Application of immune checkpoint inhibitors in hepatobiliary cancers

Abstract

Hepatobiliary cancers are primarily categorized into hepatocellular carcinoma (HCC) and biliary tract cancers (BTC). HCC is one of the most common malignant cancers, with increasing global incidence. Immune checkpoint inhibitors (ICIs) have changed the approach of HCC management in recent years. The combination of atezolizumab and bevacizumab has surpassed sorafenib as the standard regimen for first-line treatment of advanced unresectable HCC. However, the optimal choice for second-line treatment after initial treatment with ICIs lacks clinical trial confirmation. In addition, limited clinical trial data on ICIs combined with locoregional and perioperative therapies have been reported. Unlike HCC, clinical studies have found that immunotherapy is effective only for BTC with specific molecular markers. Moreover, the molecular characteristics governing immune responses and evasion remain unclear. This review provides information on the application of immunotherapy for HCC and prospects over the next few years. In addition, current evidence from clinical trials of BTC has made BTC immunotherapy a research hotspot again. Based on the findings in the literature, future clinical trials are being planned. Furthermore, biomarkers used in immunotherapies for hepatobiliary cancers that may shed light on the management of these diseases in the future were discussed.

1. Introduction

Hepatobiliary cancers, which are cancers that originate from the liver and biliary system, are mainly classified into hepatocellular carcinoma (HCC) and biliary tract cancers (BTC), including gallbladder cancer and cholangiocarcinoma. HCC is one of the most common malignant cancers worldwide [[1], [2], [3]]. Early-stage HCC can be cured with topical treatments, such as surgical resection and ablation. However, chemoresistance is prevalent in advanced HCC, whereas immunotherapy has greatly improved the prognosis of patients with advanced HCC. The incidence of BTC is at least 8 per 100,000 individuals in China, which is relatively low compared with the incidence of HCC. Cholangiocarcinoma is divided into intrahepatic, hilar, and extrahepatic cholangiocarcinoma according to different anatomical sites; all of which have a poor prognosis because of their aggressiveness and heterogeneity despite curative resection [4]. Chemotherapy is the standard first-line treatment for patients with advanced unresectable cholangiocarcinoma. However, even the standard first-line chemotherapy regimen gemcitabine plus cisplatin has limited response rates. Thus, in this review, we aim to summarize the application of immune checkpoint inhibitors (ICIs) in hepatobiliary cancers, which may help us better understand how immunotherapy would change the current standards.

Immune checkpoints are inhibitory pathways in the immune system that are regulated by the interaction of ligands and receptors. ICIs are drugs that kill tumor cells by regulating T cell activity through a series of pathways, such as co-inhibitory or costimulatory signals. The currently marketed ICIs include monoclonal antibodies against PD-1, PD-L1, and CTLA-4 [5]. In addition, several ICIs are currently in clinical trials, such as anti-LAG3 antibody and anti-TIGIT antibody.

2. Hepatocellular carcinoma

In the past few years, immunotherapy has made a breakthrough in the field of HCC. Immunotherapy has gradually developed from second- and later-line treatments of HCC to first-line treatment since 2017. In addition, the IMBrave150 study has ushered in a new era in HCC therapy [6]. ICIs combined with antiangiogenic drugs are now widely accepted to have surpassed sorafenib as the standard regimen for first-line treatment of advanced unresectable HCC. According to the latest follow-up data reported by the American Society of Clinical Oncology (ASCO) 2021, the overall survival (OS), objective response rate (ORR), and duration of response were 19.2 vs. 13.4 months (hazard ratio [HR] = 0.66; 95% confidence interval [CI] = 0.52–0.85; P = 0.0009), 30% vs. 11%, and 8.1 vs. 14.9 months in the atezolizumab + bevacizumab and the sorafenib groups, respectively. In the Chinese population, the updated OS data showed that the atezolizumab + bevacizumab group could significantly prolong OS compared with the sorafenib group with an OS of 24.0 vs. 11.4 months, respectively (HR = 0.53; 95% CI = 0.35–0.80). The combination regimen achieved the longest OS in the first-line phase 3 study in patients with advanced HCC, which serves as further confirmation that this regimen can be used as a standard treatment for patients with unresectable HCC who have not previously received systemic therapy. Furthermore, the ORIENT-32 trial is assessing the effectiveness of sintilimab in combination with IBI305 (anti-vascular endothelial growth factor monoclonal antibody, bevacizumab biosimilar) in patients with HCC as the first-line treatment compared with sorafenib (estimated end date: December 2022). At the cutoff date for data acquisition (August 15, 2020), patients in the sintilimab–bevacizumab biosimilar group had a significantly longer independent radiology review committee-assessed median progression-free survival (PFS; 4.6 months [95% CI = 4.1–5.7]) than patients in the sorafenib group (2.8 months [2.7–3.2]; HR = 0.56; 95% CI = 0.46–0.70; P < 0.0001). These results demonstrate that patients with HCC can benefit from immunotherapy combined with anti-VEGFA antibodies.

