Potential of immunotherapy for hepatocellular carcinoma
Authors: Ekaterina Breous, Robert Thimme
Journal: Journal of Hepatology (2010)

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide with limited therapeutic options. Since HCC has been shown to be immunogenic, T cell-based immunotherapy is considered a promising treatment. In this review, we summarize current knowledge of T cell responses against tumour-associated antigens, as well as the mechanisms underlying the poor quality of these responses in patients with HCC. Insights into these important aspects of HCC immunology are crucial for the further development of novel immunotherapies. Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide with limited therapeutic options. Since HCC has been shown to be immunogenic, T cell-based immunotherapy is considered a promising treatment. In this review, we summarize current knowledge of T cell responses against tumour-associated antigens, as well as the mechanisms underlying the poor quality of these responses in patients with HCC. Insights into these important aspects of HCC immunology are crucial for the further development of novel immunotherapies. Hepatocellular carcinoma (HCC) is the third leading cause of cancer deaths worldwide with over 600,000 patients dying from this disease annually [[1]Schutte K. Bornschein J. Malfertheiner P. Hepatocellular carcinoma – epidemiological trends and risk factors.Dig Dis. 2009; 27: 80-92Crossref PubMed Scopus (379) Google Scholar]. Liver transplantation and tumour resection have proven to be the most effective standard therapies and provide 5-year survival rates of 70% for patients within the Milan criteria, i.e., single tumour ⩽5 cm in size or up to three tumours ⩽3 cm in size [[2]Llovet J.M. Bruix J. Molecular targeted therapies in hepatocellular carcinoma.Hepatology. 2008; 48: 1312-1327Crossref PubMed Scopus (904) Google Scholar]. These rates reach 50% with radiofrequency ablation and transarterial chemoembolization, which are the next preferred lines of therapy. However, these therapeutic procedures most often do not provide a complete cure, as half of the treated patients experience tumour recurrence within 3 years. A further major problem is that only 30–40% of patients are eligible for the above-described treatments due to the fact that HCC frequently remains undiagnosed until an advanced stage has been reached. These patients have a median survival time ranging from 3 to 16 months [[3]Llovet J.M. Burroughs A. Bruix J. Hepatocellular carcinoma.Lancet. 2003; 362: 1907-1917Abstract Full Text Full Text PDF PubMed Scopus (3813) Google Scholar]. Thus, there is currently ongoing development and testing of alternative drug-based therapies for HCC that target tumour signalling pathways and vasculature. However, so far, the only drug that significantly prolonged survival (by nearly 3 months) in patients with advanced HCC is sorafenib, a multi-targeted tyrosine kinase inhibitor [[4]Llovet J.M. Ricci S. Mazzaferro V. Hilgard P. Gane E. Blanc J.F. et al.Sorafenib in advanced hepatocellular carcinoma.N Engl J Med. 2008; 359: 378-390Crossref PubMed Scopus (9271) Google Scholar]. In view of these facts, new strategies are required. Immunotherapy aims to provide a more efficient and selective targeting of tumour cells by inducing or boosting the existing tumour-specific immune response. The rationale for immunotherapy for HCC is based, for example, on the finding that patients with tumours containing infiltrating, presumably tumour-specific effector T cells, had a reduced risk of tumour recurrence following liver transplantation [[5]Unitt E. Marshall A. Gelson W. Rushbrook S.M. Davies S. Vowler S.L. et al.Tumour lymphocytic infiltrate and recurrence of hepatocellular carcinoma following liver transplantation.J Hepatol. 2006; 45: 246-253Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar]. Moreover, anti-CD3 and IL-2 stimulated autologous T lymphocytes infused in HCC patients significantly improved post-surgical recurrence-free survival [[6]Takayama T. Sekine T. Makuuchi M. Yamasaki S. Kosuge T. Yamamoto J. et al.Adoptive immunotherapy to lower postsurgical recurrence rates of hepatocellular carcinoma: a randomised trial.Lancet. 