Treatment failure in hepatitis C: Mechanisms of non-response

Recognition of HCV RNA by RIG-I and TLR3 and activation of type I interferon transcription. The event that is believed to trigger the innate immune response to HCV infection is the recognition of one or more pathogen-associated molecular patterns encoded in the HCV RNA. TLR3 recognizes double-stranded RNA (dsRNA) within endosomes and in turn activates the protein kinase TBK1 through the adaptor protein TRIF. RIG-I preferentially recognizes single-stranded RNA with a terminal 5′-triphosphate (ppp) over dsRNA. RIG-I activates the protein kinase IKKε through the Cardif adaptor protein. Both TBK1 and IKKε phosphorylate the transcription factor IRF-3, which leads to its homodimerization, nuclear translocation, and stimulation of interferon-β gene transcription. The HCV nonstructural protein NS3-4A specifically cleaves TRIF and Cardif, thus blocking the innate immune recognition of HCV infection.

Classification of interferon response and non-response. This plot depicts three different possible responses of the HCV serum RNA titer to interferon-based therapy in a hypothetical patient with genotype 1 HCV infection. A null response or non-response (black squares) is conventionally defined by a failure of the HCV RNA titer to fall by at least 2log₁₀ by 12 weeks of treatment. The most commonly used definition of a slow response (grey triangles) is a fall of the HCV RNA by at least 2log₁₀ by 12 weeks of treatment but detectable viremia that then clears by week 24. A rapid response (white circles) indicates that the HCV RNA is undetectable at 4 weeks of treatment by the most sensitive available test (typically quantitative RT-PCR based methods) with a typical threshold of 10IU/mL. Rapid response is highly predictive of an eventual sustained virologic response (SVR) at 24 weeks after the end of therapy.