Immunoglobulin, antibody production and mucus secretion in the skin of channel catfish, Ictalurus punctatus

Skin and cutaneous mucus of channel catfish were studied for immunoglobulin (Ig) and antibody (Ab) production and secretion and mucus secretion. Levels were examined before and after immunization or pathogenic infection. An enzyme-linked immunosorbent assay (ELISA) was developed to quantify channel catfish Ig, detecting Ig levels as low as 0.1 {dollar}mu{dollar}g ml{dollar}sp{lcub}-1{rcub}.{dollar} Serum Ig increased, independent of challenge or vaccination between 3 and 15 months of age, while mucus Ig concentration slightly decreased. Mucus Ig concentrations were highest on lateral skin, immediately caudal to opercula and lowest on ventral skin, between opercula and pectoral fin. Serum and mucus Ig concentrations in {dollar}sim{dollar}15-month-old channel catfish markedly increased following infection with Edwardsiella ictaluri. Serum Ig concentration increased 13 days post-infection, while mucus Ig increased 14 days later, 27 days post-infection. Infection with Ichthyophthirius multifiliis caused a significant increase in cutaneous mucus Ig concentration 18 days post-infection, but there was no significant change in serum Ig. Antibody was detected in serum, but not in mucus of fish following infection with E. ictaluri and following bath immunization with dinitrophenylated human serum albumin (DNP-HuSA). Parenteral immunization by intra-muscular (IM) injection produced much higher serum, than mucus anti-DNP Ab titers. Infection with I. multifiliis did not produce detectable immobilizing Ab titers in serum or mucus. Immunoglobulin producing cells were demonstrated in the epidermis of healthy channel catfish by an indirect immuno-fluorescent method. Cells producing Ig were located in the upper and lower 3-4 epidermal cell layers, in the epidermal-dermal junction and to a lesser extent in the middle of the epidermis, between alarm substance cells. Cell diameter was 5-7 {dollar}mu{dollar}m, within the size range of blood lymphocytes. Skin sections from fish infected with I. multifiliis showed Ig producing cells surrounding I. multifiliis trophonts, solitary Ig producing cells in the epidermis, and an elevated immuno-staining in the epidermal-dermal junction. Mucus producing and secreting goblet cells, dispersed at different depths of the epidermis, were visualized using an indirect immuno-fluorescence method. An ELISA was developed for the evaluation of cutaneous mucus protein secretion in the channel catfish and as a method for detecting changes in secretion. Mucus protein levels were highest on lateral skin, immediately caudal to opercula, decreasing towards caudal fin and lowest on ventral skin, between opercula and caudal fins. Infection with I. multifiliis did not affect quantity of cutaneous mucus protein secretion.