RPL is a frequent disorder among women and shows a remarkable concern for reproductive problem cause approximately one in 300 pregnancies [1, 28]. Until now, while various criteria such as anatomic, genetic, endocrine, immunologic, and microbiologic have been acknowledged that affect miscarriage, the exact etiology of RPL cases in up to 50% is unknown [2, 3, 29]. The occurrence of RPL can be controlled by genetic-related factors such as genetic polymorphisms that have been connected with poor pregnancy. Some studies have led to the consciousness that cytokines were considered as key factors in the preservation of pregnancy by regulation the immune responses . It is also found that embryo loss can be caused by improved uterine expression of some pro-inflammatory cytokines like (tumor necrosis factor (TNF)-α, interferon-gamma (IFN-γ), and interleukin (IL)-1β) [18–20, 30]. In addition, anti-inflammatory cytokines such as IL-6 and IL-10 considered as crucial factors for maintaining a normal pregnancy. Among potential co-factors, functional SNPs in which alter the expression of IL-6 play a decisive role at various stages of RPL development. The main findings of this study were an important positive association between IL-6 SNP with RPL risk. The IL-6 -634C/G polymorphism is located at the 634 base pair upstream of the IL-6 gene promoter. It might be able to control IL-6 transcription or regulates posttranscriptional modification. Genetic predisposing factors may influence the likelihood of, sensitivity to or persistence of CMV infection, as well as the rate of RPL development [31, 32].
The literature review reveals that the large number of studies investigated the role of IL-6 in the risk of RPL, focused on this SNP, with little evidence of relation [21, 26, 33]. In Iran, several studies on the relationship among various cytokine polymorphism and RPL were reported. In one study, Torabi et al. reported that there are no substantial variations between RPL cases and controls, when they investigated the role of [-592A/C, -819C/T, -1082A/G (IL-10)], -174C/G (IL-6), and -197G/A (IL-17) in the risk of RPL . But, Nematollahi et al. revealed the efficacy of the -964 A/G polymorphism in IL27 gene in the RPL etiology, especially in Iranian populations, suggesting that IL-17F (rs 763780) can have a significant role in RPL susceptibility . Besides, several studies and investigations failed to discover the connection between the prevalent polymorphisms in the IL-6 SNP and RPL risk [23, 24, 32, 34].
However, our results present a substantial association between RPL risk and G allele in the IL-6 -634 among Iranian women. This is in line with Nasiri et al. study that indicated a remarkable association between allele G and the risk of RPL among Iranian population . We obtained favorable results from our cases, as the allele G is substantially linked to incidence of RPL in Iranian women. However, it is in contrast to Saijo et al.  and Ma et al.  who reported a relation between the decreased risk of RPL and allele G, in Japanese and Chines women, respectively. It is likely that the variation of results among these studies were attributed to ethnic differences or patients enrollment criteria impact. Besides, based on several studies we suggested that this polymorphism (IL-6 -634C/G) has different affect in RPL among different populations.
In this study, we also indicated that the IL-6 -174 G/C polymorphism has no association with RPL risk. This is in line with other studies. To evaluate the effect of the IL-6 174 on incidence of RPL, Daher et al. reviewed all five research reported in Europe . In none of the studies, no variations in IL-6 genotype frequencies were found between controls and RPL patients, even in Japanese women not a allele was identifies neither in control nor in the patients . Since the race had a significant impact on findings, they could not be applied to all people, because they were not comprehensive. In comparison to other populations, the Caucasian Northern Ireland people is the only one with a lower frequency of IL-6 174 allele . One possible scientific reason that can explain these findings could be that -634C/G polymorphism is more effective than -174 G/C on IL-6 gene expression and translation. As a result, -634C/G polymorphism conceivably contributing to RPL risk among Iranian women.
On the other hand, the cytokine IL-6 has a protective role in immune responses against infections that caused by different bacteria and viruses. To cause an inflammatory reaction, pathogenic agents need access the intrauterine surroundings of the fetal tissues or endometrium, which can result in miscarriage [3, 39]. Prasad et al. investigated the role of Th1/Th2/Th17 cytokines in the immunopathogenesis process of miscarriage in Chlamydia trachomatis (Ct)-positive first-trimester aborters [19, 40]. They found that some cytokines such as IL-6, IFN-γ, IL-2, TNF-α, and IL-17A were noticeably increased in spontaneous abortion (SA) group and recurrent miscarriage(RM) group (Ct-infected) versus controls. Moreover, TNF-α, IL-17A, IL-6, and IFN-γ cytokines were considerably elevated in Ct-positive RM group compared to Chlamydia-infected SA group. Williams et al. demonstrated the TNF-alpha was produced in vivo during C. trachomatis infection and has a prime role in host defense .
In this study, our findings indicated a remarkable association between RPL with CMV positive patients and allele G in IL-6 -634 among Iranian women. Besides, the individuals with allele G in IL-6 -634 and CMV infection have greater risk of RPL in comparison with C allele. These findings are compatible with Edmunds et al. study, reported that the anti HCMV-IgM was identified in 94% of the Iranian women with the abortion background [29, 42]. Also, in the other study which conducted by Janan et al. on Iranian women with spontaneous abortion, results indicated a remarkable variation in the frequency of anti-Rubella, Cytomegalovirus, and Toxoplasma antibodies between RPL cases and healthy women . They detected that the frequency of antibodies against Cytomegalovirus (CMV-IgM and CMV-IgG) in the patient group was noticeably higher than in the control group, and found that the high titer of CMV-IgM and CMV-IgG play the remarkable role in RPL . In addition, Ebadi et al. discovered an important association between abortion and CMV infection in Iranian population . According to these results we suggested that allele G in IL-6 -634 has both directly and indirectly effects on RPL in Iranian women. Directly, it can significantly increase risk of RPL, and indirectly it can increase CMV infection, which in turn might play effective roles in the incidence of RPL among Iranian women.