Characterization of high affinity <scp>IgM</scp> and <scp>IgG</scp> monoclonal antibodies against norovirus variants <scp>GII</scp> .4 and <scp>GII</scp> .17

Human noroviruses are a leading cause of acute gastroenteritis worldwide, yet the molecular principles governing antibody recognition of their highly repetitive capsid remain poorly understood. Here, we immunized mice with virus-like particles (VLPs) from the pandemic GII.4 strain and the emergent GII.17 strain, generating monoclonal IgM and IgG antibodies via hybridoma technology. High-speed atomic force microscopy visualized IgM antibodies scanning and engaging multiple protruding (P) domains on intact VLPs. Surface plasmon resonance (SPR) analyses of engineered antibodies with identical Fab sequences but different valencies revealed that, unlike monovalent IgG, multivalent IgM exhibits dramatic affinity gains-up to 100-fold-as antigen density increases. This avidity-driven enhancement arises from the dense, repetitive P-domain architecture of the norovirus capsid, enabling IgM to achieve high functional affinity despite modest intrinsic Fab binding. Our findings define how antibody valency and epitope organization cooperate to boost viral recognition, offering a mechanistic framework for designing next-generation vaccines and antiviral antibodies that harness multivalent engagement.