2000; 356: 802-807Abstract Full Text Full Text PDF PubMed Scopus (797) Google Scholar]. These data imply a central role of T cells in modulating tumour progression and provide strong justification for T cell immunotherapy. A prerequisite for the successful development of T cell-based immunotherapeutic approaches is the identification and characterization of immune responses to tumour-associated antigens (TAAs). So far, six HCC-specific TAAs that are targeted by T cells have been identified (summarized in Table 1).Table 1TAAs triggering T cell responses in HCC patients. Open table in a new tab AFP is expressed during foetal development but is transcriptionally repressed shortly after birth. However, it is re-expressed in the majority of HCC patients. Since it is a self-protein, it has been argued that AFP-specific T cell responses are suppressed and thus difficult to activate. Nevertheless, AFP is currently the best studied and most promising target antigen for HCC immunotherapy. In pioneering studies, the group of Butterfield and Economou identified an HLA-A2-restricted AFP-specific CD8 T cell epitope and demonstrated its ability to generate CD8+ T cell responses both in human lymphocyte cultures and in HLA-A2 transgenic mice [[7]Butterfield L.H. Koh A. Meng W. Vollmer C.M. Ribas A. Dissette V. et al.Generation of human T-cell responses to an HLA-A2.1-restricted peptide epitope derived from alpha-fetoprotein.Cancer Res. 1999; 59: 3134-3142PubMed Google Scholar]. The authors further identified four dominant and 10 subdominant AFP-specific epitopes [8Butterfield L.H. Meng W.S. Koh A. Vollmer C.M. Ribas A. Dissette V.B. et al.T cell responses to HLA-A∗0201-restricted peptides derived from human alpha fetoprotein.J Immunol. 2001; 166: 5300-5308Crossref PubMed Scopus (137) Google Scholar, 9Meng W.S. Butterfield L.H. Ribas A. Dissette V.B. Heller J.B. Miranda G.A. et al.Alpha-fetoprotein-specific tumor immunity induced by plasmid prime-adenovirus boost genetic vaccination.Cancer Res. 2001; 61: 8782-8786PubMed Google Scholar, 10Meng W.S. Butterfield L.H. Ribas A. Heller J.B. Dissette V.B. Glaspy J.A. et al.Fine specificity analysis of an HLA-A2.1-restricted immunodominant T cell epitope derived from human alpha-fetoprotein.Mol Immunol. 2000; 37: 943-950Crossref PubMed Scopus (56) Google Scholar] (Table 1), all of which induced low to moderate CD8+ T cell responses in PBMCs of HCC patients [11Liu Y. Daley S. Evdokimova V.N. Zdobinski D.D. Potter D.M. Butterfield L.H. Hierarchy of alpha fetoprotein (AFP)-specific T cell responses in subjects with AFP-positive hepatocellular cancer.J Immunol. 2006; 177: 712-721Crossref PubMed Scopus (60) Google Scholar, 12Butterfield L.H. Ribas A. Meng W.S. Dissette V.B. Amarnani S. Vu H.T. et al.T-cell responses to HLA-A∗0201 immunodominant peptides derived from alpha-fetoprotein in patients with hepatocellular cancer.Clin Cancer Res. 2003; 9: 5902-5908PubMed Google Scholar]. We recently compared AFP-specific CD8+ T cell responses in PBMCs with those in tumour-infiltrating lymphocytes (TILs) and surrounding non-tumour liver tissue by stimulating lymphocytes with 18-mer overlapping peptides spanning the entire AFP protein [[13]Thimme R. Neagu M. Boettler T. Neumann-Haefelin C. Kersting N. Geissler M. et al.Comprehensive analysis of the alpha-fetoprotein-specific CD8+ T cell responses in patients with hepatocellular carcinoma.Hepatology. 2008; 48: 1821-1833Crossref PubMed Scopus (66) Google Scholar] (Table 1). Interestingly, we found T cell responses against several previously unidentified epitopes without a clear immunodominance, indicating that more AFP-specific epitopes exist and need to be identified. These results also indicate that CD8+ T cells specific for the self-antigen AFP are present in the normal T cell repertoire and are not centrally or peripherally deleted. GPC3 is a foetal oncoprotein. It belongs to a family of heparan sulphate proteoglycans that can bind growth factors (e.g., Wnt, FGF1/2, and VEGF) and thereby promote tumour growth. GPC3 was recently found to be over-expressed in HCC and is considered a promising target antigen. Two HLA-A2- and HLA-24-restricted CD8 T cell epitopes of GPC3 have been mapped in mice and shown to elicit T cell responses in HCC patients after long-term in vitro stimulation of PBMCs [[14]Komori H. Nakatsura T. Senju S. Yoshitake Y. Motomura Y. Ikuta Y. et al.Identification of