Background

Viral hepatitis refers to a type of infectious disorder that is caused by hepatitis viruses which include HAV, HBV, HCV, HDV and HEV [1, 2]. In addition to acute liver injury, these hepatitis viruses may also lead to life-threatening conditions such as liver cirrhosis or hepatocellular carcinoma (HCC) [3, 4]. The clinical course of viral hepatitis is resulted from a complex interaction between pathogen, host and environmental factors, some patients may be asymptomatic the whole life, but some patients may eventually develop liver cirrhosis or even HCC [5, 6]. Therefore, there is no doubt that individual anti-viral immunity is vital for the onset and development of viral hepatitis.

Interleukin-10 (IL-10) serves as one of the most important anti-inflammatory and immunosuppressive factor, and it plays a crucial role in regulating anti-viral immune responses [7,8,9]. Considering the immune-regulatory effects of IL-10, over the last decade, investigators all over the world have repeatedly attempted to explore the relationships between polymorphisms in IL-10 gene and the risk of viral hepatitis, yet the relationships between these polymorphisms and the risk of viral hepatitis are still inconclusive. So a meta-analysis was conducted to robustly analyze the relationships between polymorphisms in IL-10 gene and the risk of viral hepatitis by integrating the results of previous works.

Methods

The PRISMA guideline was strictly followed by the authors when designing and implementing this study [10].

Literature search and inclusion criteria

Medline, Embase, Wanfang, VIP and CNKI were throughly searched by the authors with the below terms: (Interleukin-10 OR IL-10 OR Interleukin 10 OR IL 10) AND (Polymorphism OR Polymorphic OR Variation OR Variant OR Mutant OR Mutation OR SNP OR Genotypic OR Genotype OR Allelic OR Allele) AND (Viral hepatitis OR Chronic hepatitis OR Acute hepatitis OR Hepatitis A OR Hepatitis B OR Hepatitis C OR Hepatitis D OR Hepatitis E OR HAV OR HBV OR HCV OR HDV OR HEV). Moreover, we also manually screened the reference lists of retrieved publications to make up for the potential incompleteness of electronic literature searching.

Selection criteria of this meta-analysis were listed below: (1) Studies of case–control or cohort design; (2) Give genotypic or allelic frequencies of IL-10 polymorphisms in cases with viral hepatitis and population-based controls; (3) The full manuscript with required genotypic or allelic frequencies of IL-10 polymorphisms is retrievable or buyable. Articles would be excluded if one of the following three criteria is satisfied: (1) Studies without complete data about genotypic or allelic frequencies of IL-10 polymorphisms in cases with viral hepatitis and population-based controls; (2) Narrative or systematic reviews, meta-analysis or comments; (3) Case series of subjects with viral hepatitis only. If duplicate publications were retrieved from literature search, we would only include the most complete one for integrated analyses.

Data extraction and quality assessment

The authors extracted the following data items from eligible studies: (1) Last name of the leading author; (2) Publication year; (3) Country and ethnicity of study population; (4) The number of cases with viral hepatitis and population-based controls; (5) Genotypic frequencies of IL-10 polymorphisms in cases with viral hepatitis and population-based controls. Hardy–Weinberg equilibrium was then tested by using genotypic frequencies of IL-10 polymorphisms, and the threshold of derivation from HWE was set at 0.05. The quality of eligible publications was assessed by the Newcastle–Ottawa scale (NOS) [11], and those with scores of 7–9 were considered to be publications of good quality. Two authors extracted data and assessed quality of eligible publications in parallel. A thorough discussion until a consensus is reached would be endorsed in case of any discrepancy between two authors.

Statistical analyses

All statistical analyses in this meta-analysis were performed by using the Cochrane Review Manager software. Relationships between IL-10 gene polymorphisms and the risk of viral hepatitis were explored by using odds ratio and its 95% confidence interval. The statistically significant p value was set at 0.05. The authors used I2 statistics to evaluate heterogeneities among included studies. The authors would use DerSimonian–Laird method, which is also known as the random effect model, to integrate the results of eligible studies if I2 is larger than 50%. Otherwise, the authors would use Mantel–Haenszel method, which is also known as the fixed effect model, to integrate the results of eligible studies. Meanwhile, the authors also conduct subgroup analyses by ethnic groups and disease subtypes. Stabilities of integrated results were tested by deleting one eligible study each time, and then integrating the results of the rest of eligible studies. Publication biases were evaluated by assessing symmetry of funnel plots.

Results

Characteristics of included studies

Three hundred and seventy-four literatures were retrieved by the authors by using our searching strategy. One hundred and thirty-nine literatures were then selected to screen for eligibility after omitting unrelated and repeated items. Six reviews and 48 case series were further excluded, and another nine literatures without all necessary genotypic or allelic data were further excluded by the authors. Totally 76 studies met the inclusion criteria, and were finally enrolled for integrated analyses (Fig. 1). Data extracted from eligible studies were summarized in Table 1 (Additional file 1).

Fig. 1
figure 1

Flowchart of study selection for this meta-analysis. Systematic literature search of the present meta-analysis

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Table 1 The characteristics of included studies
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Integrated analyses for rs1800871 polymorphism and the risk of viral hepatitis

Thirty-seven eligible literatures assess the relationship between rs1800871 polymorphism and the risk of viral hepatitis. The integrated analyses demonstrated that rs1800871 polymorphism was significantly associated with the risk of viral hepatitis in overall population (dominant comparison: OR = 0.89, p = 0.002; recessive comparison: OR = 1.21, p = 0.004; allele comparison: OR = 0.90, p = 0.0004) and Asians (dominant comparison: OR = 0.84, p = 0.0001; over-dominant comparison: OR = 1.14, p = 0.005; allele comparison: OR = 0.88, p = 0.0002), but not in Caucasians. Further analyses by disease subtypes revealed similar positive results for rs1800871 polymorphism in both HBV and HCV subgroups (see Table 2).

