Antiviral immune responses in OBI are continuously stimulated by persistent/intermittent low concentrations of HBV antigens and cytokines can play an important role in controlling HBV replication . In this present study, the levels of cytokines in OBI patients were heterogeneous, showing dissemination over a wide range of values with high standard deviations. In the present study, the prevalence of OBI in patients with CHC was 9.6%. Previous studies analyzing serum samples of Brazilian HCV infected patients found frequencies of OBI ranging from 0 to 24% [25-31].
OBI patients usually have a low viral load with suppression of HBV replication and thus, most OBI patients have normal liver histology or minimal fibrosis. However, they are still at risk of developing liver cirrhosis. The prevalence of OBI in cirrhotic patients varies widely from 4 to 38% between different regions of the world [22,32].
The persistence of HBV DNA in hepatocytes triggers a constant immune response, inducing mild but continuous liver inflammation, which can accelerate liver damage and favor the development of liver cirrhosis in other chronic liver diseases, such as in patients with chronic hepatitis C [22,33-35]. OBI can contribute to the development of HCC under direct and indirect mechanisms similar to those of chronic HBV infection .
In addition, the presence of OBI is believed to have an adverse effect on the response to treatment in IFN-based therapies . A study that compared patients with chronic hepatitis C with HBV DNA positive and HBV DNA negative, the presence of OBI was associated with a decrease in the success of antiviral therapy .
In our study we found a significant increase in the detection of IL-2 in occult patients when compared to healthy controls, another study found overexpression of IL-2 exclusively in patients infected with OBI when compared to healthy individuals and patients who resolved HBV infection . IL-2 plays an important role in the efficient development of effector cytotoxic CD8 + T cells, effector cells with a high expression of receptors for IL-2 (IL-2R) are cells that cause direct damage to the liver .
Baskic et al., 2017  investigating the cytokine profile in chronic hepatitis C demonstrated that median levels of cytokines TNF-α, IL-2, and IL-17A were lower in patients with HCV than in controls. In our study, low levels of IL-17A were also found in OBI and HCV co-infected patients, however high levels of TNF-α and IL-2 were found when compared to controls. Some studies have already shown that IL-17A is positively regulated in chronic HBV-mediated inflammation and may be relevant for the development of liver cirrhosis and HCC [46-48]. IL-17A can also significantly stimulate monocytes and DCs to express their ligand (IL-17R) and produce pro-inflammatory cytokines such as IL-1β, TNF-α, IL-6, etc., which are important for liver damage during progression of chronic hepatitis B .
Other cytokines possibly involved in OBI include increased interleukin 10 (IL-10), IL-10 can leads to reduced expression of IL-12, stromal cell-derived factor (SDF)-1α, and C-C chemokine receptor (CCR), which leads to the interruption of T and natural killer cells (NK cell) activation and the recruitment of immune cells to the infected liver . Our study shows a significant difference in IL-10 levels in OBI patients compared to controls. Compared to healthy individuals, IL-10 production is also increased in patients with only chronic hepatitis C. The HCV RNA load is closely associated with IL-10 expressions, and inhibition of HCV replication was accompanied by a reduction in IL-10 . However, some studies have already demonstrated reduced levels of IL-10 in HIV patients co-infected with occult HBV compared to HIV patients co-infected with chronic HBV .
In vivo levels of IL-6 have been associated with plasma ALT levels and the degree of liver fibrosis in patients with HCC in HCV monoinfected patients . We found high levels of IL-6 in the occult group and it is well known that IL-6 can play two important roles in the pathogenesis of hepatitis B, can protect the liver from virus infections by stimulating immune responses against infected hepatocytes and can inhibit the HBV entry in hepatocytes up to 90% when cells are treated with IL-6 resulting in a marked reduction in cccDNA and HBsAg secretion . But the IL-6 can also play an important role in the induction of hepatitis, cirrhosis, and HCC [53,54].
A previous study conducted with HIV co-infected individuals and in non-coinfected patients, demonstrated a low detection rate of IL-6 in patients infected with OBI when compared to healthy individuals and patients who resolved HBV infection .
We found levels of IL-4 significantly increased in occult patients. IL-4 is a cytokine that can suppress the Th1-type response, maintaining persistent HBV replication and promoting immune tolerance [42,55,56]. A study conducted by Zhang et al., 2014  showed that patients with severe hepatitis C had higher levels of IL-4 compared to milder cases.
There were some limitations in the present study. The biggest limitation was the low number of patients with OBI, but the most of studies with OBI and cytokines analyses from 12 to 30 samples [39,43,58]; and the second limitation was that there are few studies on the cytokine production profile in patients with OBI. Therefore, our study can contribute to a better understanding of the complex response process related to cytokine production in HCV coinfection with OBI.