Thus, questions arise regarding whether patients with HCC can also benefit from ICI–tyrosine kinase inhibitor ​combinations. KEYNOTE-524, a phase Ib clinical study that evaluated the combination of lenvatinib with pembrolizumab, showed a good clinical response rate. According to ASCO 2020, the median treatment duration, median OS, median PFS, and ORR were 7.9 months, 22.0 months (95% CI = 20.4–not estimable), 8.6 months (95% CI = 7.1–9.7), and 46% (95% CI = 26.6–46.2), respectively [7]. The LEAP002 phase III clinical study (lenvatinib combined with pembrolizumab) is also underway, and the final results will be published early next year. ASCO 2021 presented the latest data on regorafenib combined with pembrolizumab as the first-line treatment for patients with advanced HCC, and 10 (31%) and 18 (56%) of the 32 evaluable patients demonstrated partial response and stable disease (SD), respectively. The disease control rate (DCR) was 88%, indicating encouraging antitumor efficiency of the combination of regorafenib and pembrolizumab as a first-line treatment [8]. However, few studies have investigated HCCs with poor treatment response or primary resistance. The first-line treatment landscape of HCC in the future will likely include a triple combination (e.g., the Morpheus-Liver study) or a biomarker-based regimen enabling population enrichment.

Regarding second-line treatment, most of the available data are based on patients resistant to or progressing from sorafenib as the first-line treatment. With the advancement of atezolizumab + bevacizumab as first-line treatment, sorafenib, lenvatinib, regorafenib, cabozantinib, and ramucirumab are currently recommended by the European Society for Medical Oncology as treatment options after the failure of first-line immunotherapy-based regimens. Additionally, immunotherapy represents a new first-line option for the treatment of HCC as well as in the second-line setting. In this context, anti-PD-1, anti-PD-L1, and anti-CTLA-4 antibodies are being tested. However, the optimal choice for the second-line treatment for patients after initial treatment with ICI-based combinations lacks clinical trial confirmation and is yet to be determined. However, ICI monotherapy is likely to be discarded. Further studies for subsequent lines of therapy of new ICI-based combinations may shed light on this situation.

In addition to systemic therapy, immunotherapy combined with locoregional therapies, such as transarterial chemoembolization (TACE)/hepatic arterial infusion chemotherapy (HAIC) and radiotherapy, has progressed. TACE/HAIC and ablation may lead to tumor necrosis and cellular damage by inducing high intratumoral temperatures. This mechanism of action is responsible for the release of high levels of tumor neoantigens, thereby increasing immune recognition and activation in the tumor microenvironment [[9], [10], [11]]. Duffy et al. conducted a phase I/II study in which patients received tremelimumab combined with radiofrequency ablation or chemoablation, with an OS of 12.3 months [12]. A phase I clinical trial included nine patients who received nivolumab + drug-eluting bead TACE, achieving a DCR of 100% (PR = 22%, SD = 78%) [13]. In addition, the survival results of a phase I clinical trial of pembrolizumab in combination with TACE have not been published, but the safety of this combination has been demonstrated [14]. A retrospective clinical trial of a combination of HAIC and immunotherapy enrolled 229 patients who were divided into the HAICAP (HAIC combined with anti-PD-1 immunotherapy; n = 81) and HAIC (n = 148) groups, with an OS and PFS of 18.0 vs. 14.6 months (P = 0.018; HR = 0.62; 95% CI = 0.34–0.91) and 10.0 vs. 5.6 months (P = 0.006; HR = 0.65; 95% CI = 0.43–0.87), respectively, suggesting that HAICAP is associated with a better survival benefit for patients [15]. Moreover, the TACE-related ischemic cell damage may result in an increase in the levels of vascular endothelial growth factors. Therefore, TACE + pembrolizumab and lenvatinib (LEAP-012 trial) and TACE + durvalumab and bevacizumab (EMERALD-1 study) therapeutic combinations are being tested in this field.