HLA-A2- or HLA-A24-restricted CTL epitopes possibly useful for glypican-3-specific immunotherapy of hepatocellular carcinoma.Clin Cancer Res. 2006; 12: 2689-2697Crossref PubMed Scopus (154) Google Scholar] (Table 1). Expression of NY-ESO-1 is limited to testis in healthy subjects, however, it is often expressed de novo in HCC and other cancers such as melanoma, ovarian, and breast cancer, with no apparent gender bias. NY-ESO-1 is one of the most immunogenic TAAs known to date, with several identified epitopes currently being tested as potential vaccine candidates for cancers other than HCC. By using different approaches, three groups reported NY-ESO-1-specific CD8+ T cell responses in the peripheral blood of HCC patients to an epitope that had already been identified in melanoma patients [15Gehring A.J. Ho Z.Z. Tan A.T. Aung M.O. Lee K.H. Tan K.C. et al.Profile of tumor antigen-specific CD8 T cells in patients with hepatitis B virus-related hepatocellular carcinoma.Gastroenterology. 2009; 137: 682-690Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar, 16Korangy F. Ormandy L.A. Bleck J.S. Klempnauer J. Wilkens L. Manns M.P. et al.Spontaneous tumor-specific humoral and cellular immune responses to NY-ESO-1 in hepatocellular carcinoma.Clin Cancer Res. 2004; 10: 4332-4341Crossref PubMed Scopus (134) Google Scholar, 17Shang X.Y. Chen H.S. Zhang H.G. Pang X.W. Qiao H. Peng J.R. et al.The spontaneous CD8+ T-cell response to HLA-A2-restricted NY-ESO-1b peptide in hepatocellular carcinoma patients.Clin Cancer Res. 2004; 10: 6946-6955Crossref PubMed Scopus (66) Google Scholar] (Table 1). None of the observed T cell responses were, however, detectable ex vivo and required several weeks of in vitro lymphocyte expansion, suggesting a relatively low frequency of these cells in the circulation. SSX-2 is another cancer-testis antigen that has been recently demonstrated to be over-expressed in HCC patients. By using a CD8 T cell epitope that had been previously identified in melanoma patients, two groups detected CD8+ T cell responses in a small fraction of HCC patients after in vitro expansion of PBMCs and TILs [15Gehring A.J. Ho Z.Z. Tan A.T. Aung M.O. Lee K.H. Tan K.C. et al.Profile of tumor antigen-specific CD8 T cells in patients with hepatitis B virus-related hepatocellular carcinoma.Gastroenterology. 2009; 137: 682-690Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar, 18Bricard G. Bouzourene H. Martinet O. Rimoldi D. Halkic N. Gillet M. et al.Naturally acquired MAGE-A10- and SSX-2-specific CD8+ T cell responses in patients with hepatocellular carcinoma.J Immunol. 2005; 174: 1709-1716Crossref PubMed Scopus (85) Google Scholar] (Table 1). Antigens of the MAGE-A family were originally characterized in melanoma, but they are also widely expressed in other tumours such as breast carcinoma, glioma, colorectal carcinoma, and HCC. Zerbini et al. demonstrated in vitro induction of CD8+ T cell responses against previously identified MAGE-A1 and MAGE-A3 epitopes in tumour-infiltrating but not peripheral blood lymphocytes derived from HCC patients [[19]Zerbini A. Pilli M. Soliani P. Ziegler S. Pelosi G. Orlandini A. et al.Ex vivo characterization of tumor-derived melanoma antigen encoding gene-specific CD8+ cells in patients with hepatocellular carcinoma.J Hepatol. 2004; 40: 102-109Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar] (Table 1). Similarly, CD8+ T cell responses against MAGE-A10 epitope have also been described by Bricard et al. [[18]Bricard G. Bouzourene H. Martinet O. Rimoldi D. Halkic N. Gillet M. et al.Naturally acquired MAGE-A10- and SSX-2-specific CD8+ T cell responses in patients with hepatocellular carcinoma.J Immunol. 2005; 174: 1709-1716Crossref PubMed Scopus (85) Google Scholar]. TERT is a catalytic enzyme required for telomere elongation; its expression correlates with telomerase activity. Tumours must retain the length of their telomeres and telomerase helps them to do so by maintaining telomeric ends of linear chromosomes and protecting them from degradation. It is, therefore, not surprising that telomerase has been reported to be activated in up to 85% of all human cancers, including HCC, whereas TERT has been proposed as a universal TAA for cancer immunotherapy. Interestingly, Mizukoshi and colleagues reported ex vivo measurable weak CD8+ T cell responses to six TRET epitopes in the peripheral blood of HCC patients [[20]Mizukoshi E. Nakamoto Y. Marukawa Y. Arai K. Yamashita T. Tsuji H. et al.Cytotoxic T cell responses to human telomerase reverse transcriptase in patients with hepatocellular carcinoma.Hepatology. 