Table 2 Meta-analyses results of IL-10 gene polymorphisms and viral hepatitis
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Integrated analyses for rs1800872 polymorphism and the risk of viral hepatitis

Fifty-eight eligible literatures assessed the relationship between rs1800872 polymorphism and the risk of viral hepatitis. The integrated analyses demonstrated that rs1800872 polymorphism was significantly associated with the risk of viral hepatitis in overall population (dominant comparison: OR = 0.91, p = 0.003; allele comparison: OR = 0.93, p = 0.003) and Asians (dominant comparison: OR = 0.89, p = 0.0009; over-dominant comparison: OR = 1.10, p = 0.007; allele comparison: OR = 0.93, p = 0.004), but not in Caucasians. Further analyses by disease subtypes revealed similar positive results for rs1800871 polymorphism in both HBV and HCV subgroups (see Table 2).

Integrated analyses for rs1800896 polymorphism and the risk of viral hepatitis

Fifty-nine eligible literatures assessed the relationship between rs1800896 polymorphism and the risk of viral hepatitis. The integrated analyses demonstrated that rs1800896 polymorphism was significantly associated with the risk of viral hepatitis in overall population (dominant comparison: OR = 0.87, p = 0.02; recessive comparison: OR = 1.60, p < 0.0001; allele comparison: OR = 0.83, p < 0.0001), Asians (dominant comparison: OR = 0.88, p = 0.02) and Caucasians (recessive comparison: OR = 1.67, p = 0.009; allele comparison: OR = 0.78, p = 0.03). Further analyses by disease subtypes revealed similar positive results for rs1800871 polymorphism in both HBV and HCV subgroups (see Table 2).

Sensitivity analyses

The authors examined stabilities of integrated analyses results by deleting studies that violated HWE, and then integrating the results of the rest of studies. The trends of associations were not significantly altered in sensitivity analyses, which indicated that from statistical perspective, our integrated analyses results were reliable and stable.

Publication biases

The authors examined potential publication biases in this meta-analysis by assessing symmetry of funnel plots. Funnel plots were found to be overall symmetrical, which indicated that our integrated analyses results were not likely to be severely deteriorated by publication biases.

Discussion

This meta-analysis, for the first time, robustly assessed associations between polymorphisms in IL-10 gene and the risk of viral hepatitis. The integrated analyses results demonstrated that rs1800871 (− 819 C/T), rs1800872 (− 592 C/A) and rs1800896 (− 1082 G/A) polymorphisms were all significantly associated with the risk of viral hepatitis in Asians, whereas only rs1800896 (− 1082 G/A) polymorphism was significantly associated with the risk of viral hepatitis in Caucasians. In further analyses by disease subtypes, we noticed that the three investigated polymorphisms were all significantly associated with the risk of both HBV and HCV.

The following three points should be considered when interpreting our integrated findings. First, based on the findings of previous observational studies, it is believed that the three investigated IL-10 polymorphisms may alter mRNA expression level of IL-10 gene, impact anti-viral immune responses, and then influence the risk of viral hepatitis [12, 13]. Nevertheless, it should be noted that future experimental studies are still required to reveal the exact molecular mechanisms underlying the observed positive findings of this meta-analysis. Second, we wish to study all polymorphic loci of IL-10 gene. However, our comprehensive literature searching did not reveal sufficient eligible literatures to warrant integrated analyses for other polymorphic loci of IL-10 gene, so we only assessed associations with the risk of viral hepatitis for the three most commonly investigated polymorphisms of IL-10 gene in this meta-analysis. Third, although we aimed to investigate all subtypes of viral hepatitis in this meta-analysis, it is worth noting that the majority of eligible studies were about HBV or HCV. So future studies should continue to explore associations between polymorphisms in IL-10 gene and the risk of other subtypes of viral hepatitis.

The three major limitations of our integrated analyses were listed below. Firstly, our integrated analyses results were only derived from unadjusted pooling of previous works. Without access to raw data of eligible studies, we can only estimate associations based on re-calculations of raw genotypic frequencies, but we have to admit that lack of further adjustment for baseline characteristics may certainly impact reliability of our findings [14]. Secondly, environmental factors may also affect relationships between polymorphisms in IL-10 gene and the risk of viral hepatitis. However, most of the authors only paid attention to genetic associations in their publications, so it is impossible for us to explore genetic-environmental interactions in a meta-analysis based on these previous publications [15]. Thirdly, we did not enroll grey literatures for integrated analyses because these literatures are always incomplete and it is impossible for us to extract all required data items from these literatures or assess their quality through the NOS scale. Nevertheless, considering that we did not include grey literatures for integrated analyses, despite that funnel plots were found to be overall symmetrical, it should be acknowledged that publication biases still may affect the robustness of our integrated analyses results [16].

Conclusion

In conclusion, this meta-analysis demonstrates that rs1800871 (− 819 C/T), rs1800872 (− 592 C/A) and rs1800896 (− 1082 G/A) polymorphisms may influence the risk of viral hepatitis in Asians, while only rs1800896 (− 1082 G/A) polymorphism may influence the risk of viral hepatitis in Caucasians. In further analyses by disease subtypes, we noticed that the three investigated polymorphisms may influence the risk of both HBV and HCV. However, future studies should continue to investigate associations between polymorphisms in IL-10 gene and the risk of other subtypes of viral hepatitis.