Hepatocellular carcinoma (HCC) is one of the most common cancers, which ranks as the second leading cause of cancer-related death worldwide[15]. Is spite of recent advances in treatment options, prognosis remains quite poor, especially in patients with advanced HCC. Patients with advanced HCC have a median survival of less than one year[16]. In light of poor prognosis and resistance to chemotherapy and radiotherapy, other treatment strategies have been investigated extensively. The multikinase inhibitor (sorafenib) was the first systemic agent to show a significant improvement in overall survival for patients with advanced HCC[17]. However, anti-angiogenic agents (sorafenib and lenvatinib) only yield a modest improvement of about 3 months in overall survival[18]. Therefore, immunotherapy, which aims to interrupt immune checkpoint interaction and break immune tolerance, has come under in the spotlight.
Experimental evidence has demonstrated that PD-L1 on tumor cells can delivery inhibitory signals to PD-1+CD8+ T cells, resulting in suppression of immune response by inducing apoptosis, anergy and functional exhaustion of CD8+ T cells[19]. Further pathologic study showed that PD-L1 positive HCC was significantly associated with biological aggressiveness[20], including vascular invasion, poor differentiation, satellite nodules and high AFP levels; nevertheless, whether PD-L1 expression can influence prognosis in patients with HCC remains open to debate. Results from several studies investigating prognostic significance of PD-L1 in HCC are inconsistent[4, 5, 21]. A meta-analysis indicated that PD-L1 positive was predictive for shorter overall survival and disease-free survival[22]. However, such meta-analysis suffers several limitations[23]. First the meta-analysis did not screen our all studies in this field. Second, one included study used serum rather than tumor samples to assess status of PD-L1. Third, included patients received different treatment. All of these limitations increase heterogeneity and undermine the reliability of the results.
Our present study showed that patients with positive PD-L1 had a shorter survival than those with negative PD-L1. However, difference was not statistically significant (P = 0.116). PD-L1 expression is significantly associated with cirrhosis (P = 0.016) and TILs (P = 0.000). The remaining clinical characteristics, including gender, age, virus infection, AFP, tumor size and vascular invasion, were not significantly associated with PD-L1 expression (P > 0.05). Clinical trials of Checkmate 040 and KEYNOTE 240 indicated that anti-PD-1 monoclonal antibody could not significantly increase survival and PD-L1 expression did not influence objective response rate in patients with advanced HCC[6, 7], which might suggest that PD-L1 expression in HCC exert a modest impact on prognosis in patients with HCC. Previous studies have showed that TILs can trigger PD-L1 up-regulation in tumor cells by secreting interferon-γ[24], which is further confirmed by the result that PD-L1 positive is significantly with TILs in our present study. There has been reported that genetic variations of PD-1 predisposed patients with chronic HBV infection to cirrhosis[25]. Whether an association of tumor PD-L1 positivity with cirrhosis is a causal relationship needs to be further investigated.
The value of HLA class I molecules in HCC was rarely investigated. Down-regulation of HLA class I antigen is one of strategies for HCC-induced immune tolerance[1]. A direct evidence of tumor escape from T cell immunity cause by MHC-I down-regulation is the facial cancer in Tasmanian devil, which is transmissible to histo-incompatible companions[26, 27]. The underlying mechanism is that this cancer silenced the genes for antigen presentation at the epigenetic level. Nevertheless, our present study showed that survival was shorter in patients with low expression of HLA class I antigen than in those with high expression of HLA class I antigen, but the difference did not reach a statistical significance (P = 0.171). None of clinical characteristics, including gender, age, virus infection, cirrhosis, AFP, tumor size and vascular invasion were significantly associated with HLA class I antigen expression (P > 0.05).
Multiple pathways are involved in HCC-induced immune tolerance[1]. Logically, two immune pathways may exert a synergistic impact on immune tolerance, which recommend combination immunotherapy in cancers. Combination of immunotherapy (ipilimumab/nivolumab) also yields a better survival than single immunotherapy (ipilimumab or nivolumab) in advanced melanoma[28]. Our present study showed that coexistence of PD-L1 positive and HLA class I antigen low expression was significantly associated with worse survival (P = 0.004), which provide a rational for combination of immunotherapy in HCC. To date, there are two star drugs against PD-1/PD-L1 checkpoint (nivolumab and pembrolizumab), which have been applied clinically. Nevertheless, effective and safe drugs to recover HLA class I antigen remains under investigation, which may be a major target for future studies[29].
In conclusion, PD-L1 positive and HLA class I antigen low expression have no significant impact on prognosis when they were analyzed alone, only when they are analyzed together can they yield a significant synergistic impact on prognosis in patients with HCC. Such conclusion provide a combining immunotherapy strategy of inhibiting PD-1/PD-L1 and recovering HLA class I antigen for patients with HCC.