Ooijevaar-de Heer, P. | Search bioRxiv

bioRxiv2026

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.

immunology↗

bioRxiv2023

CD5L is a canonical component of circulatory IgM

Immunoglobulin M (IgM) is an evolutionary conserved key component of humoral immunity, and the first antibody isotype to emerge during an immune response. IgM is a large (1 MDa), multimeric protein, for which both hexameric and pentameric structures have been described, the latter additionally containing a joining (J) chain. Using a combination of single-particle mass spectrometry and mass photometry, proteomics and immunochemical assays, we here demonstrate that circulatory (serum) IgM exclusively exists as a complex of J-chain-containing pentamers covalently bound to the small CD5 antigen-like (CD5L, 36 kDa) protein. In sharp contrast, secretory IgM in saliva and milk is principally devoid of CD5L. Unlike IgM itself, CD5L is not produced by B cells, implying that it associates with IgM in the extracellular space. We demonstrate that CD5L integration has functional implications, i.e., it diminishes IgM binding to two of its receptors, the Fc{micro}R and the polymeric Immunoglobulin receptor (pIgR). On the other hand, binding to Fc{micro}R as well as complement activation via C1q seem unaffected by CD5L integration. Taken together, we redefine the composition of circulatory IgM as a J-chain containing pentamer, always in complex with CD5L.

molecular biology↗