The efficacy of atezolizumab plus bevacizumab for advanced hepatocellular carcinoma in relation to tumor-infiltrating lymphocytes: Histological assessment of CD8+ T cell spatial features as predictive biomarkers.

Abstract

4103 Background: The combination of atezolizumab and bevacizumab (Atez/Bev) has improved prognosis in advanced hepatocellular carcinoma (HCC), though its therapeutic mechanisms remain unclear. While the tumor microenvironment (TME) holds promise for biomarker discovery, current studies rely on archived diagnostic samples. We aimed to elucidate molecular determinants of treatment efficacy and identify predictive biomarkers through comprehensive TME analysis of pre-treatment samples. Methods: We analyzed biopsy samples from 94 advanced HCC patients immediately before initiating Atez/Bev treatment using immunohistochemistry (IHC), RNA-sequencing, flow cytometry, and multiplexed imaging. Our analysis focused on spatial characteristics of CD8+ T cells and effector regulatory T (eTreg) cells, with longitudinal assessment of immune responses during treatment. Results: High programmed death-1 (PD-1) positivity in CD8+ T cells was significantly associated with favorable progression-free survival (PFS) (HR 0.24, 95% CI 0.11-0.52), while CD8+ T cell density showed no significant correlation (HR 1.12, 95% CI 0.64-1.95). Through multiplexed imaging analysis using PD-1 positivity as a key indicator, we identified two critical determinants of response: CD8+ T cells localizing within tumor parenchyma rather than fibrous stroma, and maintaining diffuse distribution throughout the tumor parenchyma. These features, assessable with routine hematoxylin and eosin and CD8 IHC staining, stratified patients into four prognostic groups (p = 0.019), with median PFS ranging from 14.3 months (both favorable features) to 3.5 months (neither feature). The PD-1 positivity in eTreg cells was not associated with prognosis (HR 0.75, 95% CI 0.34-1.62). Notably, bevacizumab counteracted the potential negative effects of programmed death-ligand 1 blockade by suppressing eTreg cell activation, as demonstrated through analysis of patients who discontinued bevacizumab and in vitro experiments showing reduced expression of eTreg activation markers. Conclusions: We demonstrate that routine histological assessment of CD8+ T cell localization and distribution patterns can predict Atez/Bev efficacy in advanced HCC. The identified synergistic mechanism of bevacizumab-mediated eTreg suppression provides a framework for future combination immunotherapy development.