Abstract PO3-14-07: Characterizing Immune Infiltration in Metastatic Hormone Receptor-Positive/HER2-Negative Breast Cancer: A Comprehensive Translational Analysis from four SOLTI Clinical Trials
Authors: Seguí Elia , Sanfeliu Esther , Brasó-Maristany Fara , González-Farré Blanca , Salvador Fernando , Angelats Laura , Villanueva Lorea , Espinosa Alejandra , Galván Patricia , Fernández Esther , Gonzalez-Farré Xavier , Escrivá-de-Romaní Santiago , Chic Núria , Ciruelos Eva , Saura Cristina , Manso Luis , Oliveira Mafalda , Tabernero Josep , Prat Aleix , Pascual Tomás 
Published: May 2024

Background: Hormone Receptor-positive (HR+)/HER2-negative (HER2-) metastatic breast cancer (BC) is a highly heterogenous disease. Non-luminal subtypes within HR+/HER2- BC have been linked to poor survival in the metastatic setting. However, these subtypes might exhibit higher expression of immune-related genes, tumor-infiltrating lymphocytes (TILs), and APOBEC signatures. This study aimed to investigate the presence of tertiary lymphoid structures (TLS), TILs and immune gene expression (GE) across PAM50 intrinsic subtypes (IS) in metastatic samples from patients (pts) with HR+/HER2- BC.

Methods: We included pts with HR+/HER2- metastatic BC who participated in the molecular prescreening of the SOLTI-1904-ACROPOLI (NCT04802876), SOLTI-1502-ARIANNA (NCT04142060), SOLTI-1718-NEREA (NCT04460430) and SOLTI-1716-TATEN (NCT04251169) studies, providing metastatic samples for analysis. The proportion of TILs was centrally evaluated by a single pathologist using standard criteria. GE analysis was performed on the same FFPE tumor block using a panel of 72 genes, including 10 immune genes and the PAM50 IS. In a subset of patients, we conducted additional GE analysis using a panel of 192 genes, including the 14-gene B-cell/IgG (IGG) signature, while also retrospectively assessing the presence of TLS. Descriptive statistics, significance analysis of microarrays (using False Discovery Rate [FDR]) and logistic regressions were used to investigate the association between TLS, TILs, immune GE and PAM50 IS.

Results: A total of 401 pts were included. Distribution across IS was: 53.4% Luminal B (n=197), 27.1% Luminal A (n=100), 16% HER2-enriched (n=59), 2.7% basal-like (n=10) and 0.8% normal-like (n=3). TILs were evaluated in 375 samples (93.5%). Median (m) TILs proportion was 2% (IQR: 1-4, range 0-80). Non-luminal subtypes exhibited a higher enrichment of TILs compared to luminal subtypes (m=3% vs m=1%, p=0.002). Moreover, significant differences of TILs were also seen according to the site of metastasis (M1): m=6.5% in lymph nodes vs m=4.5% in lung M1 vs m=1% in liver M1 (p&lt; 0.001). Immune GE was significantly higher in samples with a high proportion of TILs ( &gt;10%), although the correlation between TILs and immune genes was moderate (e.g, coefficient=0.44, p&lt; 0.001, between CD8A and TILs). TLS were evaluated in 147 samples and detected in 16 pts (10.9%). Compared to TLS-negative tumors, TILs were significantly higher in TLS-positive (TLS+) tumors (m=1.5% vs m=10%, p&lt; 0.001). In addition, the presence of TLS was significantly higher in non-luminal subtypes compared to luminal subtypes (18% (11/61) vs 5.8% (5/86), p=0.019). TLS+ tumors had a significant enrichment of genes related to activated B-cells (CD19, CD79A), plasma cells (CD27), immunoglobulins (IGKC, IGLV), and T-cells (CD8A, PDCD1). Notably, the IGG signature as a continuous variable demonstrated a significant association with TLS presence, independently of the PAM50 IS and TILs (p=0.033, ROC AUC=0.817). Finally, higher expression of some immune genes was observed in non-luminal subtypes vs luminal subtypes (e.g., CD19 (p=0.016) and CD274/PDL1 (p&lt; 0.001)).

Conclusions: Our translational analysis reveals that non-luminal subtypes within metastatic HR+/HER2- BC exhibit a higher enrichment of TILs, immune GE, and presence of TLS. Importantly, we identified the IGG signature as a robust immune GE pattern that is strongly associated with TLS across PAM50 IS, independent of TILs. These findings highlight the potential significance of the IGG signature as a novel biomarker for precisely measuring TLS and selecting pts who might be more likely to derive benefit from immunotherapy.

Citation Format: Elia Seguí, Esther Sanfeliu, Fara Brasó-Maristany, Blanca González-Farré, Fernando Salvador, Laura Angelats, Lorea Villanueva, Alejandra Espinosa, Patricia Galván, Esther Fernández, Xavier Gonzalez-Farré, Santiago Escrivá-de-Romaní, Núria Chic, Eva Ciruelos, Cristina Saura, Luis Manso, Mafalda Oliveira, Josep Tabernero, Aleix Prat, Tomás Pascual. Characterizing Immune Infiltration in Metastatic Hormone Receptor-Positive/HER2-Negative Breast Cancer: A Comprehensive Translational Analysis from four SOLTI Clinical Trials [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 PO3-14-07.