Abstract 2837: A surrogate gene expression signature of tumor infiltrating lymphocytes (TILs) predicts degree of benefit from trastuzumab added to standard adjuvant chemotherapy in NSABP (NRG) trial B-31 for HER2+ breast cancer

Background: Tumor infiltrating lymphocytes (TILs) have demonstrated to be predictive of trastuzumab as well as chemotherapy benefit in adjuvant FinHER and neoadjuvant GeparQuattro phase III trials of trastuzumab in HER2-positive breast cancer patients (pts). Since TILs assessment is subject to inter-observer variations, we attempted to develop a surrogate gene expression signature of TILs and evaluate it as a potential predictive marker for the degree of benefit from trastuzumab plus standard adjuvant chemotherapy in NSABP B-31. Methods: Two hundred cases selected from the discovery cohort of B-31 with Affymetrix gene expression data were assessed for the percentage of TILs using International TILs Working Group guidelines. Gene set enrichment analysis (GSEA) was done to identify functionally enriched biologic pathways. Genes strongly associated with high TILs were identified (N = 119). Hierarchical clustering analysis of TILs signature was performed in Affymetrix gene expression data set from 731 cases randomly selected from B-31. Proportional hazard models were used to test the heterogeneity of degree of benefit from trastuzumab subgroups identified by TILs signature with disease-free survival as the end point. Results: Among 200 cases, 34 were classified as morphologic high (>50%) TILs. GSEA revealed that tumors with high TILs expressed up-regulation of genes in several immune activation pathways involving T and B cells. Tumors with low TILs had up-regulation of muscular, fibrous tissue, extracellular matrix, and estrogen receptor (ER) gene sets. ER status by immunohistochemistry (IHC) was also inversely associated with TILs (p = 0.002). Hierarchical clustering of genes most strongly associated with TILs identified a small subset (N = 86, 12%) out of 731 pts with high expression of TILs-associated genes and more benefit from trastuzumab (HR = 0.06, CI 0.01-0.47, p = 0.007) versus samples with low or intermediate TILs (HR = 0.57, CI 0.43-0.76, p