Regarding immunotherapy combined with radiotherapy, radiotherapy has been found to increase the expressions of PD-1 and PD-L1 on the surfaces of CD8⁺ T cells and tumor cells, respectively [16, 17]. Chiang et al. treated five patients with unresectable HCC using a combination of stereotactic body radiation therapy ​and nivolumab, with an ORR up to 100% (2 CRs and 3PRs) [18]. Selective internal radiation therapy consists of intra-arterial microsphere infusion with the radioisotope Yttrium-90. Several immunotherapy trials in combination with selective internal radiation therapy are currently recruiting participants, such as NCT03099564, which is a multicenter trial, and NCT03033446, which is recruiting Asian patients. Although the current data do ​not provide any evidence regarding the effect of a combination of tyrosine kinase inhibitor therapies with locoregional treatment in improving survival outcomes, a rationale for combination therapies with HCC immunotherapy was noted based on the currently available data. Acquiring more clinical and preclinical data is necessary to identify these most immunogenic techniques.

No adjuvant therapy has currently been demonstrated to improve OS in patients with early-stage HCC following surgical resection or locoregional therapies. However, the probability of a response to the administration of ICIs in the early stage may increase because the tumor burden is low, which may expand the progenitor T cell pool [50]. Kaseb et al. conducted a study that included 30 patients with resectable HCC randomized to nivolumab (arm A) or nivolumab + ipilimumab (arm B) for 6-week and 2-year therapies before and after surgery, respectively. Of the 27 evaluable patients, 21 underwent surgery (pathologic complete response [pCR] was observed in five patients [two in arm A and three in arm B] (25% pCR rate) and three patients had a major pathological response (50%–99% necrosis on histological examination). The patients’ treatment was well tolerated, without grade ≥ IV adverse reactions [19]. Further validation of larger samples will be needed for the confirmation of neoadjuvant administration of immunotherapies. Additionally, no adjuvant treatment has been demonstrated to be effective, except two randomized trials that assessed adjuvant adoptive cell therapies in patients with HCC. However, neither treatment has been clinically adopted owing to the lack of subsequent validation studies. The validation of phase III trials, such as KEYNOTE-937 (estimated end date: June 2025), are necessary for evaluating the safety and efficacy of immunotherapy as adjuvant therapy in patients with HCC after surgical resection or local ablation (Table 1).

Table 1.

Clinical trials of HCC that are ongoing.

	NCT ID	Treatment arms	Settings	Phase
Perioperative	NCT03867084	Pembrolizumab vs. Placebo	Adjuvant	III
	NCT03383458	Nivolumab vs. Placebo	Adjuvant	III
	NCT04639180	Camrelizumab + Apatinib vs. Placebo	Adjuvant	III
	NCT03847428	Durvalumab + Bevacizumab vs. Durvalumab + Placebo	Adjuvant	III
	NCT04102098	Atezolizumab +Bevacizumab vs. Placebo	Adjuvant	III
	NCT03867370	Toripalimab	Neoadjuvant	I/II
	NCT03682276	Ipilimumab + Nivolumab	Neoadjuvant	I/II
Combined with locoregional therapy	NCT05063565	TheraSphere + Durvalumab and Tremelimumab vs. TheraSphere alone	Advanced	II
	NCT04945720	HAIC + Durvalumab vs. HAIC alone	Advanced	II
	NCT04547452	SBRT + Sintilimab vs. Sintilimab alone	Advanced	II
	NCT04518852	TACE, Sorafenib + immunotherapy	Advanced	II

Limited clinical trial data on ICIs combined with locoregional HCC therapies exist, and randomized controlled clinical trials of single-agent and combination therapies both in an adjuvant setting and combined with locoregional therapies are underway and aim to surpass current results (Table 1). Every new, effective treatment strategy will inevitably go through a gradual advancement process from the later-line to the first-line treatment to the perioperative period after its emergence, and the results of phase III trials are expected to change the HCC management approach ​within the next 3–5 years.

