Oskam, N.; Keijser, J.; Streutker, M.; Keijzer, S.; Ooijevaar-de Heer, P.; van Mierlo, G.; Derksen, N.; T2B! immunity against SARS-CoV-2 study group, ; Vidarsson, G.; Rispens, T.

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.