Inefficient induction of TAA-specific CD8+ T-cell responses in hepatocellular carcinoma

In hepatocellular carcinoma (HCC), CD8+ T-cell responses targeting tumor-associated antigens (TAA) are considered to be beneficial. However, only little is known about the molecular characteristics of TAA-specific CD8+ T cells in HCC due to their low frequency. In this study, we performed peptide/MHCI tetramer-based enrichment to characterize circulating TAA-specific CD8+ T cells ex vivo in HCC patients to gain a better understanding of their phenotypical and transcriptional characteristics.

In particular, we analyzed the phenotype of circulating TAA-specific CD8+ T cells derived from the peripheral blood of age-matched HCC patients (n = 54), patients with liver cirrhosis (n = 29) and healthy donors (HD) (n = 28) by multicolour flow cytometry after peptide/MHC class I-tetramer-based magnetic bead enrichment. HLA-A*02-tetramers carrying TAA epitopes (NY-ESO-1157 – 165, Glypican-3521 – 530, AFP47 – 55 and MAGE-A3271 – 279 or viral epitopes (Cytomegalovirus (CMV) pp65495 – 503, Epstein-Barr virus (EBV) BMFL1280 – 288 and Influenza A virus M158 – 66); and HLA-A*03-tetramers carrying TAA epitopes (MAGE-A1104 – 114 and Glypican-3519 – 528) were used. Paraffin-embedded slices from 12 HCC patients were analyzed for corresponding antigen detection in tumor tissue.

Our results can be summarized as follows: First, despite applying a high-sensitivity peptide/MHCI-tetramer-based enrichment method, detection rates of TAA-specific CD8+ T cells targeting AFP, glypican-3, NY-ESO-1, MAGE-A1 and MAGE-A3 remained very low. Indeed, TAA-specific CD8+ T-cell frequencies were similar in HCC patients and in patients with liver cirrhosis or in healthy donors and lower compared to virus-specific CD8+ T cells present in HCC patients. Second, detection of low frequency TAA-specific CD8+ T-cell responses in HCC patients was linked to the presence of the respective antigen within the tumor tissue. Third, phenotypic analyses revealed that although more TAA-specific CD8+ T cells obtained from HCC patients displayed an antigen-experienced phenotype compared to healthy donors still some cells exhibited a naïve phenotype indicating lack of priming or abortive activation. Noteworthy, antigen-experienced TAA-specific CD8+ T cells did not show an exhausted phenotype suggesting restricted antigen recognition and further supporting the hypothesis of inefficient induction and activation. Finally, transarterial chemoembolization (TACE) did not induce or enhance TAA-specific CD8+ T-cell responses.

Our comprehensive analysis reveals that TAA-specific CD8+ T cells are not properly induced in HCC. This clearly highlights severe limitations of these potentially anti-tumoral T cells that may hamper their biological and clinical relevance in HCC.