2006; 43: 1284-1294Crossref PubMed Scopus (106) Google Scholar] (Table 1). In summary, these studies show that although TAA-specific T cells are detectable in patients with HCC, the frequency of these responses is low overall and in vitro stimulation is often required for their detection. Several mechanisms could contribute to the weak and often inefficient TAA-specific CD8+ T cell responses in HCC patients (summarized in Fig. 1). Tregs are the best characterized suppressor cells and have been shown to suppress tumour immunity in numerous studies. In HCC patients, Alexander and co-workers reported increased numbers of CD4+CD25+ Tregs in TILs, compared to distant tumour tissue [[21]Unitt E. Rushbrook S.M. Marshall A. Davies S. Gibbs P. Morris L.S. et al.Compromised lymphocytes infiltrate hepatocellular carcinoma: the role of T-regulatory cells.Hepatology. 2005; 41: 722-730Crossref PubMed Scopus (240) Google Scholar]. These findings were reinforced by the group of Korangy and Greten who also reported increased frequencies of CD4+CD25+ Tregs in TILs and PBMCs of HCC patients [[22]Ormandy L.A. Hillemann T. Wedemeyer H. Manns M.P. Greten T.F. Korangy F. Increased populations of regulatory T cells in peripheral blood of patients with hepatocellular carcinoma.Cancer Res. 2005; 65: 2457-2464Crossref PubMed Scopus (557) Google Scholar]. When activated with CD3 and CD28 antibodies, these Tregs were found to produce the immunosuppressive cytokine IL-10. Subsequently, increased Tregs in TILs and PBMCs were shown to express Foxp3 and to inhibit CD3/CD28 stimulated CD8+ T cell proliferation, activation, and production of granzymes and perforin [[23]Fu J. Xu D. Liu Z. Shi M. Zhao P. Fu B. et al.Increased regulatory T cells correlate with CD8 T-cell impairment and poor survival in hepatocellular carcinoma patients.Gastroenterology. 2007; 132: 2328-2339Abstract Full Text Full Text PDF PubMed Scopus (666) Google Scholar]. These combined results suggest that Tregs play a major role in the inhibition of tumour-specific T cell responses in HCC. Further, this is supported by the finding that depletion of Tregs resulted in the emergence of tumour-specific CD4+ T cell responses in six out of 16 HCC patients [[24]Greten T.F. Ormandy L.A. Fikuart A. Hochst B. Henschen S. Horning M. et al.Low-dose cyclophosphamide treatment impairs regulatory T cells and unmasks AFP-specific CD4+ T-cell responses in patients with advanced HCC.J Immunother. 2010; 33: 211-218Crossref PubMed Scopus (114) Google Scholar]. However, antigen-specificity of Tregs, their functional and phenotypical profiles, as well as mechanisms of suppression remain to be characterized in more detail. MDSCs comprise a mixture of monocytes/macrophages, granulocytes, and dendritic cells (DCs) at different stages of differentiation. In patients with HCC, blood MDSCs have recently been shown to induce Foxp3 and IL-10 expression in CD4+ T cells via arginase activity [[25]Hoechst B. Ormandy L.A. Ballmaier M. Lehner F. Kruger C. Manns M.P. et al.A new population of myeloid-derived suppressor cells in hepatocellular carcinoma patients induces CD4(+)CD25(+)Foxp3(+) T cells.Gastroenterology. 2008; 135: 234-243Abstract Full Text Full Text PDF PubMed Scopus (648) Google Scholar]. There is currently no specific drug or antibody available that selectively targets MDSCs. Since inhibition of arginase activity can cause side effects, due to the critical role of this enzyme in the urea cycle, it will be important to identify additional specific markers to target these cells. Several studies have shown that expression of HLA class I molecules and B7 costimulatory molecules is downregulated in HCC tissue [[26]Fujiwara K. Higashi T. Nouso K. Nakatsukasa H. Kobayashi Y. Uemura M. et al.Decreased expression of B7 costimulatory molecules and major histocompatibility complex class-I in human hepatocellular carcinoma.J Gastroenterol Hepatol. 