TOX promotes the exhaustion of antitumor CD8+ T cells by preventing PD1 degradation in hepatocellular carcinoma

Abstract

Background & aims: The thymocyte selection-associated high mobility group box protein (TOX) plays a vital role in T cell development and differentiation, however, its role in T cell exhaustion was unexplored. Here, we aim to investigate the role of TOX in regulating the antitumor effect of CD8⁺ T cells in hepatocellular carcinoma.

Methods: Fully functional, partially and severely exhausted tumor-infiltrating CD8⁺ T cells were sorted by flow cytometry and subjected to transcriptome sequencing analysis. Upregulated TOX expression was validated by flow cytometry. The antitumor function of CD8⁺ T cells with TOX downregulation or overexpression was studied in a mouse HCC model and HCC patient-derived xenograft mouse model. Transcriptome sequencing analysis was performed in TOX-overexpressing and control CD8⁺ T cells. The mechanism underlying the TOX-mediated regulation of PD1 expression was studied by laser confocal detection, immune co-precipitation and flow cytometer.

Results: TOX was upregulated in exhausted CD8⁺ T cells in hepatocellular carcinoma. TOX downregulation in CD8⁺ T cells inhibited tumor growth, increased CD8⁺ T cell infiltration, alleviated CD8⁺ T cell exhaustion and improved the anti-PD1 response of CD8⁺ T cells. The mechanism behind this involved the binding of TOX to PD1 in the cytoplasm, which facilitated the endocytic recycling of PD1, thus maintaining abundant PD1 expression at the cell surface. High expression of TOX in peripheral CD8⁺ T cells correlated with poorer anti-PD1 responses and prognosis.

Conclusions: TOX promotes CD8⁺ T cell exhaustion in hepatocellular carcinoma by regulating endocytic recycling of PD1. Downregulating TOX expression in CD8⁺ T cells exerts synergistic effects with anti-PD1 therapy, highlighting a promising strategy for cancer immunotherapy.

Lay summary: Abundant TOX expression in CD8⁺ T cells impairs their antitumor function in hepatocellular carcinoma. Mechanically, TOX reduces PD1 degradation and promotes PD1 translocation to the cell surface in CD8⁺ T cells, thus maintaining high PD1 expression at the cell surface. Downregulating TOX expression improves the antitumor function of CD8⁺ T cells, which shows the synergetic role of anti-PD1 therapy, highlighting a promising strategy for enhancement of cancer immunotherapy.

Keywords: Cancer; Checkpoint inhibitors; Hepatocellular carcinoma; Immunotherapy; PD1; T cell exhaustion; TOX; Tumor immunity.