Assessment of polymeric immunoglobulin A and M via detection of the joining chain

Polymeric immunoglobulin M (IgM) and A (IgA) play key roles in systemic and mucosal immunity, yet quantitative assessment of their polymeric forms has been hampered by the lack of robust, high-throughput assays. Polymerization of both isotypes implies incorporation of the joining chain (J chain), making direct detection of integrated J chain an attractive surrogate marker. Here, we report the generation and characterization of a novel panel of monoclonal antibodies targeting human J chain. Binding analyses revealed distinct antibody clusters with differential preferences for IgA-J and/or IgM-J. We developed sensitive ELISAs that allow reliable quantification of J-chain- containing IgM and IgA in recombinant preparations and complex biological samples such as serum and saliva. For IgM, assay performance in serum required mild dissociation of the IgM- CD5L complex, enabling accurate detection of integrated J chain. For IgA, clone 9G10 showed remarkable specificity for IgA-J, with minimal cross-reactivity to IgM. Application of these assays demonstrates that on average, 10% of circulating IgA is J-chain-containing, with proportional contributions of IgA1 and IgA2, and enables high-throughput measurement of antigen-specific polymeric IgA responses, exemplified by SARS-CoV-2 vaccination. These tools provide a long-needed platform to study polymeric antibody dynamics in health, infection, vaccination, and B-cell-driven diseases.