Mitogen-activated protein kinase eight polymorphisms are associated with immune responsiveness to HBV vaccinations in infants of HBsAg(+)/HBeAg(−) mothers

Hepatitis B virus (HBV) infection is a global health problem with 780,000 related deaths annually [1]. Mother-to-child transmission (MTCT) of HBV remains a major cause of chronic HBV infection. The most effective way to avoid MTCT is by carrying out a standardized and sufficient hepatitis B immunization program in newborns [2, 3]. In China, from 1992 to 2006, the prevalence of hepatitis B surface antigen (HBsAg) has been reduced by 90% among children under 5 years old, as a result of an HBV immunoprophylaxis program [4]. However, approximately 6–20% of infants born to HBsAg(+) mothers exhibited a non- or low- response to the HBV vaccine, which led to increased risk of horizontal transmission of HBV infection [5‐8]. Vaccine type, low birth weight and high maternal viral load have been identified as the most important risk factors for low immune response to HBV vaccine [9‐11]. In addition, host genetic background also plays an important role in determining the strength of immune response to vaccination.

Several genome-wide association studies (GWAS) have identified variants in human leukocyte antigen (HLA) region associated with HBV infection and with responsiveness to HBV vaccination due to their important role of immune regulation [12‐14]. However, as the genetic variation in the HLA region can only explain the 50% of the susceptibility to low vaccination response, other genetic variants outside this complex also need to be investigated [15]. Tumor necrosis factor (TNF) is a well-characterized pro-inflammatory cytokine that plays a central role through direct immunomodulatory actions. It is also known as an activator of the MAPK and NF-κB signaling pathways, leading to the up-regulation of the immune response [16, 17]. In addition, single nucleotide polymorphism (SNPs) in TNF and MAPK8 have been reported to correlate with peak anti-HBs level in Whites [18, 19]. However, the correlations between these SNPs and immune response to vaccination in a Chinese population remain uncertain.

To better explore the effects of TNF and MAPK polymorphisms on the response to hepatitis B vaccination, we conducted a case-control study in our MTCT cohort. All of the participants were infants of HBsAg(+)/HBeAg(−) mothers. These infants had an increased risk of HBV infection due to vertical and horizontal transmission. We selected tag SNPs of TNF and MAPK and genotyped these SNPs using the MassARRAY platform. We then investigated the association between our tag SNPs and low response to hepatitis B vaccination, as well as to Antibody (Ab) titers.

Antibody responses in neonates are usually lower and differ from older children or adults [20, 21]. As a result, infants might have a lower response to HBV vaccination. Meanwhile, infants born to HBsAg(+) mothers have an increased risk of HBV infection compared to infants born to healthy mothers due to a persistent exposure to HBV from their mothers. It is therefore important to investigate the genetic risk factors for low response to hepatitis B vaccination and to improve the antibody level for these infants at high risk for HBV exposure.

The MAPK pathway is activated in innate immune cells in response to microbial infection [22]. MAPK8, also known as JNK1, may promote the phosphorylation of transcription factors following up-regulation in the expression of pro-inflammatory cytokines and chemokines. MAPK8 is mainly involved in the differentiation of helper T cells into Th1 cells during the antiviral immune response. As a result, different variants of the MAPK8 gene could affect the Th1 mediated cellular immune response and ultimately affect the production of vaccine-induced antibodies [23]. Our study found that the MAPK8 rs17780725 AA genotype may increase the risk of low response to hepatitis B vaccines. Rs17780725 is in strong LD with rs2698768 (r² = 0.94), which is in the transcription factor binding site (TFBS) of MAPK8. This means that the replacement of G by A in rs17780725 might decrease the binding power between transcription factors and MAPK8, leading to down-regulation of MAPK8 transcription and the MAPK8 protein, which could ultimately down-regulate the immune response to HBV vaccination. In fact, our results also show that subjects with the rs17780725 AA genotype had a lower level of Ab titers.

TNF is a pro-inflammatory cytokine that regulates innate immunity. When a vaccine-induced immune response occurs, TNF-α may increase the expression level of certain cytokines, leading to up-regulation of immune reaction [18]. Meanwhile, when HBV infection occurs, HBV-specific CD8⁺ T cells also secrete TNF-α in the body in order to clear HBV [24]. Dhiman N et al. has found the TNF rs1799964 CC genotype to be associated with a lower measles antibody response in a Somali population [25]. Berran Yucesoy et al. [18] also have shown the TNF rs1800629 AA genotype to be correlated with a lower antibody level after HBV vaccination in a non-Hispanic white childhood cohort. However, in our study, the SNPs in TNF were not found to be associated with low response to hepatitis B vaccination. Different ethnic groups may have different genetic backgrounds. For example, the MAF of rs1800629 in Han Chinese is quite different than that found in non-Hispanic whites (0.05 vs 0.11). Additionally, there were only 41 low responders in our study. The limited number of low responders may have limited statistical power. Thus, a similar study with larger sample size study should be conducted in the future.

Our study has some additional limitations. First, we only enrolled the infants of HBsAg(+)/HBeAg(−) mothers from our MTCT cohort. Infants born to HBsAg(+)/HBeAg(+) mothers were given injections of 20 μg hepatitis B vaccine at 0, 1 and 6 months of age in our cohort, and only 7 low responders were found among these infants. Although the dose of vaccine was the most important factor affecting the immune response to HBV vaccination, too few infants with a low response would limit statistical power. As a result, we only included the infants of HBsAg(+)/HBeAg(−) mothers in this study. Secondly, antibody levels decreased gradually during the follow-up period. For non-responders, a booster immunization still needs to be administered. Therefore, future studies should include a longer follow up time and an analysis of the association between SNPs and the administration of a booster dose.