In this ethnically and racially diverse cohort of difficult-to-treat HCV-infected patients, we found that participants with the C/C genotype had a significantly steeper second-phase decline corresponding to infected cell death rate. Participants with C/C genotype also had a more rapid first-phase decline compared with those with T/T genotype. Although not statistically significant, there was a trend toward European ancestry and an association with the efficacy of suppressing the production of new virions, even after accounting for the IL28 genotype.
Infected hepatocytes can decay from a variety of causes, including natural cell death, immune-mediated decay, and noncytolytic cure. Without histological data during therapy, it is difficult to ascertain the etiology of the infected hepatocyte decay. However, we believe that an immune-mediate mechanism is responsible, based on several studies. Preliminary evidence suggests that the rs12979860 polymorphism is very close to the IL28B gene [9
], perhaps in the promoter or enhancer region of the gene. The polymorphism is also associated with natural clearance, suggesting that innate immunity is engaged [10
]. The IL28B gene produces interferon lambda, a recently discovered type III interferon, which is expressed after viral infection of the liver and signals through a similar janus kinase–signal transducer and activator of transcription (JAK-STAT) pathway as interferon-α [15
]. The JAK-STAT pathway activates interferon-sensitizing genes (ISG) such as 2’,5’-oligoadenylate synthetase (OAS) and mitogen-activated protein (MAP) kinases, which are known to cause apoptosis, growth inhibition, and viral replication [18
Interestingly, Marcello et al found that HCV subgenomic RNA replication in vitro treated with IFN-λ showed a steady increase in ISGs, whereas those HCV-infected cell cultures treated with IFN-α showed a rapid peak and then decline in ISGs [19
]. Furthermore, IFN-λ can act as an “antiviral boost” to IFN-α, by enhancing the antiviral effect [20
]. In light of the current findings and these in vitro results, we hypothesize that patients of European ancestry are inherently more sensitive to IFN-α, as reflected in the trend toward improved efficacy in suppressing virus production (ϵ), but that participants of non-European ancestry are able to still achieve viral clearance through activation of the backup IFN-λ pathway.
The results are similar to previous studies that have found that δ (second phase) was the best predictor of antiviral response and that δ is lower in African-Americans [21
]. Layden-Almer compared the viral kinetics in African-Americans and found that they had a lower ϵ and δ, compared with those of white participants [21
]. Our data suggest that the rs12979860 polymorphism explains this difference in δ. However, there may be other racial factors that are associated with a difference in ϵ. Of note, the T allele occurs more frequently in African-Americans, but the frequency explains only 50% of the variability in response, when compared with those participants with European ancestry [9
]. Thompson et al similarly found that patients with European ancestry were more likely than African-Americans to have more rapid kinetics, even in the absence of the C/C genotype [11
]. A recent study identified 4 other gene regions associated with natural clearance (TNFSF18, TANK, HAVCR1, and IL18BP); these regions may show differential expression in various races undergoing therapy [24
]. In a study of Europeans taking peginterferon or standard interferon, the second-phase decay rate was most predictive of SVR [23
]. Finally, in a multinational study of high dose induction with interferon, the infected cell decay rate was again the strongest predictor of viral cure [22
There are several unique strengths to this study. First, our cohort was heterogenous racially and ethnically, and included 3 HIV-positive patients, which makes the results more generalizable, especially to the American population. Second, we did not exclude any data in our modeling. Finally, the statistical approach to parameter estimation was rigorous, and it allows for evaluation of a diverse population of study participants. Many researchers evaluating viral dynamics in a population of patients fit the data to patients one at a time and conduct statistical analysis on the estimated dynamics parameters (this method is often referred to as the 2-stage approach). In this work, data from the aggregate population was used with nonlinear regression analysis to simultaneously estimate and compare parameters by subgroups of interest (eg, IL28 genotype). The use of regression methods with the aggregate data increases power to detect differences in model parameters and allows for adjusted analysis in our diverse population. Thus, we were able to estimate the effect of the IL28 genotype adjusted for patient ancestry. Our analysis demonstrates that genotype and ancestry must be considered simultaneously, something that is possible only when aggregate data and regression methods are used. A weakness of the study was the assumption that there is constant effectiveness of antiviral therapy. Some authors have questioned the validity of this assumption [25
In summary, the presence of the C/C genotype at IL28B rs12979860 exerts its antiviral effect by increasing the infected hepatocyte death rate. This suggests that an immune-mediated mechanism is responsible. Functional studies of the polymorphism will more clearly elucidate the mechanism of antiviral response.