B cell memory following infection and challenge of channel catfish with Ichthyophthirius multifiliis

B cell responses in channel catfish to infection with the parasitic ciliate Ichthyophthirius multifiliis were followed for 3 years. High titers of serum IgM antibodies recognizing I. multifiliis immobilization antigens were present 5weeks after immunizing infection, but by 1 year titers were at low or undetectable levels. Two to three years after infection the numbers of antibody secreting cells recognizing immobilization antigens in skin and head kidney of immune fish had decreased to the level found in uninfected controls. Challenge of immune fish showed they remained immune and that the numbers of antibody secreting cells recognizing immobilization antigens increased in skin but not head kidney. This suggests that antigen-specific memory B cells persisted for 3 years after infection and upon challenge differentiated into antibody secreting cells that localized in skin. Our results suggest that humoral immunity in channel catfish is maintained through IgM(+) memory B cells.