Development of anti-human IgM nanobodies as universal reagents for general immunodiagnostics

Nanobodies are the smallest antibody fragments which bind to antigens with high affinity and specificity. Due to their outstanding physicochemical stability, simplicity and cost-effective production, nanobodies have become powerful agents in therapeutic and diagnostic applications. In this work, the advantages of nanobodies were exploited to develop generic and standardized anti-human IgM reagents for serology and IgM⁺ B-cell analysis. Selection of anti-IgM nanobodies was carried out by evaluating their yields, stability, binding kinetics and cross-reactivity with other Ig isotypes. High affinity nanobodies were selected with dissociation constants (KDs) in the nM range and high sensitivities for detection of total IgM by ELISA. The nanobodies also proved to be useful for capturing IgM in the serodiagnosis of an acute infection as demonstrated by detection of specific IgM in sera of dengue virus patients. Finally, due to the lack of an Fc region, the selected nanobodies do not require Fc receptor blocking steps, facilitating the immunophenotyping of IgM⁺ cells by flow cytometry, an important means of diagnosis of immunodeficiencies and B-cell lymphoproliferative disorders. This work describes versatile anti-IgM nanobodies that, due to their recombinant nature and ease of reproduction at low cost, may represent an advantageous alternative to conventional anti-IgM antibodies in research and diagnosis.