We established a novel panel of 12 CTAs, that are each expressed in at least 10% of HCC tumors, while none of them are expressed in healthy tissues except immune-privileged testis. Based on mRNA analysis, approximately 80% of our HCC-patients expressed one or more of these antigens in their tumor tissues, whereas protein expression of five of these CTAs was detected in tumors of approximately 70% of these patients. This CTA-panel can therefore be safely applied for immunotherapeutic purposes in the majority of Western HCC-patients. In addition, we found that 45% of HCC-patients expressed one or more of the 12 CTAs of our panel in their histologically tumor-free liver tissue, and that expression in TFL was associated with more HCC recurrence and worse patient survival. These data suggest that occult CTA-expressing (pre-)malignant cells may remain present in non-cancerous liver tissue after tumor resection, and that these cells might be at least partially responsible for HCC recurrence after surgery.
CTA expression in tumors of HCC-patients has been studied before, however, as demonstrated by the results of our literature study (Supplementary Table S3), most studies investigated only a few CTAs, determined either RNA or protein expression but not both, and most importantly, did not exclude CTAs expressed in healthy tissues (Fig. 1B and C, Supplementary Tables S4). As far as we are aware, the present study is the most comprehensive investigation of CTA-expression in tumor and paired TFL tissues of HCC-patients performed. Moreover, we determined both RNA and protein expression and excluded any CTA that showed up to 4-log lower RNA expression compared to the mean of 3 reference genes in any healthy tissue, in order to prevent therapy-induced auto-immunity in future clinical applications. As 59% of HCC-patients expressed at least 2 CTAs, 50% expressed at least 3 CTAs, and 40% expressed 4 or more CTAs in their tumors, our CTA-panel enables therapeutic targeting of multiple CTAs in most HCC-patients, which is important to prevent escape of tumor cells that do not express a particular CTA from therapy-induced immunity.
Currently, somatic mutation-derived neo-antigens are considered to be the most promising candidate antigens for therapeutic vaccination in cancer patients.(28–30) However, Dong, et al. recently showed that in multifocal HCC neo-antigens are unique to every tumor lesion, whereas CTA expression was conserved between lesions. (31). These data indicate that effective therapeutic vaccination with neo-antigens in these patients requires analysis of mutations in every lesion and design of a vaccine consisting of neo-antigens expressed in different lesions. In addition, Dong et al found that, compared to neo-antigens, CTAs are expressed at higher levels in tumors of HCC-patients, and that CTA peptides eluted from tumor-expressed MHC class I molecules could evoke expansion of patient T cells in vitro. Therefore, CTAs may represent more suitable antigens for vaccination in multifocal HCC compared to neo-antigens. Moreover, since expression of the same CTAs is shared by different HCC patients, the use of CTAs for therapeutic vaccination allows use of off-the-shelve vaccine antigens, which can applied faster, which is important for patients with fast growing tumors, and may be more cost-effective than vaccines consisting of neo-antigens which require design and production of a personalized vaccine for every individual patient.
Importantly, several CTAs of our panel, such as the MAGE-family members, TSPY and CAGE1, are functionally involved in tumorigeneses and cancer progression by modulating gene expression, regulating mitosis and tumorigenic signaling.(16, 17) Involvement of these CTAs in cancer progression may prevent antigen loss upon therapeutic targeting.(32) Their role in cancer progression is further supported by data showing that CTA expression is more prevalent in advanced tumors.(33, 34) In HCC, MAGEA9 expression was related to the presence of distant metastasis and was an independent negative prognostic factor for disease-free survival and overall survival.(35) We found increased CTA expression, including MAGE-A9, in tumors with vascular invasion, which is associated with worse prognosis in HCC. (36) However, in this cohort we did not find any association between CTA expression in tumor tissues and survival, like Liang et al. and previous work by our group reported.(27, 37) Both studies found that patients with more tumor antigens expressed in their HCC tumor, had a better post-surgical prognosis of HCC-patients. This difference may be related to the different panels of CTA that were investigated in the referred studies and in our present study. Moreover, in contrast to our Western HCC-patient cohort, the patient cohort in the study by Liang et al. consisted mainly of chronic HBV patients.
