Varied redox forms of teleost IgM: an alternative to isotypic diversity?

Teleosts (bony fish) are thought to primarily or exclusively possess a single structural form of immunoglobulin (Ig), a tetrameric IgM. However, in species wherein intact Ig has been electrophoretically analyzed under denaturing, non-reducing conditions, a significant degree of structural diversity has been revealed. This IgM molecule appears to be assembled with great latitude in the degree of disulfide crosslinking between monomeric or halfmer subunits composing the complete IgM molecule. This heterogeneity in the basic structure (herein referred to as redox forms) is not due to isotypic differences as each B cell produces this heterogeneity within its immunoglobulin product. Additionally, in the case of the catfish, a single fish/mouse chimeric Ig H gene is capable of producing IgM with a comparable amount of structural heterogeneity within the mouse cell. Thus, the piscine B lymphocyte routinely assembles a variety of redox forms from one IgM chain. This has both profound biosynthetic implications for macromolecular assembly processes as well as intriguing possibilities for the generation of teleost Ig functional diversity.