Charlie Benson-Garnett, Mustimbo Roberts, Jenny Wu. Characterization of Immune Contexture in HR+/HER2- and Triple Negative Breast Cancer in a Real-world Cohort [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-09-07.