Abstract

Abstract The tetrameric IgM molecule is the dominant immunoglobulin found in the serum of fish. In contrast to the fully polymerized mammalian IgM, teleost IgM is variably disulfide cross-linked resulting in considerable structural diversity. Recently, it was revealed that following vaccination, rainbow trout alter the assembly of their antigen-specific IgM. It was hypothesized that these different IgM forms may allow this molecule to switch effector functions. To investigate whether there was a link between structure and function, we used serum with differing IgM structural profiles and evaluated their ability to mediate phagocytosis. We discovered that when IgM was fully disulfide cross-linked it increased phagocytosis of the antibody-opsonized bacteria. Conversely, when a larger percentage of the IgM was held together by electrostatic forces, phagocytosis was diminished. Control experiments proved that differences in antibody coating were not responsible for the differences in phagocytic potential of the antibody pools. Thus, rainbow trout IgM that is more heavily disulfide polymerized facilitates antibody-mediated phagocytosis more effectively. To our knowledge, this is the first time that IgM structural diversity has been demonstrated to affect a biological function critical for the protection against infectious disease.

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