2004; 19: 1121-1127Crossref PubMed Scopus (45) Google Scholar] and HCC cell lines [27Kurokohchi K. Carrington M. Mann D.L. Simonis T.B. Alexander-Miller M.A. Feinstone S.M. et al.Expression of HLA class I molecules and the transporter associated with antigen processing in hepatocellular carcinoma.Hepatology. 1996; 23: 1181-1188Crossref PubMed Google Scholar, 28Tatsumi T. Takehara T. Katayama K. Mochizuki K. Yamamoto M. Kanto T. et al.Expression of costimulatory molecules B7-1 (CD80) and B7-2 (CD86) on human hepatocellular carcinoma.Hepatology. 1997; 25: 1108-1114Crossref PubMed Scopus (86) Google Scholar]. Such downregulation is likely to lead to impaired processing of TAAs. Moreover, it has been shown that circulating myeloid DCs in HCC patients were decreased in numbers and had reduced cytokine production [[29]Ormandy L.A. Farber A. Cantz T. Petrykowska S. Wedemeyer H. Horning M. et al.Direct ex vivo analysis of dendritic cells in patients with hepatocellular carcinoma.World J Gastroenterol. 2006; 12: 3275-3282PubMed Google Scholar], raising the possibility that TAA presentation by DCs may also be impaired. Further work is required to determine the extent and mechanisms of such impairment. Many human cancers express PD-L1, the ligand for the inhibitory receptor programmed cell death-1 (PD-1). Tumour-associated PD-L1 has been shown to induce apoptosis of effector T cells and is thought to contribute to immune evasion by cancers. In HCC, tumour infiltrating CD8+ T cells are characterized by an increase in PD-1 expression [[30]Shi F. Shi M. Zeng Z. Qi R.Z. Liu Z.W. Zhang J.Y. et al.PD-1 and PD-L1 upregulation promotes CD8(+) T-cell apoptosis and postoperative recurrence in hepatocellular carcinoma patients.Int J Cancer. 2010; (April 19 [Epubahead of print])Google Scholar]. Interestingly, intratumoural Kupffer cells have been shown to up-regulate PD-L1 and decrease the effector function of PD-1-expressing CD8+ T cells in HCC patients [31Wu K. Kryczek I. Chen L. Zou W. Welling T.H. Kupffer cell suppression of CD8+ T cells in human hepatocellular carcinoma is mediated by B7-H1/programmed death-1 interactions.Cancer Res. 2009; 69: 8067-8075Crossref PubMed Scopus (279) Google Scholar, 32Kuang D.M. Zhao Q. Peng C. Xu J. Zhang J.P. Wu C. et al.Activated monocytes in peritumoral stroma of hepatocellular carcinoma foster immune privilege and disease progression through PD-L1.J Exp Med. 2009; 206: 1327-1337Crossref PubMed Scopus (675) Google Scholar]. It should be noted that cells with a MDSC phenotype also up-regulated PD-L1 in these studies and exerted a similar inhibitory effect on T cell activation, raising the possibility of MDSC-mediated T cell suppression. Together these data suggest that the inhibition of PD-1 may be a potential strategy in the boosting of HCC-specific immunity. It will be important to determine whether similar findings may be observed for other important inhibitory receptors such as, for instance, CTLA-4, BTLA, or Tim-3, and whether co-expression of these inhibitory receptors is present in HCC, as has been described in other tumour models [33Sakuishi K. Apetoh L. Sullivan J.M. Blazar B.R. Kuchroo V.K. Anderson A.C. Targeting Tim-3 and PD-1 pathways to reverse T cell exhaustion and restore anti-tumor immunity.J Exp Med. 2010; 207: 2187-2194Crossref PubMed Scopus (1370) Google Scholar, 34Fourcade J. Sun Z. Benallaoua M. Guillaume P. Luescher I.F. Sander C. et al.Upregulation of Tim-3 and PD-1 expression is associated with tumor antigen-specific CD8+ T cell dysfunction in melanoma patients.J Exp Med. 2010; 207: 2175-2186Crossref PubMed Scopus (952) Google Scholar]. It is known that a lack of CD4+ T cell help may lead to CD8+ T cell exhaustion. In fact, in HCC, AFP-specific CD4+ T cell responses were only present in patients with early stage disease and became exhausted as the disease progressed [[35]Behboudi S. Alisa A. Boswell S. Anastassiou J. Pathan A.A. Williams R. Expansion of anti-AFP Th1 and Tc1 responses in hepatocellular carcinoma occur in different stages of disease.Br J Cancer. 