3. Biliary tract cancers

The most ideal treatment for biliary system cancers is surgery [20]. For patients who cannot undergo surgical resection, the National Comprehensive Cancer Network guidelines recommend chemotherapy as the first-line treatment, but subsequent standardized treatment options after progression are still lacking. With the development of next-generation gene-sequencing technology, some potential targets and corresponding therapeutic drugs have been discovered, such as FGFRs and IDH1/2, which has become the focus of current clinical research [49]. Remarkable breakthroughs have been made in cancer immunotherapy in recent years, and immunotherapy represented by ICIs has shown preliminary efficacy in a few trials of advanced BTC. However, unlike HCC, for which immunotherapy is suitable in almost all patients, clinical studies have found that ICIs are effective only for BTC with specific molecular markers, suggesting that BTC may belong to immune “cold” tumors that lack T cell infiltration [21]. The results of TOPAZ-1 showed that durvalumab in combination with chemotherapy as the first-line treatment significantly improved the OS of patients with advanced BTC [36]. However, some failed results of first-line immunotherapy have been reported. A few months ago, the phase 2/3 INTR@PID BTC 055 study was announced to be terminated, which was based on a data review by an independent data monitoring committee [37]. This study aimed to evaluate the efficacy of bintrafusp-Alfa (M7824 and PD-L1/transforming growth factor-beta [TGF-β] antibody) in combination with gemcitabine + cisplatin as the first-line treatment in patients with locally advanced or metastatic BTC. Merck indicated the reason for termination as “the study is unlikely to meet the primary endpoint of OS.” A phase III clinical trial of pembrolizumab in combination with gemcitabine and cisplatin (KEYNOTE-966) is underway. These conflicting results have made BTC immunotherapy a research hotspot again, and several important lessons from the studies discussed herein will guide future clinical trials.

TGF-β plays a central role in the coordination of immune evasion during advanced cancer stages, which suggests that TGF-β pathway inhibition can overcome immunosuppressive signaling in the tumor microenvironment. However, the functions of TGF-β signaling in reducing protumorigenic inflammation during early-stage cancer are deflected into creating a permissive microenvironment during disease progression. Thus, the failure of the INTR@PID BTC 055 study does not signal the end of anti-TGF-β therapy in BTC but indicates which cell types can benefit and how distinct responses are coordinated in space and time. Second, the molecular landscapes of BTC illustrate that cancer subtypes in similar organs may arise through different extrinsic and intrinsic carcinogenic processes, which suggests the importance of conducting these molecular studies in diverse BTC populations and searching for distinct biomarkers and therapies. Finally, different molecular subtypes may have diverse immune landscapes, all of which may not benefit from immunotherapy. Thus, targeted-based therapy should be considered first for patients with BTC with potentially targetable genes.

The application of ICIs in BTC treatment has been summarized in this study. For mono-immunotherapy, biomarkers are needed for better response. dMMR/MSI-H is recognized as a molecular marker of response to immunotherapy, and patients with such a biomarker enrolled in the KEYNOTE-158 and KEYNOTE-028 trials showed good response rates to immunotherapy. Regarding monotherapy, Le et al. enrolled 86 patients with advanced mismatch repair-deficient cancers across 12 tumor types, including four patients with BTC, from September 2013 to September 2016, and treated them with pembrolizumab. The DCR for these four (one CR and three SDs) patients was 100% [22]. Similarly, 22 patients with BTC with MSI-H and/or dMMR were included in the KEYNOTE-158 study. The ORR of this population was 41% (9/22) after pembrolizumab treatment, including two CRs and seven PRs, with a median OS of 24.3 months [23]. Although ICIs showed good efficacy in patients with dMMR/MSI-H in the aforementioned studies, such patients are relatively rare [24,25]. Ju et al. analyzed the morphology and MMR status of 96 cases of intrahepatic and extrahepatic cholangiocarcinomas using hematoxylin and eosin staining (H&E) and immunohistochemical staining, respectively. They found that 6% of the samples showed MMR deficiency and the best predictive factor was a nontypical infiltrating pattern of invasion [26].

Besides dMMR/MSI-H, the correlation between PD-L1 expression and immunotherapy has been investigated. Fontugne et al. reported that PD-L1 expression by neoplastic cells was observed in nine cases (9%, five iCCAs and four pCCAs), and PD-L1-positive inflammatory cell aggregates were identified in 46% (n = 46) of the cases (31 iCCAs and 15 pCCAs) [27]. At present, there is no clearly defined criteria for PD-L1 positivity in cholangiocarcinoma, and most existing clinical studies define PD-L1 positivity as PD-L1 ≥ 1% in tumor cells or associated inflammatory cells by immunohistochemical staining [28,29]. A few clinical trials with mono-immunotherapy of patients with small BTC showed an ORR of 17%–23% [[30], [31], [32]], indicating that high PD-L1 expression may be associated with a better response to ICIs. However, the KEYNOTE-158 study, including 61 patients with PD-L1–positive BTC and 34 with PD-L1-negative BTC, showed an ORR of 6.6% (4/61) and 2.9% (1/34) in the PD-L1-positive and PD-L1-negative groups, respectively, which was significantly lower than the previously obtained results. The PFS was 1.9 and 2.1 months, respectively, and the OS was 7.2 and 9.3 months, respectively, in the two groups, with no significant difference [23,33]. The different results may be due to the differences in the methods for PD-L1 staining and criteria of PD-L1 positivity. Thus, further studies may be needed to clarify whether patients with PD-L1-positive BTC can benefit from immunotherapy and to establish standard criteria.

