Variations in IL-28B are associated with outcome of HCV infection,(12
) but the mechanistic links between the protective genotype and spontaneous outcome remain unknown. Prospective monthly follow-up of HCV-uninfected subjects who became acutely infected revealed (i) a strong correlation between IL-28B genotype and initial HCV RNA level during primary acute HCV infection (P
=0.00005), with the favorable IL-28B genotype (rs12979860-C homozygosity) correlated with higher initial viremia level, and (ii) a strong positive correlation between initial HCV RNA level and spontaneous clearance (P=0.00099). These findings are both counterintuitive and coherent with findings in other studies.
In this study, spontaneous resolution was more strongly predicted by initial HCV RNA level than by IL-28B genotype, with the former association reaching statistical significance even in this relatively small cohort. The association of protective IL28B genotype with high initial viremia resonates with recent findings from chronic infection that the protective IL-28B genotype is associated with higher (untreated) HCV RNA levels (12
) and lower intrahepatic ISG levels.(39
) Prior work demonstrated that lower baseline ISG expression predicts response to treatment.(40
) It is apparent that IL28B genotype predisposes toward a phenotype that is associated with clinical outcome, and that the association between phenotype and outcome is likely to be stronger than for genotype because other factors are likely to contribute.(15
) Taken together, the current and prior work suggest that the protective IL28B genotype is one factor that predisposes to high initial HCV RNA during acute infection and low baseline ISG during chronic infection, and that these represent measurable phenotypes in vivo that strongly predict the outcomes of interest, i.e. spontaneous resolution and treatment response, respectively. Our group recently assessed cytokine and chemokine levels in this cohort as potential markers of such a phenotype, but found no correlation between early levels of those factors and outcome or IL28B genotype.(42
These data may appear to differ somewhat from previous findings showing higher HCV RNA level is correlated with persistence of acute infection.(19
) Most studies investigating acute HCV infection have used either clinical symptoms, i.e. jaundice as well as other non-specific symptoms, or first clinical presentation/visit to identify acute infection.(19
) The current study demonstrates that the initial HCV RNA peak preceded the ALT peak by about 2 months, and the HCV RNA level dropped rapidly after the initial peak in clearance subjects. Hence, it is likely that using symptoms to identify study subjects would result in missing this early viremia peak in clearance subjects. In addition, patients presenting symptomatically might include persons with prior cleared HCV infection (which we excluded), and reinfection is associated with brief, low-level viremia.(32
The mechanisms linking IL-28B genotype, initial viremia level, viral evolution rate and outcome remain unknown. High-level HCV replication could trigger strong innate immune responses through pathways such as Toll like receptor 3 (TLR-3) (45
) and retinoic-acid-inducible gene I (RIG-I) (46
) and hence initiate strong adaptive immune responses that could eventually lead to eradication of the virus.(47
) Lower initial viremia may limit inflammation in a manner analogous to preliminary evidence suggesting that small HBV inocula can result in higher rates of persistence in chimpanzees (48
); in the current study, we could not assess inoculum size.
Accumulating data support a role for nAb responses in HCV control, though their role in spontaneous clearance remains unclear.(24
) HCV sequence evolution is shaped by selective pressures, i.e. immune pressures (positive selection) and intrinsic viral fitness constraints (negative selection), reflected in evolutionary patterns.(9
) We found that HVR1 was the only region with significantly different evolutionary rates between the two outcome groups and that these rates were significantly higher in clearance subjects than those in persistence subjects. The few sequence changes observed in HVR1 during the first year of persistent infection were convergent changes, consistent with reversion in the absence of immune pressure.(27
) In clearance subjects rapid sequence evolution in HVR1 was accompanied by evidence of strong nAb responses.(30
) Nonrandom evolution with respect to outcome suggests that pressure from nAb responses driving HVR1 evolution contribute to clearance of some viral variants.
In this study, we explored for the first time the potential linkage among IL-28B genotype, viral dynamics during early phase of HCV infection, early viral evolution patterns, and infection outcome. Detailed immunological results are not available because the inclusion criteria for this study were focused on studying viral evolution rather than the availability of large volume of blood draws.(6
) Nonetheless, our prospective sampling, stringent inclusion criteria, high resolution of early viral dynamics, and detailed analysis of hemigenomic clone sequences make this the largest and highest-resolution study of viral dynamics and evolution and their correlation with infection outcome and host genetics in humans during early phase of acute HCV infection to date.
In summary, we demonstrate for the first time that initially high HCV RNA level is predictive of spontaneous clearance, that IL-28B genotype is associated with initial HCV RNA level, and that initial early evolutionary patterns in HVR1 are correlated with infection outcome. These new links are not explained by conventional models of acute HCV infection.