Human IgM and a bovine, IgM-enriched serum fraction isolated from normal adult serum at concentrations of 0.25–1 mg/ml protein induced a pronounced increase in the metachronal wavelength of the lateral (L) cilia of the sea mussel Mytilus edulis without altering their beat frequency. This change in activity was indistinguishable from that induced by 50% adult human or bovine serum. At protein concentrations ranging from 1–9 mg/ml, human IgG or a bovine, IgG-enriched serum fraction had no or little effect on the activity of the L cilia. Similarly, neither monomeric (8S) human IgM (0.25 mg/ml) nor monospecific pentameric IgM (1 mg/ml) isolated from Waldenström's macroglobulinemia patients altered the metachrony of the L cilia. Indirect immunofluorescence demonstrated that both bovine and human IgM became attached almost exclusively to the L cilia, while very little bovine or human IgG was found to associate with these cilia.
The results of this study suggest that serum IgM specifically binds to the L cilia of Mytilus in an antigen–antibody manner and agglutinates adjacent cilia into blocks or bundles, thereby increasing the coupling between cilia. As a result, the wavelength of the metachronal coordination is increased. The origin of these ciliary antibodies and their significance to ciliary bioassays used to monitor serum for the detection of cystic fibrosis are discussed.