As shown in Table 2, clinical studies of immunotherapy combined with chemotherapy are now in full swing. A phase II clinical trial of durvalumab (D) and tremelimumab (T) combined with chemotherapy as a first-line treatment was conducted. A total of 121 patients were divided into the BMC cohort (the sequence therapy of CisGem → CisGem + D + T), CisGem + D cohort, and CisGem + D + T cohort. The primary study endpoint was ORR. The ORR was 50% in the BMC cohort and approximately 73% in both the CisGem + D and CisGem + D + T cohorts [34,38]. Moreover, the European Society for Medical Oncology 2021 has updated the results of a phase II trial of nivolumab and gemcitabine and S-1 as the first-line treatment in patients with advanced BTC. Forty-eight patients were enrolled in the trial. After a median follow-up period of 6.4 months (95% CI = 4.8–8.0), 1 and 19 patients showed pathological CR and confirmed PR, respectively. The ORR was 41.7%, with a long-term DCR of 77.1% (CR + PR + SD > 12 weeks). The median PFS and OS were 8.0 months (95% CI = 5.8–not reached) and not reached (95% CI = 10.7–not reached), respectively [35].

Table 2.

Ongoing clinical trials of BTC.

	NCT ID	Treatment arms	Line	Phase	Endpoint
Immunotherapy combined with chemotherapy	NCT05019677	GC + Tislelizumab + Ociperlimab (Anti-TIGIT)	1	II	ORR
	NCT04961788	Gemox + Toripalimab	1	II	ORR
	NCT04924062	GC + Pembrolizumab vs.GC + Placebo	1	III	OS
	NCT04989218	GC + Tremelimumab + Durvalumab	1	I/II	ORR
Immunotherapy combined with targeted therapy	NCT04809142	Anlotinib + TQB2450 vs. chemotherapy	2	III	OS
	NCT04550624	Pembrolizumab + Lenvatinib	After first line	II	ORR

Regarding immune combination targeted therapy, the KEYNOTE-098 study assessed the efficacy and safety of pembrolizumab combined with ramucirumab [39]. Hypertension, which occurred in five patients, was the most common grade 3 treatment-related adverse event. One patient experienced a grade 4 TRAE (neutropenia), and no treatment-related mortalities were recorded. Although the adverse events are tolerable, the overall response rate (4%) is relatively low, so the regimen cannot be indicated for clinical application. However, lenvatinib, another antiangiogenic drug, has shown good efficacy in combination with pembrolizumab [40,41]. In a study by Lin et al., 32 patients received pembrolizumab combined with lenvatinib as the non-first-line treatment [42]. Overall, the ORR was 25% (zero CRs and eight PRs), and the DCR, mPFS, and median OS were 78.1%, 4.9 months (95% CI = 4.7–5.2 months), and 11.0 months (95% CI = 9.6–12.3 months), respectively. Phase I/II results of later-line anlotinib + TQB2450 in patients with BTC have been reported in ASCO-GI 2021, with an OOR and DCR of 40% and 80%, respectively. Recently, targeted drugs such as FGFR2 and IDH1 inhibitors have shown good therapeutic prospects in patients with related mutations. Transcriptomic data and gene set enrichment analysis have revealed that immune response pathways are suppressed in IDH-mutant BTC. Furthermore, altered FGFR signaling was shown to play a role in immune evasion. Thus, modulating the tumor microenvironment using IDH or FGFR inhibitors, with the goal of converting these immunologically cold tumors into immunologically hot tumors, may prove beneficial for immunotherapy. Currently, studies evaluating the safety and efficacy of immunotherapy combined with FGFR or IDH inhibitors are being conducted, which would represent a viable strategy to overcome acquired resistance or perhaps even be used as a front-line therapeutic strategy in FGFR- or IDH-altered patients.

