Kalemera, Mphatso Dumisani; (2022) The interplay between HCV E1E2 glycoproteins and host receptors regulates entry efficiency and antibody evasion. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Structural and computational analyses indicate that neither E1 nor E2 (E1E2), the entry proteins of hepatitis C Virus (HCV), fall into the three classes of viral membrane fusion proteins described so far. Consequently, the detailed molecular mechanisms of HCV entry/fusion have remained elusive, so too have the structural features on E1E2 that may govern and antibody resistance. The hypervariable region1 (HVR-1) of E2 has been identified as a lynchpin for HCV humoral immune evasion and was assumed to shield the CD81 binding site from neutralising antibodies. Here, rather than acting as a simple shield, we demonstrate that HVR-1 possesses an autoregulatory function that suppresses the activity of E1E2 on free virions. This function of HVR-1 is achieved via its intrinsic conformational flexibility and high entropy. Significantly, our data suggest that interactions with SR-B1, another HCV host receptor, stabilise HVR-1, which turns off the safety catch mechanis and primes E1E2 for engagement to CD81, a prerequisite molecular event in HCV entry. As such, HVR-1 functions akin to a safety catch on E1E2 activity. Fittingly, mutations that reduce the entropy of HVR-1, and its genetic deletion, turn off the safety catch to generate hyperreactive HCV that exhibits enhanced entry. However, improvements in entry are offset by reduced thermostability and acute sensitivity to neutralising antibodies. Therefore, the HVR-1 safety catch determines the efficiency of virus entry and maintains resistance to antibodies. We also show in this work that interactions between E2 and CD81 are detrimental to the production of HCV pseudoparticles. Genetic ablation of the HCV receptor from producer cells enhanced or rescued the infectivity of most of patient clones tested without altering virion antigenic properties. We also saw improvement in SARS-CoV1/2 pseudoparticle infectivity following CD81 deletion, indicating CD81 also regulates pseudoparticle production through mechanisms other than binding viral glycoproteins.

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