Increased PD-1 expression on T lymphocytes, including CD4 and CD8 T lymphocytes, is believed to lead to T cell exhaustion and is an important mechanism for tumor cell immune defense. In this case-control study, we investigated PD-1.5 and PD-1.9 variants and soluble PD-1 levels in HBV-infected patients and controls. We observed that there is a potential clinical significance of PD-1.5 / PD-1.9 polymorphisms and plasma sPD-1 levels in HBV infection.
Several genetic association studies have investigated the association of polymorphisms of the PDCD1 gene with infectious diseases. In particular, five single nucleotide polymorphisms of PD1: PD-1.1 (-538G/A), PD-1.3 (+ 7146G/A), PD-1.5 (+ 7785T/C), PD-1.6 (+ 8669G/A) and PD-1.9 (+ 7625C/T) have been associated with human malignancies and are highly expressed in several cancers 22,33−36. Furthermore, few studies have investigated the association of these PD-1 variants and susceptibility to HBV infection and liver disease progression28,37−40. In our current study, the PD-1.9 variant but not PD-1.5, is associated with risk of HBV infection and clinical outcome.
PD-1.9 is a non-synonymous variant that results in the amino acid substitution from valine to alanine at codon 215, which may lead to a change in structure or function of PD-1. Our study has shown that PD-1.9 TT genotype may be a risk factor for HBV infection as well as for disease progression, which is consistent with our previous findings30.Two of our earlier studies which utilized different HBV patient cohorts confirm the study conducted by Fang Li et al. showing that PD-1.9 may be involved in hepatocarcinogenesis in HBV infection29. However, no significant association between PD-1.9 variant and overall cancer susceptibility has been found in meta-analyses22,25,41.
The PD-1.5 SNP is located in the exon 5 region, the transition from C to T does not alter the amino acid sequence of PD-1. The significant associations between PD-1.5 and cancer are likely due to a linkage disequilibrium of the PD-1.5 variation with other PD-1 gene polymorphisms, which can lead to altered PD-1 expression levels21. Several meta-analyses have shown that the PD-1.5 TT genotype, as well as the T allele reduce the risk of cancers22,41,42. However, all studies did not include HCC patients. Although our current study did not show a significant association between this polymorphism and susceptibility to HBV infection, we reported for the first time that the PD-1.5 CT genotype may be a protective factor for HCC predisposition.
PD-1 has a soluble form (sPD-1) that can be detected in peripheral blood14,43. In our study, the circulating sPD-1 levels of HBV patients were significantly higher than those of the control group. The results were consistent with studies in HBV infections17,44,45, chronic HCV 46 and in acute or chronic inflammatory conditions such as pancreatitis47, sepsis 48, autoimmune hepatitis and inflammatory bowel disease49. These data suggest that increased sPD-1 levels are associated with inflammation and higher inflammation tends to lead to higher plasma sPD-1 levels. Additional data from our current study was the positive correlation between plasma sPD-1 levels and indicators of liver damage such as AST and ALT. Furthermore, previous studies have shown that sPD-1 levels are elevated in HBV patients in a manner that corresponds with inflammatory factors such as AST, ALT, IL-10, IL-17, TNF-α and IFN-γ17,44,50. The above evidence could explain the significantly higher levels of plasma sPD-1 in CHB patients in this study cohort.
FIB-4 and APRI indices are considered good predictors of liver fibrosis in chronic hepatitis C51 and CHB52. In the current study, we found for the first time a positive correlation between plasma sPD-1 levels and APRI scores, suggesting that sPD-1 could be a complementary marker for the diagnosis of fibrosis in patients with chronic hepatitis B infection. In addition, this marker has many advantages, such as being less invasive and monitoring the kinetics of fibrosis progression. This shall be a useful indicator in the prognosis of HBV related diseases and in the treatment of patients. Therefore, sPD-1 could be a useful marker for monitoring the progression of HBV-related liver disease. However, studies in a larger number of patients are needed to confirm and extend the results of this study.
The current study has limitations as the results would have been more meaningful if we could have determined the expression of PD-1 membrane-bound proteins in PBMC (peripheral blood mononuclear cells) as well as protein expression in liver tissue by immunohistochemical assays. However, in this study, neither liver tissue samples nor blood samples were available for isolation of PBMC.
In summary, this study showed that plasma sPD-1 levels correlate with liver inflammation and are associated with liver fibrosis in chronically HBV-infected patients. The PD-1.5 and PD1.9 polymorphisms are associated with HBV infection and progression of HBV-related liver disease.