Structural and biochemical insights into HCV envelope proteins for germline-targeting vaccines.

Despite effective antivirals, Hepatitis C remains a global health burden, reflecting the difficulty of eliciting broadly neutralizing antibodies (bnAbs). For the Hepatitis C virus, bnAb responses are biased toward VH1-69-encoded Abs targeting a conserved epitope on the E2 glycoprotein, suggesting that effective immunogens must engage the corresponding germline (GL) B cell receptors. However, the structural basis of GL recognition remains unclear. Here, we show that GL-reverted VH1-69 bnAbs recognize the E2 core domain in a manner closely resembling mature Abs binding. High-resolution structures reveal a conserved mode of engagement, indicating that key epitope features are accessible prior to affinity maturation. Guided by these insights, we engineered E2 variants to enhance GL Ab interactions and stability. Although these designs increased affinity, they did not improve engagement of naïve B cells, highlighting a key constraint in GL-targeting strategies. Together, our findings define principles governing GL recognition and inform the rational design of an HCV vaccine.