Expression of CTAs in tumor-free liver tissues of HCC patients has been sparsely investigated before (Supplementary Table S3). Our study is the first to analyze CTA expression in tumor-free liver tissues of HCC-patients both at RNA and protein level. Moreover, this study is the first to analyze in HCC, or in any other type of cancer, whether CTA expression in non-cancerous tumor-surrounding tissues is associated with post-operative HCC recurrence and patient survival. To our surprise, we observed RNA expression of one or more of the 12 CTAs of our panel in histologically tumor-free liver tissues of 45% of our HCC-patients, while protein expression of one or more of 4 of these CTAs was detected in non-cancerous liver tissues of 40% of patients. Most notably, we found that expression in TFL was associated with faster and more HCC recurrence as well as worse patient survival after tumor resection. Aufhauser et al.(38) hypothesized that early HCC recurrence (< 2 years) after tumor resection in HCC patients is the consequence of occult multi-focality present at the time of tumor resection, but failed to find markers to identify such occult metastases. The 2-year recurrence rate in our cohort was significantly higher in patients with CTA-expression in TFL compared to patients without CTA-expression in TFL. Moreover, CTA mRNA expression profiles in TFL were similar to those in the corresponding tumors, and our preliminary immunohistochemical data show that CTA-expressing cells in TFL were either single cells or small foci. Based on these observations, we hypothesize that CTA-expressing cells in TFL of patients with early HCC recurrence represent intra-hepatic metastases. This hypothesis is supported by a study performed in colorectal cancer patients with liver metastasis. In TFL, they detected low frequencies of somatic mutations that were also observed in matched tumor samples, despite appearing normal histologically. Since these mutations were not found in the matched blood samples, it was hypothesized that either tumor DNA or tumor cells diffused or migrated into the surrounding normal tissue.(39) However, the authors did not correlate this to either HCC recurrence or survival. Conversely, CTA-expressing cells in TFL of HCC-patients with late HCC recurrence may represent de novo tumor-initiating cells. In this respect, it is interesting that we also observed CTA-expression in cirrhotic livers of non-HCC patients. A future study is needed to investigate whether such patients have an increased risk of developing HCC. Similarly, a previous study detected MAGE-antigen expression in lung tissues of former smokers at risk for NSCLC development.(40) Clearly, future research is required to further characterize CTA-expressing cells in tumor-free tissues.
Most therapeutic cancer vaccination studies have been performed in advanced cancer patients with high tumor load in which an immunosuppressive tumor microenvironment has been established, and showed modest clinical results. Based on our data showing the presence of scattered single CTA-expressing cells and small foci of CTA-expressing cells in TFL of almost half of resected HCC-patients, therapeutic vaccination with CTA after tumor resection might be a promising approach to prevent HCC recurrence in such patients. Compared to vaccination in advanced cancer, we expect that the low tumor load remaining after resection of detectable tumors may enhance the probability of effective immunological eradication of CTA-expressing (pre-)malignant cells. Analysis of expression of our panel of 12 CTAs in resected non-malignant liver tissue may allow identification of HCC-patients at risk of HCC recurrence.
A prerequisite for therapeutically targeting antigens by vaccination, is that they are immunogenic. Most of the CTAs included in our panel have previously been proven immunogenic in cancer patients.(41) More specifically in HCC patients, we and other research groups have demonstrated the presence of MAGEA1-, MAGEA10-, MAGEC2- and NY-ESO-1-specific T-cells, both in blood and in tumors. (42–46). In addition, NY-ESO-1 and TSPY-specific IgG have been detected in HCC-patients,(47, 48) while CT47A1-, PAGE1- and SLCO6A1-specific antibodies were recently detected in NSCLC patients.(49)
We acknowledge several limitations of this study. First, since the etiologies of HCC differ geographically, this CTA-panel might not be applicable to non-Western HCC-populations. Secondly, protein expression of CAGE1, MAGEB2 and TSPY needs to be confirmed. Thirdly, the reported association between CTA expression in TFL and cancer recurrence has to validated in another cohort. Finally, future research is required to investigate whether CTA-expressing cells in TFL are really (pre-)malignant cells that can give rise to cancer recurrence.