antibodies Review Structure, Function, and Therapeutic Use of IgM Antibodies Bruce A. Keyt *, Ramesh Baliga, Angus M. Sinclair, Stephen F. Carroll and Marvin S. Peterson IGM Biosciences Inc, 325 East Middlefield Road, Mountain View, CA 94043, USA;
[email protected] (R.B.);
[email protected] (A.M.S.);
[email protected] (S.F.C.);
[email protected] (M.S.P.) * Correspondence:
[email protected]; Tel.: +1-650-265-6458 Received: 16 September 2020; Accepted: 9 October 2020; Published: 13 October 2020 Abstract: Natural immunoglobulin M (IgM) antibodies are pentameric or hexameric macro- immunoglobulins and have been highly conserved during evolution. IgMs are initially expressed during B cell ontogeny and are the first antibodies secreted following exposure to foreign antigens. The IgM multimer has either 10 (pentamer) or 12 (hexamer) antigen binding domains consisting of paired µ heavy chains with four constant domains, each with a single variable domain, paired with a corresponding light chain. Although the antigen binding affinities of natural IgM antibodies are typically lower than IgG, their polyvalency allows for high avidity binding and efficient engagement of complement to induce complement-dependent cell lysis. The high avidity of IgM antibodies renders them particularly efficient at binding antigens present at low levels, and non-protein antigens, for example, carbohydrates or lipids present on microbial surfaces. Pentameric IgM antibodies also contain a joining (J) chain that stabilizes the pentameric structure and enables binding to several receptors. One such receptor, the polymeric immunoglobulin receptor (pIgR), is responsible for transcytosis from the vasculature to the mucosal surfaces of the lung and gastrointestinal tract.