Ichthyophthirius multifiliis infection and elements of mucosal immunity in the channel catfish, Iictalurus punctatus

Abstract

We are using the channel catfish (Ictalurus punctatus) infected with the ciliated protozoan parasite Ichthyophthirius multifiliis to investigate the mechanisms and cellular elements of mucosal immunity. Experiments were conducted to determine by ELISA levels of parasite-specific antibody in sera and cutaneous mucus of fish immunized against I. multifiliis. The systemic immune response of fish following surface exposure to parasites or intraperitoneal injection of purified i-antigen resulted in significant levels of I. multifiliis-specific sera antibody (Ab) at five weeks that increased over time until the end of sampling at 14 weeks. Cutaneous mucus of both treatment groups contained Ab levels that were much lower than those detected in sera. Parasite-exposed fish had mucus Ab levels beginning at three weeks post-treatment that corresponded with clearing of infection. In both treatment groups, cutaneous mucus Ab levels were not synchronous with sera levels over time suggesting that fish mucosal Ab arise by a mechanism other than passive diffusion from serum. In a second study, ultrastructural analysis of resinembedded channel catfish head kidney and skin by transmission electron microscopy (TEM) identified a specific population of cells that were labeled by goat anti-catfish immunoglobulin (Ig)-Fab-2 fragments and a biotinylated rabbit-Fab made against goat-Fab followed by avidin-gold beads. Labeling was limited to the cytoplasmic vesicles while other lymphocytes, macrophages and epidermal mucus cells were negative. The labeled cells were characterized as antibody-secreting cells (ASC) based on their morphology and staining pattern. During the course of these studies, trophonts were detected within the peritoneal cavities of fish infected by surface exposure. This result implies that I. multifiliis may invade tissues other than the exterior mucosal tissues of fish. Taken together, the TEM results are the first report of ASC in channel catfish skin while the ELISA results suggest that cutaneous antibodies may arise by local synthesis. These data support the hypothesis of a separate mucosal immune system in fish as well as the importance of cutaneous Ab in protective immunity against I. multifiliis.