2010; 102: 748-753Crossref PubMed Scopus (22) Google Scholar]. Thus, for the activation of fully functional cytotoxic T lymphocytes it will be important to identify TAA-derived CD4 T helper cell epitopes and include them in a vaccine along with CD8 T cell epitopes. It is thus clear that the mechanisms underlying weak tumour-specific T cell responses are complex, and their further in-depth exploration is required for successful vaccine development. However, a few vaccination approaches have already been tested, resulting in the generation of modest TAA-directed T cell responses in HCC patients that will be summarized in the following section. The group of Butterfield and Economou was the first to conduct an HCC vaccine clinical trial [[12]Butterfield L.H. Ribas A. Meng W.S. Dissette V.B. Amarnani S. Vu H.T. et al.T-cell responses to HLA-A∗0201 immunodominant peptides derived from alpha-fetoprotein in patients with hepatocellular cancer.Clin Cancer Res. 2003; 9: 5902-5908PubMed Google Scholar]. In this phase I trial, six HLA-A0201+ patients were immunized with AFP CD8 T cell epitopes, resulting in the generation of measurable AFP-specific T cell responses in the peripheral blood. In the consecutive phase I/II trial, the authors attempted to further enhance these responses by loading autologous DCs with AFP epitopes and administering them to 10 HLA-A0201+ patients [[36]Butterfield L.H. Ribas A. Dissette V.B. Lee Y. Yang J.Q. De la Rocha P. et al.A phase I/II trial testing immunization of hepatocellular carcinoma patients with dendritic cells pulsed with four alpha-fetoprotein peptides.Clin Cancer Res. 2006; 12: 2817-2825Crossref PubMed Scopus (202) Google Scholar]. This treatment, however, resulted only in transient CD8+ T cell responses, possibly caused by the lack of CD4 help. To overcome this limitation, a subsequent clinical trial by the group of Adams used DCs pulsed with a lysate of the hepatoblastoma cell line HepG2 before transfusion into 35 HCC patients with diverse HLA types [[37]Palmer D.H. Midgley R.S. Mirza N. Torr E.E. Ahmed F. Steele J.C. et al.A phase II study of adoptive immunotherapy using dendritic cells pulsed with tumor lysate in patients with hepatocellular carcinoma.Hepatology. 2009; 49: 124-132Crossref PubMed Scopus (229) Google Scholar]. Such an approach of using apoptotic tumour cells as a source of multivalent antigens appears to be ideal at the moment due to poor characterization of HCC TAAs, and should allow the activation of both CD8+ and CD4+ T cells. Indeed, the authors reported functional IFN-γ-producing T cells to vaccine and/or AFP in five patients, disease stabilization for 6–16 months in six patients and reductions in serum AFP levels in three patients. These positive results suggest that further improvement of DC-based vaccination strategies, such as the use of multiple TAA epitopes stimulating both CD8+ and CD4+ T cells, may translate in more effective clinical results. Although the above-described vaccine approaches yielded the most successful outcomes observed in T cell-based HCC immunotherapies so far, the achieved tumour-specific T cell responses were often not robust enough to induce clinical responses. In addition to HCC vaccines still being in early development, the observed modest clinical efficacy may be explained by the strong tolerogenic environment at the tumour site and different tumour suppressive mechanisms described above. Clearly, treatment strategies combining blockade of immunoregulatory cell types such as Tregs and MDSCs and of inhibitory receptors, with vaccine-induced activation of TAA-specific T cells may be necessary to achieve the most effective therapeutic anti-tumour activity in HCC. Also, a combination with conventional HCC treatments, such as transarterial embolization or local tumour ablation may increase HCC immunogenicity and unmask TAA-specific T cell responses [[38]Ali M.Y. Grimm C.F. Ritter M. Mohr L. Allgaier H.P. Weth R. et al.Activation of dendritic cells by local ablation of hepatocellular carcinoma.J Hepatol. 2005; 43: 817-822Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar]. In conclusion, tremendous progress has been made in the field of immunotherapies for HCC in recent years. However, the repertoire of HLA class I and class II presented TAA-specific T cell epitopes needs to be expanded, and the mechanisms of T cell failure require further investigation before immunotherapy of HCC can successfully enter the clinical stage. The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. The authors do not have a relationship with the manufacturers of the drug involved either in the past or present and did not receive funding from the manufacturers to carry out their research.