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Background <p>Hepatitis B virus (HBV) infection persists through immune evasion strategies, including engagement of the inhibitory receptor CD33 (Siglec-3) by α2,6-linked sialoglycans on HBsAg. This interaction induces ITIM phosphorylation and SHP-1/2 recruitment, dampening myeloid cell activation. The monoclonal antibody 10C8 has been identified as a potent antagonist of CD33, but the structural basis for its inhibitory activity remains unclear.</p> Methods <p>We determined the 3.2&#xa0;Å crystal structure of the human CD33 extracellular domain (CD33-ECD) in complex with the Fab fragment of 10C8 (Fab-10C8). The stoichiometry and assembly of the complex in solution were validated by size-exclusion chromatography and analytical ultracentrifugation (AUC). Structural comparisons with apo CD33 were performed to assess potential Fab-induced conformational changes, and buried surface area analyses were conducted to characterize binding interfaces.</p> Results <p>The structure reveals a 2:2 stoichiometry complex comprising two CD33-ECD molecules and two Fab-10C8 fragments, consistent with the 157&#xa0;kDa molecular mass determined by AUC. Each Fab engages the V-set domain through extensive complementarity-determining region (CDR) interactions, burying ~612&#xa0;Å<sup>2</sup> of surface area, while inter-Fab contacts stabilize a compact dimeric arrangement of CD33-ECD. This geometry differs markedly from the relaxed apo state, showing a ~21° reduction in the dimer angle. The antibody-binding epitope is adjacent to, but not overlapping with, the canonical sialic acid-binding cleft, leading to steric occlusion that prevents HBsAg engagement. Together with a potential Fab-induced ectodomain compaction, this conformation restricts CD33 clustering and likely precludes SHP-1/2 recruitment, providing a mechanistic explanation for the antagonistic activity of 10C8.</p> Conclusions <p>This study provides the structural insight into antibody-mediated inhibition of CD33. By locking CD33 into a sterically occluded, signaling-refractory conformation, 10C8 effectively reverses HBV-induced immunosuppression, thereby restoring host antiviral activity. These findings establish a structural framework for rational design of Siglec-targeted immunotherapies against chronic viral infections and other immune regulatory disorders.</p>

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Crystal structure of the CD33/Fab-10C8 complex elucidates the mechanism of antibody antagonism in HBV-induced immunosuppression

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年5月29日

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  • Yi-Hung Yeh,
  • Min-Guan Lin,
  • Pei-Shan Sung,
  • Shie-Liang Hsieh,
  • Chwan-Deng Hsiao

摘要

Background

Hepatitis B virus (HBV) infection persists through immune evasion strategies, including engagement of the inhibitory receptor CD33 (Siglec-3) by α2,6-linked sialoglycans on HBsAg. This interaction induces ITIM phosphorylation and SHP-1/2 recruitment, dampening myeloid cell activation. The monoclonal antibody 10C8 has been identified as a potent antagonist of CD33, but the structural basis for its inhibitory activity remains unclear.

Methods

We determined the 3.2 Å crystal structure of the human CD33 extracellular domain (CD33-ECD) in complex with the Fab fragment of 10C8 (Fab-10C8). The stoichiometry and assembly of the complex in solution were validated by size-exclusion chromatography and analytical ultracentrifugation (AUC). Structural comparisons with apo CD33 were performed to assess potential Fab-induced conformational changes, and buried surface area analyses were conducted to characterize binding interfaces.

Results

The structure reveals a 2:2 stoichiometry complex comprising two CD33-ECD molecules and two Fab-10C8 fragments, consistent with the 157 kDa molecular mass determined by AUC. Each Fab engages the V-set domain through extensive complementarity-determining region (CDR) interactions, burying ~612 Å2 of surface area, while inter-Fab contacts stabilize a compact dimeric arrangement of CD33-ECD. This geometry differs markedly from the relaxed apo state, showing a ~21° reduction in the dimer angle. The antibody-binding epitope is adjacent to, but not overlapping with, the canonical sialic acid-binding cleft, leading to steric occlusion that prevents HBsAg engagement. Together with a potential Fab-induced ectodomain compaction, this conformation restricts CD33 clustering and likely precludes SHP-1/2 recruitment, providing a mechanistic explanation for the antagonistic activity of 10C8.

Conclusions

This study provides the structural insight into antibody-mediated inhibition of CD33. By locking CD33 into a sterically occluded, signaling-refractory conformation, 10C8 effectively reverses HBV-induced immunosuppression, thereby restoring host antiviral activity. These findings establish a structural framework for rational design of Siglec-targeted immunotherapies against chronic viral infections and other immune regulatory disorders.