4. Immunotherapy biomarkers

Given the high response rate of immunotherapy in HCC, we mainly discuss BTC immunotherapy biomarkers in this section. Moreover, the role of MMR status and PD-L1 expression in immunotherapy in BTC has been previously discussed. TMB-H is usually considered a biomarker for effective immunotherapy in various tumors [43]. Data on biliary system tumors of TMB-H in terms of clinical response to ICIs are rare. A retrospective study included 57 patients with hepatobiliary tumors, of whom four (8%) patients had TMB-H ≥ 10 Muts/Mb, and none of them responded to immunotherapy. In contrast, six (12%) patients with TMB-L responded to ICIs [44]. Zhang et al. recently reported three cases (one iCCA and two dCCA) of TMB-H treated with ICIs. All patients responded to immunotherapy, including two PRs and one CR [45]. These studies with small samples have shown inconsistent results. Thus, the role of TMB as a biomarker of response to immunotherapy and the optimal cutoff value for TMB remains to be determined. In KEYNOTE-028, researchers found that a T cell-inflamed gene–expression profile, PD-L1 expression, and TMB predicted response to pembrolizumab in multiple tumor types. These biomarkers may help identify patients who have a higher likelihood of response to anti-PD-1 therapies in a wide range of cancers [30].

A study that enrolled 26 patients who had advanced microsatellite stable BTCs (15 with gallbladder cancers and 11 with intrahepatic cholangiocarcinoma) and received ICI treatment assessed their clinical and genomic characteristics with the clinical outcomes. Mutations in LRP1B, ERBB2, or PKHD1 were found to be closely related to the increase in PFS, and alterations in the RTK–RAS pathway were associated with improved prognosis [46]. Some studies also found that the expression of PD-L1 and the density of CD8⁺ TILs in tumor cells were significantly increased in EBV-infected patients by comparing the immune microenvironment of EBV-related intrahepatic cholangiocarcinoma and non-EBV-related intrahepatic cholangiocarcinoma, which suggests that patients with EBV-related cholangiocarcinoma respond well to ICIs [47], but further clinical validation is needed. Except for promoting factors, KRAS alteration and CIN tumor are regarded as two resistance factors of immunotherapy. A study included 121 patients with advanced biliary system tumors, of whom 48 patients received PD-1/PD-L1 monoclonal antibody treatment after systemic chemotherapy progression. The study found that the clinical benefit rate was only 5% and 39.3% in patients with KRAS-altered or CIN-type tumors and other patients, respectively (P = 0.008), with an mPFS of 2.5 vs. 1.0 month (P < 0.001), respectively. In contrast to stromal TILs, the intratumoral TIL density in the tumor center was significantly lower in BTCs with the resistance factors, indicating an immune-suppressive microenvironment [48].

These diversities in immunotherapy biomarkers for BTC may be due to several reasons, and changes should be made. First, preclinical animal models of BTC are lacking, which are required for a better understanding of the molecular mechanisms involved in the development of these diseases. Second, the use of BTC patient-derived xenografts and organoids needs to be more deeply explored for disease mechanisms and response to new therapeutic agents. Third, large samples of BTC cohorts with different molecular subtypes are lacking, which keeps important scientific and clinical questions remain unclear. Fourth, in BTC, especially gallbladder cancer, cells with ERBB2 ectopic expression had increased PD-L1 expression, indicating the potential advantages of PD-L1 blockade in these patients. Fifth, other ICIs, such as LAG-3 and T cell immunoglobulin mucin-3 (TIM-3), are in development but have yet to see widespread clinical use, which may be effective in BTC in a different microenvironment. Finally, cholangiocarcinoma often arises sporadically although several conditions, including parasitic infections, biliary tract disorders, and exposure to toxins, among which the infiltration of tumor-associated-macrophages differs. Studies in other cancers have demonstrated that tumor-associated macrophages may aid in the immune escape of cancer cells and immunosuppression by IL-10 secretion. Thus, according to etiologies, BTC patients may have different levels of immune responses. More preclinical studies and clinical trials to improve patient outcomes are needed for a better understanding of BTC and the tumor microenvironment.

5. Conclusion

With the rapid development of molecular biology, immunology, and genetic engineering technology in recent years, immunotherapy has advanced rapidly and has progressed through all lines of treatment of hepatobiliary cancers. Consequently, ICIs have become the first-line standard treatment for advanced HCC and BTCs with dMMR/MSI-H. However, to verify the application of ICIs in the treatment of BTCs and in later-line therapies of HCC, more clinical studies are warranted.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.