**3.3 Clinical implication**

What are the implications of this findings? There is a fact that some viral strains can cross the barrier of the immune response and induce the over-production of IL-6. This condition correlated with the advancement of viral activity. This

condition consequently followed by an up-regulation in the production of IL-6, this polymorphisms in the region of the IL-6 gene stimulate the overexpression of IL-6 that also correlated with viral progression. This loop correlated with the increase of the viruses virulence that damaging Th1 cell polarization and functionality. This condition caused viremia and the loss of CD8 T-cells ability to develop memory cells, thus reducing the capacity to fight viral load. Constant replication of the virus fails to grow into long-out plasma cells, limiting their ability to clear the virus Prolonged RNA virus infections increase levels of IL-6, that further makes accumulation of this pathologies state (inflammation plus cytokines and cellular presence). This state may be advantageous for several RNA viruses, mainly because it offers some opportunities for near-future infections because there are new target cells to choose from. There were a debate on how is the inflammatory cytokine, IL-6, in viral infections can be used as a biomarker for prognosis. An exploration of the IL-6 function as well as that of IL-6 inhibition in treating persistent infections might aid in developing its therapeutic benefit may reveal information about its utility [8, 9].

The study from Saji et al. showed that IL-6 is important prognosis biomarker. The study from 102 patients with moderate to severe COVID-19 showed that the 30-day was significantly higher in patients with high IL-6 (> 49 pg./mL) and SARS-CoV-2 RNAaemia (> 1.5 copies/μL) compared to those with high IL-6 or RNAaemia or without high IL-6 and RNAaemia (88% vs. 22% or 8%, *log-rank test P* = 0.0097 or *P* < 0.0001, respectively) [17].

The other study in showed similar result. Patients with hypoxemia had significantly higher concentrations of IL-6, C-reactive protein, procalcitonin, fibrinogen, total bilirubin, aspartate aminotransferase and alanine aminotransferase at initial screening. ROC analyses identified IL-6 as the most robust predictor of hypoxemia. The concentration of IL-6 > 24pg./mL predicted the development of hypoxemia with the sensitivity of 100% and specificity of 88.9%. The positive and negative predictive values were 76.9, and 100% respectively [18].

The prognostic value of clinical severity also been showed. The study in the 140 COVID-19 patients, the levels of IL-6, CRP, and PCT increased in 95 (67.9%), 91 (65.0%), and 8 (5.7%) patients on admission, respectively. The proportion of patients with increased IL-6, CRP, and PCT levels was significantly higher in the severe patients than in the mild one. Cox proportional hazard model showed that IL-6 and CRP could be used as independent factors to predict the severity of COVID-19. Furthermore, patients with IL-6 > 32.1 pg./mL or CRP > 41.8 mg/L were more likely to have severe complications [19].

#### **3.4 IL-6 inhibitor as promising treatment**

IL-6 is produced by dendritic cells, macrophages, mast cells, and other innate immune cells. Several previous studies showed that IL-6 has long been considered a marker of inflammation. The increased levels of IL-6 significantly noted in number of diseases that related with ongoing inflammatory cell activation. The IL-6 can also be produced by non-immune cells such as epithelial cells, endothelial cells, keratinocytes, and fibroblasts among others, in response to specific stimuli [4].

The presence of IL-6 may not necessarily correlate with the production of other inflammatory cytokines and it may not be just a marker of ongoing inflammation, but a direct player in the immune response. A number of studies have shown a role of IL-6 in the adaptive immune response, primarily on the differentiation fate of CD4 T cells, but IL-6 can also modulate aspects of the innate immune response [17, 18].

