Background
Tumour-infiltrating lymphocytes (TILs) are clinically informative in breast cancer, but conventional assessment is limited by observer variability and spatial heterogeneity. ECTIL (label-efficient computational tumour-infiltrating lymphocyte assessment) is a recently validated computational pathology model that estimates stromal TIL abundance from routine H&E whole-slide images (1). Using three complementary breast cancer cohorts, CPTAC-BRCA, TCGA-BRCA and TransNEO, we examined the biological, prognostic and treatment-response relevance of ECTIL. We investigated what biological information ECTIL captures, whether its correlation with immune-related signalling is consistent across cohorts, and whether ECTIL provides prognostic information beyond established transcriptomic immune signatures.
Methods
CPTAC-BRCA (2) was used as the primary molecular characterization cohort. In 110 patients with matched RNA and 273 eligible H&E slides, the ECTIL-compatible workflow included foreground segmentation, normalization to 0.5 µm/pixel, extraction of non-overlapping 512×512-pixel tiles, RetCCL feature extraction and frozen ECTIL-TCGA inference. For patients with multiple eligible slides, patient-level ECTIL was calculated as the tile-count-weighted mean.
Before transcriptome-wide analysis, ECTIL was tested against six prespecified immune RNA measures representing distinct biological axes: cytolytic activity (CYT), IFNG-6, CD8/T-cell core, T-cell-inflamed positive-17, B-cell/humoral-5 and ESTIMATE ImmuneScore. Spearman correlations were estimated with 10,000-bootstrap confidence intervals and Benjamini-Hochberg (BH) correction across the six measures. Transcriptome-wide analysis was then used to characterize gene-level relationships with ECTIL, with covariate-adjusted HC3 regression as a complementary analysis; gene-level significance was defined at BH FDR<0.05. Nested Elastic Net stability selection was subsequently used for compact RNA-score development; no clinical outcome was used during this process.
TCGA-BRCA (3, 4) provided independent RNA prognostic validation and a separate training aware image analysis. Frozen RNA scores were calculated in 1,058 primary-tumour cases. ECTIL was successfully generated for 1,122 processable H&E slides from 1,051 patients; seven slides lacking essential microns-per-pixel metadata were excluded for technical reasons. After the primary TCGA package was frozen, released ECTIL train, validation and test files were used to reconstruct the original labelled TCGA cohort of 356 slides from 342 patients and identify 710 image-RNA patients outside that cohort. Post-freeze sensitivity analyses repeated ECTIL-RNA correlations in these patients and examined receptor-phenotype consistency. Progression-free interval (PFI) was the primary clinical endpoint; DSS and OS were secondary and DFI exploratory. Cox models used a frozen clinical adjustment backbone comprising age, ordinal pathologic stage, and receptor-defined clinical phenotype. To assess complementary prognostic information, PFI models compared the frozen clinical backbone alone, clinical+ECTIL, clinical+RNA and joint clinical+ECTIL+RNA models. Incremental prognostic value of ECTIL beyond each of the six established RNA signatures, and RNA beyond ECTIL, was assessed using nested likelihood ratio tests with BH correction across the six-signature family (BH-adjusted p values reported as q values). ECTIL×RNA interactions were evaluated separately.
TransNEO (5) provided independent neoadjuvant-response validation. ECTIL was generated for 203 H&E slides from 203 patients; 147 patients had matched ECTIL, RNA and response data. Associations with pCR and continuous residual cancer burden (RCB) were tested and joint ECTIL-RNA models assessed complementary response information.
Results
Across cohorts, patient-level ECTIL ranged from 0.070–0.557 in CPTAC-BRCA, 0.034–0.617 in TCGA-BRCA and 0.051–0.604 TransNEO image cohort. In CPTAC-BRCA, ECTIL was positively associated with all six prespecified immune RNA measures. Five remained significant after multiplicity correction: IFNG-6 (ρ=0.343, q=0.002), T-cell-inflamed positive-17 (ρ=0.277, q=0.010), CYT (ρ=0.261, q=0.012), CD8/T-cell core (ρ=0.251, q=0.012) and ESTIMATE ImmuneScore (ρ=0.223, q=0.023); B-cell/humoral-5 was positive but weaker (ρ=0.149, q=0.121). Transcriptome-wide analysis identified 7,061 genes at FDR<0.05, of which 1,622 were significant in both the primary and age-, stage- and TNBC-adjusted analyses. Stability development yielded a compact nine-gene ECTIL RNA score (BATF2, IFNG, LAG3, TNFRSF13C, KIR2DL4, PLA2G2D, HMGB2, HSPD1 and CD320), with repeated held-out internal CPTAC correlation ρ=0.618; in independent TCGA RNA validation, the score was associated with longer adjusted PFI (HR per SD=0.67, 95% CI 0.53–0.84; p<0.001).
In the 1,051 patient TCGA image-RNA cohort, ECTIL was positively correlated with all six established immune RNA measures and the nine-gene score (all BH q<0.05). In the 710 patients outside the original labelled ECTIL TCGA cohort, ECTIL remained correlated with all six established RNA measures (ρ=0.235–0.317; all BH q<0.05) and the nine-gene score (ρ=0.427, q=4.83×10-32). The ECTIL–nine-gene score relationship remained positive across major receptor phenotypes, including TNBC, with no evidence of heterogeneity in the full cohort. In fully adjusted joint PFI models using a common 895-patient, 96-event analysis set, higher ECTIL remained associated with lower progression risk when modelled alongside each established RNA signature, with ECTIL HRs per SD ranging from 0.61 to 0.65. ECTIL added significant prognostic information beyond all six established RNA signatures after multiplicity correction (BH q=0.0017–0.0029). In contrast, none of the six established RNA signatures added significant prognostic information beyond ECTIL after correction, and no ECTIL×RNA interaction was significant across the six-signature family. This incremental-value pattern persisted in the post-freeze TCGA sensitivity restricted to patients outside the original labelled ECTIL cohort (n=599, 60 PFI events): ECTIL added significant prognostic information beyond each established RNA signature after BH correction, whereas reciprocal RNA-beyond-ECTIL and interaction tests were not significant after correction.
In the TransNEO response validation (n=147; 38 pCR), higher ECTIL was associated with increased odds of pCR (adjusted OR per SD=1.73, 95% CI 1.18–2.54; p=0.005) and lower continuous RCB (adjusted β per SD=−0.33, 95% CI −0.52 to −0.14; p<0.001). None of the six established RNA signatures added significant pCR or RCB information beyond ECTIL after multiplicity correction; ECTIL retained incremental information beyond B-cell/humoral-5 (pCR q=0.036; RCB q=0.034).
Conclusions
ECTIL showed reproducible correlation with multiple established immune transcriptomic signatures, retained prognostic information beyond individual RNA immune measures, and independently associated with pathological treatment response. These findings support ECTIL as a biologically interpretable computational pathology biomarker complementary to bulk immune transcriptomics. The nine-gene score may provide a compact secondary RNA representation of the ECTIL-associated transcriptomic signal.
References
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