Elevated levels of IL-6 in the lung and in serum have been found in patients infected with the influenza virus, including the 2009 H1N1 pandemic influenza [21, 22]. IL-6- and IL-6-mediated signals are essential for survival to a non-lethal dose of influenza H1N1 virus infection. Deficiency of IL-6 or IL-6R prevents clearance of the H1N1 virus in association with low numbers of neutrophils present in the lungs of infected individuals. We also show that IL-6 provides survival signals to protect neutrophils from influenza virus-triggered apoptosis. Impaired virus clearance caused by the lack of IL-6 or IL-6R signals leads to emphysema-like destruction of the lung and, ultimately, death [23, 24]. Thus, IL-6 is a protective factor against primary infection with the influenza H1N1 virus by promoting the innate phase of the immune response and virus clearance (**Figure 2**) [23, 25, 26].

With coronavirus disease 2019 (Covid-19), the role of localized inflammation was evident. Patients with severe symptoms has high interleukin-6, a cytokine produced by macrophages that induces a proinflammatory response and is often elevated in patients with Covid-19. Some studies showed the benefit of IL-6 inhibitors.

The REMACAP trials, which had an adaptive design, approximately 800 patients in need of respiratory or blood-pressure support or both were randomly assigned to placebo or a single injection of an interleukin-6 receptor blocker, tocilizumab or sarilumab. The primary outcome was a composite of in-hospital death and days free of respiratory or blood-pressure support to day 21. The group receiving an interleukin-6 receptor blocker had an in-hospital mortality of 27%, as compared with 36% in the control group, and those receiving the receptor blocker had a median of 10 to 11 organ support–free days, as compared with 0 days for control [27].

Conflicting result was shown in CONVACTA trial. This randomized, controlled trial include 452 patients with Covid-19 (oxygen saturation, ≤93%) were randomly assigned in a 2:1 ratio to receive one dose of tocilizumab or placebo. The primary outcome was clinical status at day 28; mortality was a secondary outcome. The group receiving tocilizumab had a median clinical status of 1 (discharged or ready for

#### **Figure 2.**

discharge), and the control group had a median clinical status of 2 (out of intensive care and not receiving supplemental oxygen). Mortality was 19.7% in the tocilizumab group and 19.4% in the control group [28].

The meta-analysis of 27 randomized trials of IL-6ra that included 10,930 patients with COVID-19indicate that all-cause mortality was reduced in patients hospitalized for COVID-19 and treated with IL-6ra compared with those treated with placebo or usual care. By day 28 after randomization, 1407 deaths occurred among 6449 patients randomized to receive IL-6 antagonists and 1158 deaths occurred among 4481 patients randomized to usual care or placebo (summary odds ratio [OR], 0.86 [95% CI, 0.79–0.95]; *P* = .003 based on a fixed-effects meta-analysis). Importantly, a significant mortality benefit was only found when IL-6 inhibitor were coadministered with glucocorticoids (summary OR for the association of IL-6 antagonist treatment with 28-day all-cause mortality, 0.78 with concomitant glucocorticoid administration vs. 1.09 without glucocorticoid administration). The benefits of IL-6 inhibitor were most evident among patients who received respiratory support with oxygen by nasal cannula, face mask, high-flow nasal oxygen (OR for death, 0.81 [95% CI, 0.67–0.98]), or noninvasive ventilation (OR, 0.83 [95% CI, 0.72–0.96]) vs. those who required invasive mechanical ventilation (IMV) (OR, 0.95 [95% CI, 0.78–1.16]) [29].

The review showed that, IL-6 receptor antagonist hold promise for patients hospitalized for COVID-19 with progressive disease and substantial oxygen requirements but are not yet merited for widespread use among patients with mild disease nor with prolonged invasive mechanical ventilation [30].

## **4. Conclusion**

There are evidences supporting a significant role of IL-6 during viral infections. IL-6 production that may be detrimental to the cellular immune response during viral infections. The change in IL-6 production during the immune response to viral infection are (i) the increased ability of some viral strains to overcome the immune response using a variety of evasion strategies, and consequently up-regulate the production of IL-6 as a result of increased viral loads, and (ii) polymorphisms in the IL-6 gene promoter stimulating overexpression of IL-6 during the immune response. The increased levels of IL-6 significantly related with ongoing inflammatory cell activation and poor prognosis.

*RNA Viruses Infection*
